Compressors generalized

Mechanical Expanders Reciprocating expanders are very similar in concept and design to reciprocating compressors. Generally these units are used with inlet pressures of 4 to 20 MPa. These machines operate at speeds up to 500 rpm. The thermal efficiencies (actual enthalpy difference/maximum possible enthalpy difference) range from about 75 percent for small units to 85 percent for large machines. 

The input, or male, rotor in screw compressors generally rotates at a no-load speed of either 1800 or 3600 rpm. The female, or driven, rotor operates at higher no-load... 

Air conditioners are frequently combined with heaters. A convenient place must be found for the compressor, generally outdoors. Heat pumps have been used a great deal in recent years with satisfactory results. In very dry climates, laboratories often install evaporative coolers because of their reasonable cost of installation and operation. Their one drawback, sometimes serious, is that the inside air tends to become uncomfortably humid. 

Figure 8 gives the results of these calculations. The lower set of curves shows that the investment for the heat exchanger and compressor generally decreases as the power requirement of the system increases. The three separate curves represent three different heat exchanger AkT s. This is tvpical of the conventional "trade off" expectation, but a 4°F At design has a 10% lower power requirement than either of the other choices at the same investment level. 

The reduction in steam is not without cost. We still have to pay the utilities (electricity or steam turbine) to operate the compressor. Generally, compressors are expensive to buy, maintain, and operate so you have to do some economic calculations to see if using a compressor is economical. At the usual conditions of an evaporator, the vapor can be treated as an ideal gas. Then the work required for adiabatic compression can be estimated from ... 

Here we have simplified the behavior of real compressors by assuming that when the p iston reaches the top of its travel, there is no volume left between the pis ton and the top of the cylinder. This is a zero-clearance compressor. Later we examine the consequences of the fact that real compressors generally leave a little gas in the cylinder at the top of the stroke those consequences are minor. 

Screw compressors require very little maintenance because the rotors turn at conservative speeds and are well lubricated with coolant oil. Screw compressors generally run cool, minimizing the need for both oil changes and intercoolers. Another result of injecting so much oil into the compression system is that the process is considered to be isothermal. An exploded view of a screw compressor is shown in Fig. 5.31. A capacity-control valve provides a capacity variation of between 10 and 100%. This is accomplished by allowing some of the gas in the rotors to escape through a valve located in the stator. 

Reciprocating expanders are very similar in concept and design to reciprocating compressors. Generally, these units are used with inlet pressures... 

Gas turbine driven centrifugal compressors are very efficient under the right operating conditions but require careful selection and demand higher levels of maintenance than reciprocating compressors. Compression facilities are generally the most expensive item in an upstream gas process facility. 

This reduction in permeabiUty due to cake consoHdation or coUapse may be so large that it may nullify or even overtake the advantage of using high pressures in the first place and there is then no reason for using the generally more expensive pressure filtration hardware. While a simple Hquid pump may be cheaper than the vacuum pump needed with vacuum filters, if air displacement dewatering is to foUow filtration in pressure filters, an air compressor has to be used and is expensive. 

Phosphate ester fluids are the most fire resistant of moderately priced lubricants, are generally excellent lubricants, and are thermally and oxidatively stable up to 135°C (38). Fire-resistant iadustrial hydrauHc fluids represent the largest volume commercial use. AppHcations are made ia air compressors and continue to grow for aircraft use (tributyl and/or an alkyl diaryl ester) and ia hydrauHc control of steam turbiaes ia power generation (ISO 46 esters). 

Domestic refrigeration in 1990 had a worldwide market of 56 x 10 units annually (7). The vast majority of these used CEC-12 as their working fluid, and in general these systems would operate satisfactorily for 20 years or more. Over this period of time, an average compressor might operate 60 90 x 10 h without failure. 

The steam balance in the plant shown in Figure 2 enables all pumps and blowers to be turbine-driven by high pressure steam from the boiler. The low pressure exhaust system is used in the reboiler of the recovery system and the condensate returns to the boiler. Although there is generally some excess power capacity in the high pressure steam for driving other equipment, eg, compressors in the carbon dioxide Hquefaction plant, all the steam produced by the boiler is condensed in the recovery system. This provides a weU-balanced plant ia which few external utiUties are required and combustion conditions can be controlled to maintain efficient operation. 

Since most compressors operate along a polytropic path approaching the adiabatic, compressor calculations are generally based on the adiabatic cui ve. 

To be able to decide which type of compressor would best fit the job, we should first divide the compressors into three main categories positive displacement, centrifugaf and axial flow In general terms, positive displacement compressors are used for high pressure and low flow characteristics centrifugal compressors are used for medium to high pressure dehvery and medium flow and axial flow compressors are low pressure and high flow... 

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