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任务书课题名称2HP热泵干燥机的设计院 （系）专 业姓 名学 号起讫日期指导教师毕业设计（论文）的内容和要求 课题：2HP热泵干燥机的设计本毕业设计课题结合产品开发，要求学生有一定的工程能力，本课题选题合理，工作量饱满，机械制图要求比较高。学生通过本课题的设计可以综合大学4年所学知识的运用能力，特别是工程热力学、传热学、流体力学、制冷、热泵技术及相关专业课程的知识应用，同时有要有一定创新能力。本毕业设计资料比较欠缺，所设计要求学生进行设计计算、总装图和零部件图纸的设计，通过本毕业课题的设计有利于学生工作尽快适应工作岗位的要求设计。 主要设计参数：已知环境条件：干球温度：60 相对湿度：70%压缩机功率：2HP制冷剂：R22主要内容：热泵干燥机的设计主要是单级压缩热泵循环中蒸发器和冷凝器的设计：1、查阅资料，要求查阅相关资料，中文文献25篇以上，英文文献5篇以上，了解冷除湿机工作原理，写文献综述，并作开题报告；2、环境工况及需求分析；3、热泵循环热力计算：4、蒸发器、冷凝器的设计计算；5、图纸设计，重点在总图和各换热器的设计图纸上。一、 毕业设计（论文）图纸内容及张数 设计部分：2HP热泵干燥机的设计内容：1、零部件图纸（折1#图纸6张以上） 2、完成干燥机的设计说明书； 3、完成干燥机的设计； 二、 实验内容及要求 无三、 其他 无四、 参考文献 1 制冷技术及其应用；2 制冷原理与设备；3 工程热力学；4 传热学；5 流体力学； 六、毕业设计（论文）进程安排起讫日期设计（论文）各阶段工作内容备 注文献综述、英文资料翻译开题报告系统的设计计算图纸设计写论文，准备答辩4INTERNATIONAL 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 & Sons, Ltd. Receiwd 20 Jury 1993 Re vised 2 September 1993 606 HEAT-PUMP ASSISTED DRYER by-pass air ratio, the total mass flowrate and the vented air mass flowrate were identified as key parameters which affect the system performance. An open air cycle heat-pump dryer was reported to be more sensitive than a simple dehumidifier heat-pump dryer to ambient conditions. An experimental study to validate the mathematical model reported by Jolly et al. (1990) and Jia et al. (1990) to predict the performance of a heat-pump-assisted continuous drying system was performed by Clements et al. (1993). An increase in the relative humidity of air entering the evaporator from 30% to 80% was observed to give a twofold increase in the specific moisture extraction rate, SMER, and the optimum values were found for both the total air flowrate and the by-pass air ratio. Our present work is directed towards developing a mathematical model to study the performance of a heat-pump-assisted dryer. The objective is to obtain a simplified guide to the design and selection of all-purpose heat-pump-assisted dryers. We present a description of a generic heat-pump-assisted dryer, a theoretical model developed on the basis of phychrometric equations, and performance charts to aid the selection of heat-pump-assisted dryers. THE SYSTEM The heat-pump-assisted dryer (HPD) considered in this work is made up of seven major components, namely, a compressor, an external and an internal condenser, an expansion valve, an evaporator, an outdoor air preheater and a reheat generator. Schematic diagrams of a HPD for low- and high- temperature drying applications are shown in Figures 1 and 2, respectively. The psychrometric processes involved in low- and high-temperature drying are shown in Figures 3 and 4, respectively. In low-temperature drying (LTD) applications, air absorbs moisture from the product as it flows through the dryer. Part of the humid air from the dryer is then vented out to the atmosphere. A portion of the remaining volume is then allowed to pass over the evaporator, which acts as a dehumidifier. Here, the humid air loses a part of its moisture by condensation giving up its heat to the heat pump working fluid, The dehumidified air is then mixed with the by-passed air. This new air stream mixes with the fresh air drawn into the system, and passes over the condenser, where it is heated up by condensing working fluid. The outdoor air preheater and reheat generator are generally decoupled from the system in low-temperature applications. In high-temperature drying (HTD) applications, a portion of humid air leaving the dryer is released to the atmosphere after it is allowed to flow through the outdoor air preheater to preheat fresh air drawn into the system. Preheated fresh air passing through the reheat generator is then allowed to flow through the dryer after it mixes with recirculated air at point 8, as is shown in Figure 2. In HTD applications, heat energy available at the reheat generator is fully used by fresh air drawn into the system. THEORETICAL MODEL In an HPD, the principal parameters of consideration include the capacities of the compressor, condenser and evaporator, and the percentage of outdoor air intake and the amount of air bypassing the evaporator. The variations of the parameters are limited to a very narrow range as they are required to be matched to maintain the operating conditions of the HPD and the desired air inlet conditions to the dryer, which in turn affect the quality of the