Document xzMRqjxaJ5DVBxrm4g5oQM38b

542 CHAPTER 25 1949 Guide fication required is also highly Variable.' The methods outlined previously under Heat Transfer and Resistance may be used to determine whether it is possible for a coil to perform the duty required. If entering and leaving air conditions are arbitrarily specified, the corresponding duty sometimes cannot be obtained at all without the use'of reheat. As with heating and sensible cooling coils, there are combinations of face areas, depth",, air velocity and refrigerant temperatures which will give the required perform ance. This is illustrated in Table 4. It is possible as shown in Table. 4 to perform approximately the same duty at a given refrigerant temperature with small face area and large thickness or vice versa. The large face area coil gives low air velocity and resistance but high air quantities per ton. The coil of-small face area and great depth requires small air quantities per ton of refrigeration,, high resistance and high air velocities. As shown also in Table 4 the same sensi ble, latent and total cooling capacity may be obtained with various refriger ant temperatures by proper choice of coil. This makes it possible to' keep Table 5. Capacity Balances fob Maximum and Minimum Load Conditions Conditions Capacity in Tons Total Total ................ Sensible Sensible . Latest - Required at peak load conditions..^_________ Required at minimum load conditions______ Peak load equipment balance. Same equipment balanced at minimum load Same equipment balanced at maximum load conditions with 40 per cent by-pass_____ J Same equipment balanced at minimum load conditions with 38,800 Btu per hour reheat 10.90 6.62 10.90 9.85 8.38 6.62 .7.90 3.36' 7.90 6.58 5.05 3.36 . 3.00 3.26 3.00 3.26 3.33 3.26 :: i.38 ' ' 1.98 4 1.38 ' 1.50 1.66 . 1.98 ; ' the evaporating temperature high enough to cany the load with a chosen size of condensing unit. High evaporating temperatures with correspondr ingly small compressor operating expanse can be attained but at the expense of coil surface, air quantity or both. The .choice will bedeterrniried by the1 necessities of individual installations. ... Fpr a given.quantity and condition of entering air the evaporating tem perature, of a volatile refrigerant coil is determined by a balance, between the condensing unit and the coil.. ,,The: total, sensible;and latent cooling capacity can then be determined "from the coil rating information, -if the condensing unit and cooling coil have been properly balanced for the re quired load and, due.tp miscalculated ductResistance or improper choice of fan speed, the air quantity is reduced, the total cooling capacity will also be reduced. The decrease is generally in the sensible capacity, .This is the effect'also ivhenthe air by-pass or volume, control is used^' '\ It is necessary that not only the total capacity but also the sensible, and latent cooling requirements both be met. The installation of an excess of coil results in an increase in total capacity, but not a proportipnal.gain in latent heat capacity. On installations controlled from d^-bulb tempera ture the'operating time, is shortened because of the added sensible cooling capacity. This results in less moisture pick-up and higher' relative humid ity .than calculated. If an oversize condensing unit is installed, the oppo site situation occurs. Generally,' this is not a disadvantage except that it Radiators,'.Convectors, Coils 543 results in-a load from outside air greater than calculated, as well as in in creased power consumption. If oversize equipment is furnished; a' balance should be made to assure that the ratio'of total to sensible capacity is the same as in the estimated load. .' Sometimes arbitrary air quantities are specified for ventilation or other reasons independent of the selection, of the cooling coil: As shown in Table .4, the coil selection can be altered to take care of various air quantities for the same duty. Where coil and condensing unit are selected for the peak load condition, and. the sensible.load partially disappears due to fall of outside temperature, or other cause, the condensing unit and'coil rebalance. This may result in more sensible and less latent capacity than required at the light load' condition; with an increased relative humidity in .the conditioned space,. Such a condition is'shown in Table 5. If approximately 40 per cent of the total, air is by-passed,.the condition is improved as indicated.. The situa tion may be. entirely, avoided by using reheat, where it is possible to handle any ratio of sensible and latent loads and maintain the design temperature and humidity18. Care should be taken to avoid freezing at, light loads.' In general;.freez ing occurs when the coil surface temperature falls to 32 F.. With usual coils for comfort' installations; this does not. occur unless the, evaporating tem perature at the coil outlet,is about.20 to 25 F. The exact'value'depends on the design of the coil-and the amount of-loading. ' Although it is not'. customary to choose coil; and condensing units to balance at low tempera-' tures at peak loads, there ih danger of this occurring when the load decreases. This is further aggravated if a by-pass is used so that less air is passed through the coil at fight loads. It may be even worse if the control' is arranged for decrease of inside temperature with fall of that outside. Freez ing can be avoided by making the full load balance a high evaporating temperature and checking the balance at the minimum load. Care should be exercised in the design of humidity control to minimize the cycling of the refrigerating compressor because of re-evaporation of moisture from the fins. It is.sometimes necessary to by-pass air around a coil when the compressor is not operating. REFERENCES 1 A.S.H.V.E. Code for Testing Radiators (A.S.H.V.E. Transactions, Vol. 33, 1927, p. 18). ' A.S.H.V.E. Standard Code for Testing and Rating Concealed Gravity Type Radiation (Steam), (A.S.H.V.E. Transactions, Vol. 37,1931, p. 367); (Hot Water),'' (A.S.H.V.E. Transactions, Vol. 39, 1933, p. 237). (See also A.S.H.V.E. Trans actions, Vol. 41,1935, p. 38, and Vol. 42,1936, p. 29). * A.S.H.V.E. Research Report No. 998--Factors Affecting the Heat Output of Convectors, by A. P. Kratz, M.'K. Fahnestock, and E. L. Broderick (A.S.H.V.E. Transactions, Vol. 40,1934, p. 443). 4 Factors Influencing the Heat Output of Radiators, by A. C. Davis, W. M. Sawdon and David Dropkin (A.S.H.V.E. Transactions, Vol. 42, 1942, p. 185) and Cornell University, Engineering Experiment Station Bulletin No. 29, April, 1942. 4 Heat Emission from Radiators, by K. F. Rubert (Cornell University, Engineering, Experiment Station Bulletin No. 24, 1937). Comparative Tests of Radiator Finishes, by W. H. Severns (A.S.H.V.E. Trans actions, Vol. 33,1927, p. 41).