Filter
Top Products
TRG-TRC016-EN 89
period four
Chiller-Plant Control
notes
The variable-primary-flow system, introduced in Period Three, is designed to
operate with variable flow through the chiller evaporators. Sequencing chillers
in this type of system cannot be based solely on temperature, because in a
properly-operating system the supply- and return-water temperatures will be
nearly constant. Determining when to turn chillers on or off is not a simple task.
For control stability and chiller reliability, the flow rates through the chillers,
and the rate of flow change, must be kept within allowable ranges.
Therefore, control of a variable-primary-flow system must:
n Include a method to determine system load. Many systems measure flow
rates and temperatures.
n Ensure that flow rates through the chillers are within the allowable
minimum and maximum limits. Modulation of a control valve in the bypass
pipe is commonly used to ensure minimum flow rates through the chillers.
n Control the rate at which the system flow rate changes, to ensure that it does
not change more rapidly than the chillers can adapt. This is especially critical
when turning on additional chillers.
Adequate time must be spent designing the control sequence and
commissioning the system after installation, to ensure proper operation of a
variable-primary-flow system.
Variable-Primary-Flow Systems
b
y
p
a
s
s
b
y
p
a
s
s
two
two
-
-
way
way
valve
valve
variable
variable
-
-
flow
flow
pumps
pumps
control
control
valve
valve
check
check
valves
valves
optional bypass
optional bypass
with three
with three
-
-
way valve
way valve
Figure 97