Desuperheaters & Refrigeration Heat Reclaim Systems
A refrigeration heat reclaim (RHR) water heating system links two common functions in commercial buildings to reduce purchased energy consumption and achieve cost savings. A refrigeration heat reclaim water heating system harvests heat that would normally be rejected through refrigeration system condensers and applies the heat for water heating. See Figure 1. Refrigeration heat reclaim water heating has the advantages of relatively low cost and simplicity. The primary limitation of refrigeration heat reclaim water heating systems is the fact that heat is available only when the refrigeration system is in operation. However, in many applications heat storage capacity and the operating diversity of heat source equipment remove this concern.
Note: "Refrigeration heat reclaim" refers to recovery of heat from direct expansion vapor compression cooling systems in general, including both food storage refrigeration and air conditioning. It is not limited to food storage freezers and coolers.
Desuperheaters are a particular type of refrigeration heat reclaim system that recovers only the more readily available superheat energy from the host system. They can be somewhat simpler than RHR systems that recover heat of condensation. Details are provided on the Operation page.
A typical refrigeration heat reclaim system is shown in Figures 2 and 3. Most installations are designed as preheat systems using a separate storage tank in series with a conventional water heater. The refrigeration system to which the refrigeration heat reclaim system is connected is called the host system.
A refrigeration heat reclaim device is simply a refrigerant-to-water heat exchanger installed between the host refrigeration system's compressor and condenser. On heat pumps, the heat exchanger is installed between the compressor and the reversing valve. Water is circulated through one side of the heat exchanger and hot refrigerant gas from the compressor is routed through the other side. Heat is transferred from the hot refrigerant gas to the water.
Most refrigeration heat reclaim devices are desuperheaters. Superheat refers to heat stored in the refrigerant vapor when it is heated above the temperature at which it evaporates for a given pressure. See Figure 3. Acting as a desuperheater, a heat reclaim device cools the refrigerant only to the saturation point; no condensing takes place in the desuperheater. Under typical conditions a desuperheater can remove about 10 to 30% of the total heat that would have been rejected by the condenser.
A heat reclaim device may also be designed to do condensing rather than just desuperheating. More heat can be extracted, but at a lower temperature. However, most refrigeration heat reclaim equipment manufacturers have intentionally prevented condensing to avoid problems with host equipment operation. Excessive subcooling (reduction of liquid refrigerant temperature below the saturation point) in the condenser at low outdoor temperature is the concern. With excessive subcooling, problems can occur with low compressor head pressure, improper expansion device operation from inadequate pressure drop, and liquid slugging in the compressor.
Most refrigeration heat reclaim units are designed for retrofit installation. Since installation involves cutting into the sealed refrigerant system, a qualified refrigeration mechanic should do the work. The effect of the installation on any warranties for the refrigeration system should be investigated. Some manufacturers of air conditioners and refrigeration systems place limitations on warranties if heat reclaim systems are installed.
Advantages, Disadvantages and Best Applications
Desuperheaters offer the potential for energy savings in many applications. However, it is important to understand their benefits and drawbacks in order to make sound applications decisions.
Summary of Advantages
- Retrofittable to existing vapor compression systems
- Essentially free water heating in cooling mode
- High-efficiency water heating in space heating mode (unless backup water heating is being used)
- Lower first cost than multifunction full-condensing systems
- Lower diversified electric demand than conventional electric resistance water heaters (unless backup water heating is being used)
Summary of Disadvantages
- High-efficiency water heating limited to periods when the vapor compression system is operating for its original purpose
- Backup water heating required frequently during the year (depending on the application), which lowers annual COP
- Higher first cost than conventional electric resistance or fossil-fired water heaters
- Use of aftermarket desuperheaters may void manufacturers' warranties
- Retrofit desuperheaters require infield modifications to the refrigerant circuit.
Desuperheater systems offer the best overall savings when applied to high, uniform water heating loads that coincide with consistent cooling loads served by a vapor compression system. Good applications include residences with large water heating loads and high, year-round air conditioning loads; airconditioned laundries and health clubs; dairy operations; and restaurants with ice machines or walk-in freezers. Desuperheaters are best installed on systems that have high annual cooling run-times. Greater amounts of waste heat are available from older, less efficient equipment.
Many desuperheater systems are sold by suppliers as retrofit kits. Some manufacturers offer factory-installed desuperheaters as an option for vapor compression systems. Larger vapor compression systems are often fitted with custom-made desuperheater systems designed by specialty design engineering firms. A tank-mounted dedicated beatpump water beater.
- Refrigeration, freezer, water cooler, and ice maker compressors with long daily run time that operate throughout the year.
- Applications on low-temperature refrigeration systems. They release more waste heat per ton of cooling effect than higher-temperature refrigeration systems.
- Less efficient host systems in general. They provide more heat for recovery than more efficient units.
- Where the addition of a RHR unit may improve the performance of the host machine. For example, the installation of a desuperheater in a Georgia fast food restaurant increased the capacity of the ice machine and eliminated ice runouts during the lunch period
Applications To Avoid
- Host systems with low daily run time or seasonal operation.
- Host systems using capillary tubes rather than expansion valves, usually smaller units.
- Host systems in poor condition that are likely to fail soon.
- Very high-efficiency host systems, which have less heat available for recovery.
Effect on Host System Warranty - While most air conditioner and refrigeration equipment manufacturers honor equipment warranties following the installation of refrigeration heat reclaim equipment, certain requirements and conditions may be imposed. Consult with the host system manufacturer prior to installation
- The potential heat output of a refrigeration heat recovery unit is determined by the amount of heat rejected from the host system. Lower-temperature host systems and less efficient host systems provide more heat than higher-temperature systems and more efficient systems.
- Refrigeration heat recovery from an HCFC-22 air-conditioning system will provide approximately 3000 Btu per ton-hour of host system output.
- Refrigeration heat recovery from an HFC-134a air-conditioning system will provide approximately 2200 Btu per ton-hour of host system output.
Impact on Refrigeration Device - It is sometimes claimed that refrigeration heat reclaim systems improve the host system efficiency as much as 20%, reduce electric demand, and increase the life of the compressor. These claims are often overstated. While a heat reclaim system can potentially improve compressor life, the degree of improvement has not been quantified. If installation is not done properly, the increased pressure drop in the refrigerant system can result in significant reduction of compressor life.
ANSI/ARI 470-80 Standard for Desuperheater/Water Heaters establishes equipment specifications, procedures for testing and rating, safety provisions, and product labeling requirements for external heat exchanger systems .
Strict regulations now govern the release of CFC refrigerants to the environment. Refrigerant must be recovered and careful, quality installation practices must be followed to eliminate leaks.