steam control and condensate drainage for heat exchanger

8/18/2019 Steam Control and Condensate Drainage for Heat Exchanger

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Steam Control andCondensate Drainage for

Heat Exchangers

Bulletin FHD-206

General

Heat transfer units that use steam to pro-duce hot water are known as indirect heaters. They are often shell and tubetype heat exchangers and are generallyreferred to as converters, hot water gen-erators, and instantaneous heaters. TheASME Code for Unfired Pressure Vesselsis the nationally recognized authority pre-scribing their construction for given tem-peratures and pressures. The term usedvaries with the heating medium and themanner of application. When these heatersuse steam as the heat source they areusually called steam to water converters.

In steam heated converters, the waterto be heated circulates through the tubesand steam circulates in the shell sur-rounding the outside of the tubes. Thisresults in condensate draining to thebottom of the heat exchanger shell asthe steam gives up its latent heat.

Steam to WaterHeat ExchangersThe operation of the shell and tube heatexchanger is as follows. Steam enters the

heat exchanger shell through the topvapor opening and surrounds the outsideof the tubes. As energy is transferredthrough the tubes it heats the water insidethe tubes. The heat transfer condensessteam inside the shell forming condensatethat drops to the bottom of the heatexchanger shell. The condensate flowsthrough the bottom condensate outlet andinto a steam trap.

The steam pressure in the heat exchangershell has a direct correlation to the tem-perature of the condensate formed in theshell. The properties of saturated steamare such that the steam temperature varieswith steam pressure (See Table 1 on nextpage). When the latent heat of vaporiza-tion is removed, the resulting condensatewill be close to the saturation tempera-

ture. Depending on the system load,slight sub-cooling may occur from thebottom of the heat exchanger and theinlet piping to the steam trap.

The heat exchanger should be selected tooperate at the minimum possible steampressure. This allows the lowest possiblecondensate temperature to discharge fromthe steam trap and reduces the amount of flash steam in the return system. When

heating fluids up to 200°F, the heatexchanger should be selected based on2 psig steam pressure in the shell for thmost efficient system operation.

This may require a slightly larger heatexchanger than one operating at higherpressure, however it will result in asmaller less expensive low pressuresteam trap and a smaller steam regulativalve. The low pressure selection will allimit the maximum temperature that caoccur inside the tubes, should the tempeature controller fail in an open position

TIP:When heating fluids up to200°F, se lect hea t excha nger w ith2 psi steam pressure in shell forbest efficiency.

SERIES 415CY-STRAINER

SU 84- 2 HEAT EXCHANGERwith OPTIONAL “K” HEAD

GT610-IPELECTRO-PNEUMATIC

TRANSDUCER

AIR-PRV

MODEL 62 VACUUM BREAKER

SERIES 2000PRESSURE

TEMPERATUREREGULATOR

SERIES 415C

Y-STRAINER

MODEL AP-1A

AIR PILOT

SERIES CFLOAT AND THERM OSTATIC

STEAM TRAP

CB CONDENSATE UNIT

SERIES 1INLINE FLOAT

AND THERMOSTATIC

STEAM TRAP

SERIES 415C

Y-STRAINER

STEAM

SUPPLY



It is standard practice to add a foulingfactor in the heat exchanger selection.This fouling factor adds additional tubesurface area to assure adequate heatingafter normal scale and corrosion depositson the tube surfaces. A standard .0005fouling factor will add 20 to 25% addi-tional tube surface area. When the heatexchanger is new and the tubes are cleanand shiny, the heat exchanger will operateat lower than design pressure even at fullsystem load. For example, a new heat

exchanger designed for 15 psi steam toheat water to 160 degrees will generallyheat the full system load with 0 psi steamin the heat exchanger shell.

Table 1Properties of Saturated Steam

Heat ExchangerSelection

The heat exchanger should be selectedfor operation at the minimum pressure toprovide the most efficient operation. Theproperties of saturated steam tables showa larger amount of the latent heat is avail-able at low pressure. Less energy remainsin the condensate reducing the flashsteam losses. A reasonable guide would

be to select a steam pressure that has asaturation temperature approximately30°F higher than the required outlet tem-perature of the fluid being heated in thetubes. For fluid temperatures up to 200°F,2 psig steam is recommended.

