Axial-Flow Centrifugal Compressors

Compressors applied to evaporator systems include positive-displacement, and radial-flow and axial-flow centrifugals. Each has advantages for certain applications. 

Centrifugal compressors are dynamic compressors and are essentially constant pressure, variable volume machines and must be regulated in almost all applications. Before control can be discussed, the operating characteristics must be understood. The discussion to follow will be concerned with radial-flow compressors. Axial-flow compressors have similar characteristics curves, although the change in pressure rise is much greater with equivalent changes in compressor rotational speed. 

To be able to decide which type of compressor would best fit the job, we should first divide the compressors into three main categories positive displacement, centrifugaf and axial flow In general terms, positive displacement compressors are used for high pressure and low flow characteristics centrifugal compressors are used for medium to high pressure dehvery and medium flow and axial flow compressors are low pressure and high flow... 

Continuous-flow compressors. a. Centrifugal compressors h. Axial flow compressors... 

Centrifugal Compressors The flow in a centrifugal compressor enters the impeller in an axial direction and exits in a radial direction. 

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. 

Thermocompression Evaporators Thermocompression-evap-orator calculations [Pridgeon, Chem. Metall. Eng., 28, 1109 (1923) Peter, Chimin Switzerland), 3, II4 (1949) Petzold, Chem. Ing. Tech., 22, 147 (1950) and Weimer, Dolf, and Austin, Chem. Eng. Prog., 76(11), 78 (1980)] are much the same as single-effect calculations with the added comphcation that the heat suppied to the evaporator from compressed vapor and other sources must exactly balance the heat requirements. Some knowledge of compressor efficiency is also required. Large axial-flow machines on the order of 236-mVs (500,000-ftVmin) capacity may have efficiencies of 80 to 85 percent. Efficiency drops to about 75 percent for a I4-mVs (30,000-ftVmin) centrifugal compressor. Steam-jet compressors have thermodynamic efficiencies on the order of only 25 to 30 percent. 

Compressors A compressor is a device that pressurizes the air in a gas turbine. It transfers energy by dynamic means from a rotating member to the continuously flowing air. The two types of compressors used in gas turbines are axial and centrifugal. Figures 29-25 and 29-26 depict typical industrial gas turbines. 

For ease of illustration we will consider the characteristics and behaviour of a centrifugal pump which is similar in behaviour to radial/axial flow fans and centrifugal/screw compressors. Figure 63 shows the mechanical connection of a flow valve to control the output of the pump or the discharge of the fluid through the throttle of the valve. Figure 6.39 illustrates the characteristics of the pump ... 

The inward-flow radial turbine has many components similar to a centrifugal compressor. There are two types of inward-flow radial turbines the cantilever and the mixed-flow. The cantilever type in Figure 1-34 is similar to an axial-flow turbine, but it has radial blading. However, the cantilever turbine is not popular because of design and production difficulties. 

Figure 3-10 shows a similar performance map for an axial-flow compressor. Note the smaller operational flow range for the axial-flow compressor as compared to the centrifugal compressor. Figure 3-11 shows a typical compressor map presented from a slightly different viewpoint. On this map. 

The turboprop engine has a power turbine instead of the nozzle as seen in Figure 4-2. The power turbine drives the propeller. The unit shown schematically is a two-shaft unit, this enables the speed of the propeller to be better controlled, as the gasifier turbine can then operate at a nearly constant speed. Similar engines are used in helicopter drive applications and many have axial flow compressors with a last stage as a centrifugal compressor as shown in Figure 1-14. 

Figure 1. Approximate ranges of application for reciprocating, centrifugal, and axial-flow compressors.

Here are some approximate efficiencies to use for centrifugal, axial flow, and reciprocating compressors. 

Brown, Royce N., An Experimental Investigation of a Pneumatic Closcrt Loop Anti-Surge Control for Centrifugal and Axial Flow Compressors Master s Thesis, University of Wisconsin, Madison, Wisconsin, 1966,... 

Figures 12-46 illustrates the key styles of wheels for use on the rotating shafts of multistage centrifugal, single-stage compressors and fans, and axial flow compressors (to he discussed later in this chapter). The flow-handling capability of a specific impeller geometry is descrihed hy the flow coefficient, ...

Centrifugal compressors usually have specific speeds of 1,500-3,000 rpm at the high efficiency point. The axial flow fans and blowers are high specific-speed wheels mixed flow units are lower and the centrifugal compressor wheels are the lowest in specific speed range because they have narrow impellers. 

Classifications 477. Standard Units 481. General Calculations 481. Reciprocating Compressors 484. Rotarv Compressors 487. Centrifugal Compiessors 493. Axial-Flow Compressors 494. 

In much the same manner as pumps, compressors are classified as one of two general classes positive displacement or dynamic (see Figure 3-68) [23]. These two general classes of compressors are the same as that for pumps. The positive displacement class of compressors is an intermittent flow device, which is usually a reciprocating piston compressor or a rotary compressor (e.g., sliding vane, screws, etc.). The dynamic class of compressor is a continuous flow device, which is usually an axial-flow or centrifugal compressor (or mix of the two). 

Figure 3.6. Approximate polytropic efficiencies centrifugal and axial-flow compressors...

Axial flow compressors have polytropic efficiencies up to 10% higher than centrifugal compressors for the same pressure ratio. A reasonable estimate is for the efficiency to be 5% higher than that of the corresponding centrifugal machine (i.e. multiply the estimate from Equation 13.18 by 1.05). 

In turbomachineiy the centrifugal flow and the axial-flow compressors are the ones used for compressing gases. Positive-displacement compressors such as reciprocating, gear-type, or lobe-type are widely used in the industry for many other applications such as slurry pumping. 

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