DOI: 10.3303/CET24114142 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 30 April 2024; Revised: 28 October 2024; Accepted: 6 December 2024 
Please cite this article as: Speiser F.P., Nagy B., Pan T., 2024, Energetic Analysis of Mixed-Flow Grain Dryers: A Case Study in Hungary, 
Chemical Engineering Transactions, 114, 847-852  DOI:10.3303/CET24114142 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 114, 2024 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Petar S. Varbanov, Min Zeng, Yee Van Fan, Xuechao Wang 

Copyright © 2024, AIDIC Servizi S.r.l. 

ISBN 979-12-81206-12-0; ISSN 2283-9216 

Energetic Analysis of Mixed-Flow Grain Dryers: a Case Study 

in Hungary 

Ferenc Péter Speiser*,a, Bence Nagya,Ting Panb 

aUniversity of Győr, Department of Power Electronics and E-Drives, 9026 Győr, Egyetem tér 1., Hungary 
bSustainable Process Integration Laboratory – SPIL, NETME Centre, Faculty of Mechanical Engineering, Brno University of 

 Technology - VUT Brno, Czech Republic 

 speiser.ferenc.peter@ga.sze.hu 

Convective grain drying powered by natural gas is a highly energy-intensive process with a substantial impact 

on the secure storage of harvested grain. By improving energy efficiency and reducing natural gas consumption, 

it is possible to decrease the operation's ecological footprint by lowering CO₂ emissions. However, previous 

studies often analyse the drying process as a whole, giving less attention to individual processes. For instance, 

uneven drying can lead to issues during storage, such as microbial growth and dust accumulation. This paper 

presents an energetic analysis of mixed-flow grain dryers based on a case study in Hungary for the long term. 

It examines the fundamental physical characteristics of each dryer and identifies key modifications to ensure 

proper operation. The paper also introduces a precision drying method that allows fine-tuning of process 

parameters (e.g., airflow, grain flow) to optimise grain moisture content to the desired level based on large-scale 

continuous temperature measurements. These measurements can also validate previous modifications, 

enabling ongoing monitoring of optimal operating conditions via heatmaps. 

1. Introduction 

Grain drying is a widely used preservation process, with around 1,500 drying plants in Hungary. The annual gas 

consumption depends on the volume and moisture content of the crop produced, but on average, about 

50,000 m3 of natural gas per plant can be estimated in a season, which means an annual consumption of about 

55 million m3 of natural gas (i.e. about 593,000 MWh). For volume comparison, the monthly consumption figures 

in Hungary reported by the Hungarian Energy and Public Regulatory Utility Authority are shown in Figure 1. 

below from 2022. October report of Hungarian Energy and Public Utility Regulatory Authority. Most of the energy 

usage related to crop drying typically occurs in the September-December period. By reducing the amount of 

natural gas used, it is also possible to reduce the ecological footprint of the operation by reducing CO2 emissions. 

If the efficiency of a grain dryer is increasing, it has to operate for a shorter period of time in the season, reducing 

the amount of electricity used too. 

Energy consumption for moisture evaporation is a primary concern for grain dryers. Many scholars have 

analysed energy consumption, noting that it varies between dryer types and is influenced by the product type 

and desired quality (Mondal et al., 2021). Each dryer’s design impacts energy use for efficient drying, as in the 

example of paddy drying in horizontal rotary dryers (Firouzi et al., 2017). Based on the method of energy transfer 

to the product, industrial dryers can be categorised into convective, contact, and emerging-field dryers. 

Specifically, mixed-flow dryers, a type of continuous dryer, are the most common in Hungary, making them ideal 

for analysing the energy performance of grain drying systems in this region. 

To evaluate the energy performance of industrial drying, it is essential to consider energy loss, specific electrical 

and thermal energy consumption, energy utilisation ratio, energy efficiency, and thermal performance. Mondal 

and Sarker (2024) analysed energy use among various dryers and found the efficiency of mixed-flow dryers to 

be 80 %, noting a 12 % lower efficiency compared to other dryers. Mondal et al. (2021) further examined the 

drying characteristics and energy and exergy performance of mixed-flow dryers across a temperature range of 

40-80 °C and air velocities of 3.0 and 6.0 m/s. They concluded that the energy utilisation ratio, exergy outflow, 

exergy loss, and exergy improvement potential all increased with higher drying temperatures. 

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Figure 1: Structure and volume of natural gas consumption in Hungary in 2022 

Previous studies on energetic analysis often consider the system as a whole, which can overlook specific energy 

losses and inefficiencies in individual processes in real-world applications. This study addresses this gap by 

providing a detailed energetic analysis of the drying process in a mixed-flow grain dryer over 6 y, based on a 

case study from real applications in Hungary. The contribution of the study is to propose a detailed assessment 

of a single drying process for mixed-flow grain dryers. A precision drying method is developed, allowing fine-

tuning of process parameters (e.g., airflow, grain flow) to optimise grain moisture content to the desired level 

based on large-scale continuous temperature measurements. The aim of this study is to improve low energy 

efficiency, reduce energy consumption, and lower CO₂ emissions of the dryer application. 

