Suction throttling, compressor

Suction throttle compressors rather than spillback recycle gas... 

Figure 4-17. Power demand for flooded screw compressors using slide valve and suction throttling [1]. (Reprinted by permission of the Councii of the institution of Mechanical Engineers from The Place of the Screw Compressor in Refrigeration )...

Probably the most widely used capacity control for the centrifugal compressor is speed control. The capacity curve when used with speed control covers a wide range. While electric variable speed motors offer a continuation to the speed control practice, there are some other alternatives available. Suction throttling has been widely used and offers a r sonable control range for a relatively low cost. 

The final element is a suction throttle valve that reduces the flow of air into the compressor. 

Classes II and III include all tests in which the specified gas and/or the specified operating conditions cannot be met. Class II and Class III basically differ only in method of analysis of data and computation of results. The Class II test may use perfect gas laws in the calculation, while Class III must use the more complex real gas equations. An example of a Class II test might be a suction throttled air compressor. An example of a Class III test might be a CO2 loop test of a hydrocarbon compressor. Table 10-4 shows code allowable departure from specified design parameters for Class II and Class III tests. 

Suction throttle valves are common in gas-lift service to minimi/c the action of the flare valve. Flow from gas-lift wells decreases with increased back-pressure. If there were no suction valve, the flare valve may have to be set at a low pressure to protect the compressor. With a suction valve it may be possible to set the flare valve at a much higher pressure slightly below the working pressure of the low-pressure separator. The difference between the suction valve set pressure and the flare valve set pressure provides a surge volume for gas and helps even the flow to the compressor. 

A speed controller does not ehminate the need for a recycle valve, flare valve, or suction throttling valve, but it will minimize their use. The recycle valve and suction throttling valve add arbitrary loads to the compressor and thus increase fuel usage. The flare valve leads to a direct waste of reservoir fluids and tlius loss of income. For this reason, engine speed control is rec-... 

As we will discuss in Chap. 28, increasing the compression ratio is a small effect on compressor workload, as compared to decreasing the vapor flow. Hence, suction throttling will significantly reduce the horsepower load on the compressor, as well as increase the evaporator temperature. 

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. 

If the molecular weight of the gas increases, the gas density will increase. The AP will increase. The pressure in the wet-gas drum will drop. The new suction throttle PC valve will start to close. This will restore the pressure in the wet-gas drum, without increasing the flow of gas through the spillback valve. But what happens to Px How does closing the suction throttle valve affect the actual compressor suction pressure ... 

The suction throttle valve, shown in Fig. 28.6, is analogous to the dam on the River Yeo. When we close this valve, the pressure in the wet-gas drum will increase. But the pressure at Pv the compressor suction pressure, is completely unaffected by the movement of the suction throttle valve. 

For example, let s say that the molecular weight of the gas increases by 20 percent. The AP, developed by the compressor, would then increase from 90 to 108 psi. If the absorber pressure is fixed, then the wet-gas drum pressure would drop by 18 psi. To prevent this excessive decrease in the drum pressure, we throttle on the suction PC control valve. The pressure drop through this valve would increase by 18 psi. But the pressure at P1 will drop by 18 psi, just as if we never moved the suction throttle valve. 

There are three basic ways to control throughput in a centifugal compressor spillback (bypassing), suction throttling, and variable speed. Figure 7.2 shows each of these structures and illustrates how they work in terms of their compressor curves. 

Figure 7.2 Compressor controls.

Problems still arise when the gas goes on to processing. It can be difficult, for example, to transfer hydrogen from a zone at low pressure to a compressor with suction throttling while maintaining good control of cell room header pressures. 

Suction throttling (centrifugal compressors), 460 Sulfur dioxide, 96 Sulfur fog (sulfur recovery), 116 Sulfur formation, 115 Sulfur hexafluoride, 104 Sulfur loss measurement (sulfur recovery), 114... 

Continue to pinch on the suction throttle valve until the compressor surge limit is approached, or until the spill-back is shut. 

There are a variety of computer software programs available to optimize the speed, suction throttle valve position, and spill-back valve position to minimize driver horsepower and keep a compressor safely av ay from its surge point while still controlling suction pressure. 

When 1 describe how to save compressor horsepower by suction throttling later in this chapter, you will understand the importance of the shape of the performance curve. The concept of the suction throttle valve, in fixed-speed centrifugal compressors, is the least understood aspect of all process equipment. 

Figure 41.6 is a partial answer to the problem of excessive power demand on the compressor s motor driver. We use the suction throttle valve (i.e., the "PC," control valve) shown in this figure. The use of the compressor suction throttle valve will partly offset the increase in the amp load on the compressor s motor driver, which results from... 

The main reason, however, that suction throttling reduces the amp load on the motor driver is due to the shape of the compressor performance curve (see Fig. 41.3). If we are operating on the flat portion of the curve, suction throttling will have the following effect ... 

Inlet guide vane position or suction throttle valve position will also affect compressor performance. Figure 7c shovii the effect of inlet guide vane position on compressor performance. The efiect of pinch suction throttle valve is similar to reduce compressor s speed. It will reduce compressor s head generation. 

The means available for controlling compressor capacity are suction throttling, discharge throttling, recirculation, variable guide vanes and motor speed control. Most of these controls are used in practice, and which is best depends on the application. Some of the pros and cons of these alternative control methods are as follows ... 

Suction throttling. The spinning vanes of a centrifugal compressor sling the gas outward. The centrifugal force develops a pressure proportional to the density and to the speed squared. Suction throttling reduces the density, and hence the AP thus, the machine operates as a constant-pressure ratio machine. 

---