Year 27 / Nº 43 / 2025 /
DOI: https://doi.org/10.36995/j.recyt.2025.43.004
Effect of
operating parameters on air drying potential in a prototype dryer with heat
pump
Efecto de los
parámetros de funcionamiento sobre el potencial de secado del aire en un
prototipo de secador con bomba de calor
Efeito dos
parâmetros operacionais no potencial de secagem do ar em um protótipo de
secador com bomba de calor
Rodrigo, Aparecido Jordan1; Fabricio,
Correia de Oliveira2, *; Anamari, Viegas de Araújo
Motomiya1; João, Pedro Jordan3; Rodrigo, Couto Santos1;
Valdiney, Cambuy Siqueira1; Elton, Aparecido Siqueira Martins1
1- Universidade Federal da Grande Dourados. Brasil.
2- Universidade Tecnológica Federal do Paraná (UTFPR). Brasil.
3- Universidade Estadual do Oeste do Paraná (UNIOESTE). Brasil.
* E-mail: fcoliveira@utfpr.edu.br
Received: 20/08/2024; Accepted: 20/03/2025
Abstract
In low
temperature drying, relative humidity —which expresses the relationship between air vapour pressure and
saturation pressure at the same temperature— is the
parameter with the greatest influence on the drying rate, as mass exchange is
directly related to the vapour pressure difference between the product and the
air. This study aimed to evaluate the effects of operating parameters on
relative humidity values in a prototype system based on heat pump technology.
This system was developed for low-temperature drying, cooling, and the control
of air psychrometric conditions for post-harvest storage and preservation of
agricultural products. The data was evaluated using a completely randomized
design with five replications. Two drying temperatures were employed: 25 °C
and 30 ºC. As for the air blower, rotations of 20, 25, 30, 35, 45 and 60 Hz
were used. Regarding the refrigeration compressor, rotations of 30, 35, 40, 45
and 50 Hz were used. The results showed that both compressor and blower rotations
had highly significant effects. On the other hand, relative humidity values
tended to decrease as compressor rotation increased, whereas they increased
with higher blower rotation.
Keywords: low
temperature, relative humidity, cooling, evaporation temperature.
Resumen
En el secado a
baja temperatura, la humedad relativa (que expresa la relación entre la presión
de vapor del aire y la presión de saturación a la misma temperatura) es el
parámetro que más influye en la velocidad del proceso, ya que el intercambio de
masa está directamente relacionado con la diferencia de presión de vapor entre
el producto y el aire. El objetivo de este trabajo fue evaluar los efectos de
los parámetros de operación sobre los valores de humedad relativa en un
prototipo de sistema basado en la tecnología de bomba de calor. Este sistema se
desarrolló para el secado a baja temperatura, enfriamiento y control de las
condiciones psicrométricas del aire para el almacenamiento poscosecha y
conservación de productos agrícolas. Los datos se evaluaron mediante un diseño
completamente aleatorizado con cinco repeticiones. Se utilizaron dos
temperaturas de secado: 25 °C y 30 °C. Se utilizaron rotaciones del soplador de
aire de 20, 25, 30, 35, 45 y 60 Hz. En cuanto al compresor de refrigeración se
utilizaron rotaciones de 30, 35, 40, 45 y 50 Hz. Los resultados mostraron que
tanto la rotación del compresor como la del soplador tenían efectos muy significativos.
Mientras que los valores de humedad relativa tendían a disminuir al aumentar la
rotación del compresor, ocurría lo contrario al aumentar la rotación del soplador.
Palabras claves: baja
temperatura, humedad relativa, enfriamiento, temperatura de evaporación.
Resumo
Na secagem a
baixa temperatura, a umidade relativa, que expressa a relação entre a pressão
de vapor do ar e a pressão de saturação na mesma temperatura, é o parâmetro de
maior influência na velocidade do processo, uma vez que a troca de massa está
diretamente relacionada à diferença de pressão de vapor entre o produto e o ar.
