1、Testing of a heat-pump-assisted mechanical opener dryerZuhal Oktay *Department of Mechanical Engineering,Faculty of Engineering,Balikesir University,Balikesir,10100Kampus,TurkeyReceived 15January 2002;accepted 23July 2002AbstractIn this study a heat-pump-assisted dryer with an opener mechanism was c

2、onstructed and tested at the Energy Laboratory,Mechanical Engineering Department,Balikesir University,Turkey.The test material used was wet wool,which is a brous material.The variables in the experiments were the air rate,the recirculation air ratio (RARand the bypass air ratio.In the experiments,th

3、e air mass velocities ranged from 0.78to 1.50kg/m 2s.The bypass air and recirculation air ratios varied from 20%to 80%.For this system both coecient of performance of the whole system (COP ws and the specic moisture extraction rate of the whole system (SMER ws increased as evaporator air ratios incr

4、eased.COP ws was found to range from 2to 3.5while SMER ws varied between 1.5and 2.8kg/kW h depending on RAR.2002Elsevier Science Ltd.All rights reserved.Keywords:Drying;Wool drying;Heat pumps;Heat pump dryer;Energy eciency;Performance parameters1.IntroductionHeat pumps have been extensively used by

5、industry for many years,but rarely for process drying and in particular for drying textile products.The modelling of heat pump dryers (HPDsfor design purposes has attracted research interest for more than 20years.Yet,the literature remains divided on what design features are appropriate for dierent

6、applications of HPDs 1.The studies carried out on HPDs can be classied into three groups.The rst group includes studies in which the performance analysis of these systems has been investigated 26.The second group covers studies on developing simulation models 79.The investigations on the application

7、 of HPDs to drying systems for industrial use belong to the last group 1013.In most of these studies,food products and agricultural materials were dried in various types of HPDs.*Tel.:+90-266-612-1194;fax:+90-266-612-1257.E-mail addresses:zoktay,lemanoktay (Z.Oktay.1359-4311/02/$-see front matter 20

8、02Elsevier Science Ltd.All rights reserved.PII: S1359-4311(0200112-6Applied Thermal Engineering 23(2003 153162The study reported here includes the experimental verication of a heat-pump-assisted me-chanical opener dryer.An experimental set-up described in the next section was constructed and tested

9、at the Energy Laboratory,Mechanical Engineering Department,Balikesir University,Turkey.Wet wool was used as the test material being dried.2.Experimental set-upThe facilities and procedure used in carrying out the experiments and the measurements are described blow.Nomenclature_M a air mass velocity

10、or air ow rate per square meter (kg/m 2s_Q cd heat delivered in the condenser (kW_m air mass ow rate of air (kg/s_W af power input to the axial fan (kW_W cf power input to the centrifugal fan (kW_W h power input to the compressor (kW_W c power input to the dryer cylinder (kW_V air volumetric ow rate

11、 of air (m 3/sCOP c ;h heating coecient of performance of the Carnot cycle (COP hp ;h heating coecient of performance of the heat pump (COP ws COP of the whole system (C p ;air specic heat of air (kJ/kg KRAR recirculation air ratio (%RH relative humidity (%_md drying rate in kg water per hour (kg/hS

12、MER hp specic moisture extraction rate for the heat pump (kg/kW hSMER ws specic moisture extraction rate for the whole system (kg/kW hT Ccondenser temperature (KT ci ;air average air temperature entering the condenser (CT co ;air average air temperature leaving from the condenser (CT ei ;air average

13、 air temperature entering the evaporator (CT eo ;air average air temperature leaving from the evaporator (CGreek letterq airdensity of air (kg/m 3154Z.Oktay /AppliedThermal Eng ineering 23(2003153162connected in series to the heat pump.The main components of the heat pump are an evaporator,a condens

14、er,a compressor,and an expansion valve.The drying chamber is a rotating perforated cylinder.The perforation allows dry air to enter the chamber.A small radius cylinder with an opener mechanism sits in the middle of this chamber and rotates at a higher speed than the perforated cylinder.The basic des

