THE FORMULA for determining theamount of surface required in a surfacecondenser is as follows:

Q = UA (LMTD) (1)whereQ = Heat to be absorbed, Btuh =pounds per hour of steam times 950Btu per pound;U = Overall heat transfer rate, Btu per(hr) (sq ft) (F);A = Surface required, sq ft;LMTD = Loge mean temperaturedifference between condensingtemperature and cooling water.No attempt will be made to go througha formal condenser selection. Theprocedure for condenser rating isamply covered in the publication ofHeat Exchange Institute entitled,"Standards for Steam SurfaceCondensers."* Rather we shall attemptto discuss qualitatively the factorsaffecting selections and their effectson size and price.

Turbine Steam RateTurbine steam rate is usuallyexpressed in pounds per hour ofsteam per brake horsepower and iscommonly called the water rate. Thiswater rate is a function of motive steampressure and temperature, condensingtemperature, or equivalent vacuum,and turbine efficiency. Specificinformation for a given turbineapplication can be obtained______________________________This Standard is currently out of print, but anew edition is in preparation and will beavailable in the future from Heat ExchangeInstitute, 122 E. 42nd St., 'New York, N.Y.10017.

from the turbine manufacturer.

Generally speaking, on systems wherethe motive steam pressure isapproximately 150 lb and the turbine isdischarging to a vacuum of 26.inchesof mercury (4 in. Hg absolute) , a steamrate of 14-16 lb per hr per bhp iseconomically feasible. Althoughgreater efficiency is available atincreased cost, utility costs for thistype of installation seldom justifyreducing the steam consumption bymeans of higher ef-ficiency turbines orlower condensing pressures. Whenspecial conditions are present, thespecifying engineer may considereither of the above modifications toimprove performance.

Condenser VacuumThe vacuum produced in a steamcondenser is that pressurecorresponding to the temperature atwhich the steam condenses. Thissteam condensing temperature, in turn,is dependent upon the temperature of

cooling water entering the condenserand the quantity of water available.The steam condensing temperaturecan get down to as low as 5F abovethe entering cooling water temperature,but in most air conditioninginstallations the differential is 20F,sometimes 10F, but seldom smaller, asthe closer approach would require acostly amount of condenser surface.In practice, cooling tower water is firstused in the refrigerant condenser,where it rises about 10F and is thenavailable for steam condensing. Thegreat bulk of steam-driven cen-trifugalrefrigeration systems operate with asteam condenser vacuum ofapproximately 4 in. Hg abs., which isequivalent to a condensingtemperature of 125.4F. Thus, referringto Fig 1 and presuming that 85Fcooling water is supplied to the systemfrom the cooling towers, watertemperature into the steam condenseris approximately 95F. Assuming aturbine steam rate of 14-16 lb per hr perbhp (bhp per ton of refrigeration isapproximately 1) and a condensingwater circulation rate of 3 gpm per tonof refrigeration, water rise on the steamcondenser is also about 10F. Thus, theleaving water temperature approach is20F(125F minus 105F).

by Elliot SpencerGraham Mfg.Co., Inc., Great Neck, Long Island, N.Y.

Factors affecting condenser design and selection.A typical air conditioning surface condenser specification is included.

Specifying SteamSpecifying SteamSurface CondensersSurface Condensers

If, on the other hand, we designed thesteam condenser for 3 in. Hg. abs.,which is equivalent to a condensingtemperature of 115F for the sameapproximate water temperature rise, theapproach would be 10F (115F minus105F). Thus from Eq (1), because thelog mean temperature difference isgreater, we see that the condenserdesigned for 4 in. Hg abs. requires lesssurface than the one designed for 3 inHg abs.The foregoing example is anoversimplification. In practice, thesteam rate of a turbine discharging at 4in. Hg. abs. is somewhat higher thanone discharging at 3 in. Hg. abs.Choice of a lower absolute pressuremight permit using a turbine with oneless wheel, which would result inreduced turbine cost. This reducedturbine cost might or might not beoffset by the increased condensercost. (See "Is There a Steam Turbine inYour Future," ACHV, April, 1968, pp43-47.) Similarly, condenser cost couldbe reduced if the condenser wereoperated at 5 in. Hg. abs. (133.8Fcondensing). However, turbine costmight increase more rapidly if thesteam rate had to be held to apreviously set value, which, asindicated, is generally in the range of14-16 lb per hr per bhp.Thus, the specification should simplystate the number of pounds per hourof steam to be used and the maximumtemperature and quantity of availablecooling water, omitting the preciseoperating vacuum to be maintained bythe surface condenser. This permitsthe manufacturer to offer the bestcombination of condenser, turbine andrefrigeration unit to comply with thelimitations on steam consumptionspecified.Other considerations, such as lengthlimitations and turbine vapor exhaustsize, very often have a determiningaffect on condenser cost.

