Ventilation and Airflow in Buildings
BUILDINGS | ENERGY | SOLAR TECHNOLOGY Ventilation and Airflow in Buildings Methods for Diagnosis and Evaluation Claude-Alain Roulet London .
First published by Earthscan in the UK and USA in 2008 Copyright # Claude-Alain Roulet, 2008 All rights reserved ISBN-13: 978-1-84407-451-8 Typesetting by 4word Ltd, Bristol Printed and bound in the UK by TJ International Ltd, Padstow, Cornwall Cover design by Paul Cooper For a full list of publications please contact: Earthscan 8–12 Camden High Street London, NW1 0JH, UK Tel: þ44 (0)20 7387 8558 Fax: þ44 (0)20 7387 8998 Email: earthinfo@earthscan.co.uk Web: www.earthscan.co.
Contents List of Figures and Tables Preamble Introduction vii xii xiii 1 Airflow Rates in Buildings Single-zone measurements Application to buildings, multi-zone Further interpretation of the flow matrix Equations for volume flow rates Summary of the various tracer gas methods 1 1 6 9 11 12 2 Airflow Rates in Air Handling Units Measurement of the airflow rate in a duct Airflow measurements at ventilation grilles Airflow rate measurements in air handling units Principle of the interpretation procedure Node b
vi Ventilation and Airflow in Buildings Measurement methods Determining the leakage coefficients Corrections for standard conditions Ways of expressing the airtightness Airtightness of buildings Measurement of airtightness of a duct or network 5 59 63 65 66 67 74 Measurements and Measures Related to Energy Efficiency in Ventilation Energy in buildings Energy in air handling units Heat exchangers Energy for ventilation Energy effects of indoor air quality measures 77 77 79 83 97 102 6 Contaminants in Air Ha
List of Figures and Tables Figures 0.1 0.2 0.3 0.4 0.5 0.6 1.1 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.
viii 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.
List of Figures and Tables 5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17 5.18 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13 6.14 6.15 6.16 6.17 7.1 7.2 7.
x 7.4 7.5 Ventilation and Airflow in Buildings Student distribution for 1, 2 and 5 degrees of freedom compared to the normal distribution Confidence limit divided by standard deviation versus number of measurements for various values of probability, P 158 159 Tables 1.1 1.2 1.3 2.1 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 5.1 5.2 5.3 5.4 6.1 6.2 6.3 6.4 6.
List of Figures and Tables 6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13 6.14 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.
Preamble This book includes information already published by the author in scientific journals and in an Air Infiltration and Ventilation Centre (AIVC) technical note (Roulet and Vandaele, 1991), now sold out. Part of the content of Chapters 2, 3 and 5 was also published by the author in a book edited by H. Awbi (Awbi, 2007). Roulet, C.-A. and L. Vandaele, 1991, Airflow patterns within buildings: Measurement techniques. AIVC Technical Note 34, AIVC, Bracknell, 265pp, order at inive@bbri.be Awbi, H.
Introduction Why ventilate? Without ventilation, a building’s occupants will initially be troubled by odours and other possible contaminants and heat. Humidity may rise because of indoor moisture sources such as the occupants, laundry, cooking and plants; thus enhancing moisture hazards (for example, mould growth and condensation). Oxygen will nevertheless not be missed until much later.
xiv Ventilation and Airflow in Buildings However, when the outdoor temperature exceeds indoor temperatures, it may be wise to reduce the ventilation rate, only allowing high levels of ventilation at night when the outdoor temperature is low. Ventilation is hence not only essential to ensure an acceptable indoor air quality, but is also often used to improve thermal comfort.
Introduction xv Measured [m3/(h · p)] 300 250 200 150 100 50 0 0 100 200 300 Design airflow rate [m3/(h · person)] Figure 0.1 Design and measured outdoor airflow rate per person in 12 buildings Source: Roulet et al., 1999. Measured airflow rates differ from the design values in many ventilation systems. Figure 0.2 shows the relative differences between measured and design outdoor airflow rate in 37 air handling units, i.e.
xvi Ventilation and Airflow in Buildings 100% Measured 75% 50% 25% 0% 0% 25% 50% 75% Design recirculation rate 100% Figure 0.3 Comparison of design and measured recirculation rate in 34 air handling units Source: Roulet et al., 1999. measured recirculation rates are compared in Figure 0.3. These are seldom the same. Even worse: as shown in Figure 0.4, out of 27 units planned without recirculation, 30 per cent have shown a recirculation rate of more than 20 per cent.
Introduction xvii 100% Measured 75% 50% 25% 0% -25% -10% 0% 10% Design exfiltration ratio 20% Figure 0.5 Design and measured exfiltration ratios compared in 30 units Source: Roulet et al., 2001a. the design and measured exfiltration ratios, i.e. parts of the supply air leaking through the building envelope in 30 units. Ventilation efficiency An efficient airing supplies the occupants with fresh air and blows polluted, old air in unoccupied spaces.
xviii Ventilation and Airflow in Buildings When assess airflows in buildings? Ventilation performance should be checked early to detect potential problems and to optimize the overall performance of the ventilation system. This includes: . . . . . appropriate airflow rates; negligible leakage and shortcuts; high ventilation efficiency; high fan efficiency; clean air and so on. This check should be performed: . . .
Introduction xix effectiveness of the ventilation in appropriately distributing the air in the ventilated space or in evacuating contaminants emitted at a given location can be assessed using the methods described in Chapter 3. When applied in the air handling unit, such measurements can be performed simultaneously with measurements of airflow rates, thus reducing the amount of work required.
xx Ventilation and Airflow in Buildings 1995, 2003). This can, however, be avoided by appropriate design and maintenance. Chapter 5, ‘Energy effect of IAQ measures’, lists the sources and causes of pollution in ventilation systems, proposes measurement protocols, and sets out maintenance procedures and strategies to improve the quality of the air delivered by mechanical ventilation systems.
1 Airflow Rates in Buildings This chapter intends to help the reader to measure airflow rates and air change rates in buildings and rooms, independently of a mechanical ventilation system. It presents the techniques used to measure the airflow rate entering the measured zone (single-zone measurements) and to measure inter-zone airflows (multi-zone measurements).
2 Ventilation and Airflow in Buildings outdoor environment is then: dm ¼ I þ Co Qoi Ci Qio dt ð1:1Þ where: m I C Q i o is the mass of tracer gas in the zone (kg); is the injection rate of the tracer gas (kg/s); is the tracer gas mass concentration; is the mass airflow rate (for example, Qio is airflow rate from indoor to outdoors); subscript for internal environment; subscript for external environment.
Airflow Rates in Buildings 3 This solution can be simplified, depending on the way the tracer is injected. Tracer decay, no injection A suitable quantity of tracer gas is injected to achieve a measurable initial concentration Ci;0 . At time t0 , this injection is stopped and I ¼ 0 afterwards. From Equation 1.4, it can be found that the concentration decays with time according to: Q ð1:9Þ C ¼ Cðt0 Þ exp io t M The quantity n ¼ M Qio ð1:10Þ is called the nominal time constant of the measured zone.
4 Ventilation and Airflow in Buildings Constant concentration Using an electronic mass flow controller monitored by the tracer gas analyser, the concentration of tracer gas can be maintained constant by varying the injection rate in an appropriate way. In this case, the time derivative of the concentration is zero and Equation 1.
Airflow Rates in Buildings Applying again the integral mean value theorem, we get: ð tf m ¼ Qs;i ð 0 Þ CðtÞ dt with 0 < 0 < tf 5 ð1:20Þ 0 Combining Equations 1.18 and 1.20, we get: m Qi ð Þ ¼ Qs ð 0 Þ M ð1:21Þ There are sampling pumps with controlled constant flow rate. These easily enable Qs;i to be kept constant, and in most cases it is possible to keep the airflow rate constant in supply and exhaust during the experiment.
6 Ventilation and Airflow in Buildings Figure 1.1 Records of CO2 concentration in an office room Source: Roulet and Foradini, 2002. concentration C (Ci indoors and Co outdoors) by: Qe ¼ S Ci Co ð1:22Þ where S is the CO2 source strength, i.e. about 20 l/h. The equivalent outdoor airflow rate per person can then be assessed during the periods of time when steady state can reasonably be assumed, that is when the CO2 concentration is constant.
Airflow Rates in Buildings 7 do not change during the measurement campaign. This section describes ways of interpreting the records of tracer gas injection rates and concentration in the different zones to get the airflow rates between zones, as well as airflow to and from outdoors. For more information on tracer gases and analysers, see Chapter 7, ‘Tracer gas dilution techniques’.
8 Ventilation and Airflow in Buildings where: mik Iik Cjk Cik Qij is is is is is the the the the the mass of tracer gas k in zone i; injection rate of tracer gas k in (or just upwind of ) zone i; concentration of tracer k in zone j; concentration of tracer gas k in zone i; airflow rate from node j to node i. An extension of assumption 1 above is implicit in this equation, that is: 4 The airflow entering a zone does not modify the homogeneity of the concentration of tracer gases in that zone, i.e.
Airflow Rates in Buildings 9 C contains the differences in mass concentrations Cik C0k of gas k in zone i. I is the matrix containing the mass flow rates Iik of the tracer, k, in zone i. In usual measurements, this matrix is diagonal. Q is the so-called flow matrix containing, the off-diagonal elements ( j 6¼ i) being Qij , where Qij represents the mass flow rates from zone j to zone i. The diagonal elements with j ¼ i contain the sum of the flows leaving the zone i, as defined in Equation 1.25. In Equation 1.
