2. Psychrometrics of Tobacco Curing
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Tobacco varieties cultivated in Macedonia, have short leaf, golden yellow
coloured, with characteristic aroma. At 17800 ha, a total amount of 27600 tonnes is
produced in 2014, [1]. The basic characteristics of local tobacco varieties are shown
in Table 1, [2]. Cured tobacco is one of the chief exports of the country.
Fresh collected tobacco leaf can have up to 85 % moisture content wet basis. The
desired end product of the tobacco curing ought to be correctly processed leaf with 15
to 22 % moisture content wet basis.
Table 1 Characteristics of local tobacco varieties
Characteristic
Variety
Prilep Jaka Jebel
Maximum height, cm 80 165 140
Average number of
leaves
30 40 44
Maximum leave length,
cm
26 25 18
Nicotine, % 0,9 - 1,5 0,4 - 0,9 0,3 - 0,6
Proteins, % 4,5 - 6,5 4,5 - 6,5 5 - 7
Carbohydrates, % 18 - 21,5 14 - 20,5 14,5 - 18,5
Polyphenol, % 4,25 - 6,2 3,9 - 5,8 4 - 6,15
pH factor, % 4,47 - 5,45 4,9 - 5,5 5 - 5,7
Yield, kg/ha 1200 - 2600 1200 - 2400 1200 - 2500
Tobacco curing is a process of transforming harvested tobacco leaf into basic
material for cigarettes manufacturing industry. In our country, it is conducted as
thermal treatment of the tobacco leaf by conditioned atmospheric air, [3]. The effect
of air temperature and relative humidity upon the quality of the tobacco leaf is crucial.
During the tobacco curing the leaf goes through biochemical and physical changes. A
two step process is realised: 1. tobacco yellowing, and 2. tobacco drying. In the first
phase the fresh green tobacco leaf is put in air-conditioned space. There, under the
influence of specified air states, it is yellowed. Simultaneously, in the process of
fermentation the starch and the protein are devastated at lower level compounds,
desirable for smoking. Lower nicotine content, up to 30 %, is also reached. In the
second step, if the leaf is dried with correct intensity its colour will be fixed. Every
tobacco type has characteristic colour at which owns chemical composition that
provides the highest taste features.
The normal realization of tobacco curing process depends on the accuracy of the
curing curves and the precision of their implementation.
Curing curves are specific for every tobacco variety. They are reflection of all
designer knowledge and experience on the actual problem, Figure 1, [4], [5], [6].
Using artificial heat, air temperatures are increased gradually during the curing
period. Temperatures of 30 to 50 o
C are normal for the first three days for yellowing,
than 50 to 65 o
C for two days for leaf drying, and 70 o
C for the last day. A bright
yellow to orange colour is desirable in cured tobacco leafs of Virginia bright-leaf
tobacco and Macedonian short leaf tobacco varieties. As every dried hygroscopic
material, the dried tobacco leaf must be thermally balanced with the environment in
order to avoid the absorption of water in the leaf. Therefore, at the end of curing the
temperature is decreased.
3. Filip A. Mojsovski
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1- Temperature, 2 - Relative humidity, 3 - Leaf yellowing, 4 - Leaf colour fixing and
rest of yellowing, 5 - Leaf drying, 6 - Rib drying
Figure 1 Curing curve for Virginia bright-leaf tobacco
The major task for thermal engineers is to apply the regime of air state changes
dictated by curing curves.
The problem definition for the conducted investigation was: “In what space and
with which equipment the air state changes required by the curing curves can be
reached?”.
The solution was searched by conducting field tests of designed and constructed
tobacco curing unit and air-conditioning system.
2. BUILDING AND EQUIPMENT
Gaining knowledge by keeping under consideration large numbers of existing tobacco
curing units in the country, these requirements for the building are anticipated: the
unit ought to be solidly built, located on the ground floor, with thermal insulated
ceiling, with door wall and entering air wall on the opposite sides, with the possibility
for modular extension.
