ASHRAE Journal - May 2012 - 28

factors requiring analysis include: size of peak load vs. size of heat pumps available, number of hours that will be spent at low loads, energy costs, and installed costs for the heat pumps. Since a heat pump can control either the LHWT or the LCHWT—but not both simultaneously— the machine will, by necessity, operate in either heating priority mode or cooling priority mode. • In heating priority mode, the heat pump is controlled in response to the heating load, and the LHWT is the controlled parameter. If evaporator water flow is constant, the LCHWT will vary, based on the amount of heat removed from the evaporator. • In cooling priority mode, the heat pump is controlled in response to the cooling load, and the LCHWT is the controlled parameter. If condenser water flow is constant, the LHWT will vary, based on the amount of heat rejected to the condenser. This is also known as heat reclaim mode.

Coil

Exhaust Air

Ventilation Air

42°F

52°F WTWHP Condenser

130°F 115°F

Evaporator

Figure 3: Heat pump system for exhaust air heat recovery.

Heating-Only Mode

130°F

115°F WTWHP

To aid in the understanding of heat pump Condenser system configurations, it is helpful to start with Tower Tower simpler heating-only systems before discussing 85°F Evaporator the more complicated simultaneous heating-and95°F cooling systems. Exhaust air heat recovery. Heat pumps have been used to replace or enhance “runaround” loops by extracting heat from building exhaust air and using it to warm incoming makeup air. If space exists, 85°F 95°F this system is relatively easy to add to an existing installation. Figure 3 illustrates a typical installation.4 Chiller Plant Tower water as a heat source. A heat pump can use cooling-tower water from chillers or 42°F 52°F other equipment as its heat source, as shown in Figure 4. If space exists, this arrangement can be a useful add-on to an existing installation.5 Because the temperature of the heat-source water is relatively high, a low-lift heat pump is usually Figure 4: Tower water as heat source. sufficient for this application. Here are some application considerations for this system: heat pump and one of the water chillers with a high-lift heat • Because the heat pump evaporator is connected to the pump that can provide both the cooling and the heating. This open cooling-tower circuit, its fouling factor will need to be arrangement offers several advantages. raised to an appropriate value. a. Two heat exchangers (the chiller condenser and the heat • To maintain a consistent LHWT, the tower-water temperature pump evaporator) and associated piping are eliminated, as must be artificially held at an elevated temperature, which penalare the pumping costs for both circuits. izes the chillers’ efficiency whenever the potential for lower tower b. The losses introduced by temperature approaches and water temperature exists. An option to avoid penalizing the chillers’ fouling in these two heat exchangers are eliminated, which performance is to design the heat pump for the lowest tower-water reduces lift and improves efficiency. temperature that the chiller plant can use (say, 55°F [13°C]). c. If the high-lift heat pump incorporates an interstage • Another option to avoid penalizing the chillers’ perforeconomizer, the overall cycle efficiency will be further inmance is to consider connecting the heat pump to only one of creased. the tower circuits. d. The high-lift heat pump can reduce the required cool• Another configuration for this system, which can reduce ing tower capacity, and likely has a smaller footprint than piping costs and increase efficiency, is to replace the low-lift the two units it replaces.
28 ASHRAE Journal ashrae.org May 2012

ASHRAE Journal - May 2012

Table of Contents for the Digital Edition of ASHRAE Journal - May 2012