Document RpzggnZQeQ39xMEvD4pwvKO2X

HEATINC VENTILATING AIR CONDITIONING GUIDE 1943 per hour per unit. Heavy duty furnace heaters may be arranged in battery combinations of one or more units. 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 .3500 Btu per square foot. A higher rate of heat emission tends to increase the heat loss up the chimney, and raise fuel consumption, to shorten the life of the furnace, and to overheat the air. The ratio of heating surface to grate area on furnaces for this type of work shouldfnever be less than 30 to 1 and as indicated previously may run as high as 50 to I. Control of temperature is secured through (1) controlling the quantity of heated air entering the room, (2) using mixing dampers, or (3) regu lating the fuel supply. The design of heavy duty fan furnace heating systems is in many respects similar to that of the central fan heating systems described in Chapter 21. Ducts are designed by the method outlined in Chapter 32. HUMIDIFICATION During the winter months mechanical warm air systems pffer a means of humidifying the air being supplied to,the various rooms, with increased comfort to the occupants and longer life for household furnishings. Temperatures and relative humidities should be governed within the limits of the generally accepted standards. See Chapters 2 and 27 for more detailed information on this point.- Water evaporating pans are usually located 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 part of the mixture, about 1000 Btu per pound are required. There is a trend in present practice toward heating the water in addition to heating the air. Equipment for doing this may make use of sprays, or it"may take the form of water, circulating coils placed within the combustion chamber and connected by pipes to the humidifier pans where a constant water level is maintained by some separate float device. (See Chapter 27.) Sprays for residence systems may be provided in separate housings to be installed on the inlet or outlet sidev of the fan, or they may be integral with the fan construction. They operate at water pressures of from 10 to 30 lb and use two or more spray nozzles for washing and humidification. The sprays should be adjusted to completely cover the air passages. Sprays are usually controlled by solenoid valves wired in parallel with the fan motor. The water supply may, in turn, be controlled by a humidity-controlling device located in one of the living rooms, so that the : washer will operate at all times when the fan is in operation, unless the relative humidity should rise beyond a desirable percentage. Sprays used in connection with commercial or heavy duty plants should be a regulation type of commercial spray. In all cases provision must be made to flush out accumulation of lime and dirt. Principles underlying humidity requirements and limitations for resi- 392 CHAPTER 20; MECHANICAL WARM AIR FURNACE SYSTEMS dences have been summarized in a bulletin*7 and are enumerated herewith: 1. Optimum comfort is the most tangible criterion for determining the air conditions within a residence. 2. An effective temperature of 65 deg8 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 arid limitations cannot be separated from condensations of good building construction, and the lattei; should receive serious attention in the installation of humidifying apparatus. 6. None of the types of gravity warm air furnace water pans tested proved adequate to evaporate sufficient water to maintain 40 per cent relative humidity in the Research Residence except only in moderately cold weather. 7. The water pans used in the radiator shields tested did not prove adequate to maintain 40 per cent relative humidity in a residence similar to the Research Residence when the outdoor temperature approximated zero degrees Fahrenheit. COOLING METHODS A slight cooling effect may be obtained under certain conditions by the use of the cooler basement air. A more positive cooling .effect may be obtained by the use of an air washer where the temperature of the city or well water is sufficiently low (55 F or lower), and where a sufficient volume of water can be provided. Unless the temperature of the leaving water is below the dew-point temperature of the indoor air at the time the washer is started, both the relative and absolute humidities will be somewhat increased. Coils of copper finned tubing through which cold water is pumped are available for cooling. They require less space than air washers and have the advantage that.no moisture is added to the air when the temperature of the water rises above the dew-point. Ample coil surface and fan capacity are necessary with this type of cooling. It is thoroughly feasible, to use ice or mechanical refrigeration in con nection with a warm air system and to cool the building by this method; provided the building is reasonably well constructed and insulated. . Windows and doors should be tight, and awnings should be supplied on the sunny side of the building. (See also Chapters 21, 24 and 25.) Conclusions that may be drawn from studies9 thus far completed in the Research Residence, subject to the limitations of the test are: 1. An uninsulated building of ordinary residential typie may require the equivalent of three tons of ice in 24 hours on days when the maximum outdoor temperature reaches 100 F if an effective temperature of approximately 72 deg is maintained indoors. THumidification for Residences, by A. P. Kratz (University of Illinois, Engineering Experiment Station Bulletin No. 230). 8The optimum winter effective temperature is 66 deg as recommended by the A.S.H.V.E. Committee on Ventilation Standards. (See Chapter 2). 'University of Illinois Engineering Experiment Station Bulletins Nos. 290 and 321 by A. P. Kratz, S. Koozo, M. K. Fahnestock and E. L. Broderick. A.S.H.V.E. Research Report No. 1177--Summer . Cooling in the Research Residence with a Gas-Fired Dehydration Cooling Unit by A. P. Kratz, S. Konzo and E. L. Broderick (A.S.H.V.E. Transactions, Vol. 47, 1941, p. 203). 393