The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 63
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Major outbreaks have occurred atconventions and in hospitals, for which the source of the bacteria has been traced to cooling towers ofair-conditioning systems. The bacteria require nutrients such as algae or dead bacteria in sludge, andthrive if iron oxides are present. However, properly designed, installed, and maintained cooling towershave never been implicated in an outbreak of the disease. Key requirements to be met include thefollowing:1. Mist (drift) eliminators should be effective.2.
The tower should be located so as to minimize the possibility of mist entering a ventilation system.3. Corrosion in the tower and water lines should be minimized by use of glass fiber, stainless steel,and coated steel.4. The design should facilitate inspection and cleaning, to allow early detection and remedy of sludgebuildup.5. Water treatment and filtration procedures should meet recommended standards.© 1999 by CRC Press LLC4-260Section 4ReferencesBourillot, C.
1983. TEFRI: Numerical Model for Calculating the Performance of an Evaporative CoolingTower, EPRI CS-3212-SR, Electric Power Research Institute, Palo Alto, CA.Cooling Tower Institute, 1967. Cooling Tower Performance Curves, the Institute, Houston.Kelly, N.W. 1976. Kelly’s Handbook of Cross-Flow Cooling Tower Performance, Neil W.
Kelly andAssociates, Kansas City, MO.Kintner-Meyer, M. and Emery, A.F. 1995. Cost-optimal design of cooling towers, ASHRAE J., April,46–55.Merkel, F. 1925. Verdunstungskühlung, Forschungsarb. Ing. Wes., no. 275.Mills, A.F. 1995. Heat and Mass Transfer, Richard D. Irwin, Chicago.Majumdar, A.K., Singhal, A.K., and Spalding, D.B. 1985. VERA2D-84: A Computer Program for 2-DAnalysis of Flow, Heat and Mass Transfer in Evaporative Cooling Towers, EPRI CS-4073, ElectricPower Research Institute, Palo Alto, CA.Further InformationBaker, D.
1984. Cooling Tower Performance, Chemical Publishing Company, New York.Johnson, B.M. Ed. 1990. Cooling Tower Performance Prediction and Improvement, Vols. 1 and 2, EPRIGS-6370, Electric Power Research Institute, Palo Alto, CA.Singham, J.R. 1990. Natural draft towers, in Hemisphere Handbook of Heat Exchanger Design, Section3.12.3, Hewitt, G.E., Coord Ed., Hemisphere Publishing, New York.Stoeker, W.F. and Jones, J.W. 1982. Refrigeration and Air Conditioning, 2nd ed., McGraw-Hill, NewYork.Webb, R.L. 1988. A critical review of cooling tower design methods, in Heat Transfer Equipment Design,Shah, R.K., Subba Rao, E.C., and Mashelkar, R.A., Eds., Hemisphere Publishing, Washington,D.C.Heat PipesLarry W.
SwansonIntroductionThe heat pipe is a vapor-liquid phase-change device that transfers heat from a hot reservoir to a coldreservoir using capillary forces generated by a wick or porous material and a working fluid. Originallyconceived by Gaugler in 1944, the operational characteristics of heat pipes were not widely publicizeduntil 1963 when Grover and his colleagues at Los Alamos Scientific Laboratory independently reinventedthe concept.
Since then many types of heat pipes have been developed and used by a wide variety ofindustries.Figure 4.8.12 shows a schematic of a heat pipe aligned at angle y relative to the vertical axis (gravityvector). The heat pipe is composed of a container lined with a wick that is filled with liquid near itssaturation temperature. The vapor-liquid interface, usually found near the inner edge of the wick,separates the liquid in the wick from an open vapor core.
Heat flowing into the evaporator is transferredthrough the container to the liquid-filled wicking material, causing the liquid to evaporate and vapor toflow into the open core portion of the evaporator. The capillary forces generated by the evaporatinginterface increase the pressure difference between the vapor and liquid. The vapor in the open core flowsout of the evaporator through the adiabatic region (insulated region) and into the condenser. The vaporthen condenses, generating capillary forces similar, although much less in magnitude, to those in theevaporator. The heat released in the condenser passes through the wet wicking material and containerout into the cold reservoir.
The condensed liquid is then pumped, by the liquid pressure difference dueto the net capillary force between the evaporator and condenser, out of the condenser back into the© 1999 by CRC Press LLCHeat and Mass Transfer4-261FIGURE 4.8.12 Schematic of a typical heat pipe.evaporator. Proper selection and design of the pipe container, working fluid, and wick structure areessential to the successful operation of a heat pipe. The heat transfer limitations, effective thermalconductivity, and axial temperature difference define the operational characteristics of the heat pipe.Heat Pipe Container, Working Fluid, and Wick StructuresThe container, working fluid, and wick structure of a heat pipe determine its operational characteristics.One of the most important considerations in choosing the material for the heat pipe container and wickis its compatibility with the working fluid.