When a high steam pressure source isused, the pressure should be reduced byinstalling a steam pressure regulatingvalve or by using a combination temper-ature pressure regulator.

After selecting the heat exchanger, thenext step should be planning the installa-tion. The heat exchanger should bemounted high enough to allow gravitydrainage of the condensate from thesteam trap into a vented gravity returnline. If a gravity return line is not avail-able, a condensate pump should be

installed. The heat exchanger shouldbe mounted with a pitch toward thecondensate outlet. A minimum 1 / 2 inchpitch per 10 foot length should beprovided. The heat exchanger shouldalso be located such that removal ofthe tube bundle is possible.

Steam Traps

The steam trap must be capable of com-pletely draining the condensate from theheat exchanger shell under all operatingconditions. On a heat exchanger using a

modulating temperature regulator to heatfluids under 212°F, the steam pressure inthe shell can be 0 psig. To assure conden-sate drainage, the steam trap must bemounted below the heat exchanger outlettapping and it must drain by gravity intoa vented condensate return unit. Whenpossible, the trap should be located 15inches below the heat exchanger outlet.The 15 inches static head to the trap inletwill provide 1 / 2 psig static inlet pressure tothe trap when the shell steam pressure isat 0 psig.

The trap should be sized based on this1 / 2 psig differential pressure. A safetyfactor of 1.5 times the calculated fullload capacity should be used to handleunusual start up loads. A float andthermostatic trap is normally the bestselection for a heat exchanger. Thethermostatic element quickly vents the

air from the heat exchanger shell. Themodulating float element providescontinuous condensate drainage equal tthe system condensing rate.

Failure to provide complete condensatedrainage will lead to poor temperaturecontrol and possible water hammer. Anlift in the condensate return piping aftethe trap discharge requires a positivepressure to develop in the heat exchangshell to provide condensate drainage. Fthis to occur, condensate must back upin the heat exchanger shell until enoughtube surface is covered by condensate tbuild a positive steam pressure. Whenthe positive steam pressure develops tomove the condensate through the steamtrap and up the vertical return line, oveheating can occur on the tube side of thheat exchanger due to the positive steampressure remaining in the shell. Thisresults in a wide range of outlet fluidtemperatures from the heat exchanger.

A lift in the return line as shown above

should be avoided on heat exchangers

using a modulating control valve. (Forcorrect configurations, see diagram at

top of page 4.)

A lift or back pressure in the steamtrap return piping can flood the heatexchanger shell and cause severe waterhammer as steam enters the flooded sheThe resulting water hammer can damagthe steam trap, the steam regulatingvalve, the heat exchanger tubes and cauthe gasket in the heat exchanger and trapto fail.

Trap Installation

The trap should be located below theheat exchanger shell to allow free flowof condensate into the trap. A strainercomplete with a screen blow down valvshould be installed ahead of the steamtrap. A shut off valve should be providein the trap discharge return line to isolathe unit for service. Unions should beprovided to allow trap service or replac

2

HEATED FLUID

OUTLET

FLUID

TO BE

HEATED

STEAM REGULATOR

STEAM S UPPLY

HEAT EXCHANGER

STEAM TRAP

CONDENSATE

OUTLET

15 in.

Typical Steam to WaterHeat Exchanger Installation

TIP: Fouling fac tor ass ures ad eq uatehea ting. A .0005 fouling fac tor ad ds20-25% additional tube surface.

Gage Sensible Latent Specific

Reading Saturation Heat in Heat of Volumeat Sea Temp. Water Vaporation of 1 lb. FlashLevel – Degree B.T.U. B.T.U. Steam Steam

psi °F per lb. per lb. cu.ft. %

2 219 187 966 24 0.6

5 227 195 960 20 1.6

15 250 219 945 14 3.9

30 274 243 920 9 6.5

50 298 268 912 8 9.0

100 338 309 881 4 13.2

125 353 325 868 3 14.8

TIP: Steam temperature should beselected 30° F over heat exchang eroutlet temperature.

