3.2. The Temperature Field Uniformity Analysis of TCCB and HPCB along the Horizontal Direction
The spatial distribution of temperature is of significant importance in tobacco curing. During the experiment, acquisition points 1, 2, 7, 8, 13, and 14 were selected to represent the upper shed area, and acquisition points 5, 6, 11, 12, 17, and 18 were selected to represent the lower shed area. The temperature eigenvalue, standard deviations, and temperature uniformity of the upper and lower shed areas along the horizontal direction can be calculated by substituting the temperature data of the above 12 acquisition points into Equations (1) and (2). The onset of yellowing temperature, the transition temperature from the yellowing stage to the color-fixing stage, and the temperature for the main veins of tobacco to turn brown were 38 °C, 42 °C, and 50 °C, respectively. Moreover, the drying temperature of the main veins in the stem-drying stage was 68 °C. The temperature uniformity along the horizontal direction of TCCB and HPCB is presented in
Figure 3 and
Figure 4.
Figure 3 and
Figure 4 show that at the four temperature states, the uniformity of the upper and lower sheds of TCCB along the horizontal direction was greater than that of HPCB, and the maximum uniformity occurred at 50 °C. In the upper shed, the maximum uniformity of TCCB was 4.2 °C and the maximum uniformity of HPCB was 2.9 °C; in the lower shed, the maximum uniformity of TCCB was 4.8 °C and the maximum uniformity of HPCB was 2.6 °C. Chen J L [
23] demonstrated that in the chrysanthemum heat pump drying process, the smaller the uniformity, the more stable the regional temperature field. The obtained results revealed that uniformity of HPCB is smaller than that of TCCB, indicating that HPCB provides a uniform temperature field in the curing process so tobacco leaves are cured at a stable condition, thereby improving the curing quality.
3.3. Analysis of Temperature Field Charts in TCCB and HPCB Modes along the Horizontal Direction
According to the temperature eigenvalue of each temperature point obtained from the test data, acquisition points 1, 2, 7, 8, 13, and 14 were selected to represent the upper shed area, while acquisition points 3, 4, 9, 10, 15, and 16 were selected to represent the middle shed area, and the collection points 5, 6, 11, 12, 17, and 18 were selected to represent the lower shed area. Meanwhile, the four temperatures of 38, 42, 50, and 68 °C were selected as the temperature observation points. The temperature charts were drawn using MATLAB software and the horizontal temperature variation TCCB and HPCB at four key temperature points were analyzed accordingly. The obtained results are shown in
Figure 5,
Figure 6,
Figure 7 and
Figure 8.
Figure 5 indicates that when the curing temperature was 38 °C, the tobacco leaves were in the early yellowing stage, and the temperature of tobacco leaves changes in the range of 36–41 °C. Moreover, it was found that the highest and the lowest temperature of TCCB was 40.6 °C and 36.3 °C, located in the middle region of the upper shed cross-section and in the rear region of the lower shed cross-section, respectively; the highest and the lowest temperature of HPCB was 38.1 °C and 36.2 °C, occurring in the front end of the upper shed cross-section and in the rear end of the lower shed cross-section, respectively.
Figure 5 shows that the variation range of temperature in TCCB was 4.3 °C, while it was only 1.9 °C in HPCB, indicating that the temperature field was more uniform in HPCB. This is consistent with the results of Persimmon slices heat pump drying reported by Tang, X.X. et al. [
24]. The temperature distributions of the upper shed and middle shed cross-sections in TCCB were uniform, while the stratification of the lower shed cross-section was obvious, and the temperature distribution range was 36.3–38 °C. It was found that the temperature near the return outlet was high, while the temperature near the back end was low. The temperature distributions of the upper shed, middle shed, and lower shed cross-sections in HPCB were relatively uniform, and the temperature of the upper shed was large. This is because the bulk curing barn located in the front end of the upper shed is an air inlet, in which the temperature gradient is small and the temperature stratification decreases slowly from the upper shed to the lower shed.
