Document 8RkNVL5a2yYr6va6Va7RGM43Z

844 CHAPTER 77 1962 Guide And Data Book ^ metric chart on which is illustrated a typical evaporative cooling problem. Calculations for this problem are given in Example I. Example 1: An evaporative cooling system is to be installed in the one-story office building shown in Fig. 2. Outdoor design conditions are assumed to be 95 F dry bulb and 65 F wet bulb. The ho*t gairtn which are to be used in the design of the cooling system are. All walls, roof and doors 78,500Btuh Glass area 5,060 Occupants lighting 17,000 62,700 Total sensible heat load Total latent load (occupants) 164,160 Btuh 21,250 Total beat load 185,421 Btuh Find the required air quantity, the temperature and humidity ratio of the air leaving the cooler (entering the office) and the temperature and humidity ratio of the air leaving the office. Solution: A temperature rise of 10F deg in the cooling air. is assumed. The air volume required to be supplied by the evapora tive cooler may be found from Equation 1. <2~ g 1.08(4- O '0 164 230 1.08 X 10 15 200 cfm whore Qm TM required air quantity through equipment, cubic feet per minute. q, = instantaneous sensible heat load, Btu per hour. It => indoor air dry-bulb temperature, Fahrenheit. 4, " room supply air dry-bulb temperature, Fahrenheit. This air volume represents a 2.6 min air change for a building of this sise. The evaporative air cooler is assumed to have a satura tion effectiveness of 80 percent. This is the ratio of the reduction of the dry-bulb temperature to the wet-bulb depression of the entering air. The diy-bulb temperature of the air leaving the evaporative cooler is found from Equation 2, 1, n4 - - (ti - (2) = 95 - 0.8 (95 - 65) - 71 F inhere 4 =* dry-bulb temperature of the leaving air, Fahrenheit. 4 = dry-bulb temperature of the entering air, Fahrenheit, e* humidifying or saturating effectiveness, percent. t' ** thermodynamic wet-bulb temperature of the entering air, Fahrenheit. The humidity ratio of the cooler discharge air, Wt is found Fig. 3 ... .Conditions Within Evaporative Cooler and Cooled Space for Example 1 from the psychrometric chart to be 0.01185 lb per pound dry air. The humidity ratio of the air leaving the space being cooled, Wt is found from Equation 3. + W, ' Q,, X 4840 (3) 21,250 + 0.01185 ' 15,200 X 4840 where * 0.01214 lb/lb dry air q. -- latent heat load, Btu per hour. The remaining values of wet-bulb temperature and relative humidity for the problem may be found from the psychrometric chart (see Fig. 1 for the values in this example). Fig. 3 illustrates the various relationships of the outdoor air, supply air to the space and the discharge air. RESIDENTIAL COOLING In many areas of the West and Southwest the natural tem perature of evaporation (wet-bulb) is sufficiently low so that air cooled to this temperature by the process of evaporation can be used directly for air conditioning of residences.4 The first step in designing a house cooling system is the siring of the' cooler. Although the most accurate method of determining cooler capacity requires making a heat gain calculation, the rime and effort involved in such a calculation cannot in most cases be economically justified in residential cooling. There are a number of rule-of-thumb methods that will give satisfactory results if reasonable judgment is used in applying them.4 One method that is commonly used is to divide the differ ence between the design dry-bulb and wet-bulb temperatures by 10 and consider this to be the number of minutes needed for each air change. If the heat gain is considered high because of unusually large unshaded glass areas or for other reasons, a faster air change should be used. Packaged air coolers are usually specified for residential work. Since the air flow rating of these coolers is calculated at sero external static pressure, the air delivered by the cooling system will be less than the cooler rating. Many times a house calling for a 3 min air change will require a cooler with a 2 min catalog raring. If performance data is not available from the cooler manufacturer, it is a good rule to assume that the cooler will not deliver more than 70 percent of the free air or catalog raring. In designing residential duct systems, the equal friction method or the velocity reduction method u used. Common velocities in such systems range from 1200 to 1400 fpm for trunk ducts and large branches to 800 through 1000 fpm for small branches and grilles. A consideration that is often overlooked in designing the system is the need for exhaust. If the air is not removed as rapidly as it is introduced, air pressure will build up to a point where there will be ao ;VgyaporaHve Air Cooling ^"almost complete stoppage of air flow. There will also be a - ` ooticeable increase in relative humidity. While