Document 1gn6qD5ZOe3E34XMjGx68EYnd

HEATINC VENTILATING AIR CONDITIONING GUIDE 1941 preferred as it best cares for expansion in the pipe. Cast manifolds or headers, known as branch tees, are available for this construction. OUTPUT OF RADIATORS The output of a radiator can be measured only by the heat it emits. The old standard of comparison used to be square feet of actual surface, but since the advance in radiator design and proportions, the surface area alone is not a true index of output. (The engineering unit of outputs is the Mb or 1000 Btu.) However, during the period of transition from the old to the new, radiators may be referred to in terms of equivalent square feet. For steam service this is based on an emission of 240 Btu per hour per square foot and for hot water service 150 Btu per hour per square foot. Table 1. Variation in Dimensions and Catalog Ratings of 10-Section Tubular Radiators (Steam) > No. of Tubes__ Width of Radiator....................... ...... Inches Length per SectionInches 3 4.6-5.1 2.5 4 6.0-7.0 2.5 5 8.0-8.9 2.5 6 9.1-10.4 2.5 7 11.4-12.8 2.5-3.Q Height with Legs--Inches 13-14 16-18 20-21 22-23 25-26 30-32 36-38 Heat Emission--Equivalent Square Feet 20 28.5 15.0-17.5 20.0-22.5 25.0-31.2 30 20.0-21.3 25 30.0-33.9 35 20.0-26.7 25.0-27.5 32.5-39.8 37.5-40.0 25.0-30.9 33.3-35.0 40.0-48.6 50 30.0-36.7 40.0-42.5 50.0-56.5 60 25.0-32.5 30.0-38.3 36.7-45.0 40.0-45.2 50.0-53.5 63.3-62.5 70.0-75.4 Table 1 illustrates the difficulty in tabulating tubular radiator outputs since there is so much variation in design between the products of the different manufacturers. Only on the four-tube and six-tube sizes is there any practical agreement in output value. The heat emission values appear as square feet but are entirely empirical, being based on the heat emission of the radiator and not on the measured surface. An average value of 300 Btu per actual square foot of surface area per' hour has been found for wall radiators one section high placed with their bars vertical. Several recent tests1 show that this value will be reduced from 5 to 10 per cent if the radiator is placed near the ceiling with the bars horizontal and in an air temperature exceeding 70 F. When radiators are placed near the ceiling, there is usually so noticeable a difference in temperature between the-floor level and the ceiling that it becomes dif ficult- to heat the living zone of a room satisfactorily. The heat emission of pipe coils placed vertically on a wall with the pipes horizontal is given in Table 2. This has been developed from avail able data and does not represent definite results of tests. For such coils the heat emission varies as the height of the coil. The heat emission of each pipe of ceiling coils, placed horizontally, is about 126 Btu, 156 Btu, University of Illinois, Engineering Experiment Station Bulletin No. 223, p. 30. 240 CHAPTER 12. RADIATORS AND GRAVITY CONVECTORS Table 2. Heat Emission of Pipe Coils Placed Vertically on a Wall (Pipes Horizontal) Containing Steam at 215 F and Surrounded with Air at 70 F Btu per linear foot of coil per hour (not linear feet of pipe) Single row. Two.-------Four...... -- Six............. Eight.------Ten______ Twelve...... 1 In. IK l. 132 162 252 . 312 440' 545 567 702 651 796 732 907 812 1005 IH In. 185 348 616 793 907 1020 1135 and 175 Btu per linear foot of pipe, respectively, for 1-in., 1 J^-in., and lj^-in. coils. Effect of Paint The prime coat of paint on a radiator has no material effect on the heat output, but the finishing coat may influence the radiation emission and thus affect the heat output. Within the range of temperatures at which radiators operate, color has no appreciable influence on the radiation emitted. Thus, finishing coats of oil paints of various colors will give the same results. However, a bronze paint, applied as the finish coat will change the character of the surface and reduce the amount of heat emitted by radiation. No paint has a noticeable effect on the portion of heat which is given off by convection. The larger the proportion of direct radiating surface, the greater will be the effect of any finish coat of paint which changes the character of the surface. Available tests are on oldstyle column type radiators which give results as shown in Table 3. Effect of Superheated Steam Available research data indicate that there is probably a decrease in heat transfer rate for a radiator or gravity convector with superheated steam in comparison- with saturated steam at the same temperature. The decrease is probably small for low temperatures of superheats and additional tests are necessary with varying degrees of superheat to establish accurate comparisons'for all types of radiators and convectors*. Table 3. Effect of Painting 32-in. Three Column, Six-Section Cast-Iron Radiator3 Radiator No. 1 2 3 4 Finish Abba Sq Ft. Coefficient or Heat .Trans. Btu Rbi.ahv fc Heating Value Per Cent * One"coat dull black heat resistant paint.... 27 27 27 27 ' 1.77 1.60 1.78 1.76 100.5 90,8 101.1 100.6 ^Comparative Testa of Radiator Finishes.- by W.- H. Severhs- (A.S.H.V.E. Transactions, .Vol. 33, 1927, p. 41). ` Tests of Radiators with Superheated Steam, by R* C. Carpenter (A.S.H. V.E. Transactions, Vol. 7, 1901. p. 206). 241