Book - School of Science and Technology

Types of refrigeration plant 541lithium bromide as the absorption medium. The equipment has no moving parts exceptpumps. Two-stage plant is also available, incorporating a second concentrator whichutilises heat recovery from the first stage of the cycle to provide improved efficiency. Thesource of energy being either steam, medium temperature hot water or gas, this type ofplant is suited to installation where there is a heat source of a suitable grade available,such as with combined heat and power (CHP) systems. Such plant works under a highvacuum and is available in a range from 10 kW to over 5 MW cooling capacity. The heatto be removed by the condenser water with this system is about double that of a vapourcompression plant of equivalent capacity. For example, an absorption chiller of 350 kWcapacity will use a nominal steam quantity of 220 g/s, with heat rejection of some 900 kW.The equipment is usually very heavy and does need a level surface for installation.Steam-jet plantAn interesting type of refrigerator, using water as the medium, is that shown in Figure19.9. Its operation depends on the possibility of causing water to boil at low temperaturesunder high vacua. As noted previously, water at 7 C boils at an absolute pressure of1 kPa, i.e. about one-hundredth of atmospheric pressure. The absence of any specialrefrigerant is an advantage and an economy.The energy input with this equipment is much greater than with the positive compressiontypes owing to the inefficiency of jet compression and the requirement for condensingwater flow is about five times that of a vapour compression plant.A variation of the same system, but using a centrifugal compressor in place of thesteam-jet compressor, has also been developed and this avoids the above mentioneddisadvantage of high energy input. The great difficulty with both types is the maintenanceSecondaryejectorsVentSteamNozzlePrimary ejectorEvaporatorCondenserAftercooler4.5ºC 840PaMake up38º C 6.62kPaPumpPumpPumpChilledwaterCondensateto boilerCooling waterto towerFigure 19.9 Diagram of steam-jet system