10 Ventilation and Airflow in Buildings Properties of the flow matrix The total outdoor airflow rate to each zone, i, is easily obtained by summing the columns of the flow matrix: Qi0 ¼ N X Qij ð1:31Þ j¼1 And the total exfiltration airflow rate from each zone, i, is the sum of the lines of the flow matrix: Q0i ¼ N X Qij ð1:32Þ i¼1 If there is no totally isolated chamber in the measured system, and if there is some air exchange with outside (as is the case with any usual building), the flow matrix determ
Airflow Rates in Buildings 11 sums of the matrix are the mean age of air in the corresponding rooms: h i i ¼ N X ð1:38Þ ij j¼1 This relation enables the measurement of the room mean age of air to be made, even in rooms where there are several outlets or several ways for the air to leave the room. Equations for volume flow rates All equations above are based on mass balance, and hence include mass airflow rates and mass concentrations.
12 Ventilation and Airflow in Buildings The air mass conservation (Equation 1.23) is rewritten as: q0i ¼ Ti N N X qij ð1 ij Þ X V dTi qji ð1 ij Þ þ i Ti dt T j j¼0 j¼1 ð1:43Þ These last two systems include N þ 1 equations for N þ 1 unknowns, qij , for each zone i. It should be noticed that Equation 1.43 can be simplified, and becomes similar to Equation 1.23 if indoor and outdoor temperatures are close to each other and if the internal temperature is constant.
Airflow Rates in Buildings 13 Table 1.
14 . Ventilation and Airflow in Buildings Constant injection used with long-term direct solution is simpler to use and may give, under certain conditions, unbiased estimates of an average airflow rate. The two-point decay method, and more generally the multi-zone, transient methods may lead to unacceptably large uncertainties if the measurement time period is inappropriate. See Enai et al. (1990) for two-zone, two-tracer, step-up and decay methods.
2 Airflow Rates in Air Handling Units Air handling units are designed to supply new air to the ventilated zone and to extract vitiated air from this zone. Many other airflows may be found in such units, as shown in Figure 2.1. Measurements of airflow rates in ventilation systems are useful in order to check if the air follows the expected paths and thus detect potential problems early so they can be corrected, also allowing the optimization of the performance of the airflow system.
Ventilation and Airflow in Buildings Fan 16 Extract air Ventilated space Exhaust air Outdoor air Fan Heating Humidifaction Cooling Heat exchanger Filter Recirculation dampers Supply air Figure 2.1 Schematics of a supply and exhaust air handling unit Note: The main airflow paths are shown as solid arrows, and secondary or parasitic airflow paths are shown as open arrows. Source: Roulet et al., 2000a. placed in the flow, for example a nozzle, Venturi or sharp-edged orifice (ISO, 2003).
Airflow Rates in Air Handling Units 17 is the reduction ratio, which is the ratio of the smallest diameter to the diameter of the pipe. The flow may be restricted with an orifice plate, a nozzle or a Venturi tube. The most sophisticated and expensive is the Venturi tube in which the discharge coefficient is nearly 1 and constant for Re > 2 105 and higher than 0.94 if Re > 50,000. Moreover, this device does not induce a large pressure drop in the flow.
18 Ventilation and Airflow in Buildings 0.316 R 0.548 R 0.707 R 0.837 R 0.949 R Figure 2.2 Location of the measurement points in circular and rectangular ducts Source: ASHRAE, 2001. from 0.05 to 5 m/s, and are well suited for speeds of 1–5 m/s, which are typical in ventilation ducts. Helix anemometers measure the rotation speed of a small helix that is placed perpendicular to the airflow.
Airflow Rates in Air Handling Units 19 Figure 2.3 Measuring the airflow rate in a duct with the tracer gas dilution method Source: Awbi, 2007.
Ventilation and Airflow in Buildings Airflow 20 Fan Flowmeter Differential manometer Figure 2.4 Schematics of a compensated flowmeter Note: The differential manometer adjusts the fan so that there is no pressure drop through the flowmeter. The methods described in the section on ‘Measurement of airflow rate in a duct’ (above) may of course be used to measure the airflow rates through grilles, but specific instruments may be easier to use or bring a more accurate result.
Airflow Rates in Air Handling Units C0 C4' C7 C5 C6 21 C4 2 1 C1 C1' 3 C2 C3 C3' 4 Figure 2.5 Locations of tracer gas injection (arrows), and sampling points for concentration measurements (Ci ) in a typical supply and exhaust air handling unit. Source: Roulet et al., 1999. In principle, the method described above in ‘Tracer gas dilution’ can be applied to each branch of a duct network. However, this requires as many tracer gas injections and air sampling measurements as there are airflow rates.
22 Ventilation and Airflow in Buildings expected tracer gas concentration of tracer k: Ik ¼ Ck Q01 ð2:5Þ Sampling points for concentration measurements Tracer gas concentrations are measured at several carefully chosen locations in order to obtain enough information to determine all the wanted airflow rates. It is important that there is a good mixing of tracer gas in the measured airflow. For this, several criteria should be fulfilled.
Airflow Rates in Air Handling Units 23 Figure 2.7 Example of multiple injection devices bringing additional tracer into the analyser, thus biasing the concentration measurement. To avoid this, use different colours for injection and sampling tubes. Principle of the interpretation procedure The ductwork is modelled by nodes connected by ducts.
24 Ventilation and Airflow in Buildings gas conservation equations can be rearranged so as to obtain one system of equations per node, giving all airflow rates entering in this node. At steady state Iik ¼ N X ½Cjk Cik Qji ð2:6Þ j¼0 where: Iik is the injection rate of tracer gas k in (or just upwind of ) node i, Cjk is the concentration of tracer gas k in (or just downwind of ) node j, Qji is the airflow rate from node j to node i.
Airflow Rates in Air Handling Units 25 2 Q60 Recirculation AHU 6 Q61 1' Q12 1 Q04 Q46 4' 4 Q62 3' Ventilated space Q40 0 1 3 AHU room 3 Figure 2.8 The simplified network representing the air handling unit and ducts Note: Numbers in black circles represent the nodes of the network; boxes with arrows are tracer gas injection locations; and numbered balloons are air sampling locations. Arrows represent possible airflow rates. Source: Awbi, 2007.
26 Ventilation and Airflow in Buildings Node 2, return I11 ¼ ðC11 C31 ÞQ12 þ ðC61 C31 ÞQ62 þ ðC71 C31 ÞQ72 ð2:10Þ 0 ¼ ðC12 C32 ÞQ12 þ ðC62 C32 ÞQ62 þ ðC72 C32 ÞQ72 0 ¼ ðC13 C33 ÞQ12 þ ðC63 C33 ÞQ62 þ ðC73 C33 ÞQ72 0 ¼ ðC14 C34 ÞQ12 þ ðC64 C34 ÞQ62 þ ðC74 C34 ÞQ72 Node 4, vented space 0 ¼ ðC01 C41 ÞQ04 þ ðC31 C41 ÞQ24 ð2:11Þ 0 ¼ ðC02 C42 ÞQ04 þ ðC32 C42 ÞQ24 I43 ¼ ðC03 C43 ÞQ04 þ ðC32 C42 ÞQ24 0 ¼ ðC04 C44 ÞQ04 þ ðC34 C44 ÞQ24 Node 6, recirculation 0 ¼ ðC31 C
Airflow Rates in Air Handling Units 27 This system of 27 equations when combined with the system of Equation 2.9 can be solved in various ways to provide the six main airflow rates and potentially ten parasitic flow rates. This global system of equations contains more equations than unknowns. There are several ways to address this situation, and we have found that some methods are better than others for application to air handling units. Therefore we present the tested methods below.
28 Ventilation and Airflow in Buildings Simplest way A method providing all airflow rates with the simplest solutions – hence probably the least sensitive to measurement errors – is given below. The results are provided with their confidence intervals, calculated under the assumption that random and independent errors affect the measurements of tracer gas concentration and injection rates.
Airflow Rates in Air Handling Units 29 with Q46 ¼ TðP; 1Þ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 ðC40 2 C42 Þ2 I52 þ I52 ð C420 2 þ C42 Þ ðC40 2 C42 Þ4 ð2:28Þ Outdoor airflow rate Q01 ¼ Q12 C6k C1k C6k C0k ð2:29Þ where k ¼ 1 or 2 is recommended.
30 Ventilation and Airflow in Buildings The bias resulting from the alias with Q72 is not taken into account in the confidence interval.
Airflow Rates in Air Handling Units 31 or ðC3k C4k Þ ðC C1k Þ ðC3k C4k Þ ¼ Q12 6k ðC4k C0k Þ ðC6k C3k Þ ðC4k C0k Þ p ffiffiffiffiffi ffi TðP; 1Þ f04 ¼ ðC4k C0k Þ2 Q04 ffi Q24 ð2:41Þ Q04 ð2:42Þ where f04 ¼ ðC6k C3k Þ2 ðC3k C1k Þ2 Q212 2 2 2 þ Q212 ½ðC6k C3k Þ2 C1k þ ðC6k C1k Þ2 C3k þ ðC1k þ C6k Þ2 C6k with k 6¼ 3 (recommended value: k ¼ 1).
32 Ventilation and Airflow in Buildings with k 6¼ 2 (k ¼ 4 is not recommended here). Q76 ¼ TðP; 1Þ pffiffiffiffiffiffi f76 C7k C6k ð2:52Þ where 2 2 2 f76 ¼ Q226 C3k þ Q246 C4k þ ðQ26 þ Q46 Q76 Þ2 C6k 2 þ ðC3k C6k Þ2 Q226 þ ðC4k C6k Þ2 Q246 þ ðk; 2Þ I6k where: k ¼ 1, 2 or 3; and the delta function ðk; 2Þ ¼ 1 if k ¼ 2 and 0 if k 6¼ 2. Leakage airflow rates to the technical room can be obtained from system and equation (2.19).
Airflow Rates in Air Handling Units 33 Planning tool There are many types of air handling units, and, from our experience, each new measurement poses new problems. It is hence impossible to provide a detailed measurement protocol valid for all types. Therefore, a computer program was developed that performs the following tasks: . . . . .