Degradation of the container or wick and contamination ofthe working fluid due to chemical reaction can seriously impair heat pipe performance. For example,noncondensable gas created during a chemical reaction eventually can accumulate near the end of thecondenser, decreasing the condensation surface area.
This reduces the ability of the heat pipe to transferheat to the external heat sink. The material and geometry of the heat pipe container also must have ahigh burst strength, low weight, high thermal conductivity, and low porosity.Using the proper working fluid for a given application is another critical element of proper heat pipeoperation. The working fluid must have good thermal stability properties at the specified operationaltemperature and pressure. The operational temperature range of the working fluid has to lie between itstriple point and its critical point for liquid to exist in the wicking material. The wettability of the workingfluid contributes to its capillary pumping and priming capability.
High-surface-tension fluids are commonly used in heat pipes because they provide the capillary pumping and wetting characteristicsnecessary for proper operation. Other desirable thermophysical properties include a high liquid thermal© 1999 by CRC Press LLC4-262Section 4TABLE 4.8.4Temperature(°C )Thermophysical Properties of Some Heat-Pipe FluidsLatentHeat(kJ /kg)LiquidDensity(kg/m3)VaporDensity(kg/m3)LiquidThermalConductivity(W/m°C)VaporPressure(bars)VaporSpecificHeat(kJ/kg°C)LiquidSurfaceTension(N/m ´102)0.720.780.850.910.981.041.111.191.261.311.380.010.020.040.100.250.551.312.694.987.868.941.201.271.341.401.471.541.611.791.921.921.923.262.952.632.362.182.011.851.661.461.251.040.961.041.121.191.271.341.411.491.571.650.020.070.200.471.012.023.906.4410.0416.191.811.891.911.952.012.092.212.382.622.917.287.006.666.265.895.505.064.664.293.890.150.160.160.170.170.180.190.190.200.200.210.010.010.020.050.100.190.350.610.991.552.345.325.325.325.325.325.325.325.325.325.325.329.509.048.698.448.167.867.517.126.726.325.92LiquidVaporViscosity Viscosity(cP)(cP, ´ 102)Methanol–50–30–10103050709011013015011941187118211751155112510851035980920850843.5833.5818.7800.5782.0764.1746.2724.4703.6685.2653.20.010.010.040.120.310.771.473.015.649.8115.900.2100.2080.2060.2040.2030.2020.2010.1990.1970.1950.1931.7001.3000.9450.7010.5210.3990.3140.2590.2110.1660.138Water204060801001201401601802002448240223592309225822002139207420031967998.0992.1983.3972.0958.0945.0928.0909.0888.0865.00.020.050.130.290.601.121.993.275.167.870.6030.6300.6490.6680.6800.6820.6830.6790.6690.6591.000.650.470.360.280.230.200.170.150.14Potassium35040045050055060065070075080085020932078206020402020200019801960193819131883763.1748.1735.4725.4715.4705.4695.4685.4675.4665.4653.10.0020.0060.0150.0310.0620.1110.1930.3140.4860.7161.05451.0849.0847.0845.0843.3141.8140.0838.0836.3134.8133.310.210.190.180.170.150.140.130.120.120.110.10conductivity, high latent heat of vaporization, low liquid viscosity, and a low vapor viscosity.
Table 4.8.4gives the thermophysical properties for three typical heat pipe working fluids that span a fairly wideoperating temperature range. The thermophysical properties for other heat pipe working fluids can beobtained from Dunn and Reay (1982) and Peterson (1994).The wick structure and working fluid generate the capillary forces required to (1) pump liquid fromthe condenser to the evaporator and (2) keep liquid evenly distributed in the wicking material. Heat pipewicks can be classified as either homogeneous wicks or composite wicks. Homogeneous wicks arecomposed of a single material and configuration.
The most common types of homogeneous wicks includewrapped screen, sintered metal, axial groove, annular, crescent, and arterial. Composite wicks arecomposed of two or more materials and configurations. The most common types of composite wicksinclude variable screen mesh, screen-covered groove, screen slab with grooves, and screen tunnel with© 1999 by CRC Press LLC4-263Heat and Mass Transfergrooves. Regardless of the wick configuration, the desired material properties and structural characteristics of heat pipe wick structures are a high thermal conductivity, high wick porosity, small capillaryradius, and high wick permeability. Table 4.8.2 gives the geometric properties of some commonly usedhomogeneous wicks. The properties of other wick structures, including nonhomogenous types, can beobtained from Peterson (1994).
The container, wick structure, and working fluid are used to determinethe heat transfer limitations of heat pipes.Heat Transfer LimitationsHeat pipes undergo various heat transfer limitations depending on the working fluid, the wick structure,the dimensions of the heat pipe, and the heat pipe operational temperature. Figure 4.8.13 gives aqualitative description of the various heat transfer limitations, which include vapor-pressure, sonic,entrainment, capillary, and boiling limitations.
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