Introduction
Present and ongoing research concerning the production of tobacco specific nitrosamines (TSNAs) in flue-cured tobacco recognize that TSNAs are developed by a multitude of factors during the curing and handling process. This research suggests that one of the most significant factors in the development of TSNAs is the chemical reaction occurring during the curing process between nicotine, contained in the uncured leaf, and various oxides of nitrogen (NOx), which are found in all combustion gases regardless of fuels. The significant reduction or elimination of these nitrogen compounds in the curing air has been shown to substantially reduce TSNAs in cured tobacco. In response to this research, the Tobacco Industry Leadership Group has developed these Advisory Recommendations for heating and air handling equipment that may be installed as retrofitted equipment in existing curing barns and as original equipment in new barns.
The curing of bright-leaf tobacco by the controlled application of supplemental heat was first developed over 160 years ago. Prior to 40 years ago, tobacco was cured in grower built structures that were heated either with direct-fired (products of combustion freely in contact with the curing leaf) or with some type of indirect-fired (utilizing a heat exchanger or flue) that conducted the products of combustion away from the curing leaf. Prior to recent research, barns utilizing either direct-fired or indirect-fired systems were considered to be capable of producing cured leaf of excellent and commercially indistinguishable quality.
The first commercial tobacco bulk curing barn was sold in 1960. Since that time, it is estimated that more than 60,000 barns have been sold in the flue-cured tobacco producing region of the southeastern US. It is also estimated that no more than 50,000 of these units are still in operating condition. From 1960 to the mid-1970's, a large portion of all barns manufactured employed oil-fueled, indirect fired heating systems. Many of these barns are still in use. After the mid-1970's, most new barns employed natural or LP gas, direct-fired systems. The reason for this change was the significant equipment cost and maintenance costs differentials between gas and oil fueled equipment. Further, the assumed future availability of gaseous versus liquid fuels and especially the relatively greater efficiency (20 to 40 percent) of direct-fired versus indirect-fired heating systems prompted most growers to purchase LP gas barns.
Recommendations
1. Definitions
1.1. Retrofit. The process and equipment whereby a tobacco curing barn is brought into compliance with these Recommendations.
1.2. Direct combustion curing system. A system where the combustion gases are introduced directly into the curing air stream and hence into contact with the curing leaf.
1.3. Indirect combustion curing system. A system where the combustion gases are prevented from entering the curing air stream by flues or pipes and the heat is extracted by conduction and forced-convection from a suitable heat exchange apparatus.
1.4. Curing System. The combination of heating and air handling system for curing tobacco including all auxiliary components and controls.
1.5. Heat Exchanger Assembly. Equipment including combustion chambers, flues, pipes, vents, radiators or associated parts whose function is to contain a burning gas or heated fluid and efficiently transfer the heat to a moving stream of air.
2. General
2.1 These Recommendations are voluntary and are not intended to create an express or implied obligation that they be implemented by barn owners or operators or vendors.
2.2 The accepted definitions of "shall" and "should," as included in these Recommendations, are:
2.2.1 "Shall" is intended to indicate actions that would be necessary to support a representation that a barn complies with these Recommendations.
2.2.2 "Should" is intended to indicate that which is advised, but not required, to support a representation that a barn complies with these Recommendations.
2.3 These Recommendations apply to all indirect-fired heating systems and components designed for curing flue-cured tobacco regardless of construction, fuel type or Btu output.
2.4 These Recommendations apply to existing as well as newly constructed barns.
2.5 These Recommendations apply to complete curing systems retrofitted into existing or new manufactured barns as well as heating and/or air handling equipment and components retrofitted into existing barns.
2.6 Portions of these Recommendations that do not relate to control of combustion products apply to tobacco curing systems employing external combustion such as hot water or steam systems as well as electric resistance or other types of systems where fuel combustion does not occur within the confines of the barn and the products of combustion are prevented from entering the barn.
2.7 Direct-fired curing systems of any design or fuel type do not comply with these Recommendations, except as provided under 5.1.