product. Hence, in the case of a HPD, it is necessary to know the performance of the HPD against variation of these parameters during the drying process. We employ a theoretical model based on basic psychrometric equations to simulate the performance of a low- temperature and high-temperature heat-pump-assisted dryers shown respectively in Figures 1 and 2. A term contact factor, commonly used in air conditioning applications (Norman, 19831, is used in the model to characterize the drying chamber, thus accounting for the heat and mass (moisture) transfer between the produce and the drying medium. S. K. CHOU ETAL. 607 EXTERNAL CONDENSER I I I I I I Figure 1. Schematic diagram of a heat-pump-assisted dryer for low-temperature applications Contact factor We define the contact factor, CF, of a dryer ah the ratio of the difference in the moisture content of air entering and leaving the dryer to that between the entering air and the air leaving the dryer at fully saturated condition, the condition line being along the constant enthalpy of entering air. Hence, A contact factor of unity represents air leaving the dryer at fully saturated condition (100% RH). Thus, the contact factor of the dryer determines the condition of air leaving the dryer. It is well known that the drying of a product is governed by complex heat and mass transfer mechanisms, which in turn are affected by parameters such as the flow velocity, temperature, humidity, direction of air flowing across the dryer, and the geometrical configuration of the dryer. The contact-factor concept is an attempt to treat the dryer as a black box. The value of the contact factor for a dryer, which is a function of the above mentioned parameters, can be obtained by performing experiments on various products changing 608 HEAT-PUMP ASSISTED DRYER - CamfeEU Qm#aR*R - l E R 0 - t G o t -Q- Figure 2. Schematic diagram of a heat-pump-assisted dryer for high-temperature applications those parameters. The experimental data of the drying rate of a product obtained for a dryer can be related to the contact factor of the dryer. The drying characteristics of various products reported in Strommen (19801, Saurez et al. (19801, Ratti and Crapiste (19921, Hawlader et al. (19911, Brunello and Claudio (1982 and Batsale and Puigalli (19851, and the corresponding values of the contact factor of the dryers are presented in Figures 5 to 10. The percentage weight loss of codfish given in Strommen (1980) for dryer air entering temperatures of 11C and 26C is used to determine the contact factor of the dryer used in Strommen (1980). For a given air flowrate across the dryer and the dryer air inlet conditions, the moisture content of air entering the dryer, wl, and the moisture content of air leaving the dryer at fully saturated condition, ( o ) , , % , are I Figure 3. Psychrometric processes in low-temperature drying S. h. CHOU ETAL. 609 0 - Outdoor air Figure 4. Psychrometrit processes in high-temperature drying obtained from the psychrometric chart. The percentage weight loss, given for every 10 hours, of codfish presented against the drying time in Strommen (1980) is used to calculate the moisture removal rate, (w2 - w,), of air leaving the dryer. The contact factor of a dryer can also be defined as the ratio of the actual moisture removal rate to the maximum possible moisture removal rate of the dryer. The actual moisture removal rate of the dryer, for a given air flowrate, is the difference between the moisture content, w2, of air leaving the dryer and the moisture content, wl, of air entering the dryer, and is obtained, in this case, from the percentage wcight loss of codfish. Whereas, the difference between the moisture content, (w2)100%, of fully saturated i r and the moisture content, ol, of air entering the dryer determines the maximum possible moisture removal rate that can be achieved in the dryer. The values of w, w2 and (02)100% of air thus computed tor every 10 hours of drying are used in equation (1) to compute the corresponding values of the contact factor of the dryer. Figure 5 shows the contact factor of the dryer thus computed, and the values of the percentage weight loss of codfish are also included for comparison. The contact factor of the dryer dccreases with the increase in the drying time, regardless of the dryer air inlet