Document 7OBjwvLVLrEr32x73KrEzrdNE

148 CHAPTER 7 1948 Guide k -- conductivity of pipe or tube material, Btu (square foot) (hour) (Fahrenheit degree per inch thickness).. . L =* thickness of tube wall, inches. . MTD-. abbreviation--mean temperature difference between fluid :in coil and air passing over coil, Fahrenheit degrees. Note: MTD--usually logarithmic mean. -'MTDa -- logarithmic mean temperature difference between air and coil surface. N = number of rows of coil depth; n = a constant, exponent of G in Equation 8, obtained by plotting, on loga rithmic coordinates, G against values of fca* The value of n is the slope of the line. 2a ~ sensible heat transferred, Btu per (hour) (square foot of coil face area). . ' 2t --;total heat transferred by coil, Btu per (hour) (square foot of face area). R -- ratio between externa! and internal surface of tube. average water temperature, Fahrenheit degrees. * t\ = dry-bulb temperature of air entering coil, Fahrenheit degrees. -- dry-bulb temperature of air leaving coil, Fahrenheit degrees. ! '^a .= minimum dry-bulb temperature possible without dehumidificatidn, Fahren' heit degrees. ' fdp'Cf5 dew-point of air entering coil, Fahrenheit degrees. <dp* = dew-point of air leaving coil, Fahrenheit degrees. -"h = average refrigerant temperature,'Fahrenheit degrees. ..........ts = average temperature of external surface of coil, Fahrenheit degrees. At = ts - tv. V = over-all coefficient of heat transfer, Btu per (hour) (square foot of external coil surface) (Fahrenheit degrees temperature difference between fluid in coil and air flowing over coil). : V = water velocity, feet per second. ' Z == a constant for use in Equation 8 obtained by plotting on logarithmic coordinates G against values of ha. Note: Numerical subscripts refer to condition entering and leaving respectively. REFERENCES ' '--Rational Development and Rating of Extended Air Cooling Surface, by H. B; Pownall (Refrigerating Engineering;.October, 1935. p. 211). ` Performance of Surface-Coil Dehumidifiers for Comfort Air Conditioning, by`G. L. Tuve and L.'G; Seigel (A.S.H.V.E. Transactions, Vol. 44. 1938. p. 523). *--The Effect of Turbulence Promoters on Heat Transfer Coefficients for Water Flowing in Horizontal Tubes, by L. G. Seigel (A. S.H.V.E. Journal Section, Heating, Piping andLAir Conditioning, June, 1946, p. 111). > *--Maximum Rate of Heat Transfer with Minimum Loss of Energy, by Z. Nagoaka and-A. Watanabe (Proceedings. International Congress on Refrigeration, 7th Congress. Vol. 3, No. 16, pp. 221-245, 1937). Chapter 8 AIR LEAKAGE ,Causes of Infiltration, Infiltration Due to Wind Pressure, Infiltration Through Walls,- Window and Door Leakage, Crack Method, Air-Change-Method-,- ' Infiltration Due to Temperature Difference Sealing of Vertical Openings THE air leakage which takes plaice through various apertures in buildings must be considered in heating and cooling calculations, and properly evaluated. This infiltration as it is sometimes designated takes place through cracks around doors and windows, through solid walls and through fireplaces arid chimneys. Although the latter sources of leakage may be considerable, they are often neglected on the assump tion that dampers would be closed during periods of extreme cold weather or else that the fireplace will be in use dt such times and will therefore contribute to the.heat supplied and lessen the heating load; CAUSES OF INFILTRATION The displacement of heated air in buildings by unheated outside air is due to two causes, namely, (1) the pressure exerted by the wind and (2) the difference in density of outside and inside air because of differences in temperature. The former is generally referred to as infiltration and the latter as stack or chimney effect. , In either case an exact estimate of the amount of infiltration under design conditions is difficult to make. The complicating factors include (1) variations in building construction particularly as to width of crack or size of openings through which air leakage takes place, (2) the varia tions in wind velocity and direction, (3) the exposure of the building with respect to air leakage openings and with, respect to adjoining buildings, (4) the variations in outside temperatures which influence the chimney, effect, (5) the relative area.and resistance of openings on the windward and leeward sides and on the lower floors and on the upper floors, and (6) the influence of a planned air supply and the related outlet vents. Tight construction is essential for preventing large heat loss due to infiltration. INFILTRATION DUE TO WIND PRESSURE The wind causes a pressure to be exerted on one or two sides of a building. As a result, air comes into the building on the windward side through cracks or porous construction, and a similar quantity of air leaves on the leeward side through like openings: In general the resis tance to air movement is similar on the windward to that on- the leeward side. This causes a building up of pressure within the building'and a lesser air leakage than that experienced in single wall tests as determined in the laboratory. It is assumed that actual building leakages owing to this building up of pressure will be 80 per cent of laboratory test values. While there are cases where this is not true, tests in actual buildings substantiate the factor for the general case. Mechanical ventilating systems are frequently designed to produce positive or negative pressures in an enclosure which aie greater or lower than prevalent wind pressures; In such designs, if the rate at which air is specified to be introduced to or . removed from the enclosure by positive means exceeds the infiltration rate, it is common practice to use the greater value in determining the heating capacity to warm the outside air. 149