Document qamyowbnvEm2mVbkLY59OvDvn

652 CHAPTER 28 1955 Guide r, = outer radius of insulation, inches. k = thermal conductivity of insulation, Btu per (hour) (square foot) (Fahren heit degree per inch). <i = temperature of inner surface of insulation, Fahrenheit degrees. U = temperature of outer surface of insulation, Fahrenheit degrees. It is convenient to work from the outer surface of the insulation, since the loss through the covering must be determined from the outer surface loss by means of surface loss curves such as given in Fig. 4. The curves were plotted from tests conducted at the Mellon Institute. TEMP DIFF FROM PIPE TO ROOM, F DEG Fig. 3. Heat Loss Thbough 2 In. Thick 85 pebcent Magnesia Tvpe Covebino After the true heat loss is obtained, the loss per square foot of pipe sur face can be calculated from the relationship: where g. = goOVn) qi = Btu per (hour) (square foot outer surface of pipe). The heat loss through two or more thicknesses of insulation applied to a pipe can be calculated by means of the equation: ,= u ~ l* r. log, -- r. log, -S nr (2) where Ta = outer radius of second layer of insulation, inches, r, = outer radius of last layer of insulation, inches. Pipe and Industrial Insulation 653 The method of solving Equation 2, which is the most difficult of the two, is given in Example 3. Example S: Compute the heat loss per linear foot of pipe surface per hour from a 6-in. pipe, insulated with a 3-in. thickness of diatomaceous silica, 1900 F maximum type, and a 2-in. thickness of 85 percent magnesia. The pipe is operating at a tem perature of 1200 F and is exposed to a room temperature of 80 F. Solution: In figuring the heat loss from Equation 2, it is necessary to first make an assumption for the outer surface temperature U and the temperature between the diatomaceous silica and 85 percent magnesia insulation, so that the mean tempera ture of each material can be obtained and the thermal conductivity corresponding to the mean temperature of each material substituted in the formula. First assume an outer surface temperature of 140 F and a temperature of 570 F between the two ma terials corresponding to a mean temperature of (1200 + 570) + 2 or 885 F for the dia- Table 7. Pipe Covebing Factobs to be Applied to Figs. 1, 2 and 3 Type of Pipe Insulation Mean Tempebatubb, Fahbenheit 40 70 100 200 300 500 700 900 1.05 1.05 1.05 1.05 1.05 1.05 MOULDED AMOSITE AND BINDER ---- LAMINATED ASBESTOS PAPER (35-40 Per In.) -- -- CORRUGATED AND LAMINATED ASBESTOS 0.89 0.95 1.01 1.09 1.08 1.13 1.07 1.24 _ -- __ -- PAPER 4 Ply Per In. 6 Ply Per In. 8 Ply Per In. CALCIUM SILICATE CELLULAR GLASS DIATOMACEOUS SILICA (22 lb 1 cu ft) DIATOMACEOUS SILICA (25 lb 1 cu ft) MINERAL WOOL (Rock, Slag or Glass) Low Temp. (Asphalt or Resin Bonded) U>w Temp. (Fine Fiber Resin Bonded) tvt v(Blanket, Metal Reinforced) PLASTICS (Foamed) _ _-- 1.49 1.54 1.70 1.87 1.35 1.37 1.48 1.62 __ __ _-- 1.30 1.32 1.43 1.52 __ _ -- -- 0.97 1.00 1.03 1.13 _ _ 1.05 -- -- 1.08 -- 1--.10 ---- 1.20 -- -- 1.29 -- -- 1.24 1.44 -- 1.18 1.39 _ 1.14 1.34 _ _0.80 0.83 0:89 0.98 0--.63 0.64 -- 0.65 0.78 0.68 0.90 0.73 0.98 __ 1.05 __ _ __ __ __ 0.74 0.78 0.84 -- _ __ RUBBER (Foamed) WOOL FELT 0.66 0.72 0.68 __ __ _ _ _ 0.83 0.86 0.89 -- _ _ __ __ HAIR FELT OR HAIR FELT PLUS JUTE 0.77 0.78 0.81 -- -- -- -- -- omaceoua silica and (570 -+ 140) -s- 2 or 355 F for the 85 percent magnesia insulation, ine conductivities of these two materials at mean temperatures of 885 and 355 F, t^P0 ate" ^rom Table 6, are 0.796 and 0.467 Btu, respectively. lnese values are substituted in Equation 2 and a trial calculation made. For a nominal 6-in. steel pipe: n = 3.312, r, = 6.312, and r, = 8.312. Then, 1200 - 140 8.312 log. -- 8.312 8.312 log. 3.312 6.312 0.796 + 0.467 1060 = 91.1 Btu. 6.74 + 4.89 , , 1: temperature drop from the outer surface of the insulation to the surrounding j .r a heat loss of 91.1 Btu is found from Fig. 4 to be 55 deg for a 16-in. O.D. cylin- surffl S1?r*ace> or 55 -f 80 F room temperature = 135 F surface temperature. Since a i -pCe temperature of 140 F was assumed, it is evident that a temperature closer to or, for instance, 136 F should be used for recalculation: 1200 - 136 6.74 + 4.89 91.5 Btu. actual*^6 ?emPera^ure drop through each material is equal to the heat flow times the 8ilica resistance eah material, the temperature drop through the diatomaceous is fi2nn ^ 6.74 = 617 F, or the temperature between the two insulating materials ( aju -- 617) = 583 F. Since a temperature of 570 F between the two materials was