Document k6Npw12bLNwbM1OXm56K5yNrV

Sturfy to rfstsrmlno moftl conomicfll rtheet priwurs Stsem cortrfWonn 2,OOOpslQ, IjOOOF, -- rshsot to 1000 F, 29 In. vacuum 120,000 K* sise lurblns unit Pressure to reheoter, psio 4 Optimum efftctency and hoot rate for cycle in Fig. 3 occurs ot 365 psio 5 All presiuro ond lomperoture levels hove one best iteom-reheol pressure l 7640 400 440 400 920 960 600 Pressure to rahaotsr, psio Jj Optimum efficiency and heat rots b] cycle in Fig. 6 occurs ot 460 ptbi in a moisture content less than 12 to 13%, there was e leellng among many users that the added complication and cost of reheat installations were not justified. Also, many engineers were not ready to adopt the unit arrangement of one boiler per turbine, so well suited to reheat. Until the pressure of rising fuel costs was felt recently, less alien* lion was given to reheat and more to increasing thermal efficiency through (1) improving regenerative cycle (2) topping turbines (3) maximum praclicsl throttle pressures and tempera* lures. Topping. 'Topping turbines filled a special need so a considerable number were installed in the late 1930s. Bo* cause of limited opportunities, few have been installed In recent yean. Regenerative Cycle. Also in the Isle 1930s, much progress was made in using the regenerative feedwater-heat- ing cycle. Four and five stages of fed] heating became commonplace in centrtl stations; sometimes even six ond stages were used. | Throttle Condition*. For 15 year metallurgical developments hive pc milled steady Increase in allevtbl steam temperatures. With the gabj thermal economy through higher laldi temperatures, additions! cost and cog plication of reheat were not consider justified. The present high fuel pric* however, led to considering the cot bination of reheat with high laid steam conditions, even though the pol fell below the Fig. 1 curve. ^ Reboot Operation. There is a defim trend toward wider use of the siofj boiler ingle*turbine arrongemeot b cause of proved high availability j modern boilers and the necessity fj keeping initial costs os low as potm Adoption of this setup overcomes sc? objections to the reheat cycle, for minimum complication needs * gle boiler per turbine. Power plants that have used the J beat cycle many years report no *crio difficulties. Reheat turbines boie *>, put on and off the line ns needed no greater difficulty than with n5 reheat machines. In earlier years, one objection the large steam volume eninifl reheat Does that might causa speeding with lood loss. Today, turbines bsve intercepting valves lino going to and from turbine to boiler. These valves, under coo the main speed governor, bine speed increases by about 4% "1 "1 T' 1 "1 fi8 ISC m condition*. e 30 psig, tpt10 r,, it i91/300 F 29 1 vv Ctf Ul -v *P turbiniaitxwit 1 t -- rehsotir 1 S * I 3 f | o gg Ini I 400 440 1--1 I J J 480 920 900 000 Pressure to refieoter, psio 400 440 400 820 960 600 Pressure to ohet*r, psio 8 Output of h-p turbine decreases with Q with h*p hoottr cbove reheat pressure increase In steam reheating pressure v there is on optimum vofue for latter normal. This has proved a satislactory solution to the overapeed problem with* odi undue complication of the turbine governor gear. Rsfieat Economy. Estimates indicate that gain in economy by reheat varies from 4 to 6%. Fig. 2 compares heat rates fora typical 100,000-kw nonreheat unit end a similar-site reheat unit. For'* each, throttle steam conditions are 1250 psig. 950 F; exhaust vacuum is 28% In. Hg. Fced*heaiing cycle is the same in both, and reheat is carried to 950 F in the reheat cycle. Economic analysis, not given, eon* liders initial coat, fuel cost, load factor, reliability, operating convenience, many other variables. Typical analyses show thermal gain may more than offset addi tional cost ond complicating factors. Raheot Presiuro. To find the most desirable pressure to reheat steam for the turbine and Us feed-heating cycle, calculations were based on 120,000-kw i/ulft editions of 1250 psig, [WO F at throttle, reheat to 950 F, ex- suit ot 29 in. vacuum. Calculations *.ere based on the cycle in Fig. 3. This `gram is not necessarily a recomniei1 ed arrangement, but merely indi- obi? uted (or convenience to e ,n relative results for comparison. estn for the h*p heater in each case hi t!!Uc,td *rom lh turbine at reqv , i* before going to reheat boiler. showed on extraction * *be reheat point was not .