Parallel Pumping with Optimized Impeller Pumps Part 2: Heat Transfer at Reduced Flow Rates

By Mark Bingham

Singular pump operation in a 2-pump parallel system occurs in one of two circumstances:

1. The current demand load is half of the design load, or

2. One pump has failed and/or needs to be serviced

In either case, a single pump can meet the current demand under most load conditions.

HVAC systems operate at full capacity only a few hours out of the year when weather conditions are extreme. This means that most of the time, they operate at part load conditions. Significantly reduced flows can often provide the required heat transfer during off-peak conditions. This is true of most hydronic heating and cooling systems, including water source heat pump systems (WSHP).

Figure 1. Percent of Design Flow Versus Design ΔT to Maintain 90% Terminal Heat Transfer for Various Supply Water Temperatures

Figure 1, taken from the 2023 ASHRAE – HVAC Applications, Chapter 39, “Testing, Adjusting, and Balancing, shows how this is possible. As the top curve indicates, a heating system supplied by a condensing boiler, designed for a 40°F ΔT with 140°F supply water, will yield 90% of design heat transfer at 75% of design flow. As the supply temperature increases or the temperature differential decreases, larger reductions in flow will provide 90 percent of design heat transfer. A more traditional heating system with a 180°F supply temperature and a 20°F ΔT will yield 90% of design heat transfer at just over 50% of design flow.

Reduced flows in chilled water systems require more consideration. Figure 2, also taken from Chapter 39, shows the relationship between heat transfer and flow in a typical chilled water coil. While 75% of design flow yields 90% of the total design heat transfer, it produces only 65% percent of latent heat transfer. This is because latent heat transfer increases linearly with flow, beginning with approximately 25% latent heat transfer at 40% flow and increasing to 100% latent capacity at full flow. This will likely result in a temporary loss of optimal humidity control during warm, rainy weather.

Figure 2. Chilled Water Terminal Heat Transfer Versus Flow

While this loss of dehumidification is likely to be short-lived and may only occur a few times during a cooling season, depending on the climate, some systems tolerate reduced flows better than others. WSHP systems, in particular, are great candidates for parallel pumping as the heat pumps are very tolerant of variable flow.

Table 1 below shows the performance of a typical water source heat pump (WSHP) with flow reductions of 50% over a wide range of entering water temperatures. For instance, at 70°F entering water temperature, a 2.5-ton WSHP will experience only a 1.1% drop in total cooling capacity when the flow is reduced from 6 GPM to 3 GPM.

Table 1. Typical Water Source Heat Pump Selection Data