Single Pump, Pressure Bypass, Reverse Return - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

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 27.9 kPa, but across Coil 1
where the greatest differential pressure change exists (84 kPa
at design to 103.5 kPa 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
36 kPa drop and even with the pressure bypass valve, the control
valves will be subjected to drops of near 103.5 kPa 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 9.3 kPa DROP
CHILLER
27.9 kPa
DROP
DP3
PUMP
66 L/s
150 kPa
9.3 kPa DROP
Fig. 76. Single Pump, Pressure Bypass, Reverse Return at 90 percent Flow.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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 pressure still rises to 162 kPa and the control
valve drop still rises to 132 kPa.
For 90 percent flow control, the pump still operates at 150 kPa
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 122.1 kPa as compared with the DP-3 full
flow differential pressure of 107 kPa. This small 15.1 kPa
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/ 11 L/s
HEAT/
COOL PER
COOL
COIL AHU COIL
COIL
3
2
V3
V1 V2
RETURN
351
HEAT/
HEAT/ HEAT/
COOL
COOL COOL
COIL
COIL COIL
6
4 5
V4 V5
POINT A
M15288
V6