Essentially the same basic types of mechanical equipment are used for handling gases and liquids, though the construction may be very different in two cases. Under the normal range of operating pressures, the density of a gas is considerably less than that of a liquid so that higher speeds of operation can be employed and lighter valves fitted to the delivery and suction lines. Because of the lower viscosity of a gas there is a greater tendency for leak to occur, and therefore gas compressors are designed with smaller clearances between the moving parts. Since a large proportion of the energy of compression appears as heat in the gas, there will normally be a considerable increase in temperature which may limit the operation of the compressor unless suitable cooling can be effected. For this reason, gas compression is often carried out in a number of stages and the gas is cooled between each stage.
Fans, Blowers, and Compressors
Machinery for compressing and moving gases is conveniently considered from the standpoint of pressure difference produced in the equipment. This order is fans, blowers, compressors.
The commonest method of moving gases under moderate pressures is by means of some type of fan. These are effective for pressures from 2 or 3 inch of water up to about 0.5 psi. Large fans are usually centrifugal, operating on exactly the same principle as centrifugal pumps. Their impeller blades, however, may be curved forward; this would lead to instability in a pump, but not in a fan. Since the change in density in a fan is small, the incompressible flow equations used in centrifugal pump calculations are often adequate.
The fans may be classified into three types: the propeller type, the plate fan, and the multi-blade type.
The propeller type is represented by the familiar electric fan and is of no great importance for moving gases in plant practice.
Plate fan consists of plate steel blades on radial arms inside a casing. These fans are satisfactory for pressures from 0 to 5 inch of water, have from 8 to 12 blades. Another variation of the steel-plate fan has blades curved like the vanes of centrifugal pump impellers and can be used for pressures up to 27 inch of water.
The multi-blade fans are useful for pressures of from 0 to 5 inch of water. It is claimed that they have much higher efficiencies than the steel-plate fan. These fans will deliver much larger volumes for a given size of drum than steel-plate fans.
Any pump of the rotary type can be used as a blower. When so used they generally have only two or three lobes on the rotating parts. These blowers are used for pressures from 0.5 to 10 psi. Such blowers are often used for services where very large volumes must be delivered against pressures too high for a fan. They are being replaced in many cases by centrifugal blowers.
The appearance of centrifugal blower resembles a centrifugal pump, except that the casing is narrower and larger impeller diameter. The operating speed is high, 3000 rpm or more. The reason for the high speed and large impeller diameter is that very high heads, measured in meters of low-density fluid, are needed to generate moderate pressure ratios.
Centrifugal compressors are multistage units containing a series of impellers on a single shaft, rotating at high speeds in a massive casing. These machines compress enormous volumes of air or process gas - up to 100 m3/sec at the inlet - to an outlet pressure of 20 atm. Smaller capacity machines discharge at pressures up to several hundred atmospheres. Interstage cooling is needed on the high pressure units.
Axial flow machines handle even larger volumes of gas, up to 300 m3/sec, but at a lower discharge pressures of 2 to 10 atm. In these units the rotor vanes propel the gas axially from one set of vanes directly to the next. Interstage cooling is normally not required.
Rotary positive displacement compressors can be used for discharge pressures to about 6 atm.
Most compressors operating at discharge pressures above 3 atm are reciprocating positive displacement machines. When the required compression ratio is greater than that can be achieved in one cylinder, multistage compressors are used. The maximum pressure ratio normally obtained in a single cylinder is 10 but values above 6 are unusual.
Last Modified on: 14-Sep-2014
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