Vapor recompression

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Vapor Recompression

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11 inauy industrial processes, largequantities of stearn are eithervented to the atinospliere or con-dstsed in a cooliizg tower, wastingenergy. This steam can be compressedback to II tisable presstire tisiitg only5-10 percent of the energy required togenerate the steam in a boiler, tlzerebyconserving the energy in the wastesteam. This Technology Updatedisctisses how this is accoinplishedmid outlines tlie methodology for cal-ciilatiiig the energy and financial sav-ings generated.

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BackgroundVapor (or steam) recompression is the technique of compressing avapor from a pressure too low for application use to a higher, usablepressure. Vapor recompression is analogous to the operation of a heatpump where mechanical work is used to upgrade energy from a lowertemperature level to a higher temperature level.Energy is supplied to the compressor to raise the steam pressure.However, this operation typically requires only 5-10 percent of theenergy necessary to generate the same steam in a boiler. This uniqueaspect of vapor recompression is a result of the major fraction of theenergy in the steam being available to the compressor inlet as latentheat. As a result, only a small fraction of additional energy is requiredto raise the pressure and temperature to a useful level.

In practice, vapor recompression is limited toFigure1. Relationship of compressor work to compression ratio compression ratios of less than about 2:l perstage. The compressor may be driven by an elec-tric motor, gas engine, or gas turbine.

The graph in Figure 1ndicates the energy re-quired to compress steam as a function of thecompression ratio and inlet pressure. Thecurves are based on adiabatic compression witha compressor efficiency of 70 percent. Becausethe steam leaving the compressor is super-heated, it is also assumed that water at 82.4"F issprayed into the steam to eliminate the super-heat. Figure 1can be used in conjunctionwithstandard steam tables to estimate the energysavings possible by employing vapor recom-pression. (Note: For a more accurate estimate,steam tables and/or a Mollier Chart should beused, together with efficiency data relating to aspecific make of compressor. Also, when desu-perheating is not necessary, the calculationshould be adjusted accordingly.)

Toll-free Hotline 1-800-872-3568 FAX 1-800-872-3882 Electronic Bulletin Baard 1-800-762-3319BonnevPOWE R ADM N ST RA T ION


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The nature of the industry orplant, the amount of higherpressure steam needed, the com-pression ratio, and the source oflow-pressure steam vary widely;thus a complete review must pre-cede each installation decision.ApplicationsTwo common applications usingvapor recompression are steamquality upgrading and waste-steam heat recovery, which arediscussed in the following sections.Steam Quality UpgradingA processing plant that has beenusing steam at a relatively lowpressure (say, 50 psig) may havean unexpected requirement forsteam at a higher pressure (say,100 psig). If the new steam de-mand is large, the need for newboiler capacity may be unavoid-able. However, lesser steam re-quirements could possibly be metby mechanical compression of ex-isting low-pressure steam.Operations in which steam qual-ity may be upgraded by recom-pression include food processing,vulcanizing, and clothes launder-ing. Steam recompression canalso be used in distribution pip-ing systems to boost steam pres-sures that have dropped to anunacceptably low level.The benefits of steam qualityupgrading, compared with al-ternatives, may include lowercapital cost, a lower requirementfor floor space, reduced installa-tion time and cost, and some-times a reduction in air pollutionproblems. If cheap exhaust steamis available, the operating costsmay be very favorable.

Waste-Steam Heat RecoveryIn many industrial processes,large quantities of waste energyare often liberated in the form oflow-pressure waste steam. Exam-ples of such waste energy sourcesare the following:

.Steam turbine exhaust

.Evaporators (pulp and papermanufacturing, chemical man-ufacturing, food processing)

.Refiner waste steam (thermo-mechanical pulping)

.Flash steam

.Steam cookingIt is common practice to ventsuch waste steam at pressuresbelow, say, 20 psig.In many such instances there is,at the same time, a plant applica-tion, such as the drying or heat-ing and/or evaporation ofliquids, in which steam at apressure higher than the exhaustpressure is required. Under sucha circumstance, it may well bepossible to reuse the low-pressurevapor (steam) by compressing it

to a higher pressure (vapor re-compression). Vapor recompres-sion can, therefore, result inenergy savingsby reducing theamount of steam required to begenerated from a boiler.An example of a system in whichsteam is generated in a boiler toprovide the heat needed to run anevaporator is shown in Figure 2a.Vapor from the evaporator isdriven off under atmosphericpressure at 212F. There is no va-por recompression, the evapora-tor vapor being sent to a coolingtower where it is condensed, giv-ing up heat which is wasted en-ergy.Much of the heat in the evapora-tion can be recovered by recom-pression (Figure 2b).By adding arelatively small amount of ther-mal energy to the vapor, the com-pressor raises the pressure of thevapor to about 5 psig, which cor-responds to a saturated steamtemperature of about 228F. Thesteam is then returned to thesteam chest in the evaporator tocontinue to drive the operation.