O objetivo deste trabalho foi avaliar os efeitos dos parâmetros operacionais
sobre os valores de umidade relativa em um protótipo de sistema baseado na
tecnologia de bomba de calor, desenvolvido para secagem a baixa temperatura,
resfriamento e controle das condições psicrométricas do ar para armazenamento e
conservação pós-colheita de produtos agrícolas. Os dados foram avaliados
utilizando um delineamento inteiramente casualizado com cinco repetições. Foram
utilizadas duas temperaturas de secagem: 25 ºC e 30 ºC. Para o insuflador de
ar, foram utilizadas as rotações de 20, 25, 30, 35, 45 e 60 Hz. Para o
compressor de refrigeração, foram utilizadas rotações de 30, 35, 40, 45 e 50
Hz. Os resultados mostraram que tanto a rotação do compressor como a do insuflador
tiveram efeitos altamente significativos. Enquanto os valores de umidade
relativa tendem a diminuir com o aumento da rotação do compressor, o oposto
acontece com o aumento da rotação do insuflador.
Palavras-chave: baixa
temperatura, umidade relativa, resfriamento, temperatura de evaporação.
Introduction
The search for more efficient technologies to reduce
losses, improve product quality and, equally importantly, address the
environmental issue of mitigating carbon emissions is becoming increasingly
important nowadays [1].
In the agricultural sector, a major consumer of energy
with a large share of non-renewable sources [2], this cannot be any different.
While in rural areas agricultural machinery operates practically on its own,
other sectors have seen little progress, for example, the drying of
agricultural products in Brazil, which is still predominantly performed using
direct-burning dryers, with firewood being the most commonly used fuel [3].
In addition to product contamination with toxic and cancerous
residues from direct burning [4, 5], this process also leads to the emission of
greenhouse gases [6], environmental issues related to the sourcing of the wood
used [7], socio-economic concerns related to land-use changes for energy crops [8].
Moreover, the combustion process in an environment laden with particulates also
poses the risk of accidents related to explosions [9]. In Brazil, in recent years,
there has been an increase in the number of accidents in post-harvest
processing and storage units for agricultural products due to the expansion of
grain production [10].
Another characteristic of wood-fired dryers is their
operation with high drying temperatures, which not only increase the occurrence
of physical damage [11], but also cause nutritional and sensory losses in the
products [12, 13]. These factors result in storage problems and reduced market value
due to loss of quality [14, 15].
In this context, heat pumps are an interesting option
which make rational use of the energy, generating a thermal effect that is many
times greater, as they do not convert electrical energy directly into heat, but
rather transfer heat from a low-temperature source to a high-temperature source
[13].
Brazil has an exceptionally clean electricity matrix,
with over 87% currently sourced from renewable sources [16]. This share is
continuously growing, particularly with the expansion of photovoltaic energy
following the approval of laws regulating distributed generation [17]. Such
regulations create a compensation system where electricity surplus can momentarily
be injected into the grid and retrieved for later use, creating an even more favourable
environment for the adoption of cleaner technologies, such as heat pumps.
Heat pumps stand out as very efficient equipment [18].
In addition to being environmentally friendly, as they do not use fossil fuels
but high-quality energy, they are attractive for studies in drying since they
offer the advantage of multiple operating controls over temperature and
relative humidity. The ecological advantage of using heat pumps in drying
processes lies not only in their high efficiency, but also in the precise
control of temperature and relative humidity [19, 20].
While, on the one hand, reducing the temperature is
beneficial for maintaining product quality [21, 22, 23, 24, 25, 26], it can also
greatly extend drying time, potentially having a negative impact due to the increased
energy consumption with both economic and environmental implications [27].
Therefore, in order to obtain optimal results, low-temperature
drying must be performed within a brief period [28]. Unlike high temperature
drying, where temperature is the determining factor in time, in low temperature
drying, relative humidity is the determining factor. Therefore, the lower its
value, the greater the drying potential and the shorter the as well as the less
energy consumption [13].
