Flue Gas Condensers

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Flue Gas Condensers

Introduction

Flue gases from large boilers are typically 450 -650F. Stack Economizers recover some of this

heat for pre-heating water. The water is mostoften used for boiler make-up water or someother need that coincides with boiler operation.There is a class of economizers that aredesigned to condense the flue gases and/or havethe water in direct contact with flue gases. Ihave called them 'Flue Gas Condensers'. Stack economizers and Condensers should be considered asan efficiency measure when large amounts of make-up water are used (ie: not all condensate isreturned to the boiler or large amounts of live steam is used in the process so there is no condensate toreturn) or there is a simultaneous need for large volumes of hot water.

The application difference between an economizer and condenser is that economizers are primarilyused to heat a smaller volume of water to a high temperature for boiler feed water, and condenser unitsheat a larger volume of water to a lower temperature. Condensers can be more efficient because theycan have a lower outlet exhaust temperature and take advantage of the energy in condensed flue gasses(the Latent Heat of Vaporization).

The savings potential is based on the existing stack temperature, the volume of make-up or hot waterneeded, and the hours of operation. Economizers are available in a wide range of sizes, from smallcoil-like units to very large waste heat recovery boilers. Condensers are available as small as 50 hp anda single condenser can be used on multiple boilers.

Some condensers have water in direct contact with the flue gases and others use heat exchangers. Insome applications the water that has been in direct contact with the flue gases can be directly used; inother applications, the water must be passed through a heat exchanger before it can be used.

The key to the successful application of heat recovery is the ability to put the recovered heat to use.Uses include industrial process water heating, clean-up/wash-down water heating, laundry washwater, domestic water heating, space heating, snow melt and district heating systems. PotentialApplications include: Greenhouses, Hospitals and Health Centers, Food processing, Schools andUniversities, Laundries, Breweries, Hotels, Wineries, Government Buildings, and Swimming pools.

See also Economizers

Operation
http://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizers.asphttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizers.asp


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In a direct contact unit, water is sprayed in contact with the flue gases, causing condensation andextracting most all of the heat. In units that have a heat exchanger, condensation is likely when there isenough water flow -or- at a cold enough inlet temperature to remove enough heat to causecondensation on the flue gas side of the heat exchanger. See the Manufacturer's descriptions below formore details.

Economics

The savings potential is a function of how much heat can be recovered, which is a function of howmuch cold water needs to be heated. A generally accepted "rule of thumb" is that about 10% of boilerinput capacity can be recovered with a properly sized condenser. This is a higher percentage than whatcan be recovered with aneconomizerbecause the water temperature is lower. However, there is also alot more volume of water involved, assuming there is a need for enough cold water to condense all ofthe flue gas that is available. Therefore, for 'ball parking' purposes, start by comparing boiler inputcapacity with the need to heat water.
http://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizers.asphttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizers.asphttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizers.asp


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For example: consider a 500 hp boiler with a gas input of 20 million BTUs per Hour.

20,000,000 BTUs x 10% = 2,000,000 BTUs (100% Load Factor)

2,000,000 BTUs / (900 BTUs per Gallon of 160F water) = 2,200 Gallons per Hour

(2,000,000 BTUs / 80% efficiency) = ~2.4 MCF x $7.00 per MCF Natural Gas =

$16.80 per Hour Value

Savings is reduced by 50% for a 50% Load Factor, etc.

If there is a need for that much hot water, the savings potential of $16.80 per hour would be multipliedby the number of boiler run hours, or the number of hours that the hot water can be used. In eachapplication, be sure to consider the boiler Load Factor, the efficiency that the hot water is otherwiseproduced at, the cost of natural gas, and the installation cost of the equipment.

Condensers are generally NOT used to heat boiler make-up water. Use an Economizerfor thatapplication.

A condenser that recovers 10% of boiler input should easily have a 1 year payback in a year-roundapplication.

More Information

Economizers

Exhaust Draft Control

Boiler Efficiency

DOE Tip Sheet -"Consider Installing a Condensing Economizer" PDF Format

DOE Tip Sheet -"Considerations When Selecting a Condensing Economizer"PDF Format

Manufacturers

Kemco Systems

11500 47th Street, NorthClearwater, Florida 33762Telephone: 800-633-7055Fax: 727-573-2323
http://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizers.asphttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizers.asphttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizers.asphttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Exhaust_Draft_Control.asphttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Primer/Boiler_Efficiency.asphttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizer_Cond_DOE_26a.pdfhttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizer_Cond_DOE_26a.pdfhttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizer_Cond_DOE_26b.pdfhttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizer_Cond_DOE_26b.pdfhttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizer_Cond_DOE_26b.pdfhttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizers.asphttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizers.asphttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Exhaust_Draft_Control.asphttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Primer/Boiler_Efficiency.asphttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizer_Cond_DOE_26a.pdfhttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Economizer_Cond_DOE_26b.pdf


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Go to their web site at www.kemcosystems.com

Typical gas fired heaters and boilers send up to 30% of the fuel they consume outthe exhaust stack in the form of waste heat. This loss can mean that for every dollarspent on fuel the boiler system returns only 65 to 70 cents worth of useful heat.

Kemco's revolutionary boiler Stack Economizer can easily recover over 80% of the

waste heat found in the boiler's exhaust. The dollars saved on the fuel bill in just oneyear can be enough to pay for the cost of the Economizer.

In some cases, the system reduced the flue gas temperature below the ambientcombustion air, actually taking free heat from the air supply and, in effect, creatingan apparent 100% efficient boiler system. Because the Economizer uses an externalheat sink and can reduce the exhaust temperature of the stack to within 10 F of theinlet water temperature, the maximum amount of heat is extracted.

