Book - School of Science and Technology

402 Air-conditioningVariable refrigerant volume systemsAn understanding of the operating principle of a variable refrigerant volume system willbe assisted by reference to the description of the split-system heat pump (Figure 19.24(b)).In the case of the variable refrigerant volume system, the external unit, which is usuallyroof mounted, comprises twin compressors; heat exchangers and air circulation fans. Thisexternal unit has refrigerant pipe connections to remote room terminals each of whichincorporates a refrigerant-to-air heat transfer coil, a filter and a fan to recirculate roomair (Figure 14.29).In the cooling mode of operation, the heat exchangers of the external unit function as arefrigerant condenser producing liquid which is circulated to the remote room terminals.As the refrigerant liquid passes through the coils where heat is absorbed, evaporationtakes place and the gas is returned to the compressors. In the heating mode of operation,the heat exchangers of the external unit will function as a refrigerant evaporator, absorbingheat from the outside air and boiling off the liquid prior to compression. Hot gas fromthe compressors is then circulated to the room terminals, where useful heat is rejected atthe coils and the resultant liquid is returned to the external unit.The system is also able to operate in a dual mode offering both cooling and heatingservice to satisfy the requirements of the spaces served, including the thermal balancesituation where the room heating and cooling demands are in equilibrium, in which caseExternalunitRefrigerantpipingHeatrejectionRoomunits(a) Cooling only modeHeatsource(b) Heating only modeCoolingCoolingHeatingHeating(c) Balanced heating and cooling modeFigure 14.29 Variable refrigerant volume system: modes of operation