The air supply with low relative humidity values, even
at low temperatures, is another advantageous aspect of using heat pumps in
drying [29]. In contrast with conventional dryers, where the air only undergoes
sensible heating, heat pumps first cool the air together with a process of
dehumidification and reduce its vapour pressure, then reheat it [30, 31]. In
this way, low humidity values can even be obtained at low temperatures.
Another advantage of heat pumps is the possibility of full
automation of all operations [32]. Being aware of how the operating parameters
affect the air outlet conditions, especially the relative humidity, is
important when implementing more efficient control logic. In this context, this
work aimed to analyse the operation of a prototype dryer and psychrometric controller
for agricultural product storage conditions based on heat pump technology, focusing
on the behaviour of relative humidity as a function of the main operating parameters
(compressor speed and air blower speed).
The equipment used in this study, named SIARCOMPAG (Autonomous System
for Cooling and Psychrometric Control of Grain Storage Conditions), is a
prototype developed to modulate the psychrometric conditions of the air used in
drying and post-harvest conservation of agricultural products. Its operation
focuses on modulating the temperature and relative humidity of the air [33].
Constructed with
pump technology, as depicted in Figures 1 and 2, the SIARCOMPAG system features
an air treatment duct. Within this section, the evaporator, secondary
condenser, and fan (referred to as the air blower) are located. The
refrigeration compressor and primary condenser are positioned below this duct, together
with their corresponding blower. Furthermore, the system is equipped with an
isothermal humidifier, which has the function of incrementing both the absolute
and relative humidity of the air under particular conditions. Notably, increasing
the water content of the processed product is necessary only in specific use
cases. Nonetheless, this action is not required when the system operates in its
drying mode.
Figure 1: schematic
diagram of the SIARCOMPAG.
Figure 2: perspective
view of the SIARCOMPAG.
The SIARCOMPAG
system offers different operating modes: fully automatic, semi-automatic and
fully manual. These
modes can be selected and adapted based on programmable logic controller (PLC)
settings through the human-machine interface (HMI). When operating in fully automatic
mode, users can enter specific parameters into the HMI, including product type,
desired temperature, and desired humidity of the final product. When operating in this mode, the
PLC autonomously manages operating conditions by referencing equilibrium
humidity equations, using PID (Proportional Integral Derivative) logic.
As
for manual mode, operators are required to manually configure operational
settings for each component: compressor rotation, blower rotation, auxiliary
condenser activation, and main condenser fan rotation. Alternately, semi-automatic
mode allows for a combination of manual and automatic adjustments. For
instance, some conditions are automated, like outlet temperature, while others
can be manually set.
When
the system operates in drying mode, the outlet air temperature is maintained by
adjusting the heat flow between the primary and secondary condensers. This is
done by adjusting the rotation of the main condenser fan. Since both condensers
are positioned in a sequence within the cooling circuit (an arrangement managed
by the solenoid valves as depicted in Figure 1), varying the rotation of the
main condenser fan adjusts the heat rejected by the secondary condenser,
consequently altering the drying air temperature.
For
our experiments, the system was set to semi-automatic mode: the outlet air
temperature was controlled automatically, but the compressor and blower rotations
were adjusted manually. In this configuration, the PLC automatically regulates the
rotation of the main condenser fan to regulate the heat flow to the secondary
condenser.
The
drying temperatures chosen for the tests were inspired by a previous study
[15], which found a correlation between drying temperature, relative humidity
and drying time, revealing a direct proportionality between drying time and
relative humidity. Therefore, in our study, we opted for temperatures of 25 ºC
and 30 ºC, at which the relative humidity values significantly reduced the
drying time.
The
refrigeration compressor (Elgin model CR18K6TF5) was set to operate at rotations
or frequencies of 30, 35, 40, 45, and 50 Hz. The air blower (Ventbras model
SV100) was adjusted to rotations of 20, 25, 30, 35, 45, and 60 Hz. Maximums
were limited to not exceeding the device’s designated factory specifications.
The compressor's minimum rotation was guided by manufacturer guidelines, particularly
to ensure proper lubrication. The lower limit of the blower was set to prevent
evaporator blockage due to potential freezing, which could be caused by an
insufficient airflow rate.