2. Methods and data 

Mixed-flow grain dryers are the most widely used in Hungary due to the different power requirements associated 

with high crop averages and varying plant sizes. We can achieve the reduction of energy consumption at the 

crop dryer by gathering measurement data with the appropriate resolution required for the analysis of the water 

withdrawal process.  

 

Figure 2: “tm” theoretical diagram, heat up of the crop-mass 

It becomes possible to identify deviations in the characteristics of the water extraction process based on the 

measurement data. The theoretical drying process is shown in Figure 2:  

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1. The drying medium (homogenous temperature hot air) from the gas burner is evenly distributed through 

the product, which is slowly warming up each of the grain seeds from 8-10 °C to 48-55 °C during the 

water-distraction process. 

2. All the heat given to the grain mass goes for the vaporisation of the moisture; the drying speed is 

constant. When the moisture decreases in the grain (x), the capillaries shrink, the evaporation zone 

contracts to the centre of the grain, and the mass temperature increases while the drying speed 

decreases. 

3. The drying speed is still decreasing, the mass temperature is still increasing, and the average moisture 

content is under the “secure storage minimum”. 

The measurement method provides precise data from the outlet air ducts of the grain dryer, which can be used 

to identify the configuration of the dryer (temperature distribution, heat intensity, grain flow). An industrial 

temperature measurement system is used that is able to measure air temperature at more than 500 

measurement points in parallel. However, the examples provided in the paper use fewer measurement points 

to illustrate the methodology more efficiently. It measures the temperature of hot air all over the surface passing 

through the mass of grain at the time of exit as shown in Figure 3. The gas burner is on the right side of the 

dryer shaft, and the drying air is going through the grain located in the shaft. The system measures the 

temperature of hot air passing through the mass of grain at the time of exit from the outlet air ducts; there is a 

strong correlation between the drying air temperature and the moisture content of the grain (the higher the 

temperature, the lower the moisture content of the grain). 

The measurement data is able to point out the divergence of measurable parameters of grain driers from the 

optimal level. These parameters are homogenous temperature, airspeed and speed of the grain flow [2]. In case 

of divergence, the grain may get stuck, creating jams, get fire, be over-heated (in this case, the grain becomes 

fragmented and loses inner values) or under-heated (in this case, the grain does not lose enough moisture, 

causing mould in the warehouse). The sensors can identify grain jams and measure the intensity of heat inside 

the dryer shaft. 

 

Figure 3: The measurement of temperature data 

Precision drying is based on IT tools and data processing methods and provides much more information about 

the drying process than any other tool before. It provides strict control of the drying process to preserve grain 

quality, which is important for plant development, crop production and animal husbandry. 

While preserving the quality of the grain, moisture removal must be gentle but with maximum efficiency. 

Experience shows that improvements based on the precision drying principle have, in most cases, increased 

performance and efficiency. To implement precision drying, more conditions should be kept near its optimum: 

• Homogeneous heat load over the surface of the tower; 

• Homogeneous grain-mass speed everywhere in the dryer shaft; 

• Constant air pressure over the surface of the dryer. 

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3. Results and discussion 

To show the physical causes, 3D modelling and computational fluid dynamics (CFD) are built, as shown in 

Figure 4. The applied simplification is that the grain loading and discharging part of the dryer is not included in 

the 3D model. The red part on the right is the warm side of the dryer; at the bottom, the gas is located to supply 

the heat for the drying medium. The blue part is the cold side of the dryer, where the drying medium saturated 

with moisture steps out from the grain. The condition of the homogenous pressure is the homogenous 

arrangement of air ducts on the surface. If the air ducts are not distributed uniformly on the surface of the drying 

shaft, that will cause a difference in air speeds and in drying efficiency. Simulations are performed to verify that 

there is a correlation between the distribution of air ducts, the higher temperatures and air velocity. 

 

Figure 4: The simplified 3D model for the CFD simulation the cold-side (left) and warm-side (right) 

The CFD simulation can be used to justify the effects that the so-called “thermal map” shows. The “thermal map” 

can be obtained about the vertical drying surface of the dryer shaft in front of the outlet air ducts (the “cold side” 

of the dryer), as shown in the left of Figure 5. A diagnosis can be set up based on the thermal map of the dryer. 

Conclusions can be drawn about the configuration of the hot side of the dryer (burner functionality, distribution 

of hot air in the air tunnel and on the surface, saturation of the air) and the dryer shaft (structural problems, 

airspeed). Pressure parameters are unbalanced on the drying surface (Figure 5. left), and further possible 

technical problems can be identified. 

 

Figure 5: Thermal map of the same dryer before (left) and after optimisation (right) 

The grain discharge speed and the drying air temperature are the parameters that can be set in most cases. 