34 Ventilation and Airflow in Buildings 100 N2O [ppm] 80 C3: Supply air C4: Room air C4¢: Exhaust 60 40 C6: Relief air 20 0 10:30 10:45 11:00 Time [h] 11:15 11:30 Figure 2.10 Concentrations at locations shown in Figure 2.5 resulting from injection of SF6 as tracer 1 and N2 O as tracer 2 in a leaky air handling unit Note: A shortcut through the heat exchanger dilutes exhaust air, thus decreasing the relief air concentration.
Airflow Rates in Air Handling Units 35 concentrations are in volumetric ratios. It is not possible with only one tracer injected into the ventilated space to differentiate between outdoor air from mechanical ventilation and from infiltration. Measurements in buildings with large time constants Most methods are designed to measure units with recirculation ratios below 50 per cent. This is the case of the method proposed above.
16 40 35 14 C3 30 12 C3¢ 25 10 C3¢–C3 20 8 6 15 4 10 Tracer injection 5 0 0 2 4 6 2 8 10 Time (h) 12 14 16 0 Concentration difference [mg/m3] Ventilation and Airflow in Buildings Tracer gas concentration (mg/m3) 36 Figure 2.11 Tracer gas concentrations in the supply duct, upstream (3) and downstream (30 ) of the tracer gas injection port Note: Points are measured concentrations, while lines are exponential fits. Source: Roulet and Zuraimi, 2003.
Airflow Rates in Air Handling Units 37 The recirculation airflow rate can then be calculated using: Q62 ¼ Q24 Q12 ð2:66Þ with Q62 ¼ TðP; 1Þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Q224 þ Q212 ffi TðP; 1Þ 1 þ ð1 RÞ2 Q ð2:67Þ assuming that the relative error Q=Q is the same for both airflow rates, and taking into account that Q12 ¼ ð1 RÞQ24 . Note that, in this case, Q62 decreases when R increases.
Ventilation and Airflow in Buildings Confidence interval of R 38 15% A 10% B C 5% 0% 0% 20% 40% 60% 80% Recirculation ratio 100% Figure 2.12 Confidence interval of the recirculation ratio as a function of the recirculation ratio itself for three assessment methods Note: For this figure, the relative confidence intervals of injection rate and concentrations are 5 per cent.
3 Age of Air and Ventilation Efficiency The airflow patterns should, in principle, be organized in order that new air is brought to the head of the occupants, so that they get fresh, clean air, and that contaminants be evacuated as quickly as possible, before being mixed with indoor air. However, air, as any other fluid, always follows the easiest path. This means that the airflow does not necessarily follow expected patterns.
40 Ventilation and Airflow in Buildings However, the more time a small volume of air spends in a room, the more it will be contaminated by pollutants. Since there is a large number of air particles all taking different paths, we define a probability density f( r ) that the age of particles arriving at a given location is between and þ d and, a probability F( r ) that this age is larger than .
Age of Air and Ventilation Efficiency 41 Figure 3.1 Ventilation modes with typical airflow patterns and air change efficiencies Source: Roulet, 2004. Table 3.1 Nominal time constant and room mean age of the air corresponding to the probability curves shown in Figures 3.2 and 3.3 a h i Air exchange efficiency Room mean age of the air 25% 1.44 50% 0.99 66% 0.76 80% 0.62 90% 0.55 99% 0.50 constant.
42 Ventilation and Airflow in Buildings F(τ) 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 η 25% 50% 66% 80% 90% 99% 100% 0 0.5 1 1.5 2 2.5 τ 3 Figure 3.3 Typical probability curves for the age of the air Note: These theoretical curves are for illustration. Some of them, in particular at very high efficiency, are not likely to be found in practice. corresponding to the nominal time constant. The air exchange efficiency in this very theoretical case is 100 per cent.
Age of Air and Ventilation Efficiency 43 assumes that the tracer gas behaves the same as the air: no adsorption and same buoyancy, which is the case if the tracer concentration is small. It can be readily understood that if the air is marked at the inlet by a short pulse of tracer gas, and if the tracer molecules follow the air molecules, they will arrive at a given location at the same time as the air molecules.
44 Ventilation and Airflow in Buildings Concentration [ppm] 25 C∞ 20 Step-up 15 Decay 10 5 to Injection 0 09:00 09:10 09:20 09:30 to 09:40 09:50 10:00 10:10 10:20 Figure 3.
Age of Air and Ventilation Efficiency 45 The local mean age of air at any location is the integral (or zero moment) of the probability distribution: ð1 Fr ðtÞ dt ð3:8Þ r ¼ 0 ¼ 0 The first moment of the probability distribution is, by definition: ð1 1 ¼ tFr ðtÞ dt ð3:9Þ 0 If there is only one single exhaust, the room mean age of air can be deduced from tracer concentration measurements in the exhaust duct, Ce ðtÞ: ð1 tFe ðtÞ dt 1 h i ¼ ¼ ð01 ð3:10Þ 0 e Fe ðtÞ dt 0 In this case, the nominal time c
46 Ventilation and Airflow in Buildings where: Fj is the probability distribution at time t ¼ j t, Step-up case Fj ¼ 1 Cðt0 þ j tÞ Cð1Þ Decay case Fj ¼ Cðt0 þ j tÞ Cðt0 Þ ð3:16Þ N is the last measurement integrated using the trapezium method, "n ðN; d ) is the rest of the integral, evaluated using an exponential fit on the last measurements (see below).
Age of Air and Ventilation Efficiency 47 with sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 Ce ð j t þ t0 Þ 2 Ce ð j t þ t0 Þ Ce ð1Þ Fj ¼ TðP; 1Þ þ Ce ð j t þ t0 Þ Ce2 ð1Þ Then, the confidence intervals of the moments are: qffiffiffiffiffiffi qffiffiffiffiffiffi and 1 ¼ TðP; 1Þ f 1 0 ¼ TðP; 1Þ f 0 ð3:20Þ ð3:21Þ with f 0 ¼ t 2 þ 2 ðð F0 Þ2 þ ð FN Þ2 Þ þ ð tÞ2 N 1 X Fj2 j¼1 F0 þ FN þ 2 N 1 X 2 Fj ð tÞ2 þ ð "0 Þ2 ð3:22Þ j¼1 and f 1 ¼
48 Ventilation and Airflow in Buildings Air inlets Exhaust grilles Conference room Hall Office Figure 3.6 Arrangement of the conference room and of its surroundings Source: Roulet et al., 1998. a displacement ventilation system, as shown in Figure 3.6. The conference room is 8 m by 10 m wide and 3 m high. It is completely embedded in an old, massive building. Its walls, floor or ceiling have no contact with the outdoor environment. It has no windows, but leaky entrance doors leading to a hall.
Age of Air and Ventilation Efficiency 49 Air inlets Added air inlet Exhaust grilles Added exhaust grille Added door Conference room Hall Office Figure 3.7 Arrangement of the conference room after improvement Source: Roulet et al., 1998. 100% 1000 800 700 Mean age of air Nominal time constant Air change efficiency 90% 80% 70% 600 60% 500 50% 400 40% 300 30% 200 20% 100 10% Air change efficiency Age and time constant [s] 900 0% 0 Initial values After improvement Figure 3.
50 Ventilation and Airflow in Buildings straightforward, takes time and has its cost, the theory of experimental design (Box et al., 1978) may help in providing a maximum of information through a minimum of measurements. Minimum number of measurements A map of any scalar variable, v, in a three-dimensional room is in principle obtained by measuring the variable at each node of a network and interpolating between these nodes. Such measurements are, however, very expensive and may be unfeasible.
Age of Air and Ventilation Efficiency 51 Table 3.2 Minimum number of measurements needed to obtain the coefficients of a kth degree polynomial empirical model representing a variable in a two- and three-dimensional space Model dimensions 2 3 Linear Interaction Quadratic Cubic 4th degree 3 4 4 7 6 10 10 20 15 35 for which seven coefficients must be determined. Table 3.2 summarizes the minimum number of measurements needed.
52 Ventilation and Airflow in Buildings The criteria described below are used to establish the most efficient design. Criteria for location of the measurement points The model matrix M First, let us look at the method used to obtain these coefficients. For each point, the model is applied, replacing the xi by their values given by the experimental design.
Age of Air and Ventilation Efficiency 53 If 2 is the experimental variance of the measured variable v, the variance (ve ) of the estimated variable is: 2 2 ðve Þ ¼ rT ðMT MÞ 1 r 2 ð3:36Þ A variance function can be defined: VF ¼ N 2 ðve Þ ¼ NrT ðMT MÞ 1 r 2 ð3:37Þ where N is the number of measurements. VF depends on the experimental design (M and N) and on the location r and can hence be calculated before doing any measurement.
54 Ventilation and Airflow in Buildings As mentioned above, the experimental domain is about 20 per cent smaller than the measured space, samples of air being taken at least 0.1 times the characteristic enclosure dimension from the walls. Factorial designs A factorial design for k dimensions and l levels is obtained by dividing the experimental domain (for example, the interval [ 1; 1]) on each axis into l equidistant levels.
Age of Air and Ventilation Efficiency 55 Figure 3.9 Minimum design for a 2-D quadratic model can be added to obtain a minimum design for a quadratic model, which has a condition number of 6.3 (see Figure 3.9). The 2-D full factorial design with three levels shown in Table 3.5 has a better condition number (4.4) for a quadratic model. Table 3.
56 Ventilation and Airflow in Buildings Table 3.7 Full factorial design for assessing the coefficients of a linear model with interactions No x y z 1 2 3 4 5 6 7 8 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Table 3.8 3-D centred star design No x y z 9 10 11 12 13 14 15 1 0 0 1 0 0 0 0 1 0 0 1 0 0 0 0 1 0 0 1 0 the points 8, 5 and 2 can be deleted (in that order) giving finally a design having 12 points and a condition number of 4.8.
