Reciprocating positive displacement machines

If the chamber on the discharge side was opened, the gas would stream back into the conveying chamber and would all at once compress the gas at discharge pressure level. This would frequently involve loud noise but also certain harmful thermal effects. 

Reciprocating positive displacement machines exist in versions such as piston pumps, membrane pumps, piston compressors, and membrane compressors. These machines achieve the highest pressures and the pumps are additionally able to reach a high conveying precision. The main reason for these features is the leakproof discharge chambers (operating areas of the positive displacement machines). 

Especially for high-pressure technology, however, certain additional constructional aspects have to be observed. Stroke adjustable as well as only speed adjustable pump versions are available. 

Drive Technology for Reciprocating Positive Displacement Machines 

Nearly every type of drive unit can be equipped with or without stroke adjustment. 

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Figure 13.9 Drive technologies for reciprocating positive displacement machines (a) crankshaft with stroke adjustment with the eccentric radius s(r) (b) spring-cam drive with stroke adjustment (A) (c) linear drive s(A) ...

Many users consider rotaiy compressors, such as the Rootes -type blower, as turbomachines because their behavior in terms of the rotor dynamics is very close to centrifugal and axial flow machineiy. Unhke the reciprocating machines, the rotary machines do not have a veiy high vibration problem but, like the reciprocating machines, they are positive displacement machines. 

A reciprocating compressor is a positive-displacement machine in which the compressing and displacing element is a piston moving linearly within a cylinder. Figure 10-1 shows the action of a reciprocating compressor. 

This gives a target value to the pump supplier that is worst condition. In general, for cold water duty equation (32.16) can be used for the duty flow required. Equation (32.16) is employed for reciprocating and rotary positive displacement machines with allowance made for acceleration effects. 

In general, the positive displacement machines - there are rotating and reciprocating features - show their main application range for high-pressure and lower-capacity conditions. The turbo- (centrifugal) machines, however, are best suited for high capacity but lower pressure conditions. The selection should be based on an analysis of the life-cycle costs (or costs of ownership) [1]. 

There are two general ways to compress gas in a refinery. Either a positive-displacement or a dynamic-type machine can be used. A reciprocating compressor is an example of a positive-displacement machine, whereas a centrifugal compressor is a dynamic machine. Centrifugal compressors are relatively ... 

Compressors fall into one of two fundamental types - positive displacement and turbo-machines. Positive displacement machines can be either rotary or reciprocating. They both trap the gas in a cylinder and then force it into a smaller volume and so increase its pressure. Turbo-machines impart velocity to the gas and its momentum carries it into a narrowing space and so its pressure increases. Turbo-machines can be either axial (in which the flow is parallel to the shaft) or centrifugal (in which the flow is at right angles to the shaft). Multistage turbo-machines, with intercoohng, are common. 

Positive displacement machine A reciprocating compressor or gear type pump is an example of a positive displacement machine. It increases pressure by squeezing or pushing the fluid into a region of greater pressure. 

The reciprocating compressor is a positive displacement, intermittent flow machine and operates at a fixed volume in its basic configuration. 

For reciprocating positive pumps and compressors the fluid displacement per stroke depends on the geometry, the fluid compressibility, the polytropic coefficient, and the internal leakages and volumetric losses, but basically not on the speed of machine. 

The reciprocating compressor is a positive-displacement compressor. It is cheaper to purchase and install than a centrifugal compressor. Also—in theory—far more efficient (90 percent) than a centrifugal compressor (70 percent). Certainly, reciprocating compressors are more simple to understand and engineer than centrifugal machines. Best of all, they are not subject to surge. 

Several typologies of blowers or compressors are potentially suitable for fuel cell application. In Fig. 4.4 a flow diagram of all compressor types is reported, distinguishing two main classes (dynamic and positive displacement) and different typologies for each (reciprocating and rotary as positive displacement devices, and centrifugal and axial as dynamic machines). 

To use Eq. (8.23), the integral must be evaluated, which requires information on the path followed by the fluid in the machine from suction to discharge. The procedure is the same whether the compressor is a reciprocating unit, a rotary positive-displacement unit, or a centrifugal unit, provided only that the flow is frictionless and that in a reciprocating machine the equation is applied over an integral number of cycles, so there is neither accumulation nor depletion of fluid in the cylinders otherwise the basic assumption of steady flow, which underlies Eq. (4.32), would not hold. 

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