Performance Analysis of an Electric Clothes Dryer

Improving the energy efficiency of clothes dryers has been the subject of numerous investigations. However, to date the clothes drying process has not been thoroughly tested or analyzed in a comprehensive study. Many of the techniques that have been proposed to improve energy efficiency add to dryer cost or are environmentally unacceptable. In the present study, a commercial Frigidaire dryer was instrumented and evaluated for energy performance by changing operating parameters including heater power, fan speed, drum speed, weight, and initial moisture content of the clothes. The total drying time and energy consumption was monitored as well as the effect of air leakage into the drum. The experimental results from 32 runs of the dryer were evaluated. The results from these tests are discussed and techniques are proposed in an attempt to improve the energy efficiency of this unit.     

Modeling of heat pump tumble dryer energy consumption and drying time

The use of heat pump tumble dryers is nowadays more common because they offer huge energy savings compared to conventional tumble dryers. Earlier studies made on conventional tumble dryers have shown that parameters such as heater power, fan speed, drum speed, weight and initial moisture content of textiles and air leakage have a huge impact on the energy efficiency and drying time. In the present study, a modified commercial heat pump tumble dryer was evaluated for energy consumption and drying time by changing operating parameters including fan speed, drum speed, and mass load. The total energy consumption and drying time were measured and corrected for the initial and final moisture content in the textiles. The experimental results based on 27 drying tests were evaluated to develop linear regression models for energy consumption and drying time, which show a good agreement with the experimental data. The results show that a large mass load, a high drum speed, and a low fan speed give the highest energy efficiency, i.e. the lowest energy consumption per kg of drying load. Larger loads extend the length of the drying cycle while higher fan and drum speeds result in shorter drying time.     

Enhancing the Energy Efficiency of Domestic Dryer by Drying Process Optimization

The domestic tumble dryers are becoming indispensable household appliances and responsible for up to 10% of the total residential energy use in developed countries. However, their energy efficiency is low. In this paper, the development of a multi-sensor computer-controlled prototype platform for fabric drying is described for improving the efficiency of dryers. The prototype platform enables the real-time control and recording of key drying parameters including heater power, air flow velocity, rotating speed of drying drum, and drying cycle time. These parameters are automatically adjusted according to the exhaust air humidity instead of the temperature which is used traditionally. Additionally, a new drying model of dividing the drying process into four stages based on the humidity of the exhaust air has been investigated to further increase the energy saving. The performance of this staged drying model is experimentally evaluated in respect to energy consumption, drying time, and the smoothness of fabric after drying. The results clearly indicate that the staged controlling of heating power input not only decreases energy consumption by 21.5%, but also improves the fabric smoothness by 0.9 grade compared to using a single heating power input for the whole drying process. The research outcome can enable the design and production of new dryers that are more energy efficient and lead to dried clothes that require less ironing, which in turn further reduces energy consumption.     

An experimental study on the performance of a condensing tumbler dryer with an air-to-air heat exchanger

The performance of energy consumption in the closed-loop tumbler dryer with a condenser for clothes drying is evaluated as a function of the heater capacity, the drying air flow rate inside the dryer, and the cooling air flow rate. The clothes dryer in laundries used in this study consists of a tumbling drum, condenser for condensing the humid and hot air flowing out the rotating drums, and electric heater for heating the circulating drying air. Tests were performed at the heating capacity of 1.9 kW to 2.7 kW, the drying air flow rate of 60 m³/h to 140 m³/h, and the cooling air flow rate of 100 m³/h to 240 m³/h. The total energy consumption, the drying time, and the condensate water rate were also investigated. Parametric results showed that a larger heater power resulted in shorter drying time. With increasing heater power, the air temperature and the condensate rate increased due to the higher humidity ratio in the air. The drying air flow rate and the cooling air flow rate did not have a significant effect on drying performance.     

