Simultaneous convective heat and mass transfer in impingement ink drying

Effective and economical drying of the ink is essential in the printing, packaging and converting industries. In evaporative drying, high heat and mass transfer rates are commonly achieved by means of high velocity impinging air jets To provide data for dryer designer a programme of research has been implemented to study the heat and mass transfer processes which underlie the drying of thin ink rims The heat transfer situation under impinging air jets is outlined and some experimental results are presented. Optimization of nozzle arrays for impinging air jets is analysed for practical applications. A non-contact infra-red technique for continuously monitoring the ink drying process is described and drying curves for an ink based on a single solvent (4-Methyl-2-pentanol-MIBC) are presented. Heat and mass transfer theory has been used to predict dry ing limes in the constant rate drying period These predictions have been compared with experimentally determined doing times This research has served to confirm the fundamental importance of the dry ing curve as a basis for dryer design.     

Evaluation of performance and energy consumption of an impinging stream dryer for paddy

Impinging stream dryer has proven to be an excellent alternative means for removing surface moisture of particulate materials. In this study, a coaxial two-impinging stream dryer prototype for paddy, whose surface moisture needs to be removed prior to subsequent processing, was developed and tested. The effects of various operating and geometric parameters, i.e., inlet air temperature, impinging distance, particle flow rate and particle feeding characteristics (single-point feeding vs. double-point feeding), on the overall performance (in terms of the volumetric water evaporation rate and volumetric heat transfer coefficient) and energy consumption of the dryer were then studied. It was found that the developed impinging stream dryer could reduce the moisture content of paddy by 3.4–7.7% (d.b.) within a very short period of time. The maximum value of the volumetric water evaporation rate was found to be about 198 kg_water/m³ h, while the maximum value of the volumetric heat transfer coefficient was about 7013 W/m³ K. The mean residence time of the particles (paddy) in the system was in the range of 1.81–2.42 s, leading to average drying rate in the range of 1.52–3.83 (% d.b.) s^?1, which is about 250 and 40 times higher than spouted-bed and fluidized-bed dryers, respectively. The lowest total specific energy consumption of the process was 5.1 MJ/kg_water when using double-point particle feeding at an inlet air temperature of 110 °C, an impinging distance of 5 cm and particle flow rate of 150 kg_{dry solid}/h.     

Development of a new type of solar dryer: Its mathematical modelling and experimental evaluation

A solar dryer fitted with a novel design of absorber having inbuilt thermal storage capabilities was designed, fabricated, simulated and also tested at Rajiv Gandhi College of Engineering Research & Technology, Chandrapur (MS) India. Thermic oil was used as a storage material. The main objective of the study was to reduce the drying period and enhance the quality of dried product mainly chillies and fenugreek leaves. The products were laid in a single layer. The dimensions of the dryer were arrived at using the well-defined procedure available in literature. The mass of thermic oil needed in the absorber and mass of product to be dried in trays were optimized using simulation techniques. The maximum drying air temperature required for drying agricultural products was around 65°C. The ambient conditions at the location were 25–40°C, 16–43% RH and solar radiation 105–1024 W m⁻². Experimental studies based on temperature and humidity measurements were performed on the dryer. The research concluded that the desired drying air temperature was achieved and maintained for a longer period. The length of operation of the solar air heater and the efficiency of the dryer were increased, and better quality of agricultural products in terms of colour value were obtained compared with open sun drying. Copyright © 2007 John Wiley & Sons, Ltd.     

Energy and exergy analyses of thin layer drying of mulberry in a forced solar dryer

This paper is concerned with the energy and exergy analyses of the thin layer drying process of mulberry via forced solar dryer. Using the first law of thermodynamics, energy analysis was carried out to estimate the ratios of energy utilization and the amounts of energy gain from the solar air collector. However, exergy analysis was accomplished to determine exergy losses during the drying process by applying the second law of thermodynamics. The drying experiments were conducted at different five drying mass flow rate varied between 0.014 kg/s and 0.036 kg/s. The effects of inlet air velocity and drying time on both energy and exergy were studied. The main values of energy utilization ratio were found to be as 55.2%, 32.19%, 29.2%, 21.5% and 20.5% for the five different drying mass flow rate ranged between 0.014 kg/s and 0.036 kg/s. The main values of exergy loss were found to be as 10.82 W, 6.41 W, 4.92 W, 4.06 W and 2.65 W with the drying mass flow rate varied between 0.014 kg/s and 0.036 kg/s. It was concluded that both energy utilization ratio and exergy loss decreased with increasing drying mass flow rate while the exergetic efficiency increased.     