TIP:Trap s hould b e loca ted a t lea st 15inches below heat exchanger outlet.

Heat Exchanger withModulating Control Valve

Lift



ment. The return line from the trapdischarge should be pitched into avented condensate return unit.

Vacuum Breakers

Most steam to water heat exchangersprovide a tapping in the shell to allowinstallation of a vacuum breaker. Thevacuum breaker allows air to enter theshell when a induced vacuum occurs.

Failure to install a vacuum breaker willallow the heat exchanger shell to operateat a negative pressure which may causecondensate to be held up in the shell.During light load, the heat exchanger willhave a layer of steam at the top and airunder the steam to provide just the rightamount of heat. The vacuum breakershould be mounted on a vertical pipe6 to 10” above the topping to providea cooling leg. This will protect thevacuum breaker from dirt and extremetemperatures.

Steam Regulator

The choice of the temperature regulatingvalve includes self contained temperatureregulators, pilot operated regulators andpneumatic regulators.

The steam inlet pressure to the regulatormust be higher than the required heatexchanger operating pressure to allow

flow. The available steam pressure shouldbe at least two times the heat exchangeroperating pressure to provide modulationof the regulator for good temperaturecontrol. This will also provide thesmallest size steam regulator.

The steam regulator should be sizedbased on the maximum lb./hr. of steamrequired by the heat exchanger. To prop-

erly size the regulator, the available inletsteam pressure and the heat exchangerdesign operating pressure must be known.The steam regulator should not be over-sized. Oversizing the regulator may causethe temperature to overshoot and theregulator will hunt more than a properlysized regulator. The steam regulator isnormally smaller than the connectinginlet and outlet steam piping.

Regulator Installation

A steam drip trap should be installedin the steam piping ahead of all steamregulating valves. Failure to installa drip trap will allow condensate tocollect in the steam piping ahead ofthe regulator. As the regulator opens,the mix of condensate and steampassing through the regulator may causewater hammer that can destroy the

diaphragms or bellows used to operatethe regulator.

A steam strainer should also be installedahead of the regulators to prevent dirtfrom entering the valve. Dirt can depositon the valve seat and not allow it toclose tight. The steam strainer should beinstalled with the screen pocket horizon-tally. Installation with the screen down,as commonly piped for water service,will allow a condensate pocket to formin the steam line. This condensate pocketcan carry into the main valve and causewater hammer or sluggish operation.

Shut off valves, pressure gauges, amanual bypass and unions should beinstalled to allow proper servicing of thevalves and strainers. When possible referto the manufacturer’s installation manualfor proper installation.

The temperature sensing bulb should beinstalled as close as possible to the heatexchanger outlet. It is important that thefull length of the temperature sensingbulb be inserted in the system piping.Any portion of the bulb installed in ano flow area will reduce the accuracy

of temperature control. When the sensingbulb is installed in a separable well, heattransfer compound must be installedbetween the well and the sensing bulbto aid heat transfer. The tube side of theheat exchanger should have a continuousrunning recirculation pump to providecontinuous flow past the sensing bulb.A minimum 20% recirculation shouldbe provided.

Pilot operated regulators with a pressurepilot require a downstream pressuresensing line. The pressure sensing line

connection should be connected in a non-turbulent area downstream of the mainvalve; a minimum 10 pipe diametersdownstream of the main valve is recom-mended. The steam pressure sensingconnection can also be connecteddirectly to the heat exchanger shell.

Condensate Coolers

When heat exchangers operate at highpressure, consideration should be givento the addition of a condensate cooler.The justification will be depend on thesize of the heat exchanger and the actunumber of hours per day the unit will bin operation.

With a condensate cooler, the dischargefrom the steam trap on the steam heat

exchanger outlet is piped through a wateto-water heat exchanger. A second trap then installed on the discharge of thewater-to-water heat exchanger to maintasaturation pressure and prevent flashingand water hammer from occurring inthe condensate cooler. A separate thermostatic trap is installed to allow direct aiventing of the steam heat exchanger intthe vented return line downstream of thcondensate cooler.

