Document zQeORyzzvVZk6k8Yw7zVDgwxB

and 1936American Society o/Heating Ventilating Engineers Guide, 23--Chapter Mechanical Warm Air Furnace Systems- where I = Btu per hour input. The factor 1.56 is the multiplier necessary to care for a 10 per cent heat loss in the distributing ducts and an efficiency of 70 per cent in the con version burner. SELECTING THE FAN Choose a fan which, according to its manufacturer's rating, is capable of delivering a volume of air, expressed in cubic feet per minute, against a frictional resistance, expressed in inches of water, computed by adding together the following items: 1. The frictional resistance of a warm air trunk or leader. 2. The frictional resistance of a return air trunk or duct. 1. Optimum comfort is the most tangible criterion for determining the air conditions within a residence. 2. An effective temperature of 65 deg1 represents the optimum comfort for the majority of people. Under the conditions in the average residence a dry-bulb tempera ture of 69.5 F with relative humidity of 40 per cent is the most practical for the attain ment of 65-deg effective temperature. 3. Evaporation requirements to maintain a relative humidity of 40 per cent in zero weather depend on the amount of air inleakage to the average residence, and vary from practically nothing to 24 gal of water per 24 hours. 4. Relative humidity of 40 per cent indoors cannot be maintained in rigorous climates without excessive condensation on the windows unless tight-fitting storm sash or the equivalent is installed. 5. The problems of humidity requirements and limitations cannot be separated from considerations of good building construction, and the latter should receive serious atten tion in the installation of humidifying apparatus. 3. The resistance to the flow of total volume of air through the furnace casing of hood, which is usually considered from 0.10 to 0.15 in. of water. 4. The frictional resistance through any other accessories, such as washers or filters. 1 The following conclusions were drawn from the experimental results reported in the aforementioned bulletin: 5. A factor of safety of 10 per cent of the resistance calculated above. ; X. None of the types of warm air furnace water pans tested proved adequate to HUMIDITY Mechanical warm air systems offer an excellent means of proportioning j evaporate sufficient water to maintain 40 per cent relative humidity in the Research j Residence except only in moderately cold weather. 2. The water pans used in the radiator shields tested did not prove adequate to mainj tain 40 per cent relative humidity in a residence similar to the Research Residence when and distributing moisture-bearing air; consequently, during the winter j the outdoor temperature approximated zero degrees Fahrenheit. months humidifiers may be employed to deliver water vapor to. the fan- driven air stream in proper amounts to produce a more humid atmos phere, with increased comfort for people and increased life for household furnishings. Temperatures and relative humidities should be govertied within the limits of the generally accepted standards. See Chapters 3 and PROVISION FOR COOLING SYSTEM If the system is to be used for cooling, the following provisions should be made: 11 for more detailed information on this point. 1. Where cooling is to be secured through air circulation only: In earlier types of furnaces, water evaporating pans were usually placedj in the cool portions of the air stream, but modern types usually locate/ a. Provide for an increase of 50 to 100 per cent in fan capacity through multi-speed pulleys or other means. them in air which has been heated by contact with the heating surfaces. To change water into vapor capable of being carried in an air stream as 1 b. If basement air or outside night air is to be used, provide suitable basement opening in duct system, or outdoor air intake. part of the mixture, about 1000 Btu per pound are required. Without the addition of this heat, termed the latent heat of evaporation, water injected into the air will be carried along in the form of tiny globules until it falls out of the stream or is deposited upon some surface. Furthermore, when dry air is in contact with water for a sufficient 2. Where water below 55 F or artificial refrigeration or ice is to be used: a. Provide outside air duct for circulation of cool night air for economy. b. Make provision in return duct system for cooling unit. c. Make provision lor control of the fan speed, during wrinter operation, to give a sufficient and draftless air movement. length of time without the presence of a sizable body of water or.a source other than air from which this latent heat, of evaporation can be taken, such heat is supplied from the air. There is, therefore, a trend in present ' HEAVY DUTY FAN FURNACES practice toward heating the water in addition to heating the air. Equip Fan furnaces for large commercial and industrial buildings are available ment for doing this may make use of sprays, or it may take the form of in sizes ranging from 400,000 to 3,000,000 Btu per hour per unit. Heavy water circulating coils placed within the combustion chamber and con duty heaters may be arranged in combinations of one or more units in a nected by pipes to the humidifier pans where a constant water level is battery. A few possible arrangements are shown in Figs. 6 to 13, in maintained by some separate float device. (See Chapter 11.) clusive. Humidification for Residences The principles underlying humidity requirements and limitations for residences are summarized in University of Illinois Bulletin No. 48s, as follows: Most manufacturers of heavy duty furnaces rate their furnaces in Btu per hour and also in the number of square feet of heating surface. Con servative practice indicates that at no time in the heating-up period should the furnace surface be required to emit more than an average of See Humidification for Residences, by A. P. Kratz (University of Illinois, Bulletin No. 48). 414 *66 deg Is the optimum winter effective temperature recommended by the A.S.H.V.E. Committee on Ventilation Standards. See Chapter 3. 415 =5!