improving exergetic performance parameters of a rotating-tray air dryer...
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Accepted Manuscript
Title: Improving exergetic performance parameters of a rotating-tray air dryer
via a simple heat exchanger
Author: Hamid Ghasemkhani, Alireza Keyhani, Mortaza Aghbashlo, Shahin
Rafiee, Arun S. Mujumdar
PII: S1359-4311(15)01173-4
DOI: http://dx.doi.org/doi:10.1016/j.applthermaleng.2015.10.114
Reference: ATE 7231
To appear in: Applied Thermal Engineering
Received date: 19-9-2015
Accepted date: 25-10-2015
Please cite this article as: Hamid Ghasemkhani, Alireza Keyhani, Mortaza Aghbashlo, Shahin
Rafiee, Arun S. Mujumdar, Improving exergetic performance parameters of a rotating-tray air
dryer via a simple heat exchanger, Applied Thermal Engineering (2015),
http://dx.doi.org/doi:10.1016/j.applthermaleng.2015.10.114.
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Improving exergetic performance parameters of a rotating-tray
air dryer via a simple heat exchanger
Running title: Exergy analysis of a rotating-tray air dryer fitted with a heat exchanger
Hamid Ghasemkhania, Alireza Keyhani
a, Mortaza Aghbashlo
a*,
Shahin Rafieea, Arun S. Mujumdar
b,c
aDepartment of Agricultural Machinery Engineering, Faculty of Agricultural Engineering and
Technology, University College of Agriculture and Natural Resources, University of Tehran,
P.O. Box, 4111 Tehran, Iran.
bDepartment of Bioresource Engineering, McGill University, Quebec, Canada.
cSchool of Agriculture and Food Sciences, University of Queensland, Brisbane , Queensland,
Australia
*Corresponding author: Mortaza Aghbashlo, Faculty of Agricultural Engineering and
Technology, University of Tehran, Karaj, Iran. P.O. Box: 31587-778712, Tel: +98-
2612801011, Fax: +98-261-2808138; Email address: [email protected]
Page 1 of 32
Highlights:
Exergy analysis of a rotating-tray air dryer fitted with a simple air-to-air heat
exchanger
Effect of drying air temperature and velocity on exergetic efficiency of the dryer
Promising improvement of exergetic performance parameters of the dryer using the
heat exchanger
Potential application of the proposed strategy for recovering waste energy in drying
technology
Abstract
In this study, exergy analysis was applied for a rotating-tray dryer equipped with a cross-flow
plate heat exchanger during drying of apple slices. Three drying air temperatures and tray
rotation speeds in the range of 50–80 °C and 0–12 rpm, respectively, were employed. Two
drying air velocities in the range of 1–2 m/s were adjusted for each drying temperature and
rotation speed with and without application of the heat exchanger. The experiments were
conducted to assess the effects of the experimental variables on the exergetic performance
parameters of the dryer. Also, the effect of drying conditions on the quality of dried apple
slices was assessed by determining rehydration ratio, apparent density, shrinkage, and surface
color. In general, the exergetic performance parameters of the dryer depended profoundly on
the drying air temperature and velocity. Interestingly, the exergetic efficiency of drying
process was significantly improved from a minimum value of 23.0% to a maximum value of
96.1% by using the heat exchanger. Furthermore, the incorporation of heat exchanger did not
negatively affect the quality of dried product. Therefore, the strategy presented herein could
be a promising approach for waste energy recovery in drying without any unfavorable change
in the quality of dried product.
Page 2 of 32
Keywords: Air-to-air heat exchanger; Drying; Exergy efficiency; Rotating-tray air dryer;
Quality
Notations
Nomenclatures
Redness
Yellowness
Specific heat (kJ/kg K)
Exergy flow rate (kJ/s)
Function of the independent variables
Latent heat of vaporization (kJ/kg)
Current intensity (Ampere)
Lightness
Moisture content (wet basis %)
Moisture ration (-)
Mass flow rate (kg/s)
Pressure (kPa)
Hear transfer rate (kJ/s)
Gas constant (kJ/kg K)
Temperature (°C or K)
Uncertainty in the independent variables
Uncertainty in the result
Voltage (Volt)
Mass fraction
Weight (g)
Electrical work rate (kJ/s)
Independent variables
Volume
Greek Letters
Exergy efficiency
Humidity ratio
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Relative air humidity
Efficiency
Exergetic efficiency improvement
Total colour change
Subscripts
Dead state
Air
Ash
Carbohydrate
Dried
Drying chamber
Equilibrium
Electrical
Evaporation
Electrical motor
Fat
Fibre
Fan
Fan-heater combination
Heater
Heat Exchanger
Initial volume
Numerator
Initial moisture content
Protein
Product
Rehydrated
Drying time
Vapour
Saturated vapour
water
Page 4 of 32
1. Introduction
Drying systems are the most ubiquitous components of manufacturing industry to dry
wet materials to a desired level of moisture content [1]. However, energy consumption of
industrial drying systems is generally very high, which makes the drying process as one of
the most energy-intensive unit operations of manufacturing industry. Strumiłło et al. [2] states
that industrial drying systems utilize up to 12% of the total national industrial energy used in
manufacturing processes. Nowadays, the majority of energy consumed in the drying industry
is met by fossil-based fuels. Unfortunately, the widespread utilization of such fuels has led to
the greenhouse gas emissions and has consequently brought about environmental concerns
such as global warming, climate changes, acid rain, and stratospheric ozone exhaustion [3–6].
