ENGINEERING CMANUAL OF AUTOMATION CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
236
C2557-1
MA
SA 77°F DB,
60°F WB
RA 72°F DB,
45% RH
1
2
3
4
OA
40°F DB, 35°F WB
COIL
DISCHARGE
STEAM HUMIDIFICATION PROCESS
DETERMINES SLOPE OF THIS LINE
ENTHALPY
HUMIDITY RATIO
=
PSYCHROMETRIC ASPECTS
The steam humidification process is almost isothermal.
In the following chart it is assumed that:
1. Space conditions are 72°F and 45% RH.
2. Mixed air (entering the heating coil) is 60°F and 42°F
DP.
3. Additional moisture required to maintain space conditions
is 0.0014 lbs per lb of dry air.
COOLING CONTROL PROCESSES
The following are common control arrangements for cooling.
CONTROL OF MODULATING CHILLED WATER COIL THREE-WAY VALVE
Functional Description
M10469
MA
SA
4
5
50
ON
1
CONTROL
PROGRAM
3
2
80
SUPPLY
FAN
80
Item
No. Function
1 Control system energizes when fan is turned
on (See FAN SYSTEM START-STOP
CONTROL).
2, 3 Space temperature PID control loop
modulates the three-way valve to maintain
space temperature setpoint.
4 Chilled water valve directs flow through or
around coil as needed to furnish proper
amount of cooling.
5 Control program coordinates temperature
control and fan status.
FEATURES
1. Chilled water is supplied to the coil at a constant
temperature and varying volume.
2. A reasonably constant flow is maintained through the
entire piping system.
Item
No. Explanation
1 RA and dry OA mix and enter a heating coil.
2 Heating coil leaving air; air gains
sensible heat.
3 Humidifier leaving air; air gains mostly
latent heat.
4 RA; cooled via heat loss and humidity reduced.
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