Assortment / System Description

Room pressurization and fume hood control
Pressurized room, advanced temperature control
21
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21.8.1 Temperature control
When fume hoods open and close in a VAV laboratory, the resulting changes in
supply air flow can disrupt the room temperature. The thermal power sequence
is designed to reduce that disruption by calculating and applying a
compensating change in the supply air temperature. The calculation is based
on a constant room temperature, which is assumed to be the comfort heating
setpoint.
In other modes, other temperature sequence applies and separate heating and
cooling setpoints are supported.
The room temperature PID output is a percentage that goes above and below
zero. The value represents a heat transfer rate to the room by the supply air
terminal.
Supply air temperature control loop maps the desired supply air temperature to
the supply air temperature range configured for the terminal. It is transmitted to
the terminal as percentage. The heating coil function maps that percentage
back to temperature to calculate the supply air temperature setpoint. The
supply air temperature PID calculates the valve opening needed to drive the
measured supply air temperature to the setpoint.
The discharge temperature setpoint comes from scaling the feedback value up
or down according to the current supply airflow rate. Typically, it scales up
because the actual flow is less than the maximum heating or cooling flow. If the
flow rate is low, it takes a greater temperature difference to deliver the same
heating or cooling power.
When cooling is required, the value of the temperature loop is negative and is
limited by the supply flow minimum and maximum. The output cooling demand,
in percent, is then calculated and mapped to the cooling airflow limits.
When heating is required, the value of the temperature loop is positive and is
limited by the supply flow minimum and maximum. The output heating demand,
in percent, is then calculated and mapped to the heating airflow limits.
21.8.1.1 Thermal power active
The Thermal Power Control feature is used in Comfort mode. The supply
terminal functions as a single element.
It selects flow and temperature setpoints to deliver the required heat flow at
the lowest airflow rate that satisfies configured flow and temperature limits.
Heating control sequence: 1) Supply terminal, 2) Radiator, 3) Heating ceiling,
4) Heating coil
Cooling control sequence: 1) Chilled ceiling, 2) Cooling coil
21.8.1.2 Thermal power inactive
The Thermal Power Control feature is not used outside of Comfort mode. The
feedback controller for the terminal is disabled and the element does not
participate in the heating/cooling sequence.
Heating control sequence: 1) Radiator, 2) Heating ceiling, 3) Heating coil,
4) VAV heating
Cooling control sequence: 1) Chilled ceiling, 2) Cooling coil, 3) VAV cooling
Room temperature
control loop
Room temperature
feedback
Supply air flow control
loop
Calculate supply air
temperature heating
and cooling setpoint
Calculate supply flow
setpoint for cooling and
heating
Temperature control
sequence – thermal
power active
Temperature control
sequence – thermal
power Inactive