Analysis of Grain Drying Using the Pressure the Flow of Air Heat Forced Convection Method

This research plan adopts the concept of forced convection to regulate the flow of heated air. The heat generated by combustion in the furnace is transferred to the air pipe inside the furnace. Using a blower, air is forcibly drawn through the heating pipe, where it is heated, and then directed to the heating chamber. The heating process involves conduction, convection, and radiation mechanisms as the air flows through the pipe. As the air heats up, its temperature increases, causing a decrease in density and an increase in velocity. The blower then pushes the hot air (or gas) into the heating chamber, where it is used to heat or dry the rice grains placed on the chamber. The rice grains are placed on an iron net located above the heating grid. The moisture in the rice grains is absorbed by the hot air or gas until it reaches a point where the density of the gas increases, causing a phase change. The blower installed at the top of the drying chamber discharges the moisture-laden gas into the atmosphere, while fresh hot air or gas is continuously forced into the heating chamber by the blower, maintaining circulation inside the drying chamber. This decrease in air density will cause air to flow from the drying chamber to the chimney.

Conduction is the process of heat transfer from a region of higher temperature to a region of lower temperature, occurring in one medium (solid, liquid, or gas) or across different media in direct contact [12]. In the conduction process, energy transfer occurs through direct interaction between molecules without significant movement of the molecules themselves. According to the kinetic theory, the temperature of a substance is directly related to the average kinetic energy of its molecules. Conduction is the only mechanism by which heat can be transferred in an opaque solid [13]. According to the second law of thermodynamics, heat naturally moves from a region of higher temperature to a region of lower temperature. Consequently, heat flow is considered positive when the temperature gradient is negative [14].

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Figure 1. Installation of grain dryer using convection method

Convection is an energy transfer process that combines heat transfer, energy retention, and fluid agitation. It plays an important role as a mechanism for transferring energy across the surface of a solid material and the surrounding liquid or gas [15].

Energy transfer by convection from a surface with a higher temperature than the surrounding fluid involves several stages. Initially, heat is transferred by conduction from the surface to adjacent fluid particles. This energy transfer increases the temperature and energy of these fluid particles [16].

Convection heat transfer is categorized into two types: free convection and forced convection [17]. Free convection occurs when the fluid movement results only from the density difference caused by the temperature gradient. The greater the density difference, the faster the air flow, accelerating the drying process, as in Figure 1. On the other hand, forced convection occurs when the fluid movement is driven by an external device, such as a pump or fan [18]. The energy balance in the system can be calculated by considering several factors: heat from the flue gas entering the heating chamber (Q_gbi), heat absorbed in the grain drying chamber (Q_abs), heat escaping through the fan to the surrounding air (Q_gbo), heat lost through the walls of the heating chamber (Q_d), and total additional heat losses (Q_losses) [19].

1). Total heat from flue gas to the heating chamber

$Q_{\mathrm{gbt}}=m_g \times \mathrm{C}_{\mathrm{pg}} \times \mathrm{T}_{\mathrm{gbt}}(\mathrm{kJ})$                (1)

2). The total heat from the flue gas is absorbed

$Q_{\mathrm{abs}}=m_a \times \mathrm{C}_{\mathrm{pa}} \times\left(T_s-T_a\right)+m_a \times \mathrm{h}_{\mathrm{fg}}(\mathrm{kJ})$              (2)

3). The total heat of the flue gas goes to the heat exhaust fan to the environment

$Q_{\mathrm{gbo}}=m_g \times \mathrm{C} \mathrm{p}_g \times \mathrm{T}_{\mathrm{gbo}}(\mathrm{kJ})$                (3)

4). Other heat losses (losses)

$Q_{\mathrm{gbb}}=Q_{\mathrm{abs}}+Q_{\mathrm{gbo}}+Q_{\text {losses }}(\mathrm{kJ})$               (4)

$\mathrm{Q}_{\text {losses }}=\mathrm{Q}_{\mathrm{gbt}}+\left(\mathrm{Q}_{\mathrm{abs}}+\mathrm{Q}_{\mathrm{gbo}}\right)(\mathrm{kJ})$              (5)

5). Exhaust gas mass flow rate

$\dot{m}_g=\rho \times U_m \times A_i(\mathrm{~kg} / \mathrm{s})$               (6)

6). Reynolds number    

$R_n=\frac{G \times d_i}{\mu}$               (7)

7). Nusselt numbers

$N_u=\frac{h_o \times d_o}{k}=5+0.016 \times R_e^a \times P r^b$                  (8)

