翻译原文QCX.pdf

This article was downloaded by East China Normal University On 15 January 2014 At 18 09 Publisher Taylor 1532 2300 Online Copyright Continuous drying Heat pump Low temperature drying SMER INTRODUCTION Drying is an energy intensive operation Jay and Oliver 1994 estimated that energy used in drying operations was about 20 of the total energy used for industrial production in the U K Thermal drying is often conducted at high temperatures However many agricultural and food materials specialty crops are sensitive to high temperatures A drying system that is both energy effi cient and preserves product quality is desired Drying systems incorporating a dehumidifi cation cycle have been developed that conserve energy and handle the material gently Alves Filho and Strommen 1996 Sosle et al 2000 These dryers operate using heat pumps where sensible and latent heats are recovered from the exhaust air The heat is then recycled back through the dryer by heating the air entering the dryer using the heat pump condenser The objective of this paper is to test the use of small inexpensive house hold type dehumidifi ers for drying high moisture material Alfalfa that was readily available was used to evaluate the performance of the drying system EXPERIMENTAL SYSTEM AND TEST PROCEDURE Re circulating Cabinet Dryer Two dryer setups were considered In the fi rst setup a cabinet dryer consisting of a chamber and a dehumidifi er were assembled Fig 1 The chamber was partitioned into left and right side compartments with air circulating through the compartments in a clockwise direction Five trays were stacked in the left compartment to support the drying material A household type dehumidifi er was placed in the right compartment for air conditioning The dehumidifi er had a condenser and an evaporator coil which provided the heating and cooling dehumidifi cation for the process air in the dryer 1674ADAPA SOKHANSANJ AND SCHOENAU Downloaded by East China Normal University at 18 09 15 January 2014 2002 Marcel Dekker Inc All rights reserved This material may not be used or reproduced in any form without the express written permission of Marcel Dekker Inc MARCEL DEKKER INC 270 MADISON AVENUE NEW YORK NY 10016 A fan was built into the dehumidifi er which was used to drive the air through thecoils Thefanalsoprovidedairfl owforthere circulatingloop Thewallsof the chamber were insulated with 51mm thick fi berglass insulation The doors of the cabinet were sealed to avoid air leaks using foam rubber strips Six preliminary experiments were performed using the arrangement of equipment shown in Fig 1 setup 1 The results obtained and the operating conditions of this dryer led to a modifi ed dryer setup Fig 2 The new setup setup 2 also had left and right side compartments similar to the fi rst setup Nine trays were stacked in the left compartment while two dehumidifi ers of the same capacity were stacked in the right compartment An improved layout of instrumentation was done to record appropriate parameters instrumentation will be discussed in the next section The household dehu midifi er specifi cations are given in Table 1 Instrumentation Figure1showsthelocationswheretemperatureandrelative humidity RH readings were recorded using LCD digital hygrometer Figure 1 Block diagram of the experimental dehumidifi er system showing one dehumidifi er PERFORMANCE OF RE CIRCULATING CABINET DRYER1675 Downloaded by East China Normal University at 18 09 15 January 2014 2002 Marcel Dekker Inc All rights reserved This material may not be used or reproduced in any form without the express written permission of Marcel Dekker Inc MARCEL DEKKER INC 270 MADISON AVENUE NEW YORK NY 10016 Modelno 3309 50 Cole ParmerInstrumentCompany Chicago IL 60648 USA Temperature and RH relative humidity readings were taken at the entrance and exit of the stacked trays and between the con denser and evaporator coils Atmospheric temperature and RH values were also measured using this instrument A hot wire anemometer was used to measure the velocity of process air between trays 4 and 5 the bottom tray was considered as tray 1 It was assumed that the air velocity would be Figure 2 Block diagram of the experimental dehumidifi er system showing two dehumidifi ers Table 1 Heat Pump Dehumidifi er Specifi cations Marketed by Canadian Tire CorporationTMModel 43 5404 8 D1 Refrigerant usedR134a Power rating424W High side pressure condenser 2 03MPa Low side pressure evaporator 0 97MPa Dehumidifying