Two-Position Control Of Direct Expansion Coil System-Modulating Face And Bypass Damper - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

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