Honeywell AUTOMATIC CONTROL Engineering Manual page 248

AIR HANDLING SYSTEM CONTROL APPLICATIONS
Item
No. Function
1 Control system energizes when fan is turned
on (See FAN SYSTEM START-STOP
CONTROL).
2 Space temperature inputs to control program.
3 Setpoint determines refrigerant on mode.
4 Refrigerant solenoid valve closes when
fan is off.
5 Relay controls liquid line solenoid valve.
6 Relay enables compressor start control system.
7 Temperature differential determines
compressor off mode.
8 Compressor minimum on and minimum off
times prevent short cycling.
9 Control program coordinates cooling, safety,
and fan interlock control.
FEATURES
1. The refrigerant liquid line solenoid valve is closed and
the compressor cannot be energized when the supply fan
is off.
2. Software time delays for compressor protection.
CONDITIONS FOR SUCCESSFUL OPERATION
The differential timers are set wide enough and the software
timers are set high enough to prevent short cycling of the
compressor under light load.
LIMITATIONS
1. Direct expansion coils are difficult to control from leaving
air due to the large and rapid temperature drop when
energized.
2. Compressor operating and safety controls must be
incorporated into the control system.
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
The DX control system shall be enabled anytime the fan
operates.
Cooling system shall be cycled by a temperature control loop
with a 2.5 degree (adjustable) differential to maintain the space
temperature setpoint. When the system is commanded on by
the control program, 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.
ENGINEERING CMANUAL OF AUTOMATION CONTROL
On a rise in space temperature to the setpoint, the refrigerant
liquid line valve shall open and a relay shall enable the
compressor to start under it's controls.
When the space temperature drops to a value equal to the
space temperature setpoint minus a differential, the liquid line
solenoid valve shall close, and the compressor shall continue
to operate until shut down by it's low-pressure cutout.
Refrigerant system control and interlock wiring shall be as
recommended by the compressor manufacturer.
PSYCHROMETRIC ASPECTS
With on-off control, either cooled air or mixed air is supplied
into the space.
In the following chart it is assumed that:
1. Desired space and RA condition is 78°F DB and 50%
RH (65°F WB).
2. Design OA temperature is 95°F DB and 75°F WB.
3. Air entering the system is from the ECONOMIZER
CYCLE DECISION application. The system operates on
a minimum of 35 percent OA during the cooling cycle.
4. Coil leaving air temperature is 55°F.
RA 78°F DB,
50% RH
3
55°F DB
The following results are obtained:
Item
No. Explanation
1 Mixed air temperature at cooling design
condition.
2 Air entering the coil 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 has the DX cooling
energized, coil leaving air is at this value.
4 Air supplied to the space will alternate
between Point 1 and Point 3 as determined by
the space thermostat.
238
OA
1
2
95°F DB,
75°F WB
MA
SA
4
C2559-1