EXPERIMENTAL STUDY ON DRYING OF CHILLI IN A COMBINED MICROWAVE-VACUUM-ROTARY DRUM DRYER

This paper presents new data on drying chilli in a microwave-vacuum-rotary drum dryer. This novel technique is designed to combine the advantages of vacuum drying and evenly dispersed microwave energy in a rotary drum. The drying kinetic and the specific energy consumption at particular product moisture content were measured experimentally. Moreover, the effect of pressure inside the chamber and the rotational speed of the drum were also determined.     

EXPERIMENTAL STUDY ON DRYING OF CHILLI IN A COMBINED MICROWAVE-VACUUM-ROTARY DRUM DRYER

ABSTRACT

This paper presents new data on drying chilli in a microwave-vacuum-rotary drum dryer. This novel technique is designed to combine the advantages of vacuum drying and evenly dispersed microwave energy in a rotary drum. The drying kinetic and the specific energy consumption at particular product moisture content were measured experimentally. Moreover, the effect of pressure inside the chamber and the rotational speed of the drum were also determined.     

Simulation of Wood Particle Motion Through a Concurrent Triple-Pass Rotary Dryer

The retention time of solids in a drum is an important parameter for the design of rotary dryers, since it directly influences the mass and heat transfer rates. If it is too short, the wood particles do not become adequately dried. If it is too long, they become over-dried. Therefore, having an appropriate retention time is useful in terms of both energy and plant capacity. Wood particle mean retention time in a rotary dryer is affected by several variables, such as dryer dimensions, solid characteristics, and operational parameters. The purpose of this work was to simulate the effects of some wood particle characteristics and operational parameters on the mean retention time, drum holdup, and velocity of the wood particles during drying in a pilot-scale, closed-loop, triple-pass rotary dryer by means of a computer code. The simulation results of wood particle motion can be used for modeling, design, and optimization of closed-loop, triple-pass rotary dryers.     

用于中药浸膏干燥的滚筒干燥机

对滚筒干燥机在罗布麻浸膏干燥上成功应用进行了研究,并与现有中药浸膏干燥机进行了对比分析,探讨了滚筒干燥机在中药浸膏干燥领域应用的可行性。     

SIMULATION MODEL FOR INDUSTRIAL DRYERS: REDUCTION OF DRYING TIMES OF CERAMICS & SAVING ENERGY

ABSTRACT

A procedure was outlined to optimize industrial dryers for ceramics. The procedure consists of drying experiments on full-size products in a lab dryer, measurements of characteristics of the dryer and by simulations with DrySini. DrySim is a flexible simulation program in which a user can model his own dryer with predefined components. Two examples are given, the optimization of a chamber dryer and the optimization of a tunnel dryer. In both examples the production of the existing dryers could be increased and at the same time cost of energy could reduced by optimal use of waste air of kilns and minimizing mixing of kiln air with ambient air.     

Modeling and simulation of drying characteristics on flexible filamentous particles in rotary dryers

Experiments were conducted to demonstrate the effects of the drum wall temperature on the heat and mass transfer in rotary dryers. The drying characteristics of flexible filamentous particles in rotary dryers were further explored. In addition, the inlet and outlet temperatures and moisture contents of granular particles were measured. As a result, the good agreement between the simulations and experiments verified the rationale and feasibility of the numerical method. Therefore, the approach was adopted to evaluate the temperature and moisture content of wet granular particles in a rotary dryer in different conditions, for instance, drum wall temperature and rotational speed. The results revealed that the higher drum wall temperature led to hotter particles with lower outlet moisture content. Conversely, the higher rotational speed resulted in cooler particles with higher outlet moisture content due to the decrease of residence time in the rotary dryer.     

Gaseous Carbon Dioxide as the Heat and Mass Transfer Medium in Drying

Using available correlations for heat transfer, a comparative analysis of drying rates in CO₂ and in air was performed for several basic types of dryers. Higher heat transfer rates were found for dryers with active hydrodynamics, which translates into shorter drying time for materials dried in the first drying period. These results were validated by experiments on drying wheat kernels fluidized by air and by CO₂. Shorter drying times by about 20% were confirmed for CO₂, which offers energy savings of about 3% of the heat input to the dryer. Additional energy savings of 4% of the heat load can be expected for drying at temperatures below 100°C because of the lower wet-bulb temperature for CO₂ than that for air. The potential for CO₂ abatement was evaluated based on a case study for drying of distillers' spent grain.     

