Electric motors compressor drives

A relatively new innovation for use in electric motor compressor drives is the variable frequency power source. Fundamentally, the power source converts an existing three-phase source into DC then uses an inverter to convert back to a variable frequency supply. Thyristors or transistors are used to switch the output at the required frequency. 

For most of the rotary compressors in process service, the driver is an electric motor. Compressors in portable service, however, particularly the helical-lobe compressor, use internal combustion engines. Many of the rotary compressors require the high speed that can be obtained from a direct-connected motor. The dry type helical-lobe compressor is probably the main exception as the smaller units operate above motor speed and require a speed increasing gear which may be either internal or external (see Figure 4-1). The helical-lobe compressor is the most likely candidate for a driver other than the electric motor. Aside from the portables already mentioned, engines are used extensively as drivers for rotaries located in the field in gas-gathering service. Steam turbines, while not common, probably comprise most of process service alternate drive applications. 

In a gas engine-driven chiller, the engine produces mechanical shaft power that is used for operating a chiller compressor. Such chillers are very similar to conventional electiic-diiven chillers. The only difference is that the electric motor that drives the compressor in an electric chiller is replaced with a reciprocating engine. 

The direct current (DC) output of a fuel cell stack will rarely be suitable for direct connection to an electrical load, and so some kind of power conditioning is nearly always needed. This may be as simple as a voltage regulator, or a DCIDC converter. In CHP systems, a DC to AC inverter is needed, which is a significant part of the cost of the whole system. Electric motors, which drive the pumps, blowers, and compressors mentioned above, will nearly always be a vital part of a fuel cell system. Frequently also, the electrical power generated will be destined for an electric motor - for example, in motor vehicles. 

Using other criteria, compressors are classified as open, semiher-metic accessible), or hermetic. Open type is characterized by shaft extension out of compressor where it is coupled to the driving motor. When the electric motor is in the same housing with the compressor mechanism, it could be either hermetic or accessible (semihermetic). Hermetic compressors have welded enclosures, not designed to be repaired, and are generally manufac tured for smaller capacities (sel-... 

For 3,600-rpm compressor drives below 5,000 hp, simplicity of installation almost dictates using the two-pole induction motor. No gear e. required, and the overall electrical and mechanical installation is the simplest possible. 

Figure 12-5F. Lubricated and nonlubricated balanced opposed process reciprocating compressors, designed to API 618 code. Fixed- and variable-speed drives using gas or diesel engines, steam or gas turbines, or electric motor. Note power drive to connect to right side of cross-head box in center. (Used by permission Bui. PROM 635/115/95-11. Nuovo Pignone S. P. A., Florence, Italy New York Los Angeles and Houston, Texas. All rights reserved.)...

The centrifugal compressor is well established for the compression of gases and vapors. It has proven its economy and uniqueness in many applications, particularly in which large volumes are handled at medium pressures. This compressor is particularly adaptable to steam turbine or other continuous speed change drives, as the two principles of operation and control are quite compatible. It is also adaptable to the electric motor, gas engine, and gas turbine with each installation being specific to a particular problem or process. Installation as well as operating costs can be quite reasonable. 

Electric motors are the most common drivers for the m ority of pumps, compressors, agitators, and similar equipment in the process industries. Process engineers should obtain the assistance of a qualified electrical engineer before completing motor specifications ior the wide variety of equipment applications and respective power sources. The use of standard specifications for the various types and classes of motors is helpful and reduces repetitious details. Be certain that the type of motor is properly matched to the service, atmosphere, load characteristics, and available type and power factor of the electrical energy to drive the motor. Some basic guides are summarized, but they cannot be used as all-inclusive rules to fit all plant or equipment condi-... 

Compressors having external drive require a gland or seal where the shaft passes out of the crankcase, and are termed open compressors. They may be belt driven or directly coupled to the shaft of the electric motor or other prime mover. 

A steam turbine is a machine with an ancient genealogy. It is a direct descendant of the overshot water wheel, used to kick off the industrial revolution in England, and the windmill still used in Portugal. Turbines are widely used in process plants to drive everything from 2-hp pumps to 20,000-hp centrifugal compressors. They are versatile machines, in that they are intrinsically variable-speed devices. Electric motors are intrinsically fixed-speed machines. It is true that there are a variety of ways to convert AC (alternating-current) motors to variable speed, but they are all expensive and complex. 

A two-stage double-acting compressor with water cooled cylinder jackets and intercooler is shown in Figure 7.18(c). Selected dimensional and performance data are in Table 7.7. Drives may be with steam cylinders, turbines, gas engines or electrical motors. A specification form is included in Appendix B. Efficiency data are discussed in Section 7.6, Theory and Calculations of Gas Compression Temperature Rise, Compression Ratio, Volumetric Efficiency. 

The three main compressor types are rotary, reciprocating, and centrifugal. The compressor drives can be constant or variable speed, and can be driven by electric motors, steam turbines, gas turbines, gasoline, or diesel engine drives. For a particular application, the type of compressor is selected by considering the required capacity and discharge pressure. 

COi Compressor Drive E = electric motor, ST = steam turbine e Air is needed to provide oxygen for passivation of stainless steel equipment. 

This problem was taken from Reference 5.56. Assume that the electric motor efficiency is 94%. Calculate the power required for an electric motor drive for a compressor to compress a process gas containing propane, butane and methane from 5 °C (41°F) and from 1.4 to 7.0 bar (20.3 to 101 psia). The composition of the gas in mole percent is CsHg = 89.0, n-C4Hio = 6.0, and C2H6 = 5.0. The flow rate is 1090 kgmoEh (2403 Ibmol/h). 

Most chemical-plant-size compressors are electrically driven [43]. Moore [25] discusses the characteristics of squirrel-cage induction and synchronous electrical motors. Wound rotor induction motors have not been used for compressor drives. For 370 to 4500 kW (500 to 6,000 hp), the induction motors are the first choice. The squirrel-cage induction motor is the most coimnonly used driver in the process industries from 1/8 to 1,5000 hp (0.0932 to 1,120 kW [25]. From 15,000 hp (149 to 11,200 kw) the synchronous motor could be used [25]. If the compressor is operated at 7,500, 11,000, and 23,000 rad/s (1,200, 1,800 and 3,600 rpm), no step-up gears are required. The least costly speed for an induction motor is 1,000 rad/s (1800 rpm) so that this speed is usually selected. Step-up gears are used to obtain higher speeds. 

Compressor drives. Compressors and blowers in fluid cracking units may be either steam or electrically driven (209). Steam turbines were preferred over electric-motor drives in the design of early units, partly because of fear of a power failure. However, under some circumstances, the economics favor the use of electric motors. Several units have been built with electrically driven main air blowers, and at least one refiner has selected electric motors for the air blower, gas compressor,... 

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