windows and 'y0oTs located at distant points may be used, a better method to exhaust the air to the attic, thus forcing out the hot air in .,'tfie'attic. This reduces attic temperature and improves the 'cooing conditions in the house. An attic fan can be used to se- --cure optimum exhaust : . ^Evaporative cooling is often combined with forced air heat' - -fag* fig. 4 illustrates a typical combination system. Since the cooler usually handles at least five times as much air as is re quired on the heating cycle, the ducts and registers must be 'ssed.to fit the cooling requirements. This results in low register velocities on the heating cycle. However, the areas where residential evaporative cooling is widely used are char'acterued by mild weather, and register velocities are not as ' critical as in cold winter areas. If it should be necessary or ad visable to have higher register velocities and a better air dis tribution balance than would normally prevail on the heat cycle in a combination system, registers are available with bladesso arranged that all of the register may be used for cool ing,-while only V6 or of the register is used for heating.7 Registers for evaporative cooling systems should have the blades set on at least J4-in. centers and preferably on 1-in. centers to minimize resistance to air flow. The use of combi nation systems may cause problems of furnace corrosion unless the heat exchange element of the furnace is made of stainless steel or coated with porcelain enameL This problem can be overcome by locating the cooler and the furnace in the system so that they may be isolated from each other. Manual slide dampers or motor operated dampers should be installed in the trunk duct at the cooler outlet and at the furnace plenum. In summer, the furnace damper is closed and the cooler damper is opened, with damper positions reversed in the winter. Most residential cooling systems are operated with manual controls. Evaporative cooling systems differ from other types of heating and cooling systems in that they are largely selfregulating. Since the lowest possible temperature that can re- M.A. CftLLC ELEVATION 845 suit is several degrees above the prevailing wet-bulb tempera ture, it is impossible to undercool to the extent that can occur with other types of equipment. Furthermore, the cost of operation is so nominal that there is no particular cost incen tive to regulate the system automatically. A simple means of providing better regulation is to install the cooler with a twospeed motor that can be operated on high speed during the heat of the day and on low speed during the evening and night when the cooling load is low and quiet operation is espe cially desirable. In general there is no need for a hinmdistat in residential cooling as the effective temperature will always be lower'regardless of the outdoor relative humidity, if the satu ration media is wet. COMMERCIAL COOUNG Evaporative cooling systems for commercial applications are divided into two categories: (1) those in which the cooling load is primarily external, and (2) those that must handle heavy internal loads. The design of the former is similar to the residential systems covered in the preceding section. If they are large and complex it is advisable to make a heat gain calculation and to have precise knowledge of local wet- and dry-bulb temperatures. Coincident values of these tempera tures and the number of hours that any given wet-bulb tem perature will be exceeded in a statistically normal summer are essential. These will permit a rational analysis of the condi tions that can be maintained by evaporative cooling. Two books8** that have appeared in the literature provide much useful data on climatic conditions for the design of evapora tive cooling systems. Frequency of occurrence of wet-bulb temperatures are given for a large number of cities as well as coincident wet- and dry-bulb temperatures for representative localities. Two-Stage Cooling Tire effectiveness of evaporative cooling in producing com fortable conditions depends on the weather, the general limitations are related to the prevailing outdoor dry-bulb and wet-bulb temperatures. The use of two-stage systems for com mercial applications can extend the range of atmospheric con ditions under which comfort requirements can be met. (For the same design conditions, two-stage cooling will provide lower coo! air temperatures and thereby reduce the required air flow rate.) The simplest arrangement is a combination of precooling coils using cooling tower water and evaporative cooling. Fig. 5 is a schematic arrangement of such a system.19.-- Fig. 4 .... Layout of Combination Heating and Evaporative -Cooling System for Residence Fig. 5 .... Two-State Evaporative Cooling with Precooling Coil and Cooling Tower