Hot Water From Refrigerant Hot Gases


A typical refrigeration machine with a water-cooled condenser rejects approximately 15,000 BTU/hr for each 12,000 BTU/hr of refrigeration. An air-cooled condenser rejects up to 17,000 BTU/hr for each 12,000 BTU/hr of refrigeration. Up to 5,000 BTU/hr of this heat rejected from either system can be recaptured. To recover the heat of compression, a heat exchanger can be installed in the hot-gas line between the compressor and the condenser of the chiller. A typical arrangement in conjunction with a domestic hot- water system is shown in the figure below. Hot-gas temperature depends on head pressure but is usually around 180°. Heat pumps, operating year-round for cooling or heating, offer a fine opportunity to use a hot-gas heat exchanger. Cold water is circulated through the heat exchanger by the circulating pump. When hot water is not being used, water is pumped back through the heat exchanger, the water heater or both. A mixing valve is provided to maintain the desired temperature. Energy can suitably be reclaimed from refrigerant-system hot gas when there is a steady and concurrent demand for refrigeration and heat and when refrigeration systems operate for 750 or more hours per year. With refrigerant gas-to-water heat exchangers, care must be taken not to reduce superheat to a point that allows liquid slugging and the heat exchanger must be located after the hot gas bypass or other unloading devices. If the exchanger is located outdoors, drains must be provided to prevent freezing.

Energy savings are dependent on many variables such as how much and when excess heat is available and how much and when excess heat can be used. In general, up to 80 percent of excess heat can be recovered. Therefore, savings can range from 80 percent of the available heat to 0 percent if there is no use for the heat. Paybacks range from three years and up depending on the quantity of heat available and the use for the heat.

Installation (retrofit) cost estimates vary with the size of equipment and the installation specifics. Costs range from $1,000 per compressor for simple systems where all the components are close together to over $3,000 per compressor for more complicated installations.