8). Overall heat transfer coefficient

$h_i=\frac{N_u \times k}{d_0} W / m^2{}.{ }^{\circ} \mathrm{C}$               (9)

9). Air section (hot fluid) outside the pipe

$T_f=\frac{\left(T_{u i}+T_{u o}\right)}{2}{ }^{\circ} \mathrm{C}$                  (10)

10). Overall Heat Transfer Coefficient (U)

$U=\frac{1}{\frac{d_o}{d_i h_o}+\frac{d_o}{2 \times k} \times \ln \frac{d_o}{d_i}+\frac{1}{h_i}} \mathrm{~W} / \mathrm{m}^2 .{ }^{\circ} \mathrm{C}$               (11)

11). Logarithmic Mean Temperature Difference (LMTD)

$\Delta T_{L M T D}=\frac{\Delta T_1-\Delta T_2}{\ln \left(\frac{\Delta T_1}{\Delta T_2}\right)}{ }^{\circ} \mathrm{C}$                (12)

12). Total heat transfer area

$A_{t o t}=\pi \times d_o \times L \times n\left(\mathrm{~m}^2\right)$              (13)

13). Total heat transfer in the heat exchanger

$Q_{t o t}=A_{\text {tot }} \times U \times L M T D(\mathrm{~kJ} / \mathrm{h})$              (14)

This drying system uses the principle of forced convection (Forced Convection Drying), where hot air is forced to flow through the pile of rice with the help of a fan. Hot air is produced from a biomass (rice husk) combustion furnace and is channeled into the drying chamber through an isolated air duct. The main principles of this design include:

1). Heating the air using a biomass-fueled furnace.

2). Even distribution of hot air with a fan and air duct system.

3). Control of temperature and humidity in the drying chamber.

4). High energy efficiency, with the use of rice husks as the main fuel.

Main components of the dryer installation and design parameters and technical calculations:

a. Furnace

i. Fuel: Rice husk or other biomass

ii. Combustion efficiency: 80%

iii. Fuel calorific value: 3,800 kCal/kg

iv. Burning capacity: 3 kg of husk/ton of grain

The combustion furnace uses rice husk or biomass fuel, temperature 450℃ and is equipped with air heating pipes.

Calculation of the heat energy produced:

$Q_{\text {losses }}=Q_{\mathrm{gbt}}+\left(Q_{\mathrm{abs}}+Q_{\mathrm{gbo}}\right)(\mathrm{kJ}) Q=m \times C V$            (15)

where,

Q=total heat energy (Kcal)

m=mass of rice husk burned (kg)

CV=calorific value of rice husk (kCal/kg)

So we get:

Q=3 kg×3800 Kcal/kg=11400 Kcal

b. Hot Air Duct

i. Material: Heat-resistant steel plate

ii. Duct length: 4 meters

iii. Duct diameter: 0.3 meters

iv. Air velocity in the duct: 3 m/s

Centrifugal fan (blower) with 2 HP power to produce optimal air pressure, Air velocity 2-5 m/s, depending on the thickness of the grain pile, air distribution is equipped with ducting so that the air flow is evenly distributed throughout the grain layer.

Calculation of air flow rate:

$\mathrm{Q}_{\mathrm{air}}=\operatorname{Axv}\left(\mathrm{m}^3 / \mathrm{s}\right)$              (16)

where,

Q_air=air flow rate (m³/s)

A=duct cross-sectional area (πr2)

υ=air velocity in the duct (m/s)

$Q_{\mathrm{air}}=\left(\pi \times(0,15)^2 \times 3\right)=0.21 \mathrm{~m}^3 / \mathrm{s}$

c. Centrifugal Fan

i. Fan power (P): 1.5 kW

ii. Air flow velocity: 2-3.5 m/s

iii. Hot air discharge (Q): 0.21-0.3 m³/s

iv. Fan efficiency (η): 85%

Fan power calculation:

$P=\frac{Q \times \Delta P}{\eta}\text{(Watt)}$                (17)

$P=\frac{0.25 \times 100}{0.85}=29.4 \,\mathrm{Watt}$

d. Drying Room

i. Capacity (Q): 12 tons of grain

ii. Dimensions of drying room: 6 m×3 m×2.5 m

iii. Initial grain moisture: 22%

iv. Final grain moisture: 12%

v. Inlet air temperature: 55-65℃

vi. Exit air temperature: 40-50℃

$\mathrm{Q}_{\text {drying }}=\mathrm{m} \times \mathrm{Cp} \times \Delta \mathrm{T}($ Kcal $)$               (18)

where,

Q_drying=total energy for drying (Kcal)

m=mass of grain dried (kg)

