Assortment / System Description

Room pressurization and fume hood control
Pressurized room, temperature control
21
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21.7.4 Air flow control
The pressurized room operates a damper, using closed loop control to drive the
measured flow to an airflow setpoint that it calculates based on demand
signals received for heating, cooling and ventilation. It can control airflow with a
settling time of 1 to 2 seconds when applied with the right peripheral devices. It
can also be set up for slower operation.
The main output is a modulating output for damper position that is generated
by a PID or external airflow controller. Functions to support air balancing are
included.
For each supply terminal, separate minimum and maximum flow rates exist for
heating, cooling, and ventilation. Each of these HVAC functions has its own set
of configuration objects and properties.
Airflow limits for the terminal are related to – but are not the same as – the
airflow levels required to ventilate the room. The room ventilation function sets
a ventilation demand level that maps to the terminal’s ventilation flow limits.
Type of airflow Normal use Other possible use
Max cooling flow
(VavSuAirFlMaxC
)
Highest flow rate the designer allows for the terminal or space. It usually
corresponds to the design cooling load found on the box schedule.
--
Min cooling flow
(VavSuAirFlMinC
)
For this application, min cooling flow is usually not the value found on the
box schedule. It can be / should usually be set to zero.
If the box schedule lists minimum cooling flow, it is probably a ventilation
limit and not the value needed here.
May be higher to support cooling
coil or chilled beam, or to ensure
adequate air diffusion.
Nominal air flow
(AirFlNom)
AirFlNom is a parameter of the supply or extract damper/venturi AF. It is
typically not used (the default value of zero has no effect).
May be used to select the scaling value that converts physical flow values
to relative flow. If used, should be set equal to or greater than the largest
configured max flow value.
May be used to scale flow and
setpoint values for an external
airflow controller.
Max heating flow
(VavSuAirFlMaxH
)
Highest flow rate the designer wants for reheat. The energy codes may
require a flow level equal or less than half of max cooling flow.
In some rooms, increasing primary
VAV airflow may not be a desirable
way to add heat. In such cases
airflow will not vary with heating
load (max heating flow and min
heating flow are set equal). Follow
the sequence of operation specified
for the project.
Min heating flow
(VavSuAirFlMinH
)
Below this flow rate, heating will not function properly. The coil may
overheat or the supply air will not diffuse correctly. May be higher or lower
than ventilation flow.
If the box schedule or other HVAC design document specifies a min
heating flow, consult with the engineer to find out what is the intended
purpose: is it for heating or for ventilation.
In certain situations, max heating
flow and min heating flow may be
set equal. In special cases may
equal zero.
Max ventilation
flow
(VavSuAflMaxVnt
)
Highest flow rate the terminal can apply to a DCV loop or a ventilation level
selected by room operating mode. Sometimes equal to maximum cooling.
- or -
Highest flow rate needed to make-up exhaust (the rate of supply airflow
needed to maintain proper pressurization when extract flow is at or near
maximum). If maximum make-up needed exceeds max cooling flow, set
max ventilation high enough to cover. This might happen in a room with
lots of fume hoods and not a lot of other equipment.
Can be set lower (than max cooling)
if reheat equipment is too small to
temper full cooling flow. Can be set
higher if primary airflow is not the
main cooling source.
Min ventilation
flow
(VavSuAflMinVnt
)
In this application, min ventilation flow is not the required ventilation rate.
Min ventilation flow can usually be set to zero.
--
Required
ventilation
(dynamic value,
not configured)
In most systems, the required ventilation rate is a dynamic value driven by
occupancy and possibly other things. In addition to occupancy, issues
affecting required ventilation can include code requirements and
standards, design decisions, the type of space being used, air
contaminants and OA fraction to name several.
Might be set as the low end of the
range allowed by a DCV algorithm.
Flow control limits