Book - School of Science and Technology

540 Refrigeration: water chillers and heat pumpsCentrifugalThe centrifugal compressor is used chiefly where large duties are required. The advantages are:. saving of space as compared with reciprocating machines. reduced vibration (thus suitable for a roof-top plant chamber). reduced maintenance due to there being no wearing or reciprocating parts. efficient part load operation with a control range down to 10%.The most common refrigerant used in centrifugal compressors is R134a, replacing thoseusing CFCs R11 and R12 in earlier models, which for air-conditioning temperaturesoperate over a small pressure range, thus reducing slip losses between blades and casing.An important consideration is the turn-down range, i.e. the ability to follow loadvariations. The two-stage machine shown in Figure 19.7 has a turn-down to 10% of fullload, thus making it highly flexible in operation. Single- or multi-stage machines areavailable, some driven through gearing to achieve the required speed of rotation. Thesemachines are available in the capacity range of 500 kW to 4.5 MW and are used invariablyfor water or ethylene glycol solution chilling and may be water or air cooled. Capacitycontrol is by throttling at the suction inlet or speed control. These machines are normallyof the semi-hermetic or open type with the condenser and evaporator close-coupled to thecompressor as one complete unit.Absorption plantA single stage absorption plant is shown in Figure 19.8. This type uses either ammonia asthe refrigerant and water as the absorption medium, or water as the refrigerant andCondenser 46 ºCCondenser water outEvaporator 4 ºCConcentrator 99 ºC35 ºCSteam 118 ºCCondenseFlashchamberAbsorberpump7 ºCAbsorber 40 ºC‘Out’ ‘In’chilledwater12 ºCEvaporatorpumpRefrigerantsumpTemperatures given are typical30 ºCCondenserwater inHeatexchangerConcentrator pumpFigure 19.8 Diagram of single stage absorption system