Age of Air and Ventilation Efficiency 57 Table 3.9 Condition number of MT M for some experimental designs and three models Experimental design Number of points Quadratic model Interactions model Linear model 3 4 5 6 9 – – – 6.3 4.4 – – 1.0 1.0 1.0 2.0 1.0 1.0 1.0 1.0 4 8 10 15 – – 4.3 4.4 – 1.0 3.2 1.0 1.0 1.0 1.0 1.
4 Airtightness Why check airtightness? Controlled airflows, having adequate flow rate and passing at the appropriate locations are essential for good indoor air quality. Leakage, allowing the uncontrolled air to follow inappropriate paths, should therefore be reduced as much as possible. This requires an airtight building envelope and airtight ductwork: building and ductwork airtightness is a prerequisite for efficient natural or mechanical ventilation.
Airtightness 59 Infiltration ratio 60% 50% 40% 30% 20% 10% 0% 1 2 3 4 5 6 7 8 9 10 11 Figure 4.1 Measured part of outdoor air that is not supplied by the system in mechanically ventilated buildings, shown with uncertainty band ventilation rate. Maintaining air quality requires an increase in supply air, leading to energy wastage. Checking the airtightness of a building envelope or a duct network should therefore be performed at each commissioning of a building or ventilation system.
60 Ventilation and Airflow in Buildings column) and the airflow rate through the fan is measured using any of the following methods: . . . The airflow rate through a fan depends on the pressure differential and its rotation speed. Measuring these two quantities allows assessment of the airflow rate from the characteristic curve of the fan. Blower doors use this method. A suitable airflow meter such a nozzle or a sharp-edged orifice is installed in the airflow circuit (see Figure 4.5).
Airtightness 61 Airflow rate [m3/h] 1000 800 600 400 Measured points Power law Quadratic law 200 0 0 10 20 Pressure [Pa] 30 40 Figure 4.
62 Ventilation and Airflow in Buildings flow rates at each pressure using the method described in section ‘Determining the leakage coefficients’, below.
Airtightness 63 Figure 4.3 Principle of the guarded zone technique applied to several walls of a room Note: At the left, only the external wall is measured, while at the right, the right partition wall is included in the measurement. experiments is performed, enough equations can be written to compute the airflows through the various measured parts for each pressure step. For that purpose, the pressure steps should be the same in each experiment, for example, 10, 30, 50 and 70 Pa.
64 Ventilation and Airflow in Buildings its absolute temperature, and as long as the pressure differential remains small with respect to the atmospheric pressure: q ¼ qm T Tm ð4:5Þ where T and Tm are the absolute temperatures of the air going respectively through the measured elements and through the fan or the airflow-measuring device. This assumes that the variations of air moisture do not significantly change the density. Before any further analysis, Equation 4.
Airtightness 65 log(airflow rate) 5.5 5.0 slope n 4.5 log(C) 4.0 3.5 0 1 2 3 Log(pressure difference) 4 Figure 4.4 Logarithmic plot of airflow rates and pressure differences Note: The slope of the best-fit line is an estimate of n and its ordinate at origin is an estimate of logðCÞ. intervals. If the coefficients a and b are known, the airflow coefficient C and the exponent n are calculated using: C ¼ expðaÞ and n¼b ð4:11Þ The Etheridge model in Equation 4.
66 Ventilation and Airflow in Buildings Using the subscript o for these standard conditions and no subscript for the measurement conditions, then: ð2n 1Þ ð1 nÞ ¼ C ð4:15Þ Co o o where is the viscosity (kg s 1 m 1 ), the density of air and n the power law exponent.
Airtightness 67 Virtual air change rate By dividing the airflow rate at conventional pressure by the internal volume of the tested enclosure gives a virtual leakage air change rate at that pressure. For this figure, 50 Pa is the most used pressure difference, and the figure is then noted n50; in [h 1 ]. This value is less than 1 h 1 in airtight buildings but, depending on the climate and building habits, buildings may have figures larger than 10 h 1 .
68 Ventilation and Airflow in Buildings in order to create a pressure difference large enough to minimize influences from wind and temperature differences on the results. This pressure differential is built up and maintained by means of a fan, forcing airflow through the envelope or component to be evaluated.
Airtightness 69 The overall airtightness of the structure and the size of the available fan govern the maximum volume of enclosure that may be pressurized. Even if large fans are available, in large leaky structures it may be possible to only achieve a limited range of pressure differentials. Several researchers have used trailer mounted fans with maximum flow capacities of about 25 m3 /s to examine buildings with volumes as large as 50,000 m3 .
70 Ventilation and Airflow in Buildings Figure 4.7 Roof corner under depressurization Source: Roulet, 2004. case, the airtightness is not good enough and cold air enters the inhabited space through cracks between wooden panels. The stack effect method This simple and easy-to-install method to estimate the air leakage distribution in tall buildings is based on the pressure distribution induced in buildings by the stack effect (Tamura and Wilson, 1966).
Airtightness 71 there is a priori only one neutral plane at the height z0 . The neutral plane is the generally horizontal plane in the building or part of it where the indoor– outdoor pressure differential is zero. Its height depends on the size and position of the ventilation and leakage openings. It is such that the airflows going in and out of the building are balanced.
72 Ventilation and Airflow in Buildings measuring the air speed at several locations and integrating over the whole opening or using a tracer gas. ð zn n dð r Cr pðzr Þn Þ ¼ 0 ð4:26Þ g Qg þ t Ct pðzt Þ þ zrb 3 Windows open at the top level – the airflow through these windows, Qt , is measured. We have similarly: ð zrt n dð r Cr pðzr Þn Þ ¼ 0 ð4:27Þ g Cg pðzg Þ þ t Qt þ zrb The neutral plane is now at the top level. Assuming that n is 0.
Airtightness 73 Figure 4.8 Principle of the neutral height method for assessing leakage area Note: The left shows no leak (except the test opening), while on the right, the leakage is above the test opening. Source: Roulet, 2004. top of the opening to observe the flow direction. The neutral level is located between the ingoing and outgoing flow directions. Sensitivity can be increased or decreased by reducing or increasing the width of the opening.
74 Ventilation and Airflow in Buildings that Ti > To , by: 1=2 3=2 To z 3=2 zn Q ¼ QA 1 n ffi QA ð1 aÞ3=2 a3=2 Ti H H with a¼ ð4:29Þ zn H where zn is the height of the neutral level. The opening area between this neutral level and the mid-height of the opening is close to the equivalent leakage area. If the leakage is small, the sensitivity can be increased by reducing the width of the opening, for example by partly closing the door.
Airtightness Seal 75 Seal Sampling Tracer A Tracer B Seal Seal Sampling Figure 4.9 Location of tracer injection and sampling tubes for the measurement of leakage airflow rates in a ventilation system Source: Roulet and Vandaele, 1991. example, using plastic sheeting and adhesive tape. Inflated balloons are also well suited to seal circular ducts. Tracer gas injection and air sampling tubes are installed at appropriate points in the main supply or exhaust ducts, as shown in Figure 4.
76 Ventilation and Airflow in Buildings upstream end of the duct and its concentration is measured at both ends to give the flow rate at each. The leakage of the whole supply or exhaust network may be determined by measuring the difference between the airflow rate in the main duct (close to the fan) and the sum of all the flow rates at the individual inlet or extract terminals.
5 Measurements and Measures Related to Energy Efficiency in Ventilation Energy in buildings Energy uses and indoor environment quality Energy is used in buildings for many purposes such as: . . . . . . . heating and cooling; drying and humidifying; ventilation (moving the air); hot water supply; lighting; building systems such as lifts, escalators, communication networks; cooking, washing, leisure, producing goods and services.
78 Ventilation and Airflow in Buildings Passive and active ways to get high quality buildings. Passive ways are architectural and constructive measures that naturally provide a better indoor environment quality without or with much less energy use. Examples are: . . . . .
Measurements and Measures Related to Energy Efficiency in Ventilation 79 indoor air quality at a lower energy cost. Once again, measurements may help in commissioning and diagnosing failures.
80 Ventilation and Airflow in Buildings 30 Sat. 90 kJ/kg Water content [g/kg] 25 90% 80 80% 70 20 70% 60 60% 50 15 50% 40 40% 30 10 30% 20 5 20% 10 10% 0 0 10 20 30 ºC Figure 5.1 Psychrometric chart with constant relative humidity curves and constant enthalpy lines Note: It is shown that air at 208C and 50 per cent relative humidity contains about 7.5 g of water vapour per kilogram. Its enthalpy is 39 kJ/kg and its dew point is close to 108C. Figure 5.
Measurements and Measures Related to Energy Efficiency in Ventilation 81 30 Sat. 90 kJ/kg Water content [g/kg] 25 90% 80 80% 70 20 70% 60 60% 50 15 50% 40 40% 30 10 30% 20 5 10 20% Humidifying Heating 10% 0 0 10 20 30 °C Figure 5.2 Paths in the psychrometric chart for heating and humidifying outdoor air in winter to reach 208C and 50 per cent relative humidity Figure 5.
82 Ventilation and Airflow in Buildings 30 Sat. 90 kJ/kg Water content [g/kg] 25 90% 80 70 20 15 80% Cooling 60 70% 50 Drying 60% 50% 40 40% 30 10 30% 20 5 10 20% Reheating 10% 0 0 10 20 30 °C Figure 5.3 Paths in the psychrometric chart for heating outdoor air in winter or cooling it in summer to reach 208C and 50 per cent relative humidity slowly if the air is in contact with massive structures that were cooled down before, for example, by strong airing during the cool night.
Measurements and Measures Related to Energy Efficiency in Ventilation . . . 83 From the indoor environment, heat loads and solar gains. This way, common in tropical climates, saves the investment of the heating system, and heating energy is free. It has, however, the disadvantage of blowing cold air into the occupied spaces, often leading to draughts. In such systems, recirculation is often very large and temperature control is obtained by varying the supply airflow rate.
