Presentation on Flash Steam RecoveryBy: Steven McNeil V.P. of Marketing/Penn Separator Corp.

What Is Flash Steam:

In a steam boiler heat is added to the water to generate steam. The water under pressure can only hold somuch heat. This is the sensible heat of the liquid at the operating pressure. When additional heat is added thewater then turns to steam. The amount of additional heat required to create the steam is the latent heat ofevaporation. The steam produced then contains the full BTU’s of the sensible heat of the liquid and the latentheat of evaporation.

The opposite occurs when flashing condensate. The condensate that is under pressure contains energy that ismore than the water can hold when the pressure is reduced. This energy from the condensate is released in theform of flash steam. The greater the difference in the initial pressure verses the flash to pressure the greater theamount of flashing that occurs.

The amount of flashing that will occur can also be calculated using steam tables. The amount of heat that theliquid can hold at the higher pressure Enthalpy Sat. Liquid in (BTU’s/lbs.) minus the amount of heat in the lowerpressure also (BTU’s/lbs.) divided by the latent heat of evaporation at the lower pressure is equal to thepercentage of flashing that will occur. The flash Chart “F” provides the results of this calculation for variousflash pressures.

The steam tables also show the heat content of flash steam at the lower pressure. The total heat Hg (SatVapor) is the BTU’s/lbs. of energy in the steam for every lbs. of flash steam it will contain. Because this energyis so high for every lbs. of steam that flashes, recovering this steam is very advantageous.

An example of the flash calculation is 100 psig condensate flashing to a 5 psig low pressure steam line. OR:

309 - 196 = 11.8% Flashing to Steam.960

For a condensate load of 1000 #/hr. X 11.8 % = 118 #/hr. would flash to steam. This leaves 882 #/hr. ofcondensate at 5 psig.

Why Recover Flash Steam:

The best reason to use a flash tank is to reclaim the energy from the flashing steam. The flashsteam contains the full BTUs of saturated vapor. The steam tables showed that flashingsteam to low pressure of 5 psi can provide 1156 BTU’s/lbs. of steam. As the flash pressureincreases so does the BTU’s that the flashing steam contains.

If we use our previous example of 11.8% flash steam going from 100 psig to 5 psig and see how1,000 #/hr. of condensate flashes steam equal to 118 lbs./hr. This Flash steam. of 118 #/hr.X 1156 btu’s/lbs = 136,408 BTUs per hour recovery.

Our heat recovery survey shows the fuel savings to produce this steam, using a $ 5.00/millionbtu fuel cost and a boiler efficiency of 80%. The savings equal $ .85 per hour. For a boileroperating 24 hrs. a day 365 days a year this would save $ 7,468.60

When a Flash Tank is used and flash steam is returned to the system it not only recovers the fullbtu’s of the steam but also has other benefits.

1. The steam is actually soft water that would need to be replaced if vented from the system.In the example above the steam will return .24 gpm of water to thesystem.2. The condensate at low pressure now 5 psig is easier to return to the system.3. It eliminates transferring a high temperature mixed flow media.4. Provides an area for air separation.5. Reduces condensate line sizing due to excessive steam loads.6. Creates a cushion in return lines.7. Reduces cooling water requirements.

Flash Tanks Provide the area:

Flash tanks provide the area required for the efficient release of the flash steam to a lowerpressure. Flash Tanks are available in a variety of designs and are available in a horizontal orvertical style. Vertical designs are preferred because of their ability to produce cleanersteam to the vent. Traditional flash tanks use the surface area of the water and a low upwardvelocity (10 fps) to separate the steam and condensate.

Penn Flash Separators can be smaller because they use a tangential inlet that creates a centrifugal spin-ning action to mechanically separate the steam and condensate. The cyclone action also provides a lowpressure area for the clean steam to rise to the vent.

Normal features of a vertical flash tank or separator are a tangential inlet and wear plate, centrally locatedsteam vent, condensate drain, a tank clean-out, and inspection ports. Other connections could include a levelgauge, safety valve connection, air vent, level controller connections, or a pressure gauge connection. All flashtanks and separators should be designed and build in accordance with ASME Code Sec. VIII, Div. 1 for anunfired pressure vessel. Drain and Vent sizing are also critical to flash tank selection and should be chosen tolimit velocity and pressure drop as the steam and condensate travels down the lines.

The pressurized condensate can come from a variety of sources:(Sketch #1-5)

A flash tank as shown (Sketch #1) can provide an point where condensate can be collected before beingreturned to the feedwater system. Several lines can use a common flash tank minimizing uneven flows andslugs of water while at the same time returning flash steam to a low pressure steam application saving valuableBTU’s. Even if the lines contain different pressures of condensate in the inlets, the tank will operate at theflash-to pressure. The remaining condensate will then be all at the same pressure. On separators with severalinlets, swing check valves should be used on each inlet to prevent back flows from entering the line. The flashtank also provides a dead air space which can be used to vent unwanted air from the system. A safety valvecan be provided to limit the flashing steam pressure protecting the low pressure user down stream or toprotect the tank from over pressurizing.

Another common application is condensate from steam traps (Sketch #2). Traps are designed to drain liquidunder pressure from a steam line or equipment while leaving the steam in the system. Because the condensateis pressurized a flash tank can separate flashing steam and supplement steam to a lower pressure application.This application shows process traps supplying steam to low pressure heaters.

