COMPUTER-BASED SELECTION PROGRAMS

Software programs that make parallel-pump selection quick and easy are available. If the type of system required (heating or cooling, building type, etc.) is known, a load-profile library can be used to estimate the energy-cost savings of parallel- and single-pump operation. The software programs can help avoid pump selections that would cause the single-pump curve to fail to intersect the system curve, resulting in unsatisfactory single-pump operation.

CONSTANT- AND VARIABLE-SPEED PUMPS IN PARALLEL

Often, the objective in using parallel constant- and variable-speed pumps is to minimize a system's initial and operating costs. For example, let's assume that two pumps in parallel each have been selected for full system head and half of full system flow. The system also requires a controller and sensor to provide automatic operation, but initial costs are reduced because only one variable-speed drive is needed and the constant-speed pump requires only a standard motor starter. In a variable-volume hydronic system, the variable-speed pump handles part-load flows up to its full revolutions-per-minute capability and the constant-speed pump is turned off. If flow increases, the constant-speed pump stages on, and the variable-speed pump slows to meet the actual flow requirement. As flow continues to increase, the variable-speed pump again speeds up to the full revolutions-per-minute capacity so that at design flow both pumps are operating at full speed, splitting the flow between them.

A problem arises when a variable-speed pump is making up the difference between a constant-speed pump's capability and required system flow. Point A in Figure 3 represents a requirement for 4,000 gpm at 80 ft of head in a system designed for 4,500 gpm at 100 ft of head. The variable-speed pump cannot meet that point by itself, even running at full speed, so the control system must turn on the constant-speed pump at full speed.

FIGURE 3: Constant-and variable-spead pumps in parallel

The two pumps then operate at Point B in Figure 3, providing more flow than necessary. The variable-speed pump can be ordered to slow from the full revolutions-per-minute capacity to 1,550 rpm to meet the current flow requirement. At 80 ft of head, the constant-speed pump provides only 2,900 gpm, so the variable-speed pump must make up the difference (1,100 gpm). The variable-speed pump could provide 1,100 gpm by rotating at 900 rpm, but it would develop only 25 ft of head at that speed. Unless the control system maintains a speed of 1,550 rpm, the variable-speed pump will be deadheaded and its check valve will be held closed by the 80 ft of discharge pressure from the constant-speed pump. At 900 rpm, the variable-speed pump would contribute nothing to the system flow and eventually overheat.

The 1,100 gpm provided by the variable-speed pump rotating at 1,550 rpm is an artificially low flow compared with the head developed in the pump volute. It is likely that large, unbalanced pressures will exist in the pump volute, causing excessive loads on the pump shaft and bearings. The shaft and bearings are not fragile; they can withstand excessive loads for some period, but will start to fail as the loads get higher for longer durations.

PRESSURE-BOOSTER SYSTEMS

A variable-speed controller

A variable-speed pressure-booster pump is controlled best by sensing a single point of pressure, preferably near the top of a plumbing system. Compared with an HVAC system, the minimum control head of a plumbing system is greater. Therefore, its control curve is flatter and the range of revolutions-per-minute variation in the variable-speed pump is smaller. A variable-speed pump in a plumbing system cannot slow as much as an HVAC pump. For these reasons, constant- and variable-speed pumps sometimes are operated in parallel by a suitable control system.

DOES CONSTANT-/VARIABLE-SPEED PUMPING MAKE SENSE?

In the early days of variable-speed pumping, adjustable-frequency drives were costly, so using a combination of constant- and variable-speed pumps made sense. Now, an adjustable-frequency drive and a large motor starter cost nearly the same amount, and the primary reason for using constant- and variable-speed pumps in parallel largely has disappeared. In addition, running both pumps at the same variable speed reduces pump-bearing wear and provides energy savings at a cost comparable to the constant-/variable-speed combination. Parallel pumping can offer significant benefits and should be considered for industrial HVAC systems when appropriate.

For past HPAC Engineering feature articles, visit www.hpac.com.

An HVAC training consultant, Roy Ahlgren is a columnist for Plumbing Systems & Design, a publication of the American Society of Plumbing Engineers. A retired director of ITT Corp.'s Little Red Schoolhouse, he is a member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers and a past chair of its hydronics technical committee.