Document 2jGayMjzJeojoOgD4nNw3ZNY5

492 (CHAPTER 25 .... 1948 Guide depending on the.application.-. (See Chapter-43.) Since required ratios may demand wide variations in air velocities; refrigerant1 temperatures, and coil depth, general rules as to their 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 ordinarily, vary between 40 and 50 F where, cooling, is accompanied by dehumidification. Water velocities range from 2 to about 6 fps. * When no dehumidification is desired, for which condition the dew-point . of .the entering air is equal to or lower than the cooling coil surface tem: perature, the: coil selection is made on the basis'of dry-bulb temperatures and sensible heat `transfer .only, the same as with heating coils. It is possible also to choose . various; arrangements. of facearea, depth, air velocity, etc., for the same duty. - : .... Dehumidifying Coils The selection of coils for combined cooling and dehumidifying duty is more; involved; than, for heating or sensible, cooling . and requires con- Table 4. Various Cooling Coil Arrangements Selection 1 2 r` 1 - .- - 4 Total cooling capacity, tons__ -- 100 Sensible cooline capacity^ tons... 69 Latent cooling capacity,- tons.;.-- 31 Ratio total to sensible heat-...... 1.45 Air quantity, cfm.__............ ......... 47,800 -Cfm per total ton-i -.......... ......... : 478 Face velocity, fpm......................... 325 Resistance, in. water..................... 0.11 Coil face area, sq ft....................... 147 Coil rows deep.________________ 4 Coil evaporator temp. F deg...... 45 100 . 100 100 ; 69 ; . .69 " ; ' 69 . 31 ... 31 ; - 31 1.45 1.45 : 1.45 41,700 37,100 46,800 417 - 371 ! 468 423 goo ; -. 600 0.27 : . 0.51. . 0.37 99.0 74.2 . -78.1 6 8' - 4 45 r 45 - 38 sideration of both dry- and wet-bulb air temperatures. (See Chapter 7.) (t 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 coilto 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 performance. .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 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 caii beattained but 1 Radiators, Convectors, Coils . ,:V - __________________________________ 493 ' 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 aii excess of -cbil results in an increase in total'capacity, but not a proportional gain in latent heat capacity; On installations controlled from dry-bulb tem- Table 5. Capacity Balances for- Maximum and Minimum Load Conditions Capacity in Tons Total Sen^ble' Latent ; .= * RAX,a- . .. - ^Sensible Required'at.peakload: conditions:. 10.90 Required at minimum load <nditions,,...J_.._. '6.62 Peak load equipment balance............ .10.90 Same equipinent balanced at minimum load 9.85 -Same equipment-balanced at maximum loakl conditions with 40 per cent by-pass_______ 8.38 Same equipment balanced at minimum load conditions with 38,800 Btu per hour reheat 6.62 7;90 3.36 3.00 3.26 - 1.38 : 1.98 -7.90-:; . 3.00 .: .. 1.38 6.58 3.26 1.50 5.05 3.33 1.66 3.36 3.26 1:98 . perature the operating time is shortened because of:.the addbti 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. disadvantage except that it results in a load from outside air greater than calculated, as well as in increased power consumption... If oversize equipment, is furnished, a balance should be made to assure thatthe ratio of totai tb'sen'sible 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 cooilng .coil. As shown in Table 4, the cbil 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 5. 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 18. Care should be taken to avoid freezing at light loads. In general, freezing, occurs when the coil surface temperature falls to 32 F. With