Thermal Storage - Ice Harvesters
Ice harvesters circumvent the problems associated with liquid overfeed ice-on-pipe systems by combining all of the components and accessories required for ice production in a single manufactured package. This piece of equipment, called an ice harvester, is installed above an open tank that stores a combination of water and flakes of ice.
To produce ice, 32°F water is drawn from the storage tank and delivered to the ice harvester by a recirculation pump at a flow rate of 8 to 12 gallons per minute per ton of ice-producing capacity. Once inside the ice harvester, the recirculated water flows into a drain pan positioned over a series of refrigerated plates. Each of these plates is constructed of two stainless steel sheets welded together at their circumference. A refrigeration system integral to the ice harvester maintains the plates at a temperature of 15 to 20°F.
As the water leaves the drain pan, it flows freely over both sides of the refrigerated plates where it freezes to a thickness of 1/8 to 3/8 inch. On reaching a given thickness - or at the initiation of a time clock - the ice is dislodged from the plates by a hot gas defrost cycle, and falls into the tank below. When cooling is required, a transfer pump draws ice water from the storage tank and delivers it to a building heat exchanger.
It is possible to use the ice harvester as a water chiller by raising the suction temperature of the refrigeration system and pumping warm water from the building heat exchanger over the refrigerated plates. In fact, operating at this higher suction temperature improves the ice harvester's efficiency. Unfortunately, the ice harvester cannot produce chilled water without destroying ice stored in the storage tank. This inability to operate as a true water chiller in a chilled water system poses a significant efficiency penalty on ice storage systems with ice harvesters. To get around this inefficiency, ice harvesters are commonly used in tandem with conventional water chillers.
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