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Positive Displacement Pumps
Jet pumps
Air-lift Pumps
Propeller Pumps and Fan
Centrifugal Pumps and Fans

In pumps and fans, mechanical energy from some other source is converted into pressure or velocity energy in a fluid. The food technologist is not generally much concerned with design details of pumps, but should know what classes of pump are used and something about their characteristics.

The efficiency of a pump is the ratio of the energy supplied by the motor to the increase in velocity and pressure energy given to the fluid.

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).

Fig. 4.3 Liquid pumps
Figure 4.3 Liquid pumps

Jet Pumps

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.

Air-lift Pumps

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:

Air introduced into the fluid as shown in Fig. 4.3(e) to pump water from an artesian well.
Air introduced above a liquid in a pressure vessel and the pressure used to discharge the liquid.
Vapours produced in the column of a climbing film evaporator.
In the case of powdered solids, air blown up through a bed of powder to convey it in a "fluidized" form.

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.

Propeller Pumps and Fan

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.

Centrifugal Pumps and Fans

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.

Figure 4.4 Characteristic curves for centrifugal pumps
Adapted from Coulson and Richardson, Chemical Engineering, 2nd Edition, 1973.

For a given centrifugal pump, the capacity of the pump varies with its rotational speed; the pressure developed by the pump varies as the square of the rotational speed; and the power required by the pump varies as the cube of the rotational speed. The same proportional relationships apply to centrifugal fans and these relationships are often called the "fan laws" in this context.

EXAMPLE 4.3. Centrifugal pump for raising water
Water for a processing plant is required to be stored in a reservoir to supply sufficient working head for washers. It is believed that a constant supply of 1.2 m3 min-1 pumped to the reservoir, which is 22 m above the water intake, would be sufficient. The length of the pipe is about 120 m and there is available galvanized iron piping 15 cm diameter. The line would need to include eight right-angle bends. There is available a range of centrifugal pumps whose characteristics are shown in Fig. 4.4. Would one of these pumps be sufficient for the duty and what size of electric drive motor would be required?

Assume properties of water at 20°C are density 998 kg m-3, and viscosity 0.001 N s m-2

Cross-sectional area of pipe A = (p/4)D2
p/4 x (0.15)2
                                             = 0.0177 m-2
                    Volume of flow V = 1.2 m3 min-1
                                             = 1.2/60 m3 s-1
                                             = 0.02 m3 s-1.

                Velocity in the pipe = V/A
                                             = (0.02)/(0.0177)
                                             = 1.13 ms-1

                               Now (Re) = Dvr/m
                                             = (0.15 x 1.13 x 998)/0.001
                                             = 1.7 x 105
and so the flow is clearly turbulent.

From Table 3.1, the roughness factor e is 0.0002 for galvanized iron
and so
         roughness ratio   e/D = 0.0002/0.15 = 0.001
So from Fig. 3.8,

                                                 ƒ = 0.0053

Therefore the friction loss of energy = (4ƒv2/2) x (L/D)
                                                    = [4ƒv2L/2D]
                                                    = [4 x 0.0053 x (1.13)2 x 120]/(2 x 0.15)
                                                    = 10.8 J.

For 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.

                   velocity energy = v2/2
                                         = (1.13)2/2
                                         = 0.64 J
So total loss from bends and discharge energy
                                         = (6 + 1) x 0.64
                                         = 4.5 J

Energy to move 1 kg water against a head of 22 m of water is
                                      E = Zg
                                         = 22 x 9.81
                                         = 215.8 J.

Total energy requirement per kg:
                                   Etot = 10.8 + 4.5 + 215.8
                                         = 231.1 J
and theoretical power requirement
                                         = Energy x volume flow x density
                                         = (Energy/kg) x kgs-1
                                         = 231.1 x 0.02 x 998
                                         = 4613 J s-1.

Now the head equivalent to the energy requirement
                                         = Etot/g
                                         = 231.1/9.81
                                         = 23.5 m of water,

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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Unit Operations in Food Processing. Copyright © 1983, R. L. Earle. :: Published by NZIFST (Inc.)
NZIFST - The New Zealand Institute of Food Science & Technology