Modelling of Specific Moisture Extraction Rate and Leakage Ratio in a Condensing Tumble Dryer

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1 Modelling of Specific Moisture Extraction Rate and Leakage Ratio in a Condensing Tumble Dryer Lena Stawreberg, Lars Nilsson To cite tis version: Lena Stawreberg, Lars Nilsson. Modelling of Specific Moisture Extraction Rate and Leakage Ratio in a Condensing Tumble Dryer. Applied Termal Engineering, Elsevier, 2010, 30 (14-15), pp < /j.appltermaleng >. <al > HAL d: al ttps://al.arcives-ouvertes.fr/al Submitted on 16 Jan 2012 HAL is a multi-disciplinary open access arcive for te deposit and dissemination of scientific researc documents, weter tey are publised or not. Te documents may come from teacing and researc institutions in France or abroad, or from public or private researc centers. L arcive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recerce, publiés ou non, émanant des établissements d enseignement et de recerce français ou étrangers, des laboratoires publics ou privés.

2 Accepted Manuscript Title: Modelling of Specific Moisture Extraction Rate and Leakage Ratio in a Condensing Tumble Dryer Autors: Lena Stawreberg, Lars Nilsson P: S (10) DO: /j.appltermaleng Reference: ATE 3123 To appear in: Applied Termal Engineering Received Date: 21 October 2009 Revised Date: 21 April 2010 Accepted Date: 24 May 2010 Please cite tis article as: L. Stawreberg, L. Nilsson. Modelling of Specific Moisture Extraction Rate and Leakage Ratio in a Condensing Tumble Dryer, Applied Termal Engineering (2010), doi: / j.appltermaleng Tis is a PDF file of an unedited manuscript tat as been accepted for publication. As a service to our customers we are providing tis early version of te manuscript. Te manuscript will undergo copyediting, typesetting, and review of te resulting proof before it is publised in its final form. Please note tat during te production process errors may be discovered wic could affect te content, and all legal disclaimers tat apply to te journal pertain.

3 Modelling of Specific Moisture Extraction Rate and Leakage Ratio in a Condensing Tumble Dryer Lena Stawreberg 1*, Lars Nilsson 2 1 Department of Energy, Environmental and Building Tecnology, Karlstad University, SE Karlstad, Sweden, 2 Department of Cemical Engineering, Karlstad University, SE Karlstad, Sweden, *Corresponding autor. Tel: ; fax address: lena.stawreberg@kau.se (L. Stawreberg) ABSTRACT Te use of tumble dryers in ouseolds is becoming more common. Tumble dryers, owever, consume large amounts of electric energy. A statistical model over te tumble dryer is created from a design of experiments. Te model will be used to find te best settings for te power supply to te eater, te internal airflow and te external airflow in order to reac a ig specific moisture extraction rate (SMER) and a low leakage ratio of water vapour. Te aim also involves explaining te trends of te SMER and te leakage ratio based on te pysics of te tumble dryer drying process. A statistically significant model, wic can be used for improving te SMER and leakage ratio, was establised from 19 experiments in a condensing tumble dryer. Te results sow tat a ig power supply to te eater, a ig internal airflow and a low external airflow give te igest SMER values. Tis combination of settings also results in te largest leakage ratio values for te dryer. Leakage is most affected by te external airflow. 1

4 Keywords: design of experiments, energy efficiency 1. ntroduction Tumble dryers are common devices in ordinary ouseolds. Tey offer a fast and convenient way of drying textiles, independent of weater conditions. Teir disadvantage, owever, is teir large consumption of energy, i.e. electricity. Te energy use of one unit is small considering te large number of existing dryers and ow often tey are used, te total energy use is considerable. Since 1990, te clotes drying energy consumption in te OECD countries as risen by 32.7% to 77.1TW in 2000, nternational Energy Agency [1]. t is terefore igly desirable to reduce te energy use of tese products. Tere are mainly two types of dryers available on te market today; te venting tumble dryer and te condensing tumble dryer. Te condensing tumble dryer is becoming more frequently used in ouseolds. Tis type of dryer as te advantage tat all eat supplied to te dryer eventually ends up in te laundry room. Tis is especially beneficial in te Scandinavian climate. Additionally, it does not require ducting to exaust te moist air to te outside making it easy to install. Tere is, owever, a leakage of water vapour from te internal system of te dryer, wic sould be minimised in order to avoid ig umidity in te laundry room. Te tird type of dryer available is equipped wit a eat pump. According to Braun et al. [2] and Bansal et al. [3], te air cycle eat pump dryer was found to be significantly more efficient tan te traditional tumble dryer. Te cost of tis dryer, owever, is considerably iger tan te cost of a traditional tumble dryer. 2

5 A few studies are found in te literature in wic te condensing tumble dryer as been investigated regarding its energy use. According to Bansal et al. [3], te energy efficiency of a condensing tumble dryer is 7% iger tan tat of te venting tumble dryer. Tis was determined from a computer model of te dryer. Conde [4] and Hekmat and Fisk [5], owever, state tat te condensing tumble dryer as an increased energy use compared wit te venting tumble dryer. Conde [4] used a computer model in combination wit experiments, wereas te investigation made by Hekmat and Fisk [5] was based on measurements. Cocran et al. [6] found a significant improvement in te energy efficiency of te condensing tumble dryer by incorporating surface tension elements instead of te traditional air to air eat excanger. According to Liu et al. [7], te optimal performance of a condensing tumble dryer using termoelectric elements instead of te air to air eat excanger strongly depends on te intensities of te drying air temperature, te electric power and te total weigt of te clotes. n order to improve te performance of te condensing tumble dryer, a model of te process can be useful. Teoretical models ave been made by Deans [8], Lambert et al. [9], Yadav and Moon [10], and A Bing and Siming [11] regarding te venting tumble dryer. Due to te recycling of air, a more complex teoretical model is required to describe te drying process of te condensing tumble dryer as compared to te venting tumble dryer. Te components of te condensing tumble dryer are densely placed, wic makes it difficult to perform accurate measurements in te dryer, measurements tat could be used for validating a teoretical model. Tis speaks for te use of a design of experiments, i.e., a metod tat provides a way to create a statistical model of te 3

6 process using a low number of experiments. Te design of experiments as been found to be a useful tool for studying drying processes, and especially for making improvements to existing dryers. Tis metod was used for optimising a combined drying process (infrared and infrared-convective drying) of a porous medium in a study carried out by Dutournie et al. [12]. Tey performed experiments wit four factors and tree responses, and tey cose a full factorial central composite design. Dutournie et al. [12] concluded tat te use of experimental designs could be a powerful metod for obtaining optimal operating conditions for a dryer, especially in combined drying. Te response surface metod as been applied by Teppaya and Prasertsan [13] to optimise te drying of rubber wood, by Madamba and Lopez [14] to optimise te osmotic deydration of mango slices and by Hammami and Rene [15] to determine te freeze-drying process variables for strawberries. Nitz and Taranto [16] investigated te drying of beans in a pulsed bed dryer using an experimental design wose results were used for creating a model of te process. An experimental design was also performed by Lopes da Cuna et al. [17], wo studied te drying of mango powder in a spout fluidised bed dryer. Te aim of tis study is to use a design of experiments in order to create a statistical model over te condensing tumble dryer. Te model will be used to find te best settings for te power supply to te eater, te internal airflow and te external airflow in order to reac a ig specific moisture extraction rate (SMER) and a low leakage ratio. Te aim also involves explaining te trends of te SMER and te leakage ratio based on te pysics of te tumble dryer drying process. 4

7 2. Material Te condensing tumble dryer consists of te main components as sown in Fig. 1, i.e., a rotating drum containing te wet textiles, two fans, a eat excanger and an electric eater. Te drying process in a condensing tumble dryer can be described in terms of tree stages considering te drying air: umidification, deumidification and eating. Humidification of te air takes place inside te drum wen te internal airflow encounters te wet textiles. Te temperature of te air decreases as te umidity of te air increases. Te air is deumidified in te eat excanger wic is cooled wit room air. Te umidity of te air is reduced and te temperature is lowered. Finally, te drying air is eated in te electric eater at constant umidity leading to a reduction in relative umidity. Te drum as an inner diameter of 570 mm and a lengt of 390 mm. Te size of te cross flow plate eat excanger is 230 mm wide, 90 mm ig wit a lengt of 320 mm and consists of 10 orizontal plates. Four different periods are recognised during te drying cycle of a condensing tumble dryer: (1) te eating period, (2) te constant drying rate period, (3) te falling drying rate period, and (4) te cooling period. n te eating period, te wet textiles are eated until an equilibrium temperature is reaced and te constant drying rate period begins. Te drying rate remains constant until te water at te surface of te textiles as evaporated. During te falling drying rate period, wen water is transported from witin te textiles to te surface, te temperature of te drying gas leaving te drum increases. Wen te textiles ave reaced te desired moisture content according to te cosen drying program, te cooling period begins during wic time no power is supplied to 5

