08-16-2017, 10:43 PM
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Heat-Recovery Water Heating
Heat recovery is the capture of energy contained in fluids or gases that would otherwise be lost from a facility. Heat sources may include heat pumps, chillers, steam condensate lines, hot air associated with kitchen and laundry facilities, power-generation equipment (such as microturbines or fuel cells), and wastewater drain lines. Opportunities There are two basic requirements for heat-recovery water heating: (1) hot water demand must be great enough to justify equipment and maintenance costs, and (2) the waste heat temperature must be high enough to serve as a useful heat source. Large facilities such as hospitals and military bases often have the perfect mix of waste heat and demand for hot water to effectively use waste-heat-recovery systems for water heating. Consider heat-recovery water heating whenever adding or replacing large heating or air-conditioning equipment. For example, double-bundle chillers can easily provide for the recovery of heat normally lost to a cooling tower. The simplest heat-recovery water preheaters can even work with small commercial kitchens and housing units. Technical Information How waste heat is captured and utilized depend upon the temperature of the waste heat source. Where water temperature of 140 180 F (60 82 C) is required, waste heat sources with higher temperatures should be used. Lower-temperature sources, such as hot kitchen air or drainline water, may require mechanical systems to concentrate the heat or supplemental heating using another fuel (i.e., the waste heat serving to preheat the water). Hot gas heat exchangers. The refrigeration cycle of an air conditioner or heat pump provides an opportunity to recover heat for water heating. HVAC compressors concentrate heat by compressing a gaseous refrigerant. The resultant superheated gas is normallypumped to a condenser for heat rejection. However, a hot-gas-to-water heat exchanger may be placed into the refrigerant line between the compressor and condenser coils to capture a portion of the rejected heat. In this system, water is looped between the water storage tank and the heat exchanger when the HVAC system is on. Heat pumps operating in the heating mode do not have waste heat because the hot gas is used for space heating. However, the heat pump system can still heat water more efficiently than electric resistance heating.