Mathematical modelling and experimental investigation of tropical fruits drying

A mathematical model has been developed to solve the heat and mass transfer equations for convective drying of tropical fruits. The model takes into account shrinkage of material and moisture content and shrinkage dependant effective diffusivity. Heat and mass transfer equations for the dryer, termed as equipment model, have also been developed to determine the changes of drying potential of the drying medium during drying. The material model is capable of predicting the instantaneous temperature and moisture distribution inside the material. The equipment model, on the other hand, describes the transfer process in the tunnel dryer and predicts the instantaneous temperature and humidity ratio of air at any location of the tunnel. Thus, the model is capable of predicting the dynamic behaviour of the dryer. The predicted results were compared with experimental data for the drying of banana slices dried in a solar dryer. Experimental results validated the model developed.     

Solar drying of foods: Modeling and numerical simulation

A simulation code was developed to predict the batch drying performance of a packed bed of particles, e.g. cylinders or slices of carrot, apples, etc., subjected to time-varying air conditions. This model allows for shrinkage of the particles. The time-dependent inlet drying air conditions permit the simulation of the case of a solar dryer in which the inlet air temperature is necessarily a function of the hour of the day. All the parameters involved in the model were obtained independently from experimental solar dryer data. The results compared well with published experimental data for solar drying of diced carrot. Effects of various key parameters of this process are presented and discussed.     

Experimental study and analysis of porous thin plate drying in a convection dryer

In this study experiments were conducted to study the porous thin plate drying process. An analysis was also carried out to determine the moisture transfer coefficients that vary with the moisture content of the plate during drying. The study is useful for analyzing the transient drying process of porous thin plates. Furthermore, energy and exergetic analyses based on the measured results were carried out to determine the dryer performance. It is found that, although the drying rate increases, the energy utilization ratio of the drying process decreases with either an increase in the rate of air flow or an increase in the size of the drying air inlet. The results indicate that a significant amount of the energy provided to the system was lost and not utilized for drying, especially for high drying rate dryers. An exergetic analysis showed that by adjusting the configuration of the air inlet and flow rate, the exergetic efficiency of the dryer can be significantly increased (from 0.5% to 10%). An overall transport phenomena analysis and performance evaluation including the transient moisture transfer coefficients, drying rate, energy utilization ratio, and exergetic efficiency in the current study provide comprehensive and important insights to the thermal physics of porous thin plate drying processes and the design of efficient dryer systems.     

Response surface optimization of a novel pilot dryer for processing mixed forest industry biosludge

As a promising sludge handling alternative capable of utilizing the secondary energies in industrial environments, we investigated the use of a novel pilot‐scale cyclone dryer for processing industrial mixed sludge from the forest industry. Attainable sludge dry solids contents (%) and respective specific energy consumption of drying (kWh kg^?1 H₂O) were successfully modelled by response surface methodology based on a constructed design of experiments. Predicted sludge dry solids and the specific energy consumption of drying varied between <30–65% and <0.4–1.8 kWh kg^?1 H₂O depending on controlled inlet air temperature, sludge feeding rate and humid air recirculation levels. The response models were further optimized for efficient combustion of processed sludge with inlet air temperatures corresponding to potentially available secondary heat. According to the results, energy efficient drying of mixed sludge with a specific energy consumption <0.7 kWh kg^?1 H₂O can be performed with inlet air temperatures ≥60 °C corresponding with pilot‐scale feeding capacities between 300–350 and 550 kg h^?1 depending on inlet air temperature. These findings suggest that the introduction of novel drying systems capable of utilizing the available secondary energies of industrial environments could significantly improve the energy efficiency of sludge drying and potentially allow considerable cost savings for industrial operators. Copyright © 2015 John Wiley & Sons, Ltd.     

