Category: Centrifugal Air Compressors

The delivery pressure v’s mass flow rate in a dynamic compressor is shown in Fig. 14.13. Suppose a discharge valve is put in the line for this compressor. The mass rate of flow will be zero when the discharge valve is closed and the static pressure developed is that delivered by the impeller to the air…

Let us consider the following variables for the performance of the compressor. p3 = pressure at the outlet of diffuser p1 = inlet pressure T1 = inlet temperature D = diameter G = mass rate of flow N = r.p.m of rotor Then, p3 = f (p1, T1, D, G, N) The following dimensions are chosen: M = mass (kg), L = length (m), t = time (s) Number of variables, n = 6 Basic…

In order to avoid acceleration to sonic or supersonic velocities, the Mach number of flow entering the impeller eye should be kept below the value of 0.9. When the absolute velocity of approach is high enough or the static absolute temperature of the entering air is low enough to make the Mach number more than…

The following types of blade shapes are used for impellers of centrifugal compressors: The characteristics of these blades are shown in Fig. 14.8. Centrifugal effects on the curved blades create a bending moment and produce increased stresses which limit the maximum speed at which the impeller can run. Slightly backward-curved impeller blades (i.e., β2 < 90°) give optimum…

There are two types of losses in a centrifugal compressor: internal and external. The internal losses sustained in the compressor are manifested by an increase of enthalpy of air. The internal losses which occur in compressor are as follows: The compressor sustains an external loss in the form of bearing friction and windage loss. The…

In the ideal velocity diagrams, we had assumed that vw2 = u2. However, in actual operation, this condition is not satisfied in actual practice due to secondary flow effects. Actually, vw2 < u2. The difference (u2 – vw2) is called the slip. The slip factor is defined as the ratio of actual whirl component to the ideal whirl component and is denoted by ϕs.…

The velocity diagrams at inlet and outlet of the impeller for the centrifugal compressor are shown in Fig. 14.6(a). Figure 14.6 (a) Actual velocity diagrams Let u1 = mean blade velocity at entrance u2 = mean blade velocity at exit va1 = absolute velocity at inlet to rotor va2 = absolute velocity at outlet to rotor vr1 = relative velocity at inlet…

Isentropic efficiency may be defined as the ratio of isentropic temperature rise to actual temperature rise. where  During compression process, work has to be done on the impeller. The energy balance then gives,   cpT01 = cpT02 – W

For an adiabatic compression process, there is no heat exchange with the surroundings. If the adiabatic process is reversible (frictionless), then the process is called isentropic process in which the entropy of the system does not change. However, in an actual compressor, during adiabatic compression, there are losses due to friction in air and blade…

The velocities of air encountered in centrifugal compressors are very high as compared to that in reciprocating compressors. Therefore, total head quantities should be taken into account in the analysis of centrifugal compressors. The total head quantities account for the kinetic energy of the air passing through the compressor. Consider a horizontal passage of varying…