Centrifugal compressors suction temperature

Spillback bypasses the compressor discharge gas, back to the compressor suction. This is a relatively energy-inefficient way to increase the evaporator temperature. Spillback is discussed in Chap. 28, Centrifugal Compressors and Surge. ... 

Throughout our discussion of motor-driven, constant-speed centrifugal compressors, I have assumed that the suction temperature was constant. But let s refer back to Fig. 28.1. Assume that the temperature in the drum increases. How will this affect the pressure in the drum ... 

The pressure coefficient = 0.5 to 0.65 assume 0.55. The pressure differential increases with increase in suction gas density (increased molar mass or suction pressure, or decrease inlet temperature, decrease in k). Power Up to 7.5 kW/mVs. Efficiency of large centrifugal compressors 76 to 78%. 

Centrifugal compressors Centrifugal compressors deliver actual volumetric flows (cubic decimeters per second and performance should not be expressed as mass, mols or standard volumetric flow). Assume compression ratios equal in all stages. Maximum number of stages that can be on one shaft or fit in the frame = 8 minus 1 stage for each side nozzle. Compression ratio 2.5-4. The pressure coefficient = 0.5-0.65 assume 0.55. The pressure differential increases with increase in suction gas density (increased molar mass, suction pressure or decrease inlet temperature, decrease in k). Power up to 7.5 kW/m /s. Efficiency of large centrifugal compressors 76-78%. 

Let s say we have a centrifugal refrigeration compressor, driven by a motor. The motor is tripping off because of high amperage. Should we open or close the suction throttle valve shown in Fig. 22.2 Answer— close it. Of course, both the evaporator vapor outlet temperature and the process fluid outlet temperature will increase. But that is the price we pay for having too small a motor driver on the compressor. Does this mean that when our home air conditioner gets low on freon, our electric bill drops Correct. But the price we pay is a hot home. 

As we will discuss in Chap. 41, increasing the compression ratio is a small effect on compressor workload as compared to decreasing the vapor flow as long as we are operating on the flat part of the compressor s flow vs. the head performance curve, which is almost always the case for a centrifugal machine. Hence, suction throttling will significantly reduce the horsepower load on the compressor, as well as increase the evaporator temperature for a motor-driven compressor. 

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