The tobacco curing unit needs certain thermal inertness to be able to eliminate the
outdoor influence. Because the door wall is bordered by corridor, the entering air wall
is bordered by machine room, the unit is one module in a series of neighboring units,
only the thermal insulation of the ceiling was mentioned. The filling of the unit with
palettes which is always done by fork-lift truck raises the question of its location.
Modular construction is used to allow the increase of the available capacity. The basic
4. Psychrometrics of Tobacco Curing
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criterion for curing space planning is: 20 m3
volume in the unit for tobacco collected
from 1 ha area.
The architecture of the curing unit used as a sample in the realized investigation is
shown in Figure 2.
1 - Curing space, 2 - Machine room, 3 - Corridor, 4 - Air-conditioning unit,
5 - Supply air duct, 6 - Return air duct, 7 - Relief air duct, 8 - Measuring levels
(3x3x5 =45 points)
Figure 2 Tobacco curing unit
The selection of air handling and distribution system is carried out according to:
(1) general requirements for long life with low maintenance and operating costs in
order to provide low life-cycle cost, and (2) specific requirements, such as needed
quantity of supplied air, the quality of entering air, the location of air distributive
elements to provide the specified air state changes in the curing unit.
Selected air-conditioning system includes heating and humidifying sources. The
air-conditioning unit consists of mixing section for atmospheric air and recirculating
air, filter, heater, humidifier and fan, Figure 3.
By applying constant airflow, single duct, constant volume air-conditioning
system the changes of the entering air state ware obtained in accordance with the
curing curves. This system responded completely to the space needs and the control
system maintained air temperature and relative humidity closely to the values of the
curing curves, [7], [8]. A return fan is eliminated because the over pressurization is
relieved by damper.
5. Filip A. Mojsovski
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1 - Tobacco curing unit, 2 - Air-conditioning unit, 3 - Mixing section, 4 - Filter, 5 -
Heater, 6 - Humidifier,
7 - Fan, 8 - Inlet air, 9 - Outlet air, 10 - Atmospheric air, 11, 14, 15 - Measuring points
(Pitot tube),
12, 13 - Measuring points (Thermometer, Hygrometer, Anemometer)
Figure 3 Functional scheme of single duct, constant volume air-conditioning system
3. FIELD PERFORMANCE TEST
Three curing units, located side by side, were available for the planned investigation.
Each curing unit was equipped with different air distribution system. This air
distribution strategy was applied to compare the influence of air supply locations on
the curing process, [9], [10], [11].
An under floor air distribution system, with air supply at floor level, was used in
the first curing unit.
The second curing unit had one side wall with inlet air and across side wall with
outlet air grilles.
The third curing unit had downside double duct for inlet air and upside double
duct for outlet air.
Testing procedures for a tobacco curing unit are not set by government standards.
In the actual investigation two basic test models were used to evaluate a facility:
(1) tobacco curing unit without palettes, and (2) operational. The fully evaluation
cannot be reached until the tobacco curing unit is operated in operational test mode,
that means including all the palettes. Techniques for conducting initial performance
test and operational monitoring were similar.
The both test models were conducted through the same procedure. Measurements
were realized for the curing medium (flow rate, moisture and temperature) and for the
cured product (moisture and temperature). The Pitot tube in conjunction with a
precise manometer was applied for airflow rate measurement at measuring points 11,
14 and 15. Air conditions were determined with psychrometers positioned at
measuring points 12 and 13. The used digital thermometers for measuring points 12
6. Psychrometrics of Tobacco Curing
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and 13 had accuracy 0,2 % of measured value, a thermocouple NiCr/Ni sensor,
reaction time 0,3 seconds and sensor diameter of 1,5 mm. Alternatively, the relative
humidity was measured with the use of digital hygrometers. The initial, internal and
final moisture content of tobacco leaf was inspected by taking representative leaf
samples in measuring points at measuring levels 8. Then, the results were obtained in
laboratory with electric moisture meter, [12]. The measuring points at measuring
levels 8 were used for temperature control, too.
