Fluid Machinery
Pumps and fans all belong to fluid machinery which can be used to transport fluids. Pumps are always used to transport liquids and fans are always used to transport gases. In this chapter, we will first discuss the characteristics of centrifugal pump, inc
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Fluid Machinery
Abstract
Pumps and fans all belong to fluid machinery which can be used to transport fluids. Pumps are always used to transport liquids and fans are always used to transport gases. In this chapter, we will first discuss the characteristics of centrifugal pump, including its head rise, efficiency, performance curves. Then we will introduce system curve and pump selection. Finally, we will indicate some basic concepts about centrifugal fan. Keywords
Centrifugal pump
7.1
Head rise Efficiency Operating point Centrifugal fan
Centrifugal Pump
Centrifugal pumps are a sub-class of dynamic axisymmetric work-absorbing fluid machinery [1, 2]. Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. As shown in Fig. 7.1, fluid enters axially through eye of the casing, is caught up in the impeller blades, and is whirled tangentially and radially outward until it leaves through all circumferential parts of the impeller into the diffuser part of the casing. The fluid gains both velocity and pressure while passing through the impeller. The doughnut-shaped diffuser, or scroll, section of the casing decelerates the flow and further increases the pressure.
© Metallurgical Industry Press, Beijing and Springer Nature Singapore Pte Ltd. 2018 H. Song, Engineering Fluid Mechanics, https://doi.org/10.1007/978-981-13-0173-5_7
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Fluid Machinery
Fig. 7.1 Centrifugal pump [3], (1-impeller 2-impeller blades 3-suction side 4-casing 5-outlet)
7.1.1 Theoretical Considerations 7.1.1.1 Head Rise of the Pump As shown in Fig. 7.2, section 1-1 is the inlet of the pump with vacuum gauge 3, and section 2-2 is the outlet of the pump with pressure gauge 4. The energy difference Fig. 7.2 Centrifugal pump in a piping system [3]
7.1 Centrifugal Pump
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of per unit weight fluid between outlet and inlet e2 e1 , or the energy head added to the flow, is called head rise of the pump, which is denoted by H. Namely
H ¼ e2 e1
As shown in Fig. 7.2, based on the surface O–O of the reservoir 1, the energy of per unit weight fluid in sections 1-1 and 2-2 is e 1 ¼ hs þ
p1 v2 þ 1 c 2g
e2 ¼ hs þ z2 þ
p2 v2 þ 2; c 2g
where c is the specific weight of the liquid. Assuming that the atmospheric pressure is pa , and vacuum gauge indicates pv , pressure gauge indicates pM , then p1 ¼ pa pv þ czv p2 ¼ pa þ pM þ czm Thus H ¼ e2 e1 ¼ hs þ z 2 þ
pa þ pM pa pv v2 v21 þ zm h s zv þ 2 c c 2g
Namely H ¼ ðz2 þ zm Þ zv þ
pM þ pv v22 v21 þ c 2g
ð7:1Þ
ðz2 þ zm Þ zv ¼ Dz is the elevation height difference between pressure gauge and vacuum gauge, which is always small and can be neglected. Usually v1 and v2 are about the same, then the head rise H¼
pM þ pv c
ð7:2Þ
Therefore, the head rise of the pump can be calculated by pressure gauge and vacuum gauge. According to Fig. 7.2, based on section O–O, we write Bernoulli equation between the surface of the reser
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