Document MJDmOw7QMDVJwz3X4RmK8GbXV

HEATING VENTILATINC AIR CONDITIONING GUIDE 1944 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 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. 'Rebeating by Means of Refrigerant Compressor Discharge Gas. by S. F. Nicoll (A.S.H.V.E. TransACTIONS, Vol. 47, 1941, p. 239) . 520 hi CHAPTER 27 lcfyuipnient Air 'Washers, Humidification with Air Washer, Apparatus for Direct Humidification, Air Dehumidification with Washers, Water Main Temperatures, Atmospheric Water Cooling Equipment, Design Wet-bulb Temperatures for Water Cooling, Cooling Ponds, Winter Freezing AIR humidification is effected by the vaporization of water and always requires heat from some source. This heat may be added to the water prior to the time Vaporization occurs or it may be secured by a transformation of sensible heat of the air being humidified to latent heat as the vapor is added to the air. The thermodynamics of the process are discussed in Chapter 1. The removal of moisture from air may or may not involve the removal of heat from die air-vapor mixture. With spray equipment dehumidification of air always necessitates the removal of heat. , AIR WASHERS Air washers may be used as either humidifiers or dehumidifiers de pending upon the method of operation and the temperature of the spray water. The functions of an air washer are to regulate the moisture and heat content of air passing through it and to remove dust and dirt from the air. Air washers are not as effective as air filters in the removal of dust and dirt. The construction of commercial air washers is indicated in Figs. 1 and 2. Any air washer consists essentially of a chamber through which the air passes in intimate contact with water. The lower portion of the washer chamber serves as a sump for the spray water. Contact between the air and the washer water is secured: (1) by breaking the water into a very fine mist, (2) by passing the air over surfaces which are continuously wetted by water, or (3) by a combination of water sprays and wetted plates. Scrubber-plate types of washers are used largely to wash heavy reclaimable products from the air, and are generally composed of one to three eliminator-type baffle scrubber plates across the air stream. Water is supplied at the tops of the scrubber plates by flooding nozzles placed across the top of the washer. Spray washers have one or more banks of water atomizing nozzles placed in the air stream above the level of the water in the sump. The direction of the water sprays may be against the air stream, with the air stream, or with 521