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INTERNATIONAL JOURNAL OF ENERGY RESEARCH VOL 18 605 622 1994 PERFORMANCE OF A HEAT PUMP ASSISTED DRYER S K CHOU M N A HAWLADER J HO N E WIJEYSUNDERAAND S RAJASEKAR Depariment of Mechanical and Production Engineering National Uniwrsiiy of Singapore 10 Kent Ridge Crescent Singapore 051 I SUMMARY 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 KEY WORDS Heat pump Drying Contact factor Modelling INTRODUCTION Drying is one of the oldest forms of food prebervation and a common unit operation in many chemical and process industries In conventional dryers humid air from the dryer is vented to the atmosphere which results in the loss of both the sensible and latent heat of vaporization of its moisture content Instead with the incorporation of a heat pump to a dryer humid air leaving the dryer can be recycled dehumidified mixed with fresh air stream and preheated before it is returned into the dryer A heat pump assisted dryer is thus an integration of a heat pump system with a dryer Strommen 1980 studied the drying characteristics of codfish using a fully closed heat pump dryer and proposed a semi empirical model to predict the drying rate for codfish Zyalla et af 1982 reviewed the various types of dryers and reported that a heat pump dryer has advantages over the others when RH230 is required inside the dryer An experimental study on the performance of a heat pump dehumidification dryer system was reported by Tai et af 1982a 1982b Dry air was used to dry wet linen cloths suspended in the dryer The system attained maximum coefficient of performance COP when the approach velocity was 1 6 m s The minimum specific power consumption SPCh for an approach velocity of 1 6 m s was obtained when superheat was at 19 K Skevington et af 1987 reported two novel applications of a heat pump dryer in food processing namely apple crisp drying and deodorization of mutton A mathematical model to predict the performance of an integrated heat pump assisted dryer was reported by Pendyala et af 1990a The performance of a heat pump assisted dryer was studied by Pendyala et af 1990b with two different refrigerants R11 and R12 The effects of the approach velocity of air to the evaporator and the superheat of the working fluid on the performance of the heat pump as sisted dryer were studied The coefficient of performance COP and the specific power consumption values SPC obtained using R11 were 3 5 and 3 500 kJ kg respectively and the corresponding values for R12 were 2 5 and 1800 kJ kg A detailed mathematical model to investigate the performance of a heat pump assisted continuous drying system was reported by Jolly et af 1990 and this model was used by Jia et af 1990 to study the performance of a heat pump assisted continuous drying system against several key system aspects such as the evaporator air by pass ratio and the use of recuperators The CCC 0363 907X 94 060605 18 0 1994 by John Wiley oc 10 RH t 1 OC 20 RH Specific power consumption of dryer kW grams Grams of moisture removed In the dryer per kg per sec of dry air Figure 17 Performance chart for T at 75 C 3 Now to predict the required compressor capacity a horizontal line drawn from point a to module 2 is extended until it meets the curve drawn for the same dryer air inlet conditions chosen in module 1 at point b From point b a vertical line is drawn to the x axis of module 2 to select the compressor capacity 4 From point b to select the evaporator capacity a vertical line is drawn to module 3 to point c From point c a horizontal line is drawn to the y axis of module 3 to read the evaporator capacity The condenser capacity which includes the capacities of the internal and external condensers can now be calculated based on the capacities of the compressor and evaporator obtained from modules 2 and 31 respectively 5 To obtain the capacity of the internal condenser a horizontal line drawn from point c to module 4 meets the curve drawn for the dryer air inlet conditions chosen in module 1 at point d From point d a vertical line is drawn to the x axis of module 4 to read the capacity of the internal condenser From the values of the total capacity of the condenser and the capacity of the internal condenser the capacity of the external condenser can be calculated The use of an external condenser is often required in LTD applications 620 HEAT PUMP ASSISTED DRYER It should be noted that as this chart requires information on the moisture removal rate of the dryer the moisture removal rate that can be achieved under constant drying rate condition which will usually be higher than that under falling drying rate condition should be used in the selection of these components to ensure the effective functioning of the dryer under adverse conditions The contact factor of the dryer