The water-to-water heat exchanger desigdiffers from a steam heat exchanger.

The water-to-water heat exchanger hasinternal baffles to direct the water flowacross the tubes to improve heat transfeWater-to-water heat exchangers areexternally distinguishable as the shellinlet and outlet tappings are the samesize; steam heat exchangers havea large vapor opening in the top of theshell and a smaller condensate outlet inthe bottom.

The fluid in the condensate coolertubes may be the inlet water to the steaheat exchanger tubes. When the initialtemperature of the fluid is too high to

cool the condensate below 212°F, aseparate fluid may be heated. Preheatindomestic hot water or preheating boilermake up water are two possibilities.

TIP: Insta ll a vac uum brea ker on a llstea m c onverters.

TIP: For good control, take at least a50% pres sure drop ac ross the c ontrolvalve.

THERMOSTAT

TRAP

REGULATOR

STEAM SUPPLY

HEAT EXCHANGER

C

O

TRAP

COOLER

TRAP

Installation withCondensate Cooler



STEAM INLET

HEAT EXCHANGER

CONDENSATE COOLERPUMPED DISCHARGE

CONDENSATE

PUMP

THERMOSTATIC

TRAP

4

Heat exchanger installations depending on operating pressuresand the of type of return pump.

Low Pressure (2 psig or less) with Standard Condensate Unit

STEAM INLET

CONDENSATE

PUMPHEAT EXCHANGER F&T TRAP

Heat exchangers operating at 2 psig or lesscan be drained into a standard low costfloor mounted condensate returned pump.

High Pressure with Flash Tank and Low NPSH Condensate Unit

STEAM INLET

ELEVATED CONDENSATE

UNIT WITH LOW NPSHPUMP

HEAT EXCHANGERF&T TRAP

FLASH TANK

Heat exchangers operating at higher pres-sure require a flash tank to vent the flashsteam. An elevated condensate pump unitequipped with 2-foot NPSH pumps are re-quired to handle the condensate at satura-

tion temperature.

High Pressure with Condensate Cooler and Standard Condensate Unit

When it is necessary to operate a heatexchanger at high pressure (above 15 psig)a condensate cooler can be added to

sub-cool the condensate below 212°F. Theillustration shows the proper steam trap-ping for a condensate cooler. The incom-ing fluid to the steam heat exchanger maybe used to sub-cool if the temperature islow enough. A separate cooling sourcemay also be used.



5

Heat exchanger installations depending on operating pressuresand the of type of return pump.

Low Pressure and Pressure Powered Pump

HEAT EXCHANGER

RESERVOIR TANK

PRESSURE POWERED

PUMP

PUMPED DISCHARGE

DRIP TRAP

STEAM SUPPLY

STEAM SUPPLY

DRIP TRAP

F&T TRAP

A pressure powered pump unit may also bused to return condensate. The installationshown would be used on low pressure heaexchangers. The receiver tank is vented toatmosphere on this unit.

High Pressure and Pressure Powered Pump

HEAT EXCHANGER

THERMOSTATIC VENT

STEAM SUPPLY

STEAM S UPPLY

DRIP TRAP

TO RETURN LINE

PRESSURE POWERED PUMP

RESERVOIR

TANK

PUMP

DISCHARGEF&T TRAP

Heat exchangers operating at higher pressures may use a closed pressure poweredpump system. The installation shown wilallow condensate to discharge directlythrough the steam trap when the pressureon the heat exchanger is higher than thereturn line pressure. When the heaexchanger pressure is not sufficient, the

pressure powered pump receiver will filand operate to discharge condensate.

The F&T steam trap can be sized based onthe differential pressure from the pressurepowered pump discharge pressure less thereturn line back pressure.

For additional information or assistance in selecting components,contact your Fluid Handling “Steam Team” representative.

steam control and condensate drainage for heat exchanger

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