Figure 2a. Single effect evaporator system-no vapor recompressionCooling water/

,Boiler steam I \I S*

Liquor

9ondensate


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Figure 2b. Single effect evaporator system-with vapor recompressionVapor 212F

0 psig

9ompressorSteam 228F5 psig 3ondensatehe economics of waste-steam heat recovery by vapor recompressionepends mainly on the cost of owning and operating the compressor.lthough the capital costs for a compressor installation may be high,the reduced boiler steam requirements can result in sufficient energysavings to pay for the compressor installation and running costs in anacceptable time.

SavingsThe dollar savings per year (S) available by using the vapor recompres-sion process for heat recovery is calculated by the formula:

s= ( t x f ) (Cb s - CW)where:t = operating time (hr./yr.)f = team flow (lb./hr.)Cbs = cost of boiler steam production ($/lb.)Cvr = cost of vapor recompression ($/lb.)

ensitivity analysis shows that the factor Cbs is the most sigruficant inchieving savings and the cost of compression Cur is about 1 12th ofCbs as shown in the example calculation in the next section. The savingsan be calculated from the formula:

11, - hfx c, - ( E c x A x C e )( n x 0,)s = f x f ) xwhere: L _I

12, = enthalpy of steam (Btu/lb.)\i f = enthalpy of water (Btu/lb.)Co = cost of oil ($/gal.)12 = efficiency of boilerOe= energy content of oil (Btu/gal.)Ec = energy required to compress steam (Btu/lb.) (see Figure 1)A = factor to convert B tu to kw h (0.000293kWh/Btu)Ce= average electricity cost ($/kWh)


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v Th e Electric Ideas Clearinghotise isa comprehensive informationsotirce for cotnmercial and indus-trial energy users. It is operated bythe Wasliington State Energy Of-fice and is part of the Electric Ideastechnology transfm program spon-sored by participating utilities andthe Bonneville PowwAdministration.Technology Update C H - 34Electric Ideas ClearitighorrseP.O. Box 431 71Olympia, WA 98504-3171Toll-free Hotlirze:Fax: 1-800-872-3882Electronic Bulletin Board:1-800-872-3568

1-800-762-3319DOEiBP 2020January 199315C

Adapted from B.C. Hydro. Vapor Recom-pression. Guides t o Energy Management.Gem No . 1601.July 1984. With permis-sion from B.C. Hydro, Vancouver, BritishColumbia.

Example CaIculat on of Energy and Cost SavingsA plant vents 14.9 psig steam to the atmosphere. Concurrently, inanother application, there is a requirement for 4,960 lb./hr. of 39.9 psigsteam from the boiler.The following calculations provide an estimate of energy and cost savingselectrically driven compressor rather than generating the 39.9 psig steamfrom a boiler. Calculations are based on the following (assumed) data.to be realized by compressing the 14.9 psig steam to 39.9 psig, using an

Boiler feed water temperature = 82.4"FBoiler efficiency= 80%Annual operating time = 5,200 hr.Fuel = #6 oilFuel heating value = 181,840 Btu/gal.Fuel cost = $1.364/gal.Electricity cost = $O.O4/kWhFrom steam tables:

li f = 50.4 Btu/lb. for 82.4"F water11 = 1175.7 Btu/lb. for 39.9 psig steam

Annual Energy Requirement of Boiler Fuel4,960 lb&. X (1,175.7- 50.4) x 5,200 hriy .= 36,280 Btu0.8

Annual Cost for Raising Boiler Steamx $1.364/gal. = $272,1006,280 x lo6Btu

181,840 Btu/gal.Compressor Work39.9 psig + 14.7 psia14.9 psig + 14.7psia = 1.84

(From Figure 1,the compressor requires 67 Btu/lb. of steam delivered.)Annual Energy Required for Driving Compressor

67 Btu/lb. X 4,960 lb&. X 5,200 '"./Yr. = 1,730 X lo6BtuAnnual Cost for Compressor Energy

1,730 x l o 6B tu x 0.000293 kwh/Btu x $ 0 * 0 4 A ~ ~$20,300Annual Savings

Energy: (36,280 - 1730)x lo6Btu = 34,550 x o6BtuCost:$272,100 - $20,300 = $251,800Return on InvestmentThe cost for a compressor package to handle the installation in thisexample would be in the order of $250,000. On this basis, the simplepayback would be approximately 1year:

= 0 . 9 9 ~ .250,000$251,80Q/yr.Note: The above estimate is for illustrative purposes only, and numbers have beenrounded off for simplicity.

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Vapor recompression