Water TemperatureAs cooling water temperaturedetermines the vacuum that can beproduced, it is imperative that themaximum anticipated water temperature

be specified. This temperature isdetermined by the water source andvaries with the location of theinstallation. Water flow rate to thecondenser is usually determined bythe refrigeration manufacturer and isgenerally set at approximately 3 gpmper ton of refrigeration when coolingtowers are used. However, when riverwater or water at relatively lowtemperature is available, water flowrate varies and specifications shouldtake these variations into account.A recent trend in large multi-storyoffice buildings has been to use 2 gpmper ton of refrigeration. Although thisaffects condenser, and turbineadversely due to higher condensingtemperatures, there is a saving incondenser water piping.Too often, specifications state that thesurface condenser shall be capable ofhandling turbine exhaust whilemaintaining the desired vacuum with a5% increase in normal steam load.Compliance with this requirement isalmost impossible. Specificationsseldom provide the 5% greater waterquantity needed to balance this 5%loading factor. Further, usually noprovision is made for the extra load onthe cooling tower and the change insteam rate of the turbine whenoverload hand valves are used.Furthermore, the refrigeration machineis operating at a different point on itscurve and may be rejecting more heatwhich, in turn, means hotter water tothe steam condenser. The net resultunder the 5% excess load condition isthat the steam condenser receivescooling water that is more than 5%above the initial design temperatureand the resulting excess heat impartedto the water overloads the coolingtower, further aggravating thesituation.If it has been decided that the surfacecondenser should have additionalsurface in it, the most practicalapproach would be to state that theunit shall have 5% excess surface or10% or whatever is needed. In general,a surface condenser will handle theentire excess load at a slight increasein back pressure. If this is permissible(and it must happen unless all

accessory items have built-in excesscapacity), no statement of excesscapacity need be made.

Water Pressure DropWater side pressure drop is a functionof tube velocity, condenser length andthe number of passes the water has tomake in a given condenser. Airconditioning practice permitsvelocities as high as 10 fps in the tubeside of refrigerant condensers and, inrecent years, this has been extended toapply to steam surface condensers forair conditioning installations.It has been common practice to limitwater pressure drop to 10 ft. However,if this limitation is rigidly enforced,velocities must be reduced to complywith this pressure drop requirement,resulting in lower heat transfer ratesand higher condenser costs. Generally,water pressure drops of approximately15 ft can be attained in most single-pass condensers operating at 4 in. Hgabs. Most two-pass condensers,usually those running at 3 in. Hg abs.,require pressure drops as high as 20-30ft.Specification of water velocity andmaterials of construction for tubes andtube sheets where river water orbrackish water is involved must becarefully evaluated. Informationshould be obtained on the experienceof other users in the area havingsimilar design conditions before aspecific material is selected. Generallyspeaking, for brackish waterapplications, velocities in excess of 6.5fps can seriously reduce tube life, butfor cooling tower applications a 10 fpsvelocity is acceptable.

