Document 2b8Lgv9OgJbrqLzpq3eVr01b

ehaves as it is heated to steam at different constant pressureSj Superpressures: The Next Step to Top Efficiency Using pressures above the critical mokes steam behave differently. The usual boiling process disappears completely. Here's a quick run down on steam facts and figures By B O A SKROTZKI, Aaacklo Editor Big Volume Marks low Pressure Recent announcements reveal wo'rc at the start of a great stride forward in power-plant condition*--a move into Lire region far above the critical pressure. For years our attention centered on the steady advance of throttle temperature. Now the focus shifts to pressure--to a region about which most of us ere somewhat liaty. So we believe most engineers wiii welcome a refresher ' on the physics and thermodynamics of steam at pressures above the critical On the first two pages, we review the behavior of one pound of water as it's healed under con stant pressure. On the next two pages, we study the action of steam in different cycle layouts and compare thermal efficiencies and other 'characteristics. On the last two, there are preliminary details of a projected steam plonl to run ol superpressure, together with an outline of some of the problems being studied. In the continuing struggle to hold down power cost in spite of rising fuel prices we naturally look for ways of improving thermal efficiency. We well know, however, that this usually means using more complicated equip ment in the power plant. Often this entails greater invest ment ond consequent carrying charges. So the fuel sav ing we realize by using more efficient apparatus is partly offset by the additional financial burden. In any event, to achieve oiir aim, the total production and ihvestment charges against a kwhr must be reduced. That this has been consistently done in the past is the greatest tribute to the genius of our power-plant design engineers. Let's pour one pound of 32-F water is our cylinder and put a total pressures! 250 psi on the phton. Thai places the piston at a. Now we'll heat the water and find it expands very slightly to s', while its temperature goes up to 400.95 F as shown above in the lower graph. The upper graph shows that vie ban put in 975.2 Blu of heal, which is the enthalpy of the water. As we heat beyond ibis point, boiling begins and the volume increases greatly to b, while the temperature stays con stant. We also find that we have pul ia a great deal of heal to boil off the water to steam, as shown in tho upper graph, 625.1 fitu. After all the water has changed to steam, the volume continues increasing on further heating, and the temperature agnin increases. We call ibis superheat ing. The upper graph shows us that lor `each Btu pul into tho superheated steam we get a greater temperature rise ihto for on equal amount put Into the water. If we could look into our cyUadff during the boiling process, we woaW see weler covering the lower part. 1 the water small steam bubbles form* which rise and break through the water surface to join the steam filling the upper part of the cylinder. Siam formation causes large volume Increase 10 SNOfNieaiNO ANO MANAOfcMCNT SECTION POWf ..__.....___ d:.. ii>. Tamnoraiiirs I Nn Rollina ol Critical Pressure Water Or Starling again with one pound of 32-F water, let's now load our piston to develop a pressure) ol 2000 psia. As wo heat the water it again expands very (lightly while its temperature rises, the pressure staying constant. The water becomes saturated ot 695.82 F. Upper gtsph shows we have pul in 665.7 Btu .ol hesi io bring it to the boiling point, compared to 975.2 Btu at 250 psia. Healing the saturated water to boil -it off to steam, we find that (he volume expands by only 0.1621 cu ft, compared to 1.62S1 cu ft at 250 psia. Comparing the total heat input from 32 F, ii has required 1129.1 Btu to form saturated esm at 2000 psia and 12003 fitu at 2S0 psia--not very different even though the volumes differ greatly. Adding heat to the saturated steam, ** superheat it as shown by the rising tempmtoie. Getting up into the hightemperature range we find that the entbslpy is not greatly different at the different pressures. For instance, at 1200 F, 2000-psia steam has 1596.1 Dlu U> while 250-psia steam has 1632.7 Bra. ^b best or enthalpy of vaporisation water has decreased markedly for higher pressure. It is only 463.4 jhu- (or the 2000-psia steom, but 825.1 t0 for ZSO-ptla steam, neatly double. Now lei us really load our piston by applying the critical pressure of 3206.2 psia. We start heating our 32-F water and again the volume increases slightly while the temperature rises. As we reach s temperature around 680 F tho /ate of volume increase rises slightly til! we reach the critical temperature of 705.4 F. After that, the volume-in crease rate drops off aomeuditt with the temperature still rising. At the critical pressure we didn't get any volume increase at a constant tem perature. Looking at the heat-input graph above, wo find a point of inflec tion in the 3206.2-psla pressure curve, but no horizontal, constant-temperature boiling process. In other words, the boiling or evaporation process has dis appeared. Just what does this mean? We start off with a fluid we reeognizo as water at moderate room temperature*. As It is heated, molecule* of the water keep expanding the spaee they need to nioinloin o constant pressure. Finally, we get to temperatures at which we know the fluid acta like steam or a vnpot. At no time does a water level or meniscus appear. Neither do we see any forma tion of steam bubbles. As we heat the water it limply keeps expanding in a smooth manner until it become* steam. At superpressures it's a matter of ar bitrary definition whether we call the fluid water or steam. We could adopt the rule that at temperatures above the erilicsl we call the fluid steam, while below, it's water. The graphs above summarize HjO behavior at many states. The experiment below demonstrates the disappearance ol water level be cause water and steam densities becomo equal ot superpressurea. TNI OflAPFEAXINO WATC8 IIVS1I Temparalvrs, F Prtsure, ptSo vapor (Wntllp, Ibper <u It 02237 11.66 1980 Meniscus- WoHr dimity, lb p*r i It 564 &9e 1968 HeotlnQ voter ond its vapor, seated in quartz tube, raises the pressure ond temperature. Wofor level becomet Indistinct, disoppeors at crilkol point ENOINCEIINO AND MANAOEMEW SECTION TUSH PASS i ii *