84 Ventilation and Airflow in Buildings To improve energy efficiency, mechanical ventilation systems are often equipped with heat recovery for recovering the heat contained in exhaust air. This heat is in most cases given back to supply air. Such heat recovery exchangers are efficient during both cold and hot seasons, saving heating and cooling energy. Some of these heat exchangers also transfer humidity, thus decreasing the energy used to humidify or dehumidify the air. As shown in Figures 0.3, 0.4 and 0.
Measurements and Measures Related to Energy Efficiency in Ventilation 85 Figure 5.5 Top half of a rotating heat exchanger can be transferred from the warm side to the other. The heat recovery efficiency of these exchangers ranges from 60 to 80 per cent, depending on the type and size. A variant of this exchanger is the heat pipe exchanger, in which heat pipes are used to transport heat from warm to cold air. The air leakage between both sides of such heat exchangers should be zero.
86 Ventilation and Airflow in Buildings Figure 5.6 Relative position of fans and rotating heat exchangers Placing both fans on the same side results in a large pressure differential through the rotating heat exchanger, thus increasing leaks.
Measurements and Measures Related to Energy Efficiency in Ventilation 87 The enthalpy flow, H, is the product of mass airflow rate and specific enthalpy, h: H ¼ Qh ð5:5Þ where is the density of air. At ambient temperature, a numerical expression of Equation 5.3 for air is: h ¼ 1004:5 þ xð2;500;000 þ 1858:4 Þ ð5:6Þ where: is the air temperature, x is the humidity ratio, that is the mass of water vapour per kg dry air.
88 Ventilation and Airflow in Buildings Exhaust air 6 Extract air 4 - + Outdoor air 1 3 Supply air Figure 5.7 Schematics of an air handling unit, showing location of pressure taps for pressure differential measurements Leakage through heat exchangers Some heat exchangers let some air leak between both air channels. This is in most cases not expected, since there are very few air handling units equipped with both recirculation and a heat exchanger.
Measurements and Measures Related to Energy Efficiency in Ventilation 89 rates. If these pressure differentials are significantly larger than the nominal values, the wheel should be cleaned. Indication on how to measure pressure differentials is given in ‘Measurement of pressure differences’, below.
90 Ventilation and Airflow in Buildings In simplified methods to calculate heating (or cooling) demand of buildings, ventilation heat loss, V , is often calculated by (CEN, 1999, 2007): V ¼ m_ ðhx ho Þð1 G Þ ð5:15Þ where: m_ is the mass flow rate of outdoor air in kg/s, hx is the specific enthalpy of extract air, which is considered as representative of the average indoor air, ho is the specific enthalpy of outdoor air, G is the global efficiency of the heat recovery system.
Measurements and Measures Related to Energy Efficiency in Ventilation 91 We have mentioned above the following recirculation rates: External Re ¼ m_ i m_ o m_ e m_ a ¼ m_ e m_ e Rie ¼ Inlet to exhaust m_ i m_ rs m_ e m_ re ¼ m_ i m_ i Rxs ¼ Extract to supply m_ s m_ rs m_ x m_ re ¼ m_ x m_ x ð5:20Þ ð5:21Þ ð5:22Þ The mass flow balance for the whole building is: m_ a þ m_ exf ¼ m_ o þ m_ inf ð5:23Þ Combining this equation with the definition of the external recirculation rate, we get: m_
92 Ventilation and Airflow in Buildings where: x ¼ m_ x m_ x þ m_ exf ð5:32Þ is the extraction efficiency, i.e. that part of the air leaving the ventilated volume, which is extracted through the air handling unit, and re ¼ 1 Rxs ¼ m_ re m_ x ð5:33Þ is the air recovery efficiency, or that part of the extract air that passes through the heat recovery unit. Looking at Equation 5.32, it seems at first glance that the global heat recovery efficiency depends only on extract and exfiltration airflow rates.
Measurements and Measures Related to Energy Efficiency in Ventilation Global efficiency 1.0 93 Recirculation Rxs 0.8 0.0 0.6 0.2 0.4 0.4 0.2 0.6 0.0 0.0 0.8 0.2 0.4 0.6 0.8 Exfiltration ratio γexf 1.0 Figure 5.9 Relative decrease of global heat recovery efficiency as a function of exfiltration ratio exf and internal recirculation rate Rxs Source: Roulet et al., 2001. which depends on all parasitic airflow rates. When there is no external recirculation (Re ¼ 0), Equation 5.
94 Ventilation and Airflow in Buildings Net energy saving and performance index Heat recovery systems recover thermal energy but use electric energy for the fans. The net energy saving should therefore take into account the primary energy needed to produce electricity and the fact that the losses of the fans heat the air.
Measurements and Measures Related to Energy Efficiency in Ventilation 95 Table 5.
Ventilation and Airflow in Buildings Global recovery efficiency 96 100% Large units Small units 80% 60% 40% 20% 0% 0% 20% 40% 60% 80% 100% Heat exchanger efficiency Figure 5.10 Global heat recovery efficiency versus nominal heat exchanger effectiveness measured in several units 8 7 6 5 4 3 2 1 0 Good Poor Unacceptable COP in the three small units, and external recirculation above 20 per cent is measured in three large units.
Measurements and Measures Related to Energy Efficiency in Ventilation 97 Best net energy savings in large units (7 and 8 in Table 5.2) are 80,000– 90,000 kWh per winter season, but unit 10 actually wastes as much energy. Small units (a, b and c) save between 80 kWh and 350 kWh during an entire season. From an energy and economic lifetime analysis perspective, such units are disadvantageous. Note that these results are obtained when the heat recovery is functioning.
98 Ventilation and Airflow in Buildings where: is the density of air, c is the heat capacity of air, is the temperature difference between exhaust air and supply air. The kinetic energy given to the air by the fan is, sooner or later, degraded into heat by viscosity and friction on the surfaces of ducts, room walls and furniture.
Measurements and Measures Related to Energy Efficiency in Ventilation 99 consumption of electrical energy by the fan motor, and on the other hand the kinetic energy given to the air in duct.
100 Ventilation and Airflow in Buildings q 265 Pa Dp Figure 5.13 Installation of the differential manometer to measure the pressure differential across the fan The two ports of this manometer are connected to pressure taps located on both sides of the fan (see Figure 5.13). Care should be taken to avoid too much dynamic pressure on these taps.
Measurements and Measures Related to Energy Efficiency in Ventilation 101 simultaneously the r.m.s. voltage, U, between phase and neutral point, the r.m.s. current, I, running into each motor coil and the phase shift, ’, between voltage and current. The power is calculated by: e ¼ 3 X Uj Ij cosð’j Þ ð5:54Þ j¼1 the sum being on all three phases.
102 Ventilation and Airflow in Buildings 100% Efficiency 80% 60% 40% 20% 0% 0 2,500 5,000 Measured power [W] 7,500 Temperature increase [K] Figure 5.16 Fan efficiencies as a function of actual fan motor power 2.5 2.0 1.5 1.0 0.5 0.0 0 500 1,000 Pressure differential [Pa] 1,500 Figure 5.17 Air temperature increase as a function of pressure differential across the fan on which the frequency, the voltage, the current and the fan motor power can be displayed.
Measurements and Measures Related to Energy Efficiency in Ventilation 103 indoor environment quality and health or paying attention to possible damages to buildings. If a decrease of thermal comfort was implicitly accepted, cases of mould growth, increased indoor pollution and health hazards were not expected but often observed. Since then, the idea that saving energy in buildings decreases the indoor environment quality still prevails.
104 Ventilation and Airflow in Buildings Table 5.
Measurements and Measures Related to Energy Efficiency in Ventilation 105 Cold ceiling Fan Heating Humidifaction Cooling Heat exchanger Filter Office space Radiators Figure 5.18 The HVAC system in the simulated building For each variant, the effect of the following changes in design and operation was simulated: . . . . . . . . .
106 Ventilation and Airflow in Buildings per day), ventilation strategies have a minor influence on heating energy demand. The air may be either heated by coils in the supply air or by radiators in the room. The energy use for heating does not change significantly in all climates. The tightness of the building envelope has a large influence, up to a factor of two, on the heating energy need. When high infiltration occurs, humidity is also reduced in winter.
Measurements and Measures Related to Energy Efficiency in Ventilation 107 a rotating heat exchanger (see Chapter 6, ‘Rotating heat exchangers’), has a negligible effect on energy demand. Infiltration or exfiltration through a leaky building envelope strongly reduces the efficiency of heat recovery (see ‘Effect of leakages and shortcuts on heat recovery’, above).
6 Contaminants in Air Handling Units The purpose of mechanical ventilation systems is to supply appropriate amounts of clean air and to evacuate vitiated air. However, in field audits it was seen that ventilation systems often host contaminant sources and are, in the worst cases, the main source of air pollution in buildings (Fanger, 1988; Bluyssen et al., 1995, 2000b). Components in the mechanical ventilation system may considerably pollute the passing air.
Contaminants in Air Handling Units 109 16 Cassette Cellulose Glass fibre Odour intensity 14 12 10 8 6 4 2 0 0 500 1000 Airflow rate [m3/h] 1500 Figure 6.1 Olfactive pollution of various new filters as a function of airflow rate Source: Bluyssen et al., 2000a, 2003. Ducts The duct material and the manufacturing process has the biggest effect on the perceived air quality (Björkroth et al., 2000).
110 Ventilation and Airflow in Buildings The effect of airflow on the perceived air quality from ducts was relatively small and is probably insignificant in normal applications. Increasing the airflow rate in the duct does not, surprisingly, reduce the odour intensity: the additional airflow rate certainly dilutes the evaporated oil but the increased air velocity also evaporates more oil.
Contaminants in Air Handling Units 111 Odour intensity 6 5.5 5 4.5 4 3.5 200 300 400 500 600 Surface concentration of bacteria [thousands of CFU/cm2] 700 Figure 6.4 Bacteria concentration at inner surface of a humidifier correlated with the odour intensity Note: CFU ¼ colony forming unit. Source: Müller et al., 2000.
