Document wqx08mGDyyyrKqNKY8N95r6OJ

588 CHAPTER 26 1951 Guide ASME Power Bdiler Code, American Society of Mechanical Engineers. American Standard Code for Pressure Piping, ASA B-31.X--1942, American Standards Association. Shipping*6 Engineering Regulations of the Coast Guard, American Bureau of General Specifications for Inspection of Material, Appendix VII, Welding U S Navy. Specifications for Welding, Appendix 5, Part 1-General--for vessels of the U. S. Navy, Bureau of Shtps, April, 1940. 7 American Standard, Steel Butt-Welding Fittings. ASA B16.9-1940, American btandards Association. American Standard, Steel Socket-Welding Fittings, ASA B16.11-1946, American btanaards Association. CHAPTER 27 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 that 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 8 show how the annual heat loss from uncovered pipe and its dollar value may be computed from the data in Table 1. Example 1. Compute the total annual heat loss from 165 ft of 2 in. bare pipe in service 4000 hr per year. The pipe is carrying steam at 10 lb pressure and is exposed to an average air temperature of 70 F. Solution. The pipe temperature is taken as the steam temperature, which is 239.4 F, obtained by interpolation from Steam Tables. The temperature difference be tween the pipe and air = 239.4 -- 70 = 169.4 F. By interpolation of Table 1 between temperature differences of 157.1 and 227.7 F, the heat loss from a 2-in. pipe at a tem perature difference of 169.4 F is found to be 1.624 Btu per (hr) (linear ft) (F deg). The total annual heat loss from the entire line = 1.624 X 169.4 X 165 (linear ft) X 4000 (hr) = 181,600 Mb. (Mb = 1000 Btu.) Example S.' Coal costing $11.50 per ton and having a calorific value of 13,000 Btu . per pound is being burned in the furnace supplying steam to the pipe line given in the previous example. If the system is operating at an overall efficiency of 55 percent, determine the monetary value of the annual heat loss from the line. Solution. The cost of heat per 1000 Mb supplied to the system = 1,000,000 X 11.5 (dollars) -s- [13,000 (Btu) X 2000 (lb) X 0.55 (efficiency)] = $0,804. The total cost of heat lost per year -- 0.804 X 181.6 (thousand Mb) = $146.00. 589