* m,. 61 ^or this throttle-steam pres* p* with an economiser. * moiiM,1 ,tutJ7 'uulu to find the "kfcnt reheat pressure for this rating. Variation in heat rate over en tire range of reheat pressure U only a fraction of 1%. For tho conditions set up, optimum reheat pressure is about 365 psia. Since reheat pressure can vary considerably without appreciable change in turbine heat rate, other (ac tors like piping and valve sites or boiler performance may indicate a dif ferent pressure to be more economical from the over-all plant view. These data check with pressures and with throttle to reheat pressure ratios on existing machines. Fig. S shows a similar study for an 80,000-kw turbine lor throttle condi tions of 1290 psig, 850 F, and 650-F reheat temperature. This machine was actually built to give reheat pressure of 329 psia at the load point for which the onslysis was made. Consideration is being given to large reheat turbines designed for throttle steam, conditions in the 2000-pslg 1050F range, reheating to 1000 F. Fig. 7 shows the cycle for making a reheat pressure analysis of a 120,000-kw tur bine with these conditions. Unit is the cross-coinpound type with the h*p 3600rpm turbine exhausting steam to the reheat boiler and h-p feedwater heater. Steam from the reheater expands in the intermediate-pressure turbine down to 25 psia, then exhausts to the double flow 1-p turbine in tandem with the i-p turbine at 1800 rpm. Fig. 6 shows again that efficiency variation with pressure is only a frac tion of 1%. Curves indicate a most effi cient pressure of about 460 psia for turbine and its feed heating cycle. In addition to over-all consideration of valve and pipe sizes and boiler per formance, with the cross-compound turbine, generator sixe also depends on reheat pressure, which is the h*p tur bine exhaust pressure. Fig. 8 shows output of the h-p 3600*rpm element as a percentage of total output at different exhaust pressures for this element. While this curve was plotted from cal culations for the 120,000-kw rating, it can be used for close approximations for larger ratings with the same steam conditions. Since reheat pressure can vary considerably without affecting efficiency, reheat pressure may be selected to give standard ratings for the two generators. For the results of Fig. 6, steam for tho highest-pressure heater was taken from the h-p turbine's exhaust. With the cross-compound arrangement, space is available for an extraction nozxle'on the h-p turbine et o point giving a pres sure higher than reheat pressure, and another heater can be added to the beating cycle. This can reduce turbine heat rate about 0.6%. Whether over all plant economics justifies this ex pense, higher-pressure extraction must be analyzed in each case. With an economizer before the boiler, this heater probably cannot be justified, but some power-plant designers are giving serious consideration to these extremely high-extraction pressures. Fig. 9 shows results of determining optimum reheat pressure when a final heater at e higher pressure produces final feed* temperature. Feed tempera ture was not held constant in this study, but was varied eo an approximately equal rise occurred in.the highest-pres* sure heater in each In this at in all calculations for data on reheat performance, a 10% pressure drop was taken in the reheat lines.and reheat boiler. Today, problems of the steam-plant designer are more complex than ever, since not only have fuel costs increased, but construction and equipment costs have also jumped in varying propor tions. Further increases in pressure and temperature with the straight regener ative cycle have reached the point of diminishiog returns.' In this newlight, designs making use of the reheat cycle are being reconsidered. Naturally, re heat ia not the answer to oil problems of reduction in power-generation coat but a number of eases can easily justify the additional cost of reheating. BiWlogrepby. "The Reheat Cycle," by W Wedkin, Mechanical IFortd, March 30, 1945. "Is Reheat Coming Backf" Combustion, September 1947. 100 (466) POWER Au9<ti Auguit 1946 (467) tot