112 Ventilation and Airflow in Buildings Exhaust air Outdoor air Extract air Supply air Figure 6.6 Schematics of the purging sector Note: A part of the outdoor air cleans the porous structure and then is sent back to the exhaust air. such that a sector of it that contains exhaust air passes first through the purging chamber. The author has seen wheels turning the wrong way! In addition, contaminants can be transferred from exhaust to supply ducts by adsorption–desorption.
Contaminants in Air Handling Units 113 Figure 6.7 shows that certain categories of volatile organic compounds (VOCs) are easily transferred by a sorption transfer mechanism. Among the tested VOCs, those having the highest boiling point were best transferred. The largest transfer rate in a well-installed unit was found for phenol (30 per cent).
114 Ventilation and Airflow in Buildings Selecting the panel The subjects are selected from a group of at least 50 applicants of ages ranging from 18 to approximately 35 years old. There is no restriction on distribution of gender. Participants should abstain from smoking and drinking coffee for at least one hour before any test. Also, they are asked not to use perfume, strong smelling deodorants or make-up, and not to eat garlic or other spicy food the day before the tests and on the day of the tests.
Contaminants in Air Handling Units 115 Bottle Jar Fan Figure 6.9 Recommended locations of small bottles in PAP meter Source: Bluyssen, 1990. The 2-propanone gas is evaporated in the PAP by placing one or more 30 ml glass bottles filled with 10 ml of 2-propanone and making different holes in the caps of these bottles. The concentration (in parts per million) of 2-propanone obtained with one small bottle is about three times the diameter of the hole in millimetres.
116 Ventilation and Airflow in Buildings Table 6.1 PAP values and 2-propanone concentrations in PAP meters used as milestones Value 1 (no odour) 2 5 10 20 . . Concentration [ppm] <1 5 19 42 87 mechanical air supply with filtered air; mixing ventilation with a certain minimum ventilation rate. During the tests, the background level of 2-propanone should not be more than 1 ppm. Five different 2-propanone concentrations generated by five PAP meters are used as milestones for the training.
Contaminants in Air Handling Units 117 pollution of concentrations of 2-propanone unknown to them by making comparison with the milestones. On the third, fourth and fifth days, training includes 2-propanone concentrations and other sources of pollution. Since the pollutants have different characters to 2-propanone it is of great importance that the subjects understand that they are exposed to the intensity by comparing the intensity of the milestones.
118 Ventilation and Airflow in Buildings Exhaust air C6 Extract air C4 - + Outdoor air C1 Supply air C3 Figure 6.10 Schematics of an air handling unit showing location of VOC injection and sampling points, Ci , for concentration analysis Source: Roulet et al., 2000. of Figure 6.10. The concentration of these VOCs in the air is analysed at the four locations shown in Figure 6.10.
Contaminants in Air Handling Units Stainless steel turnings 119 Syringe Hot air blower (200 °C) Ø 6 mm copper tube Figure 6.11 Flash evaporation device for injecting the VOCs Source: Roulet et al., 2000. Air sampling and analysis Air at the four locations is sampled with a pump through small tubes filled with an adsorbing medium (for example, activated charcoal, TENAX). The sampling rate is about 0.1 l of air per minute.
120 Ventilation and Airflow in Buildings Table 6.
Contaminants in Air Handling Units 121 Table 6.
122 Ventilation and Airflow in Buildings Table 6.4 Pressure differentials in the units [Pa] Auditorium unit Filters Across wheel in Inlet – supply 88 5 Extract – exhaust 107 5 Laboratory unit out 80 5 110 5 EMPA unit in out in out 85 5 60 5 82 5 67 5 97 2 94 1 109 3 88 1 125 5 54 5 30 5 21 5 230 2 283 5 72 5 125 5 125 5 137 5 423 2 475 5 Between Cold side supply and Warm side exhaust and rotates at 5 rpm.
Contaminants in Air Handling Units 123 Table 6.6 VOC transfer rate in the experiments performed in both EPFL units (%) Auditorium unit Purging sector Experiment no.
124 Ventilation and Airflow in Buildings 70% 60% 50% 40% 30% 20% 10% 0% n- D ec an e 1Bu ta no 1l H ex an ol Ph e 1- nol H ex Be an nz al 1, al 6d D e h ic yd hl e or oh ex D an ip e ro py le th er Li m on en e m -X yl en M e es ity le ne No purging sector With purging sector Figure 6.14 Average VOC recirculation rates measured in the EPFL auditorium (leaky) unit, with and without purging sector Source: Roulet et al., 2000. experiments, the smallest recirculation rates are for limonene.
Contaminants in Air Handling Units 125 Transfer ratio 70% 60% 50% Alcohols 6 C chains 6 C cyclic 40% 30% 20% 10% 0% 0 50 100 150 Boiling point [°C] 200 250 Figure 6.16 Transfer ratio as a function of the boiling point for three families Note: The filled square corresponds to dichlorohexane. 100% EMPA 80% y = 1.09x + 0.22 R2 = 0.63 60% 40% 20% 0% 0% 20% 40% 60% 80% EPFL laboratory Figure 6.
126 Ventilation and Airflow in Buildings Table 6.8 General IAQ strategies for HVAC systems Design Operation Prevent pollution from outdoor air coming into the system Select appropriate filtering system Discontinuous sources: ventilate mainly Locate outdoor air intake at a clean site, when the source intensity is small far from potential pollution sources Continuous sources: use an appropriate filtering system Prevent pollution Avoid recirculation Install appropriate filtering system in extract duct.
Contaminants in Air Handling Units 127 Table 6.
128 Ventilation and Airflow in Buildings Table 6.
Contaminants in Air Handling Units 129 Table 6.
130 Ventilation and Airflow in Buildings Table 6.
Contaminants in Air Handling Units 131 Table 6.
7 Common Methods and Techniques Expressing concentrations and flow rates Coherent units When using equations, such as Equation 2.7, to model ventilation systems, coherent units should be used to get the correct results. Some examples are given in Table 7.1, and Annex A gives conversion tables. If the analysers and tracer gas flowmeters do not provide coherent units, the measured data should be converted to coherent units before further interpretation. Corrections for density changes Note that Equation 2.
Common Methods and Techniques 133 Table 7.1 Examples of coherent units Airflow rate kg/s m3 /h m3 /h Injection rate Concentration kg/s m3 /h cm3 /h Mass concentration Volume concentration ppm with P i M i ni M¼ P i ni ð7:3Þ ¼ 28:96 g/mole. being the average molar mass of the mixture. For dry air, M The relative change in density is then: p T M ¼ þ p T M ð7:4Þ As long as the tracer gas is present only in trace concentration, the temperature has the largest effect on the density.
134 Ventilation and Airflow in Buildings In a mixture, every gas, x, occupies the whole volume, V: V¼ nx RT px ð7:8Þ The relations between mass concentration, Cm , molar concentration, CM , and volume concentration, Cv of component x, are then: m M n M M Cm ¼ P x ¼ P x x ¼ x CM ¼ x CV M M i mi i Mi ni ð7:9Þ Molar and volume concentration are the same, since at a given pressure and temperature, one mole of gas always occupies the same volume.
Common Methods and Techniques 135 Table 7.2 Properties of the gases most frequently used as tracers Tracer name Chemical formula Molecular Density/air MAC weight @NTP [ppm] MDCy [ ] Helium Neon He Ne 4 20 0.14 0.69 – – >6 10 6 >20 10 12 Carbon dioxide Nitrous oxide CO2 N2 O 44 44 1.53 1.53 5000 25 3 10 6 50 10 9 Sulphur hexafluoride SF6 146 5.
136 Ventilation and Airflow in Buildings Table 7.3 Background concentration of some gases Gas Formula Rural concentration Water vapour Argon H2 O Ar 20 10 3 9.3 10 3 Carbon dioxide Helium Methane CO2 He CH4 350 10 6 5.24 10 6 1.
Common Methods and Techniques 137 Table 7.4 Qualities of some tracer gases Name Compliance with the quality Low No No Ease BackNo fire Low Density hazard toxicity close to reactivity of ground local cost use conc.
138 Ventilation and Airflow in Buildings maintained under pressure. It is noted, however, that these two conditions complicate the experimental arrangement since control leads must extend to the end of the injection tube, and the system, under pressure, will be more sensitive to leaks. In buildings with a large internal volume it may be necessary to discharge large amounts of tracer.
Common Methods and Techniques 139 Sampling methods Samples of air containing tracer gases need to be taken for analysis. There are several sampling methods, each one being adapted to a particular purpose. Grab sampling using hand pumps and bags is very cheap, easy to install and needs few materials in the field. This method can be used for decay measurements in no more than a few zones, and for constant emission provided conditions remain constant.
140 Ventilation and Airflow in Buildings tight caps. Properly used passive samplers adsorb all the tracers that are in the air entering the sampler. They are used to obtain a quantity of tracer that is nearly proportional to the dose (that is, the time integral of the concentration) received during the measurement time. Passive (or diffusive) sampling is initiated by opening one end of the tube for hours, days or weeks.
Common Methods and Techniques 141 As an example, for an airflow rate of 100 l/h, that is 28 10 6 m3 /s, the minimum pipe inner diameter will be 2.6 mm to have an air speed of 5 m/s. In this case, the pressure drop will be 420 Pa/m, which may be too large in most buildings. A pipe with a 4 mm inner diameter will have an air speed of 2.2 m/s and a pressure drop of 80 Pa/m, which allows for 12 m long pipes with a pump allowing 1000 Pa under pressure at 10 l/h.
Purge injection Tracer gas Analysis Pre-purge Tracer gas Analysis Purge Ventilation and Airflow in Buildings Purge 142 Figure 7.1 Two strategies for injection and sampling Note: Left is one zone at a time; right is time shared. Remember, however, that, in order to achieve good mixing, it is advantageous to inject the tracer continuously whenever possible. A series of short pulses spread evenly over time can simulate this continuous injection.