Fouling FactorMost surface condensers for airconditioning and power plantapplications are designed with acleanliness factor of 85%. This meansthat the heat transfer rate used indesigning the condenser is 85% of theclean heat transfer coefficient. Forthose installations where cooling watersources produce rapid fouling of

the tubes, a higher factor must beused. This holds true for both therefrigerant condenser and the steamcondenser.For most applications using cleancooling tower water, refrigerationcondensers should be specified with afouling factor of 0.0005 and steamsurface condensers designed for an85% clean tube coefficient. If therefrigerant fouling factor is increased,the surface condenser cleanlinessfactor should be increased by the samepercentage. Thus, if the refrigerantcondenser is specified as 0.001 vs.0.0005, the corresponding cleanlinessfactor for the surface condenser is 70%clean vs. 85 % clean.

Steam Inlet Velocity

Velocity at the exhaust of the steamturbine is limited by National ElectricalManufacturers Association to 450 fps.Should the exhaust line from turbine tocondenser be long, there can beexcessive pressure drop in the vacuumline, which means that pressure at thecondenser inlet is penalized by thepressure drop between turbine andcondenser. Condenser manufacturersusually guarantee to maintain thespecified vacuum at the inlet to thesteam condenser. However, pressuredrop between turbine exhaust andsteam condenser must be calculatedby the design engineer andcompensated for by specifying thatthe condenser shall maintain a vacuumslightly below the vacuum at theexhaust of the turbine. A specificationthat calls upon the supplier to supply aspecified steam consumption obviatesthis problem. Where a given velocity isused to determine the pressure drop,this should be stated in thespecification so that it is not exceeded.In any event, velocities in excess of450 fps should not be permitted.Impingement plates should beprovided below the condenser steaminlet to protect tubes from the effect oflocally high inlet velocities andimpingement of condensate. Steaminlet velocities should not exceed 400fps and provision should be madebased upon design experience in this

field for proper vapor velocities withinthe shell.Calculation of condenser heat load isbased on the fact that steam enteringthe condenser has a value ofapproximately 950 Btu per lb. Moreprecise calculations can be made if thespecific turbine characteristics areavailable, but this figure is sufficientlyaccurate for the sizing of the coolingtower and condenser.

Space Limitations

Space considerations often influencethe cost of a condenser. For example, ifthe surface condenser is limited in tubelength, a multipass design may berequired in place of a single-passdesign. This results in a short, stubbyunit, which is generally more expensivethan a long, thin unit.Specifications of head room limitationsand permissible condenser tube lengthshould take into consideration thedistance beyond the tube sheets of thecondenser required to allow removingtubes for replacement. Condensersmust be mounted so that liquid levelon the hotwell is sufficiently high topermit removal by condensate pumps.For air conditioning installations, theliquid level should be at least 5 ftabove the inlet to the condensatepump and preferably 1 or 2 ft higher.This permits an economical pumpselection.

Air Leakage

As both the exhaust of the steamturbine and the condenser areoperating under a substantial vacuum,air is bound to leak into the system.This leakage occurs through the glandseals on the steam turbine and throughminute holes in the piping connectionsassociated with the surface condenseritself. Over a period of years, HeatExchange Institute has determined thenormal quantity of air that should leakthrough properly designed turbinesand piping systems, and these arespecified in their "Standards forSurface Condensers." Manufacturershave similarly standardized theirejector sets so that several standardsizes are available for specific air

leakage quantities based on variousteam quantities entering thecondenser. Specification that theejector shall be designed for thequantities of air leakage as designatedin HEI Standards is sufficient.

Codes

Steam surface condensers operateunder a vacuum and are, therefore, notconsidered pressure vessels. TheASME Code is a pressure vessel codeand is not, strictly speaking, applicableto surface condensers operating undera vacuum. However, the tube side of asurface condenser is considered apressure vessel, as it is subjected tothe full water pressure. Whennecessary, this side of the condensercan be designed and constructed toASME Code requirements. Mostsurface condensers are designed andconstructed in accordance with HEIStandards. They can be built to ASMECode requirements and so stamped bya qualified inspector. This type ofconstruction, which requires specificmethods of welding, specially qualifiedwelders and keeping of materialrecords with certified copies of theserecords to be supplied, results in amore expensive surface condenser.However, if the unit is to be built toASME Code requirements, it should bestated that both the shell and tubesides, or simply the tube side, shall bedesigned, constructed and stamped inaccordance with ASME Code and shallbe accompanied by certificationssigned by a qualified insuranceinspector. Do not be mislead by thestatement, "Condenser is constructedin accordance with applicable ASMECodes." It is common for somemanufacturers to indicate that, asASME is a pressure vessel code, it isnot applicable to vacuum condensersand therefore their equipment complieswith applicable ASME Codes.