capacity of each heat pump9 5L of water per day Internal fan model no IS 4420WCA1125rpm 1676ADAPA SOKHANSANJ AND SCHOENAU Downloaded by East China Normal University at 18 09 15 January 2014 2002 Marcel Dekker Inc All rights reserved This material may not be used or reproduced in any form without the express written permission of Marcel Dekker Inc MARCEL DEKKER INC 270 MADISON AVENUE NEW YORK NY 10016 relatively uniform at that point The mass of moisture condensed at the evaporator and crop material dried was measured using two electronic weighing scales All the measurements were taken and recorded manually Figure 2 shows the location of thermocouples installed to measure air temperature at the entrance and exit from the stacked trays and in the space between the condenser and evaporator coils in setup 2 Thermocouples were also mounted on the evaporator and condenser coils to measure the pipe surface temperatures The ambient air relative humidity was measured using the wet bulb and dry bulb temperature sensors Two RH sensors were located one at the air entrance and the other at the air exit from the trays to measure the relative humidity A hot wire anemometer was used to measure the velocity of process air between the trays The velocity measurements were done in between tray 7 and tray 8 the bottom tray was counted as tray 1 It was assumed that the air velocity would be relatively uniform at that point Instantaneous power was measured by a wattmeter The mass of the moisture condensed at the evaporator and crop material dried was measured using a load cell and an electronic weighing scale respectively Two ports one for inlet air and the other for outlet air were made These ports were used to maintain a constant temperature steady state condition inside the drying chamber by periodically opening and closing them The opening and closing of the inlet and outlet ports at the top of the drying chamber was done manually using butterfl y valves The mass fl ow rate of air at the inlet port was measured using a Pitot tube Temperatures relative humidity and the mass of water condensed were monitored continuously using a data logger Details of all the instrumentation used are provided in Table 2 Due to the practical diffi culties in installing a fl ow sensor in the heat pump the measurement of the refrigerant mass fl ow rate was not done The refrigerant mass fl ow rate was calculated as a part of a larger simulation model Adapa 2001 Experimental Procedure Experiments were performed with batch drying fi xed tray and conti nuous bed moving tray drying The alfalfa was chopped to a size of 50mm Using an electronic balance 0 14kg and 0 40kg of alfalfa was weighed and spread uniformly over trays for setup 1 and setup 2 respectively The thickness of the layer was 20 30mm The initial moisture content of alfalfa was 70 wet basis following the ASAE standard S358 2 DEC93 1997 for moisture content measurement Three fi xed tray batch drying experiments were performed for each setup 1 and setup 2 In fi xed tray drying trays fi ve trays in setup 1 and PERFORMANCE OF RE CIRCULATING CABINET DRYER1677 Downloaded by East China Normal University at 18 09 15 January 2014 2002 Marcel Dekker Inc All rights reserved This material may not be used or reproduced in any form without the express written permission of Marcel Dekker Inc MARCEL DEKKER INC 270 MADISON AVENUE NEW YORK NY 10016 Table 2 Instrumentation Used in the Tests for the Second Setup InstrumentManufacturerRange Calibrated RangeUncertainty 1 T type thermocouples Omega Engineering Inc 60 C to 100 C0oC to 100 C 1 C 2 Relative humidity sensors Vaisala HMP233 0 to 100 RH5 to 95 RH 0 95 RH 3 Hotwire anemometer TSI Incorporation VelociCalc 0 to 50m s0 to 50m s0 015m s 4 Pitot tubeAir Flow Development0 33 to 1 65m s0 33 to 1 65m s 0 013 5 Data loggerSciemetric Instrument 8082A 0 5VN A N A 6 Wattmeter Valhalla Scientifi c Model no 2101 3 8113 N A N A N A Not applicable 1678ADAPA SOKHANSANJ AND SCHOENAU Downloaded by East China Normal University at 18 09 15 January 2014 2002 Marcel Dekker Inc All rights reserved This material may not be used or reproduced in any form without the express written permission of Marcel Dekker Inc MARCEL DEKKER INC 270 MADISON AVENUE NEW YORK NY 10016 nine trays in setup 2 were placed in the drying chamber at the beginning of the experiment and the doors were closed The material in each tray was weighed at 15 and 30min time intervals for setup 1 and setup 2 respectively This was