Document v1jzB05Bk1DnLJJBoOnyrxVdw

156 ' CHAPTER 7 . 1949 GuideY fluid is fairly constant, being affected principally by pressure drop through the tubes, by superheat of the evaporated refrigerant, and by the presence of oil in solution. To obtain maximum coil capacity it is necessaiy to keep the pressure drop through the tubes at a minimum, to keep the superheat as low as possible without carrying liquid back to the compressor, and to arrange for good separation and return of oil to the compressor. Another important factor is the removal of gas to keep the tube surface flooded with liquids as much as possible. The internal film coefficient is markedly in creased by heavy heat loads, because the. increased turbulence and gas velocity cause good contact of the liquid with the tubes. . Values of Ar usually lie between 150 and 450. For rating of dehumidifying coils, satis factory results are obtainable by first determining the average external sur face temperature from Equation 7, and then using the difference between the external film temperature and the refrigerant for evaluating A* in Equation 91 A,= ATN(,t.-,r,) (9) where hT = internal film coefficient of heat transfer, Btu per (hour) (square foot of. internal tube surface) (Fahrenheit degree). 1, = average refrigerant temperature, Fahrenheit degrees. The term ft, -- t,) is commonly written At. To evaluate h, by this method the same tests that were required to determine K. may be used. When water is the cooling medium in tubes, the rate of heat transfer is a function of its velocity, which influences the number of contacts of the water molecules with the tube surface, per unit of time. Increased water, velocity and reduced tube diameter 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 in smooth tubes is as follows: hr = 1.5(1 + 100) -- (10) where V t= water velocity, feet per second. D = internal diameter of tube, inclies. ' t = average water temperature, Fahrenheit degrees. Equation 10 should not be used when Reynolds Number is less than 2000. Since, in the case of finned tubes using water as a refrigerant, test values of At based on the calculated surface temperature for the entire coil may be lower than those obtained by use of Equation 10, actual test results are preferred if available. When saturated steam is condensed in the tubes'of coils, the film coef: ficient At varies from 1000 to 2000, depending on freedom from air in tpe steam, and upon good drainage of the tubes. The coefficient is fairly con stant for a particular coil, giving values of At that are directly proportional to <?t- However, if water, coil test results are analyzed on a row-by-row basis good agreement with Equation 10 will result.' Mr Heatijjg and CoolmgJCoils 1 157 tubesat tlie expense of.pressure drop. . The inctrheeasvbalue nf * V--r "q,uid in the type of turbulence promoter and the rate^flowYYdepends uPon meat can be made regarding their use and it is besf'fe , state' papers on this subject for further information w t0 re!er detailed Determining Size of Cooling Coil To illustrate the use of individual film coefficient , tehxeitparoircceodnudreitfioonr,sceoleilcstiunrgfathcee tpermoppeerrastizuerecotooltiangl ccooiill Wi" ftor determining temperature is outlined in Exampl^l" 0011 load and refrigerant y )4-- 1NTGBN6 H CONA */. cJ\ TION > yoV/ -*> ESDIHQfL T \ \ \A \ X X.KIR GQN tttOM L vwo < ' *\ X7 \ \\\\ A Y v\ \ Y 7 > e 913 tO Fig. 2. Psychbometbic Layout for Coil Selection - Example t. An industrial application requires.the coolingof a certain quantity.of air from a condition of 102 F dry-bulb and 85 F wet-bulb to a final condition of 80.5 F dry-bulb and 73 F wet-bulb. The air velocity across the coil is to be 400 fpm and coil data are as follows: A* = 10.7 at 400 fpm, hr = 325, external surface area: = 15 s'q.ft; per (square foot of face) (row of coil depth), ratio of external surface area to internal surface area =15. .. i1 Solution. (1) Lay out the problem p$ychrometry as indicated in Fig. 2 and note, that the minimum horizontal aistance between the load ratio line and the saturation: curve is 1.8 F dry-bulb at.point A Fig. 2. This means that t* -- 1dpi in Equation 7 must not be less than 1.8. Therefore, Equation 7 should be solved for N to deter mine the proper number of rows to be used for the coil. : hiAN-\ .-U '(dpl = log. 102. ,, - = log. 12.22 = 2.5 . 0.243ft.= log. -- ldp! 1.8 Then substituting values for A., A, and G, N may be found as follows: = 2.5 from whioh, N = 6.58 0.243 X 1740 R-