COMPRESSORS

COMPRESSORS

CENTRIFUGAL COMPRESSOR:

The centrifugal compressor comprises a rotating impeller and a fixed set of diffuser. The impeller is a rotating disc with radially set of diffuser. The impeller is flange disposed vanes .Air entering the eye of impeller is flange outwards by the vanes which impart kinetic energy to it. This energy is then converted to pressure as the airflow through the fixed diffusers ,which also turn of airflow from a radial to an axial direction.








Figure (1) Typical centrifugal compressor.

COSTRUCTION:

The impeller may be either single or double sided. Double sided impeller :have
1. Smaller diameter for given air flow.
2. Greater intake losses.
3. More air flow instability.
4. Higher stresses by two sets of blades.
5. Low rotational speed.

Modern impeller are machined from solid forging of titanium alloy and polished .They are attached to the shaft flanged fittings and balanced statically and dimension and tested to (5%) over speed.






Figure (2) Double-entry main stage compressor with side-entry compressor for cooling air. (Courtesy of Rolls-Royce, Ltd.)

THE EFFECT OF BLADE SHAPE ON PERFORMANCE

There are three types of vanes used in impellers. They are: forward- curved, backward-curved, and radial vanes, as shown in Fig. (3).The impellers tend to undergo high stress forces. Curved blades, such as those used in some fans and hydraulic pumps, tend to straighten out due to centrifugal force and bending stresses are set up in the vanes. The straight radial blades are not only free from bending stresses, they may also be somewhat easier to manufacture than curved blades.



Figure (3) Shapes of centrifugal impeller blades: (a) backward-curved blades, (b) radial blades, and (c) forward-curved blades.

Figure (3 )shows the three types of impeller vanes schematically, along with the velocity triangles in the radial plane for the outlet of each type of

Figure 4.4 represents the relative performance of these types of blades. It is clear that increased mass flow decreases the pressure on the backward blade, exerts the same pressure on the radial blade, and increases the pressure on the forward blade. For a given tip speed, the forward-curved blade impeller transfers maximum energy, the radial blade less, and the least energy is transferred by the backward-curved blades. Hence with forward-blade impellers, a given pressure ratio can be achieved from a smaller-sized machine than those with radial or backward-curved blades.




























Figure (4) Pressure ratio or head versus mass flow or volume flow, for the three blade shapes.

4.4 VELOCITY DIAGRAMS


Figure 4.5 shows the impeller and velocity diagrams at the inlet and outlet. Figure 4.5a represents the velocity triangle when the air enters the impeller in the axial direction. In this case, absolute velocity at the inlet, C1 = Ca1. Figure 4.5b represents the velocity triangle at the inlet to the impeller eye and air enters through the inlet guide vanes. Angle u is made by C1 and Ca1 and this angle is known as the angle of prewhirl. The absolute velocity C1 has a whirl component Cw1. In the ideal case, air
comes out from the impeller tip after making an angle of 90O (i.e., in the

radial direction), so Cw2 = U2. That is, the whirl component is exactly equal to the impeller tip velocity. Figure 4.5c shows the ideal velocity triangle. But there is some slip between the impeller and the fluid, and actual values of Cw1 are somewhat less than U2. As we have already noted in the centrifugal pump, this results in a higher static pressure on the leading face of a vane than on the trailing face.