Figure 6 shows that when the temperature of flue-cured tobacco was 42 °C, the tobacco leaves were in the late yellowing stage to the early color-fixing stage, and the temperature range was 37–46 °C. Under these conditions, the highest and lowest temperature of TCCB, which occurred in the middle area of the upper and lower shed cross-sections, reached 45.9 °C and 40.0 °C, respectively. Furthermore, the maximum and minimum temperature of HPCB, which occurred in the front end and back end of the upper and lower shed cross-sections, reached 41.9 °C and 37.0 °C, respectively.
Figure 6 reveals that the temperature range of TCCB was 5.9 °C, which is slightly higher than 4.9 °C for HPCB, indicating that the temperature field distribution uniformity of HPCB was slightly higher than that of TCCB. In TCCB, the temperature distributions of the upper shed, middle shed, and lower shed cross-sections were relatively uniform in each layer. The overall temperature of the upper shed was higher, followed by the middle shed, and the lower shed had the lowest temperature. In the HPCB mode, the temperature distributions of the upper shed and the middle shed cross-sections were relatively uniform in each layer. The temperature stratification of the lower shed cross-section varied in the range of 37.0–41.0 °C. The temperature near the return air outlet was high, while there was a low-temperature zone near the back end.
Figure 7 reveals that when the temperature of flue-cured tobacco was 50 °C, the tobacco leaves were in the middle and late stage of color-fixing, and the temperature varied in the range of 45–56 °C. The highest temperature of TCCB reached 55.7 °C, which occurred in the middle area of the upper shed cross-section and the lowest temperature was 48.5 °C, which was located in the middle area of the lower shed cross-section. The maximum and minimum temperatures of HPCB, which occured in the front end of the upper shed cross-section and the back end of the lower shed cross-section, were 51.7 °C and 45 °C, respectively.
Figure 7 indicates that the temperature fluctuations of TCCB and HPCB were consistent. The temperature range of TCCB was 7.2 °C, while that of HPCB was 6.7 °C. In TCCB, the temperature distribution of the upper, middle, and lower shed cross-sections were relatively uniform in each layer. The highest temperature occurred in the upper shed, followed by the middle shed, and the lowest occurred in the lower shed. In HPCB, the temperature distributions of the upper and middle shed cross-sections were relatively uniform in each layer. The temperature stratification of the lower shed cross-section was 45–48 °C, in which high-temperature and low-temperature zones occured near the return air outlet and the back end, respectively.
Figure 8 shows that when the temperature of flue-cured tobacco reached 68 °C, the tobacco leaves were in the late stage of stem-drying and the temperature range was 62–72 °C. The highest temperature of TCCB reached 71.2 °C, which occurred in the middle area of the upper shed cross-section and the lowest temperature was 64.8 °C, which occurred in the back end of the lower shed cross-section. Moreover, it was found that the maximum and minimum temperatures of HPCB reached 69.7 °C and 62.0 °C, which occurred in the front end and back end of the upper and lower shed cross-section, respectively.
Figure 8 shows that the temperature fluctuation of TCCB was 6.4 °C, which is slightly higher than 5.7 °C for HPCB. In TCCB, the temperature distributions of the upper and middle shed cross-sections were relatively uniform in each layer, while the temperature of the lower shed cross-section was lower and varied in the range of 64.8–68 °C. Sun L [
25] demonstrated that this distribution meets the requirements of curing tobacco leaves during the stem-drying stage. In HPCB, the temperature distributions of upper and middle shed cross-sections in each layer were relatively uniform and the overall temperature distribution met the requirements of curing tobacco leaves in the stem-drying stage.
According to the temperature field charts of TCCB and HPCB in the horizontal direction, the maximum temperature of TCCB was higher than that of HPCB with a relatively uniform temperature distribution in each shed. However, at some key temperature points, the temperature of the lower shed was low and stratified. This was especially more pronounced in the back end of the lower shed. In general, HPCB can effectively form a stable temperature flow field in the curing barn and significantly improve the curing quality of tobacco leaves.
3.4. Analyzing the Dehumidification Capacity of the Heat Pump
During the experiment, the dehumidification capacity at each stage of tobacco curing was measured by the humidity detection system. Then, the dehumidification efficiency at each stage was calculated by Equation (6). The results are presented in
Table 6.