The installed air-conditioning unit is relatively small, with capacity of 10000
m3
/h. Measured values of distributed airflow in the curing space were in the range of
2 m3
/s. Air thick with artificial colored fume was applied to provide visual effect.
Temperature sensor usually shows the thermal state within a relatively small
volume. To determine the temperature fields change in a curing space, a net of
measuring points was defined. Forty-five measuring nodes at three vertical and three
horizontal levels were included in data acquisition process, Figure 2, Position 8.
Applied building and equipment allowed stabile following of required parameters
by curing curves. In a few temporary withdrawals from the curing curve regime, by
the operational research, the elasticity of the system was proved.
The obtained data, completed with laboratory quality analysis of cured tobacco,
was decisive in the selection of suitable curing unit.
The curing unit with under floor air distribution system was eliminated because of
relatively higher costs for underground ducts construction and the difficulties with the
ducts cleaning. The curing unit with lateral air inlet grilles also showed impassable
results.
Table 2 Tobacco curing unit identification
Component, parameter Value or description
1. Curing space
- Dimensions: length, width,
height (m)
16, 10, 4
- Capacity: palettes (pieces) 56
- Capacity: tobacco (kg) 25000
- Air state Pressurized
2. Air handling system
- Air-conditioning unit, capacity
(m3
/h)
10000
- Air-conditioning unit,
components:
Filter, Heater, Humidifier, Fan
- Air distribution, supply air Downside, lateral ducts with 12
grilles
- Air distribution, return air Upside, lateral ducts with 12
grilles
- Air velocity, supply duct, (m/s) 6
- Air velocity, supply grilles,
(m/s)
2,0 - 2,2
- Air velocity, return grilles, (m/s) 1,6 - 1,7
- Air pressure control Relief duct and damper
3. Tobacco handling system Metal palettes, 2x1x3 m
7. Filip A. Mojsovski
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The tobacco leaf received best quality when the curing process was conducted in
the curing unit with downside air supply, Figure 2. This type of curing unit, with
attested thermal capability, was selected as a model for building in the future. The
relevant parameters for model curing unit are presented in Table 2.
4. CONCLUSION
All the knowledge and experience of local agricultural experts on curing of short leaf
tobacco varieties “Prilep”, “Jaka” and “Jebel”, manifested by curing curves, was
implemented in the investigation for a proper design of the curing space and
equipment.
In the situation when there is a lack of standards on testing procedures for tobacco
curing units, techniques of initial performing test and operational monitoring were
used for evaluation of curing unit quality.
A curing unit with attested thermal capability is proposed as a model for building
in the future.
REFERENCES
[1] Statistical Year Book of the Republic of Macedonia, State Statistical Office,
Skopje, Macedonia, 2015, (in Macedonian).
[2] Tobacco chart of the Republic of Macedonia, Ministry of Agriculture, Forestry
and Water Economy, Skopje, Macedonia, 1985.
[3] Mojsovski A., Mojsovski F., Tobacco drying, Technical report for court
expertise, FME-ITE.A-09.07-1., Faculty of Mechanical Engineering, Skopje,
Macedonia, 2007, (in Macedonian).
[4] Maltry W., Pötke E., Sneider B., Drying in Agriculture, Chapter 9: Tobacco
drying, VEB Verlag Technik, Berlin, Germany, 1975, (in German).
[5] Kröll K., Kast W., Drying and Dryers in the Production, Chapter 2: Tobacco
drying, Springer Verlag, Berlin, Germany, 1989, (in German).
[6] ASHRAE Handbook Fundamentals, Chapter 10: Physiological Factors in Drying
and Storing Farm Crops, American Society of Heating, Refrigerating and Air-
Conditioning Engineers, Atlanta, USA, 1985.
[7] Ono T., Yoshida T., Designing a Dryer in Tobacco Manufacturing Process, in
Drying ’86 edited by A. S. Mujumdar, Hemisphere Publishing Corporation,
Washington, USA, 1986.
