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In most cases, sustaining the flow of a fluid through piping or ducting requires a work input to a suitable fluid machine. When the fluid is a liquid or slurry, the machine is called a pump. Gas and vapor handling units are called fans, blowers, or compressors, depending on the pressure rise. In harmony with the presentations of Sections 2.6.2 and 2.6.3, we limit the present discussion to pumps.

Two commonly employed types of pumps are positive-displacement pumps and centrifugal pumps. In positive-displacement machines, volume and pres- sure changes occur while the liquid is confined within a chamber or passage.

Machines that direct the flow with blades or vanes attached to a rotating member (impeller) are called radial-flow or axial-flow turbomachines, de- pending on whether the flow path is essentially radial or nearly parallel to the machine centerline. Centrifugal pumps are radial-flow turbomachines. As centrifugal pumps are widely used for industrial applications, we further limit the present discussion to this type.

When selecting a pump for a given service, it is necessary to know the nature of the liquid being handled. Corrosive or reactive characteristics of the liquid requiring special materials of construction should be understood. Also, the presence of solids in the liquid may introduce complexities related to erosion and agglomeration. Knowledge of the temperature, pressure, viscosity, and other liquid properties is also generally required. Additionally, the de- signer must consider the relationship to the flow rate of key parameters such as of the pressure rise (or head), power requirement, and pump efficiency.

Measured performance data for centrifugal pumps are commonly repre- sented compactly on plots called characteristic curves giving the variation of total head versus volumetric flow rate. Figure shows characteristic curves for centrifugal pumps operating at a fixed speed with different impeller sizes.

Figure shows characteristic curves at various speeds for a fixed impeller

106 THERMODYNAMICS, MODELING, AND DESIGN ANALYSIS

Pump efficiency

Volumetric flow

rate-Figure 2.6 Centrifugal pump performance charts: (a) characteristic curves of centrifugal pumps operating at a constant speed for two impeller diameters; characteristic curves of centrifugal pump at various speeds: (RPM) (RPM)' for fixed impeller diameter.

2.7 CLOSURE 107 size. It is important to note that, at any fixed speed, the pump operates along a particular characteristic curve and at no other points. For this ma- chine, the head decreases continuously as the flow rate increases; the power required increases as the flow rate increases; and the pump efficiency in- creases with capacity until a best efficiency point is reached (80% for the largest diameter and greatest speed cases of Figure and then decreases as flow rate increases further. Performance charts such as Figure 2.6 may also contain additional information. An often included parameter is the net positive suction head (NPSH), discussed later.

Centrifugal pumps may be combined in parallel to deliver greater flow or in series to provide a greater head. It is common practice to drive pumps with electric motors at nearly constant speed, but variable-speed operation can lead to electricity savings in certain applications. For pumps with variable-speed drives, it is possible to change the characteristic curve, as illustrated in Figure Various operating features influence pump performance. Wear is a note- worthy example. As a pump wears with use, the characteristic curve tends to move downward toward a lower pressure at each flow rate. Other factors that can adversely influence pump performance include pumping hot liquids, phase mixtures, or liquids with high viscosities. As the presence of even a small amount of entrained gas can drastically reduce the pump performance, particular attention must be given to preventing air from entering at the suc- tion side of the pump. Another effect that adversely affects the performance is cavitation.

Cavitation may arise when the local pressure falls below the vapor pressure of the liquid. If this occurs, liquid may flash to vapor, forming vapor cavities.

The growth and collapse of the vapor cavities not only disrupts the flow but also may cause mechanical damage. Since cavitation is detrimental to both pump efficiency and pump life, it must be avoided. Cavitation can be avoided if the pressure everywhere in the pump is kept above the vapor pressure of the liquid. This requires in particular that a pressure in excess of the vapor pressure of the liquid be maintained at the pump inlet (the suction). A measure of the required pressure difference is provided by the NPSH. By locating a pump so that the NPSH is greater than a specified value obtained from man- ufacturers’ data, cavitation can be avoided. As noted before, NPSH data is often shown on pump performance charts.

For further discussion concerning pump selection, including operating data and solved examples, see references such as [7] or contact manufacturers’

representatives directly.

2.7 CLOSURE

In this chapter, we have presented some fundamental principles of engineering thermodynamics and illustrated their use for modeling and design analysis in

108 THERMODYNAMICS, MODELING, AND DESIGN ANALYSIS

applications involving both engineering thermodynamics and fluid flow. In

Chapter 3, the current presentation continues, but with emphasis on the exergy concept. In Chapter 3, we also begin to develop one of the central themes of this book: Greater use should be made in thermal system design of the second law of thermodynamics. Modeling and design analysis considerations related specifically to heat transfer enter the discussion beginning with Chapter 4.

