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Tobacco: Tobacco Barn Retrofit

Heat Exchanger Information

Detecting Heat Exchanger Leaks

Nitrosomines in the cured leaf became an issue for growers in late 1999. Since that date all of the flue-cured bulk tobacco barns being used have been updated with indirect fired curing units. These units were built by major tobacco barn manufacturers and local metal shops. Depending on the metal used and quality of construction will determine the life of these units.

Two full harvest and curing seasons has past for the retrofitted bulk barns. Tobacco companies have been sampling the cured leaf for Nitrosomines in the cured leaf. By in large they have not found Nitrosomines levels in cured leaf. Nitrosomine levels in cured leaf have been greatly reduced. But there have been cases where nitrosomine levels were found and the growers were notified of the problem. Some reasons for high nitrosomine levels still being detected in cured leaf are as follows:

  1. The flue gas exit point is less than 2 feet above the highest point of the barn. Growers should make sure that flue gases are not allowed to enter the barn through the air inlet vents during curing.
  2. The welds in and on the metal heat exchanger has separated (cracked). This could be a result of thin metal being used, more than one type of metal used, poor welds, and inappropriate bracing. These heat exchangers experience several thousand heating/cooling cycles during a six- or seven-day cure. All metals expand and contract upon heating and cooling.

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How Do I Find Cracks?

Use a flash light look at the heat exchanger welds, this may take crawling under the floor and looking down over the furnace area. It will be very hard to examine all of the welds.

A better way is use an electronic gas meter to detect carbon dioxide gases being released into the curing air. Recently, North Carolina State University (Grant Ellington) has conducted test using a commercial Carbon Dioxide (CO2) meter to measure levels in the curing barn. A slide show (PDF) was used to report the results. Any CO2 meter having a range of 0 -10,000 ppm can be used. The probe must be placed in the air stream in the barn. All fresh air vent must be closed. Operate the barn for 5 to 10 minutes with the fan on only. The carbon dioxide levels in the unheated barn should read approximately 350 to 500 ppm (ambient air). Next, advance the thermostat 30° to 40° F above ambient air temperature. The maximum temperature setting during this test should not exceed 140° F. Monitor the CO2 level for 10 to 20 minutes after the burner initially ignites. If the CO2 levels increase significantly above the initial ambient background levels during this time, then the heat exchanger should be inspected closely for cracks. Some units levels of CO2 may change very rapidly, increasing by a factor of 2 or more above initial ambient levels in only minutes. Other units may only increase by 200 or 300 parts per million in ten minutes or longer, but an inspection for cracks is still recommended.

If high CO2 levels or cracks are observed, contact the installer and/or manufacturer of heat exchanger. Under the agreement, they are to repair the unit for three years.

Even though there may be elevated levels of CO2s, and therefore other combustion by-products present inside the curing chamber, the relationship between CO readings and TSNAs levels is not known at this time. However, we do know that combustion by-products (NOx) are responsible for TSNA formation in flue-cured tobacco.

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High Temperature Sealant

A high temperature sealant (Exhaust sealant - for temperatures up to 2300°F) may be needed in addition to these mechanical fasteners to properly seal the connection from leaking exhaust gases. The best solution would be to minimize or eliminate the flue-pipe connections located inside the barn.

A source for this sealant is Cotronics Coproration.

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Problems with Heat Exchangers

A lot of tobacco has now been successfully cured in retrofitted barns. Many growers are reporting only a small or insignificant increase in fuel use and are pleased with the quality of the cured tobacco. The drier heat in a heat exchanger barn (direct fired barns dump over 1 gallon of water into the barn for each gallon of fuel used) has probably resulted in less down stalk barn rot than would have occurred in this rather wet year.

As more and more tobacco is cured in heat exchanger barns, we have occasionally heard of some problems possibly associated with heat exchangers. These have included barn fires, improper fuel/air adjustments, igniter problems, under capacity and some problems with barn rot.

If you are aware of such problems, we would appreciate an e-mail describing the situation and as much information as possible.

E-mail Paul Sumner at psumner@uga.edu or J. Michael Moore at jmmoore@uga.edu.

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