Document 2R15BBE32o53DbGORGa6gpwXa

American Society of Heating and Ventilating Engineers Guide, 1935 llisr- 3 What factors determine the dew point of the air entering the space? The maximum dew point desired in the conditioned space, and the moisture gain j,, space per unit weight of air supplied. 4 Why must the air leaving a dehumidifying type air washer he reheai,j| before delivery? ,1 The air leaves the dehumidifying air washer saturated at a relatively low temperati| which in most cases is lower than the allowable delivery dry-bulb temperature. 3SI the air may possibly be carrying a small amount of entrained water which might seSI out in the ducts near the washer and cause corrosion difficulties. 5 What methods are used for reheating air? 1. Passing it over reheating coils. 2. Mixing it with by-passed air at a higher temperature. 6 What determines the final temperature of the spray water in a deliuniidi^| Because of the effectiveness of the heat transfer between air and finely divided water in a well designed dehumidifier, the air will be cooled to within 1 or 2 F of the i^l water temperature, provided the air velocity through the washer does not exceed 600fp " This final temperature should then be taken as 1 or 2 F lower than the required f point of the air leaving the washer. 7 What arc the advantages of using counter flow of air and water in surf* coolers? Counter flow results in a higher mean temperature difference than does parallel flovjL the same range of air and water temperatures, which means that less cooling surfactH required. Counter flow permits higher initial water temperatures and also allowa greater temperature rise tor the water. These factors combine to reduce the costf circulating and refrigerating the cooling water. 8 What factors other than cost should be considered in determining whetla to use a central system or another type? a. Appearance: The equipment must be designed to harmonize with the architei of the building. b. Distribution: The system must maintain adequate and uniform air motion overt! entire conditioned space. c. Control: The control system must be designed to give effective partial load operat; 9 Can the central cooling and dehumidifying system be used as an all-yai round conditioner? By modifying the control system and adding blast coils or a water heater to the sp type system, the cooling system will function as one for heating and humidifying. T surface cooling type may be transformed by modifying the control, and adding anot set of coils and a humidifier. 10 Will the tons of refrigeration-effect per day be the value calculatedj Example 4 of this chapter times the hours of operation? No. The tons of refrigeration-effect are functions of the load. The components old load vary, that is, the number of people occupying the space, the outdoor conditions, the solar radiation will change from hour to'hour and from day to day. The calcnl load represents the maximum required for design peak conditions. 11 Will the quantity of return air required in Example 4 of this chapWg used all season? No. When the outdoor wet-bulb temperature becomes lower than the maintained^ bulb temperature, it is more economical to use all outside air than to dehumioiiTg return air. 164 Chapter 10 COOLING METHODS Methods of Cooling Air, Evaporative Cooling, Dehumidification, Silica Gel System, Alumina System, Design of System, Operating Methods. Steam Jet System, Compressors, Refrigerants, Methods of Cooling, Condensers BY using any of the following four methods, or any combination of them, effective temperature (see Chapter 2) may be reduced. o Sensible cooling: Lowering of the dry-bulb temperature by the removal of sensible bat'''without change of the dew-point temperature. - y* Dehumidifying: Lowering of the dew-point temperature by the removal of.mois ture without change of the dry-bulb temperature. c:'.Evaporative cooling: Lowering of the dry-bulb temperature through the evapor ation of moisture without the addition or the subtraction of heat. i-Air motion: Increasing the air motion over the body with the resulting higher evaporation from the skin. As an example, let the condition be considered of 92 F dry-bulb, with a 40 per cent relative humidity, corresponding to a wet-bulb temperature of 72.8 F, and an effective temperature for still air of 81.1 F. This effective temperature may be reduced 3.1 F by any of the four basic methods mentioned, as follows: First, by lowering the dry-bulb temperature to 85.5 F without changing the dew-point of 64.2; this gives an effective temperature of 78 F. ^Second, by reducing the moisture content of the air to 46 grains per pound of dry air .-.without changing the dry-bulb temperature; this gives an effective temperature of 78 F. 'Third, by reducing the dry-bulb temperature to 83.8 F without changing the total heal of the air. This requires the evaporation of 14 grains of moisture per pound of dry - air, and the effective temperature will become 78 F. ::Fourth, by increasing the air movement from still air to 460 fpm, a velocity which will 1 reduce.the effective temperature 3.1 F from 81.1 F to 78 F. Method to Employ -. best method of reducing the effective temperature in any specific depend on the accompanying circumstances and can be deter- .mined only by a thorough analysis made by a competent engineer, 'jenerally speaking, the removal from the air of the sensible heat, or !59Jffi1.re> pr both, by sensible cooling or dehumidifying is the most sa factory method. Adequate results by the utilization of air motion or rioLi^POratiVe coolin are difficult to obtain because of the dependence -auV' ,me*"ds upon climatic conditions beyond the engineers'control r.'ii!1" these methods are much less expensive than the first two 165