Heat and mass transfer of seed drying in a two‐pass infrared radiation vibrated bed

Based on the investigation of drying of seeds by one‐pass infrared radiation vibration, the present study developed a two‐pass infrared radiation vibrated dryer. One of its characteristics was that during the drying process both seeds and the inlet hot air went through outer and inner cylinders (two‐pass) so that the drying travel distance of seeds was extended by 20%. Unsteady drying dynamic experiments showed that by increasing the inner travel distance, the seed dehydration rate increased by 87%, the total drying period was 1/1.8 times that of the one‐pass dryer, and the seed temperature was 0.82 times that of the one‐pass dryer. With the constant‐drying‐rate period prolonged and the drying rate increased due to the addition of the inner cylinder, the whole drying process was better adaptable to the physiology and biology of seeds and thus kept the seeds viable and enhanced heat and mass transfer. Against the drying rate equation, the paper experimentally studied the effects of radiation, convection, and conduction on the seed drying process. The study provided theoretical and technical bases for the drying of various vegetable seeds. © 2002 Scripta Technica, Heat Trans Asian Res, 31(2): 141–147, 2002; DOI 10.1002/htj.10024     

Energy analysis in fluidized‐bed drying of large wet particles

Energy analysis of a fluidized‐bed drying system is undertaken to optimize the fluidized‐bed drying conditions for large wet particles (Group D) using energy models. Three critical factors; the inlet air temperature, the fluidization velocity, and the initial moisture contents of the material (e.g., wheat) are studied to determine their effects on the overall energy efficiency to optimize the fluidized bed drying process. In order to verify the model, different experimental data sets for wheat material taken from the literature are used. The results show that the energy efficiencies of the fluidized‐bed dryer decrease with increasing drying time and become the lowest at the end of the drying process. It is observed that the inlet air temperature has an important effect on energy efficiency for the material where the diffusion coefficient depends on both the temperature and the moisture content of the particle. Furthermore, the energy efficiencies showed higher values for particles with high initial moisture content while the effect of gas velocity varied depending on the material properties. A good agreement is achieved between the model predictions and the available experimental results. Copyright © 2002 John Wiley & Sons, Ltd.     

Thermodynamic analysis of fluidized bed drying of carrot cubes

In this study, the energy and exergy analyses of fluidized bed drying of carrot cubes were investigated. Drying experiments were conducted at inlet air temperatures of 50, 60, and 70 °C, BD (bed depths) of 30, 60, and 90 mm and square-cubed carrot dimensions of 4, 7, and 10 mm. The effects of drying variables on energy utilization, energy utilization ratio, exergy loss and exergy efficiency were studied. The energy utilization and energy utilization ratio varied between 0.105–1.949 kJ/s and 0.074–0.486, respectively. The exergy loss and exergy efficiency were found to be in the range of 0.206–1.612 kJ/s and 0.103–0.707, respectively. The results showed that small particles, deep beds and high inlet air temperatures increased energy utilization, energy utilization ratio, and exergy loss due to high value of heat and mass transfer. Also, the exergy efficiency had maximum value when higher drying air temperature, larger CS (cube size) and shorter BD were used for drying experiment.     

Analysis of a new drying chamber for heat pump mint leaves dryer

Drying is an energy intensive and time consuming process, so reducing amount of demanded energy and drying time are important issues for drying technology. The main aim of this paper is to analyze the drying characteristics of mint leaves in a new cylindrical form of drying chamber at low drying air temperature and by emphasizing on energy analysis. The dryer consists of air source heat pump system, air to air heat recovery unit and proportional temperature controller. Experiments were performed at 2, 2.5 and 3 m/s air velocities and at 35 °C cabin inlet air temperature. Mint leaves were dried from 9 g water/g dry matter to 0.1 g water/g dry matter. Designed drying chamber, with three stainless steel cylinders in circular nested form, has a positive effect for drying technology. This system has some advantages such as: drying of product by accessing a uniform air flow and preventing spread of light weight samples like mint leaves over drying system. Calculations based on experimental data show that in the best case, by consuming 3.164 kWh energy in a heat pump with 3.94 coefficients of performance, 4.56 kWh energy had been gained by heat recovery unit. Average 48% of energy was saved by means of heat recovery unit. Effective moisture diffusivity values varied from 3.50E?11 to 5.88E?11 for mint leaves.     