8 te eater of te dryer. 3. Metod 3.1. Statistical model for SMER and leakage ratio A design of experiments was used to identify significant factors and to determine ow tey interact. Te power supply to te eater, te internal and te external airflows were studied as factors affecting te performance of te dryer. Te experiments were laid out in a symmetrical fasion around five standard reference experiments. Eac factor was varied between a maximum and minimum value as sown in Table 1. Te software used for te study was MODDE 7.0. Te responses were: (1) te specific moisture extraction rate SMER [g/kj] determined by SMER m m Q Wi Wo =, (1) tot and (2) te leakage of water expressed in percentage of te total evaporated mass of water according to L V = m Wi m Wo m C m Wi m Wo. (2) Te SMER sows te ratio between te amount of evaporated water and te total energy supply. A ig SMER would indicate an efficient drying process wit low 6

9 energy losses. Te leakage of water from te tumble dryer is an important parameter, bot for reducing energy losses from te process and for preventing a umid environment in te laundry room. For tis study, a quadratic regression model was cosen wit a central composite facecentred (CCF) design. Tis is a standard design supporting a quadratic model describing linear relations, quadratic relations and interactions between te factors according to (Eriksson et al. [18]): 2 y = β0 + β1q& + β2v& + β3v& E + β11q& + β22v& + β Q& V& + β Q& V& + β V& V& + ε E 23 E 2 + β V& 2 33 E (3) n tis investigation, 19 test runs were made including five standard reference experiments. All tests were performed wit 4 kg of dry cotton (conditioned in room air at 13 C wit te air relative umidity at 31%) wetted to a moisture content of 60% (db). Te weigt of te drying load was registered before and after eac test run. Te weigt of te condensate was also registered. Temperatures of te air in te internal system were measured every second during te process. Mean values from ten seconds were registered. Temperature sensors wit an accuracy of ± 0.5 C were placed between te eater and te drying drum, T, next to te fan tat transports te internal airflow, T f, and between te eat excanger and te 7

10 eater, T x, see Fig. 1. Te temperatures of te inlet and te outlet external airflow, T HXi and T HXo (also indicated in Fig. 1.), and te temperature of te condensed water leaving te dryer, T C, were measured. Te power supply to te eater and tat to te motor rotating te drum and te fans of te dryer was registered for eac test run. Mean values over ten seconds were measured wit an accuracy of ± 1%. Te power supply to te eater was adjusted by on-off regulation wit a set target value. Te mean value of te measured power supply during eac test was used in te analysis. All power supplied to te eater was assumed to increase te temperature of te internal airstream. Te power supply to te motor rotating bot fans and te drum was 145W. Tree different impellers were used to regulate te internal airflow according to te values in te experimental design set at 88m 3 /, 77m 3 / and 66m 3 /, respectively. Te internal airflow over te electric eater was determined from tests performed wit a dry load according to Q& V& =. (4) ρ c T T ) p ( x Te external airflow was reduced by covering te inlet duct and it was determined from te assumption tat te energy loss from te internal airflow passing troug te eat excanger was transferred completely to te external airflow. As te tests were performed wit a dry load, te external airflow was determined by 8

11 V& ρ ( T f Tx ) V& E =. (5) ρ ( T T ) E HXo HXi Te external airflow in te original dryer was 155 m 3 /. By covering te inlet duct te external airflow was reduced to 113 m 3 / and 72 m 3 / Pysical model for SMER Te following assumptions were made in te pysical model over te drying process in te condensing tumble dryer: relations are determined only for te constant drying rate period; te entalpy of te drying air is constant over te drum, = f ; te relative umidity of te air leaving te drum is 100%; te relative umidity remains at 100% trougout te eat excanger; te internal mass flow of dry air is constant trougout te internal system; and te eat excanger is considered infinitely large. n a condensing tumble dryer all parameters depend on eac oter, wic made it necessary to find a starting point for te calculations. An internal airflow temperature between te eat excanger and te eater, T x, was terefore set to a value representing a test run in te dryer. Tis value for T x was later verified by a calculated value produced from material and energy balances for all components in te system. Te fundamental equations used in te model can be found in Pakowski and Mujumdar [19]. Te umidity of te air between te eat excanger and te eater, Y x, was calculated 9

