Document v6Y9j8o6E0p3wQaOJnKjqZkJE

Relative Capabilities of Vacuum ftimps |..i Handling Ao Air-Vapor Mixture 'Sosref on 1.SF Temperature Depression Curvts ofconstant -hfr--o* at-ir--fet--atwree-, /singft element, 2stage q I #teamjtf air ejvoter, i 1 nominally rafeet A 7Sefrn\ 7-notory pumps,\ b Single stags hogging *,' Jet, nominally rated i J 1500cfm air vapor 4-* mixture at tOin. hj oAi * 0.1 0.2 as I 2 3 57 Pressure of pump auction, In. Hg abs Relative capabf/Hfes of vocuum pumps depend on type and Individual character istics; the rotory-pump copaclty remains relatively constant at. all ptessuret In view of the email eienm end power requirement* this bee been disregarded. Making Decision. An ommenta can* lamination problem may dictate selec tion of the rotary pump. Since an elr ejector can readily furnish greater air* removal capacity than n rotary pump this may lead to its selection in other cased. Use of a rotary pump necessi tates keeping air leakage within limits of best practice to ovoid loss in vacuum. At lower loads, where alt pumps rather than the condenser control vacuum, the air ejector may produce an overage of 0.1 in. Hg better vacuum. ComblnoHon Unir. An interesting ar* rangement, in which both high vacuum capability under normal operation and high pumping capacity when starting ore combined in one set of equipment, occurs in the primary steam jet--sec* oodery reciprocating- vacuum pump unit. This has been in successful op* eratlon in one modern station for s number oi years. The secondary pump* Ing unit that operates down to about 4-in. Hg aba is used to evaeuate the condenser when alerting, and then con tinues to run os the eecond stage of a steam jet, which operates at about l In. Hg aba. The primary jet can be op erated with low-pressure extraction steam, thereby improving the economy of this combination unit AIB-RIMOVAI CAPABILITY Generally speaking, the air-removal capability of a rotary vacuum pump decreases with (1) higher vacuums, (2) a decrease in temperature depression (beiow the temperature corresponding to the condenser vacuum) of the airvapor mixture os it posses over the olrcooler tubes, and with increased air leakage. Air leakage in a tight surface con denser. with the turbine glands in good condition, should not normally exceed 3 to 4 cfm. Some condensers perform consistently with only 15- lo 2-cfm leak age. Others may tun as high as 5 to 6 cfm, but greater leakage is un usual with o condenser in good condi tion. i Condenser manufacturers base their air ejectors on the requirements of the Heat Exchange Institute. The following repreaentative figures ore from the Con denser Section of its Standards: Moximutn free Steam Condensed, Air Leakage lb per hr cfm at 70 F 100.001-150,000 iso.ooi-250,000 6.5 as 250.001-350,000 10.0 350.001-450,000 US 450.001-600,000 135 600,001 and up * 16.0 As applied to Urge surface con densers, this table would bear revision, since It requires designing for mueh higher air-leakage rates than are nor mally experienced. As an example, if an 80,G00-kw tur bine la selected for which the full-load condenser flow is 49S.000 lb per hr, the air leakage for which sir ejectors sre supplied under the above regulation is 13.5 cfm. This requirement i| q.' cessive. But furnishing two 2-iUge steam jets, each rated 6.75 cfm sod operating only one under nokmal coo.. diUona, would be a reasonable interpr* taiion of the Heat Exchange Instilate requirements. Unless otherwise re quested, a condenser manufacturer may furnish double this capacity. If la supplied in two jets, the previously mentioned annual operating costs would bo doubled. When operating in conjunction with \ a surface condenser a vacuum pump ' handles an air-water vapor mixture that has been subcooled several degrees be low the saturation temperature by pais.1 . ing over the tubes in the sir-cooler see- ; tion of the condenser. Under DaltenY law of partial pressures, each com ponent of this mixture (air and water vapor) occupies the same total volume** as (bat of ibe entire mixture, bat esci 2 component is present si its own\psr-. tiol pressure. Sum of the two partis]:* pressures makes up the total preuuii# at the air offtake. This partial pres-'S sure of the air at condenser-air off-take has e profound efleet on capacity of air-'* removal apparatus. fn all computations involving partial pressures, temperature depression of; the alr-vapor mixture below the satora-/ (ion temperature of (he vapor-must be either measured or assumed. (If sstur*-' tion temperature were used, the mlx-s ture would be 100% vapor.) Tho Heat' Exchange Institute specifies e 75-F; temperature depression. This is con-', servative, and good condensers should do better. Another criterion being con sidered in the industry stsamea the' air-vapor offtake at definite elevation to' temperature above that of the inlet dr-; culating water. A figure of 5 F bis.been mentioned, but it requires s goodj air-cooling section to accomplish this, Such an assumption Implies a greater! subcooling of the eir-vapor mixture tij high loads than 75 F below saturation, temperature, this bung more favorable! to either type of air removal equlpmcat., Under the pressure and temperature conditions In a condenser, both sir and water yapor obey the perfect get low and Dalton's law with sufficient ac curacy. Thus (or a given weight of oh at constant temperature U) v. - r, ! where y% air volume under reduced pressure V| free sir volume, or "sir leakage" P * etmoepberlo pressure, or 30 lo. Hg sbs -t P, partial pressure of the sir at sir wn take, In. Hg sbs 4 |472) POW6R August 1*4*2 for example, if the volumetric capacity ar pumping speed of a rotary pump is 550 cfm at 29-in.