Temperature
and relative humidity values at the air inlet (Ent), between the evaporator and
the secondary condenser (Meio), and at the air blower outlet (Saida) were
supplied using the RHT sensors integrated in the SIARCOMPAG system (Figure 3),
These sensors provide data on air conditions to the PLC, enabling the automatic
adjustment of operating parameters.. This information can be accessed via a
specific HMI (human-machine interface) screen (Figure 4).
Figure 3: sensor
locations: (A) RHT sensors at the entry, (B) sensor between the evaporator and
secondary condenser, and (C) sensor at the air blower outlet.
Figure 4: human-machine
interface screen with temperature and relative humidity data: input (Ent),
midpoint (Meio) and output (Saida).
Figure 5: electronic
expansion valve Carel model E2V (A) and its corresponding controller
Carel model EVD (B).
In tests where compressor rotation was adjusted,
blower rotation was consistently set to its nominal value. Similarly, when
blower rotation was modified, compressor rotation remained fixed at its nominal
rate. For both cases, nominal rotations were equated to the maximum value
employed during testing, which stood at 60 Hz.
For each setting condition, once parameters were
adjusted, a stabilisation period for the equipment was observed, which ranged between
10 and 15 minutes. Upon stabilisation, measurements were taken and, to do so,
each condition was subjected to five repetitions conducted at 2-minute
intervals, totalling a 10-minute duration for each test.
Graphic presentations were employed to streamline the
analysis, focusing on output conditions (relative humidity and temperature
values) as a function of operating parameters (compressor and blower
rotations).
Statistical evaluations were conducted using R
statistical software, version 4.0.2 [34], based on a completely randomized
design with five replications per assay. Data underwent tests to measure normality
(Shapiro-Wilk test) and variance homoscedasticity (Bartlett test). After
verifying these facts for each test, a variance analysis (ANOVA) was executed,
gauging significance with the F test (p≤0.01).
When F values were deemed significant, data were
subjected to regression analysis, evaluating both linear and polynomial models
(p≤0.01). Best-fitting equations were selected based on the significance of
model coefficients together with the optimal determination coefficients (R²).
As already mentioned [35], these statistical estimators effectively outline the
structure of spatial dependence, which is influenced by the variables under analysis,
as well as the shape and volume of the samples.
Results and discussion
Table 1 provides a summary of the variance analysis
for output relative humidity values, considering both blower and compressor
rotation. F-values
are highly significant (p ≤ 0.01) for both factors, indicating statistically
distinct group means. The coefficient of variation (CV) is notably low,
under 1%, reflecting strong experimental control and consistent data
repeatability. As highlighted by Singh et al. [36], in agricultural
research, it is preferable for CV values to be below 10%.
Table
1: F-values of the variance analysis (ANOVA) for average output relative humidity
as a function of the blower and compressor rotations.
|
Blower rotation |
Compressor rotation |
|||||
|
|
Drying temp. 25 °C |
Drying temp. 30 °C |
|
Drying temp. 25 °C |
Drying temp. 30 °C |
|
|
SV |
DF |
Mean squares |
DF |
Mean squares |
||
|
Rotation |
5 |
114.925** |
189.056** |
4 |
50.431** |
14.5536** |
|
Error |
24 |
0.053 |
0.014 |
20 |
0.049 |
0.0038 |
|
CV (%) |
|
1.02 |
0.57 |
|
0.61 |
0.23 |
**: significant at p ≤ 0.01 for the F-test; SV: sources
of variation; DF: degrees of freedom; CV: coefficient of variation.
Figure 6 shows the air conditions at the psychrometric
controller inlet, representing the average temperature and relative humidity
values during the tests.
Figure 7 illustrates the behaviour of the average
evaporation temperature in relation to blower rotation (A) and compressor
rotation (B) at drying temperatures of 25 ºC and 30 ºC. The
evaporation temperature was notably influenced by these parameters. In particular,
the temperature showed a direct proportionality regarding blower rotation and
an inverse proportionality regarding compressor rotation.
