Document Rj9ga7pDppwYmwvjaR53yx0gz

4512 CHAPTER 36 1954 Guide cooling and dehumidification, the logarithmic difference does not apply strictly, and such problems should be handled as described in a later section oil Performance of DehUmidifying, Coils. With volatile refrigerants there is often an appreciable pressure drop and corresponding change in evaporat ing temperature through the refrigerant circuit. The problem is further complicated ;by the ;fact that the refrigerant is evaporating in part pf tpe circuit, and superheating in the. remainder.' I n spite'of `this, heat transfer and ratings :for coils using volatile refrigerants are usually ihased on'a refrigerant temperature corresponding to the average pressure in-the coil. The design and surface arrangement of the coil include,such items as materials, type, thickness, height and spacing of-the finis, and the ratio of this surface to that of the tube, the use of the staggered or in-line 'tube "arrangement, and provisions to increase the air, turbulence such, as the use `of comigated as against flat fins.' Staggered tubes`increase the;total heat 'tfahsfer, as against the in-line arrangement, and corrugated fibs may be more effective than flat. This design and surface arrangement has a large affect on the air film beat transfer resistance. r/' i 1! - ' !i Thie, velocity of the air usually considered is: the; coil face velocity.,t This .bears, & varied relaliontothe actual velocity over th.ei surface, depending upon the individual .coil design. ' As long, as a fixed design of coil is under con sideration face, velocities may be,used, hilt they may he unsatisfactory in comparing different designs, as itis the acttiai surface,velocity that is signifi cant. The air volume is often based on standard air at 70,F and,,a baro metric. pressure of 29.92 .in. Hg, . The use of .air volume .in. cm] ratipg information may be misleading. The significant value is mass .velocity.in. pounds per {minute) (square foot of face area) and not .cubic feet per minute, 'because for a .'fixed 'yoliime the corresponding weight may vary widely, ^depending upon the temperature and barometric pressure. ` ' ' .-..At the same mass air velocity, varying;-performance-can be obtained .depending upon the turbulence .of the air flow into the coil, and upon the . uniformity of 'distribution of air over the coil face. The latter is. very im portant in: obtaining,reliable test ratings, and in realizing rated performance ;in practical, installations. The resistance through the icoils will. assist m . distributing the air : properly, but where the. inlet, duct connections are brought in at sharp angles to the coil face, the effect-is frequently .bad and there may even be reverse air currents through the coils. This reduces the capacity, but can be avoided.by proper layout:or by the use ofidirecting baffles. ,. ;. Heat transfer depends1 also upon the velocity of the medium in the tubes and upon its character, whether flowing water, condensing steam or evapo rating volatile refrigerant. Heat transfer rates expressed as Btu per (square foot of internal surface) (degree logarithmic mean effective tem perature difference bfetweeh the fluid'and tube'wall) are; forexample: about .150 to 300 for evaporating dichlorodifluoromethane, about 350 to 1200 for water at 2 and 6 fps, and about 1200 for condensing steam.The.ihfluence of the medium in the tubes on the overall heat transfer rate is therefore apparent. Because of these variables, reliable rating and performance information for any design of coil must be. based on actual tests on that coil, under the expected conditions of operation. A comparison between the performance of two designs, unless based On such tests on each, may lead to-entirely erroneous conclusions. Details on coil calculation and performance follow- Afr Heating and Cooling Coils u;. >Wf 813 PERFORMANCE OF HEATING AND DRY COOLING COILS The performance ,of heating and dry cooling coils depends in general upon: ' : 1. The overall coefficient of heat transfer from the fluid'within the coil to the air it heats or cools. ..., . . ........... 2. The,mean,,temperature .difference between the fluid within the coil and the air flowing over the coil. ` ( L ., '3. The physical dimensions of the coil. ; Thus,-for any , one-definite operating condition, the heating or cooling capacity of > a-given 'coil is: expressed by the -following basic' formula: where ' ' '. qy = .u x (&C) k A x n :[ f (D .. qt = total heat transferred by the coil, Btu per. (hour) (square foot-of coil face . s;,area)... . ' - , :. {/ -- overall coefficient tjf; heat! transfer,- Btu: per (hour) (square, foot of external . coil, surface){(Fahrenheit; degree, temperature, difference between the fluid within the coil,and.the airiflowing over, the coil).; ; = mean temperature difference, Fahrenheit degrees, between the fluid within !the coil'and the:air,passing over.it. .(Thisris commonly taken as the loga- i rithmic mean temperature difference). .: ,A..= external surface area:of the'givenicoil; square feet,per (square foot of, coil '- face'area)'(row:ioficoil depth).' .!.>> N = number, of; rows'df,coil;'depth. - i ;.. - :-1! Overall Coefficient of Heat Transfer , Of all factors affecting the performance of heating or dry cooling coils, the overall coefficient of heat transfer is the most difficult to determine, as it is influenced by several.factors' which depend upon coil design and con ditions of operation.. Considering'any coil' whether of bare pipe or cif' finned type, the overall heat transfer coefficient for a given size and des'ign of cOil cad hlways be considered as a combined effect of three individual heat transfer coef ficients, namely: ... ......,,,,,, ^ucuiucub ui uransier Deiween air ana tne external surface of the coil, usually given in Btu per (hour) (square foot external surface) (Fahrenheit degree mean- temperature, difference). , ;: .. .1. etc2.. The coefficient of heat transfer through the coil material--tube wall, fins, ribs, `3: The film1 coefficient of; hedt'transfer between'the internal surface of the coil and the fluid flowing within'the,coil; usually-givehiin Btu per (hour) '(square foot internal surface),(Fahrenheit,degrqejmean temperature difference). ; . .... ., These three individual coefficients acting in series, result. in an overall coefficient of Kea^transfer in accordance with the basicjaws given in Chap ters 5 arid 9. For a bare pipe coil the overall coefficient of heat transfer, whether for heating or for-cooling (without, dehumidification), can be ex pressed by a .simplified basic formula as follows: