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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). 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