done by rapidly removing the trays from the cabinet weighing them on an electronic scale and returning them back to the cabinet Moisture content at each weighing interval was calculated from the initial moisture content of alfalfa and the mass loss from each tray The fi xed tray drying continued until the moisture content of the material in all the trays was lower than 10 wet basis Three moving tray drying experiments were performed for each of the two setups In setup 1 alfalfa in tray 1 was weighed at 15min intervals After drying tray 1 for a further 45min tray 2 was introduced on the top rack moving tray 1 one rack down The procedure was repeated until tray 6 was introduced into the chamber and tray 1 was removed from the bottom The content of tray 1 was dried to below 10 mc In setup 2 moving tray drying experiments were conducted immedi ately following the batch fi xed tray drying This was done because the system was at steady state at the end of the fi xed tray drying period Nine fresh trays of alfalfa were placed in the chamber at the beginning of fi xed tray drying Drying was continued until the bottom tray tray 1 reached 16 moisture content Tray 1 was removed from the chamber all other upper trays were moved down on the racks and a fresh tray of material was placed on the top rack The door was closed and the material in all the trays was dried for another 30min The entire set of trays were removed rapidly from the cabinet and weighed When the bottom tray reached near 10 moisture content the removal and weighing were repeated This procedure was repeated until the 18th tray had been reached to the bottom and it dried to below 10 moisture content At this point the drying experiment ended RESULTS AND DISCUSSION A combination of two setups one or two humidifi ers and two drying confi gurations fi xed tray or moving tray were tested Each combination was repeated three times for a total of 12 experiments listed in Table 3 The total number of trays passed through the system for each test is listed The total mass is the cumulative mass of alfalfa on all trays before and after each drying setup The average mass of alfalfa was 100 to 150g per tray in setup 1 one dehumidifi er The mass increased to about 400g in setup 2 where two humidifi ers were used The initial moisture content of alfalfa chops ranged from 65 to 71 Thefi nalmoisturecontentvariedfromonetesttothenext Fortests2and3in PERFORMANCE OF RE CIRCULATING CABINET DRYER1679 Downloaded by East China Normal University at 18 09 15 January 2014 2002 Marcel Dekker Inc All rights reserved This material may not be used or reproduced in any form without the express written permission of Marcel Dekker Inc MARCEL DEKKER INC 270 MADISON AVENUE NEW YORK NY 10016 Table 3 Summary of Data from Experimental Drying of Alfalfa Chops in a Re circulating Air Cabinet Using Household Dehumidifi er Test no Drying Confi guration No of Dehumidifi er No of Trays Mass of Alfalfa Tested kg Moisture Content Drying time min Instantaneous Power Consumption W Temp C RH Drying Air Velocity m s Mass of Condensed Water kg InitialFinalInitial FinalMax Min Max Min First setup 1 fi xed tray150 590 1970 010 2180 33 021 4 27 920 10 320 235 2 fi xed tray150 450 1470 02 2s195 32 520 1 27 820 60 310 125 3 fi xed tray150 510 1570 52 3210 35 819 1 28 420 20 310 229 4moving tray181 010 3869 818 8360 39 621 3 30 125 80 310 181 5moving tray181 050 3871 020 1360 39 524 1 29 826 40 320 190 6moving tray181 030 3768 912 5360 39 720 7 29 926 50 320 187 Second setup 7 fi xed tray293 601 1870 08 531142851 124 1 80 520 40 361 279 8 fi xed tray293 601 5265 117 330041340 119 2 71 120 50 370 666 9 fi xed tray293 601 4165 010 633041540 218 4 72 021 20 381 122 10 fi xed followed by moving tray 22710 803 8669 013 378042346 820 1 77 119 80 363 413 11 fi xed followed by moving tray 22710 803 8068 410 178042447 019 8 78 020 00 373 305 12 fi xed followed by moving tray 22710 803 9468 814 578042346 220 0 77 421 30 363 613 Not measured 1680ADAPA SOKHANSANJ AND SCHOENAU Downloaded by East China Normal University at 18 09 15 January 2014 2002 Marcel Dekker Inc All rights reserved This material may not be used or reproduced in any form without the express written permission of Marcel Dekker Inc MARCEL DEKKER INC 270 MADISON AVENUE NEW YORK NY 10016 setup 1 the fi nal moisture content was extremely low We are not sure whether this represents the actual moisture content or errors in experiments and or calculations The listed drying times