Motor-driven compressor

Eig. 7. Schematic flow diagram of a basic horizontal-tube vapor compression (VC) desalination plant, shown (a) with a mechanical, motor-driven compressor and (b) with a thermocompressor, using an ejector, where (------) represents vapor (—), brine and (-), product. 

Motor-driven compressors usually operate at constant speed, and other methods of controlhng the capacity are necessary. On reciprocating compressors discharging into receivers, up to about 75 kW (100 np), two types of control are usually available. These are auto-matic-start-and-stop control and constant-speed control. 

Rgure 3-26. Mantfbld-fype volume txnttles are used where cylinders are operated in parallel, as on this two-stage, motor driven compressor. (Courtesy of Dresser-fland)... 

A motor-driven compressor is used as an oxidation system for a chemical unit. The control scheme for this arrangement is shown in Figure 8-40(c). Description of the control blocks follows. 

Fl = frame loss for motor-driven compressors only, values range 1.0-1.05 (note, this is not a driver efficiency factor). 

Usually a constant speed, motor-driven compressor will be equipped with inlet guide vanes to the first wheel to allow suction volume control. If the guide vanes are not used, some other throtding device should be available. Figure 12-62 presents the constant speed performance of a centrifugal machine with inlet guide vanes. 

Size the discharge surge drum and associated piping for a single-acting, single-cylinder, motor-driven compressor with the following performance characteristics ... 

The motor-driven compressor boosts the pressure of the refrigerant vapors from the evaporator pressure, up to the condenser pressure. 

Wouldn t it be more simple to just slow down the compressor as the molecular weight increases After all, it seems as if we need less feet of head when the gas density increases. As we slow the compressor by 10 percent, the feet of polytropic head would drop by 20 percent. Fine. But 99 percent of motor-driven compressors are fixed-speed machines. 

A reciprocating motor-driven compressor rated for 2000 brake hp 1,120,000... 

Utility costs per barrel of feed (assuming an electric-motor-driven compressor) are ... 

For motor-driven compressors, the control can become more intricate, especially for the usual constant-speed motors. Motor-driven compressors can be controlled as follows ... 

FIGURE 25—2 A motor-driven compressor with antisurge controls... 

For a motor-driven compressor, the maximum amperage rating on the motor defines when the compressor is work- or horsepower-limited. Check the amp load in the motor "breaker or "switch" room. 

Many refrigeration compressors are horsepower-limited. This means the driver cannot put out enough work to drive the compressor as fast as desired. A motor-driven compressor is horsepower-limited when it trips off, because it is drawing too much current. 

Let s assume w e have a constant-speed, motor-driven compressor. The compressor is starting to surge, or stall, or slip, or become unstable. The... 

Refrigerant motor-driven compressor Refrigerant condenser Refrigerant letdown valve Evaporator Refrigerant receiver... 

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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