Document xzYKB8Lrg5xZ0vjQ122Rw5XnE

heating ventilating air CONDITIONING CUIDE 1941 heating season. Objectionable drafts are likely to occur when the velocity of the air current is 40 fpm and the temperature of the air current 2 F or more below the customary winter room temperature. Higher velocities are desirable in the summer time when the air temperature exceeds 80 F. Variations in air movement and temperature in different parts of occupied rooms are often indicative of relative air distribution. The work of. the A.S.H.V.E. Research Laboratory indicates that an air movement between 15 and 25 fpm with a temperature variation of 3 F or less in different parts of a room, 36 in. above floor, represent satisfactory distribution. THE FOUR VITAL FACTORS From the preceding discussion it is clear that thermal environment cannot properly be adjusted to the requirements of human health and comfort without control of all the four basic factors concerned. According to the recommendations of the Sub-Committee on the Hygiene of Environmental Conditions in the Dwelling63, it is of great importance in all research studies to make an accurate record of each of the four independent factors governing bodily heat exchanges, tempera ture, movement and humidity of the air, and mean radiant temperature of the surrounding surfaces. For this purpose the committee suggested in the interest of comparability the use of the following four types of instruments or others yielding similar data: 1. Silvered dry-bulb thermometers or hair-pin thermometers (Bargeboer). 2. Silvered dry Kata-thermometers or the hot wire anemometer. 3. Psychrometer, wet- and dry-bulb, whirling or ventilated. 4. Globe thermometer (Vernon) or the dry resultant thermometer (Missenard). - Such instruments as these, when properly calibrated and their readings are compared, can be used for determining the four basic physical factors concerned separately or in certain combinations. The results of the fourphysical measurements thus determined can generally be translated into the terms of any special instrument combining two or more of them. In some instances it may be important to record not only the movement and temperature of the air at various levels, but also the temperature of each wall and window, of the flooring, and of the ceiling, and to measure the total effective radiation of the surroundings in 6 directions; in order to trace the exact causes of defects in the building which have an un favorable influence on the heat exchanges of its inhabitants. Facts of this type are of great practical importance. In all fundamental studies of air conditions records should be obtained, therefore, showing: 1. True air temperature (free from radiation effects). 2. Air movement. 3. Humidity. 4. Mean radiant temperature of surrounding surfaces. In interpreting the results of such studies, the clothing and the physical activity of the subjects are of primary importance. "Housing Commission of the League of Nations, adopted at Geneva, June 25. 1937. Chapter 3 HEAT TRANSMISSION COEFFICIENTS AND TABLES ,Methods of Beat Transfer, Coefficients, Conductivity of Homogeneous Materials, Surface Conductance Coefficients, Air Space Conductance Practical Coefficients, Table of Con ductivities and Conductances, Tables of Over-all Coefficients of Heat Transferfor.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 4, 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. 69 -. I