Figure 6: air conditions
at the psychrometric controller inlet during tests: (A) average temperature
with varied blower rotation, (B) average temperature with varied compressor
rotation, (C) average relative humidity with varied blower rotation, and (D)
average relative humidity with varied compressor speed.
The trend observed in the evaporation temperature in
relation to the blower rotation can be attributed to the heat exchange in the
evaporator and the capacity of the refrigeration system. Given that the
compressor rotation remained constant, any variation in the air flow produced a
change in the temperature of the evaporator outlet. This, in turn, prompted the
expansion valve to avoid superheating by regulating the flow of refrigerant
fluid within the evaporator. Consequently, this process altered the evaporation
temperature to optimise the heat exchange process. As highlighted in a study by
Wang et al. [37], optimizing heat exchange is crucial for heat pump
systems, emphasizing the high efficiency of the dryer.
|
A. |
B. |
|
|
|
Figure 7: average
evaporation temperature as a function of blower speed variation with a compressor
speed fixed of 60 Hz (A) and compressor speed variation (B) with a blower speed
fixed of 60 Hz.
The differences in behaviour (linear with 30 °C
and second order with 25 °C) for the evaporation temperature observed in
Figure 7 may be related to the difference in condensation pressure (Figure 8A),
which affected the opening rate of the expansion valve (Figure 8B). The
increase in drying temperature implies an increase in condensation temperature
and, consequently, in condensation pressure, due to the reduction in the main
condenser fan rotation to increase the heat flow that is rejected into the air
in the secondary condenser.
|
A. |
B. |
|
|
|
Figure 8: changes
in condensing pressure (A) and expansion valve opening rate (B) as a function
of varying blower rotation with compressor rotation fixed at 60 Hz.
As blower rotation increased, there was a noticeable
decline in condensation pressure. This is attributed to a greater volume of air
moving through the secondary condenser, enhancing the heat exchange process.
Patel [38] highlighted the importance of this aspect, noting that refrigeration
system performance relies significantly on efficient heat exchange, with its energy
efficiency intricately linked to the pressure within the condenser.
The increase in drying temperature is achieved by
reducing heat exchange in the primary condenser through a decrease in its fan
speed, which in turn increases heat rejection in the secondary condenser. As
the secondary condenser has a smaller capacity than the primary one, this
explains the observed differences in pressure, the opening rate of the
expansion valve and the slight variation in the slope inclination between the
adjusted lines.
When examining the effects of changes in compressor
rotation (Figure 7A), the trends in evaporation temperature appeared to be strongly
associated with fluctuations in the refrigerant flow, which is controlled by
the compressor. This relationship then affected the degree of opening of the
expansion valve (Figure 9A). Inlet conditions during these tests were both
smaller and more stable (Figures 6C and 6D). Additionally, changes in
compressor rotation affected condensation pressure (Figure 9B). However, this
influence was less pronounced on the opening rate of the expansion valve when
the drying temperature was kept constant and only the blower rotation was
varied.
|
A. |
B. |
|
|
|
Figure 9: changes
in condensing pressure (A) and expansion valve opening rate (B) as a function
of varying compressor rotation with blower rotation fixed at 60 Hz.
A significant observation across both scenarios
(altering blower rotation and adjusting compressor rotation) was that the
opening rate of the expansion valve was higher when setting the drying
temperature to 25 ºC. This trend can be attributed to the lower condensation pressure
since the flow of refrigerant fluid is inherently dependent on the pressure
difference between the condensation and evaporation stages. A reduced pressure
difference requires the expansion valve to open further in order to maintain balanced
flow.
Salazar-Hincapié et al. [18] assessed a heat pump for
dehydrating aromatic herbs. They noticed that a decrease in condensation
pressure was accompanied by a drop in evaporation temperature. Moreover, they further
observed that an increase in condensation pressure in one of their tests resulted
in greater compression work, related to the gap between the pressures during
condensation and evaporation.
