Design and Thermal Analysis of an Air Source Heat Pump Dryer for Food Drying

sustainability

Article Design and Thermal Analysis of an Air Source Heat Pump Dryer for Food Drying

Haolu Liu, Khurram Yousaf, Kunjie Chen *, Rui Fan, Jiaxin Liu and Shakeel Ahmed Soomro

College of Engineering, Nanjing Agricultural University, Nanjing 210031, Jiangsu, China; [email protected] (H.L.); [email protected] (K.Y.); [email protected] (R.F.); [email protected] (J.L.); [email protected] (S.A.S.) * Correspondence: [email protected]; Tel.: +86-139-5100-7707

Received: 18 July 2018; Accepted: 3 September 2018; Published: 8 September 2018

Abstract: In this study, an experimental heat pump dryer was designed. The speciﬁc moisture extraction rate and moisture extraction rate were used as performance indicators to explore the inﬂuence of environmental factors and the style of the hot air cycle on heat pump drying. The average temperature and humidity in Nanjing’s summer, winter, and throughout the whole year were taken as the experimental ambient temperature and humidity. Garlic slices 3 mm thick, with an initial moisture content of 66.714% w.b., were dried until the end moisture content was 10% w.b. Experimental results and thermal analysis showed that the open and semi-open heat pump dryers were greatly affected by ambient temperature and humidity. The closed heat pump drying system was greatly affected by the bypass air rate.

Keywords: heat pump dryer; ambient temperature and humidity; speciﬁc moisture extraction rate; moisture extraction rate

1. Introduction Food drying is an important part of production. There are signiﬁcant drawbacks to the traditional way in which heat for drying is gained by burning fuel: it consumes a large amount of energy and it causes serious environmental problems, such as haze problems in China [1–4]. A heat pump is a device that converts medium–low temperature heat energy into medium–high temperature heat energy. Its characteristic is to obtain a large amount of medium–high temperature heat energy using a small amount of high-grade energy, [5] which is energy saving and environmentally friendly. Using a heat pump as a heat source in food drying can solve many problems caused by the burning of fossil fuels and has attracted the attention of many researchers. Heat pump drying has the characteristics of high energy utilization, low drying temperature, and easy control, which makes it widely used in the drying process of wood, grain, and agricultural and sideline products [6–9]. A heat pump dryer (HPD) consists of a heat pump system and a dryer, and the performance of the HPD is greatly affected by the performance of the heat pump system. A different refrigerant can affect the performance of the heat pump system. It can improve the performance of the heat pump system by selecting the right refrigerant as the working medium of the heat pump system, according to the ambient temperature. Shen et al. [10] designed an air source HPD in which refrigerants R22 and R134a are adopted as the working medium for the high-stage and low-stage, respectively. According to their study, the supplying temperature can increase to 70 ◦C. Lee et al. [11] designed a two-cycle HPD, where one cycle used the refrigerant 124 to get a temperature greater than 80 ◦C and the other cycle used the refrigerant 134a. Two cycles are performed in the HPD: (1) the refrigerant circulation in the heat pump system and (2) the drying medium circulation in wind tunnel and drying chamber. The heat exchange between the