-Hg vacuum, (from Hg- 4) whet corresponding rate of leakage of free atmospheric sir can It handle and still maintain this vacuum? Assume 75-F temperature depression of the air-vapor mixture. Condenser pressure, in. Hg abs 1.0 Sslurotion temperature, F 79.0 Temperature depression, F 7,5 Temperature at air offtake, F 715 Partial pressuro ol vapor at 715 F, in. Hg aba 0.76 Partial pressure of air, in. Hg abs 0.22 From equation (1) end neglecting ef fect of temperature change on volume: 0 72 550 X 4.0 dm Thus s rotary pump can handle an airteekoge rate of 4 cfm of free atmos pheric air and maintain a vacuum of 29 in. Hg. Since the action of a steam-jet air ejector is based on the transformation of tbo jet kinetic energy into pressure energy of the air-vapor mixture in the diffuser, such jets are always rated in terms of their weight-rate capacity to handle an eir-vapor mixture. A repre sentative "6.75-cfm" jet with capacity in lb-per-hr plotted againat vacuum is shown on the left-hand curve of Fig. 3. If the temperature depression of the sir-vapor mixture as U enters the jet suction is known or assumed, the ca pacity of the jet in equivalent volume of free sir may be determined as shown io the right-hand curvo of Fig. 3. For this transformation use the following: U) y. - + where If* - total weight of alr-vapor mixture, lb weight of air in tho alr-vapor mixturtj lb P, * partial pressure of vapor Pi m partial pressure of sir Fig. 4 shows the volumetric capacity ol both types of pumps In term* of their capabilities under actual vacuum conditions rather than in terms of freeair handling ability. Tbe rotary pump exhibits an almost constant, although restricted, volumetric capacity through the usual operating range of vacuum in surface condensers, extending from atmospheric pressure into a range of low absolute pressures never realised la power-plant practice. The 2-stage team-jet air ejector, on the other hand, exhibits its characteristics of high ca pacity over a more limited range of pressure with rapid reduction in capac ity when passing into pressure regions, both above and beiow the operating range. Table I shows the effect of vacuum variation on air-pump performance, and Table II the effect of variation in temperature at the condenser air off take. These tables were calculated us ing equation (1) and show the effect of 4-cfm and 6.7S*cfm free air leak age. The volume of the alr-vapor mixture to be handled should be compared with tbe capacities of the two types of air pumps. Tbe two pumps in Fig. 4, al though of different capabilities, have been compared because the rotary pump is, to the author's knowledge, the larg est made at present aod the one usually used for this service. Tbe "6.75-cfm" oir ejector coold be supplied for this condenser,' although it is larger than the rotary pump, but it is smaller than (hose often furnished in the past. Fig. 4 includes curves of constant free air leakage, fntersections of these curves with the characteristic of cither vacuum pump indicates the limiting vacuum obtainable. The figure also indicates the "starling range" of pump suction pressure ond the comparative performance of a large hogging jet, rated 1350 lb per hr or 1500-efm airvapor mixture at 10 to. Hg abe with one and two rotary pumps. Note that theta performances are comparative only; air ejectors and hogging jets are available both larger and smaller In capacity, at prices and with operating easts in proportion. The higher the vacuum and the lower the temperature depression at the eir offtake, the less favorable are condi tions for either type of air pump. Un der the assumed air leakage, when the volume to be handled exceeds the airpump capacity at a given vacuum (asturning the condenser capable of pro ducing that vacuum) the air-removal equipment will be unoble to bold tbe vacuum and (l will fall off until the smaller air-vapor specific volume comes into equilibrium with the air-pump capability. If desired, air-removal ca pacity can be doubled by operating two units. Remember that some of the more extreme conditions in the tables, and those in which the air pump ca pacity b apparently exceeded may be beyond tbe reach of the condenser or uneconomical to attain. Large turbines, unless designed for unusually high vacuum, reach a point where an in crease in vacuum produces very little additional power output with constant throttle flow. Fig. 2 demonstrates the relative abili ties of the condenser and Its air pumps to "control" the vacuum over tbe load range, using an 80,000-kw steam tur bine and a 75,000-iq-ft 70,000-gpm con denser unit as an example. In this case, the temperature of the air-vapor mixture has been taken 5 F above that of the circulating-water inlet, instead of 75 F below saturation temperature. Solid lines show the condenser char acteristic at 65- and 80 F inlet circulat ing-water temperatures corresponding to average and summer conditions. Dashed ond dotted lines show air-pump performance with 4-cfm free air leak age for the various pump combinations designated. For the assumed rate of air leakage, at all points to the right of tbe intersection of the two seta of curves, the air pump can handle the air leakage, and has no effect on con denser performance, fn other words, (Continued on page 146) Table I--Effect of Vacuum oo Alr-Vapor Mixture Volume nom, in. Hg........................ 27.0 21.0 294 l*J Cm4im, prtsiuf*. I*. Hp ob*.1.0 2.0 14 03 Jr-*opor WiIvn, tltrt, to bo >;ts- ***m !? lookoet.............................. MS^fa fnn .......................... 101 IS* 1*0 1000 j# ^ 020 1700 'owe# Agqml 1948 Tatilo U--Effect of Vactnmi and Temperature on Air-Vapor Mixture Volume Tratweioro tfoprottlo* of olr-oopw alilen below Htwitli* (omptrotero, f............ Vofemo # fr-*por n/vfore, rim, to bo AeaSM WU trot off feefcepo tor eoerfoottr pt*- `I 7.1 10 TTa. h bt............................. .............. mo * too 2 lo. He ab............................................................ 700 410 U0 (47)| 25