Cp=specific heat of grain (Kcal/kg℃)

ΔT=change in air temperature (℃)

Then obtained

Q_drying=12000×1.5×(65-30)=630000 Kcal

Energy Efficiency Analysis of drying is calculated as follows:

$\eta=\frac{Q_{\text {drying }}}{Q_{\text {input }}}$              (19)

$\eta=\frac{630000}{760000} \times 100 \%=82.9 \%$

Drying efficiency reaches 82.9%, indicating optimal energy utilization. This system only requires 3kg of husk per ton of grain, making it fuel efficient. With optimal temperature and air velocity settings, moisture content can be reduced by up to 10% in 10 hours.

Estimated Drying Time Based on testing Drying from 30% moisture content to 12%-14% takes 6-8 hours at a temperature of 50°C and an air velocity of 3m/s. Drying is 70% faster than conventional methods, which usually take 2-3 days.

The advantages of the PFAHFC Dryer Design are that it is faster than natural drying, which takes 40 hours, energy efficient, with an efficiency of 82.9%, low operational costs, because it only uses husk as fuel and produces better quality grain, with a more stable moisture content.

The dryer design is designed to dry 12 tons of wet grain with high efficiency using a forced convection system. The following are the main parameters used as listed in Table 1.

Table 1. Dryer design parameters

Table 4. Comparison with traditional drying

The following is the initial experimental data obtained from testing the rice drying system using Pressure The Flow of Air Heat Forced Convection (PFAHFC) technology.

Based on the initial data in Table 3, sample data was then taken for traditional drying as a comparison material as shown in Table 4.

The following is a trend of changes in water content over time with variations in hot air temperature based on Table 3.

This graph shows that the higher the hot air temperature (℃), the shorter the time required for the drying process, at a temperature of 55℃, the drying time is around 8 hours, while at a temperature of 70℃, the drying time drops to 6 hours, This indicates that increasing the temperature accelerates the evaporation of moisture in the grain, making the drying process more efficient.

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Figure 2. Relationship between hot air temperature and drying time

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Figure 3. Relationship between water content and and drying time

Figure 2 and Figure 3 above show the effect of hot air temperature on the drying time of the grain. Additional data shows that the higher the temperature of the hot air used, the faster the water content in the grain decreases to reach the standard of 12%.

The fan speed in the grain drying system plays an important role in regulating the distribution of heat and humidity in the drying room. The higher the fan speed, the faster the hot air circulates, which can speed up the drying process. However, improper settings can cause energy inefficiency and decrease the quality of the grain [20].

The trial was conducted with three variations of fan speed:

a. Low (V1=2 m/s)

b. Medium (V2=4 m/s)

c. High (V3=6 m/s)

At each fan speed, the air temperature in the drying room and air humidity were measured every 30 minutes during the drying process. The target final moisture content of the grain is 12-14%.

Analysis and Interpretation based on Table 5 as follows:

1). Effect on temperature

a. The higher the fan speed, the higher the average temperature in the drying room.

b. At high speed (V3=6 m/s), the average temperature reaches 55℃, which accelerates the process of water evaporation from the grain.

2). Effect on humidity

a. The final humidity decreases faster with increasing fan speed.

b. At low speed (V1), air humidity remains high because the circulation of hot air is less than optimal.

3). Drying time efficiency

a. Higher fan speeds significantly reduce drying time.

b. The fastest drying occurs at V3 (6 m/s) with a time of 4.5 hours, compared to 8 hours at V1.

Higher fan speeds increase drying efficiency by accelerating the flow of hot air and reducing moisture more quickly. However, temperatures that are too high (>55℃) have the potential to damage the quality of the grain, so optimizing the fan speed setting needs to be done to achieve a balance between efficiency and quality. A fan speed of 4-5 m/s is recommended to maintain a balance between drying efficiency and grain quality. Further testing can be done by adding parameters such as energy consumption and post-drying grain quality. In the drying efficiency analysis, we will consider the energy input-output ratio to assess the extent to which the energy input into the system is used effectively in drying the grain. This ratio can be calculated using the thermal efficiency of the system, which measures how much energy is used to dry the grain compared to the energy required to increase the air temperature [21].