2.8 A curing system that otherwise complies with these Recommendations is not compliant if it has features that impair the results intended by these requirements (i.e., the significant reduction of TSNA’s from cured leaf).
2.9 To comply with these Recommendations, equipment shall first comply with all applicable sections of ASAE Standard: ASAE S248.3, latest revision, "Construction and Rating of Equipment for Drying Farm Crops."
2.10 All curing systems, equipment or components complying with these Recommendations shall also comply with any other applicable recommendations, laws, rules, guidelines or codes. A partial list of potentially applicable standards are shown in Attachment A below. Standards other than those shown may also apply.
3. Construction
3.1 Each curing system or component should be constructed of entirely new unused parts and materials.
3.2 The Heat Exchanger Assembly shall be constructed of materials suitable for the intended purpose and of sufficient strength and durability to ensure reliable service for a minimum of three years.
3.3 All equipment shall have all shields, screens and other safety devices for the protection of life and property.
3.4 Provisions shall be made to permit ready observation of and reasonable access to all parts for assembly, maintenance and repair.
3.5 Curing systems shall be designed in such manner as to prevent the products of combustion from entering the curing air in any quantity and in any manner for the warranted life of the system. Means by which combustion gases may enter the curing air include, but are not limited to the following: heat exchanger cracks, weld voids, drafts, improper flue seals, corrosion openings, flame erosion openings and improper mountings.
3.6 Curing systems purporting to comply with these Recommendations shall be subject to tests and examinations by a cooperative tobacco state land grant university program for the levels of nitrous oxides and other combustion gases in the curing air. The same or additional tests of performance, efficiency, reliability and general fitness for the intended application may be conducted by application only from either the equipment manufacturer or barn owner or operator.
3.7 The full results of any test, in writing, will be provided by the testing agency to the persons who requested the test and the equipment owner. Additionally, these test results may be published or otherwise made available to the public through the proper channels of a cooperative tobacco state land grant university program.
4. Applicability
4.1 Barn owners or operators representing that their tobacco is cured in a manner satisfying these Recommendations are responsible for ensuring compliance with these Recommendations.
4.2 To the extent that a vendor of curing equipment, either new air handling and heating systems in new barns, new air handling and heating systems as retrofits to existing barns, or new components as retrofitted equipment to existing barns, represents that it is selling a package compliant with these Recommendations:
4.2.1 The vendor shall ensure that such equipment is compatible with the other components of the barn.
4.2.2 Where systems or components are retrofitted into an existing barn, the vendor shall ensure such equipment meets or exceeds the performance of the original equipment and that the heating and air handling system, in general, likewise meets or exceeds the performance of the original equipment in the original barn in terms of heat output, air handling capacity (i.e., static pressure and flow rate), electrical requirements and general suitability for its intended functions. In particular, and without limitation, a vendor of a retrofitted heat exchanger shall acknowledge and correct for the added air resistance of that heat exchanger if the added resistance reduces the static pressure and flow rate of the existing air handling system to a materially lower level for the proper curing of tobacco.
4.2.3 The vendor shall ensure that equipment complying with these Recommendations shall have little or no effect on the curing schedule in terms of time to complete the cure or the proper control of temperature and humidity as is customarily practiced by the grower with that particular model of barn and tobacco in terms of stalk position, loading capacity or other variables as they arise.
4.2.4 The vendor shall ensure that nothing in the design, installation, maintenance or operation of this equipment negatively impacts the merchantability of the cured leaf removed from these barns.
5. Alternative Systems
5.1 Any barn owner or operator or
vendor may nonetheless comply with these Recommendations if it can demonstrate
to the satisfaction of the Tobacco Industry Leadership Group or its
designee that its barn or process eliminates NOx products or otherwise
reduces TSNAs commensurate with a level achieved through compliance with
Sections 1-4 of these Recommendations.
ATTACHMENT A
Reference List of Applicable Recommendations
The following is a partial list of reference
specifications and recommendations pertaining to the design, manufacture,
installation, maintenance, testing or use of equipment or potential components
for curing tobacco or similar applications.