Document ExLxLL9XvRoaD2y9JzxKEqQgN

American Society of Heating and Ventilating Engineers Guide,~1937" COOLING METHODS A slight cooling effect may be obtained under certain conditions by the use of basement air. A more positive cooling effect may be obtained through air washers where the temperature of the 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 is necessary with this type of cooling. It is thoroughly' feasible to use ice of mechanical refrigeration in con nection with the fan and duct system for the heating installation, 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 10'and 11. Results at Research Residence The following conclusions may be drawn from the studies thus far completed in the Research Residence, subject to the limitations of the conditions under which the tests were run6. 1. An uninsulated building of ordinary residential type may require the equivalent of three tons of ice in 24 hr on days when the maximum outdoor temperature reaches 100 F if an effective temperature of approximately 72 deg is maintained indoors. . '2. The use of awnings at all windows in east, south, and west exposures may result in Savings of from 20 to 30 per cent in the required cooling load. 3. The cooling load per degree difference in temperature is not constant" but increases as the outdoor temperature increases. 4. The heat lag of the building complicates the estimation of the cooling load under any specified conditions and makes such estimates, based on "the-usual methods of computation, of doubtful value. 5. The seasonal cooling requireitierits are extremely variable from year to year, and ; the ratio between the degree-hours of any two seasons occurring within a lO year period may be as high as 7.5 to 1.. Hence an average value of the degree-hours cooling per season is comparatively meaningless. 6. The duct system in a forced-air heating installation can be successfully converted to a system for conveying cool air for the purpose of cooling the structure. No con densation of moisture was.observed when the duct temperatures were not less than 65 F. 7. Cooling by means of water at a temperature of 60 .F is riot satisfactory? unless an indoor temperature of less than 80 F is maintained. 8.` In the selection of cooling coils, the frictional resistance of the coil to flow of air must be given careful consideration. ' ' 9. Cooling the structure by introducing large quantities of air from outdoors at night tended to reduce the amount of cooling required on the following day and was a practical means of providing more comfortable conditions in those homes where cooling systems were not available. -- - : I i Study of Summer Cooling in the Research Residence at the University of Illinois.'by A. P: Kratz and S. Kqjizo (A.S.HiV.E. Transactions. Vol. 39. 1933) ;:Study of Summer Cooling in the Research Residence' for the Summer of 1933. by A. P. Kratz and S. Konzo (A.S.H.V.E. Transactions, Vol. 40. 1934). 440 Chapter 24--Mechanical Warm Air Furnace Systems METHOD OF DESIGNING COOLING SYSTEM The general procedure for the design of a cooling system in a forced-air installation is as follows: 1. Calculate heat gain for each room or space to be conditioned. (See Chapters 5 and 8). Allowance for addition of outside air must be included in this calculation. 2. Select a temperature of air leaving supply inlets. In Research Residence tests7 a value of from 65 to 70 F was found satisfactory. 3. Determine indoor conditions to be maintained. In Research Residence 80 F dry bulb and 45 per cent relative humidity was found satisfactory. 4. Determine the quantity of air to be introduced into each room. (See Chapter 10, Equations 1 and 2.) 5. Estimate heat loss in duct system between cooling unit and supply registers. 6. Calculate the heat to be removed by the cooling unit, in the form of sensible heat and latent heat. - 7. Determine size of ducts in duct system, and size of registers, as explained in this chapter under the heading of Method of Designing Forced-Air Heating Systems. 8. Determine pressure loss in duct system and select fan as also explained in the same section. 9. Select cooling unit from manufacturer's data. Specify temperature and pressure of available cooling water, voltage and characteristics of electrical supply, and method of control of apparatus. 10. Select cooling coils from manufacturer's data to take care of latent heat load and to give required drop in air temperature with the weight of air flowing. See Chapter 10 on section Surface Cooling Problems. 11. If system is to be used for both winter heating and summer cooling, duct sizes must be checked to insure that velocities and friction losses are reasonable for both conditions of operation. Adjustable dampers will be necessary to make changes in air distribution for the two seasons. Provision must also be made for changing fan speeds for summer and winter operation. Loc. Cit. Note 6. PROBLEMS IN PRACTICE 1 A residence furnace, having a ratio ofheating surface to grate area equal to ' 20 to 1, is to be selected to heat a house which has a computed load of 225,000 Btu per hour. If coal having a calorific value of 12,000 Btu per pound is to be burned, if the furnace will burn 7.5 lb of coal per square foot of grate per hour, and if the furnace efficiency is 65 per cent, determine the square feet of grate area necessary in the furnace to be selected. Substituting in Equation 5:. G = 225,000 12,000 X 7.5 X 0.65 X 0.85 = 4.53 sq ft of grate area. *------ ' " ----------------- A furnace having at least 4.5 sq ft of grate area should therefore be selected. 2 I Why should secondary surface be designed for easy cleaning? If the combustion is not perfect, soot is formed immediately above the fire and is apt to form a deposit on the secondary surface from which it should be removed. If the secondary surface is so designed that there are horizontal passages, fine gray ash will settle out in these to form an insulation between the hot gases of combustion and the metal of the furnace; consequently, these should be readily cleaned. If the passages are vertical they are largely self-cleaning of ash, but provision should be made for easy and thorough cleaning of the collection chamber below them. 441 J