Page:The American Cyclopædia (1879) Volume XV.djvu/340

 328 STEAM - 1 * 48, and log. V=2'516 0-939 log. P. In the following table constant multipliers are given, the product of which into the initial pressure will give the mean or the terminal pressure for the grade of expansion selected : MEAN AND TERMINAL PRESSURES (SALTER). POINT OF CUT-OFF. CONSTANT. DBT AND SATURATED. CONDENSING BY WORKING. Mean. Terminal. Mean. Terminal. Mean. Terminal.

0-886 0-465 0-522 0-597 0-748 0-847 0-966 0-125 0-167 0-200 0-250 0-875 0-500 0-750 0-869 0-449 0-506 0-532 0-732 0-839 0-964 0-110 0-149 0-181 0-229 0-353 0-479 0-787 0-857 0-487 0-495 0-571 0-728 0-833 0-962 0-099 0-187 0-167 0-214 0-836 0-468 0-726

A mixture of steam and other gas has a ten- sion which is equal to the sum of the tensions of the two components. Thus, if a cubic foot of air at atmospheric pressure be enclosed in a vessel of that capacity, and if a cubic foot of steam of the same tension be introduced with it, the pressure upon the walls of the vessel will be two atmospheres, the temperature of both gases being the same. Steam formed from sea water is liberated at a higher temper- ature than when formed from pure water. The boiling point of water is raised about 0*04 F. for each increment of 1 per cent, of its own weight of salt. Sea water, containing -^ of its weight of salt, boils at 213'2 under atmos- pheric pressure. The maximum proportion of salt permitted in marine steam boilers is usu- ally /j, the boiling point being raised 2 -4 F. Steam, as worked in the steam engine, if not dried by superheaters, is wet ;. <?., it carries in suspension fine particles of water. The amount of water so suspended has been found by Prof. Thurston to be from 0'03 to 0'20 of the weight of the mixture. Ten per cent, is a usual proportion with -good boilers. The amount was determined by condensing in a calorimeter a determinable weight of the mix- ture, by the use of a known weight of water, and noting the rise in temperature of the lat- ter. Knowing the temperature due to the steam pressure, the weights of steam and wa- ter can be determined. The principal advan- tage of superheating is an increase of economy due to the thorough expulsion of water from the vapor, and consequent reduction of loss by condensation and revaporization in the steam engine cylinder. A less degree of improve- ment is due to the simple increase of tem- perature, and to the consequent widening of the range of temperature within which it is worked. The most elaborate and most accu- rate experimental determination of the coin- cident temperatures, pressures, and volumes of saturated steam were made by Regnault, at the expense of the French government, and under the auspices of the academy of sciences, and published in the Memoirea de V academic for 1847. The following table gives a summary of the properties of steam based upon Regnault's determinations. Pressures are given in pounds per square inch above a vacuum, and in inches of mercury measuring from the same point. Volumes are relative to water at its greatest density. Weights are given in pounds, and specific gravity is referred to air as unity at a temperature of 32 F. The distribution of heat in each pound of steam evaporated at 212 F. is given as follows: Unit* Mechanical of equivalent, heat. In foot pounds. A. The sensible heat : 1. To heat the water from 82% or through 180 ISO'9* = 189,655 B. The latent heat: 2. To convert the water to vapor, ir- respective of pressure on surface 892-9 8. To advance against and remove the incumbent atmosphere, whe- ther air or previously generated steam, its pressure being 2,116-8 Ibs. per square foot of surface 72'8 55,815 Total latent heat 965'2 = 745,057 Total heat of steam 1,146' 1 = 884,712 It is evident that the total latent heat of steam cannot be taken as in any way the measure of the energy or work in, or that can practically be obtained from, the steam. Much the larger part of such heat is expended in merely over- coming the cohesion of the liquid ; and at all temperatures but a small fraction of the latent heat can be made available in perform- ing work. Of the total, seven tenths is lost through the existence of natural conditions over which man can probably never expect to obtain control, two tenths through imperfec- tions of mechanism, and but one tenth is util- ized in even good engines. PROPERTIES OF SATURATED STEAM. PRESSURE. Temperature in degrees. Latent heat in degrees. Total heat in degrees from zero. il Weight of l cubic foot. ft i* Inches of mercury. i 2-0 102-0 1,048-0 1,145-0 20,620 0-0030 Ib 0-037 5 10-2 102-3 1.000-7 1,168-4 4,585 0-0187 0-170 10 14-7 20-4 80-0 193-2 212-0 979-0 966-1 1.172-9 1,178-6 2,860 i 0-0264 1,644 0-0379 0-827 0-470 15 20 80-5 40-7 218-0 227-9 965-0 954-4 1,178-9 1,168-5 1,612 0-0387 1,220 1 0-0511 0-480 0-633 m 50-9 240-0 945-8 1,187-1 985 0-0684 0-786 80 61-1 250-2 938-9 1,190-3 827 0-0755 0-935 85 71-8 259-2 932-2 1,193-0 713 0-0875 1-085 40 81-4 26M 926-5 1,185-4 628 0-0994 1-282 45 91-6 274-3 921-3 1,197-6 562 0-1111 1-377 50 101-8 280.9 916-6 1,199-6 508 01227 1-521 55 112-0 2869 912-8 1,201-4 465 0-1848 1-C64 60 122-2 292-5 9082 1,208-2 428 0-1457 1-605 65 132-3 297-8 904-5 1,204-8 8<JS 0-1570 1-946 70 142-5 802-7 900-9 1,206-8 371 0-1682 2-084 To 152-7 807-4 897-5 1,207-7 fa 0-1792 2-221 80 162-9 811-8 894-8 1,209*0 828 0-1 101 2-867 85 173-1 816.0 8918 1,210-8 810 0-2010 2-492 90 1S5-2 820-0 888-4 1.211-6 2S5 0-2118 2-C25 95 198-4 823-9 885'6| 1,212-7 281 0-2225 2-757 100 208-6 327-6 882-9 1.218-8 268 0-280 2'887 105 213-8 83M 880-8 1.214-9 256 0-2484 8-016 110 224-0 884-6 877-9 1.210-0 246 8-144 115 234-1 337.8 B76-B 1.217-0 236 0-2640 8-272 120 244-8 341-0 878-1 1,218-0 228 0-2748 8-809 125 '-':.4-r> 8441 sT(>! 1,218-9 2-20 0-2648 8-5'J8 150 805-4 858-2 860-6 1,223-2 187 0-840 4-139 200 407-2 881-6 843-4 1,280-8 147 0-4250 5-2C6
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