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AIR HANDLING SYSTEM CONTROL APPLICATIONS
FEATURES
1. Use of the heat recovery system makes it energy efficient
to use 100 percent OA by transferring heat from RA to
supply air during heating operation and transferring
cooling from RA to supply air during cooling operation.
2. This system can be used to preheat air for an MA system
or as a 100% OA system.
CONDITIONS FOR SUCCESSFUL OPERATION
1. A non-freezing medium should be circulated between
coils.
2. Heat recovery pump should be shut down during periods
when low OA and RA differentials exist and when the
HVAC system demands for free cooling are not exceeded
by cool OA.
3. Improved control under heating light load conditions may
be obtained with an SA PI control loop and a three-way
valve. Since the exhaust temperature is never low enough
to satisfy the cooling demand, the valve should not bypass
the coil during cooling operation.
4. Entering air to each coil may require filtration to keep
heat transfer coefficients high.
5. Building exhaust must be centralized.
6. If, after heat recovery, the SA is still below setpoint, a
heat exchanger may be added to increase water
temperature to the supply coil.
NOTE: This reduces the recovery system efficiency by
lowering the differential temperature between
the water and OA.
SPECIFICATIONS
OA and exhaust fans shall start, their OA dampers open, and
the water flow and temperature controls shall be enabled
anytime the HVAC system requires OA.
In the cooling season, the recirculating pump shall run with
full flow in the coils anytime the OA temperature is greater
than six degrees above the RA temperature.
In the heating season, the recirculating pump shall run
anytime the OA temperature is less than three degrees below
the supply fan SA temperature setpoint.
A pump inlet mixing valve shall be modulated during heating
operation if necessary to prevent the supply fan DA temperature
from rising above it's setpoint (determined by the HVAC system
demands). The mixing valve shall also modulate to prevent the
exhaust coil entering water temperature from falling below 0 C
to prevent the coil from frosting.
PSYCHROMETRIC ASPECTS
In the following chart it is assumed that:
1. The three-way valve is full open to the coil because the
exhaust air heat does not exceed the supply coil heat
requirements.
2. The heat recovery pump is operating.
3. Heating RA condition is 24 C DB and 17 C WB.
4. OA temperature is 1.5 C DB and –1.5 C WB.
EA
1
1
SA FROM SUPPLY FAN
OA 1.5 F DB, -1.5 C WB
The following results are obtained:
Item
No.
Explanation
1
Temperature rise of the SA coil and temperature
drop of the EA coil are a function of system
design, flow rates, and component efficiency.
A 8 kelvins rise is shown as an example.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
234
HEATING RA CONDITIONS
24 C DB, 17 C WB
C3245
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