REFERENCES

1. M. J. Moran and H. N. Shapiro, Fundamentals of Engineering Thermodynamics, 3rd ed., Wiley, New York, 1995.

2. A. Bejan, Advanced Engineering Thermodynamics. Wiley-Interscience, New York, 1988.

3. R. B. Bird, W. E. Stewart, and E. N. Lightfoot, Transport Phenomena, Wiley, New York,

4. R. W. Fox and A. T. McDonald, Introduction to Fluid Mechanics, 4th ed., Wiley, New York, 1992.

5. R. C. Reid and K. Sherwood, The Properties of Gases and Liquids, 2nd ed., McGraw-Hill, New York, 1966.

6. 0. Knacke, 0. Kubaschewski, and K. Hesselmann, Thermochemical Properties of Inorganic Substances, 2nd ed., Springer-Verlag, Berlin, and Verlag Stahleisen,

1991.

7. R. H. Perry and D. Green, Chemical Engineers’ Handbook, 6th ed., McGraw-Hill, New York, 1984.

8. Handbook 1993 Fundamentals, American Society of Heating, Refrig- erating, and Air Conditioning Engineers, Atlanta, 1993.

9. S. W. Churchill, Friction-factor equation spans all flow regimes, Chem. Eng., 7 November, 1977, pp. 91-92.

10. “Flow of fluids through valves, fittings, and pipes, Crane Company Technical Paper No. 410, New York, 1982.

1 1. Engineering Data Book, Hydraulic Institute, Cleveland, 1979.

12. B. K. Hodge, Analysis and Design of Energy Systems, 2nd ed., Prentice-Hall, Cliffs, NJ, 1990.

13. D. J. Wood and A. G. Rayes, Reliability of algorithms for pipe network analysis, Hydraulic Div., Proc. Vol. 107, No. 1981, pp. 1145-1161.

PROBLEMS

2.1 Consider an automobile engine as the system. List the principal irre- versibilities present during operation. Repeat for an ordinary forced air, natural-gas-fired household furnace.

PROBLEMS

At one location the ocean surface temperature is while at a depth of 540 m the temperature is 2°C. An inventor claims to have developed a power cycle having a thermal efficiency of 10% that receives and discharges energy by heat transfer at these temperatures, respectively.

There are no other heat transfers. Evaluate this claim.

A patent application describes a device that at steady state generates electricity while heat transfer occurs at temperature only. There are no other energy transfers. Evaluate this device thermodynamically.

A gas flows through a one-inlet, one-outlet control volume operating at steady state. Heat transfer at the rate occurs only at a location on the boundary where the temperature is If 0, determine whether the outlet specific entropy is greater than, equal to, or less than the inlet specific entropy. What can be said when O? Discuss.

Determine the changes in specific enthalpy and specific entropy for each of the following changes of state of water:

(a) = 0.15 = 280°C; = 0.15 = 0.9

(b) T , = = 0.3 = 2.5 = 140°C

(c) = 20 ⁼ 500°F; ⁼ 20 ⁼ 14

(d) = = 0.5 ⁼ 500 ⁼ 320°F

Water vapor at 1.0 300°C enters a turbine operating at steady state and expands to 15 The work developed by the turbine is 630 per of steam flowing through the turbine. Ignoring heat transfer with the surroundings and kinetic and potential energy effects, deter- mine (a) the isentropic turbine efficiency, (b) the rate of entropy gen- eration, in per of steam flowing. the change in specific entropy, in using (a) Equation 2.41 and (b) Equation Compare the calculated results, and for operation at steady state interpret the negative sign.

Derive Equation and (b) Equations

Two approaches are under consideration for the production of hydrogen (H,) and carbon dioxide (CO,) by reacting carbon monoxide (CO) with water vapor in an insulated reactor operating at steady state:

THERMODYNAMICS, MODELING, AND DESIGN ANALYSIS

2.11

(a) The carbon monoxide and water vapor enter the reactor in separate streams, each at 400 K, 1 atm. The products exit as a mixture at

1 atm.

(b) The carbon monoxide and water vapor enter the reactor as a mix- ture at 400 K, 1 atm. The products exit as a mixture at 1 atm.

In each case determine the rate of entropy generation, in per kmol of CO entering. Compare and discuss these results.

Coal with a mass flow rate of 10 and mass analysis C, 88%; H, mine the minimum theoretical power required by the device, in kilo- joules per kilogram of coal burned.

2.12 Referring to Figure 2.3, determine the maximum theoretical work, in at 1 bar, if the fuel is (a) C, (b) H,, (c) CH,.

2.13 The molar analysis of a gas mixture at 850 K, 9.623 bars is 77.48%

N,, 20.59% 0,, 0.03% CO,, and 1.90% For the mixture de- termine (a) molecular weight, (b) specific enthalpy, in (c) specific entropy, in Obtain enthalpy and entropy data for the components from Table C.l. diameters, in inches, and the weight in pounds per linear foot?

2.16 A piping layout carrying liquid water at 70°F at a volumetric flow rate of 0.2 is composed of four sections of 4-in. diameter steel pipe having a total length of 550 ft, three 90" rounded elbows, and a fully open valve. Evaluate Equation 2.31, in if the valve is (a) a globe valve, (b) a gate valve.

A desalination plant requires piping for the sea water that has been drawn into the plant. What piping material might you specify for this application? Discuss.

2.18 Design frequently requires choices between different varieties of the same type of device. For what applications might

2.15

2.17