Cylinder sizing, compressors

Figure 10-5 shows a very large integral compressor. Tbi.s would he typical of compressors in the 2.000 hp to 13,000 hp size. The size of this unit can be estimated by the height of the handrails above the compressor cylinder on the walkway that provides access to the power cylinders. This particular unit has sixteen power cylinders (eight on each side) and four compressor cylinders. 

Each frame is designed for a maximum number of cylinders. The frame itself does not indicate the number of stages or the duty of the compressor. An individual frame can be used for many different sizes o compressor cylinders and for a wide range of applications. Frames are typically classified as separable (balanced-opposed) or integral-type, as shown m Figure 11-3. 

Typically, in specifying a unit, the suction and discharge pressures, capacity (MMsefd), inlet temperature, and gas properties are given. The actual sizing of the cylinders is left to the manufacturer from his specific combinations of standard cylinders, pistons, and liners. However, once a proposal is received from a manufacturer, sometimes it is beneficial to check the cylinder sizing and make sure that indeed the compressor will perform. Sometimes it is necessary to size a new cylinder for an existing compressor or to verify that an existing compressor will perform in a different service. 

The amount of heat rejected hy compressor jackets varies with the size and type of machine. This heat rejection is usually given as Btu/hr/hhp. Heat rejection to the compressor cylinder will average about 500 Btu/hr/hhp. Some go as low as 130 though, and it is necessary to check with the manufacturer to obtain an accurate figure. 

This arrangement is based on compressor cylinders operating in parallel and being of the same size and characteristics. 

Carbon monoxide. Carbon monoxide is a colorless, odorless gas, liquefying at -191.5°. It is readily available, at pressures of up to about 100 bar, in a variety of cylinder sizes, so that experiments can be run at 80 bar without a compressor. The acute toxicity of carbon monoxide reflects its high affinity for hemoglobin. It is stated" that 400-500 ppm of carbon monoxide in air can be breathed for an hour without appreciable effect, while levels above 1000 ppm are dangerous, and at more than 4000 ppm it causes death within an hour. The previously cited reference gives a Threshold Limit Value of 50 ppm. Carbon monoxide must always be used in an efficient fume cupboard, keeping emissions into the laboratory atmo-... 

Compressors up to around 75 kW (100 hp) usually have a single center-throw crank, as illustrated in Fig. 10-83. In larger sizes compressors are commonly of duplex construction with cranks on each end of the shaft (see Fig. 10-87). Some large synchronous motor-driven units are of four-corner construction i.e., they are of doubleduplex construction with two connecting rods from each of the two crank throws (see Fig. 10-88). Steam-driven compressors have one or more steam cylinders connected directly by piston rod or tie rods to the gas-cyhnder piston or crosshead. 

Another feature necessary to the reciprocating compressor is cylinder cooling. Most process compressors are furnished with water jackets as an integral part of the cylinder. Alternatively, particularly in the smaller size compressors, the cylinder can be designed for air cooling. 

Most of the drivers used with the sliding vane compressor are electric motors. Variable speed operation is possible within the limits of vane speed requirements. The vanes must travel fast enough to seal against the cylinder wall but not so fast that they cause excessive wear. For the smaller units, under 100 hp, V-belts are widely used. Direct connection to a motor, however, is possible for most compressors and is used through out the size range. 

The pulsations can cause the use of excess horsepower when compared to the ideal or a system design that reduces pulsations and thereby improves cylinder performance and efficiency. The pulsation shaking forces in the suction and discharge dampeners (bottles) can be evaluated by computer analysis, and the magnitude and frequency in hertz can be reduced to an acceptable level by adjusting the dimensions (size) of the dampeners. The magnitude of the internal forces directly affects the mechanical stress on the nozzles of the cylinder and of the dampeners. Compressor... 

Use either a 40-in. l.D. header for these three compressors (two running or equivalent with three) or bring in two parallel headers, cross tied, of about half the flow area each, which equals (5,210) (3.408)/ (2,000) (2) or 4.53 fti. This corresponds to a pipe diameter of 2 ft, 5 in., say 2 ft, 6 in. This is still a large pipe, and space arrangements may dictate which is preferred. If this header is coming to the compressors from a great distance, the pressure drop must be checked to be certain that the system drop will ensure specified pressure at the suction of the cylinders. In some cases, the header size can be made slightly smaller if the pressure... 

Use 22-in. or 24-in. pipe, or check size of suction opening and if reasonably close (in building, close to compressor), within about 2 in., use the same pipe size as cylinder suction. Do not use this approach if the suction drum is removed from the compressor and the suction connection is smaller than the size calculated. Use the larger pipe size. 

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

Positive-Displacement Compressors Reciprocating compressors are built in different sizes (up to about one megawatt refrigeration capacity per unit). Modern compressors are high-speed, mostly direct-coupled, single-acting, from one to mostly eight, and occasionally up to sixteen cylinders. 

Actual Values of Efficiencies.—Tests of piston compressors show extreme variations of mechanical efficiency from 76 to 97 per cent, with approximate averages for the more common sizes of 85 per cent. The true volumetric efficiency of piston air compressors will vary from 80 to 97 per cent, and the cylinder efficiency for water-jacketed compressors from 80 to 85 per cent. This will result in efficiencies of compression varying from 64 to 82 per cent. 

It is usual to allow % cu. ft. of displacement in the vacuum apparatus to every square foot of filtering area. To obtain the displacement of a compressor subtract one-half the net cross-section of the piston rod from the area of the cylinder, convert the latter into square feet and multiply by twice the stroke expressed in feet or fractions of a foot and the number of revolutions per minute. The Tree air capacity of compressors at sea level as given by the manufacturers is the displacement and can be used as a practical factor in determining the proper size compressor to use for vacuum purposes. 

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