Specific energy consumption in microwave drying of garlic cloves

The convective and microwave-convective drying of garlic cloves was carried out in a laboratory scale microwave dryer, which was developed for this purpose. The specific energy consumption involved in the two drying processes was estimated from the energy supplied to the various components of the dryer during the drying period. The specific energy consumption was computed by dividing the total energy supplied by amount of water removed during the drying process. The specific energy consumption in convective drying of garlic cloves at 70 °C temperature and 1.0 m/s air velocity was estimated as 85.45 MJ/kg of water evaporated. The increase in air velocity increased the energy consumption. The specific energy consumption at 40 W of microwave power output, 70 °C air temperature and 1.0 m/s air velocity was 26.32 MJ/kg of water removed, resulting in about a 70% energy saving as compared to convective drying processes. The drying time increased with increase in air velocity in microwave-convective drying process; a trend reverse to what was observed in convective drying process of garlic cloves.     

Performance of indirect solar cabinet dryer

In this paper, the development and testing of a new type of efficient solar dryer, particularly meant for drying vegetables and fruit, is described. The dryer has two compartments: one for collecting solar radiation and producing thermal energy and the other for spreading the product to be dried. This arrangement was made to absorb maximum solar radiation by the absorber plate. In this dryer, the product was loaded beneath the absorber plate, which prevented the problem of discoloration due to irradiation by direct sunlight. Two axial flow fans, provided in the air inlet, can accelerate the drying rate. The dryer had six perforated trays for loading the material. The absorber plate of the dryer attained a temperature of 97.2 °C when it was studied under no load conditions. The maximum air temperature in the dryer, under this condition was 78.1 °C. The dryer was loaded with 4 kg of bitter gourd having an initial moisture content of 95%, and the final desired moisture content of 5% was achieved within 6 h without losing the product colour, while it was 11 h for open sun drying. The collector glazing was inclined at a particular angle, suitable to the location, for absorption of maximum solar radiation. A detailed performance analysis was done by three methods, namely ‘annualized cost method’, ‘present worth of annual savings’ and ‘present worth of cumulative savings’. The drying cost for 1 kg of bitter gourd was calculated as Rs. 17.52, and it was Rs. 41.35, in the case of an electric dryer. The life span of the solar dryer was assumed to be 20 years. The cumulative present worth of annual savings over the life of the solar dryer was calculated for bitter gourd drying, and it turned out be Rs. 31659.26, which was much higher than the capital cost of the dryer (Rs. 6500). The payback period was calculated as 3.26 years, which was also very small considering the life of the system (20 years). So, the dryer would dry products free of cost during almost its entire life span. The quality of the product dried in the solar dryer was competitive with the branded products available in the market.     

Performance of a heat-pump assisted dryer

We present a simple mathematical model of a heat-pump-assisted dryer developed from psychrometric processes. A term ‘contact factor’ is used in the theoretical model to characterize the drying chamber. The experimental data of the drying rates of different types of products are used to predict the values of the contact factor of the dryers. We examine the effect of various parameters such as the contact factor, air inlet conditions, and the moisture removal rate on the performance of the heat-pump-assisted dryer. It has been shown that the non dimensional contact factor of a dryer is insensitive to dryer air inlet temperature. Finally, a performance chart to guide the selection of the heat-pump dryer components is proposed.     

Thermodynamic study of wet cooling tower performance

An analytical model was developed to describe thermodynamically the water evaporation process inside a counter‐flow wet cooling tower, where the air stream is in direct contact with the falling water, based on the implementation of the energy and mass balance between air and water stream describing thus, the rate of change of air temperature, humidity ratio, water temperature and evaporated water mass along tower height. The reliability of model predictions was ensured by comparisons made with pertinent experimental data, which were obtained from the literature. The paper elaborated the effect of atmospheric conditions, water mass flow rate and water inlet temperature on the variation of the thermodynamic properties of moist air inside the cooling tower and on its thermal performance characteristics. The analysis of the theoretical results revealed that the thermal performance of the cooling tower is sensitive to the degree of saturation of inlet air. Hence, the cooling capacity of the cooling tower increases with decreasing inlet air wet bulb temperature whereas the overall water temperature fall is curtailed with increasing water to air mass ratio. The change of inlet water temperature does not affect seriously the thermal behaviour of the cooling tower. Copyright © 2005 John Wiley & Sons, Ltd.     