conditions. It should also bc noted that the variations in the percentage weight loss of codfish against the drying time, for air inlet conditions of 11C, 60% RH and 26C, 60% RH, are not reflected in the corresponding values of the contact factor of the dryer. The drying characteristics of soybean obtained by Saurez et af. (1980) and the predicted values of contact factor are shown in Figure 6. The drying behaviour of soybean was studied by Saurez et al. (1980) under different dryer air inlet conditions. For a given time, t, the values of w1 and (w),% obtained from the air inlet conditions, and the values of w2 predicted from the moisture content of soybean are used in equation (1) to compute the contact factor of the dryer. Both the moisture content of soybean and the contact factor of the dryer decrease with the increase in drying time. It is evident from Figure 6 that the drying rate of soybean is strongly influenced by the temperature of entering air, whereas the contact factor is least affected by the temperature of entering air. Figure 7 presents the moisture content ratio, X,/Xo, of potato slices obtained by Ratti and Crapiste (1992) for two different air inlet conditions, 45C, 50% RH and 45C, 11% RH. The values of contact factor are also shown in Figure 7. The moisture content ratio X,/Xo which is used in the prediction of the amount of moisture removed at time t, is the ratio of the moisture content of potato disks at time t and the initial moisture content of potato disks. The values of both XJX, and contact factor are strongly affected by the relative humidity of entering air, and decrease with the increase in drying time. Figure 8 presents the drying characteristics of tomatoes reported in Hawlader et al. (1991) and the corresponding values of the contact factor of the dryer. For a given dryer air inlet conditions, Hawlader et al. (1991) studied the effect of air velocity on the drying characteristics of tomatoes under dryer air inlet 610 I I I - - 11 OC, 60% RH - - 26 OC, 60% RH - 0.50 I /a HEAT-PUMP ASSISTED DRYER c I 30 27 24 ae 2 18 v) 0 I - 15 .- u7 12 U s9 6 3 0 - - - - 0,40 3 0 0 Y- 0.30 0 C 4 0.20 00 0.10 I / - 41.5 O c , 18% RH - - 53 OC, 17X RH c, d - Time Vs Contact factor d 0.50 L I I I I I I 0.00 10 20 30 40 50 60 70 80 Time, hrs Figure 5. Variations of percentage weight loss and contact factor with time for the dIying of codfish (Strommen, 1980) conditions of 80C DBT, 355C WBT and 80C DBT, 36.6 C WBT. Figure 8 presents the values of the weight loss of tomatoes obtained for air velocities of 0-4 m/s and 1.8 m/s and the corresponding values of the contact factor predicted by equation (1). As can be seen in Figure 8, the air velocity has significant impact on the weight loss of tomatoes and the contact factor of the dryer. The increase in air velocity increases the weight loss of tomatoes and decreases the contact factor of the dryer. Thus, Figures 5 and 6 present the effect of the dry bulb temperature of entering air on the contact factor of the dryer, while Figures 7 and 8 illustrate, respectively, the impact of the relative humidity of air 5.00 0 , C A3 r 0 v, 0 C a , 4.00 3.00 U 2.00 0 a , L cn 0 I 3 1.00 .- a, b - Time Vs Moisture content c, d - Time Vs Contact factor - - - d-c- 0.40 5 0.30 0.20 0.1 0 0 0 Y- 0 c 4 I I I I I 0.00 0.00 80 160 240 320 400 480 5 60 Time, min Figure 6. Variations of moisture contents and contact factor with time for the drying of soybean (Saurez et el. 1980) S. K. CHOU E T A . o, 250 vi Q) 0 * 200 ii 0 -c, 611 - 00C WT, 36.6 C WET, V, - 0.4 m / m - - WC er, 30.8 C WT, V, - 1.0 m/o - 048 - 1 0 - 0.6 2 a, b - llme Va molature content c, d - Tlme Va Contact factor - Y- 0 X x 300 - - - 1 .oo 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.1 0 1 ,o Y 0 0 . 