Materials of Construction

Materials used in construction ofsteam surface condensers are given inTable 1.Use of copper alloy tube sheets insteam surface condensers with theaccompanying requirement that tubesheets be bolted to the shell by meansof collar bolts, is a carryover frommarine practice. There is no reasonwhy a steel tube sheet cannot be used,in view of the fact that the refrigerationcondenser just upstream of the steamcondenser uses this type ofconstruction. When a steel tube sheetis used, the specification shouldindicate that it may be welded to theshell.

Specification Considerations

When tube alloys that differ from thestandard admiralty material are used,ratings must conform to HEIStandards. These standards reduce theheat transfer rate to conform with thecharacteristic performance of variousalloys. Similarly, when tube gagesmust be increased above the normal 18BWG, heat transfer rates should becorrected in accordance with HEIStandards to compensate for increasedtube wall resistance.It is not uncommon in power plantpractice to use a divided water boxconstruction on surface condensers,but this type of construction isunusual in air conditioning practice, asair conditioning equipment is seldomcritical. However, where cooling wateris particularly dirty, fouling of thetubes may be rapid and, under thesecircumstances, a divided water boxcondenser might have someadvantage.Corrosive effects on water boxesshould be compensated for by use ofalloy cladding, fiber glass reinforcedpolyester lining or similar procedure.

Auxiliary Equipment

Selection of auxiliary equipment to beused in conjunction with the surfacecondenser can be very important.Failure of the ejector set or condensatepump is critical to the operation of theentire system. The condensate pumps,

which are the only moving parts in asurface condenser system should besupplied in duplicate. Power plantpractice usually requires twin airejector sets, one a standby, but for airconditioning installations, a single setis sufficient.

Ejectors

For condensing pressures of 5 in. Hgabs. (25 in. of vacuum) and lower, atwo-stage ejector system isrecommended because, as thecondenser water temperature drops,the two-stage ejector permits operatingat reduced condenser pressure that, inturn, reduces overall steamconsumption of the turbine. Also, asthese systems are designed on thebasis of maximum summertemperatures, a two-stage ejectorprovides operating economies duringthose days when maximum watertemperature levels are not reached.It is important in specifying steamejectors that the minimum anticipatedsteam pressure be stated. Allowanceshould be made for pressure dropbetween the source of steam and thepoint at which steam enters theejectors. If an ejector is designed for100 lb steam and lower pressure steamis delivered, the ejector may notfunction properly; if the pressure ismore than 10% below design level itwill fail to operate altogether.Specifications should state that designair leakage for the ejector system be inaccordance with HEI Standards. Whenmore than one turbine discharges intoa common condenser, the ejector mustbe designed for 1 1/2 times the normalair leakage for the total steam quantity.For more than two turbines, othercorrection factors are applicable.The cooling medium for the inter andaftercondensers of the two-stageejector system can be cooling waterfrom the cooling tower or condensatefrom the hotwell of the surfacecondenser. As intercondensercondensing temperature is about 145Fand aftercondenser condensingtemperature is about 212F, fouling ofthe inter and aftercondensers will bemore rapid than that of the maincondenser.

It is therefore recommended thatcondensate be used as a coolingmedium for the inter andaftercondensers, as condensate ispractically pure water and containslittle or no solids. A typical schematiccontrol diagram using condensate overthe inter and aftercondensers is shownin Fig. 2.Condensate from the intercondensermust be drained back to the hotwell, asthe intercondenser is under a vacuum.This can be done by means of acondensate trap or, where headroompermits, by means of a loop seal, whichmust be at least 8 ft high. Condensatefrom the aftercondenser can be drainedback to the hotwell through a trap ormay simply be drained to an opensight drain. The writer prefers thesecond method

because it eliminates the possibility ofa malfunction of this trap, which couldbleed air back to the hotwell.