Furthermore, the commonly used convective or conductive drying methods suffer
from various shortcomings such as long processing time and high energy costs. The energy
issue of drying systems could be overcome to a large extent if energy saving strategy is
applied to recover a portion of the energy in the waste outflow. This is ascribed to the fact
that the major component of energy losses in conventional drying systems occurs because of
the exhaust of moist air from dryers [7]. One of the most promising technologies in this
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regard is the application of heat-pump assisted drying system. However, the heat-pump
assisted drying systems suffer from several drawbacks such as higher capital and operational
costs, possibility of refrigerant leak and consequent environmental issues, and the need for
regular maintenance [8]. Another approach to reuse the waste energy from exhaust is to mix a
known portion of outlet air with fresh incoming air. However, the exhaust is humid which
lowers the driving force for moisture evaporation. Often the outflow air is contaminated and
hence must be cleaned. Thus, there is an increasing demand for innovative engineering
approaches and new drying techniques to save energy and minimize production costs in the
drying operation. Fortunately, it is well-documented that by installing a simple heat recovery
systems and then utilizing the recovered energy to preheat fresh inlet air [9]. It is possible to
recover a part of the heat in exhaust air. Nevertheless, advanced engineering tools can be
employed to assess the sustainability and efficiency of the heat recovery equipment in
industrial drying systems.
Traditionally, various energy conversion processes in industry are often assessed via
energy analysis. However, energy analysis based on the first law of thermodynamics does not
give information on the quality of different energy forms [10, 11]. Analysis based on the
second law of thermodynamics, namely exergy analysis, overcomes limitations of the first
law analysis [12]. As well, it has emerged as a tool for designing, analyzing, optimizing, and
retrofitting energy-intensive unit operations. Exergy analysis utilizes the concepts of
conservation of mass and energy together with the second law of thermodynamics. Several
studies have been published on the use of exergy analysis for various drying processes and
systems [13–23]. The outcomes of previously published researches indicate that drying
processes can be satisfactorily designed and optimized using exergy analysis. However, very
little information is available on exergy analysis of convective drying systems fitted with heat
recovery systems to reuse a portion of the thermal energy in the exhaust. It is worth pointing
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out that air-to-air cross-flow plate heat exchangers can be a low-cost way of reducing the
heating load in commercial air flow dryers. The objective of this study was to present exergy
analysis for a rotating-tray convective dryer for apple slices fitted with an air-to-air cross flow
plate heat exchanger. Some qualitative measurements such as rehydration ratio, apparent
density, shrinkage, and surface color of dried product were made to evaluate the effect of
drying variables. It is worth pointing out that a rotating-tray design avoids the problem of
non-uniformity in air flow distribution often encountered in conventional cabinet dryers.
2. Material and methods
2.1. Sample preparation
Apples (Golden Delicious variety) purchased from a local market were stored in a
refrigerator at 4 °C prior to the drying experiments. The apples were washed, peeled, and cut
into 3 mm slices, after 1 h of stabilization period at the ambient temperature of 25 °C. At the
start of each experiment, 750 g of apple slices were weighed and uniformly distributed on the
trays in thin layers. The initial moisture content of the samples was determined by drying a
known amount of the apple slices at 105 °C in a vacuum oven for 24 h. The average moisture
content of the samples was found to be 84±0.3% (wet basis %).
2.2. Drying equipment
A rotating-tray convective dryer fitted with an air-to-air cross-flow heat exchanger
was designed and fabricated to recover waste heat from outflow air and improve air
distribution within the drying chamber (Fig. 1). The dryer consisted of an adjustable
centrifugal blower, a heat exchanger, an electrical heater, a control panel, a drying chamber, a
closure, an air flow pipes, a shaft and two bearings, an inverter, and a DC electric motor.
Page 7 of 32
Figure 1
The cylindrically-shaped drying chamber with 86 cm diameter and 40 cm height was
constructed using 1.2 mm thick stainless steel sheet. The closure was created on the drying
chamber to place and remove the sample trays during drying experiments. The closure was
sealed with a gasket to prevent the heat loss from drying chamber. Four screened stainless
steel trays having dimension of 30×30 cm were located within the drying chamber with
an angle of 90° relative to each other. The shaft was rotated using a 24V DC electromotor and
the tray rotation speed was adjusted via voltage control. The drying air was heated using nine
U-type electrical elements having capacity of 4.5 kW (9×500 W). The air mass flow rate was
regulated by controlling the speed of the blower’s motor using a frequency modulation
device. The air-to-air cross-flow flat-plate heat exchanger was developed to recover waste
heat from the outflow air for preheating the inflow air (Fig. 2). The aluminum-made flat
plates having dimension of 40×40 cm and thickness of 0.3 mm were placed in parallel on a
square frame with 6 mm distance.
Figure 2
The whole body of the dryer was completely insulated with glass wool wrapped with
aluminum foil to avoid undesirable heat loss. The air velocity was measured using a hot film
sensor with accuracy of ±0.1 m/s placed in the connection pipe between the heater and drying
chamber. As well, the relative humidity of drying air was recorded at this point with accuracy
of ±2% RH. Furthermore, various SHT15 temperature sensors having an accuracy of ±0.4 ºC
were installed on different positions of the dryer to control drying process and record the
required data for exergy analysis. Weight loss of the samples were measured by means of two
aluminum single point load cells (Zemic, model L6D) located under the bearings with an
accuracy of ±0.1 g. The control of drying process was performed using an AVR
microcontroller. Labview software was used to communicate with the dryer, observe drying
Page 8 of 32
process, and save the required data for exergy analysis including temperatures, relative
humidity, air velocity, and samples mass with 30 s time intervals. The microcontroller was
interfaced to a PC. Generally, the temperature of drying air was controlled with an accuracy
of ±1°C during drying experiments using the developed controller and the temperature sensor
located at the middle of the heater and drying chamber connection pipe.
2.3. Experimental procedure
Experiments were performed at air temperatures of 50, 65 and 80°C, air velocities of
1 and 2 m/s, tray rotation speeds of 0, 6, and 12 rpm with and without application of the heat
exchanger. Each experiment was repeated twice. The dryer was run for one hour in order to
achieve desirable steady-state conditions before each drying experiment. Drying process was
continued until the samples reached to the moisture ratio of 0.1. It is worth mentioning that
the moisture ratio of wet products during drying process can be determined using the
following equation [24]:
(1)
However, due to the high moisture content of fresh fruits, the above-mentioned can be written
as follows [24]:
(2)
Moreover, the heat exchanger was detached from drying system during non-heat
exchanger trials using the connection pipe between the drying chamber and heat exchanger.
2.4. Experimental uncertainty
Uncertainty analysis is required to demonstrate the repeatability and accuracy of the
experimental data [25]. The experimental errors and uncertainties can occur due to the
Page 9 of 32
instrument selection, condition, calibration, environment, reading and recording, and test
planning [26]. Uncertainty analysis was carried out using the methodology developed by
Holman [27].
(3)
2.5. Theoretical considerations
Figure 3 shows a schematic illustration of the main features of the drying system
including heat exchanger, fan-heater combination, and drying chamber with input and output
terms. It was assumed that the dryer operated at a steady-state condition. Moreover, the
variations of kinetic and potential exergies were ignored through this research due to their
negligible contribution on the magnitude of input and output exergies.
Figure 3
The exergetic efficiencies of heat exchanger, fan-heater combination, and drying chamber
were computed using the following equations:
(4)
(5)
(6)
The exergy rate of the air at points 1–5 was determined using the following equation [28]:
(7)
Page 10 of 32
Based on mass conversation principle, the following equation can be written for drying air at
different points within the drying system:
(8)
The recorded relative humidity of air at point 3 was used for computing the humidity ratio of
air at points 1–3 as follows [29]:
(9)
(10)
The humidity ratio of the vented air from drying chamber was obtained using water balance
equation for drying process.
(11)
and,
(12)
The power rate consumed by fan-heater combination was computed using the following
equations:
(13)
(14)
(15)
The power rate utilized by electromotor for rotating the trays was determined as follows:
(16)
For solid and liquid materials, the specific exergy can be computed as follows:
(17)
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The specific heat of the material at the inlet and outlet points was obtained by Choi and
Okos’s equation [30]:
(18)
(19)
The equations describing the specific heat capacity of applied components based on
temperature are listed in Table 1.
Table 1
The exergy efficiency of the drying process with heat exchanger was obtained using the
following equations:
(20)
As well, the exergy efficiency of the drying process without heat exchanger was found as
follows:
(21)
The rate of exergy utilized for drying of the product was obtained using the following
equation:
(22)
It should be mentioned that the temperature of the product being dried was considered to be
equal to that of the wet bulb temperature and drying air temperature at the first and second
stages of drying, respectively. The possible error raised from this assumption was negligible
since the samples were very thin (3 mm).
The rate of heat transfer due to the evaporation was computed as
(23)
Page 12 of 32
where the mass flow rate of the evaporated water was:
(24)
The latent heat of vaporization at saturation state based on absolute temperature was
determined using the equations developed by Brooker et al. [32].
273.16≤ (K) ≤338.72
(25)
338.72≤ (K) ≤533.16
The reference state temperature and pressure were considered to be 25 °C and 101.315 kPa,
respectively.
Consequently, the following equation was applied to calculate the exergetic efficiency
improvement of drying process at a given drying condition due to the incorporation of heat
exchanger into drying system.
(26)
2.6. Qualitative measurements
In order to determine the rehydration ratio of dried product, a given amount of dried apple
slices were immersed for 50 min in water at room temperature. In this study, 100 mL of
distilled water was used per gram of dried product. Then, the slices were weighed again after
draining the water during 2 min. The rehydration ratio of dried apple slices was computed
using the following equation [33]:
(27)
Toluene displacement method was employed to measure the volume of samples before and
after drying. The following equation was applied to calculate the shrinkage of dried samples
at the end of drying process [34]:
Page 13 of 32
(28)
As well, the apparent density was determined by measuring the mass of the samples used for
shrinkage measurement before and after drying.
A computer vision system was employed to measure the surface color of the samples in CIE
L*a*b* space. The detailed information on the apparatus structure was completely explained
in Dowlati et al. [35]. A CCD color camera (Canon G9 digital color camera, Tokyo, Japan)
with remote capturing capability was used to provide images from the randomly selected
dried and wet samples [36]. The images were analyzed according to the methodology
comprehensively illustrated in Nadian et al. [37]. Finally, the total color difference of dried
apple slices with respect to the fresh state was determined as follows [38]:
(29)
All the above mentioned measurements were carried out in duplicate.
2.7. Statistical analysis
Statistical evaluation of the results was performed using a 3×3×2×2 split factorial
design (three temperatures, three tray rotation speeds, two air velocities, and two modes of
heat exchanger) with two replications for each treatment. Analysis of variance was carried
out to find the effects (p<0.05) of air temperature, air velocity, tray rotation speed, heat
exchanger on the exergetic performance and qualitative parameters. Multiple comparison
tests were performed using Duncan test at 95% confidence level. All the analyses were
carried out using SAS (Statistical Analysis System 9.2) computer program.
3. Results and discussions
Page 14 of 32
Table 2 lists the detailed uncertainty analysis for experimental measurements of
parameters and overall uncertainties of predicted values. The results demonstrated that all
uncertainties were below an acceptable error level (<5%).
Table 2
The effect of drying air temperature on the variation of moisture ratio of apple slices
at drying air velocity of 1 m/s and tray rotation speed of 6 rpm with application of the heat
exchanger is illustrated in Figure 4. It is obvious from this figure that the drying rate
increased as the drying air temperature increased. Similar trends were obtained for other
drying experiments.
Figure 4
Exergy analysis of the dryer and its main components were carried out by using data
obtained from the drying experiments. Figure 5 indicates the variations in the exergetic
performance parameters of the drying system as a function of drying time at drying air
temperature of 65 °C, air velocity of 1 m/s, and tray rotation speed of 6 rpm with and without
application of the heat exchanger. Similar trends were found for other drying experiments.
All data reported in this paper are the mean of the two replications. Obviously, the exergy
efficiency of fan-heater combination and drying process profoundly improved with
application of the heat exchanger. The effect of drying variables on the exergetic performance
parameters of the dryer and its components is comprehensively discussed in the following
sentences based on time-averaged values.
Figure 5
Figure 6 represents the effect of drying variables on the exergy efficiency of fan-
heater combination. Clearly, the heat exchanger significantly improved (p<0.05) the exergy
efficiency of fan-heater combination, indicating the effective recovering of the exergy of the
outlet air. In another word, incorporation of the heat exchanger to the drying system
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profoundly lowered the utilization of high quality electrical energy and subsequently
enhanced the exergy efficiency of fan-heater combination. It was envisaged that an increase
in the drying air temperature and air mass flow rate would reduce the exergy efficiency of
fan-heater combination due to the higher heat transfer and more corresponding exergy
destruction. However, the higher exergy efficiency was achieved (p<0.05) for the higher
drying air temperature and velocity. It could be paraphrased by the fact that the exergy
magnitude of the hot air is directly proportional to its temperature, according to the Eq. 7.
