Document dQ89BeDG0MXYEkER0KrXZZnN5

162 CHAPTER 7 1946 Guide correspondingly small compressor operating expense can be attained but at the expense of coil surface, air quantity or both. The choice will be determined by the necessities of. individual installations. For a given quantity and condition of entering air the evaporating temperature 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 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 results in an increase in total capacity, but not a proportional gainin latent heat capacity. On installations controlled from dry-bulb tem- 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._______________ 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 . Sensible 10.90 6.62 10.90 7.90 3.36 7.90 Latent 3.00 3.26 3.00 9.85 6.58 3.26 8.38 .5.05 3.33 6.62 3.36 3.26 _ Total Ratio =----------------- Sensible 1.38 1.98 > 1.38 1.50 1-.66 1.98 . . perature the operating time is shortened because of the added sensible cooling capacity. This results in less moisture pick-up and higher relative humidity than calculated. If an oversize condensing unit is installed, the opposite situation occurs. Generally, this is not a disad vantage except that it results in a load from outside air greater than; calculated, as well as in.increased power consumption: If oversize equip ment 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 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 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 2. If approximately 40 per cent of the total air is by-passed, the condition is improved as indicated. The situation 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 humidity *. Care should be taken to avoid freezing at light loads. In-general; Heat'Transfer Surface Coils -r,. ,, ^ / '.163' freezing occurs, when the. coil surface temperature falls ..to-32 F. With usual coils for comfort installations, this does not occur unless the evapo rating temperature 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 temperatures 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 Fig. i4. Psychrometric Layout for Coil Selection of moisture from the fins.. It is sometimes necessary to by-pass air around a coil when the compressor is not operating. Determining Size of Cooling Coil The procedure for selecting the proper size of coil for cooling and'for determining exit air condition, coil surface temperature, total coil load and refrigerant temperature is illustrated in Example, 1/ Example 1. An industrial application requires the cooling of 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;. Aa = 10.7 at 400 fpm, ht - 325, external surface area^ = 15 sq ft per (square foot of face) (row of coil depth), ratio of external surface .area'to internal surface area = 15. Solution. (1) Lay out the problem psychrometry as indicated in Fig. 14 and note that the minimum horizontal distance'between the load ratio line and the saturation* curve is 1.8 F dry-bulb at point A Fig. 14.- This means that. It -- tdm in Equation 7 must not be less than 1.8. .-Therefore, Equation 7 should be solved for N to'determinethe proper number of rows-to be used for the coil.