Compressor types

Fig. 5. Compressor types used in refrigerator/free2ers (a) typical arrangement (b) reciprocating piston mechanisms connecting rod and Scotch yoke and...

In a V/f control generally, only the frequency is varied to obtain the required speed control. Based on this frequency, the switching logistics of the inverter control circuit control the inverter s output voltage using the PWM technique to maintain the same ratio of V/f. A W/control is, however, not suitable at lower speeds. Their application is limited to fan, pump and compressor-type loads only, where speed regulation need not be accurate, and their low-spccd performance or transient response is not critical and they are also not required to operate at very low speeds. They arc primarily used for soft starts and to conserve energy... 

Figure 1-2. Typical application ranges of compressor types.

Standard materials for the compressor are cast iron for the cylinder and carbon steel for the shaft. The rotor parts are steel. The liquid pistf f compressor has another feature that compensates for low efficiency, using special materials of construction and compatible liquid compr sant, unusual or difficult gases may be compressed. By using titanium internal materials and water as a compressant, gases containing wet chlorine can be compressed. This is a very difficult application for most o( the other compressor types. 

Figure 7-4. Speed-torque curve for two compressor types. Each type of compressor has its own curve [7].

API again offers some help. API 671 covers the special purpose coupling as either a specification or guideline. It has been common practice for the compressor vendor to furnish the couplings. Most compressor vendors have no problem accepting this responsibility, while driver vendors sometimes prefer not to furnish them. In some compressor types, the coupling style will be somewhat influenced by the user. When torsionals become a major consideration, the vendor may have to make the choice based on system need. In all cases, the user should establish his right of approval if the vendor performs the selection. 

Either soleplates or baseplates should be furnished for most of the compressor types. Reciprocating compressors are sometimes directly... 

Check valves are required in the piping system at any point where backflow of gas after a shutdown has the ability to restart the compressor, running it backwards or, for that matter, even in the normal direction. Reverse rotation is totally bad, as many components of the various compressor types are not designed for reverse rotation, and there is some possibility, generally remote, that the compressor could reach a destructive over speed. Forward rotation is bad primarily because the intent was to stop the compressor, and it is now operating out of control. This is a problem, particularly if the shutdown was caused by a compressor failure indication, and the need to stop was to prevent further damage. In this mode, it is unlikely that the compressor can attain an overspeed condition. An application with a high potential for backflow is the parallel operation of two or more compressors. 

All compressors have an operating envelope. The size is dependent on the compressor type and the application. If the appropriate design considerations were implemented, then operation within the envelope should be problem-free. The first problem stems from a communication gap. Engi... 

The first step in the dimensioning process is to choose the mixture ratio jx. For the compressor type, which is not considered here, we choose fx = 3.0, i.e., fii, = 9.1 kg/s. Next we have to decide D, the diameter of the tube, i.e., we need a criterion for the choice of velocity of air. A crucial aspect which affects the choice of velocity is the wearing of the tube the higher the velocity is the greater is the wear. On the other hand, the velocity cannot be too low because of the risk of pluming (the velocity difference v - c has to be positive in all parts of the piping). Here we choose = 50 m/s with =1.2 kg/m. This gives D = 0.44 m. 

Once the required horsepower and number of stages are estimated, a choice of compressor type can be made from the considerations included earlier. Some example selections are included in Table 10-1, The selections listed in this table are meant as common types that would normally be specified for the given conditions. It must be emphasized that the.se are not recommendations that should be accepted without consideration of the advantages and disadvantages listed earlier. In addition, local foundation conditions, type of drivers available, cost of fuel, availability of spaie... 

Figure 12-ID. Basic compressor types. (Used by permission Coker, A. K. Hydrocarbon Processing, V. 73, No. 7, p. 39, 1994. Gulf Publishing Co., Houston, Texas. All rights reserved.)...

Compressor Type Press., psia Stage or Machine... 

Figure 12-95. Axial compressor Type AV100-16, during erection. Note stationary and rotating blades. Two identical steam turbine-driven machines supply air to blast furnace at steel works. Suction volume = 560,000 NmVh discharge pressure = 6.2 bar power input = 52,000 kW each. (Used by permission Bui. 26.13.10.40-Bhj. Sulzer Turbo Ltd.)...

Each of the two general classes of compressors and their subclass types have certain advantages and disadvantages regarding their respective volumetric flow capabilities and the pressure ratios they can obtain. Figure 3-69 shows the typical application range in volumetric flowrate (actual cfm) and pressure ratio several of the most important compressor types can obtain [23]. 

Figure 3-70. General performance curve for various compressor types [23].

Figure 35.3 Compressor types - approximate capacity and pressure limitations...

All compressor types have moving parts, high noise levels, high pressures, and high-temperature cylinder and discharge-piping surfaces. 

Determining if two or more compressors should have separate or single foundations depends on the compressor type. A combined foundation is recommended for reciprocating units since the forces from one unit usually will partially balance out the forces from the others. In addition, the greater mass and surface area in contact with the ground damps foundation movement and provides greater stability. 

A compressor must operate within a system that is designed to acquire and compress a gas. These systems must include the following components regardless of compressor type ... 

The lubrication system has two basic functions to lubricate the compressor s moving components and to cool the system by removing heat from the compressor s moving parts. While all compressors must have a lubrication system, the actual design and function of these systems will vary depending on compressor type. 

A large variety of oils is available, and recommendations for any set of conditions, compressor type and refrigerant can be obtained from the refiners. They are naphthene or paraffin-based oils. Synthetic lubricants have been developed for ultra-low-and high-temperature systems, especially for process heat pumps. 

The choice of compressor type is now a wide one, and at least two alternatives should be considered before making a final selection. 

Optimal compressor- types for the various power ranges are plotted in Fig. 4.1-29, which was calculated on the bases of 1 bar intake pressure and an isothermal efficiency of 64% [24]. It can give only approximate reference points for the most favourable area of application, which varies depending on the manufacturer. If the intake pressure is greater than 1 bar, as is the rule, it is necessary to recalculate. For example the compression of ethylene at a flow-rate of 64000 kg/h 51 130 Nm3/h (Standard cubic metre per hour) from 231 to 2151 bar, corresponds to a real intake volume flow-rate at 231 bar of 161 m3/h, and a power consumption of 8200 kW. 

Fig. 4.1-29. Power range of various compressor types for high-pressure [24], D, Diaphragm compressor PI, Piston compressor, dry running (piston ring, labyrinth) P2, Piston compressor, lubricated (piston ring) P3, Ultra-high-pressure compressor, lubricated (plunger piston) TR, Radial turbo-compressor TA, Axial turbo-compressor S, Screw compressor.

The different volumetric capacities and the cooling efficiencies of both compressor types are compared in Figure 2.27. 

Fig. 2.27. Cooling capacity of both compres- capacity for both compressor types at different sors operating with R404A at a liquefying tern- evaporating temperatures, perature of 30 °C. The curves show the cooling...

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