Flash Separators can be used in staging (Sketch #3) where a steam system uses high, medium, and lowpressure steam. A separator used on the high pressure condensate return can be used to flash steam to amedium pressure steam application and the medium pressure condensate remaining can be flashed to a lowpressure steam application. The condensate then at a low pressure becomes easier to dispose of in the normalmanner.

On other applications condensate must be flashed to atmosphere (Sketch #4). This is done when the mixedflow of condensate and steam that can not be handled by the return line or equipment. When flashing toatmosphere the remaining condensate will be at 212 deg. F. In this instance the condensate will be gravitydrained. A trap on the outlet is not required. A condensate aftercooler can be included to cool the condensateto an acceptable temperature before it is directed to a drain.

Some other unique applications for flash tanks have included engine cooling through circulation of coolants,deaerator or other equipment overflows, process draining, autoclaves, laundry drains, pharmaceutical, andgeothermal applications.

Using Flash Tanks and Separators: (Instructions C-6)

Flash tanks used on pressurized systems should be located as close to the application as possible to mini-mize back pressure in the line, prevent radiant heat losses, and loss of steam. The condensate line, tank, andpiping should be insulated to prevent unnecessary heat loss.

Condensate piping should be pitched toward the inlet of the tank. Where two or more lines share a commonflash tank they should be protected from back flow using a check valve. Because the flash steam will be ventingto a low pressure application the tank will operate at the vent-to pressure.

A trap or level controls would be required to drain the condensate while keeping the steam in the system. Draintraps should be invented bucket or Float & Thermostatic type sized for 3 times the normal flow rate to com-pensate for slugs of condensate and to allow for constant drainage from the tank. The trap should be locatedbelow the water level in the tank to accomplish a gravity drain. Other more elaborate controls can be used toaccomplish the draining of condensate. They could include a external float cage or a pressure differentialtransmitter with a pneumatic or mechanical control valve.

The vent of the separator would be piped to a low pressure header. The steam demand should be greater thenthe amount of flash steam to prevent over pressurizing. A check valve should be used in the vent if the conden-sate flow rate to the tank is intermittent. A safety valve on the tank or located in the vent line can be used toprotect the low pressure application from over pressurizing. A pressure gauge can be installed in the vent line ortank to visually check the venting pressure of the separator.

Some flash tanks come with level gauge connections that can be used to visually check the level. A high levelalarm switch can be included to warn the operator of high condensate levels in the tank.

The tank provides an air space just above the inlet were air can collect. A thermostatic air vent could be usedto vent this air from the system.

A clean-out coupling is normally provided for periodic blowdown of the flash tank to clean out sediment thathas collected. This should be piped to a sewer drain.

Flash Tanks that vent to atmosphere, to flash off unwanted steam, are less complicated to install. The inletscome directly into the tank and do not require check valves. The steam vent piping should be the same size orlarger then the tank vent and be piped as direct as possible to the atmosphere to minimize back pressure. Thetank shall be located above the floor drain, or equipment, high enough to allow for gravity drainage. Insulationwould only be required for personnel protection. An aftercooler fitting can be used on the drain to automati-cally temper the water before entering a sanitary sewer.

Continuous Boiler Blowdown Heat Recovery (Sketch #5)

The final application for a flash tank is boiler continuous blowdown. Most boiler manufactures recommendautomatic TDS (Total Dissolved Solids) control.

This is done by continuously taking boiler water from 4-6” below the surface of the boiler water where TDS isat its highest level. Continuous blowdown is the only acceptable way to control boiler TDS. Even so blowdownlosses can be tremendous and heat recovery is required.

The first stage of continuous blowdown heat recovery uses a flash tank to flash part of the blowdown waterinto steam. Most of the time a deaerator or feedwater heater operating at 5-15 psi and can use all the addi-tional steam. The heat recovered from the flashing continuous blowdown, is 50-55% of the heat recovered.Returning the steam to the system also saves make-up water that would otherwise need replaced.

A second stage can be added to the flash tank and heat recovery system that includes a heat exchanger thattransfer the remaining heat into the boiler feed water. This stage of the heat recovery can provide 3 5-40% ofthe heat recovery. Besides transferring heat to make up water the blowdown is cooled to 100-110 deg. F andwill be acceptable in most drain systems.

A heat recovery system reclaims more of the BTU’s then just a heat exchanger. Penn’s Technical bulletinPSC486 shows the savings over time and the payback for a complete system verses just a heat exchanger. Asyou can see from the graph a shell and tube heat exchanger can only cool blowdown to 110 deg. F underpressure. The complete heat recovery system after flashing can not only cool to the same 110 deg. F but, cancool the blowdown even further providing additional savings over just a heat exchanger.

If we look at the savings example we had before from the flash steam and add the additional savings from thetransfer of heat from the condensate the savings for 1,000 #/hr. continuous blowdown would be $ .85 + .71 =$ 1.56 per hour. For a boiler operating 24 hours a day 365 days a year this would total $ 13,665.00 savingsper year. The savings from continuous boiler blowdown can be 1/2 to 1 ¼% overall boiler efficiency. Consid-ering the cost to produce steam the payback on a system of 3-6 months is not uncommon. Penn can providea free heat recovery survey with out obligation to show you possible saving for your boiler system.

Additional information on Penn products are available from our Sales Representative. We hope that thisinformation is useful and provides information that will lead to more efficient boiler operation and customersavings.