4 : 0 0 0.2 0.0 0.00 1 I 10.40 L c. d - Time Vs Contact factor 4 0.30 00 200 300 400 500 600 Time, min Figure 7. Fractional decrease in moisture content and the variation of contact factor with time (Ratti and Crapiste, 1992) and the air velocity on the contact factor. The contact factor is least affected by the temperature of entering air, as shown in Figures 5 and 6, whereas it is strongly influenced by the relative humidity of air and the air velocity, as shown in Figures 7 and 8, respectively. This suggests that for a given product, the contact factor of a dryer is independent of the dryer air inlet temperature, and may depend on the flow velocity and the relative humidity of air entering the dryer. This means that a dryer contact factor obtained for a given dryer air inlet temperature and humidity may be used to describe the drying rate of the product under other dryer air inlet conditions having the same relative humidity. Figure 9 presents the moisture content of sorghum grains reported by Brunello and Claudio (1982) Figure 8. Decrease in total mass of tomatoes and the variation of contact factor with time (Hawlader et al. 1991) 612 c, d - Time Vs Contact factor 1 I 1 I I HEAT-PUMP ASSISTED DRYER 0.20 I i 0.60 0.1 6 Y- om - -z - - wp = 0.1959 g/g of product - - wP = 0.1963 g/g of product Air Inlet condition. - 42 C, 60% RH I c/ d a, b - Time Vs Moisture content 0.50 0.40 5 0.30 $ 0 U Y- 0 t a 0.20 8 0.10 Time, min Figure 9. Variations of moisture content and contact factor with time for sorghum grains (Brunello and Claudio, 1992) and the corresponding values of contact factor predicted by equation (1). Brunello and Claudio (1992) studied the drying behaviour of sorghum grains of different moisture contents. As expected, the moisture content of sorghum grains and the contact factor decrease with the increase in drying time. It is evident from Figure 9 that the initial moisture content of the samples of sorghum grains tested does not have significant impact on the drying rate of sorghum grains as well as on the contact factor of the dryer. In Figure 10, the experimental results of the drying characteristics of almond reported by Batsale and Puigalli (1985) and the corresponding values of the contact factor of the dryer are presented. The values of the moisture content of almond and the contact factor decrease with increasing drying time. Formulation In low-temperature applications, dry air entering the drying chamber at point 1 absorbs moisture from the product in the dryer and reaches point 2. The dry bulb temperature, T2, of humid air at point 2 can be computed by equation (1) which is a function of the contact factor. From the definition of the contact factor, where Part of the humid air (1 - X) is vented out to the atmosphere in the low-temperature application, and in high-temperature application, this air stream is passed through the outdoor air preheater to recover heat before it is released to the atmosphere. For a known compressor capacity W, and the coefficient of performance COP, the condition of air drawn into the high temperature heat pump dryer at point 8, after passing through the outdoor air preheater and reheat generator is calculated using the equations given below. The temperature of air leaving the outdoor air preheater, T, is given as (1 -XCpa (T7 - To) = (1 -X)Cpa (TI - To) T7 = (T2 - To) + To (3) S. h. CHOU ETAL. 0.40 + Air conditions - 45 C, 18% RH r C d 3 0.30 - a - Time Vs Moisture content - b - Time Vs Contact factor w- O U 5 0.20 - - 4 S 0 0 g 0.10 - - G b (n 0 .- I / I I 1 I I I I I 0.00 613 0.40 0.30 b 0.20 3 CI 0 0 Y- 0 4 5 0 0 0.10 0.00 The temperature of air leaving the reheat generator, T, is given as Q, HLG X QG The following equation recommended by ASHRAE (1989) is used to predict the temperature of air at point 8 (T8). H,-2501 0 , T8 = 1 + 1-805 w8 (4) In high-temperature calculated based on the applications the condition of air at point 6 leaving the condenser can be air conditions at point 1 and point 8, thus TI =,YT6 + (1 -X)T8 and TI - (1 -X)T, X T6 = In low-temperature applications, the condition of air leaving the condenser is the same as that of air entering the dryer. Since the condition of air leaving the condenser and the capacity of condenser are known, the condition of air entering the condenser at point 5 can be computed as Q, =X(H, -Hs) H, = H6 - (%) The temperature of air at point 5, T, predicted from a correlation presented in ASH= (1989) is given as H, - 2501 W, 1 + 1.805 w, T, =: 614 HEAT-PUMP ASSISTED DRYER The condition of air at point 4 in the low-temperature application can be predicted using the mixing stream equations as the conditions of air at point 5 and point 0 are known. Thus, T5=XT4+(1-X)To and T, - (1 - X)To X T4 = (7) In high-temperature drying applications, as there is no mixing between the dehumidified air and The evaporator capacity, Q, computed for known