User Manual

12
OTHER IMPORTANT FACTORS RELATING TO COMBUSTION
• The three T’s of combustion
– Time
• Amount of time that the fuel and oxygen are together in the combustion chamber
– Temperature
• How high the temperature is determines the rate of oxidation, or speed of the combustion
– Turbulence
• How well the fuel and air are mixed
• These three factors are all interrelated, and will move your results along the combustion curves.
COMBUSTION MEASUREMENT TERMS
Other parameters measured include net temperature, draft and efficiency.
Net Temperature
Net temperature is the difference between the combustion air entering the combustion chamber and the flue gas temperature past
the heat ex
change. This is used to determine how efficient the system is extracting heat from the combustion process in addition to
the performance of the combustion process. On sealed systems that have ducted inlet air for combustion air, the net temperature
must compare this air stream temperature with the flue gases. If the appliance simply uses room air for the combustion air, our
analyzers have an internal temperature sensor in the handset, so it will use this temperature when calculating net temperature.
The most accurate results for efficiency are obtained when measuring flue gases at the point where flue temperature (not flame tem-
perature) is the highest.
Draft
Draft is the difference between the ambient pressure level and the pressure level in the flue. This is created either by the natural
buoyancy of the hot gases created in combustion lifting, or by an inducer fan that assists the flow of flue gases up the stack. Most
combustion equipment will specify the amount of draft that is r
equired for proper operation.
Draft helps draw combustion air into the combustion chamber, and also helps in mixing the fuel and oxygen. Without proper
draft, the combustion process can spill poisonous by-products into the space where the appliance is located. This can be a risk to
those in the area, or create a danger to residents or employees working near the combustion equipment.
Efficiency
Efficiency is a measure of how well the fuel is burned to create heat, and how well the generated heat is captured for the intended
use.
The information used to create this v
alue are based on the fuels heating value, the heat lost up the flue and the gas components in
the flue gas. The original method to determine efficiency included many manual methods and lookup charts. As an example you
would measure the CO2 level and the stack temperature and then reference a slide scale that would give you the relative efficiency
number. UEi’s electronic combustion analyzers perform the measurements on a continuous basis, and can calculate the efficiency
as adjustments are being made. Combine this with a printout and you are able to provide a before and after comparison of the
combustion equipment in relatively little time as part of normal servicing.