Table 5. Fan speed test results

The efficiency of drying paddy is greatly influenced by the ratio of energy used to the water content that can be removed in a certain time. This analysis aims to calculate the energy efficiency of a forced convection-based drying system by considering the energy input-output ratio and other metrics such as thermal efficiency, fuel consumption, and drying time [22]. The main parameters used in this calculation are:

a. Input Energy (Q_in)

$\mathrm{Q}_{\text {in }}=\mathrm{m}_{\text {fuel }} \times \mathrm{CV}$             (20)

where,

m_fuel=mass of fuel (kg)

CV=fuel calorific value (MJ/kg)

b. Energy Absorbed by Air (Q_air)

$\mathrm{Q}_{\mathrm{air}}=\mathrm{m}_{\mathrm{air}} \times C p a i r \times \Delta \mathrm{T}$              (21)

where,

m_air=mass of heated air (kg)

Cp_air=specific heat of air (1.005 kJ/kg·K)

ΔT=change in air temperature (K)

c. Energy Used for Evaporation of Water from grain (Q_evap)

$Q_{\text {evap }}=m_{\text {evap }} \times h_{\text {evap }}(\mathrm{kJ})$                (22)

where,

m_evap=mass of water evaporated (kg)

h_evap=latent heat of vaporization of water (2260 kJ/kg)

a. Input Energy (Q_in)

Q_in=25×14.5=362.5MJ/hour

b. Energy Absorbed by Air (Q_air)

Q_air=150×1.005×(55−30)

Q_air=150×1.005×25=3.77 MJ/hour

c. Mass of Evaporated Water

m_evap=m_grain×(initial water content−final water content)

m_evap=12,000×(0.22−0.12)=1,200kgm

d. Energy Used for Evaporation (Q_evap)

Q_evap=1,200×2260=2,712,000 kJ=2712 MJ

f. Thermal Efficiency of Dryer

$\begin{gathered}\eta=\frac{Q_{\text {evap }}}{Q_{\text {in }} \times \text { time }} \\ \eta=\frac{2712}{362,5 \times 6} \times 100 \% \\ \eta=79.1 \%\end{gathered}$

The experimental data carried out are shown in Table 6 below.

Table 6. Experimental data and calculations

Thermal efficiency of 79.1% indicates that most of the energy is used to evaporate water from the grain, while a small portion is lost in the form of residual heat and conduction to the environment.

Table 7. Comparison parameters of dryers

Efficiency can be improved by optimizing the flow of hot air to be more even, improving insulation in the system to reduce heat loss and adjusting the fan speed to suit drying needs [23].

Natural drying (using sunlight) and forced drying (using Pressure The Flow of Air Heat Forced Convection technology), based on the main parameters that affect efficiency and quality of results.

Based on Table 7, a Quantitative Analysis can be made:

1). Drying time

a. Natural drying takes 2-3 days depending on weather conditions.

b. Forced drying only takes 6-8 hours per batch, increasing efficiency by 70-80%.

2). Energy efficiency

a. Forced drying has a thermal efficiency of around 80%, meaning that most of the heat energy is used effectively to dry the grain.

b. Natural drying has no energy control, but does not require fuel input.

3). Crop loss

a. Natural drying causes 10-15% of grain to be damaged, due to rain, dust contamination, or mold.

b. Forced drying only experiences <3% loss, increasing yields and farmer profits.

4.  Production capacity

a. Natural drying can dry around 4-5 tons of grain/day.

b. Forced drying can dry up to 12 tons/day, increasing almost 3 times.

Comparatively, it can be concluded that forced drying is superior in terms of time, capacity, and quality of results, natural drying is cheaper because it does not require additional energy, but is very dependent on the weather and has a higher risk of losing results and in terms of economic efficiency, although forced drying has operational costs, increased capacity and higher selling prices of grain make it more profitable in the long term [24].

A more specific quantitative analysis of the comparison of natural drying and forced drying using the Pressure The Flow of Air Heat Forced Convection (PFAHFC) method [25]:

1). Drying time efficiency

Natural drying is highly dependent on weather and sunlight conditions, while forced drying can speed up the drying process with controlled hot air flow.

a. Natural drying: 30-48 hours (depending on the weather).

b. Forced drying: 8-12 hours (controlled).

c. Increased time efficiency: >70%.

2). Grain Moisture Content Before and After Drying

a. Natural drying:

Initial moisture content: 24-26%.

Final moisture content: 14-15% (varies, less uniform).

b. Forced drying:

Initial moisture content: 24-26%.

Final moisture content: 12-14% (more uniform).

3). Post-harvest losses

a. Natural drying: 10-15% yield loss due to mold, fermentation, and contamination.

b. Forced drying: <3% yield loss due to more even and controlled drying.