12 from a correlation for air saturated wit water vapour, and te entalpy, x, was determined by x = c T + Y ( c T ) 0. (6) pa x x pv x + Te mass flow of dry air in te internal system was determined by V& ρ & =, (7) Y +1 m A x were te density of te umid air was determined from tabled values of pressure and temperature at a relative umidity of 100% and te ideal gas relation. Te umidity of te air remains constant over te eater, and te temperature of te air after te eater, T, was determined from T = m& A ( c p Q& + c A p V Y x + T ) x. (8) Te entalpy, = f, was determined from te temperature and te umidity of te air between te eater and te drum similarly to equation (6). Te umidity of te air leaving te drum, Y f, was calculated from a correlation for air saturated wit water vapour. 10

13 Te eat transfer of te eat excanger was determined bot from te internal and te external airflow. Te eat transfer gained by te external airflow over te eat excanger was Q& = V& ρ c T T ). (9) HX E E p ( HXo HXi Te room air temperature was set to 20 C and te eat excanger was assumed to be infinitely large. Tus, te temperatures of te external and te internal airflow leaving te eat excanger ad te same value. Te eat transfer from te internal airflow was Q & = m& [ + ( Y Y ) c T ]}. (10) x { f x f x p x A W Te condensed water leaving te eat excanger was assumed to ave te same temperature as te internal airflow leaving te eat excanger. n te ideal process, te internal and external eat transfer sould be equal as tere would be no eat losses or leakages. However, to be able to relate te results from te model to te investigated tumble dryer, te ratio of te external eat transfer to te internal eat transfer was set to 0.78 in te model due to eat losses and leakages. Tis value was cosen to give te temperatures in te model a value not exceeding C off te measured values of T x. n te ideal drying process te SMER for te constant drying rate period is determined by 11

14 Y Y SMER & A & f x = m. (11) Q 3.3. Leakage ratio Te leakage ratio depends on te umidity of te internal airflow, te pressure difference between te internal system and te surroundings as well as on te size of te cavities. Te area of te cavities will not vary between te different tests implying tat tere sould be a correlation between te umidity, te static pressure and te leakage ratio. By using te measured temperature between te drum and te eat excanger in combination wit te assumption of 100% relative umidity at te drum outlet in te dryer, te umidity of te airflow was determined from a Mollier cart. Te influence of te pressure differences on te leakage was not furter investigated in tis study. As a first approximation, ten, leakage ratio depends on te umidity of te circulating air. 4. Results Te results from te tests performed in te condensing tumble dryer are sown in Table 2. Te mean value of te measured power supply to te eater deviates sligtly from te target values in te experimental design and as terefore been adjusted to te measured value. Te measured values of te leakage ratio deviated from te normal distribution and were terefore logaritmically transformed. A normal distribution of te response is required in te regression analysis. Te statistical model sows good significance. Wen interpreting te results from te model, it is important to point out tat te deviation between te target values of te power supply to te eater and te actual 12

15 measured values will make te model unsymmetrical. Tis can be studied troug te condition number, wic was 3.7 for tis study. For tis type of design, it sould be in te range of 3 8 according to Eriksson et al. [18]. Tis would mean tat te model is acceptable. Anoter important issue is te interval cosen for te experiments, wic is below te settings used for te original dryer. Results from te five standard reference experiments sow a good precision as te mean value of SMER was ± 0.002g/kJ and te leakage ratio was 37 ± 1.3%. nteractions including te external airflow ( V & V& and V & E Q& ) and te quadratic 2 E coefficient of te external airflow ( V & ) were statistically insignificant in te description of SMER. Te correlation between te investigated factors and SMER is terefore, according to equation (3), described by E SMER = Q& V& V& E Q& V& Q& V&. (12) No interactions were significant in te correlation for te leakage ratio and only te quadratic coefficient of te power supply to te eater was significant. Te logaritmically transformed leakage ratio is, according to equation (3), described by L V = 10 exp( Q& V& V& E Q& ). (13) 2 An ANOVA test sows tat te regression models are significant and statistically good for bot SMER and leakage ratio. Bot te goodness of fit (R2) and te goodness of 13

