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688 CHAPTER 48 1960 Guide tion is not always used. The amount of induced air varies with unit design. A fourth less common arrangement is similar, but makes use of hot water in the induction unit in winter and cold water in the induction unit in summer. Control of the valve on the unit is obtained by use of a summer-winter type thermostat. The unit is provided with drip pan and drain piping to remove condensation. No recirculation is used. On cargo ships, tankers and vessels not carrying pas sengers or not having air conditioning, heating of crews' spaces and officers' staterooms is usually obtained through a central system having ffiters, preheaters and reheaters. A minimum temperature of air leaving the preheater is controlled by a duct thermostat. The reheater is controlled by a sub-master discharge-duct thermostat, reset by out side master control. Relationship between sub-master and master control (wherein discharge temperature is raised as outdoor temperature drops), is set according to a schedule based on the ship's itinerary. Ductwork for all systems described is designed for con ventional velocity. However, if power is available, and suit able duct construction and adequate sound absorbing facilities are provided, high velocity systems may be used. BIBLIOGRAPHY Rothray Passenger Cart Report on Performance and Cost of Operation of 1937 In ternal Combustion Engine Mechanical Compression Equipment for Air Conditioning Railroad Passenger Cars (Division of Equipment Research, Association of American Railroads, May 1, 1937). Report on Relative Performance of Air Filters (Mechanical Division, Association of American Railroads, January 15, 1938). L. W. Wallace and G. G. Early, Jr.: Air conditioning of railroad passenger cars (ASME Transactions, November 1937). Kenneth Cartwright: Passenger car cooling methods (Refrigeratmg Engineering, February, p. 83; March, p. 158, 1936). J. R. Homaday: Diesel drive for passenger air conditioning (Refrigerating Engineering, March 1942, p. 139). G. T. Wilson: Railroad air conditioning {Refrigerating En gineering, May 1943, p. 323). M. R. Eastin: Railway air conditioning (Railway Elec. Engr, August-December 1942). F. L. R*Mmann and E. M. Bill: Head-end power for railway cars (Railway Elec. Engr^ May 1939). J. D. Loftis: Head-End Power for Streamlined Passenger Trains (ASME Raleigh Section, October 26,1946). P.C.C. Car Ventilation (Westinghouse Electric Corp., B-3697, September 1946). Sum* and Automobiles E. T. Todd and F. O. Gadd: Bus beating (Heating and Ventilating, December 1946, p. 83). L. W. Child: Air conditioning of automobiles and buses (Society of Automotive Engineers Journal, June 1938). Jerry Hicke: Bus air conditioning (Heating, Piping and Air Conditioning, October 1938, p. 639). A. J. Mallinckrodt aTM! Lars Hanson: Bus air conditioning (Refrigerating Engineering, June 1939, p. 388). O. G. Tinkey: What has been done in auto air conditioning (Refrigerating Engineering, January 1953, p. 31). M. W. Baker, D. C. McCoy, H. V. Joyce, and P. J. Kent: Cars that beat the heat (Society of Automotive Engineers Journal, July 1953, p. 19). P. J. Kent: Automobile air conditioning--progress and prob lems (ASHVE Transactions, VoL 60,1954, p. 37). M. W. Baker and D. C. McCoy: Passenger automobiles (ASRE Air Conditioning Refrigerating Data Book, 1954-55, Chapter 50). Airplanes D. W. Tomlinson: Comfort in high altitude flying (ASHVE Transactions, Vol. 47, 1941, p. 57). A. J. Hess: Heat exchangers for .aircraft (Refrigerating En gineering, September 1944, p. 192). B. M. Brod: Heating and ventilating for transport airplanes (ASHVE Transactions, VoL 52, 1946). A. A. Amhym: Comfortization of Aircraft (Pitman Publish ing Corp., New York, 1945). B. L. Messinger: Refrigeration for air conditioning pres surized transport aircraft (Heating and Ventilating, January 1946, p. 63). Ventilation and air conditioning of the S. S. Panama (Heating and Ventilating, September 1939, p. 47). Air conditioning the new Mauretania (Heating, Piping and Air Conditioning, July 1939, p. 431). J. H. Clarke: Heating, ventilating and air conditioning on shipboard (Healing, Pipmg and Air Conditioning, August, p. 467; September, p. 529; October, p. 610, 1940). O. D. Colvin, W. H. E. Hahne, M. R. Colby: Care of cargo at sea (Society of Naval Architects and Marine Engineers Transactions, Part I, Vol. 46, 1938, p. 109; Part II, Vol. 49,1941, p- 208). Modem Marine Engineers Manual (Cornell Maritime Press, Cambridge, Maryland, 1943, Vol. II, Sections 16-19). Comdr. T. H. Urdahl, USNR^ and W. C. Whittlesey: War ship ventilating, heating and air conditioning (ASHVE Trans actions, Voi. 49, 1943, p. 35). Comdr. R. H. Urdahl, USNRn and W. C. Whittlesey: DeU-S.N.R.: Standardized heating and ventilating equipment for fighting ships (Heating, Piping and Air Conditioning, July 1943, p. 333). Comdr. R. H. Urdhal, USNR, and W. C. Whittlesey: De signing warship ventilation with standardised equipment (Heat ing, Piping and Air Conditioning, August 1943, p. 419). J. W. Markert: Modem air conditioning (Marine Engineer ing and Shipping Review, November 1945; p. 177). W. H. Carrier and L. E. Starr: Modem marine refrigeration and air conditioning (Marine Engineering and Shipping Re view, April 1946, p. 132). Capt. T. H. UrdahJ and Comdr. E. R. Queer: Dehumidifica tion protects U. S. Navy's inactive