Suction throttling

There are two basic methods for controlling vacuum pumps suction throttling and using a load gas. Mechanical pumps may also use variable speed drive single stage jets may also use a regulating spindle. 

The ideal load gas is air, but some systems cannot tolerate air. The discharge gases from a steam jet will be wet and some systems cannot tolerate moisture. Discharge gases from a mechanical pump may contain oil or water, either of which may not be tolerated by the system. If the process requires, dry nitrogen or other suitable inert gas can be used but these may represent a significant operating expense. 

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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. 

A process pressure increase over a set value would cause a signal to reach the suction throttle valve and would close the valve in order to reduce the inlet pressure. 

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. 

Suction throttling is usually done with a butterfly control valve, which has a very low pressure drop when it is wide open. 

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. 

Figure 22.2 Suction throttling to control evaporator temperature.

Figure 28.6 is a partial answer to Jane s predicament. It is a suction throttle valve. Use of the suction throttle valve will partly reduce the... 

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. 

Controlling the pressure in the drum by suction throttling allows the compression ratio P2/P1 to increase in response to an increase in the molecular weight of the gas. As the compression ratio increases, according to Eq. (28.2), the amperage on the motor driver also increases. But not by a lot. 

If the molecular weight rises by 20 percent and we control the drum pressure by suction throttle control, the motor amps will increase by about 10 percent. 

Both the suction throttle pressure control (PC) valve and the spillback [flow-control (FC)] valve are in a nice operating position. All we wish to do is save electricity. 

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. 

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