Document K6ZYQpv4gDV37ZEddJ94adzGx

114 CHAPTERS 1950 Guide Since the calculation is based on a 1 ft pipe length: = 21 Btu per hour. The temperature drops through the various resistances are now readily evaluated by Equation 12 as: L -- ta air to insulation surface = R, q,t = 0.22 X 21 = 4.6 F. ta -- tmi through the insulation = R, q,, = 3.9 X 21 = 82 F. tn -- t,\ through the pipe wall = R, q,,, = 8.5 X 10~* X 21 = 0.02 F. Ui -- It pipe wall to cold water = R, q,, = 2.8 X 101 X 21 = 0.06 F. The solution was obtained on the assumption that the air temperature and the outside temperature differed by 20 deg. In order to obtain a slightly better estimate of the rate of heat transfer, the numerical solution should be repeated using the temperatures calculated from the. previous listed temperature differences. The foregoing problem serves to illustrate a general method of solving steady-state heat transfer problems. There are many problems which cannot be approximated by steady-state solutions, for instance: the problem of pipe line insulation in transient service ;6 the behavior of auto matically controlled thermoflow circuits; or the periodic absorption of solar energy by roof and wall structures during the day and nocturnal radiation to the cold sky at night.8 The transient heat transfer problem differs from the steady-state in that energy storage rates need to be con sidered. Thus thermal capacity, in addition to resistance effects, is signifi cant. The vector sum of the thermal capacitance and resistance is the thermal impedance. It is not. within the scope of this chapter to deal with these problems. There are, however, solutions available in graphical form for certain special cases.4-7,8 Also a general approximate method may be employed, which is analogous to the treatment of capacity-resistance lumped parameter electrical circuits.8 REFERENCES 1 Absorption and Extraction, by T. E. Sherwood (McGraw-Hill Co., 1937). 1 The Transmission of Heat by Radiation and Convection, by Griffith and Davis (Special Report No. 9, 1922, Department of Scientific and Industrial Research, His Majesty's Stationery Office, London, England). * Heat Insulation in Air Conditioning, by R. H. Heilman (Industrial and Engineer ing Chemistry, Vol. 28, July, 1936, p. 782). 4 Heat Transmission, by W. H. McAdams (McGraw-Hill Book Co., 2nd edition, 1942). 4 Transient Heat Conduction in Hollow Cylinders after Sudden Change |of InnerSurface Temperature, by R. L. Perry 'and W. P. Berggren (University] of California Publications in Engineering 5,59; 1944). 'Periodic Heat Transfer at the Inner Surface of a Homogeneous Wall, by H. A. Johnson (A.S.H.V.E. Journal Section, Healing, Piping and Air Conditioning, May, 1948, p. 121). 7 Heat Transfer, Vol. 1 by Max Jakob (John Wiley and Sons, New York, 1949). * Heat Transfer Notes by L. M K. Boelter, V. H. Cherry, H. A. Johnson and R. C. Martinelli (University of California Press, Berkeley and Los Angeles, 1946). * Method for Determining Unsteady-State Heat Transfer by Means of Electrical Analogy by V. Paschkis and H. D. Baker (A.S.M.E. Advance Paper, June 20-21,1941). BIBLIOGRAPHY Industrial Heat Transfer, by Alfred Schack (John Wiley & Sons, 1933). Calculation of Heat Transmission, by M. Fishenden and O. A. Saunders (His Majesty's Stationery Office, London, 1932). CHAPTER 6 PHYSIOLOGICAL PRINCIPLES Chemical Vitiation of Air, Physical Impurities in Air, Thermal Interchanges Between the Body and Its Environment, High Temperature Hazards, Acclimati zation, Upper Limits of Heat for Men at Work, Application of Physiologic Principles to Air Conditioning Problems, Effective Temperature Index and Comfort Zones VENTILATION is defined in part as the process of supplying air to, or removing air from, any space by natural or mechanical means. The word in itself implies quantity, but air must be of the proper quality also. The term air conditioning in its broadest sense implies control of any or all of the physical or chemical qualities of the air. The A.S.H.V.E. Code of Minimum Requirements for Comfort Air Conditioning1 defines it "as the process by which simultaneously the temperature, moisture content, movement and quality of the air in enclosed spaces intended for human occupancy may be maintained within required limits. If an installation cannot perform all of these functions, it shall be designated by a name that describes only the function or functions performed." CHEMICAL VITIATION OF AIR People living indoors bring about certain physical and chemical changes in the air about them. The oxygen content of the air diminishes and the carbon dioxide increases, but these changes are too slight to be significant except in air tight spaces as in submarines. Organic matter which is usually perceived as odors, comes from the body or clothes. Moisture and heat are given off by the body. There is no evidence of any toxic volatile material given off by man to the ambient air. Stale air may be offensive because of odors and may induce loss of appetite and loss of energy. Objectionable body odors have the same effects. These reasons, whether esthetic or physiological, usually make it desirable in the design of air conditioning systems to provide for the elimination or control of odors arising from occupancy, cooking, or other sources. This may be accomplished by introducing odor-free air in sufficient quantities to reduce odor concentrations by dilution to a level which is not objectionable. Odor-free air may be outdoor air or air which has been cleared of odors by sorption, washing, or other appropriate means. In the case of vitiation by a few hazardous gases such as carbon mon oxide from heating, cooking, and certain industrial processes, no satis factory chemical treatment for the elimination of the impurity has been found. The only satisfactory solution is elimination at the source by local exhaust ventilation; or, if this is impossible, reduction to a safe concen tration by dilution. (See Chapter 8.) In the case of contamination by other matter, including volatile vapors and gases, chemical treatment for the removal or reduction of the impurities has been made available through air cleaning methods, which are discussed in Chapter 33. When the only source of contamination is the human occupant, and over heating is not a problem, the minimum quantity of outdoor air needed ap pears to be that required to remove objectionable body odors, or tobacco smoke. The concentration of body odor in a room, in turn, depends 115