4). Energy consumption

Natural drying: Does not require fuel or electricity, but requires a large area and is highly dependent on weather.

Table 8. Quantitative comparison

Forced drying: Uses biomass fuel (rice husk) or gas, with a consumption of around 2.5-3.5 kg of husk/100 kg of grain

With the quantitative data in Table 8, we can conclude that forced drying is much more efficient and produces higher quality grain compared to natural drying methods. This study focuses on the development and implementation of forced convection-based grain drying technology (Pressure The Flow of Air Heat Forced Convection - PFAHFC) to improve drying efficiency and effectiveness compared to traditional methods. The scope of this study includes:

1). Energy efficiency analysis

a. Evaluation of biomass fuel consumption (rice husk) in producing hot air.

b. Comparison of energy efficiency between natural and forced drying methods.

2). Drying system performance

a. Measurement of air temperature and humidity during the drying process.

b. Evaluation of drying speed and its impact on grain quality.

3). Drying design optimization

a. Calculation of key parameters such as inlet air temperature, fan speed, and air distribution.

b. Identification of factors affecting drying effectiveness.

4). Field scale application

a. Field trials to assess tool performance under real conditions.

b. Analysis of the impact of technology on farmer productivity.

To understand the effectiveness of the PFAHFC system, this study compares natural drying (without tools) and forced drying (using tools) based on several key parameters.

Look at Table 9, from the table, it can be seen that forced drying is superior to traditional methods because it is faster, more efficient, and produces better quality grain. The application of PFAHFC technology in the real world includes several important aspects:

1). Small and medium farmer production scale

a. Farmers who were previously only able to dry 4 tons per day using traditional methods can now increase their capacity to 12 tons per day.

b. With higher energy efficiency, farmers can save fuel and reduce operational costs.

c. This technology does not depend on the weather, so production is more stable and is not hampered by the rainy season.

2). Sustainability and energy efficiency

a. This system uses rice husks as fuel, which is an abundant agricultural waste.

b. With an efficiency of 82.9%, the heat energy produced is optimally utilized for the drying process.

c. Utilizing renewable energy sources can help reduce dependence on fossil fuels.

3). Economic impact on farmers

a. With shorter drying times, farmers can increase production frequency and accelerate planting cycles.

b. The quality of the rice increases, so that the selling price is higher on the market.

c. Farmers' income increases, because crop losses due to bad weather can be minimized.

4). Implementation on an industrial scale

a. This system can be used in agricultural cooperatives, rice mills, or rice storage warehouses.

b. With faster and more uniform drying, rice stocks can be managed better.

c. Potential for further development with automatic sensors to control temperature and humidity, so that the drying process is more precise.

To improve dryer performance, this study identified and optimized:

a. Design of the combustion furnace and hot air duct.

b. Adjustment of fan capacity and speed to suit drying needs.

c. Use of thermal insulation to reduce heat loss and improve energy efficiency.

Table 9. Comparison of natural and forced drying

This study not only evaluated the technical aspects but also their impact on production and farmer welfare. Some aspects analyzed include:

a. Increasing drying capacity from 4 tons/day to 12 tons/day.

b. The effect of this technology on the quality and selling price of grain.

c. Savings in operational costs compared to traditional methods.

As a final step, this study tested the application of the PFAHFC system on a field scale to ensure that this technology:

a. Can be easily operated by farmers without the need for high technical skills.

b. Has lower operational costs compared to conventional methods.

c. Is able to increase production efficiency and food security in areas that often experience weather constraints.

Challenges and Solutions in Implementation are as follows:

a. Challenges:

1). The initial investment cost is quite high for small farmers.

2). Technical skills are needed to operate

3). Maintenance of equipment to keep it functioning optimally.

b. Solutions:

a. Assistance from the government to help farmers access this technology.

b. Training and technical assistance for farmers in the use and maintenance of equipment.

Forced convection-based rice drying technology (PFAHFC) has proven to be more efficient and effective than traditional methods. With an efficiency of 82.9%, drying time can be shortened to 8-10 hours, with more uniform results. Implementation of this technology in the field can increase farmer productivity, crop quality, and farmer income. For wider implementation, technical and financial support is needed from the government and the private sector so that this technology can be used massively.

This research was supported by the Wonerejo Village Farmers Group, Kurik District, Merauke, South Papua (Grant No.: 145.1/258/WNR/III/2024) and the Department of Mechanical Engineering, Musamus University, Merauke (Grant No.: 849/UN52.1/AM/2024).