Table of Contents
AIR HANDLING SYSTEM CONTROL APPLICATIONS
Item
No.
Function
1
Control system energizes when fan is turned
on (See FAN SYSTEM START-STOP
CONTROL).
2,3
Space temperature sensor inputs to PID control
loop to maintain setpoint.
4
Face and bypass damper modulates for cooling
control.
5
Damper position determines compressor mode.
6,7
Refrigerant liquid line solenoid valve cycles
for cooling.
8
Relay energizes compressor.
9
Timers protect compressor from short cycling.
10
Control program coordinates temperature,
compressor, and fan interlock control.
FEATURES
1. The proportions of air passing through and around the
coil are varied by modulating face and bypass dampers.
2. Lowering air velocity through the coil lowers the moisture
content of air leaving the coil, thus producing lower space
RH than systems that only cycle the refrigeration at a
constant air volume.
CONDITIONS FOR SUCCESSFUL OPERATION
1. Capacity control of refrigeration provided to avoid icing
under light load.
LIMITATIONS
1. Direct expansion coils are difficult to control from SA
due to the large and rapid temperature drop when
energized.
2. Compressor operating and safety controls must be
incorporated into the control system.
3. The system may be controlled from SA if necessary, but
only if the sensor is located far enough downstream of the
coil to ensure complete mixing of the air and software
timers are provided to prevent compressor short cycling.
Hot gas bypass may be required.
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
The DX control system shall be enabled anytime the fan
operates.
Space temperature PI control loop shall modulate DX coil
face and bypass dampers to maintain setpoint.
Anytime the face damper modulates up to 80% open, the
refrigerant valve shall open and a relay shall enable the
compressor to start under compressor controls. When the
compressor system is commanded on, it shall remain on at least
eight minutes and when it is commanded off (or drops off during
power interruptions) it shall remain off at least ten minutes.
Anytime the face damper modulates down to 30% open, the
refrigerant valve shall close and the compressor relay shall open.
When the solenoid valve is closed, the compressor shall
continue to operate until shut down by it's low-pressure cutout.
PSYCHROMETRIC ASPECTS
Cooled air from the coil is mixed with bypass air to provide
the necessary DA temperature.
In the following chart it is assumed that:
1. Desired RA condition is 27 C DB and 50% RH
(19.5 C WB).
2. Design OA condition is 35 C DB and 24 C WB.
3. Air entering the system is from the ECONOMIZER
CYCLE DECISION application. The system operates on
25 percent OA during the cooling cycle.
4. Coil leaving air temperature is 13 C.
SA
4
3
DX COIL
DISCHARGE
13 C DB
The following results are obtained:
Item
No.
Explanation
1
Mixed air temperature at cooling design enters
the cooling system.
2
Air enters the coil and is cooled along a line of
constant moisture content until saturation is
approached. Near saturation the moisture con-
tent is reduced as the air is cooled. This process
involves both latent and sensible cooling.
3
When the space thermostat calls for DX
cooling to be energized and the face damper
greater is than 30% open, air leaving the DX
coil is at this value.
4
Supply air will vary between Point 3 and Point
1 as the space thermostat positions the face
and bypass dampers.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
240
OA
2
1
35 C DB,
MA
24 C WB
RA 27 C DB,
19.5 C WB
C3251
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