The variance analysis of the output relative humidity
values, based on both blower and compressor rotations, showed significant
results for the two assessed temperatures (Table 1). The low coefficient of
variation in the results indicates an elevated level of experimental control
and consistency.
Following the same trend as the evaporation
temperature, the relative humidity of the air leaving the psychrometric
controller tended to increase with higher blower rotation (Figure 10A) and to
decrease as the compressor rotation increased (Figure 10B). This is because
relative humidity values are directly related to evaporation temperature values
(Figures 11 and 12) due to the cooling process with dehumidification, which takes
place in the evaporator. As a consequence, the greater the cooling below the
dew point, the lower the relative humidity of the air in the reheating process
(outlet). Once again, the influence of compressor rotation on the pressure
differential across the expansion valve becomes evident, resulting in models
with different behaviour for the drying temperatures of 25 ºC and 30 ºC (Figure
10B).
|
A. |
B. |
|
|
|
Figure 10: average
output relative humidity as a function of varying blower rotation (A) at a
fixed compressor rotation of 60 Hz, and (B) varying compressor rotation at a
fixed blower rotation of 60 Hz.
Minimum observed values for relative humidity were
16.6% at a drying temperature of 25 ºC and 14.8% at 30 ºC. These values were
observed at the lowest blower rotation while the compressor maintained a
consistent rotation (Figure 10A). In a study by Aktas et al. [29], they
achieved around 18% of relative humidity at a 36 ºC drying temperature using a
recirculating heat pump dryer. Salehi [39] recorded 16% of relative humidity at
40 ºC-45 ºC during their fruit drying experiment with a heat pump.
Meanwhile, Escalona et al. [40] recommended a minimum relative humidity
of 12% for mild drying conditions at a drying temperature of 30 °C using a heat
pump.
At 25 ºC, maximum values for relative humidity (40.9%)
were obtained at the slowest compressor speed using a steady blower speed
(Figure 10B). Meanwhile, at 30 ºC, the peak relative humidity (31.7%) was recorded
at the highest blower speed with the compressor speed remaining unchanged
(Figure 10A). Zlatanović et al. [41] reported 30% of relative humidity
at 35 ºC while drying several items, including potatoes, apples, and bananas, with
a heat pump. Similarly, Getahun et al. [42] reported approximately 35% of
humidity at 50 ºC during their drying experiments with chilli peppers.
|
A. |
B. |
|
|
|
Figure 11: average
relative humidity based on the evaporation temperature obtained during tests
with a varying blower rotation and a fixed compressor rotation of 60 Hz: (A)
drying at 25 ºC and (B) drying at 30 ºC.
|
A. |
B. |
|
|
|
Figure 12: average
relative humidity based on the evaporation temperature obtained during tests
with a varying compressor rotation and a fixed blower rotation of 60 Hz: (A)
drying at 25 ºC and (B) drying at 30 ºC.
The gap in relative humidity values between the two
drying temperatures was more pronounced when adjusting the compressor rotation,
despite only minimal variations in evaporation temperatures (Figure 7B). This
suggests that the dehumidification effect associated with elevated evaporation
temperatures (Figure 11) is less pronounced, implying that the primary factor
in reducing relative humidity is the heating phase.
In contrast, when adjusting the blower rotation, the
gap in relative humidity values between drying temperatures was narrower. Lower
evaporation temperatures during the drying process of 25 ºC (Figure 10)
amplified dehumidification, compensating for the decrease in relative humidity attributed
to heating.
According to Zhang et al. [31], an increase in
evaporation temperature led to a reduced drying rate, primarily due to
diminished air dehumidification efficiency at higher evaporation temperatures.
Mołczan & Cyklis [43] explored the effect of evaporation temperature on
specific drying rates, discovering that a 10 ºC decrease in evaporation
temperature enhanced water removal by more than four times, shifting from 0.44
kg kW h-1 to 1.90 kg kW h-1.
