TESTS OF RADIATORS WITH EXTENDED SURFACE SO AS TO FORM AIR - FLUES, COMPARED WITH PLAIN CAST-IRON RADIATORS.* d' D' Same as D with ex tended surface re- 9 38 84 I 38 84 41.2 3.97 225 71.7 153 0.292 0.00191 1.85 284 285 4.50 4.0 222 66.2 159 0.365 0.00231 2.24 355 312 52. Tests of Indirect Heating Surfaces.-The tests which have been made on indirect heating surfaces show very great difference in results, varying from those given by Peclet for the loss due to convection alone, to results which are 8 or 10 times as great. This difference in result is no doubt due in each case to the velocity of air which comes in contact with the surface. When the indirect radiators are not freely supplied with air, or the velocity is low, the amount of heat which is discharged is small; when the velocity of the air is high, the amount of heat taken up is proportionally greater. According to experiments made by the writer, the coefficient of heat transmission increases as the square root of the velocity of the air. The amount of air passing over a given surface of the radiator can be estimated quite accurately by the amount of heat given off, which we can reasonably suppose in this case to be Test by Denton & Jacobus, July, 1894. all utilized in warming the air. At a temperature of about 60 degrees, I heat-unit will warm 55 cubic feet of air 1 degree (see Table VIII), so that the number of cubic feet of air warmed is equal to 55 times the total number of heat-units given off from I square foot of heating surface per hour, divided by the difference of temperature of entering and discharge air. NOTE. Let T = -- temperature discharge air, t' that of entering air, H = total number of heat-units given off per square foot of surface, a the number of square feet of surface. Then, Cubic feet of air per square foot heating surface = 55H (T-t')a The following tests, made under the direction of the writer, give actual results obtained in testing steam-pipes in a current of air moving at different velocities: SUMMARY OF RESULTS.-TEST OF 2" STEAM-PIPE WITH COMPARISONS OF WATER AND STEAM CIRCULATION, WITH INDIRECT RADIATORS, NATURAL AND FORCED AIR-SUPPLY. BY J. H. MILLS. 9 Staggered Tube Coil Radiator, Shakelton's, water. 10 J. H. Mills, Mills' indirect, Shakelton's. water.... 1885. Gold's pin, Shakelton's, water. 192 192 176 134 99 49 1.97 106 99 1.00 192 195 I 5 85 143 105 58 1.5 117 13: 1.25 78 202 172 38 128 149 90 4.5 123 213 1.43 145 251 1.82 144 206 186 78 150 118 72 149 206 1.75 144 206 194 78 166 122 88 2.5 148 250 2.05 144 214 178 28 143 168 115 5. 146 323 1.92 144 214 181 28 138 169 110 5.3 157 332 1.96 50 198 159 34 116 144 82 7.8 158 249 1.73 144 196 155 40 111 135 71 6.6 204 279 2.07 144 196 155 40 114 135 74 7 211 300 2.22 Averages. 202 173 53 134 135 82 4. 6 151 239 1.75 Comparison of Steam and Water under similar Conditions and Temperatures. Nason, 10-foot flue NOTE.-For Nos. 5, 15, 17, and 19 the heat recorded is that due to the amount of steam condensed (see Table XII). 85 217 228 178 83 133 145 50 10.1 534 513 3-54 From the general results shown in the table page 80 it is seen that the heat-units given off per square foot per degree difference of temperature equals very nearly the square root of four times the velocity in feet per second. That is, h = √4v. The tables pages 81 and 82 contain an extensive summary of tests of indirect radiators, abstracted from Mills' work on Heating and Ventilation, and are of especial interest as showing the close agreement in results, whether water or steam is used. The higher results in this table agree fairly well with the rule stated; those for natural draught are much smaller, and approximately equal to the square root of the velocity in feet per second. 53. Conclusions from Radiator Tests.-The general results of radiator tests can be summed up as follows: First, that the values for heat transmission in recent tests of direct radiators vary greatly and differ more from an average result than from those given by Péclet, and consequently his results can be used with confidence as applying to modern radiators. Second, the results of the test show greater differences in favor of low radiators as compared with high ones than was shown in the experiments of Péclet. Third, the experiments do not show any sensible difference for different materials used in radiators or for hot water or steam, provided the difference in temperature between the air in the room and that of the fluid in the radiator is the same. Fourth, the internal volume of radiators is of value only in lessening the friction of the fluid. It has no special influence on the results. Fifth, the extended surface radiators, or radiators in which the cast iron projects. from the surface into the air, show large results when estimated on the basis of projected or plain surface, but show very small results when estimated on the basis of measured surface. Sixth, thin radiators, or those with one row of tubes, always show higher efficiency than thick ones or those with numerous rows of tubes. Seventh, comparative tests of radiators should only be made between radiators of similar forms, or at least those which have about the same amount of surface. |