Fig.F-16 An example of practical layout of a series-manufactured radial
pump and its characteristic - the dependence between mass flow rate and
generated difference in fluid specific energy (compare with Fig. F-9). The
characteristic determines the behaviour of the pump in a system.
Radial (centrifugal) pumps, blowers, ventilators, and exhaustors:
Strictly speaking, the expressions discussed here are valid for liquid flow in pumps. It is, however, possible to use them (with some tolerance) also for flow of gas (in particular: air) in ventilators - as long as pressure differences are low. There is a conventional (not always observed) limit of pressure ratio / = 1.1, above which the machine is called blower and it is usual for its design to take into account compressibility. For pressure ratios above / = 4 , calculation with compressibility is absolutely necessary and the particular machine is called a compressor. The basic function principle of all these machines - as long as they belong into the radial machine category is, however, the same. The absolute pressure level is not important - the same machine may be used also as a vacuum pump if its inlet pressure is subatmospheric. The differences between the individual categories are mainly in design and manufacturing details (pump inner space must be usually sealed to keep liquid inside while leaking air from a blower need not be of concern; higher pressure in a pump requires a cast case while a ventilator body may be a cheaper sheet metal one etc. ...).
Fig.F-15
Essential information about pump or blower behaviour for its user is contained in the (loading) characteristics , as shown here in the bottom part of Fig.F-16. Equation of such characteristic for the primitive single-duct pump (or blower) was derived in Fig.F-9. The curve in Fig.F-16 is valid only for a single rotational speed (cf. Fig.F-10). This usually suffices for machines in stationary industrial applications, where they are usually driven by an asynchornous electric motor (the speed of which does vary with loading, but only within a small, practically negligible range). More complicated situation is encountered in vehicle applications where they are driven by variable speed engine.

Fig.F-17 The most important part of radial turbomachines (axial ones are not treated in the present chapter) is their rotor (in pumps often called a runner ). The spaces between each two neighbouring blades may be treated (in a simplified, one-dimensional approach) as a rotating duct from Fig.F-14. At the rotor inlet (X), the shape of the blade is determined by the vector addition of velocities in Fig.F-17a. If the condition of radial inlet absolute velocity is fulfilled, then the expression for work imparted to fluid, as derived for the single duct in Fig.F-14, is also applicable for a rotor with many ducts.

























Fig.F-17a Vectors of velocity components at the inlet of a pump (or blower) rotor. The fluid does not rotate before entering the rotor - so that its absolute inlet velocity must be radial, without any tangential component. Since the absolute velocity results from the vector addition of the relative and the rotational (carry-away) component, the vector of relative velocity must be inclined. This vector must be tangent to the rotor blade, so that its backward inclination dictates the backwards inclined shape of blades (Fig. F-17) at the entrance.


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This is page Nr. F06 from textbook Vaclav TESAR : "BASIC FLUID MECHANICS"
Any comments and suggestions concerning this text may be mailed to the author to his address tesar@fsid.cvut.cz

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