A water-to-water heat pump extracts heat from one water loop, raises the temperature of the heat, and transfers it to another water loop. A large-capacity, water-to-water heat pump is an electric-drive, vapour-compression water chiller which has been modified to accommodate the higher pressures required of a heat pump. lt removes heat from water in its evaporator, and raises the temperature of the heat, so it can produce hot (120-170°F, 49-77°C) water in its condenser, and deliver the hot water at a controlled temperature. In selecting large water-to-water heat pumps, it is important to consider these factors including hot water temperature, fouling factors, turndown, sizing.
Hot water temperature
The maximum Leaving Hot-Water Temperature (LHWT) of a given heat pump is typically limited by the design working pressure of the unit for a given refrigerant. While higher LHWTs reduce the size of the airside heat exchange equipment (coils), the operating cost, and possibly the capital cost, of the heat pump will increase. For each application, the best balance between airside and waterside capital costs will differ. For the highest efficiency, use the lowest possible LHWT. The difference between the LHWT and the Leaving Chilled-Water Temperature (LCHWT) is an indication of the pressure difference, or lift, against which the compressor must work. For centrifugal heat pumps, each stage of compression can provide about 70°F (40°C) of lift. A single-stage centrifugal heat pump is sometimes called a ‘low-lift’ unit, while a centrifugal heat pump with two or more stages is referred to as a ‘highlift’ unit. scroll and screw heat pumps can provide about 85-150°F (47-83°C) of lift.
Fouling factors
Water chillers are normally designed for waterside fouling factors of 0.00010 and 0.00025 ft2-hr-°F/btu in the evaporator and condenser, respectively, assuming the condenser operates in an open coolingtower loop. In a heat pump, however, the condenser loop is normally closed, while the evaporator loop may be closed or open. Therefore, 0.00010 fouling should be used for the heating loop, and the evaporator should be assigned an appropriate factor, based on the water quality.
Turndown
Because heat pumps operate at essentially constant lift (the entering chilled water temperature and LHWT remain relatively constant), the turndown (available load reduction) can be significantly less than the turndown for water chillers. This is especially true for centrifugal compressors, where minimum capacity may be 40-50% of the design heating load, as dictated by the surge boundary. Surge is not an issue for positive-displacement compressors; therefore single-compressor screw heat pumps are able to unload to a minimum capacity of 25-35% of the design heating load. Scroll and screw heat pumps that utilize multiple compressors offer greater turndown capabilities.
Sizing
It is vitally important to remember that a heat pump condenser must always be able to fully reject the heat absorbed in the evaporator, plus the compressor work. For this reason, a heat pump should never be oversized for the load. If the facility’s heating requirement is insufficient to accept the minimum heat output of a given heat pump, the temperature in the hot water loop will rise uncontrollably until the heat pump shuts down. In heating-only applications, the sizing of the heat pump is relatively simple: if the heat source is not a limiting factor, the heat pump can be sized for the maximum heating requirement.
In simultaneous heating-and-cooling applications, the sizing of the heat pump requires greater analysis. Consider a facility with year-round heating and cooling requirements. The heating load may include reheat, domestic hot water and other heating requirements. Figure 1 shows the load profiles, plus the curve for the Heat Rejection from the Cooling Load (which is the sum of the cooling load and the compressor work). Remember, when sizing heat pumps for simultaneous heating-andcooling, the heating output is never independent of the cooling requirement. In this example, one cannot select a heat pump to cover the entire heat requirement because the coincident cooling requirement (the heat source) is not sufficient to support it. Following this logic, the proper size of the heat pump is at the cross-over point of the curves for the Heating Load and the Heat Rejection from the Cooling Load (point A in Figure 1).
A heat pump sized at this point will result in a unit which will provide a portion of the total heating and cooling requirements of the building. It cannot completely replace the cooling or heating equipment in the building, but it can replace a portion of it in a highly efficient manner, as shown in Figure 2. 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. Besides these factors, system configurations including controls also need to be considered to optimized a water-to-water heat pump. In summary, large-capacity, water-to-water heat pumps offer a number of desirable economic and environmental benefits. Building owners and designers interested in applying heat pumps in their facilities have a variety of system configurations to choose from to meet their specific needs. However, care must be exercised with the selection of the equipment and how it is controlled.
Courtesy: Johnson Controls, Inc.