15、ign and operating characteristics of the system are summarised as follows: high0:08m wide3rows deep.The evaporator has15circuits of three tubes each.Air cir-culation was achieved using a centrifugal fan with an output power of65W.The dryer is500mm in long and550mm in diameter.Fresh air entering the

16、system and humid air exhausting from the system are regulated by dampers.The temperature and relative humidity of fresh air are taken to be1520C and60%,respectively.In the experiments,the wool to be dried hadve dierent weights ranging from200to600g. Thebreneness of the wool is31.72l m and the coecie

17、nt of variation is21.39%14.In the experiments,the following measurements were carried out on both air and refrigerant cycles for an overall performance verication of the HPD.Dry bulb temperature andrelative humid ity of air were measured at the inlet and outlet of the drying chamber,inlet and outlet

18、 of the condenser and evaporator using a portable temperature and relative humidity measurement device(with an accuracy of2%.The wet bulb temperature of air was obtained from the dry bulb temperature and relative moisture of air.Refrigerant circuit temperatures were measured with copper-constant the

19、rmocouples(with an accuracy of0.2 inserted into the refrigerant piping,the inlet and outlet of the compressor,condenser and evap-orator.Air velocities at the inlet duct of dryer,evaporator and bypass duct were measured with an anemometer(measurement range:030m/s and the reading accuracy:3%.The rotat

20、ional speed of the cylinder was varied with a beltpulley mechanism and measured with a tachometer. Electrical power input to the compressor was measured with a wattmeter.Fan power was calcu-lated from the air rate and the fan revolution number using the fan laws.Measurement of water removal was perf

21、ormed in the following manner:Moisture removed from the exhaust air was collected in the evaporator pool as it passed through the evaporator.The condensed liquid in this pool was then drained into a graduated measuring cylinder.The reading value on the cylinder was taken as the extraction moisture r

22、ate.3.Evaluation of heat pump dryer performanceThe overall performance of a HPD may be characterised by several criteria.Among them,the coecient of performance(COPand the specic moisture extraction rate(SMERhave been used by Jia et al.9,Clements et al.4,Schmidt et al.6,Hawlader et al.12,Gilmour et a

23、l.15. For an ideal refrigeration system operating between a condenser temperature of T C and an evaporator temperature of T E,the maximum heating coecient of performance,COP c;h was obtained from the Carnot cycle as16:COP c;hT CT CT E1156Z.Oktay/AppliedThermal Eng ineering23(2003153162The COP is den

24、ed as the ratio of the heat recovered at the condenser to the work required by the compressor;COP hp;h_Qcd_Wh2where_Q cd is the heat delivered in the condenser and_W h is the power input to the compressor.The heating COP is the ratio between the heat supply and the compressor input,and also may be e

25、xpressed by the respective enthalpy dierences9.The system COP can be dened with energy required for compressor,fans and the electrical devices,and can be represented as:COP ws_Qcd_Wh_W af_W cf_W c3The massow rate on the refrigerant side was not measured.Therefore,the heat delivered in the condenser_

26、Q cd,was estimated using the experimental values of the following items as: _Qcd_m air C p;airT co;airT ci;air4_m airq air_V air5where_m air is the massow rate of air,C p;air is the specic heat of air,_V air is the volumetricow rate of air,q air is the density of air,T ci;air and T co;air are the av

27、erage air temperatures entering and leaving the condenser,respectively.The SMER can be dened as the energy required to remove1kg of water and may be related to the power input to the compressor(SMER hpor to the total power to the dryer including the fan power and the eciencies of the electrical devi

28、ces(SMER ws,as given by Jia et al.9,Schmidt et al.6,Hawlader et al.12.SMER hp_m d_Wh6andSMER ws_m d_Wh_W af_W cf_W c7where_m d is the drying rate in kg water per hour.4.Experimental studyThe recirculation air ratio(RARis dened as the ratio of recirculation air to the total air(mass basisin the syste

