Document 2RaaBjgKdDDdoDyxwbJzawNo7

HEATING VENTILATING AIR CONDITIONING GUIDE. 1940 ments of the same design of coil depending upon the relative importance of space, cross-sectional area, and air resistance. Table 2 shows an example. Cooling Coils The usual range of ratings for cooling and dehumidifying coils are enumerated herewith: ; . Entering Air Dry-Bulb--60 tp 100 F. .Entering Air Wet-Bulb--50 to 80 F. I Air Face Velocities--300 to 800 fpm, (sometimes as low as 200 and as high as 1200). Volatile Refrigerant Temperatures--25 to 55 F, at coil suction outlet. Water Temperatures--40 to 65 F. Water Quantities--2 to 6 gpm per ton, or equivalent to a water temperature range of from 4 to 12 F. Water Velocity--2 to 6 fps. . The ratio of total to sensible heat removed varies in practice from 1.00 to about 1.65, i.e., sensible heat is from 60 to 100 per cent of total, depending on the application. (See Chapter 21, Table 1). Required ratios may demand wide variations in air velocities, refrigerant tempera tures, and coil depth, so that general rules as to these values may be misleading. On usual comfort installations air face velocities between 400 and 600 fpm are frequent, 500 being a common value. Refrigerant temperatures will ordinarily vary between 40 and 50 F where cooling is accompanied with dehumidification. Water velocities will range from 2 to about 6 fps. When no dehumidification is desired, for which condition the dew-point of the entering air will be equal to or lower than the cooling coil tempera ture, the coil selection is made on the basis of dry-bulb temperatures and sensible heat transfers only, the same as with heating coils. It is possible also to choose various arrangements of face area, depth, air velocity, etc., for the same duty, as illustrated in Table 2 for a steam coil. Dehumidifying Coils The selection of coils for combined cooling and dehumidifying duty is more involved than for heating or sensible cooling and requires con sideration of both dry- and wet-bulb air temperatures. It is further complicated by the fact that the proportional amount of dehumidification 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 per formance.- This is illustrated in Table 3. J It is possible as shown in Table 3 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 will give low air velocity and resistance but high air quantities per ton. The coil of small face area and great depth will require small air quantities per ton of refrigeration, 484 CHAPTER 25. HEAT TRANSFER SURFACE COILS Table 3. Various Cooling Coil Arrangements Selection 1 Total cooling capacity, tons __ 100 Sensible cooling capacity, tons... 69 Latent cooling capacity, tons...... 31 Ratio total to sensible heat......... 1.45 Air quantity, cfm............................ 47,800 Cfm per total ton........................... 478 Face velocity, fpm..... .................. 325 Resistance, in. water..................... 0.11 Coil face area, sq ft..... ........... . 147 Coil rows deep................................. 4 Coil evaporator temp, deg F....... 45 .! 2 100 69 31 1.45 41,700 417 423 0.27 99.0 6 45 3 100 69 31 1.45 37,100 371 500 0.51 74.2 8 45 4 100 69 31 1 45 46,800 468 600 0 37 78 1 4 38 high resistance and high air velocities. As shown also in Table 3 the same sensible, latent and total cooling capacity may be obtained with various refrigerant temperatures by proper choice of coil. This makes it possible to keep the evaporating temperature high enough to carry the load with a chosen size of condensing unit. High evaporating temperatures with 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 will be 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 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 Table 4. 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 10.90 6.62 10.90 Sensible 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 p,,TmT?TAL Sensible 1 38 1 98 1.38 1.50 1.66 1.98 485