Document Qg5zEaZ0ObVabk10oXNZbx9o

490;Chapter 25 . 1945 Guide i -- - i' Table 2. Capacity Balances for Maximum and Minimum Load Conditions Conditions Required at peak load conditions.___ Required at minimum load conditions............ Peak load equipment balance.. __ . I__ 1 Same equipment balanced at minimum load conditions____ _______ ,, . __ .. Same equipment balanced at maximum load conditions with 40 per cent by-pass.___ Same equipment balanced at minimum load conditions with 38,800 Btu per hour reheat Capacity in Tons Total 10.90 6.62 10.90 Sensible' ' Latent 7.90 3.36' 7.90 3.00 3.26 3.00 9.85 6.58 3.26 8.38 5.05 3.33 6.62 3.36 3.26 _ Ratio T^-o--t--a--l------ Sensible 1.38 . 1.98 1.38 .1.50 1.66 , 1.98 between the condensing unit and thp 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 required load and, due to miscalculated duct resistance 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 when the 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 will result in an increase in total capacity, but not a proportional gain in latent heat capacity. On installations controlled from dry-bulb tem perature the operating time will be shortened because of the added sen sible cooling capacity. The result will be less moisture pick-up than calculated, and higher relative humidity. If an oversize condensing unit is installed the opposite situation will take place. The relative humidity will be lower than estimated. This is not generally a disadvantage except that it results in a greater load from outside air than calculated, as well as in increased power consumption. If oversize equipment is furnished, a balance should be made to assure that the ratio of total to sensible capa city 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 1, 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 tempera ture or other cause, the' condensing unit and coil rebalance. This may result in more sensible capacity than required at the light load condition and less latent in proportion, with an increased relative humidity, in the conditioned space. Such a condition is shown in Table 2. If approxi mately 40 per cent of the total air is by-passed, the condition will be improved as indicated. The situation could 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 humidity7. Care should be taken to avoid freezing at light loads. In general, freezing occurs when the coil surface temperature falls to 32 F. With usual coils for comfort installations, this will not occur unless the evapo- 'Reheating by Means of Refrigerant Compressor Discharge Gas. by S. F. NIcoU (A.S.H.V.E. Trans* actions. Vol. 47.1941. p. 239). . Heat-Transfer'Surface Coils 491' rating temperature at the coil outlet is about 20 to 25 F. The exact value depends on the design of coil and the amount of loading. Although it is not customary to choose coil and condensing units to balance at low tem peratures. at peak loads, there is 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 light loads. It may be even worse if the control is arranged for decrease of inside temperature with fall of that outside. Freezing 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. I