Proportional pressure is a control method that varies the speed of the pump according to a pressure algorithm. It generally requires the pump motor to be controlled by a variable frequency drive (VFD). The VFD changes pump speed based on an external instrument signal (typically a pressure sensor), or the VFD controls speed based on mapped performance of differential pressure and power as a function of flow rate over a VFD output frequency range programmed into the VFD controller. The latter approach indirectly determines pressure, so it can allow for pressure control without an external system instrument.
A common application for proportional pressure control is a hot water circulating system with variable friction head due to multiple heating zones. Image 1 shows four system curves that represent 1-, 2-, 3- and 4-zones or heating areas respectively. Each zone is a living space with radiators that the pump circulates hot water through, and the zones have valves that open or close based on a thermostat measuring the temperature in the heated space.
Therefore, the zones calling for hot water could range from zero (pump off because no heat is needed) or one (highest friction and lowest flow rate) up to all four (lowest friction and highest flow rate). Also illustrated in Image 1 are two pump curves. Pump 1 (solid black line) illustrates the pump differential pressure versus flow rate with no speed control, and pump 2 (green dashed line) illustrates the same pump but with variable speed proportional pressure control. The green dots represent the flow rate and pressure for the variable speed proportional pressure control. The red dots represent the flow rate and pressure for the pump at constant speed.
Note that the pumps would operate at the same flow rate and pressure with all four heating zones open, so a single green dot is illustrated, but when 3-, 2- or 1-zone is open, the proportional pressure control reduces the speed along a linear control algorithm. The point where the green proportional pressure control curve intersects the vertical axis (pressure at zero flow) can often be adjusted higher to better meet part load requirements or lower to save additional energy compared to operating at full speed (red dots).
For more information on controls, refer to the introductory, basic and intermediate level pump and system training offered at training.pumps.org.
