Food and beverage plant operators face a number of head-scratching issues, perhaps none more perplexing than the energy cost riddle. On one hand, they pay for and consume electrical energy to remove heat from their refrigerated spaces via an ammonia refrigeration system while routinely rejecting that heat back into the atmosphere. On the other hand, they pay for and consume natural gas or other fossil fuels to heat the water used for facility and equipment sanitation and processing. If the rejected heat could be captured and used to provide water heating, substantive amounts of energy would be saved and the attendant operational and societal benefits would be realized.

The advantages of ammonia vs. HFC refrigerants in large industrial installations and food preservation are widely known. It is more efficient and provides better heat transfer properties. Ammonia and ammonia refrigeration systems are less costly to buy and operate. As a natural refrigerant, ammonia is the most environmentally friendly, with a global warming potential (GWP) and ozone depleting potential (ODP) of zero. But, as a medium for heat recovery, it does provide obstacles.

The highest temperatures and pressures in the refrigeration system (i.e., compressor discharge gas) provide the optimal source for heat to be transferred to the cleanup water. Ammonia at typical condensing pressures, however, condenses at low temperatures: 75 to 95°F (24 to 35°C). As such, the transfer of energy to city water (through conventional heat exchangers) to create 145 to 165°F (63 to 74°C) washdown water is only effective for limited preheating of the cold water supply. The largest quantity of potential heat energy in the ammonia goes un-harvested.

If, however, the refrigeration system compressor discharge gas — at relatively high pressures of 180 psig (13.2 atm) — could be fed directly into the suction of a heat pump and compressed into even higher pressures — between 450 to 800 psig (32 to 55 atm) —  condensing the higher pressure ammonia in a heat exchanger would capture much larger quantities of heat energy. This would allow raising the temperature of the city water, in one case at least, to the requisite 145°F (63°C) needed for washdown.

But, how do we obtain the required compressive force to operate with ammonia at extremely high condensing temperatures? The answer comes from high pressure, single-screw compressors designed specifically for heat pump waste heat recovery.

Using an Ammonia Heat Pump System for Waste Heat Recovery

The ammonia heat pump system comprises two skid-mounted packages:

• One package consisting of the ammonia heat pump components, including a desuperheater, the single-screw heat pump compressor unit, compressor motor, oil separator, oil cooler, condenser, liquid subcooler and a PLC controller.
• One package consisting of the secondary coolant pumping system and plate-and-frame  heat exchanger.

It should be noted that the system described here reflects the prohibition of potable water to be in direct heat exchange with ammonia, per local code. System variants can be designed to comply with local codes, process preferences/protocols, etc. Always follow local codes.

In the ammonia heat pump system, ammonia gas discharged from the existing refrigeration system’s compressors enters the ammonia heat pump through the operation of a motorized control valve. The gas is drawn into the heat pump system desuperheater. The two-pass desuperheater lowers the temperature of the superheated gas to within 20°F (6.7°C) of saturation while providing a source of heat to the secondary loop. A three-way valve is provided to bypass gas if the approach drops below 20°F (6.7°C). The lower temperature gas helps ensure that oil temperatures in the heat pump compressor do not exceed recommended levels. The gas is then directed to the suction of the ammonia heat pump compressor.

The operation of a single-screw compressor, which can receive suction gas up to 400 psig (27 atm) or higher, is not limited by suction pressure. The highest efficiency and greatest amount of heat delivered from the heat pump occurs when operating at the highest possible suction pressure, which corresponds to the highest discharge pressure from the existing refrigeration system.

After compression to 512 psig (35.8 atm), the ammonia is routed through the heat pump impingement oil separator. Oil from the separator is recirculated, filtered and cooled via an external oil cooler, which removes some of the heat of compression and transfers that heat to the secondary loop.

After leaving the oil separator, the high pressure gas enters the shell of a shell-and-tube condenser, transferring heat to the secondary loop via the glycol/water mixture flowing through the tubes of the condenser and condensing as high pressure liquid.

Further heat is transferred to the secondary loop by subcooling the high pressure condensed liquid in the ammonia heat pump subcooler.

Figure 1 depicts the relative value of each of the heat sources. It is clear that the greatest source of heat energy for water heating is provided by the condensing of ammonia at the high pressures and temperatures of the heat pump system.

How Much Can I Save If I Use an Ammonia Heat Pump System for Waste Heat Recovery?

The cost categories and savings estimates of operating a heat pump waste heat recovery system in large, industrial, ammonia refrigerant installations are:

• Hourly operating cost ratio of a direct contact water heater vs. a heat pump is greater than 3 to 1, or usually between $250,000 and 500,000 annually.
• Approximately 10 million to 30 million gallons of water are saved annually (condenser water consumption and sewer) along with associated water treatment savings.
• Energy cost savings of 30 to 90 percent annually.
• Reduction of greenhouse gases (GHG) emissions and subsequent carbon tax (where applicable).
• ROIs of 35 to 50 percent, or higher.

 In conclusion, food and beverage processing requires both refrigeration and hot water. Each of these needs has been met by independent utilities: the consumption of electricity to extract and reject low-grade heat and fossil fuels for the production of high-grade heat. The addition of an industrial heat pump with a proven high-pressure, single screw compressor results in a more than five-fold reduction in energy consumed to produce an equivalent amount of hot water. Further savings are achieved through the reduction of power and water consumed by the system’s existing compressors and condensers.