compressor and condenser capacities can be (8) outdoor air, the condition of air at point 4 remains the same as that at point 5. expressed as Q, =XY(H, -H3) - ( 0 2 - w ) H I The enthalpy of condensate, H3w, at point 3 can be expressed as H3w = 0-168 15 + 4*19T3 (9) The substitution of equation (9) in equation (8) yields where C, = 4.19, and C, = 0.168 15. The percentage of air passing over the evaporator, Y, can be calculated as w4-02 x 100 Y = ( w3 - ,) (11) Soluing the mathematical model For a given dryer air inlet conditions and the contact factor of the dryer, the condition of air leaving the dryer is computed by equation (2). The contact factor, which varies from zero to unity, is used to define the amount of moisture removed from the product. A contact factor of unity corresponds to the maximum possible moisture removal rate that can be achieved by the dryer, while a contact factor of zero value represents a drying process/dryer with no moisture flow from the product to the drying medium. Thus, the use of the non-dimensional contact factor simplifies the calculation procedure. The amount of air recirculated in the HPD, X, is assumed to solve this model, and the value of X, which is one of the principal parameters that determines the capacity of the system, is usually chosen based on the type of product dried and outdoor air conditions. In LTD applications, the condition of air entering the condenser is predicted by equation (6) for a known compressor capacity, W, and the coefficient of performance, COP. The known conditions of air at point 5 and 0 are used in equation (7) to predict the condition of air at point 4. In HTD applications, the generator heat losses and the effectiveness of outdoor air preheater are assumed, and equations (3) and (4) are used to calculate the condition of fresh air drawn into the system at point 8. Equation (51, which is a function of flowrates and temperatures of air at points 1 and 8, is used in the prediction of the temperature of air at point 6. As there is no mixing of air streams between points 5 and 4, unlike in LTD applications, where the outdoor and recirculated air streams are mixed at point 5, the condition of air remains unchanged at point 4. In both LTD and HTD applications, the conditions of air at points 4 and 2 thus computed are used in equation (10) to predict the temperature, T3, of air leaving the evaporator at point 3 which is then used in equation (11) to calculate the percentage of air passing over the evaporator. S. h. CHOU ETAL. 615 RESULTS A computer program has been developed to calculate the influence of important variables on the performance of the HPD. Figure 11 shows the effect of the contact factor on the moisture removal rate of the dryer for dryer air inlet conditions of 15C, 60% RH and 15C, 70% RH. The moisture removal rate increases with the increase in contact factor. It can be deduced from Figure 11 that, for a given air inlet temperature to the dryer and a fixed value of the contact factor, the moisture removal rate is a function of relative humidity of air, and decreases with the increase in the relative humidity. The change in the specific moisture extraction rate, SMER, which is defined as the ratio of the moisture removed from the dryer to the energy input to the dryer, with the increase in the moisture removal rate is presented in Figure 12. As expected, the SMER, increases with the increase in the moisture removal rate. The variation in the specific power corisumption, SPC, of the dryer with the increase in the moisture removal rate is shown in Figure 13. The SPC, is the ratio of energy input to the dryer to the moisture removed from the dryer. With the increase in the moisture removal rate, the SPC, initially starts decreasing at a faster rate, and then tends to decrease slowly with further increase in the moisture removal rate. The effects of the specific power consumption of condenser, SPC, (ratio of energy used at condenser to the moisture removed from the dryer), and specific power consumption of evaporator, SPC, (ratio of energy used at evaporator to the moisture removed from the dryer) on the moisutre removal rate of the dryer are presented in Figures 14 and 15, respectively. The values of both SPC, and SPC, decrease with the increase in the moisture removal rate of the dryer. PERFORMANCE CHART Based on a parametric analysis, a performance chart that can be used directly to select the various components of the heat-pump dryer is prepared. Two performance charts, each consisting of four modules marked (0, (2), (3) and (41, drawn for air inlet conditions at 15C and 75C, respectively, are shown in Figures 16 and 17. The charts are drawn for a heat pump dryer handling 95% recirculated air 2 .o 1.8 1.6 4 6 1.4 - 1.2 g 1.0 0.8 2 0.6 .