This means that the exergy of electrical work was effectively converted to the energy with
better quality or higher exergy content as the temperature and mass flow rate of drying air
increased (see also Eq. 5). However, the rotation speed of tray did not affect (p>0.05) the
exergy efficiency of fan-heater combination due to the trivial effect of the tray rotation speed
on the temperature of exhaust air from the drying chamber.
Figure 6
Figure 7 depicts the effect of drying variables on the exergy efficiency of drying
chamber. Evidently, the exergy efficiency of drying chamber increased (p<0.05) when the
heat exchanger was applied for waste heat recovery because of a profound reduction in the
rate of exergy destruction. It could be related to a better distribution of drying air through
drying chamber, since the heat exchanger acts as a barrier against the outlet air. This in turn
led to a uniform distribution of drying air and lowered the rate of exergy destruction within
drying chamber and elevated the exergy efficiency. Increasing drying air temperature
significantly decreased (p<0.05) the exergetic efficiency of drying chamber. Higher exergy
efficiency of drying chamber means that the outflow exergy is still comparable to the inflow
exergy, indicating the potential of outlet air for accomplishing the work. Moreover,
increasing the rotation speed of tray decreased (p<0.05) the exergetic efficiency of the drying
chamber. It could be attributed to the more useful application of inflow exergy for moisture
Page 16 of 32
evaporation from apple slices, leading to a reduction in the outflow exergy. However,
increasing the mass flow rate of drying medium did not affect (p>0.05) the exergy efficiency
of the drying chamber.
Figure 7
Table 3 tabulates the exergetic efficiency values of various drying system (drying
chamber) previously cited in the literature. The exergy efficiency of the rotating tray air dryer
was well above the average values reported in the published literature. Thus, this
thermodynamically-efficient drying chamber can be a promising alternative to the other
available drying system for sustainable, cost-effective, and eco-friendly drying process.
Table 3
The effect of drying variables on the exergy efficiency of heat exchanger is exhibited in Fig.
8. Increasing drying air velocity led to an increase (p<0.05) in the exergy efficiency of heat
exchanger because of the effective recovery of the exergy of the vented air from drying
chamber based on the Eq. 4. Interestingly, the exergetic efficiency of heat exchanger was not
significantly influenced (p>0.05) by the drying air temperature. Moreover, the rotation speed
of tray did not affect (p>0.05) the exergy efficiency of heat exchanger due to an insignificant
effect of tray rotation on the temperature of exhaust air from the drying chamber.
Figure 8
The effect of drying variables on the exergy efficiency of the drying process is
demonstrated in Fig. 9. The exergy efficiency of drying process varied from a minimum
value of 0.43% to a maximum value of 2.19% under the drying conditions investigated,
indicating that convective drying process is an exergetically low efficiency process. The
considerably small amount of moisture evaporation during convective drying process is
responsible for this low exergetic efficiency. It is interesting to note that the inclusion of heat
exchanger profoundly improved (p<0.05) the exergy efficiency of drying process. This can be
Page 17 of 32
explained by the fact that some part of outflow exergy from drying chamber was effectively
recovered via the heat exchanger. Increasing drying air temperature increased (p<0.05) the
process exergy efficiency because of an increase in the magnitude of moisture evaporation,
according to the Eqs. 22–24. However, the exergy efficiency of the drying process did not
significantly affect (p>0.05) by increasing the rotation speed of tray. This could be ascribed
to the fact the tray rotation marginally increased the mass transfer from the product being
dried. On the other hand, the process exergy efficiency decreased (p<0.05) by increasing the
air mass flow rate since the process exergy efficiency is inversely proportional to the exergy
rate of inlet drying air.
Figure 9
Figure 10 illustrates the effect of different drying conditions on the exergetic
efficiency improvement of drying process due to the incorporation of heat exchanger into
drying system according to the Eq. 26. This improvement varied from a minimum value of
23.09% to a maximum value of 96.13% at the drying conditions studied due to the recycling
of a considerable amount of outflow exergy. It could be concluded that the incorporation of a
simple heat exchanger to the hot air drying system remarkably improved the second law
efficiency of the process without high expenditure. Therefore, further surveys should be
carried out to increase the exergy recovery from outflow air via efficient heat exchangers and
embody low-cost heat exchangers to industrial drying systems for improving the
performance.