Exergetic performance assessment of a pilot‐scale heat pump belt conveyor dryer

In this study, olive leaves were dried in a pilot‐scale heat pump (HP) belt conveyor dryer as a thin layer. Drying experiments were carried out at the drying air temperature range of 45–55°C with the drying air velocity range of 0.5–1.5 m s⁻¹. The performance of the system and the process was evaluated using exergy analysis method. The exergy loss and flow diagram (the so‐called Grassmann diagram) of the dryer system was presented to give quantitative information regarding the proportion of the exergy input that is dissipated in the various system components. Effects of the drying air temperature and the velocity on the performance of the drying process were discussed. The actual coefficient of performance values were obtained to be 2.37 for the HP unit and 2.31 for the overall system, respectively. The most important component of the system for improving the efficiency was determined to be the compressor. Exergetic efficiencies of the drying of olive leaves were in the range of 67.45–81.95%. It was obtained that they increased as the drying air temperature decreased and the drying air velocity increased. Copyright © 2009 John Wiley & Sons, Ltd.     

Exergetic evaluation of drying of laurel leaves in a vertical ground-source heat pump drying cabinet

This paper is concerned with the exergy analysis of the single layer drying process of laurel leaves in a ground-source heat pump drying cabinet, which was designed and constructed in the Solar Energy Institute, Ege University, Izmir, Turkey. The effects of drying air temperature on exergy losses, exergy efficiencies and exergetic improvement potential of the drying process are investigated. The results have indicated that exergy efficiencies of the dryer increase with rising the drying air temperature. Moreover, the laurel leaves are sufficiently dried at the temperatures ranging from 40 to 50°C with relative humidities varying from 16 to 19% and a drying air velocity of 0.5 m s⁻¹ during the drying period of 9 h. The exergy efficiency values are obtained to range from 81.35 to 87.48% based on the inflow, outflow and loss of exergy, and 9.11 to 15.48% based on the product/fuel basis between the same drying air temperatures with a drying air mass flow rate of 0.12 kg s⁻¹. Copyright © 2006 John Wiley & Sons, Ltd.     

Energy analysis of hazelnut drying system‐assisted heat pump

In this experimental study, a proportional integral derivative (PID) controlled heat pump dryer was designed and manufactured. Heat pump dryer was tested drying of hazelnut and energy analyses were made. Drying air temperatures were changed as 50,45 and 40°C in the drying system. Drying air velocities were changed as 0.25 m s^?1 for 50°C, 0.32 m s^?1 for 45°C and 0.38 m s^?1 for 40°C. Heating coefficient of performance of whole system (COP_ws) of the heat pump dryer was calculated as 1.70 for 50°C, 1.58 for 45°C and 1.40 for 40°C drying air temperatures. Energy utilization ratio changed between 24 and 65% for 50°C, 17 and 63% for 45°C and 14 and 43% for 40°C drying air temperatures in the heat pump dryer. Copyright © 2008 John Wiley & Sons, Ltd.     

The effect of the impingement air drying on print mottle and other coated paper properties

The objective of this work is to study the influence of effective initial impingement drying (one-sided high intensity air impingement drying) on the quality of blade-coated paper and to compare it with IR (infrared) drying. A specially designed 1.31 m-long impingement air dryer unit was installed 2 m downstream of the coating station of the pilot coater for more efficient paper drying. Different drying strategies using different combinations of three impingement temperatures (300, 450 and 550 °C) and three impingement velocities (25, 40 and 60 m/s) were tested in the impingement dryer unit to determine the influence of drying on paper quality. Drying effects are compared with those obtained with two rows of an electrical IR at same position. The results of the investigation indicate that backtrap (BT) mottle was reduced with increasing drying power of the impingement dryer. The paper quality parameters, gloss and smoothness of the coated samples were better with air-drying than IR drying. The interesting finding of this study is the improvement of mottle with a high drying rate in the consolidation phase for coated WF (wood-free) paper. Drying section configuration with effective impingement drying unit directly after the coating station showed good performance and is proposed as a possible solution for the future dryer designs.