Furthermore, Salazar-Hincapié et al. [18] detected
challenges with ice formation in the evaporator at particularly low evaporation
temperatures, which triggered heat transfer difficulties by altering the
convective coefficients. Indeed, evaporator freezing is a significant issue; not
only does the insulating nature of ice reduce thermal exchange, but it can also
obstruct airflow.
In this study, ice buildup in the evaporator became
apparent when the blower rotation dropped to 35 Hz. This accumulation hampered
the heat transfer and widened the gap between the evaporator outlet and the
evaporation temperature (Figure 13).
|
A. |
B. |
|
|
|
Figure 13: air temperature
at the evaporator outlet as a function of evaporation temperature for different
blower rotations and a fixed compressor rotation of 60 Hz: A) drying at 25 ºC
and B) drying at 30 ºC.
Ice formation on the surface of the evaporator, which
is more pronounced at 25 ºC (Figure 12), is a factor that may also have
contributed to the differences in the slope of the adjusted lines for the
linear functions, as well as to variations observed in the adjusted models
(linear or quadratic).
For drying at 25 ºC (Figure 13A), assuming no ice
formation and that the outlet air temperature follows the trend set by the
evaporation temperature up to a blower rotation of 45 Hz, the predicted
evaporator exit air temperature would be around -7 ºC with a rotation of 20 Hz.
Nevertheless, due to the disruptive effect of ice on heat exchange, the actual
exit temperature at this rotation was recorded as -1.7 ºC.
By modelling a scenario with an evaporator outlet air
temperature of -7 ºC and then reheating it to 25 ºC, a simulation using the
Grapsi v.8.1.1 software predicted an exit relative humidity of 11%. This value
is considerably lower than the 16.6% observed under real conditions where ice
formation occurred.
To enhance air cooling and mitigate issues related to
evaporator freezing, one potential solution could be the implementation of a multi-stage
cooling system. This approach could involve using a heat pump system equipped
with multiple evaporators, as suggested by Du & Long [44]. By employing
this strategy, the majority of the moisture in the air would be removed in the
first stage, with the initial evaporator operating at a positive temperature. Subsequently,
the temperature could be further reduced in the second stage with the secondary
evaporator operating at sub-zero temperatures. This sequential process would
not only improve the cooling efficiency but also significantly reduce the probability
of ice formation.
1. Varying compressor and blower rotations, even with
a constant drying temperature, resulted in a variation of almost 40% in
relative humidity values of the outlet air.
2. Increasing the compressor rotation from 30 Hz to 50
Hz produced a reduction of nearly 20% in the relative humidity values, whereas increasing
the blower speed from 20 Hz to 60 Hz led to an approximate 50% increase. Therefore,
lower blower rotations and higher compressor rotations help to reduce relative
humidity values and, consequently, increase the drying potential of the air.
3. Varying the blower rotation implied a greater
variation in relative humidity values, due to an existing correlation between
relative humidity and evaporation temperature —lower evaporation temperatures
resulted in lower relative humidity values. Consequently, the reduction in air
flow produced a decrease in evaporation temperature values.
4. Decreasing temperature and air flow rate, combined
with an increasing compressor rotation (that is to say, a greater mass flow of
refrigerant), produced more efficient heat exchange, ensuring that the air is
cooled below the dew point. The greater the degree of cooling below the dew
point, the more water is removed, resulting in lower absolute humidity and
lower vapour pressure. As a result, after the reheating stage, the relative
humidity of the air is reduced, thereby enhancing its drying potential at the
same temperature.
5. On the other hand, reducing the blower rotation below
35 Hz caused the evaporation temperature to drop below 0 °C, leading to ice
formation and accumulation in the evaporator. This consequence interfered with
the downward behaviour of the relative humidity values of the drying air.
Acknowledgments
We express our deep gratitude to the Programa de
Pós-Graduação em Engenharia Agrícola at the Universidade Federal da
Grande Dourados (PGEA/UFGD). We extend our sincere gratitude to the
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES—Finance
Code: PDPG 155 and 16/2022) for their financial support. We
would also like to thank the team at STTA for their valuable feedback and
review of the definitive version of this article.
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