29、m,while the bypass air ratio(BARis dened as the ratio of bypass air to the total air(mass basisin the system.The total air massow rates,RAR and BAR were adjusted by the circulating fan speed and air dampers,located in the evaporator and bypass air ducts.Z.Oktay/AppliedThermal Eng ineering23(20031531

30、62157Although BAR could be adjusted to100%of the totalow rate in practice,high BARs reduced the eciency of the dryers and caused instabilities in the heat pump unit4.Therefore,the BARs were varied only up to80%of the total massow rate through the system.5.Results and discussionIn this study,the eect

31、 of the air mass velocities,the case where the system is stable,the case of bypass and recirculation,bypass and recirculation air ratios on the HPD with opener mechanism were examined and described(Fig.2.In the case where bypass is fully open and closed,illustrated in Figs.3and4,respectively,the tim

32、e to reach the steady state was shorter when the air mass velocity was0.78kg/m2s.When air mass velocities were increased,the temperatures were unstable.The values were stable at lower mass velocities and the heat pump drying system reached steady state in a shorter time.When the bypass cycle was ful

33、ly open,the air temperature entering the condenser was40C.When RAR was fully open,the air temperature entering the condenser was found to be32C for mass ve-locity of0.78kg/m2s.In the case when the RAR was fully open,the values for the relative humidity of the air entering the condenser were lower th

34、an those in the BAR system.When the system was operated with mass velocity of_M a1:50kg/m2s,the air temperature dierence between the inlet and outlet of the evaporator and the condenser signicantly decreased,because there was not enough time to heat air through the condenser and to condense humidity

35、 of air through the evaporator.In the operating mode with bypass fully open,the inlet and outlet temperatures of the evaporator were higher with the bypass fully closed.In this case,the inlet relative humidity of the air to dryer was high.This is not wanted because it results in lower COP and SMER v

36、alues.Fig.6indicates the eect of RAR on COP,COP ws,SMER and SMER ws of the HPD.For the case studied,the SMER increased with increasing the COP values at increasing evaporator air ratios.When the RAR was increased from20%to40%,to60%and to80%,the increase in performance indicators(COP ws/SMER wswere20

37、%/26%,50%/53%,75%/86%respectively.If the heat pump and the dryer are examined separately,one might speculate that the working conditions for optimum performance of the heat pump and of the dryer are identical.Tai et al.17reported that the maximum SMER was obtained when the COP was maximum.However, l

38、ater studies presented by Jia et al.9and Prasertsan et al.18reported that maximum COP and SMER did not necessarily occur at the same working condition.This phenomenon was ex-plained by the interaction between the process air in the dryer system and the refrigeration in the heat pump.Because of this

39、interaction,the heat pump and the dryer should be examined together. RARs. s.6.ConclusionsA prototype heat-pump-assisted mechanical opener drying system was designed,constructed,and tested for the drying of wool.Using the heat pump drying system developed,the ambient conditions may be varied and con

40、trolled.The drying time of the material may also be reduced by four times over natural drying,while the hygienic and homogeneous drying requirements are maintained.The opener mechanism is placed in the drying chamber.The material is opened in the chamber allowing dry air to ow evenly through the ber

41、s.As the thickness of the material is de-creased with opener mechanism,constant drying period was long and decreasing drying period was short.This case illustrates that dryer with opener mechanism requires less energy and drying time.In similar,commercial HPDs,the COP values vary from 3to 52022.In a

42、ddition,the values for the SMER and SMER ws of a convential wool dryer with a continuous conveyor dryer is given as 4and 2by a Turkish company 23.It may be concluded that the performance of the system studied remains within the range of commercial heat pumps.References 187204. s.and dehumidication s

43、ystems.II:An experimental assessment of the heat pump characteristics of a heat pump dehumidication system using R112,International Journal of Energy Research6(1982323331.3P.Baines,C.Carrington,An assessment of heat pump timber drying,International Symposium on the Large ScaleApplication of Heat Pump,England,2025March,1987,pp.231236.4S.Clements,X.Ha,P.Jolly,Experimental verication of a heat pump assisted continuous dryer simulation model,International Journal of Energy Research17(19931928.5S.Prasertsan,P.Sae