Honeywell AUTOMATIC CONTROL Engineering Manual page 363

The greatest change at the sensor provides the most tolerant
and robust control. For this reason the sensor is located, not
across the chiller with a setpoint of 9.6 ft, but across Coil 1
where the greatest differential pressure change exists (28 ft at
design to 34 ft at 90 percent of design).
NOTE: With the controller pickups in these locations, it does
not matter where in the system the load is located,
what value it has, or whether it is symmetrical or
non-symmetrical.
Also note that the Load 1 balancing valve takes a full flow
12 foot drop and even with the pressure bypass valve, the control
valves will be subjected to drops of near 34 ft for light loads.
For larger and more extended systems, both of these values
must be considered when evaluating reverse return and control
valve high differential pressure solutions.
These values lead to the conclusion that a differential pressure
bypass solution may satisfy the light-load flow through a chiller,
but may not adequately relieve control valve light-load
differential pressures on larger systems. Also be aware that
pressure drop changes due to scaled chiller tubes effect the
bypass valve operation.
SUPPLY 3.2' DROP
CHILLER
9.6' DROP
DP3
PUMP
1080
GPM
50' HEAD
3.2' DROP
Fig. 76. Single Pump, Pressure Bypass, Reverse Return at 90 percent Flow.
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
SYSTEM WITH 90% FLOW THROUGH COIL
HEAT/
COOL
COIL
1
DP2
DP1
POINT B
Single Pump, Pressure Bypass, Reverse Return
A reverse return system analysis equivalent to the direct return
analysis of Figure 74 would show that at half flow and no
bypass, the pump head still rises to 54 ft and the control valve
drop still rises to 44 ft.
For 90 percent flow control, the pump still operates at a 50 foot
head with the sensor located as far away from the pump as practical
to take advantage of as many friction drop changes as possible.
The preferred sensor location (Fig. 76) is DP-1 if point A is
near Load 1. Sensor location DP-2 is second best and again if
point B is near Load 1. If location DP-3 is selected by default,
with a setpoint of 40.4 ft as compared with the DP-3 full flow
differential pressure of 36 ft. This small 4.4 foot change requires
a higher quality sensor and more frequent calibration checks
than for locations DP-1 and DP-2. Locate the pressure bypass
valve and sensor as far from the chiller/pump as possible , but
no closer than DP-3.
In all cases it is still preferable to locate the system strainer
outside the control loop as shown in Figure 75.
HEAT/ HEAT/ 180 GPM
COOL PER
COOL
COIL AHU COIL
COIL
2 3
V1 V2 V3
RETURN
353 ENGINEERING MANUAL OF AUTOMATIC CONTROL
HEAT/
HEAT/ HEAT/
COOL COOL COOL
COIL
COIL COIL
6
4 5
V4 V5
POINT A
M15063
V6