Primary pumps
Primary pumps in a primary/secondary pumping system can be used to maintain a constant flow through devices
that encounter operation or control difficulties when exposed to variable flow. The primary/ secondary pumping
technique decouples the “primary” production loop from the “secondary” distribution loop. This allows devices such
as chillers to obtain constant design flow and operate properly while allowing the rest of the system to vary in flow.
As the evaporator flow rate decreases in a chiller, the chilled water begins to become over-chilled. As this happens,
the chiller attempts to decrease its cooling capacity. If the flow rate drops far enough, or too quickly, the chiller
cannot shed its load sufficiently and the chiller’s low evaporator temperature safety trips the chiller requiring a
manual reset. This situation is common in large installations especially when two or more chillers in parallel are
installed if primary/ secondary pumping is not utilized.
The VLT solution
Depending on the size of the system and the size of the primary loop, the energy consumption of the primary loop
can become substantial.
A frequency converter can be added to the primary system, to replace the throttling valve and/or trimming of the
impellers, leading to reduced operating expenses. Two control methods are common:
The first method uses a flow meter. Because the desired flow rate is known and is constant, a flow meter installed
at the discharge of each chiller, can be used to control the pump directly. Using the built-in PID controller, the
frequency converter will always maintain the appropriate flow rate, even compensating for the changing resistance
in the primary piping loop as chillers and their pumps are staged on and off.
The other method is local speed determination. The operator simply decreases the output frequency until the
design flow rate is achieved.
Using a frequency converter to decrease the pumps speed is very similar to trimming the pumps impeller, except
it doesn’t require any labor and the pumps efficiency remains higher. The balancing contractor simply decreases
the speed of the pump until the proper flow rate is achieved and leaves the speed fixed. The pump will operate at
this speed any time the chiller is staged on. Because the primary loop doesn’t have control valves or other devices
that can cause the system curve to change and the variance due to staging pumps and chillers on and off is usually
small, this fixed speed will remain appropriate. In the event the flow rate needs to be increased later in the systems
life, the frequency converter can simply increase the pumps speed instead of requiring a new pump impeller.
CHILLER
F
CHILLER
F
Flowmeter
Flowmeter
VLT
®
6000 HVAC Series
22
MG.61.B5.02 - VLT
®
is a registered Danfoss trademark
Содержание VLT 6000 HVAC Series
Страница 31: ...Ordering form VLT 6000 HVAC Series MG 61 B5 02 VLT is a registered Danfoss trademark 31 Introduction to HVAC ...
Страница 63: ...Mechanical dimensions VLT 6000 HVAC Series MG 61 B5 02 VLT is a registered Danfoss trademark 63 Installation ...
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