Document npqJXLGMLgmnG5RVXVvbDKj6

624 CHAPTER 27 1954 Guide tractors National Association, Second Edition, 1951). Welding Handbook (-American Welding Society, 1942).. . :/ 4 ASME Power Boiler Code, American Society of Mechanical Engineers. ` American Standard Code for Pressure Piping, ASA B-31:1--1942, American Sta8nMdaardrisneAsEsnogciinaetioenri.n- g Regulations of the Coast Guard, American Bureau of Ship pinGg.eneral Specifications for Inspection of Material, Appendix VII, Welding, U. S. Navy. Specifications for Welding, Appendix 5, Part 1--General--for vessels of the U. 7SA.mNearvicya, nBuSretaanudoafrdS,hSiptse,eAl Bpruilt,t-1W94e0l.ding Fittings, ASA B16.9-1940, American Sta8nAdamredsri-cAasnsoSctaiantidoanr.d, Steel Socket-Welding Fittings; ASA B16.11-1946, American Standards Association. CHAPTER 28 PIPE INSULATION Heat Losses from Bare and Insulated Pipes,-Low Temperature Pipe Insulation, Insulation of Pipes to Prevent Freezing, Economical Thickness of Pipe Insulation, Underground Pipe Insulation THE heat loss from uninsulated pipes may be of considerable magni tude if the temperature of the surrounding medium differs appre ciably from that of the fluid conveyed. Losses are increased by rapid motion of the surrounding air or by contact of the pipe with bodies of high conductivity. Careful consideration must, therefore, be given to this factor in a properly designed system, and adequate insulation pro vided, if necessary. HEAT-LOSSES FROM BARE PIPES Heat losses from horizontal bare steel pipes, based on tests at Mellon Institute and calculated from the fundamental radiation and convection equations (Chapter 5), are given in Table 1. Heat losses from horizontal copper tubes and pipes with tarnished surfaces, are given in Table 2.1 Heat losses from bare pipe of materials having lower emissivities may be calculated from data appearing in Chapter 5. The area in square feet per linear foot of pipe is given in Table 3 for various standard pipe sizes, and Table 4 for copper tubing, while Table 5 gives the area in square feet of flanges and fittings for various standard pipe sizes. These tables can be used to advantage in estimating the amount of insulation required. . Very often, when pipes are insulated, flanges and fittings are left bare so as to allow for easy access to the fittings in case of repairs. The fact hat a pair of 8-in. standard flanges having an area of 2.41 sq ft would lose, at 100 lb steam pressure, an amount of heat equivalent to more than a ton of coal per year, shows the necessity for insulating such surfaces. Examples 1 and 2 show how the annual heat loss from uncovered pipe and its dollar value may be computed from the data in Table 1. - Example /; Compute the total annual heat loss from 165 ft of 2 in. bare pipe in rvice 4000 hr per year. The pipe is carrying steam at 10 lb pressure and is exposed w an average air temperature of 70 F. F StHn: The pipe temperature is taken as the steam temperature, which is 239.4 tw ,ine<^. ky interpolation from Steam Tables. The temperature difference be, en pipe and air = 239.4 -- 70 = 169.4 F. By interpolation of Table 1 between ,, Per.at*jre differences of 157.1 and 227.7 F, the heat loss from a 2-in. pipe at a tem- ture difference of 169.4 F is found to be 1.624 Btu per (hr) (linear ft) (F deg). ThA jrn-ai annual heat loss from the entire line = 1.624 X 169.4 X 165 (linear ft) X 4000 = 181,600 Mb. (Mb = 1000 Btu.) Pe^Do"1^ Pa' costing $11.50 per ton and having a calorific value of 13,000 Btu Previn d 18 l*'11 burned in the furnace supplying steam to the pipe line given in the determ-8 examPie- H the system is operating at an overall efficiency of 55 percent, . o , 8e tae monetary value of the annual heat loss from the line. (dol?0U*N0n' The cost of heat per 1000 Mb supplied to the system = 1,000,000 X 11.5 ofhi!?V + f13-000 (Btu) X 2000 0b) X 0.55 (efficiency)] = $0,804. The total cost - eat lost per year = 0.804 X 181.6 (thousand Mb) = $146.00. 625