Document 0g6kV136gEj2QvapqkjeXeJqJ

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 resistance to heat flow inherent in the insulation itself. The maximum increase in heat loss due to air velocity ranges from about 30 per cent in the case of 1-in. thick insulation, to about 10 per cent in the case of 3-in. thick insulation, provided that the insulation is thoroughly sealed so that air can flow only over the surface. If the conditions are such that the air may circulate through cracks and crevices in the insulation, the increases may be far greater than those given. Therefore, it is essential that insulation be sealed as tightly as TEMPERATURE DIFFERENCE FROM PIPE TO ROOM, DEG FAHR Fig. 4. Heat Loss Through 2 In. Thick 85 per cent Magnesia Type Covering . possible. Pipe insulation exposed to the elements should be thoroughly waterproofed. Example S. If the steam line given in Examples 1 and 2 is covered with 1 in. thick 85 per cent magnesia, determine the resulting total annual loss through the insulation. Also compute the monetary value of the annual saving and the percentage of saving over, the heat loss from the bare pipe. Solution..- By referring to Fig. 2, the coefficient for 1 in. magnesia on a 2 in. pipe is found to be 0.285 Btu per hour per linear foot of pipe per degree temperature difference at a temperature difference of 169.4 F. The total hourly loss per linear foot of pipe will then be 0.285 X 169.4 = 48.3 Btu. The total annual loss through the insulation = 48:3 X 165 (linear feet) X 4000 (hours) = 31,900 Mb. The annual bare pipe loss as 782 CHAPTER 43. PIPE AND DUCT HEAT LOSSES determined in the solution of Example 1 was found to be 181,600 Mb. The saving due to insulation is then 181,600 31,900 = 149,700. Mb per year. From the solution of Example 2, it was found that the heat supplied to the system cost 80.804 per thousand Mb. Therefore, the monetary value of the saving = 0.804' (dollars) X 149.7 (thousand Mb) = $120.36, or 82.4 per cent of the cost when using uninsulated pipe. Table 8. Conductivity (k) of Various Types of Insulating Materials for Medium and High Temperature Pipes3 Types of Insulating Materials 100 85 per cent Magnesia Type............... ....... :..... Corrugated Asbestos Type...... '....................... (4 Plies per 1 in. thick) Corrugated Asbestos Type. .............. .............. (8 Plies per 1 in. thick) Laminated Asbestos Type'....... ........................ (30-40 Laminations per 1 in. thick) Laminated Asbestos Type............................... (14-20 Laminations per 1 in. thick) Mineral Wool Type.-.............. .......................... High Temperature Type....... ......................... - (Diatomaceous Earth and Asbestos) Brown Asbestos Type....................:................... (Felted Fiber) 0.359 ,0.495 0.505 0:326 0.374 0.350 0.576 0.338 Mean Temperature, Deg F 200 0.403 0.618 0.598 0.380 0.445 0.410 0.614 0.396 300 0.448 0.741 400 0.493 0.864 0.692 .0.786 0.434 0.488 0:518 0.589 0.470 0.530 0.652 0.689 0.453 0.510 500 0.539 -- -- 0.543 0.662 0.590 0.726 0.568 "From tests conducted at Mellon Institute. Table 9. Pipe Covering Factors Types of Insulating Materials Temperature Difference, Pipe to Air, Deg F . 100 200 300 400 500 600 85 per cent Magnesia Type.___________ 1.050 Corrugated Asbestos Type........ ............. 1.425 (4 Plies per 1 in. thick) Corrugated Asbestos Type...... ................ 1.435 (8 Plies per 1 in. thick) Laminated Asbestos Type--.................... 0.969 (30-40 Laminations per 1 in. thick) Laminated Asbestos Type______ _____ 1.103 (14-20 Laminations per 1 in. thick) Mineral Wool Type. _ .................. ..... 1.023 High Temperature Type.-- ............... 1.560 (Diatomaceous Earth and Asbestos) Brown Asbestos Type....................... : 1.003 (Felted Fiber) 1.024 1.465 1.437 0.960 1.104 1.028 1.489 0.997 0.997 1.505 1.438 0.951 1.105 -i.033 1.418 0.990 0.971 1.545 1.440 0.942 1.106 1.038 1.347 0.984 0.944 0.933 1.107 1.043 1.276 0.977 0.918 . --------; 0.924 1.108 1.048 1.205 0.971 LOW TEMPERATURE PIPE INSULATION , Surfaces maintained at temperatures lower than the surrounding air are insulated to reduce the flow of heat and' to prevent' condensation and frost. The insulating material should absorb a minimum amount of moisture, because the absorption of moisture substantially increases the conductivity of the material. This property is particularly'important in the insulation of surfaces that are below the dew-point of-the surrounding 783