Common Methods and Techniques . 143 up to several minutes for gas chromatographs or multi-tracer infrared analysers. Faster analysis will enable more frequent sampling of each zone and hence provide more detailed data. Frequent sampling (for example, every five to ten minutes) is essential for the constant concentration technique to maintain accurate control of concentration. Accuracy – last but not least, the accuracy of the concentration measurement directly influences the accuracy of the results.
144 . . Ventilation and Airflow in Buildings vapour and CO2 present at high concentrations in the air. Filters are used to minimize the effect but humidity should be measured simultaneously to some tracers, such as N2 O, to allow for corrections. Analysis time – 10–50 s. Accuracy – 1 per cent of full scale if the zero drift is controlled. Photo-acoustic detector This analyser is also an infrared absorption spectrometer, but uses a different detector.
Common Methods and Techniques 145 Table 7.5 Tracer gases most used in the mass spectrometer technique Mass Comments 127 51 7.6% of mass 127 peak. Interferes with Freon 22 Freon R22 þ CHF CHClFþ CHClþ 2 51 69 85 Interferes with SF6 2.1% of peak 51. Interferes with R14 and R13B1 1.5% of peak at mass 51.
146 Ventilation and Airflow in Buildings Gas chromatography A puff of the sampled air is injected into a separating (chromatographic) column, a tube in which adsorbent material is packed. This column is heated and the pulse of sample is pushed with a flow of inert carrier gas. The various components of the sample pass through the column at various speeds according to their affinity for the adsorbent material.
Common Methods and Techniques 147 By making the prescribed number of strokes of the hand-held bellows, the correct amount of air is drawn through the tube. This enables the tracer gas evaluation to be made. The glass tube has graduation marks on it, and the length of the discolouration caused by the reaction indicates the concentration of tracer in the room air. Detector tubes can only be used once and must be discarded after each sample taken.
148 Ventilation and Airflow in Buildings Linear least square fit of the first kind Such methods are used to find the coefficients of leakage models of Equations 4.1 or 4.2 in fan pressurization (see Chapter 4, ‘The fan pressurization method’). The regression of the first kind assumes that the abscissa, xi , of each measurement is well known and that the distribution of the ordinates around the regression line is Gaussian with a constant standard deviation.
Common Methods and Techniques 149 Confidence in the coefficients The variances on the linear coefficients of the regression of the first kind are usually estimated using the following relations, which assume that the dispersion around the line is Gaussian with a constant standard deviation and is the result of the measurement errors: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N 2 u1 X 1 sy nsxy x2 ð7:17Þ and sa ¼ sn t sn ¼ N 2 sx N i¼1 i If T(P; ) is the significance limit of the two-sided Student distribution
150 Ventilation and Airflow in Buildings Regression of the second kind When there are uncertainties in both axes, there is no reason to emphasize the x axis, and the same procedure can be followed commuting the roles of x and y.
Common Methods and Techniques 151 where "y and "x are the experimental errors on y and x respectively and deduce a corresponding value of a using Equation 7.16. This recipe does not show clearly which quantity is minimized by the fit. Another more physical way is so-called ‘orthogonal’ regression.
152 Ventilation and Airflow in Buildings will be obtained, and a correlation between C and n will be found: the larger C values correspond to the smaller n and vice versa. A good identification technique should give the most likely couple of coefficients together with the probability density fðC; nÞ. Such a technique exists (Tarantola, 1987) and is summarized below.
Common Methods and Techniques the a posteriori information. This new distribution: h i T 1 ðzÞ ¼ C exp 12 f ðzÞT C 1 T ðzÞ þ ðz zp Þ C z ðz zp Þg 153 ð7:41Þ From this distribution, the z vector presenting the maximum likelihood can be found. It is the vector, z, that minimizes the exponent: T 1 ðzÞT C 1 T ðzÞ þ ðz zp Þ C z ðz zp Þ ð7:42Þ This most probable vector contains the identified model parameters and the most probable values of the measured quantities.
154 Ventilation and Airflow in Buildings Generally, an instrument does not directly give the required information. In most cases, several measurements are combined to obtain the needed value. For example, in tracer gas measurements, several concentrations, gas flows, time and volume measurements are combined in equations that are solved to get the airflow rates. The errors accompanying the measured values propagate through the interpretation formulae and finally give a probable error on the final result.
Common Methods and Techniques 155 0.5 0.4 0.3 0.2 -3 -2 -1 0.1 Confidence interval 0 0 1 2 x- 3 s Figure 7.2 Significance limits and confidence interval domain. The statistical method allows one to obtain more information on the reliability of the results. Because of random reading errors and uncontrolled perturbations, the test values will follow a given distribution. We can model such distributions by treating x as a stochastic variable.
156 Ventilation and Airflow in Buildings Variance and standard deviation A figure representing the importance of the scattering around the average value is the mean square deviation or variance: P 2 P 2 2 ðx Þ Nhxi2 i ðxi hxiÞ ¼ i i ð7:51Þ Sx ¼ ðN 1Þ ðN 1Þ The square root of Sx is the estimate, sx , of the standard deviation, x : pffiffiffiffiffiffi ð7:52Þ sx ¼ Sx ffi x The larger the number of measurements, the better the estimate.
Common Methods and Techniques 0.5 1 0.4 0.8 0.3 0.6 0.2 0.4 0.1 0.2 0 0 -3 -2 -1 157 0 x – s 1 2 3 -3 -2 -1 0 x – s 1 2 3 Figure 7.3 Normal (or Gaussian) distribution (left) and its probabillity function (right) The confidence interval [ c; c] of the normal distribution is obtained by solving the equation: pffiffiffi P ¼ erfðc= 2Þ ð7:57Þ for a given value of P.
158 Ventilation and Airflow in Buildings 0.4 Normal 5 2 1 0.3 0.2 0.1 0 –4 –3 –2 –1 0 1 2 3 4 Figure 7.4 Student distribution for 1, 2 and 5 degrees of freedom compared to the normal distribution more detail in statistical tables such as Zwillinger (2003)) and in most mathematical software packages.
Confidence limit/standard deviation Common Methods and Techniques 159 5 4 P = 99.9% 3 99% 2 90% 1 0 50% 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Number of measurements Figure 7.5 Confidence limit divided by standard deviation versus number of measurements for various values of probability, P Hence, we can state: ¼ hxi Ic ð7:65Þ P is the probability that the confidence interval contains the ‘true’ value. P is chosen a priori, in practice between 0.9 and 0.
160 Ventilation and Airflow in Buildings the infinitely small increments dxi by the absolute error xi and by summing the absolute values: X @fj ð7:67Þ x yj ¼ @x i i i If only arithmetical operations are used, the rules simplify to the following: . . If the result is obtained by adding or subtracting the measurements, the absolute error on the result, y, is the sum of the absolute errors, x, of each measurement.
Common Methods and Techniques 161 represented by a vector x and a matrix A. The question is: which is the resulting error y on the vector y, which is the vector containing the final results? If the matrix A and the vector x were known, we could write: ðA þ AÞð y þ yÞ ¼ x þ x ð7:72Þ and, taking Equation 7.
162 Ventilation and Airflow in Buildings the partial derivatives are computed as above and we get finally: X X 2 2 s2yi ; yj ¼ ik yl jm yn sakl amn þ ik jl sxk xl klmn þ X kl ð ik yl jm jk yl 2 im Þsakl xm ð7:79Þ klmn which simplifies, if the variables are independent (that is, if the covariances are zero, which is not always the case): X X 2 2 2 ð7:80Þ s2yi ; yj ¼ ik jk yln saki akl þ ik jk sxk xk kl kl Upper bound of the errors The vector y contains a large number of data, but it is hel
Common Methods and Techniques and the matrix norm is subordinated to the vectorial norm jxj if: jAxj for any x 6¼ 0 jAj ¼ max jxj 163 ð7:85Þ The subordinated norm is the smallest matrix norm compatible with the norm jxj.
164 Ventilation and Airflow in Buildings where 1 and n are respectively the largest and the smallest eigenvalues of AH A. This number is the spectral condition number. Constant absolute error If the absolute error is constant: A ¼ e1 and y ¼ y1 ð7:93Þ where 1 and 1 are respectively a matrix of order N and an N-component vector with all elements equal to 1 (they are not the identity matrix and the unit vector).
Common Methods and Techniques 165 Table 7.8 Airflow rates [m3 /h], calculated from the data given in Table 7.7 Flow going to Flow coming from Outdoors Zone 1 Zone 2 Outdoors Zone 1 Zone 2 – 21.4 22.2 11.0 – 20.1 32.6 9.7 – From which, using the method described in Chapter 1, ‘Zone by zone systems of equations’, we get the airflow rates to and from each zone [m3 /h] shown in Table 7.8.
References Aeschlimann, J.-M., C. Bonjour and E. Stocker, eds, 1986, Me´thodologie et Techniques de Plans d’Expe´riences: Cours de Perfectionnement de l’AVCP, vol. 28, AVCP, Lausanne. Andersson, B., K. Andersson, J. Sundell and P.-A. Zingmark, 1993, Mass transfer of contaminants in rotary enthalpy exchangers, Indoor Air, vol. 3, pp. 143–148. ASHRAE, 2001, Handbook – Fundamentals, ASHRAE, Atlanta.
References 167 Brown, S. K., M. R. Sim, M. J. Abramson and C. N. Gray, 1994, Concentrations of volatile organic compounds in indoor air: A review, Indoor Air, vol. 4, pp. 123– 134. Carrie, F. R., P. Wouters, D. Ducarme, J. Andersson, J. C. Faysse, P. Chaffois, M. Kilberger and V. Patriarca, 1997, Impacts of air distribution system leakage in Europe: The SAVE Duct European Programme, 18th AIVC Conference vol. 2, Athens, pp. 651–660.