PumpsCondensate pumps for air conditioningare customarily of the horizontal type,designed for approximately 4 1/2 - 5 ftnet positive suction head (NPSH) andtotal dynamic heads of from 80-150 ft.Actual NPSH available for thesepumps depends upon the physicallocation of the condenser. Thedischarge head is affected by wherethe condensate is pumped after itleaves the condensate pump; that is,to an open condensate tank or back tothe cooling tower, etc. The importanceof providing sufficient NPSH forproper operation of the condensatepumps cannot be overemphasized. Thecondenser should be locatedsufficiently high off the floor to put asmuch head on the inlet of thecondensate pumps as is physically

possible. The greater the availableNPSH, the easier job the pump has andthe less danger there is of cavitation.Condensate pumps are generally of thehorizontal close coupled type, 3450 rphand with open drip-proof motors.

Relief ValvesSurface condensers most commonlyuse atmospheric relief valves of thewater-sealed or O-ring type, althoughbursting discs are occasionallyapplied. Size and rating characteristicsfor these valves are given in HEIStandards. In operation, a smallamount of water is kept above therelief disc to prevent the inleakage air.Water in the valve must besupplemented periodically, as a certainamount will evaporate. A sight glass isprovided, but because relief valves areusually located at a considerableheight and the sight glasses arefrequently obscured by sediment, theyare difficult to read. This author prefersto supply a small constant drip ofwater to the valve, permitting the

overflow to drop into an open sightdrain. Thus, as long as there is anoverflow, we know that the valve issupplied with water.

Liquid Level ControlThe method of control used dependsupon whether or not condensate isused as the cooling medium for theinter and aftercondensers of theejectors. When condensate is used asthe cooling medium, a scheme such asthat shown in Fig. 2 should be used. Ifcondensate is simply dumpedoverboard, then a simple mechanicallyoperated level control on the dischargeof the condensate pump is sufficient.The arrangement shown in Fig. 2requires that all of the condensate bepumped over the inter andaftercondensers before diverting aportion back to the hotwell, oroverboard, depending upon loadconditions.

Minimum Simplified Specification forSteam Surface Condensers in

Air Conditioning Practice

Vacuum at inlet to steam condenser__________________Steam flow to condenser, lb per hr___________________Condenser water inlet temperature, F_________________Condenser water gpm_____________________________Permissible condenser water pressure drop, ft__________Number of water passes___________________________

(Single-pass for 26 in. of vacuum and two-pass for .27 in. Should be left up to supplier) .

Maximum water velocity, fps_______________________10 fps for cooling tower applications)

Tube diameter and gage___________________________(3/4 in. X 18 BWG)

Cleanliness factor________________________________(85% clean for cooling tower applications)

Waterbox design pressure__________________________Water test pressure________________________________

(1.5 times design pressure except for cast iron which is 5 Lb above design pressure)___________

Shell side design pressure ___________________________(Full vacuum test to 20 Lb)

Steam pressure and temperature at ejector inlet___________Condensate pump capacity, gpm ______________________Condensate pump net positive suction head______________Condensate pump dischargehead______________________

(Add pressure drop through inter- and aftercondensers and overboard valve to total system loss.)

Electrical characteristics of pump motors ________________

1. The steam condenser shall be of the shell and tube type, single or multipass, designed and constructed in accordance withHeat Exchange Institute (HEI) Standards, except as noted herein, and shall be suitable for the design conditions noted above.Condenser manufacturer shall demonstrate successful installations of similar equipment that has been in operation for atleast__________(5 or 10) years.

2. Condenser shall have cast iron or steel waterboxes, steel tube sheets, steel tube supports, admiralty tubes and a steel hotwellcomplete with screen, strainer and hand-hole clean-out. Hotwell shall be of the vertical tank type, suitable for one-minutestorage capacity.