This can be done by some control algorithms but also can be managed manually. The differences are always 

caused by some physical phenomenon. (Gottschalk, 2010). The left of Figure 6 shows an example where the 

asymmetry of the heat load is caused by the improper operation of the gas burner. Although the burner is not 

working properly, there is still quite a large space above the burner where the cold and hot air should mix, 

resulting in a homogenous temperature drying medium. The measurements clearly show that this expectation 

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is not fulfilled; the drying medium is not mixing properly in this case, and there is a big temperature difference 

between the two sides. This temperature difference will cause an obvious difference in the moisture content of 

the grain coming from the two sides of the dryer, too. On the right side of the drying shaft, instead of 40 °C, 

90 °C can be measured in whole seasons, which can cause enormous energy waste, over-drying and problems 

during warehousing (5-6 % difference of moisture content within one discharge cycle). This difference can be 

corrected by optimising the configuration of the warm side of the dryer, forcing the drying medium to mix 

properly. 

Long-term measurements (more than ten drying seasons) are available for different types of grain dryers (20 

different manufacturers). Some patterns can be identified in the thermal maps if the operation of the dryer differs 

from the optimal, as shown in the asymmetry of heat load in the dryer section (Figure 6 left) and the asymmetry 

of the discharge mechanism (Figure 6 right). The grain flowing into the centre of the dryer clearly flows faster 

than the layers at the side walls (Kocsis et al., 2011), where wall friction hinders the flow (Iroba, 2012). If this is 

possible in the grain dryer, the discharge mechanism should be adjusted to slow the grain flow in the middle. 

 

Figure 6: Asymmetry of heat load during maise drying (left), Asymmetry of the discharge mechanism (right) 

The difference between the theoretical process (Figure 2) and the measurements is in the second step of the 

drying process. The drying speed is not constant in the second section but continuously increases with the 

decrease of the moisture content of the grain. The mixed-flow dryers are well established in the commercial 

market, so it is important to find out whether the drying process is optimal or, with further optimisation, if the 

energy efficiency and product quality can be improved. Unfavourable designs can cause uneven mass flow and 

air flow distributions, resulting in locally different drying conditions and, hence, uneven grain drying (Wiegler et 

al., 2011). 

The issue of non-uniform distribution of air ducts is prevalent in many grain dryers in Hungary, as shown in 

Figure 7. If the air ducts are not distributed uniformly on the surface of the dryer, that will cause differences in 

air speeds and drying efficiency, too. It is necessary to find better solutions through development, but that is 

more important to increase the energy efficiency of the dryers established already in order to ensure 

sustainability. In this case, the over-heated horizontal sections occur because of the nonuniform distribution of 

air ducts; at irregular places, the air speed increases, and even the maise can be pulled out by the airflow from 

the dryer. The horizontal red bars mean energy loss. If the overheating can be stopped, then up to 30 % energy 

saving can be realised. According to Maier and Bakker-Arkema, there is still a need for research because the 

optimal shape, size, and location of air ducts in the drying bed of a mixed-flow dryer have not been established 

yet (Maier et al., 2002). 

  

Figure 7: Uneven distribution of air ducts on the surface of the dryer (left) and the 3D model (right) 

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4. Conclusion 

This paper analyses the energy efficiency of mixed-flow grain dryers through a long-term case study in Hungary. 

It focuses on identifying key modifications and precision drying methods to optimise grain moisture content, 

utilising large-scale temperature measurements to validate improvements and ensure optimal, eco-friendly dryer 

operation. The results of this study demonstrate that the non-uniform distribution of air ducts significantly 

impacts the drying efficiency and temperature uniformity in grain dryers. Through 3D modelling and CFD 

simulations, it was shown that uneven air duct distribution leads to differences in air speeds and drying 

performance. Thermal maps provided diagnostic insight, revealing issues such as asymmetry in heat load and 

discharge mechanisms. These disparities can result in uneven moisture content and significant energy losses, 

as highlighted by the 30 % potential energy savings if overheating is mitigated. 

Precision drying makes the drying process transparent and more secure since it supplies information from all of 

the surfaces where the heated drying medium meets with the grain. This results in a transparent drying process, 

which ensures better quality of dried grain (proteins, vitamins not destructed), a more secure drying process 

(reduced fire hazard) and warehousing (no microbes, no dust) and less energy consumption and less CO2 

emission. Some examples were introduced that show the possible differences in the drying process. It’s 

important to optimise the configuration of the warm side of the dryer and ensure proper mixing of the drying 

medium to achieve a homogeneous temperature profile. It’s suggested that existing grain dryers be 

upgraded through improved air duct distribution, precision drying systems, enhanced mixing mechanisms, and 

advanced controls to boost energy efficiency, grain quality, and sustainability. The limitation of this work is that 

we have the observed data but do not yet have the simulation results. Future work will involve refining CFD 

models to simulate various dryer configurations and assessing the impact of air duct placement and airflow 

optimisation. 

Acknowledgements 

The research was supported by the European Union within the framework of the National Laboratory for 

Autonomous Systems (RRF-2.3.1-21-2022-00002). 

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