168 Ventilation and Airflow in Buildings Hakajiwa, S. and S. Togari, 1990, Simple test method of evaluating exterior tightness of tall office buildings, in E. M. H. Sherman, ed., ASTM STP 1067, ASTM, Philadelphia, pp. 231–248. Hanlo, A. R., 1991, Use of tracer gas to determine leakage in domestic heat recovery units (HRV), in Air Movement and Ventilation Control within Buildings, AIVC 12th Conference proceedings, vol. 3, Ottawa, pp. 19–28. Hodgson, A. T.
References 169 Presser, K. H. and R. Becker, 1988, Mit Lachgas dem Luftstrom auf der Spur Luftstrommessung in Raumlufttechnischen Anlagen mit Hilfe der Spurgasmethode, Heizung Luftung Haustechnik, vol. 39, pp. 7–14. Riffat, S. B. and S. F. Lee, 1990, Turbulent flow in a duct: Measurement by a tracer gas technique, Building. Serv. Eng. Res. Technol., vol. 11, pp. 21–26. Roulet, C.-A., 2004, Sante´ et Qualite´ de l’Environnement Inte´rieur dans les Bâtiments, PPUR, Lausanne. Roulet, C.-A. and R.
170 Ventilation and Airflow in Buildings Sandberg, M. and C. Blomqvist, 1985, A quantitative estimate of the accuracy of tracer gas methods for the determination of the ventilation flow rate in buildings, Building and Environment, vol. 20, pp. 139–150. Seibu Giken Co. Ltd., 1999, Technical Information on Ion Power Total Heat Exchanger, Seibu Giken, Fukuoka. Sherman, M. H., 1990, Tracer gas techniques for measuring ventilation in a single zone, Building and Environment, vol. 25, pp. 365–374. Sherman, M. H.
Annex A Unit Conversion Tables Introduction SI units are used throughout this book. Non-SI units are, however, of general use in air infiltration and ventilation, like the air change rate in l/hour or US units. To expedite the unit’s translations, some tables are given below. Only physical quantities which are of general use in air infiltration and ventilation measurement techniques are listed.
172 Ventilation and Airflow in Buildings Area Name 1 1 1 1 1 m2 cm2 sq in sq ft sq yd 1 10 4 6.4516 10 4 0.092903 0.836127 104 1 6.4516 929.0304 8361.27 1550 0.3937008 1 144 1296 10.7639 0.0328084 1/144 1 9 1.19599 0.01093613 1/1296 1/9 1 Symbol square metre square centimetre square inch square foot square yard 2 m cm2 sq in sq ft sq yd Volume Name 1 cubic metre 1 litre 1 millilitre 1 cubic yard 1 cubic foot 1 cubic inch m3 Symbol 3 m l ml cu yd cu ft cu in ml cm3 l 1 0.001 0.
Unit Conversion Tables 173 Pressure Name Symbol 1 Pascal 1 millibar 1 mm water column Pa mbar mm H2 O 1 inch water column in H2 O 1 pound per square inch lb/in2 or psi Pa mbar mm H2 O in H2 O psi 1 100 9.81 0.01 1 0.0981 0.102 10.2 1 0.004 0.422 0.0393 145.037 10 6 14.5037 10 3 1.42 10 3 249 6894.76 2.5 68.9476 25.4 703 1 27.7 36 10 3 1 Volume flow rate Symbol m3 /s l/min m3 /h cu ft/s cu ft/min cu ft/h m3 /s l/min m3 /h cu ft/s cu ft/min cu ft/h 1 16.667 10 6 277.
Annex B Glossary Items in italics are additional entries in the glossary. Age of the air (or age of a contaminant) Average time period since the fresh air (or a contaminant) entered the room or the building. This age depends on the location in the building. The room mean age of air is the average of the age over the whole room. Air change performance Coefficient defined by ASHRAE, which is the double of the air exchange efficiency.
Glossary 175 adventitious openings), caused by pressure effects of the wind and/or the effect of differences in the indoor and outdoor air density. Air infiltration characteristic The relationship between the infiltration airflow rate into a building and the parameters that cause the movement. Air leakage Airflow rate through a component of the building envelope, or the building envelope itself, when a pressure difference is applied across the component.
176 Ventilation and Airflow in Buildings Building component General term for any individual part of the building envelope. Usually applied to doors, windows and walls. Building envelope The total of the boundary surfaces of a building, through which heat (or air) is transferred between the internal spaces and the outside environment. Calibration Operation where the output of a measuring device is compared with reference standards, to accurately quantify the results provided by the measuring device.
Glossary 177 Constant concentration technique A method of measuring ventilation rate whereby an automated system injects tracer gas at the rate required to maintain the concentration of tracer gas at a fixed, predetermined level. The ventilation rate is proportional to the rate at which the tracer gas must be injected. Constant injection rate technique A method of measuring ventilation rate whereby tracer is emitted continuously at a uniform rate.
178 Ventilation and Airflow in Buildings Distribution effectiveness Ratio of the average tracer gas or contaminant concentration to the concentration that could be reached, at equilibrium, in the same zone or building with the same tracer or contaminant sources. Also the ratio of the contaminant or tracer turnover time to the room mean age of air. It is the inverse of the relative contaminant removal effectiveness. Door panel Panel adapted to a door or a window on which the pressurization fan is mounted.
Glossary 179 Fan pressurization General term applied to any technique involving the production of a steady static pressure differential across a building envelope or component. Often referred to as dc pressurization. Flame ionization detector Detector used in conjunction with a gas chromatograph, in which the change in ionic current caused in a hydrogen–air flame by a tracer or contaminant is detected. This detector is sensitive to organic compounds.
180 Ventilation and Airflow in Buildings Indoor air pollution Pollution occurring indoors from any source, i.e., from outside as well as inside the building. Infrared gas analyser Instrument used to determine tracer gas concentrations by determining the transmission of infrared radiation at an absorption frequency through a fixed path length. Inter-zonal airflow General term applied to the process of air exchange between internal zones of a building. Leakage area See equivalent leakage area.
Glossary 181 Mixing The degree of uniformity of distribution of outdoor air or foreign material in a building. Mixing fan Small electric fan used to aid the mixing of room air and tracer gas before and/or during a measurement. Multiple tracer gas technique General term applied to measurement methods using two or more tracer gases. These methods are often used to evaluate inter-zonal airflows. Multi-zone A building or part of a building comprising a number of zones or cells.
182 Ventilation and Airflow in Buildings Piston-type ventilation See displacement flow. Pitot tube Anemometer measuring the difference between the pressure in a tube facing the flow, in which the flow is stopped, and the pressure along a side of the tube. Pollutant removal effectiveness See ventilation efficiency. Pollution migration Descriptive term for the movement of indoor air pollutants throughout a building.
Glossary 183 Reductive sealing method A method of determining the leakage of specific building components by pressurizing the building and recording the leakage changes as components are sealed successively. When all the major outlets and component cracks are sealed, the remainder is the background leakage.
184 Ventilation and Airflow in Buildings Site analysis Applied to any tracer gas measurement technique where tracer gas concentrations and air exchange rates are determined directly at the measurement building. Smoke leak visualization A method of detecting leaks in the building fabric by pressurizing the building and using smoke to trace the paths followed by the leaking air.
Glossary 185 Turnover time of a contaminant Ratio of the mass of contaminant contained in an enclosure to the mass flow rate of the contaminant source in this enclosure. Ventilation The process of supplying and removing air by natural or mechanical means to and from any space. Ventilation efficiency An expression describing the ability of a mechanical (or natural) ventilation system to distribute the outdoor air in the ventilated space. Ventilation energy Energy loss from a building due to ventilation.
Index absolute error, 154 active ways, 78 adsorption, 123 age matrix, 10 age of the air, 39, 42, 174 air age of, 39, 42, 174 change efficiency, 40 leakage rate, 67 performance, 174 rate, 1, 174 conditioning, 79 exchange efficiency, 174 exchange rate, 174 handling unit, 20 airflow rates in, 15 energy in, 79 permeability, 59 speed, 17 airflow assessment, xiv coefficient, 60, 174, 179 meter, 20 rate, xiii, 1, 174 equivalent outdoor, 5 exfiltration, 31 exfiltration, xvi exhaust, 31 extract, 28 in air handling unit, 15 i
188 Ventilation and Airflow in Buildings conductance, 176 confidence interval, 28, 149, 154, 155 conservation equation, 1, 7, 177 constant concentration, 3, 12, 176 constant injection rate, 3, 12, 43, 177 contaminant, 108, 177 in heat exchangers, 117 transfer, 10 cooling, 79, 105 coil, 83 covariance, 156 decay method, 3, 12, 43, 177 density, 177 correction for, 132 of tracer gas, 138 detector tubes, 146, 176 discharge coefficient, 177 displacement flow, 177 duct, 15, 129 airtightness, 74 contamination, 109 effic
Index air change rate, 67 and heat recovery, 89 area, 67, 178 characteristics, 180 coefficients, 60, 64 heat exchangers, 88 visualization, 69 least square fit, 27, 147 manometer, 180 mapping experiments, 49 mass conservation, 1, 7, 177 spectrometer, 144, 180 matrix age, 10 flow, 9 norm, 162 mechanical power, 97 minimum airflow rate, xiii mixing, 137, 181 model matrix, 52 multi-zone, 181 airflow rates measurements, 6 pressurization, 62 network, 140 neutral height, 72 node by node, 23 nominal time constant, 3, 40,
190 Ventilation and Airflow in Buildings tracer gas (Continued) injection, 21 mixing of, 137 pulse injection, 4, 12, 182 sampling, 22 techniques, 12 trained panel, 113 transfer of contaminants, 10 units, 132 conversion tables, 171 variance, 52, 156, 160 vectorial norm, 162 velocity traverse, 17 ventilation, xiii, 185 balanced, 175 efficiency, xvii, 39, 178, 185 energy, 97 grilles, 19 system, 15 Venturi tube, 16, 185 visualization of air leakage, 69 VOC, 119 volume flow rates, 11 water vapour pressure, 87 wel
