In air- and water-distribution systems employing variable-frequency drives and utilizing closed-loop control with pressure feedback for fans and pumps, energy savings can be significant. They also can be somewhat elusive, undone by the improper placement of pressure-feedback transmitters.
This article discusses the proper placement of pressure-feedback transmitters — the most important factor in realizing projected energy savings — in HVAC systems.
REDUCING FLOW AND PRESSURE
Transmitter location “two-thirds of the distance” into a distribution system or “at significant loads” is recommended with little understanding as to why. Broader explanation is needed if such rules of thumb are to be effective.
The equation for power required by a fan or pump is:
Power = (Flow × Pressure) ÷ Efficiency
From this equation, it can be seen that a reduction in either flow or pressure will contribute to a reduction in power, while a reduction in both flow and pressure will contribute to a greater reduction in power.
Flow is modulated directly by control valves or dampers in response to applied-load changes. As flow decreases from its design value, the system's resistance to that flow decreases as well. Thus, less pressure is needed from a fan or pump operating under part-load conditions.
MEASURING PRESSURE AT LOADS
A transmitter should be placed so that it measures pressure where a load is located — in distribution-system extremities. This pressure allows a drive's proportional-plus-integral-plus-derivative (PID) controller to take advantage of decreased resistance to flow as flow is reduced.
With this placement (Figure 1), the controller-set-point requirement is the actual pressure needed by the load. There is no need for a controller set point based on “worst-case” (maximum load, maximum pressure) conditions.
Sometimes, pressure transmitters are located at a pump or fan discharge (Figure 2), usually to reduce installation costs or take advantage of a standard-equipment package. Such placement fails to maximize energy savings. For instance, the placement shown in Figure 2 results in savings of 5 percent, while the placement shown in Figure 1 results in savings above 30 percent.
The pressure required at the farthest significant load in a system is constant, regardless of load changes. It is the pressure necessary to maintain design flow through a load device and a control valve or damper that is open to handle design load in a conditioned space. To deliver this pressure and flow, a pump or fan must be capable of overcoming system resistance to flow.
As flow to a load increases, pump or fan discharge pressure must be great enough to overcome increasing system resistance to flow. Maximum system flow, therefore, requires the highest pump or fan discharge pressure.
If a pressure transmitter is located near a pump or fan discharge, as in Figure 2, the controller set point must be established at the worst-case high discharge pressure to ensure there will be enough pressure at the load when flow is greatest. At lower flows, there will be more pressure than necessary “out at the loads.”
Controller set point is a single value, so if a pressure transmitter is located at a pump or fan discharge, it must be set at a high level — the system design pressure. Such a high controller set point ignores significant potential energy savings that would be produced by variable flow with variable pressure at the pump or fan discharge.
If, on the other hand, a pressure transmitter is located at the farthest significant load in a distribution system, as in Figure 1, the controller set point can be established at the much lower pressure required by the load. In this case, as flow is reduced by control valves or dampers, a pump or fan needs only to maintain the lower pressure requirements of the load.
As flow and system resistance to it decrease, so does pump or fan discharge pressure — by the same amount as the falloff in resistance. In this control scheme, pump or fan discharge pressure is uncontrolled, while the pressure that is important to system operation is under closed-loop control.
Placing a pressure transmitter at the extremities of a distribution system, as in Figure 1, improves energy conservation by allowing the drive PID controller to utilize a lower set point. This means system pressure at the control valves or dampers never will be more than is required. The potential for energy savings resulting from variable flow with variable pressure will be realized because the pump or fan discharge pressure decreases as flow decreases.
When a transmitter is located at a pump or fan discharge, as in Figure 2, the ability to sense what is happening in the system is lost. To prepare for the worst-case (full-flow) condition, the control system must always provide design pressure at the pump or fan discharge.
With a transmitter “out in the system” at loads, as in Figure 1, discharge pressure is allowed to vary as needed. This is a “smart” system because the controls automatically adapt to variations in system resistance to flow occurring with changes in flow rate. The lower the controller set point relative to the design pressure of a pump or fan, the less power that is required.
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The Yaskawa Electric America building-automation team has more than 75 years of experience in providing application support, harmonic analysis, payback analysis, quotation assistance, and commercial HVAC adjustable-frequency-drive technical support. It can be contacted at firstname.lastname@example.org.