Correlation of the Area-Mass Transfer Coefficient Inside the Drum of a Clothes Dryer

The mass transfer process inside the drum of a clothes dryer affects its performance. The area-mass transfer coefficient was defined and correlated to the weight of the clothes, drum speed, the Reynolds number, Schmidt number, and Gukhman number using the experimental results of 32 runs on a Frigidaire clothes dryer. The coefficient of correlation was 0.994. A separate correlation for the Sherwood number was evaluated for a single piece of cloth stretched perpendicular to the flow in a pipe. This second correlation was used to compare the mass transfer inside the actual dryer with that of the “idealized” mass transfer process for a single piece of cloth. The results indicated that the average mass transfer efficiency for clothes in the dryer was only 26.4% of the ideal process. Means to enhance the area-mass transfer coefficient and the reasons for the poor performance of an actual dryer are discussed.     

Correlation of the Area-Mass Transfer Coefficient Inside the Drum of a Clothes Dryer

Abstract

The mass transfer process inside the drum of a clothes dryer affects its performance. The area-mass transfer coefficient was defined and correlated to the weight of the clothes, drum speed, the Reynolds number, Schmidt number, and Gukhman number using the experimental results of 32 runs on a Frigidaire clothes dryer. The coefficient of correlation was 0.994. A separate correlation for the Sherwood number was evaluated for a single piece of cloth stretched perpendicular to the flow in a pipe. This second correlation was used to compare the mass transfer inside the actual dryer with that of the “idealized” mass transfer process for a single piece of cloth. The results indicated that the average mass transfer efficiency for clothes in the dryer was only 26.4% of the ideal process. Means to enhance the area-mass transfer coefficient and the reasons for the poor performance of an actual dryer are discussed.     

OPTIMUM OPERATING CONDITIONS IN DRYING FOODSTUFFS WITH SUPERHEATED STEAM

It is inferred from experimental data that in drying foodstuffs with superheated steam, the initial drying rate has a direct effect on the rate at which the overall drying takes place. That is, the faster the initial drying rate, the shorter the overall drying time. This criterion is very convenient because at the beginning, water moistens the sample external surface so evaporation does not depend on internal sample characteristics, but only on external convective heat and mass transfer rates. Mass and energy balance equations are solved and the result converted into a general initial drying rate equation, in which all dryer characteristics are grouped into one dimensionless parameter. The initial drying rate equation is mathematically maximized and the optimum working conditions determined. The result shows that initial drying rate always increase with increases of either the superheated steam temperature or velocity, but once these two variables are fixed, there exists at least one “optimum” pressure at which the initial drying rate is a maximum. Finally, the initial drying rate and optimum condition equations are applied to three model dryers, a dryer for a flat sheet, a fixed bed dryer and a rotary dryer. In each case, numeric values are computed and plotted as drying rate versus pressure curves, in which the optimum drying rate is also included. Also presented is a chart to compare the optimum pressures as functions of temperature and steam velocity for the three dryers.     

Gaseous Carbon Dioxide as the Heat and Mass Transfer Medium in Drying

Using available correlations for heat transfer, a comparative analysis of drying rates in CO₂ and in air was performed for several basic types of dryers. Higher heat transfer rates were found for dryers with active hydrodynamics, which translates into shorter drying time for materials dried in the first drying period. These results were validated by experiments on drying wheat kernels fluidized by air and by CO₂. Shorter drying times by about 20% were confirmed for CO₂, which offers energy savings of about 3% of the heat input to the dryer. Additional energy savings of 4% of the heat load can be expected for drying at temperatures below 100°C because of the lower wet-bulb temperature for CO₂ than that for air. The potential for CO₂ abatement was evaluated based on a case study for drying of distillers' spent grain.     

