Variable speed pumps offer several benefits and their installation is not as difficult as you may think, says Wayne Rose.
Replacing constant speed pumps with variable speed alternatives is a popular method for reducing energy consumption. The payback is fast (typically one year) and changes to pipework and connections are minimal in comparison to other upgrades.

There are advantages beyond carbon reduction too. As the pump is not operating at full speed at all times, it can be expected to have a longer life. The lower water velocity reduces water-borne noise, and there is no check valve chatter, as the variable speed pumps work on soft start and soft stop.

Despite the popularity, however, a large percentage of conversions from fixed to variable speed in recent years have failed to achieve their full carbon reduction potential. The reason is that, when planning the migration to variable speed, certain misunderstandings must be avoided. For example, it is often assumed that a system can be converted to variable speed by simply installing an adjustable frequency drive onto the pump motor, or straight replacement of a fixed speed pump with a variable speed model.

In reality though, it is the flow system that is the 'make or break'. In order to achieve the true energy saving potential, the system piping inside and/or outside of the plant room may require modifications. A common pitfall is where the flow conditions limit the system to constant speed operation, despite the presence of variable speed pump technology. For example, with threeway valves installed, there is no opportunity to reduce the pump flow, as the valve will divert flow as demand is reduced.

This results in constant pump flow, even though the pump is equipped to deliver more efficient partload pumping.

A simple adaptation, however, can rectify this. By closing the bypass line valve, you effectively make the threeway valves operate as two-way valves.

With the bypass closed, when the three-way valve modulates to attempt to divert the flow through the bypass, it will restrict the flow rate instead.

There is a caveat, however. Variable flow hvac systems converted from threeway to two-way valve control can experience certain challenges when the two-way valves are shut off. For example, the pump may overheat if it continues to run against a closed valve at its minimum speed, with the risk of premature seal failure or thermal shock cracking issues if cold water is suddenly allowed into a hot cast iron casing. Or the temperature of the conditioned water in the headers and remote risers, now stationary, will decay to ambient. This means that when heating or cooling is needed, there could be a delay in delivering freshly heated or cooled water from the plant room, up the riser, and along the floor to the terminal.

A simple solution is to leave open the most remote three-way valve bypass line on each riser. If there are few loads in the zone, close the most remote three-way valve by 50 per cent so that energy is not wasted, with too much conditioned water being returned unused to the chiller or boiler. In this way, water will only be bypassed when the remote load (typically the least served in the system) is satisfied.

Another effective (although more complicated) bypass control option is to mount both the bypass and the differential pressure sensor remote to the pump on the index line. To operate effectively, the minimum frequency on the drive control must be set high enough at low load to ensure the pump will operate at its minimum set speed and move left, up the performance curve increasing the pressure at the index, causing the bypass valve to open. The closed valve pressure of the pump at its minimum frequency must be 25 per cent higher than the control pressure setting of the pressure transducer at the index.

It is also important to ensure that the method of control for the variable speed pumps has been carefully thought through. These decisions will be dictated by the choice of equipment and type of installation.

• Wayne Rose is a director of the product solutions group at Armstrong Integrated