in Table 3 represent duration of a drying test The tests in setup 2 lasted longer than the tests in setup 1 due to increase in the number of trays tested The instantaneous power consumption recorded in setup 2 ranged from 413 428W The minimum temperature represents the ambient temperatures during a test averaged 20 C Higher temperatures and relative humidity were recorded in setup 2 where two humidifi ers and larger mass of alfalfa were in the chamber A higher air velocity was also recorded in setup 2 The mass of condensed water was almost half of the mass of alfalfa mass loss The other half likely leaked to the outside during opening and closing the door This is evident from a larger mass of water condensed in the fi xed tray confi guration than in moving tray confi guration Preliminary data were collected using setup 1 which was not fully instrumented These initial experiments provided a basic understanding of the process In setup 2 which was fully instrumented moving tray drying experiments were performed immediately following the fi xed tray drying test The results obtained from the moving tray drying experiments in setup 2 are discussed in the remainder of this paper These results are a representation of both fi xed and moving tray drying experiments Figure 3 shows the temperature and relative humidity of the air as the material dried The air temperature at the inlet and outlet of the drying section increased and reached an asymptote Initially the temperature was low because the material was wet and cool As time progressed the material was heated and drying commenced The maximum temperature at the entrance to the trays was 45 C The temperature was maintained at 45 C by opening and closing the inlet and outlet butterfl y valves manually Fig 2 There were two reasons for not allowing the temperature to exceed 45 C For specialty crops the optimum drying temperatures at which no struc tural damage and nutrient losses occurs lies between 30 45 C The relative humidity of air at the entrance and exit of the trays decreased almost exponentially from an initial value of 79 down to 19 by the end of fi xed tray drying Fig 3 The initial high moisture content of the material and low temperature of air resulted in high air relative humidity values The air became drier due to the onset of falling drying rate The emulation of continuous fl ow moving tray drying was initiated after 270min of fi xed tray drying During moving tests temperature and relative humidity were constant because on an average the moisture load ing to the test chamber was also constant As the moving tray drying started PERFORMANCE OF RE CIRCULATING CABINET DRYER1681 Downloaded by East China Normal University at 18 09 15 January 2014 2002 Marcel Dekker Inc All rights reserved This material may not be used or reproduced in any form without the express written permission of Marcel Dekker Inc MARCEL DEKKER INC 270 MADISON AVENUE NEW YORK NY 10016 the relative humidity at the exit of trays which was below 20 increased and reached 30 and remained constant thereafter Fresh material was introduced into the dryer every 30min and dry material was removed maintaining a near steady rate of moisture evaporation Figure 4 shows cumulative moisture condensation for fi xed tray drying followed by moving tray drying The initial moisture condensation was high for fi xed tray drying and decreased as the drying progressed reaching almost zero at about 270min end of fi xed drying As the relative humidity of air decreased condensation rate decreased as well leading to almost zero at relative humidity values below 20 In moving tray mode fresh moist material was introduced every 30min which increased the relative humidity of the air to about 30 The result was an initial increase in the condensa tion followed by a constant rate of condensation The amount of moisture condensed at the evaporator was approximately 50 of the total moisture evaporated from the alfalfa The remaining 50 of the moisture was mainly lost to the ambient air through inlet and outlet ports with some additional loss occurring when the chamber door was opened to introduce fresh alfalfa and remove dried alfalfa Figure 5 shows the measured specifi c moisture extraction rate SMER and instantaneous power consumed during the drying process SMER is defi ned as the mass of water removed per unit of energy expended The high SMER of 1 02kg kWh in the fi rst 40min was due to the high Figure 3 Typical measured temperature and re