16 prediction (Q2) are ig. For SMER, R2 is and Q2 is For leakage, R2 is and Q2 is A confidence level of 95% was used. Tere was no lack of fit for any of te models. Te model error is in te same range as te pure error and te model sows good fit to te data. n Fig. 2, te SMER of different power supplies and different external and internal airflows is sown in a surface response plot. Low external airflow and ig power supply to te eater gives te igest SMER value. t can also be seen tat an increased internal airflow also increases te SMER, especially at ig power supply to te eater. Low internal airflows in combination wit ig external airflows give te lowest values for te leakage ratio. Te power supply to te eater as a minor effect on te response, as sown in Fig. 3. Te SMER as determined from te pysical model over te tumble dryer is sown in Fig. 4 plotted against te values from te statistical model. Te relation between te pysical model and te statistical model generated from MODDE can be described by SMER Pysical =1.17SMER Statistical (14) wit R 2 =

17 Based on te assumption tat te umidity of te internal airflow will affect te leakage of water from te dryer, te leakage ratio and te umidity of te internal airflow exiting te drum are plotted against te external airflow in Fig. 5. Te leakage ratio presented in te figure is determined from te statistical model, and te umidity is determined from measurements of temperature in te dryer. Te relative umidity of te air exiting te dryer was assumed to be 100%. An increased external airflow leads to a decrease in te umidity of te air and tereby also to a decreased leakage ratio. 5. Discussion Te results from te statistical model sow tat it is not possible to find settings tat simultaneously give a low leakage ratio and a ig value of SMER for te investigated tumble dryer. Bot te external airflow and te power supply to te eater affect te SMER strongly. n Fig. 2 it can also be seen tat tere is a significant interaction between te power supply to te eater and te internal airflow; te iger te power supply, te more a cange in internal airflow affects te SMER. Tis could be explained by te increased temperature in te tumble dryer due to a ig power supply to te eater, wic will lead to increased moisture contents in te internal airflow. An increased internal airflow will terefore be required in order to transport te evaporated moisture from te textiles. According to te results from te statistical model, te leakage ratio as a strong correlation wit te external airflow. A ig external airflow gives a low leakage ratio. n Fig. 3 it can be seen tat bot a low and a ig power supply to te eater give approximately te same leakage. Tis can be a result of te required drying time. A low 15

18 external airflow togeter wit a low power supply to te eater will result in a longer drying time. f te process is long, larger leakages can be found if te umidity of te internal airflow is ig. t can, owever, also be a result of te effectiveness of te eat excanger related to an increased power supply to te eater. f te cooling of te eat excanger becomes too low as te umidity of te internal airflow increases, less water will condense in te eat excanger. Te efficiency of te eat excanger is an important factor, wic as also been pointed out by Cocran et al. [6] and Liu et al. [7]. Canges in te internal airflow will increase or reduce te leakage ratio similarly, independent of external airflow or power supply to te eater. Tis indicates tat canges in te internal airflow will cange te static pressure in te internal system. An increased pressure resulting from a iger airflow will lead to an increased leakage ratio. Pressure differences in te system sould be furter investigated. Te pysical model presented in tis study is a simplified version using ideal assumptions, for te beaviour of te components and is limited to modelling te conditions during te constant drying rate period. According to Fig. 4 it can be determined tat, trougout te study, te values for SMER from te pysical model are iger tan te ones from te statistical model. t seems to be possible to use a simple ideal model in order to see a trend, but not for acieving accurate values. Te pysical model sows ow te umidity, togeter wit te temperature, will affect te SMER, assuming drying takes place in te period of constant drying rate. t can be seen tat tere is a correlation between te two models te iger te temperature and umidity, te iger te SMER. Terefore, it can be concluded tat te constant drying rate period as a large influence on te drying process in te condensing tumble dryer. 16

