Cooling Control Processes; Control Of Modulating Chilled Water Coil Three-Way Valve - Honeywell AUTOMATIC CONTROL Engineering Manual

AIR HANDLING SYSTEM CONTROL APPLICATIONS
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.
STEAM HUMIDIFICATION PROCESS
DETERMINES SLOPE OF THIS LINE
ENTHALPY
=
HUMIDITY RATIO
RA 72°F DB,
45% RH
MA
1
OA
40°F DB, 35°F WB

COOLING CONTROL PROCESSES

The following are common control arrangements for cooling.

CONTROL OF MODULATING CHILLED WATER COIL THREE-WAY VALVE

Functional Description
3
5
80
CONTROL
PROGRAM
80
2
50
MA
ENGINEERING CMANUAL OF AUTOMATION CONTROL
SA 77°F DB,
3
4
60°F WB
2
COIL
DISCHARGE
C2557-1
SA
4
1
ON
SUPPLY
FAN
M10469
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.
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.
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