10

Fig. I.17 Depth factor for number of tube rows crossed in convection banks.

«Calculated from Crane Co. Tech. Paper 409, May 1942.

Pumps, compressors, fans, and blowers are extensively used to move gases and liquids through the process network and over heat-exchanger surfaces. The general constraint in equipment selection is a matching of fluid handler capacity to pressure drop in the circuit connected to the fluid handler.

Pumps are used to transport liquids, whereas compressors, fans, and blowers apply to gases. There are features of performance common to all of them. For purposes of illustration, a centrifugal pump will be used to discuss performance characteristics.

Centrifugal Machines. Centrifugal machines operate on the principle of centrifugal acceleration of a fluid element in a rotating impeller/housing system to achieve a pressure gain and circulation.

The characteristics that are important are flow rate (capacity), head, efficiency, and durability. Qf (capacity), hp (head), and np (efficiency) are related quantities, dependent basically on the fluid behavior in the pump and the flow circuit. Durability is related to the wear, corrosion, and other factors that bear on a pump's reliability and lifetime.

Figure 1.19 shows the relation between flow rate and related characteristics for a centrifugal pump at constant speed. Graphs of this type are called performance curves; fhp and bhp are fluid and brake horsepower, respectively. The primary design constraint is a matching

Fig. I.18 Friction factor / as affected by Reynolds number for various in-line tube patterns, crossflow gas or air, do, tube diameter; Z±, gap distance perpendicular to the flow; Zjj, gap distance parallel to the flow.

of flow rate to head. Note that as the flow-rate requirement is increased, the allowable head must be reduced if other pump parameters are unchanged.

Analysis and experience has shown that there are scaling laws for centrifugal pump performance that give the trends for a change in certain performance parameters. Basically, they are:

Efficiency:

Dimensionless head:

Fig. I.19 Performance curve for a centrifugal pump.

Dimensionless brake horsepower:

YD 2n 3 \D 3n where D is the impeller diameter, n is the rotary impeller speed, g is gravity, and y is the specific weight of fluid.

The basic relationships yield specific proportionalities such as Qf o n (rpm), hp o n2, fhp o n3, Qf rc —2

11 D

For pumps, density variations are generally negligible since liquids are incompressible. But for gas-handling equipment, density changes are very important. The scaling laws will give the following rules for changing density:

fhp

><xp

Qf,

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