[8] CER Factory for Thermal Engineering, Tobacco Dryer SD-352, Čačak, Serbia,
1990, (in Serbian).
[9] Gatley D. P., Understanding Psychrometrics, American Society of Heating,
Refrigerating and Air-Conditioning Engineers, Atlanta, USA, 2005.
[10] ASHRAE Handbook Fundamentals, Chapter 1: Psychrometrics, American
Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, USA,
2013.
[11] Mojsovski F., Analysis of humidity level in psychrometric thermal processes,
Ph.D. Thesis, Faculty of Mechanical Engineering, Skopje, Macedonia, 2007, (in
Macedonian).
[12] ASAE S487 DEC1987 (R2008), Moisture Measurement-Tobacco, American
Society of Agricultural and Biological Engineers, St. Joseph, USA, 2008.
![Psychrometrics of Tobacco Curing
http://www.iaeme.com/IJMET/index.asp 25 editor@iaeme.com
Tobacco varieties cultivated in Macedonia, have short leaf, golden yellow
coloured, with characteristic aroma. At 17800 ha, a total amount of 27600 tonnes is
produced in 2014, [1]. The basic characteristics of local tobacco varieties are shown
in Table 1, [2]. Cured tobacco is one of the chief exports of the country.
Fresh collected tobacco leaf can have up to 85 % moisture content wet basis. The
desired end product of the tobacco curing ought to be correctly processed leaf with 15
to 22 % moisture content wet basis.
Table 1 Characteristics of local tobacco varieties
Characteristic
Variety
Prilep Jaka Jebel
Maximum height, cm 80 165 140
Average number of
leaves
30 40 44
Maximum leave length,
cm
26 25 18
Nicotine, % 0,9 - 1,5 0,4 - 0,9 0,3 - 0,6
Proteins, % 4,5 - 6,5 4,5 - 6,5 5 - 7
Carbohydrates, % 18 - 21,5 14 - 20,5 14,5 - 18,5
Polyphenol, % 4,25 - 6,2 3,9 - 5,8 4 - 6,15
pH factor, % 4,47 - 5,45 4,9 - 5,5 5 - 5,7
Yield, kg/ha 1200 - 2600 1200 - 2400 1200 - 2500
Tobacco curing is a process of transforming harvested tobacco leaf into basic
material for cigarettes manufacturing industry. In our country, it is conducted as
thermal treatment of the tobacco leaf by conditioned atmospheric air, [3]. The effect
of air temperature and relative humidity upon the quality of the tobacco leaf is crucial.
During the tobacco curing the leaf goes through biochemical and physical changes. A
two step process is realised: 1. tobacco yellowing, and 2. tobacco drying. In the first
phase the fresh green tobacco leaf is put in air-conditioned space. There, under the
influence of specified air states, it is yellowed. Simultaneously, in the process of
fermentation the starch and the protein are devastated at lower level compounds,
desirable for smoking. Lower nicotine content, up to 30 %, is also reached. In the
second step, if the leaf is dried with correct intensity its colour will be fixed. Every
tobacco type has characteristic colour at which owns chemical composition that
provides the highest taste features.
The normal realization of tobacco curing process depends on the accuracy of the
curing curves and the precision of their implementation.
Curing curves are specific for every tobacco variety. They are reflection of all
designer knowledge and experience on the actual problem, Figure 1, [4], [5], [6].
Using artificial heat, air temperatures are increased gradually during the curing
period. Temperatures of 30 to 50 o
C are normal for the first three days for yellowing,
than 50 to 65 o
C for two days for leaf drying, and 70 o
C for the last day. A bright
yellow to orange colour is desirable in cured tobacco leafs of Virginia bright-leaf
tobacco and Macedonian short leaf tobacco varieties. As every dried hygroscopic
material, the dried tobacco leaf must be thermally balanced with the environment in
order to avoid the absorption of water in the leaf. Therefore, at the end of curing the
temperature is decreased.](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)