fleet (Heating, Piping and Air Conditioning, March 1946, p. 71). Robert Tate: Reconversion of Liner S. 8. LurUne (Society of Naval Architects and Marine Engineers, May 12, 1949). J. W. Markert: Air conditioning of P-2 American President liners (Pacific Marine Review, August 1946). J. W. Markert: Export lines air conditioning of four aces (Marine Engineering, March 1949). CHAPTER 49 SNOW MELTING Design: Heating Requirement, Hydraulic Requirement, Installation: Safety, Internal Corrosion, S/ab Construction, Thermo! Stresses, Control, Testing, Draining, Drifting Snow, Design Example THE practicability of melting snow by means of heated the effects of these four factors, it is necessary to consider the coils has been demonstrated in a large number of instal insulating effect of the snow before it is melted. As stated lations in sidewalks, roadways, ramps, and runways. In ad previously, the first flakes fall on a dry, warm surface, and dition to eliminating the need for snow removal, other ad are then warmed to 32 F and melted. During the time that vantages gained are greater safety to pedestrians and vehicles, the flakes are being warmed, and before they are completely and reduction of labor in removal of slush from floors. melted, they can be considered as tiny blankets or insulators. The design of a snow-melting system must determine and The effect of this insulation has been measured1 and can be satisfy two primary requirements: (1) the heating require of considerable magnitude. Since the snowflakes cover a frac ment, and (2) the hydraulic requirement. Each of these is tion of the surface area, it has been convenient to think of treated in this chapter. Several points regarding installation the insulating effect as an area ratio. The area covered by the practices are also discussed. snowflakes is the insulated area, and the uncovered area is the DESIGN uninsulated area. The term free area ratio Ar has been adopted to represent the ratio of the uncovered, or free, area to the Heating Requirement total area or, The heating requirement for snow malting is affected by four atmospheric factors: (1) rate of snowfall, (2) air temper ature, (3) wind velocity, and (4) humidity. The effects of these factors can be evaluated by consideration of the action of snow falling on a warmed surface. The first flakes fall on a dry, warm surface, nH are then wanned to 32 F and melted. The water from the melted snow -soon forms a him over the entire area and starts to evapo^ rate. The evaporation of the film is a mass transfer from the surface to the atmosphere. In addition, there is a heat transfer from the film to the ambient air and surfaces. Mass transfer due to evaporation. The rate of evaporation of the melted snow from the snow-melting is affected by the wind speed and the vapor-pressure difference between the air and the melted snow. The vapor pressure of air, however, is fixed by the relative humidity and the temperature of the air. If the slab surface temperature is fixed, then the evapora tion loss varies with changes in air temperature, relative humidity, and wind Bpeed. Heat transfer by convection and radiation from the melted snow to the ambient air and surfaces. A combined filni coefficient has been used with sufficient accuracy to determine the com bined convection and radiation loss. This coefficient is based on a heat transfer from a wetted surface, such as the film of melted snow, to air. The coefficient is a function of wind speed alone. The heat transfer, of course, is dependent upon the film coefficient and the temperature difference between the surface and the air. Since the surface temperature is fixed, the convection and radiation losses vary with changes in air temperature and wind speed. To determine evaporation and heat transfer from the melted snow to tire air it is necessary to know three of the four climatic factors: (1) wind speed, (2) air temperature, and (3) relative humidity, and (4) rate of snowfall. The fourth factor, rate of snowfall, is necessary to determine the heat required to warm the snow to 32 F, and to melt it. Before deriving equations to give quantitative values for hence At = Free area ratio. Af = Free area, aq ft. A, e* Total area, sq ft. 0 < A, < 1 For Ar -- l, the system would have to melt the snow so rapidly that the snow accumulation would be absolutely sere. This is impossible from a theoretical standpoint, but for all practical purposes it is permissible to have Ar = 1 as a max imum. For Ar -- 0, the surface would be completely covered with snow to a depth sufficient to completely prevent evapo ration and heat transfer kisses. Research on the insulating effects of snow indicate that there are just three practical values for the free area ratio, 0,0.5, and 1. This will be covered in greater detail in a later section. The equations for the heating requirements of a snow melting system have been derived and thoroughly explained in Reference 2. The general equation for slab output, q., as derived and discussed thoroughly in Reference 1, is ft - ft + $ + A,(q, + ft) (2) where q, " sensible beat transferred to snow, Btu per (hour) (square foot). ft> * heat of fusion, Btu per (hour) (square foot). Ar * ratio of snow-free area to total area, dimensionless. q, -- heat of evaporation, Btu per (hour) (square foot), ft " heat transfer by convection and radiation, Btu per (hour) (square foot). 669