under constant drying rate condition can be used in equation 1 to predict the moisture removal rate that can be achieved under constant drying rate condition CONCLUSIONS A theoretical model is developed based on basic psychrometric equations to study the performance of a HPD The concept of contact factor is used in the mathematical model to describe the heat and mass moisture transfer process between the product and the drying medium The values of the contact factor predicted for different types of products dried under different air inlet conditions are presented Results indicate that the specific moisture extraction rate ShfERh and the specific power consumption SPCh are strongly influenced by the contact factor of the dryer The contact factor of the dryer is sensitive only to the relative humidity and velocity of air entering the dryer This information will be useful in experiments as it allows the drying rate of a product at different temperatures and same humidity to be estimated from a single test The performance charts prepared on the basis of information generated using the mathematical model are presented as a selection guide for the components of the HPD Experiments will be conducted with different types of products for various dry air inlet conditions and air flow and dryer parameters to validate the mathematical model and to predict the contact factor of those dryers NOMENCLATURE contact factor dimensionless coefficient of performance dimensionless dry bulb temperature C specific heat capacity of air kJ kg C enthalpy of humid air at point 2 kJ kg enthalpy of humid air at point 3 kJ kg enthalpy of humid air at point 4 kJ kg enthalpy of air at point 5 kJ kg enthalpy of air at point 6 kJ kg enthalpy of air at point 7 kJ kg enthalpy of outdoor air at point 8 kJ kg enthalpy of water condensed at point 3 kJ kg generator heat losses percentage condenser capacity kW evaporator capacity kW power input to generator kW reheater capacity kW specific moisture extraction rate kg kWh specific power consumption kJ kg dry bulb temperature of outdoor air at point 0 C dry bulb temperature of dry air at point 1 C dry bulb temperature of humid air at point 2 C S I CHOU ETAL 621 dry bulb temperature of dehumidified air at point 3 C dry bulb temperature of air at point 4 C dry bulb temperature of air at point 5 C dry bulb temperature of air at point 6 C dry bulb temperature of outdoor air at point 7 C dry bulb temperature of outdoor air at point 8 C wet bulb temperature of humid air at point 2 C specific humidity of humid air at point 2 g kg of air specific humidity of dehumidified air at point 3 g kg of air specific humidity of air at point 4 g kg of air specific humidity of air at point 5 g kg of air specific humidity of air at point 8 g kg of air wet bulb temperature C compressor work kW moisture content per gram of product fractional quantity of air recirculated initial moisture content of a product g moisture content of a product at time f g percentage of air passing over evaporator condenser compressor dryer evaporator heat pump dryer REFERENCES ASHRAE 1989 Fundamentals Handbook ASHRAE Atlanta GA Batsale J C and Puigalli J R 1985 Drying of crop products with a shell experimental approach and modelling its applications Brunello G and Claudio A 0 1982 The kinetics of sorghum grains drying in a mechanically stirred bed dryer Drying 82 Clements S Jia Xiguo and Jolly Peter 1993 Experimental verification of a heat pump assisted continuous dryer simulation Hawlader M N A Uddin M S Ho J C and Teng A B 1991 Drying characteristics of tomatoes Journal of Food Jia X Jolly Peter and Clements Shane 1990 Heat pump assisted continuous drying Part 2 Simulation results Znt Journal of Jolly P Jia Xiguo and Clements Shane 1990 Heat pump assisted continuous drying Part 1 Simulation model Znt Journal of Keey R B 1978 Introduction of indusmul drying operatiotrr Pergamon Press Oxford Mujumdar Arun S 1987 Handbook of industrial drying Marcel Dekker New York Norman C Harris 1983 Modern air conditioningpractice McGraw Hill New York Pendyala V R Devotta S and Patwardhan V S 1990a Heat pump assisted dryer Part 1 Mathematical model Int Journal of Pendyala V R Devotta S and Patwardhan V S 1990b Heat pump assisted dryer Part 2 Experimental study Znt Journal of to hazelnuts Drying 85 Hemisphere Publishing Corporation New York 410 414 Hemisphere Publishing Corporation New York 56 60 model Int Journal of Energy Research 17 19 28 Engineering 14 259 268 Energy Research 14 771 782 Energy Research 14 757 770 Enem Research 14 479 492 Energy Research 14 493 507 Ratti C and Crapiste G H 1992 A generalized drying curve for shrinking food materials Drying 92 Elsevier New York Part A 864 873 Saurez C Viollaz Pascual and Chirife Jorge 1980
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