Head and power requirement

The values of compressibility factor and isentropic exponent (k or heat capacity latio) arc usually decreased as gas is compressed. It is usually use average compressibility factor to caJcuIaie compressor head and power. Compressor head calculation is not sensitive to isentropic or polytropic exponent. Therefore, isentropic or polytropic exponent at compressor inlet or its average ean be used for compressor head and power calculation. 

Isentropic expone nt of heavy gas such propone, butane, propylene is about 1-14, For air, it is about 1.4, For the same compression ratio, power required to compress air with high isentropic exponent is about 5-10% higher than compress heavy gas with low isentropic exponent. This is true for all type of compressors 

For low molecular weight gas (MW less than 10) and high head requirement, coitrifuga] compressor may not be a good candidate for the compression service, and reciprocating or notary compressor may be required. 

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Figure 7.8. Topical capacity-head ranges of some centrifugal pumps, their 1978 costs and power requirements. Suction and discharge are in inches (Evans, 1979, Vol. 1).

Determine the required correction factors. A centrifugal pump handling a viscous liquid usually must develop a greater capacity and head, and it requires a larger power input than the same pump... 

These ideas of impeller flow, head and power input as related to operating variables have some merit for a qualitative description of the effects of the operating variables on the process. However, it requires extensive experience, and usually actual experiments, to decide whether a system performance is favored by a particular combination of flow and head. (Rushton and Oldshue (R12) note that high values of Q/3Care preferred for blending and solid suspension, low ratios for liquid-liquid and gas-liquid operations.) This approach still requires the systematic study of impeller speed and diameter as process variables. 

Most PD compressors are connected to constant-speed motors, because variable-speed motors are much more expensive. This poses a problem in controlling the compressor flow rate. One way to control a compressor with a constant-speed motor is to vary the clearance volume by means of clearance pockets, which are connected to or disconnected from the head of the compressor by remotely operated valves. For a 100 percent effleient, adiabatic compressor, what is the effect of such a pocket on the pressure ratio, flow rate, and power requirement ... 

It is possible to improve the mixing performance of single-screw machines by the use of additional mixing heads or by the use of in-line static mixers described earlier. Several such devices are available commercially and mixing will be improved in comparison with the single screw alone. However, additional capital cost and power requirement will be incurred. Figure 7.18... 

The purchaser of the compressor has to clearly mark the point on the datasheet on which he wants the compressor vendor to guarantee the performance. Capacity, head, speed and power required should be discussed with vendor as the rated point can be different from normal operating point. 

Manually adjusted screw or ratchet take-ups that adjust the position of the tail pulley to control belt tension can be used on relatively short, light duty conveyors. Automatic take-ups are used on conveyors over about 25 to 30 m long. The most common is the weighted automatic gravity take-up (see Fig. la). Other types of automatic take-ups have hydrauHc or pneumatic powered devices to adjust a snub pulley position and maintain a constant belt tension. The requited take-up movement varies according to the characteristics of the belt constmction and the belt length. Typically, take-up movements for pHed belts are 2% to 3% of the center distance between head and tail pulley, and about 0.5% for steel cable belts. The take-up movements requited for soHd woven belts are usually shorter because of the lower elastic stretch. Take-up requirements for a particular situation should be confirmed by the belt manufacturer. 

Viscosity (See Sec. 5 for further information.) In flowing liquids the existence of internal friction or the internal resistance to relative motion of the fluid particles must be considered. This resistance is caUed viscosity. The viscosity of liquids usuaUv decreases with rising temperature. Viscous liquids tend to increase tlie power required by a pump, to reduce pump efficiency, head, and capacity, and to increase Friction in pipe lines. 

Reducing cylinder wall and cylinder head temperatures removes more heat from the gas during compression, lowering its final temperature and reducing the power required. 

Taking into account the pump efficiency rj, the overall power requirement is obtained from the product of the mass flowrate G, the total head h and the acceleration due to... 

A pump, built for these operating conditions, has a measured maximum overall efficiency of 70%. The same pump is now required to deliver water at 30 m head. At what speed should the pump be driven if it is to operate at maximum efficiency What will be the new rate of delivery and the power required ... 

Most pump manufacturers provide composite curves, such as those shown in Fig. 8-3, that show the operating range of various pumps. For each pump that provides the required flow rate and head, the individual pump characteristics (such as those shown in Fig. 8-2 and Appendix H) are then consulted. The intersection of the system curve with the pump characteristic curve for a given impeller determines the pump operating point. The impeller diameter is selected that will produce the required head (or greater at the specified flow rate). This is repeated for all possible pump, impeller, and speed combinations to determine the combination that results in the highest efficiency (i.e., least power requirement). Note that if the operating point (Hp, Q) does not fall exactly on one of the (impeller) curves, then the... 

The vendor shall take test data, including head, capacity, power, and vibration, at a minimum of five points. These points will normally be (a) shutoff (no vibration data required), (b) minimum continuous stable flow, (c) midway between minimum and rated flow, (d) rated flow, and (e) maximum allowable flow (as a minimum, 120 percent of BEP). 

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