Document Yj34gBOYGwZgLx669Vo1Zg7JV

American Society of Heating and Ventilating Engineers Guide, 1924-25 heat loss per square foot of insulated pipe is somewhat dependent upon the pipe size it cannot be said that the data given by this figure is exact for all pipe sizes, but it is known that it is sufficiently so for all practical purposes in the heating and ventilating field. These curves are exact for 3-in. pipe. Similar curves for flat surfaces and surfaces of large radius, such as boilers, tanks, heaters, and ducts, are given in Fig. 54. When coverings are used other than those for which curves are given reference should be made to Table 84 which gives the conductivities of the various materials. That curve should.then be used which represents the covering the conductivity of which is the nearest to the conductivity of the covering to be used. If greater accuracy is desired, interpolation may be resorted to. Example.--What is the heat loss through a pipe covering of conductivity 0.9 at 200 deg. temperature difference? The conductivity of air cell is 0.802, and that of vitrified air cell is 1.087. The heat loss through air cell is 132 B.t.u., that through vitrified air cell is 165 B.t.u., so the heat loss through the covering under consideration is given by A-132 0.9-0.802 165-132 1.087 -0.802 or X --143 B.t.u. per sq. ft. of pipe per hr. While 1-in. thick covering is economically sufficient for most purposes in connection with heating and ventilating piping, greater thicknesses should be used on all boilers, piping, tanks, etc., where high pressure steam is used. Table 85 shows the thickness of covering required for maximum net saving with coal at $4 per ton. TABLE 85. THICKNESS OF COVERING FOR MAXIMUM NET SAVING WITH COAL AT *4 PER TON Pipe Size In. H \a 3 6 12 Flat Hot Water. IN. i i i i i . `K 5 Lb. In. 1 1 1 m H4 2 100-200 Lb. In. 114 2 2 214 3 200 Lb. and 150 Dec. Fahr. Superheat In. ' 2 2 2 3 3)4 4 When coverings thicker than 1 in. are used the losses vary consider ably for the different thicknesses and reference must be made to the hand books of the various manufacturers as lengthy tables are required for each thickness and kind of covering. RADIATING SURFACE OF PIPES In order to use the curves in Fig. 53 it is necessary to know the number of square feet of radiating surface in 1 linear ft. of pipe. Table 86 gives this data for the various standard pipe sizes. 146 American Society of Heating and Ventilating Engineers Guide, 1924-25 TABLE 86. RADIATING SURFACE IN 1 LINEAR FOOT OF PIPE Pipe Size In. 'A % 1 Hi H4 Surface Sq. Ft. 0.22 0.274 0.344 0.435 0.498 Pipe Size In. 2 214 3 314 4 SURPACE Sq. Ft. 0.622 0.751 0.917 1.047 1.178 Pipe Size In. 5 6 8 10 12 Surface Sq. Ft. 1.455 1.733 2.257 2.817 3.33s THICKNESS OF MAGNESIA COVERINGS Magnesia coverings are not made exactly 1 in. thick, but their thickness varies with the size pipe for which they are intended. This is done for the reason that it has been demonstrated that the larger the pipe the more economical it is in the long run to use a thicker covering. The thickness for different pipe sizes are given in Table 87. Fig. 54. Heat Transmission in B.t.u. Per Hour Per Square Foot of Flat Insulated Surface. (1 In. Covering) Coverings made in thicknesses as shown in Table 87 are known as Standard Thick coverings. Since these thicknesses do not vary greatly from 1 in., the curves of heat loss for 1-in. thick coverings in Fig. 53 can be used in the case of standard thick coverings without error or practical TABLE 87/ THICKNESS OF MAGNESIA COVERINGS FOR VARIOUS PIPE SIZES Pipe Size. IN. 14 to l}4incl. 2 to 3>4 " 4 to 6 7 to 10 12 Thickness In. 14 114 iy* 114