Positive displacement machine

Continuous-Flow Compressors Continuous-flow compressors are machines where the flow is continuous, unlike positive displacement machines where the flow is fluctuating. Continuous-flow compressors are also classified as turbomachines. These types of machines are widely used in the chemical and petroleum industiy for many services. They are also used extensively in many other industries such as the iron and steel industry, pipeKne boosters, and on offshore platforms for reinjection compressors. Continuous-flow machines are usually much smaller in size and produce much less vibration than their counterpart, positive displacement units. 

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. 

Rotary compressors are positive-displacement machines. Figure iO-7 shows a typical vane compressor. The operation is similar to that of a vane pump shown schematically in Figure 10-10 of Volume 1, 2nd Edition (Figure 10-9 in 1st Edition). A number of vanes, typically from 8 to 20, fit into slots in a rotating shaft. The vanes slide into and out of the slots as the shaft rotates and the volume contained between two adjacent vanes and the wall of the compressor cylinder decreases. Vanes can be cloth impregnated with a phenolic resin, bronze, or aluminum. The more vanes the compressor has, the smaller the pressure differential across the vanes. Thus, high-ratio vane compressors tend to have more vanes than low-ratio compressors. 

Screw compressors are rotary positive displacement machines. Two helical rotors are rotated by a senes of timing gears as shown in Figure 10-8 so that gas trapped in the space between them is transported from the suction to the discharge piping. In low-pressure air service, non-lubri-cated screw compressors can deliver a clean, oil-free air. In hydrocarbon service most screw compressors require that liquid be injected to help provide a seal. After-coolers and separators are required to separate the seal oil and recirculate it to suction. 

The spiral-lobe and helical-screw compressors are rotary positive-displacement machines and quite adaptable to a wide assortment of process and refrigeration gases. This class of equipment is usually built to comply with the American Petroleum Institute Standard 619. These units oper-... 

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. 

This is a statement of the NPSH that the pump can sustain by its own operation, so that the operating requirement is that NPSHr > NPSHa- NPSHr is more difficult than NPSHa to determine. Reference 7 suggests working relations for positive displacement machines that are outlined below. 

Rotary, positive displacement machines in which two intermeshing rotors, each in helical configuration displace and compress the air available in lubricated and non-lubricated construction the discharge air is normally free from pulsation high rotation speed. 

Positive-displacement machines use safety valves. There are ASME standards of materials, sizing, and only ASME stamped valves should be used. The relieving capacity of a given size of safety valve varies materially with the make and design. Care must be taken to assure proper selection. 

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]. 

We recommend using an Eppendorf multipette plus (Eppendorf) or similar positive displacement machine to repeat dispense quickly and accurately. 

Vacuum blowers (vacuum pumps) are used to draw waste nitrogen, carbon dioxide, and water vapor from the adsorber vessels during the evacuation step. There are two rotary-lobe blowers operating in series. They are positive displacement machines. A timing gear arrangement maintains the close tolerances that are essential for ef cient operation. 

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. 

Compressors are designed as turbo or positive displacement machines. The two types have different operahonal behaviour. With turbo machines the amount of compressed gas decreases with increasing pressure, while the machines of the second group deliver, owing to their displacement principle, nearly a constant mass flow independent from the discharge pressure. Piston and screw compressors are the most prominent displacement machines. 

Multiple-stage turbo compressors are by far the most frequently used machines in cryogenic air separation. Positive displacement machines are foimd in niche applications, such as piston compressors for oxygen compression and screw compressors for small plants processing less than 4000 m h of air. 

Figure 4.25 Constant speed characteristics of aerodynamic and positive displacement machines.

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. 

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