Centrifugal compressors operating curve

Centrifugal compressors operate on a curve, just like centrifugal pumps. A typical curve is shown in Fig. 41.3. The horizontal axis is actual cubic feet per minute (ACFM). This is analogous to GPM, used on the horizontal (x) axis of centrifugal pump curves. The vertical axis is Hj, (polytropic feet of head). This is analogous to the feet of head used on the vertical (y) axis of the centrifugal pump curve. 

Caution should be taken to prevent excessive acceleration. One solution is to provide for opening of the compressor discharge vent valve to increase the compressor flow and increase blower horsepower. This is effective with a centrifugal compressor, however, not with an axial compressor unless the compressor is provided with adjustable stator vanes that are reliable. This can be seen from the performance curve. The head versus flow curve for a given vane setting is extremely steep and opening the vent valve is ineffective. However, if the vanes operate fully open and the vent valve opens, the combined effect is satisfactory. The vent valve must not be oversized. 

The Operating System. Regardless of calculated centrifugal compressor performance, the machine will operate only on or along its operating curve to fit the system of which it is a part. This is quite similar to the system performance of a centrifugal pump. Friction, other pressure drops of the system, and how friction varies with operating conditions determine machine performance. 

On the other hand, suppose we are working with a centrifugal compressor. This particular machine has a very flat operating curve. This means that a small decrease in discharge pressure will result in a large increase in vapor flow. Therefore, the load on this compressor s driver will increase. 

The centrifugal compressor, unless it is dirty or mechanically defective, has to operate on its curve. As the compressor discharge pressure increases, then Hp, the feet of polytropic head required, must also increase. Also, as can be seen from the compressor curve, the volume of gas compressed (ACFM) must decrease. When the volume of gas drops below a critical flow, the compressor will be backed up to its surge point. 

The operating point is located where the centrifugal compressor curves cross the system curve of the process. The system curve can be a constant pressure one (horizontal line), a mostly friction one, or any other. 

Centrifugal compressors have head-capacity curves similar to those of the more familiar centrifiigal-pump curves. Within the lower limit necessary to avoid surge and the upper limit of impeller capacity, each unit can in theory operate at any point on its curve. 

Centrifugal compressors are dynamic compressors and are essentially constant pressure, variable volume machines and must be regulated in almost all applications. Before control can be discussed, the operating characteristics must be understood. The discussion to follow will be concerned with radial-flow compressors. Axial-flow compressors have similar characteristics curves, although the change in pressure rise is much greater with equivalent changes in compressor rotational speed. 

This means that an operating change that forces the compressor back up its curve will reduce the driver horsepower. Later in this chapter, you will see how critical this bit of information is. Most constant speed centrifugal compressors are designed to operate on the flat portion of their curves. 

Where the driven machine is a centrifugal type i.e. pump or compressor, the shaft power may be taken as that which occurs at the end of curve operating point. This rule-of-thumb point is defined as being 125% of the power required at the maximum operating efficiency point on the designed curve of pressure (head) versus fluid flow rate, at the rated shaft speed. 

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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