Figure 10
Table 4 summarizes the measured quality parameters of apple slices dried under
different operating conditions. In general, the quality parameters of dried apple slices were
significantly influenced (p<0.05) by the air temperature. However, the other parameters did
not significantly affect (p>0.05) the quality parameters of the dried apple slices. A similar
Page 18 of 32
finding has been previously reported by Santos-Sánchez et al. [33] for the effect of air
velocity and tray rotation speed on the L* and b* values of tomato slices. Increasing drying
air temperature decreased both shrinkage and density of the dried apple slices because of the
rapid moisture removal at higher drying air temperatures. This in turn does not give adequate
time to the viscoelastic matrix of the product to shrink back completely into the voids
previously filled the water. Similarly, Lewicki and Jakubczyk [45] and Bai et al. [46] found
that increasing drying air temperature decreased shrinkage and density of the dried apple.
Moreover, the rehydration capacity of apple slices dried at higher temperatures was
somewhat better than of those dried at lower temperatures. It could be attributed to the fact
that higher drying temperatures lead to dried slices with more porous structure, thereby
facilitating the rehydration process [47]. A similar finding was reported by Sacilik and Elicin
[48] for hot air drying of apple slices.
It is interesting to note that the normalized lightness of the dried apple slices at higher
air temperatures is slightly higher than for those dried at lower air temperatures. It might be
due to the degradation of chlorophyll and carotenoids in apple at higher drying air
temperature, as discussed by Nadian et al. [37]. Additionally, the normalized redness of the
dried apple slices increased remarkably with increasing air temperature due to promotion of
the Maillard reaction at higher temperatures. Sacilik and Elicin [48] and Vega-Gálvez et al.
[49] found similar results for the effect of drying air temperature on the normalized redness
of the dried slices. The normalized yellowness of the dried slices increased slightly with
increasing drying air temperature. This is ascribed to the fact that an increase in air
temperature increased the rate of non-enzymatic browning reaction and accordingly increased
the yellowness. Finally, the total color difference of the dried apple slices increased
considerably with temperature of the air. An almost similar finding was obtained for the
effect of drying air temperature on the total color difference of the dried organic apple slices
Page 19 of 32
[48]. It can be concluded that the higher drying air temperature unfavorably changed the
color of the dried product. Therefore, special caution is required in selecting an appropriate
operating condition for apple drying.
Table 4
4. Conclusions
The exergetic performance of a rotating-tray convective dryer fitted with an air-to-air
cross-flow plate heat exchanger was assessed via exergy analysis at different drying air
temperatures, air velocities, and tray rotation speeds. Furthermore, the effect of the operating
variables on the quality of the dried product was evaluated through quantitative
measurements of the quality e.g. rehydration ratio, apparent density, shrinkage, and surface
color. The incorporation of a heat exchanger into the system improved the exergetic
parameters of the overall system, by raising the exergy efficiency of the process.
Nevertheless, the quality of the dried product was not unfavorably altered by the use of a heat
exchanger. Generally, the exergetic efficiency of the drying process was very poor and lower
than the exergy efficiency of the whole drying system due to the lower drying rate.
Nevertheless, the process exergy efficiency was found to be a very useful tool for
thermodynamic analysis of a drying system compared to the system exergy efficiency. The
results of this study can be applied in design and optimization of industrial-scale heat
exchangers for recovering the waste heat from exhaust air and improving the performance of
convective dryers.
Acknowledgment
The authors would like to extend their appreciations for financial support provided by the
University of Tehran.
Page 20 of 32
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Page 26 of 32
Fig. 1. Schematic of the dryer. 1, frame; 2, drying chamber; 3, closure; 4, control panel; 5,
heater; 6, heat exchanger; 7, blower; 8 and 9, air flow pipes; 10, inflow pipe; 11, outflow
pipe; 12, DC electric motor; 13, load cell; 14, sample trays; 15, bearing; 16, tray rotation
mechanism.
Fig. 2. Heat exchanger: (A) a schematic view indicating the flow directions (B) actual picture
(C) interior view (D) triangular plate between smooth plates.
Fig. 3. Schematic of the drying system with input and output terms. 1. fresh air; 2. heated air;
3. hot air; 4. moist air; 5. exhaust air; 6. fresh product; 7. dried product.
Fig. 4. The variations of moisture ratio of apple slices at drying air velocity of 1 m/s and tray
rotation speed of 6 rpm with application of the heat exchanger.
Fig. 5. The variations of exergetic performance parameters of the drying system at drying air
temperature of 65 °C, drying air velocity of 1 m/s, and tray rotation speed of 6 rpm with and
without heat exchanger as a function of drying time.
Fig. 6. Effect of drying variables on the exergy efficiency of fan-heater combination.
Fig. 7. Effect of drying variables on the exergy efficiency of drying chamber.
Fig. 8. Effect of drying variables on the exergy efficiency of heat exchanger.
Fig. 9. Effect of drying variables on the exergy efficiency of drying process.
Fig. 10. Effect of drying variables on the exergetic efficiency improvement of drying process
due to the incorporation of heat exchanger into drying system.