19 As te pysical model is created from an ideal process, te result from tis study sows tat te performance of te components in te dryer mainly follows te ideal process in te investigated interval. Te pysical model, owever, will not give any information about wic factor is te most important. t sould also be pointed out tat bot te presented models are valid for a specific interval and for a specific tumble dryer. n Fig. 5 it is sown tat an increased external airflow will lead to decreased umidity as well as to a reduced leakage ratio. Tis agrees wit results from bot models: te statistical model sows tat te leakage ratio decreases wit an increasing external airflow, wereas te pysical model sows tat increasing external airflow (better cooling) leads to a decreased umidity of te circulating air. Unfortunately tis poses a problem wen trying to combine a ig SMER and a low leakage. A iger SMER is acieved as te umidity of te circulating airflow is increased. Tis, owever, directly leads to iger leakage, wic is undesirable in te laundry room. Te results from tis study imply tat te use of a statistical model is a good coice wen studying an existing dryer. Te presented model can be used for improving te performance of te tumble dryer by finding profitable settings, but also for improved knowledge about te process. Wen investigating a different type of tumble dryer, a new design of experiments sould be made to detect differences between te dryers regarding sealings, te efficiency of te various components, etc. t is difficult to point out te best settings for te dryer as regards te igest possible SMER and a low leakage ratio, as tey do not correlate. Today, te focus of te 17

20 manufacturers primarily lies on increasing te SMER. t is important to reduce te energy use considering te energy classifications on te market. Tere is still no official classification system regarding leakage, but te figures are neverteless often sown in market tests. Leakage is tus an important parameter to reduce considering te environment in te laundry room. 6. Conclusions A design of experiments performed in MODDE gives a statistically significant model tat can be used as a tool for improving te SMER and reducing te leakage from te condensing tumble dryer. A ig power supply to te eater, a ig internal airflow and a low external airflow give te igest SMER value. Bot power supply to te eater and te external airflow ave a large effect on te SMER according to te model presented by MODDE. A simplified ideal model confirmed tat calculations made over te constant drying rate period are representative for te drying process in identifying trends of ow SMER canges, but tey are not sufficient to provide accurate values. Te leakage ratio is most affected by te external airflow. Tere is a correlation between te moisture content of te air in te internal system and te leakage ratio at different external airflows. A ig moisture content leads to increased leakage. 18

21 Nomenclature c p specific eat capacity [kj/kg C] entalpy [kj/kg] 0 evaporation entalpy, 0 C [kj/kg] L V leakage of water vapour [kg] m mass [kg] m& mass flow [kg/s] Q tot total energy supply [kj] Q & eat transfer [W] SMER specific moisture extraction rate [g/kj] T temperature [ C] V & volume flow [m 3 /s] Y umidity [kg/kg] ρ density [kg/m 3 ] Subscripts A C cover E f x HX i o V W air condensate outer cover of te dryer external between fan and eat excanger (internal system) between eater and drum (internal system) between eat excanger and eater (internal system) eat excanger external airflow (external system) internal inlet outlet vapour water 19

22 References [1] nternationalenergyagency, Cool Appliances - Policy Strategies for Energy Efficient Homes, EA Publications, Paris, [2] J.E. Braun, P.K. Bansal, E.A. Groll, Energy Efficiency Analysis of Air Cycle Heat Pump Dryers, nternational Journal of Refrigeration 25 (7) (2002) [3] P.K. Bansal, J.E. Braun, E.A. Groll, mproving te Energy Efficiency of Conventional Tumbler Clotes Drying Systems, nternational Journal of Energy Researc 25 (15) (2001) [4] M.R. Conde, Energy Conservation wit Tumbler Drying in Laundries, Applied Termal Engineering 17 (12) (1997) [5] D. Hekmat, W.J. Fisk, mproving te Energy Performance of Residential Clotes Dryers, Te 35t Annual nternational Appliance Tecnical Conference, Oio State University, [6] M. Cocran, J. Goodnigt, B. Babin, S. Eckels, Condensing Dryers wit Enanced Deumidification Using Surface Tension Elements, Applied Termal Engineering ) [7] D. Liu, F.-Y. Zao, G.-F. Tang, Modeling and Performance nvestigation of a Closed-Type Termoelectric Clotes Dryer, Drying Tecnology ) [8] J. Deans, Te Modelling of a Domestic Tumbler Dryer, Applied Termal Engineering 21 (9) (2001) [9] A.J.D. Lambert, F.P.M. Spruit, J. Claus, Modelling as a Tool for Evaluating te Effects of Energy-Saving Measures. Case Study. A Tumbler Drier, Applied Energy 38 (1) (1991) [10] V. Yadav, C.G. Moon, Modelling and Experimentation for te Fabric-Drying Process in Domestic Dryers, Applied Energy ) [11] N. A Bing, D. Siming, A New Termination Control Metod for a Clotes Drying Process in a Clotes Dryer, Applied Energy 85 (9) (2008) [12] P. Dutournie, P. Salagnac, P. Glouannec, Optimization of Radiant-Convective Drying of a Porous Medium by Design of Experiment Metodology, Drying Tecnology 24 (8) (2006) [13] T. Teppaya, S. Prasertsan, Optimization of Rubber Wood Drying by Response Surface Metod and Multiple Contour Plots, Drying Tecnology 22 (7) (2004)

