Variable flow pipework systems

This publication explains how to design re-circulating heating or cooling water systems incorporating variable speed pumps. Correctly designed, these systems have the potential to deliver worthwhile energy savings over the lifetime of a building.Most constant speed pumps operate with fixed flow rate and energy consumption for their entire lifetime. When zones are satisfied, 3- or 4-port valves divert heating or cooling water away from terminal units through bypasses. Overall flow through the pump remains roughly constant. In systems where pump speed is allowed to vary, the flow rate constantly changes in response to demand. When zones are satisfied, 2-port controlvalves close enabling the pump to reduce speed and save energy.Since the mid-1990s variable speed pump drives have become a viable alternative to constant speed drives. However, there has been much debate regarding how to maximise their energy-saving potential. There are many ways to make a system work with variable speed pumps. However, if not designed correctly, the energy savings achieved may be small relative to the increased cost of the system.In order to be worthwhile, the design must: — achieve a significant reduction in pump energy consumption (relative to that for an equivalent constant flow system)— incur a minimal increase in installed cost— not reduce the effectiveness of internal space temperature controls.With regard to the last point, it is important to remember that any savings in pump energy will easily be negated if the control of internal space temperatures is compromised. A variation of 1 °C from the internal design condition is likely to result in a bigger energy penalty than any savings achieved by the pump.This publication begins by explaining the main issues relevant to any variable flow design, including how to calculate pump energy savings, size control valves (in systems requiring modulating control of water flow rates) and avoid valve noise or cavitation in valves. It also presents two cost-effective solutions for designing variable flow heating and cooling systems which should achieve worthwhile pump energy savings without compromising the effectiveness of temperature controls.