Specifications

2.7.21 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 2 or more chillers in parallel are installed if primary/secondary pumping is

not utilised.

2.7.22 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. 2 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 always maintains 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 pump speed is very similar to trimming the pump impeller, except it does not

require any labour and the pump 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 operates at this speed any time the

chiller is staged on. Because the primary loop does not 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 remains

appropriate. In the event the flow rate needs to be increased later in the systems life, the frequency converter can simply

increase the pump speed instead of requiring a new pump impeller.

Introduction to VLT® HVAC D... Design Guide