Honeywell AUTOMATIC CONTROL Engineering Manual page 250

AIR HANDLING SYSTEM CONTROL APPLICATIONS
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 80°F DB and 50% RH
(66.5°F WB).
2. Design OA condition is 95°F DB and 75°F WB.
ENGINEERING CMANUAL OF AUTOMATION CONTROL
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 55°F.
SA
4
3
DX COIL
DISCHARGE
55°F 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
content 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.
240
OA
1
2
95°F DB,
MA
75°F WB
RA 80°F DB,
66.5°F WB
C2560-2