Traditional finned-tube and plain-tube units cannot reduce boiler or heater stack losses below 280 Fwithout self-destructing from cold-end corrosion. But Kemco's Stack Economizer safely reduces these

stack gases as low as 40 F.

That's because the gases from the boiler travel in a counter-flow pattern to the water, absorbing theheat and being pre-heated. There is a direct contact between the gases and the water, and because thefinal heat transfer mechanism is an "alternate surface" heat transfer, the stack gases can be cooled towithin 5 F of the initial temperature of the inlet water.

This results in a double savings. First, the flue gas is cooled to a lower temperature with the Kemcosystem, so more sensible heat is taken from the dry flue gases, water and vapor. Second, heat energy isrecovered from the heat of vaporization of the moisture formed during combustion by the fuel and thecombustion air.

Sidel USA Systems

PO Box 1868Atascadero, CA 93423Telephone: 805-462-1250 or800-668-5003

Go to their web site at www.sidelsystems.com

Sidel Systems has been in operation in Canada since 1978, in the United States the company wasincorporated in 1992. Sidel Systems specializes in the design and installation of hot water heatingsystems for commercial greenhouses.

The Sidel SRU series waste heat recovery units are built in North America in accordance with theASME codes. An SRU flue gas waste heat recovery system can be installed with any natural gas orLPG fired power burner boiler or heating unit. When the heating unit is required to operate on oil theflue gases will have to be bypassed from the waste heat recovery unit. Flue gas recovery systems havebeen used around the world for over 20 years. In most applications savings of 12-15% are realized.
http://www.kemcosystems.com/http://www.sidelsystems.com/http://www.kemcosystems.com/http://www.sidelsystems.com/


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Standard sizes are for equipment with inputs from 2.1 to42 million BTU per hour, 50 to 1,000 Boiler HorsePower.

The waste heat recovery unit is designed to transfer thewaste heat from the forced draft heating unit into usableheat. The waste flue gases from the heating unit are

redirected to the recovery unit, which is placed beside theheating unit or alongside the unit's chimney. The recoveryunit cools the flue gases to the point where sensible aswell as latent heat is recovered.

The waste heat recovery unit is a finned tube heatexchanger. The hot waste flue gas is diverted into thebottom receiver of this unit and then flows upwards acrossand through a series of specially designed finned tubes.The water that is being heated flows on the inside of thesetubes. The SRU series is designed so that waste flue gas

temperatures even below the water dewpoint (58C - 136)can be reached.

The SRU waste heat recovery unit has a large gas-sideheat transfer surface of aluminum fin tubes, with the finsand the tubes formed as one piece. Pure aluminum is avery good heat conductor; is it also exceedingly resistantto the mildly acidic flue gas condensate. The aluminum fin

tubes are fitted around a stainless steel tube. Stainless steel is used to prevent corrosion on thewaterside surface. Compared to other materials, stainless steel will not allow the build-up of ironparticles, which would act as an insulator on the tube surface.

Because of the high conducting properties of aluminum, the fins on the heating tubes are very thin.This allows more room for the flue gases to pass through. The result is an optimal ratio of the flue gasflow with respect to the fin surface area, which keeps the resistance of the flue gas heated side low.Because there is only a small increase in pressure, in most cases the heating unit is still capable ofusing its existing blower fan.

Thermal Energy International Inc.36 Bentley Avenue

Ottawa, Ontario, K2E 6T8 CanadaTelephone: 613-723-6776 Fax: 613-723-7286

Go to their web site at www.thermalenergy.com

FLU-ACE can be configured to handle any volume rate from boilers fueled by natural gas, oil, coal,or biomass. FLU-ACE technology greatly improves the fuel efficiency of boiler operations, providesa significant return on investment, and reduces environmental emissions.
http://www.thermalenergy.com/http://www.thermalenergy.com/


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Up to 90% of the heat normally lost through boiler flue gas stack emissions is recycled by FLU-ACE. This is possible because FLU-ACE's unique direct contact (gas/liquid) design enables optimalrecovery of both sensible (dry) heat and latent (wet) heat, even in widely varying operating conditions.

Conventional heat recovery technologies require a dedicated piece of equipment for each boilerexhaust. Now the varying flow of flue gases emitted from multiple boilers [1] can be efficientlyprocessed by a single FLU-ACE unit [2]. This means a lower initial investment, lower operatingcosts and a higher return on investment. FLU-ACE will have an ongoing positive effect on yourbottom line throughout its long operating life.

FLU-ACE's unique internal structure [3] ensures maximum condensing heat and mass transfer. It alsoenables an unobstructed flow of liquids and gases through the tower, guaranteeing virtually continuousoperation with limited downtime.

FLU-ACE is equipped with a variable speed, induced draft fan [4] at the tower outlet toautomatically maintain the optimum flue gas static pressure set point at the tower inlet [5].

Primary hot water is produced when boiler flue gas heat and pollutants are transferred and water vaporis condensed in the FLU-ACE. The primary water (at up to 63C/145F) accumulates in the receiver[6] where it is chemically treated to neutralize acids.

FLU-ACE uses variable speed pumps [7] to keep the temperature of the hot primary water leavingthe receiver at the desired level. Control valves regulate distribution of the primary water to the heatexchangers [8] where the heat is transferred either to secondary water [9] (for direct process make-upor boiler make-up usage) or to secondary glycol fluid [10] (for direct plant make-up air heating orboiler combustion air preheating).

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Flue Gas Condensers