- % 0.4 VI 0 I 3 2 0.2 - 15OC,60%RH - 15C,70%RH 0.0 - 1 I 0.0 0.2 0.4 0.6 0.8 1 .o Contact factor, dimensionless Figure 11. Effect of the contact factor on the moisture removal rate 616 HEAT-PUMP ASSISTED DRYER f $ 12*0 Y ei 10.0 e 5 8.0 2 6.0 f 4.0 E 2.0 0) 0.0 u rj 0 0 X 9) 9) u m 0 0 2 0 .- .- - 15OC, 60%RH - m o C , 70% RH I I 1 I I 1 I I I I I I 1 I I 1 I I I $ 0.0 0.2 0.4 0.6 0 8 1.0 1.2 1.4 1.6 1.8 2.0 Moisture removal rate, g/s Figure 12. Influence of the moisture removal rate on SMER and 5% outdoor air. The assumed values of other parameters are as follows (i) the coefficient of performance of HPD (ii) the effectiveness of outdoor air preheater (iii) generator heat losses :60% (iv) transmission losses :20% :8 :O-7 (from generator to compressor) 5.0 4.5 4.0 3 . 5 3.0 2.5 2.0 1.5 1 .o 0.5 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Moisture removal rate, g/s Figure 13. Influence of the moisture removal rate on SPC S. K. CHOU ETAL. 9 - I 7 6 - 617 5 - 4 - 3 - 2 - 1 - - 15C, 60% RH - 15CI70%RH OI- L I I I I I 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Moisture removal rate, g/s Figure 14. Effect of the moisture removal rare on the condenser capacity for unit moisture removal rate The module marked (1) in Figures 16 and 17, also known as the input module, presents the effect of the amount of moisture removed in the drying chamber per kilogram per second of dry air on the amount of moisture condensed per second at the evaporator. For a known moisture removal rate in the dryer, the amount of moisture condensed at the evaporator can be obtained from this module. Information on the capacity of compressor can be obtained from module (2), or the compressor module in which the specific power consumption, SPC, of the heat-pump dryer is related to the amount of condensate at the evaporator. Hence, the compressor capacity can be read directly from this module based on the amount 0 I I 1 I , I 1 I I 1 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Moisture removal rate, g/s Figure 15. Effect of the moisture removal ratc on the evaporator capacity for unit moisture removal rate 618 HEAT-PUMP ASSISTED DRYER of condensate at the evaporator. Module (3) or the evaporator module provides information on the evaporator capacity as it shows the relation between the specific power consumption, SPD, and the evaporator capacity. Module (4) or the condenser module relates the condenser and evaporator capacities. For a given evaporator capacity and condensate at the evaporator obtained from module (l), the condenser capacity can be obtained from this module. The procedure for the selection of the heat pump components of the dryer is as follows: (1) Firstly, for a given product to be dried, the amount of moisture to be removed per kilogram per second of dry air is to be chosen. (2) In module (0, the amount of moisture removed per kilogram per second of dry air is presented against the amount of moisture condensed at the evaporator for various dryer air inlet conditions. So, for a known moisture removal rate per kilogram per second of dry air, a vertical line has to be drawn to point (a) to select the dryer air inlet conditions. From point (a), a horizontal line drawn to the y-axis of module (1) determines the amount of moisture condensed at the evaporator. Amount of energy used ot condenser per grorns o f moisture removed per second In the dryer. kW/gome I t.- atolrm t- I ,st olr M a t n m -lpo -m -= 1 -za y - k P F -m -%la 1: - l a -im 9 0 fb Pa -M 6 & ID - I -QLI -oo 10 Speclflc power consumptlon o f dryer. kW/grams &oms o f molehrre removed In the dryer per kg per aec of dry alr Figure 16. Performance chart for TI at 15 “C S. K. CHOU ETAL. 619 Amount of energy used at condenser per grams of moisture renoved per second In the dryer, kWlgroms Y UYYT U u Y YL 0 I 0 I I I I 75;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