Document 2jvx8LK099qbrrkyxwdEmKRxR

American Society of Heating and Ventilating Engineers Guide, 1937 * 4 9 a. What are the advantages of roof ventilators? b. How are proper sizes determined for roof ventilators? a. Roof ventilators offer the best utilization of the inductive force of the wind, and the? may be very economically fitted with built-in fans to supply the necessary circulation when the force of the wind is not sufficient. b. Because of the many factors affecting the flow through roof ventilators no accurate formula can be given. It is usual practice to make the combined throat area of all roof ventilators between one-half area and full area of the air inlets as determined bv Formula 1. ' 5 9 What methods of control are used in ventilating systems? Hand control, control by a thermostat located in the ventilated space or in the venti. lator, or wind velocity control designed to keep the air discharge constant regardless of wind velocity. 6 9 How is the quantity of air required for a budding determined? Sufficient air must be supplied to carry away the heat and impurities generated within j building. The temperature rise and concentration of impurities in the exhaust air must be held within specified limits. (See Example 2.) 7 9 What measurements are necessary to determine the capacity of a venti. lating system? Wind velocity and air velocities through openings, determined by suitable anemo meters; outdoor air temperatures, measured by a shaded thermometer not near objects heated by the sun or near exhaust air openings; indoor air temperatures, measured at various heights to secure a good average. 8 9 How much air must -be supplied for dissipating the heat generated in a dairy barn housing 100 cows if the outside temperature is 20 F and the inside temperature is to be maintained at 45 F? The total heat generated is 100 X 3000 = 300,000 Btu per hour. Then from Formula 3, ^ cm (t-to) 13.5 X 300,000 0.24 X 60 (45 - 20) = 11,250 cfm. This amount of air should also keep down humidity and odors. 9 9 a. What precaution is necessary in the ventilation of garages using natural ventilation? b. How much window area is required for a garage with 50 x 100 sq ft flow area if natural ventilation is used? a. The carbon monoxide content of the air should be kept below 1 part in 10,000 and windows should be kept open at all times. b. The window area should aggregate 5 per cent of the floor area. 0.05 X 50 X 100 = 250 sq ft of window area. This area should be evenly distributed along two sides of the building. 98 ' Chapter 5 heat transmission coefficients AND TABLES Methods of Heat Transfer, Coefficients, Conductivity of Homogeneous Materials, Surface Conductance Coefficients, Air Space Conductance, Practical Coefficients, Table of Conductivities and Conductances, Tables of Over-all Coefficients of Heat Transfer for Typical Building Construction, Combined Coefficients of Transmission IN order to maintain comfortable living temperatures within a building it is necessary to supply heat at the same rate that it is lost from the building. The loss of heat occurs in two ways, by direct transmission through the various parts of the structure and by air leakage or filtration between the inside and outside of the building. The purpose of this chapter is to show methods of calculation and to give practical trans mission coefficients which may be applied to various structures to deter mine the heat loss by direct transmission. The amount lost by air filtration is determined by different methods, as outlined in Chapter 6, and must be added to that lost by direct transmission to obtain the total heating plant requirements. METHODS OF HEAT TRANSFER Heat transmission between the air on the two sides of a structure takes place by three methods, namely, radiation, convection and conduction. In a simple wall built up of two layers of homogeneous materials separated to give an air space between them, heat will be received from the high temperature surface by radiation, convection and conduction. It will then be conducted through the homogeneous interior section by con duction and carried across to the opposite surface of the air space by radiation, conduction and convection. From here it will be carried by conduction through to the outer, surface and leave the outer surface by radiation, convection and conduction. The process of heat transfer through a built-up wall section is complicated in theory, but in practice it is simplified by dividing a wall into its component parts and considering the transmission through each part separately. Thus the average wall may be divided into external surfaces, homogeneous materials and interior air spaces. Practical heat transmission coefficients may be derived which will give the total heat transferred by radiation, conduction and convec tion through any of these component parts and if the selection and method of applying these individual coefficients is thoroughly understood it is usually a comparatively simple matter to calculate the over-all heat transmission coefficient for any combination of materials. 99