Page 27 of 32
Table 1. The specific heat capacity of the apple components [31]. Component Equation, T(°C)
Carbohydrate
Protein
Ash
Fat
Fiber
Water
Page 28 of 32
Table 2. Uncertainties of the experimental measurements and overall uncertainties for
predicted values.
Parameter Unit Comment Nominal
value
Experimental measurements
Uncertainty in the temperature measurement °C ±0.905 –
Uncertainty in the weight measurement g ±0.141 –
Uncertainty in the air velocity measurement m/s ±0.173 –
Uncertainty in the air relative humidity
measurement
% ±2.828 –
Uncertainty in the time measurement s ±0.023 –
Uncertainty in the measurement of moisture
content
g ±0.021 –
Uncertainty in the tray rotation speed rpm ±0.054 –
Predicted measurements
Total uncertainty for moisture ratio Dimensionless ±1.21% 0.53
Total uncertainty for exergy efficiency of fan-
heater
Dimensionless ±2.64% 7.05
Total uncertainty for exergy efficiency of drying
chamber
Dimensionless ±3.24% 86.61
Total uncertainty for exergy efficiency of heat
exchanger
Dimensionless ±3.31% 21.00
Total uncertainty for exergy efficiency of overall
drying system
Dimensionless ±4.27% 11.09
Total uncertainty for exergy efficiency of overall
drying process
Dimensionless ±4.11% 1.78
Total uncertainty for shrinkage % ±2.12% 74.60
Total uncertainty for rehydration ratio Dimensionless ±2.74% 1.78
Total uncertainty for density measurement g/cm3 ±2.87% 0.616
Page 29 of 32
Table 3. Comparison of exergetic efficiency values of various drying system (drying
chamber). Type of dryer Product Exergy efficiency of drying
chamber (%)
References
Solar dryer Shelled/unshelled pistachios
10.86–100 Midilli and Kucuk
[39]
Cyclone type dryer Pumpkin slices 30.81–100 Akpinar et al. [40]
Industrial-scale hot air dryer Pasta 65.4 Ozgener [41]
Ground-source heat pump
dryer
Mint 15.44–100 Colak et al. [42]
Semi-industrial continuous
band dryer
Carrot slices 55.27–93.29 Aghbashlo et al.
[14]
Tray dryer Broccoli florets 59.7–81.92 Icier et al. [26]
Fluidized bed dryer Carrot cubes 10.30–70.70 Nazghelichi et al.
[43]
Fluidized bed dryer Paddy 41.30–58.14 Sarker et al. [44]
Rotating-tray air dryer Apple slices 72.63–92.26 Current study
Page 30 of 32
Table 4. Quality parameters of apple slices dried at different drying conditions. 1
Drying condition Quality parameter
Temperatur
e (°C)
Air
velocity (m/s)
Tray
rotatio
n speed
(rpm)
Heat
exchanger mode
Shrinkage
(%)
Density
(g/cm3)
Rehydratio
n ratio
50
1
0 Yes 73.74±1.4
6 0.638±0.0
4 1.67±0.08
1.08±0.01
0.70±0.09
1.22±0.01
10.20±0.56
0 No 73.21±0.4
1
0.638±0.0
2 1.76±0.13
1.08±0.0
2
0.67±0.0
3
1.24±0.0
3
10.62±0.3
2
6 Yes 74.94±0.1
3 0.637±0.0
1 1.72±0.10
1.09±0.03
0.69±0.02
1.23±0.02
11.12±0.59
6 No 74.20±0.5
2
0.639±0.0
3 1.71±0.04
1.11±0.0
2
