Document 6BmKEeYka2Mg3gD5NrvbY9kEo

482 '_________ Chapter 25 . 1945 Guide temperature as previously described, and then using the ; difference between the external film temperature and the refrigerant for. .evaluating hi in Equation 15. ' ' = -T~: 9-------- ~R (<s - h) V ~_. ' " (15) The term {U -- tr) is commonly written Af. The usefulness of the fore going equation is impaired by the fact that both hT and At must be evalu- Heat Transfer Surface Coils . 483 where ... Aw = internal film coefficient of heat transfer,. Btii per (hour) (square foot of internal tube surface) (degreeFahrenheit). V = water velocity, feet per second. D -- internal diameter of tube, inches. t = average water temperature, degrees Fahrenheit. ; In the case of finned tubes, values of h,, may be lower than those obtained by use of Equation 17. Accurate results can be obtained by. using Equation 15, if the logarithmic mean temperature difference between surface and water is used in place of Af. When saturated steam is condensed in the tubes of coils, the film Fig. 14. Determination of Surface Temperature ated experimentally. More direct results can be obtained by ignoring hi and determining the.relation between Af and total coil capacity: At = <3 -- tT = mqn where ' n'and n -- constants determined by tests. (16). When water is used as a cooling medium in tubes, the rate of heat transfer is a function of water velocity, because this results in an increase in the number of contacts of the water molecules with the tube surface, per unit of time. Thus increased water velocity and reduced tube dia meter cause increased heat transfer. Heat transfer is also greater at higher temperatures of the water. The basic formula .for the film coefficient of heat transfer for flow of water is as follows: ' y o.8 Aw= 1-5 (i + 100) DO .2 (17) Fig. .15. Typical Curves Showing Relation Between Total Capacity and Temperature Difference for Refrigerants coefficient hi varies from 1000 to 2000, depending on freedom from air in the steam, and upon good drainage of'the tubes. The coefficient is fairly constant for a particular coil, giving values of Af that are directly propor tional to q. GRAPHICAL ANALYSIS OF COIL PERFORMANCE In testing coils, determination of surface temperatures is most im portant. A convenient way of determining surface temperatures is illustrated in Fig. 14. Test points A and B are made without'varying the wet-bulb temperature of entering air, the air velocity, the refrigerant temperature, and the total capacity of the coil. Only the dry-bulb and dew-point temperatures of the entering air are varied. A straight line is drawn between points A and B, and is extended to the ordinate of zero moisture removal, giving point C which represents the moisture content of saturated air that corresponds to the surface temperature. Points D and E are similarly plotted, the only, difference being that another