23 [14] P.S. Madamba, R.. Lopez, Optimization of te Osmotic Deydration of Mango (Mangifera ndica L.) Slices, Drying Tecnology 20 (6) (2002) [15] C. Hammami, F. Rene, Determination of Freeze-Drying Process Variables for Strawberries, Journal of Food Engineering 32 (2) (1997) [16] M. Nitz, O.P. Taranto, Drying of Beans in a Pulsed Fluid Bed Dryer: Drying Kinetics, Fluid-Dynamic Study and Comparisons wit Conventional Fluidization, Journal of Food Engineering 80 (1) (2007) [17] R. Lopes da Cuna, A.G. de la Cruz, F.C. Menegalli, Effects of Operating Conditions on te Quality of Mango Pulp Dried in a Spout Fluidized Bed, Drying Tecnology 24 (4) (2006) [18] L. Eriksson, E. Joansson, N. Kettane-Wold, C. Wikström, S. Wold, Design of Experiments Principles and Applications, Umetrics AB, Umeå, [19] Z. Pakowski, A.S. Mujumdar, Basic Process Calculations and Simulations in Drying, in: A.S. Mujumdar(ed.) Handbook of ndustrial Drying, Vol. Taylor & Francis Group, Pp

24 Table 1. Experimental design levels of te factors. Factor Low Centre Hig Power supply, Q & 1400 W 1550 W 1700 W nternal airflow, V & 66 m 3 / 77 m 3 / 88 m 3 / External airflow, V & E 72 m 3 / 113 m 3 / 155 m 3 /

25 Table 2. Tests performed in te condensing tumble dryer wit varying values of power supply to te eater, internal airflow and external airflow. Te measured values of SMER and leakage ratio are presented. Experiment Factors Responses number Q & V & V & SMER L E V [W] [m 3 /] [m 3 /] [g /kj] [%]

26 Heater (1.9 kw) T T x Heat excanger Drum T f T HXi nternal airflow (88 m 3 /) T HXo External airflow (155 m 3 /) Fig. 1. Scematic description of a condensing tumble dryer. Temperature sensors are indicated.

27 Fig. 2. Variations of SMER at different external airflows and power supplies to te eater according to te statistical model. Te two surfaces represent two different internal airflows.

28 Fig. 3. Variation of te leakage ratio at different external airflows and power supplies to te eater according to te statistical model. Te two surfaces represent two different internal airflows.

29 SMER -pysical model [g/kj] 0,425 0,415 0,405 0,395 0,385 0,375 0,365 0,355 0,345 0,335 0,325 0,315 0,305 0,295 0,285 0,275 0,265 0,255 0,245 0,235 0,235 0,24 0,245 0,25 0,255 0,26 0,265 0,27 0,275 SMER -statistical model [g/kj] Fig. 4. SMER determined from te pysical model, plotted against SMER determined from te statistical model.

30 60% 0,25 Leakage ratio [%] 50% 40% 30% 20% 10% 0,2 0,15 0,1 0,05 ] Humidity at te drum outlet [kg/kg Humidity at te drum outlet [kg/kg] Leakage ratio Humidity 0% External airflow [m 3 /] Fig. 5. Leakage ratio and umidity of te internal airflow exiting te drum, plotted against te external airflow. Te leakage ratio determined from te statistical model, and te umidity determined from measurements.