0.75±0.0
7
1.21±0.0
8
10.73±0.3
1
12 Yes 75.50±1.0
6 0.646±0.0
5 1.82±0.06
1.08±0.03
0.63±0.08
1.23±0.08
10.78±2.65
12 No 74.26±1.9
8
0.640±0.0
1 1.70±0.13
1.08±0.0
1
0.76±0.0
4
1.21±0.0
2 9.40±0.43
2
0 Yes 73.83±0.5
2 0.631±0.0
1 1.71±0.07
1.09±0.02
0.73±0.09
1.20±0.07
10.19±1.27
0 No 74.70±0.6
9
0.627±0.0
5 1.86±0.13
1.10±0.0
3
0.71±0.1
3
1.21±0.0
6
10.44±2.2
3
6 Yes 74.45±1.3
2 0.643±0.0
2 1.73±0.02
1.09±0.00
0.67±0.02
1.22±0.04
10.84±1.13
6 No 73.41±1.4
7
0.643±0.0
1 1.70±0.08
1.08±0.0
1
0.76±0.0
3
1.21±0.0
5 9.65±0.94
12 Yes 74.46±0.5
3 0.627±0.0
3 1.83±0.07
1.08±0.02
0.70±0.11
1.25±0.09
10.53±2.59
12 No 74.88±1.0
3
0.618±0.0
2 1.77±0.31
1.08±0.0
2
0.76±0.0
5
1.24±0.0
2
10.73±1.0
7
65
1
0 Yes 73.76±1.6
0 0.609±0.0
2 1.76±0.10
1.08±0.05
0.71±0.06
1.24±0.02
11.44±0.97
0 No 72.60±1.9
8
0.628±0.0
4 1.75±0.06
1.08±0.0
3
0.87±0.0
5
1.27±0.0
6
10.94±0.7
2
6 Yes 74.60±1.7
7 0.616±0.0
2 1.78±0.05
1.11±0.05
0.75±0.19
1.27±0.01
12.40±2.00
6 No 73.13±1.8
1
0.619±0.0
1 1.82±0.07
1.10±0.0
2
0.83±0.0
1
1.25±0.0
1
11.00±0.9
2
12 Yes 72.73±0.3
1 0.634±0.0
6 1.81±0.05
1.09±0.03
0.77±0.11
1.24±0.05
10.72±1.76
12 No 73.26±1.4
0
0.615±0.0
2 1.82±0.09
1.08±0.0
3
0.76±0.0
4
1.28±0.0
3
11.24±1.2
7
2
0 Yes 73.45±1.3
3 0.613±0.0
1 1.82±0.09
1.09±0.02
0.65±0.04
1.20±0.05
10.61±0.71
0 No 73.30±0.9
6
0.614±0.0
2 1.91±0.20
1.07±0.0
1
0.63±0.0
4
1.26±0.0
8
11.34±0.3
5
6 Yes 73.79±0.6
7 0.622±0.0
1 1.81±0.02
1.09±0.02
0.73±0.17
1.28±0.01
12.10±0.45
6 No 72.30±0.9
4
0.619±0.0
1 1.81±0.06
1.10±0.0
2
0.82±0.0
2
1.24±0.0
2
11.16±1.2
0
12 Yes 73.72±0.8
8
0.617±0.0
4 1.81±0.02
1.11±0.0
3
0.86±0.0
5
1.25±0.0
7
11.57±1.2
3
12 No 73.80±0.5
2
0.604±0.0
4 1.86±0.02
1.07±0.0
1
0.80±0.0
9
1.27±0.0
1
10.97±0.2
7
80 1 0 Yes 72.08±1.7
9
0.595±0.0
1
1.91±0.08 1.11±0.0
0
0.83±0.0
2
1.23±0.0
5
11.76±1.1
7
0 No 71.88±0.7
5
0.603±0.0
2
1.82±0.02 1.11±0.0
2
0.85±0.1
0
1.25±0.0
3
11.57±0.4
9
6 Yes 72.64±1.7
8
0.582±0.0
2
1.84±0.03 1.11±0.0
5
0.97±0.0
3
1.26±0.0
6
11.80±2.6
5
6 No 71.22±2.3
6
0.587±0.0
2
1.95±0.07 1.11±0.0
2
0.87±0.0
2
1.27±0.0
2
11.97±0.9
8
12 Yes 72.79±0.3
5
0.599±0.0
2
1.93±0.06 1.09±0.0
3
0.73±0.1
7
1.26±0.1
0
12.18±1.3
5
12 No 71.91±1.2
0
0.599±0.0
3
1.99±0.29 1.09±0.0
0
0.79±0.0
6
1.28±0.1
0
11.69±2.4
3 2 0 Yes 71.82±1.4
6
0.589±0.0
1
1.99±0.05 1.12±0.0
1
0.80±0.0
2
1.25±0.0
1
12.66±0.5
0
0 No 71.98±1.38
0.591±0.03
2.01±0.11 1.11±0.02
0.77±0.09
1.25±0.08
12.07±0.86
6 Yes 72.42±1.3 0.582±0.0 1.84±0.02 1.11±0.0 0.85±0.1 1.24±0.0 11.43±1.1
Page 31 of 32
7 1 2 4 2 3
6 No 72.70±0.2
2
0.588±0.0
3
1.96±0.02 1.09±0.0
1
0.85±0.0
4
1.28±0.0
4
11.39±1.1
3
12 Yes 71.01±1.2
6
0.600±0.0
1
1.92±0.08 1.10±0.0
0
0.82±0.2
4
1.24±0.0
2
11.63±1.0
5
12 No 71.95±0.71
0.584±0.02
1.94±0.28 1.08±0.02
0.84±0.10
1.28±0.05
11.01±0.47
2
3
4
Page 32 of 32