PUMPS AND FANS
Positive Displacement Pumps
In a positive displacement pump, the fluid is drawn into the pump and is then forced through the outlet. Types of positive displacement pumps include: reciprocating piston pumps; gear pumps in which the fluid is enmeshed in rotating gears and forced through the pump; rotary pumps in which rotating vanes draw in and discharge fluid through a system of valves. Positive displacement pumps can develop high-pressure heads but they cannot tolerate throttling or blockages in the discharge. These types of pumps are illustrated in Fig. 4.3 (a), (b) and (c).
In jet pumps, a high-velocity jet is produced in a Venturi nozzle, converting the energy of the fluid into velocity energy. This produces a low-pressure area causing the surrounding fluid to be drawn into the throat as shown diagrammatically in Fig. 4.3 (d) and the combined fluids are then discharged. Jet pumps are used for difficult materials that cannot be satisfactorily handled in a mechanical pump. They are also used as vacuum pumps. Jet pumps have relatively low efficiencies but they have no moving parts and therefore have a low initial cost. They can develop only low heads per stage.
If air or gas is introduced into a liquid it can be used to impart energy to the liquid as illustrated in Fig. 4.3 (e). The air or gas can be either provided from external sources or produced by boiling within the liquid. Examples of the air-lift principle are:
introduced into the fluid as shown in Fig. 4.3(e) to pump water from an
A special case of this is in the evaporator, where boiling of the liquid generates the gas (usually steam) and it is used to promote circulation. Air or gas can be used directly to provide pressure to blow a liquid from a container out to a region of lower pressure. Air-lift pumps and air blowing are inefficient, but they are convenient for materials which will not pass easily through the ports, valves and passages of other types of pumps.
Propellers can be used to impart energy to fluids as shown in Fig. 4.3 (f). They are used extensively to mix the contents of tanks and in pipelines to mix and convey the fluid. Propeller fans are common and have high efficiencies.
They can only be used for low heads, in the case of fans only a few centimetres or so of water.
The centrifugal pump converts rotational energy into velocity and pressure energy and is illustrated in Fig. 4.3(g). The fluid to be pumped is taken in at the centre of a bladed rotor and it then passes out along the spinning rotor, acquiring energy of rotation. This rotational energy is then converted into velocity and pressure energy at the periphery of the rotor. Centrifugal fans work on the same principles. These machines are very extensively used and centrifugal pumps can develop moderate heads of up to 20 m of water. They can deliver very large quantities of fluids with high efficiency. The theory of the centrifugal pump is rather complicated and will not be discussed. However, when considering a pump for a given application, the manufacturers will generally supply pump characteristic curves showing how the pump performs under various conditions of loading. These curves should be studied in order to match the pump to the duty required. Figure 4.4 shows some characteristic curves for a family of centrifugal pumps.
Assume properties of water at 20°C are density 998 kg m-3, and viscosity 0.001 N s m-2
area of pipe A = (p/4)D2
Velocity in the pipe = V/A
Now (Re) = Dvr/m
From Table 3.1, the
roughness factor e
is 0.0002 for galvanized iron
ƒ = 0.0053
Therefore the friction
loss of energy = (4ƒv2/2) x (L/D)
the eight right-angled bends, from Table 3.2 we would expect a loss of
0.74 velocity energies at each, making (8 x 0.74) = 6 in all. There would
be one additional velocity energy loss because of the unrecovered flow
energy discharged into the reservoir.
Total energy requirement
Now the head equivalent
to the energy requirement
and from Fig. 4.4 this would require the 150 mm impeller pump to be safe, and the pump would probably be fitted with a 7.5 kW motor.
Fluid-flow applications > SUMMARY & PROBLEMS
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