Document nN3Bo8OoG2gwGGeJ8yJwE61rX

CHAPTER 18 Undulation 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 transfer through uninsulated pipes may be of considerable. magnitude if the temperature of the surrounding medium differs appreciably from that of the fluid conveyed. Careful consideration must, therefore, be given to this factor in a properly designed system and adequate insulation provided, 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 3), are given in Table 1. Heat losses from horizontal copper tubes and pipes with bright, lacquered and tarnished surfaces, are given in Tables 2, 3 and 41. ' The area in square feet per linear foot of pipe, is given in Table 5 for : various standard pipe sizes, and Table 6 for copper tubing, while Table 7 gives the' area in square feet of flanges and fittings for various standard pipe sizes. These tables can be ifsed 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 2 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 hours 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 Table 8, Chapter 1. The temperature difference between the pipe and air = 239.4 -- 70 = 169.4 F; By interpolation of Table 1 between tem perature differences of 157.1 and 227.7 F, the heat loss from a 2-in. pipe at a temperature difference of 169.4 F is found to be 1.624 Btu per hour per linear foot per degree tem perature difference. The total annual heat loss from the entire line = 1.624 X 169.4 X 165 (linear feet) X 4000 (hours) = 181,600 Mb. Heat Loss from Copper Piping, by R. H. Heilman {.Heating, Piping and Air Conditioning, September. 1933, p. 458). 344 CHAPTER 18. PIPE INSULATION Example 2. Coal costing S11.50 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 per cent, determine the monetary value of the annual heat loss from the line. . -e i--. innn Mh snnnlied to the system = 1,000,000 X 11.5 345