Fig. 37-24 Representative Single-Stage Centrifugal Compressor Performance Characteristics Curves Over a Range of Inlet Vane Positions. Source: The Trane Company characteristic curves for a single-stage centrifugal compressor operating at constant speed over a range of inlet vane positions. The pressure differential between the inlet (evaporator) and outlet (condenser) of the compressor are on the vertical axis and the compressor refrigerant flow rate capability at various pressure differences and vane positions are on the horizontal axis. The surge line represents the flow rate and pressure difference conditions that produce surge. Any compressor operating point that falls to the right of this line is satisfactory from a stability standpoint.

The two principal concerns for operation at low load are compressor stability and volumetric efficiency. As the refrigerant flow is reduced in response to falling load, the inlet vanes are modulated. This stabilizes the evaporator pressure and temperature by balancing the compressor pumping capacity with the new gas flow rate. As the system load decreases, the reduction in refrigerant flow reduces the amount of heat transferred to the condenser. This increases the heat transfer capability of the condenser relative to the load, meaning it is capable of condensing the refrigerant at a lower temperature and pressure. This reduces the difference between the stabilized evaporator pressure and the new condensing pressure, moving the operating point. With effective modulation, compressors can typically operate down to a minimum of 10 or 20% of design load, depending on design and operating factors. Below this level, a low-temperature control will stop the compressor and then restart it when a rise in temperature indicates the need for additional cooling. This process is highly undesirable, particularly for large machines. Typically, performance starts to fall off rapidly below 30 to 40% of full load. To extend the stable operating range, a false load could be put on the system through hot gas bypass. Alternatively, a design featuring multiple impellers or speed control could also be used to extend the operating range and improve compressor volumetric efficiency.

Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

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