ENGINEERING MANUAL OF AUTOMATIC CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
239
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 22°C DB and 50%
RH (18°C WB).
2. Design OA temperature is 35°C DB and 24°C 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 13°C.
TWO-POSITION CONTROL OF DIRECT EXPANSION COIL SYSTEM—
MODULATING FACE AND BYPASS DAMPER
Functional Description
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.
C3250
SA
OA
35°C DB,
24°C WB
3
2
1
4
13°C DB
RA 25°C DB,
50% RH
MA
M15171
7
MA
SA
ON
1
9
8
10
MINIMUM
ON TIME
MINIMUM
OFF TIME
COMPRESSOR
(MINUTES)
8
5
10
4
6
RELAY
RELAY
OPEN
CONTROL
PROGRAM
3
2
24.5
24.5
81
SUPPLY
FAN
TO COMPRESSOR
START CIRCUIT
80
30
FACE DAMPER
PERCENT OPEN
SOLENOID
POSITION
OPEN
CLOSED
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