3. Provision for tube expansion shall be made either by means of bowed tubes or shell side expansion joint. Covers of thewaterboxes shall be removable without disturbing water piping. Connections shall be provided in the hotwell for ejector drain,condensate outlet, liquid level control bypass, fresh water startup, gage glass and high level water alarm. Connections shall alsobe supplied in the condenser shell for pump vent, vacuum gage, pressure gages to be located on waterboxes, an atmosphericrelief valve and steam inlet from the turbine drive. Waterboxes shall be supplied with a plugged vent and drain and shall haveflanged water connections. All vapor side flanges shall be 125 lb FF. Water side flanges shall be______________ lb (raised orflat face, depending upon pressure).

4. A single-element, two-stage ejector assembly, complete with internal interconnecting water and steam piping with surfaceinter- and after-condenser shall be provided. Ejector shall be cast iron. Ejector motive steam nozzles shall be stainless steel. Thetwo-stage ejector shall be a self-contained package, complete with steam valves, interconnecting steam piping, steam strainerand interconnecting vapor piping, and shall be provided with mounting bracket so that ejector can be mounted directly on thesurface condenser.

5. Condensate pumps shall be supplied in accordance with design conditions and shall be close coupled, single suction,enclosed impeller type with cast iron casings, bronze or cast iron impellers, stainless steel shafts, shaft seals suitable for theservice, grease lubricated bearings and open drip-proof motors. Pump motor starters will be provided by others. Electricalcharacteristics will be as specified under _____________herein.

6. A hotwell gage glass shall be provided.

7. A steam inlet expansion joint shall be provided, sized to conform with the diameter of the turbine exhaust with corrugations.Expansion joint shall have copper or stainless steel bellows and shall be provided with a liner if steam velocity exceeds 300 fps.

8. An atmospheric relief valve shall be furnished, sized for protection in accordance with HEI Standards.

When condensate quantity exceedsthat required by the inter andaftercondensers, a suitable bypassshould be supplied.Selection of air-operated levelcontrol valves is dependent uponinformation supplied by theconsulting engineer; namely, thedischarge head beyond theoverboard level control valve andthe total dynamic head the pumpmust develop. Specifications shouldindicate whether the total dynamichead stated for the pump has takeninto account the condensatepressure drop through the inter andaftercondensers and the overboardvalve. Generally, pressure dropthrough the inter andaftercondensers is less than 15 ftand that on the overboard valve is10-30% of the available discharge

head beyond the overboard controlvalve.Gages and ThermometersA combination pressure andvacuum gage of the dial type shouldbe supplied to give a roughindication of condenser vacuum.Exact vacuum can be properlydetermined only by use of a mercurymanometer, which can be specified.Pressure gages should be installedon the inlet and outlet water boxes.It is also common practice to installpressure gages at the condensatepump discharge, as well as at theinlet to the condensate pump, so theoperating engineer can easily checkthe performance of pump andcondenser. In addition,thermometers should be placed sothat inlet and outlet watertemperatures, as well as temperatureof the condensate in the hotwell ofthe surface condenser, may be read.Air Leakage MeterMost operating engineers of airconditioning installations judgeexcessive air leakage by the amountof steam and droplets of condensateissuing from the aftercondenservent. However, an air leakage meterplaced on the vent of theaftercondenser is more reliable and,as it represents an investment ofapproximately $150, is well worth theexpense.

Expansion JointA single or multi-corrugationexpansion joint should be placedbetween the exhaust of the turbineand the inlet to the surfacecondenser. These expansion jointsusually have copper or stainlesssteel bellows and a steel flange.However, as the cost of copperincreases, stainless steel joints arereplacing them. The number ofcorrugations is determined by theamount of expansion that must beaccommodated by the joint andshould be determined by theengineer prior to completing thespecification. When the number ofcorrugations is not stated, a single-corrugation joint is supplied, whichmay or may not be sufficient for thejob.When steam velocities through theexpansion joint exceed 300 fps, it isadvisable to install a stainless steelliner to prevent noise and excessvibration of the joint.It is possible to obtain reducedsteam consumption simply bydesigning the surface condenser fora higher vacuum. Relative costs ofcondensers designed for 26 and 27in. of vacuum are given in Fig. 3.