A Diffusion Model for a Drum Dryer Subjected to Conduction, Convection, and Radiant Heat Input

Drum dryers are commonly used for production of a flaky dry powder from thick suspensions. This article presents results of a simple diffusion-based model to predict the drying performance of a pilot-scale twin-drum dryer. Numerical results are compared with experimental data obtained for a biological sludge whose initial moisture content varied from 1.0 to 2.3 kg/kg db. The agreement of model predictions with the pilot-scale experimental data is favorable. Effects of film thickness, drum rotational speed, external air flow velocity, and its humidity are examined parametrically. Sludge film thickness is identified as the most critical operating parameter to control the final moisture content and productivity of the dryer. The validated model is used to predict performance of a drum dryer subjected to heat input by convection and radiation along with conduction through the drum wall. It is shown that dryer output can be enhanced significantly by increasing the film thickness and applying radiant heating in the initial period of drying. A simple mathematical model of this type can be used for the purpose of design and analysis as well as scale-up of industrial drum dryers based on simple laboratory-scale experiments.     

ANALYSIS OF MECHANICALLY AGITATED FLUID-PARTICLE CONTACT DRYERS

Incorporation of agitation to spouted and fluidized bed dryer result in significant increases in the drying capacity (Q_s), although product retention persist, which is reduced by increasing the air flow. The physical phenomena occurring in these dryers with several liquid substrates was analyzed and the residence time distributions (RTD) were obtained by the use of dye tracers. The residence time (τ) was found to be a function of the rate of agitation (n) and reaches a minimum at n = n_opt, which was characteristic for each type of substrate, and where maxima also appeared for the drying capacity (Q_s = Q_{s max}) and the heat transfer coefficient (Nu_p = Nu_{p max}). The RTD can be modeled by series of consecutive dryers and a modified Vanderschuren and Delvosalle model can be employed to calculated moisture of the dry product.     

A Novel Design of Crop Dryer for Use in Developing Countries

The dryer is required for drying of grain as well as drying of the processed products in small catchment agro processing centers in the developing world. However, due to varied material characteristics of grain and secondary processed product, two entirely different types of dryers are required. The grain is dried in a recirculatory dryer, whereas processed product is dried in a tray dryer, where it is frequently mixed and trays are also intermittently changed. To avoid the need for two dryers, a novel design of a low-cost hot air dryer was developed where just by changing the trays the dryer can be converted from an LSU grain dryer to a tray-type product dryer. The dryer was tested for drying soybean grain as well as processed soy products like blanched soybean dal and soyflakes. The capacity of the dryer was 100 kg/batch in a tray dryer with each tray accommodating 10 kg of wet material. In case of LSU mode, the capacity of the dryer was 250 kg of grain per batch. The drying time required was 5 h for 250 kg of wet soybean from 24 to 10% moisture content, whereas in a tray dryer 100 kg blanched soybean dal was dried from 60 to 10% in 5 h and 100 kg of soyflakes from 25% moisture content to 10% moisture in 1.75 h. The cost of the dryer is estimated at US$580.00 and it can be fabricated in a moderately equipped workshop in developing countries.     

Model-Based Analysis of Convective Grain Drying Processes

ABSTRACT

Simulation results for convective drying processes in cross-flow packed bed grain dryers are discussed in this article. A mathematical model developed in order to enable easier design of convective dryers and optimization of operating conditions for agricultural materials (wheat, corn, sunflower seed, etc.) is used in the study. On the basis of calculated values of the state variables of the gas phase and the solids, a clear image of the process itself can be obtained, as well as an overview of advantages and disadvantages of a specific design, supporting and facilitating decisions about the choice of dryer type and operating scheme. The case of double passing of drying agent, with and without additional heating, for a cross-flow continuous dryer, as well as the case of different materials processed in a series of cross-flow batch dryers, is analyzed.