Table 6 indicates that the temperature of the drying medium gradually increased in the barn. Moreover, mean dehumidification capacity
Wn and dehumidification efficiency
ηw increased first and then decreased, which can be interpreted by water loss of tobacco leaves in the drying process. During the experiment, the maximum and minimum
Wn were 32.51 kg/h and 3.12 kg/h, respectively, and the total
W was 3591.41 kg. In the yellowing stage, the temperature of the drying medium of tobacco leaves was low, the moisture of fresh tobacco leaves was high, and the relative humidity of the tobacco loading chamber was high. Moreover, the total
W was 1259.16 kg (342.96 + 410.60 + 505.60), accounting for 35.06% of the total
W of the whole process. When leaves entered the color-fixing stage, the water loss of tobacco leaves accelerated and the dehumidification capacity and efficiency improved significantly. The maximum dehumidification capacity reached 32.51 kg/h, and the dehumidification efficiency exceeded 0.85%/h, indicating that the dehumidification efficiency was high. During the color-fixing stage, the total dehumidification capacity was 2057.71 kg (665.07 + 780.24 + 612.40), accounting for 57.3% of the total dehumidification in the drying process. In the stem-drying stage, most of the initial moisture was discharged and a little amount of moisture remained in the veins of leaves. Accordingly, the water loss rate after this stage was low. Subsequently, the dehumidification capacity was low (3.12 kg/h) and the dehumidification efficiency dropped significantly and reached 0.09%/h. The total dehumidification capacity in the drying period was 274.54 kg (205.90 + 68.64), accounting for 7.64% of the total dehumidification capacity. Furthermore, the total dehumidification capacity during the stem-drying stage was 274.54 kg, accounting for 7.64% of the total dehumidification.
The HPCB is an effective scheme to resolve the problems of high energy consumption in curing barns by recycling the energy with the condensate water (dehumidification capacity). The more the condensate water is discharged from the system, the more energy will be recycled and then return to the heating arrangement system again.
3.7. The Energy Efficiency and Economic Analysis of Bulk Curing Barn
During the curing process, the coal consumption, electricity consumption, and labor costs of TCCB and HPCB were recorded. In this regard,
Table 9 shows that the mass of fresh tobacco, dry tobacco, and water removal in TCCB was 4376, 660.9, and 3715.1 kg, respectively. The coal consumption was 1.13 tons, and the power consumption of TCCB is 350 kWh. The total energy consumption cost was 1492 CNY and the labor cost was about 270 CNY, so the total cost was 1762 CNY, and the cost per unit mass of dry tobacco was 2.67 CNY/kg. Moreover, the mass of fresh tobacco, dry tobacco, and water removal in HPCB was 4221, 629.59, and 3591.41 kg, respectively. The power consumption of HPCB was about 1420 kWh. The total energy consumption cost was 781 CNY and the labor cost was 54 CNY, so the total cost was 835 CNY and the cost per unit mass of the dry tobacco was 1.34 CNY/kg. It is found that the unit cost of HPCB was only 50.19% that of TCCB.
The thermal efficiency of the curing barn can be calculated by substituting the data of
Table 9 into Equations (7) and (8). The obtained results in
Table 10 show that the exergy value of TCCB was 9,622,109 kJ, the coal energy consumption was 21,636,176.47 kJ, and the electricity consumption was 1,260,000 kJ. Accordingly, the total energy consumption was 22,896,176.47 kJ, and the thermal efficiency was 42.02%. Similarly, the energy value of HPCB was 8,598,387 kJ, the total energy consumption was 12,924,000 kJ, and the thermal efficiency was 66.53%. Accordingly, the thermal efficiency of HPCB was obviously higher than that of TCCB.
The performed analyses demonstrated that, compared with TCCB, with a thermal efficiency of 42.02%, the thermal efficiency of HPCB reached 66.53%. This remarkable difference originates from different structures of HPCB and TCCB, and energy saving in the HPCB mode. For TCCB, the heat loss includes the hot flue tail gas and the hot air that is discharged from the curing barn, which causes huge energy waste and may even lead to environmental issues. There is no hot air vent in HPCB and energy is recovered in the whole process. Compared with the TCCB, HPCB has higher energy efficiency and lower waste of energy.