For heating or chilled water systems serving air conditioning plant, the requirement for maximum heating or cooling occurs only at start up, or on infrequent peak design days. For the majority of the time, a reduced heating or cooling output will suffice. During these periods there is potential to pump less water, thereby reducing the annual pump energy consumption.
Pump energy can be saved because there is a useful correlation between pump speed, pressure, flow rate and power. For any pump that is pumping against a fixed resistance, the consequences of changing pump speed (from N1 to N2) can be predicted from the pump similarity laws:
where N is the pump speed (rev/s), Q is the flow rate (m3 /s), Δp is the differential pressure across the pump (Pa) and P is the pump power (W). In other words, if pump speed is reduced to 25% of its previous value then:
- flow rate (Q) is also reduced to 25% of its previous value
- pump pressure generated (Δp) is reduced to 6.25% (i.e. one sixteenth of its previous value)
- pump power consumption (P) is reduced to 1.6% (i.e. one sixty-fourth of its previous value)
The same consequence can be seen when these relationships are applied to the standard equation for determining pump power:
where η is the overall pump efficiency (%).
It can be seen that if pump speed is reduced to 25%, causing flow to be reduced to 25% and pump pressure to be reduced to 6.25%, then, as predicted by the pump similarity laws, pump power reduces to 1.6% (i.e. 0.25 times 6.25).
This relationship holds true provided the pump is pumping against a fixed resistance because, for this situation, pump efficiency usually remains fairly constant regardless of changes in pump speed. Therefore, if the pipework system is serving a uniform heating or cooling load then it should be possible to keep the system resistance constant and regulate pump speed up and down in response to demand, thereby achieving all of the 98.4% energy saving predicted at 25% flow.
However, most systems serve multiple zones with variable loads each requiring individualised control of terminal units. This control is typically provided by 2-port control valves which modulate flow as required to suit the zone. In a system with 2-port control valves, the overall system resistance will not be fixed but will increase and decrease as valves open and close.
In this situation the actual pump energy savings achievable will depend on the way in which pump speed is controlled. The easiest way to control pump speed is to make it respond to a differential pressure signal between two points somewhere in the system. The best energy-saving options are:
- vary pump speed based on the pump differential pressure and using an integral speed control characteristic designated by the pump manufacturer.
- vary pump speed to maintain pressure constant at system extremities (using remote differential pressure sensors).
The consequences of each option in terms of pump and system resistance characteristics are shown in Figures 1a and 1b. For each example, a minimum system flow rate of 25% has been assumed.
It can be seen from Figure 1a that pump integral controllers are able to generate their own speed control characteristics which determine how the pump will respond to changes in system resistance. It can be seen from Figure 1a that pump integral controllers are able to generate their own speed control characteristics which determine how the pump will respond to changes in system resistance. The pump operating point will always lie somewhere on this characteristic. Figure 1a shows a straight line control characteristic, but pump manufacturers can also provide curved characteristics which give larger reductions in pump speed for the same operating conditions.
Pumps controlled in this way have the advantage that they avoid the need for remote differential pressure sensors. However, with all integral controllers there is an assumption that the system has a fairly uniform and predictable load pattern and that all 2-port valves will open and close roughly together. If the load pattern is not uniform, i.e. some circuits are likely to remain fully open whilst the majority close down, then there is a risk that the fully open circuits may be starved of flow as pump speed reduces.
The use of remote differential pressure sensors at system extremities is a more precise way of controlling pump speed. Pump speed is controlled such that the minimum design pressure is always available at each extremity. Therefore, as shown in Figure 1b, the part load pump operating point could lie anywhere within a range of values between maximum and minimum load conditions. Multiple sensors are required because in a variable flow system where 2-port valves may close down in random order, the system index may not remain in one location but could move around to different parts of the system.
It can be seen from Figure 1b that for a system controlled in this way the minimum load operating point is not fixed by any pre-determined control characteristic but is free to drop by as much as required. It is therefore likely that the use of remote sensors will achieve larger energy savings than if integral speed controllers are used.
For each pump speed control method, the pump energy saving achievable between maximum and minimum load conditions will be equal to the difference between maximum and minimum load pump power, i.e:
Pump energy saving = (Δp1 Q1 / η1) – (Δp2 Q2 / η2)
By plotting maximum and minimum load pressure loss and flow rate conditions on the pump manufacturer’s pump curve, the change in pump efficiency and consequent energy saving can be determined. However, to complete this calculation, pump duties need to be estimated for both maximum and minimum load conditions. This may require repeating the pump sizing exercise.
CIBSE Knowledge Series — Variable flow pipework systems