流体力学与传课件Pump priming.ppt

Figure also shows an optimum point. The optimum point at which the losses are minimum is selected as the design point for the compressor. Surging Surge occurs in a turbo compressor when discharge head cannot be sustained at the available suction flow. for example, A centrifugal compressor is designed to operate between a given evaporator and condenser pressures. Due to variations either in the heat sink or refrigerated space, the actual evaporator and condenser pressures can be different from their design values. For example, the condenser pressure may increase if the heat sink temperature increases or the cooling water flow rate reduces. If the resulting pressure difference exceeds the design pressure difference of the compressor, then refrigerant flow reduces and finally stops. Further increase in condenser pressure causes a reverse flow of refrigerant from condenser to evaporator through the compressor. As a result the evaporator pressure increases, the pressure difference reduces and the compressor once again starts pumping the refrigerant in the normal direction. Once the refrigerant starts flowing in the normal direction, the pressure difference increases and again the reversal of flow takes place, as the pressure at the exit of compressor is less than the condenser pressure. This oscillation of refrigerant flow and the resulting rapid variation in pressure difference gives rise to the phenomenon called “surging”. Thus it can be seen that for a given gas with a fixed density, the pressure rise depends only on the tip speed of the blade. The tip speed of the blade is proportional to the rotational speed of the impeller and the impeller diameter. The maximum permissible tip speed is limited by the strength of the structural materials of the blade and the sonic velocity of the gas. Under these limitations, the maximum achievable pressure rise (hence maximum achievable temperature lift) of single stage centrifugal compressor is limited for a given gas.