Book - School of Science and Technology

Other systems 401Simultaneous heating and cooling can be provided by individual units to suit thethermal loads around the building. There is a running cost benefit arising from suchoperation due to condenser heat from those units performing as coolers being rejectedinto the water circuit, thereby reducing the heat input required from central boilers. Adiagrammatic arrangement of the system is given in Figure 14.28, from which it can beseen that a two-pipe closed water circuit is maintained at around constant temperature,typically 27 C, to provide the heat source and heat sink for the heat pumps.The coefficient of performance of the smaller distributed refrigeration compressors islower than that obtained from central plant; the ratio being of the order of two to one.However, the distribution losses and the electrical power absorbed by the chilled waterpumps associated with a central plant have the effect of reducing the effective differencebetween the energy requirements of the alternative methods of cooling.Outside air may be introduced to the space through a central plant either independentfrom the units, connected to each unit, or alternatively drawn from outside directly intoeach unit. Air supplied from a central plant is preferred since this provides better controland reduced maintenance.Temperature control is normally by a thermostat sensing return air to the unit whichsequences the compressor and reversing valve. Only coarse temperature control isachieved and control over humidity is poor with such a system. Noise can be a problemwith the units located in the space, particularly since the compressors may start and stopfairly frequently.Larger heat pump units may be used to serve areas such as shops and departmentstores. Typically, these would operate on the reverse cycle principle, but would use outsideair both as the heat source in the heating cycle and as the heat sink for the cooling cycle.Optional – heat rejectionto other systemReject heat throughcooling towerPumpT 2MV MV MVT 2T 3T 3Typical operating temperaturesT 1 = 27ºCT 2 = 18ºCT 3 = 34ºCOptional –reclaimheatsourceT 2T 1Heat exchangerReverse cycle unitsHeating modeCooling modeBoiler plantMVPumpMotorised valveFigure 14.28 System arrangement for a reverse cycle heat pump system