Sustainability 2018, 10, 3216; doi:10.3390/su10093216 www.mdpi.com/journal/sustainability Sustainability 2018, 10, 3216 2 of 17 drying medium and the refrigerant is carried out by an evaporator and condenser. The drying medium absorbs moisture from material in the dryer and has a heat exchange with the heat pump system in the HPD, so its nature will have a great impact on the performance of the HPD. Chapchaimoh et al. [12] dried ginger in a closed heat pump dryer separately, using air and nitrogen as the drying medium. According to their study, when the supplied air temperature is 50 ◦C, the specific moisture extraction rate (SMER) of ginger drying in air is 0.06 kg water/MJ compared with 0.07 kg water/MJ. In this paper, the drying medium is air. Hot air circulation can affect the heat exchange between the drying medium and the heat pump system, as well as affect the inlet temperature and humidity of the drying chamber. According to the style of the hot air cycle, the HPD can be divided into an open type, semi-open type, and closed type. Liu et al. [13] used a closed heat pump drying system to dry carrot slices with a thickness of 3 mm. In the experiment, researchers compared the effects of different supplied air temperatures and airflow rates on the drying performance of the heat pump dryer. Ta¸seriet al. [14] dried pomace with drying air at a temperature of 45 ◦C and different air velocities in open type HPD and closed HPD. According to their study, the drying air velocity was slightly effective on reducing the drying time; however, there was no significant effect on the power consumption. They compared the energy consumption of the HPD and convective dryer and found that the energy consumption was reduced by up to 51%. Specific moisture extraction rate and moisture extraction rate (MER) are commonly used to evaluate an HPD. Ganjehsarabi et al. [15] performed exergy and exergoeconomic analyses of a heat pump tumbler dryer by using actual thermodynamic and cost data. They dried wet cotton fabric, and the results showed that the SMER was equal to 1.08 kg/kWh. Mortezapour et al. [16] used a hybrid photovoltaic–thermal solar dryer equipped with a heat pump system for drying saffron. According to their study, a maximum dryer efficiency of 72% and a maximum SMER of 1.16 were obtained at an air flow rate of 0.016 kg/s and air temperature of 60 ◦C when using the heat pump. Control strategy is very important in improving the performance of a heat pump [8]. Wei et al. [17] used the Artificial Neural Network (ANN) model for predicting the performance of the Heat Pump (HP). Their study indicated that the ANN model was reliable and robust. Based on the ambient temperature and water temperature, a new dual fuzzy controller was studied to understand the effect of the initial opening and superheat of the electronic expansion valve on the performance of the air source heat pump water heater [18]. Yang et al. [6] proposed a synchronous control strategy to improve the control accuracy of a closed-loop HPD’s superheat and drying temperature. Ju et al. [19] provided an evaluation method for the convection hot air-drying method to improve drying efficiency and reduce energy consumption by controlling relative humidity. Although many studies have been reported on HPD, the performance of HPD influenced by the hot air circulation method in different ambient temperature and humidity conditions was seldom reported. In places like Nanjing, which is located in a subtropical monsoon climate zone, there are large changes in temperature and humidity. Thus, it is very significant for industrial manufacturers to explore the effect of different hot air circulation modes on the performance of the HPD under different ambient temperatures and humidity conditions. However, few existing works have considered the combined effects of ambient temperature and humidity and different hot air circulation methods on the drying performance of HPD. In order to fill the gaps in this research, this work studied the drying performance of HPD with different hot air cycle modes under different environmental conditions and carried out thermal analyses on this. In order to explore this question, an experimental HPD was designed. In this machine, the style of the hot air cycle can be changed by switching the air duct valve, so we can evaluate the performance of HPD with different styles of the hot air cycle. In the experiment, SMER and MER are used as performance indicators to explore the influence of environmental factors and hot air circulation on heat pump drying. The average temperature and humidity in Nanjing’s summer, winter, and throughout the whole year were the experimental environmental conditions, and garlic slices were used as the dried material. In the selected ambient temperature and humidity, garlic slices were dried Sustainability 2018, 10, x FOR PEER REVIEW 3 o f 17 Sustainability 2018, 10, 3216 3 of 17 selected ambient temperature and humidity, garlic slices were dried using different hot air circulation methods of HPD, and an enthalpy–humidity diagram of circulated air and drying using different hot air circulation methods of HPD, and an enthalpy–humidity diagram of circulated kinetics were used to analyze the experimental results. air and drying kinetics were used to analyze the experimental results.

2.2. Design Design of of the the Heat Heat Pump Pump Dryer Dryer ToTo improve improve the the performance performance and and develop develop a a new new heat heat pump pump system, system, many many studies studies have have focused focused onon imp improvingroving the performance of conventional heat heat pump pump systems systems through through different different methods, methods, such such as asimproving improving compressor compressor performance, performance, using using a new a heat new pump heat working pump working medium, ormedium, using multi-stage or using multicompression.-stage compression. The performance The performance of HPD is notof HPD only is inﬂuenced not only influenced by the performance by the perfo of thermance heat of pump the heatsystem pump but system also the but style also of the its style hot air of cycle,its hot therefore air cycle, wetherefore designed we adesigned heat pump a heat dryer. pump The dryer. HPD Thestructure HPD structure diagram isdiagram shown inis shown Figure 1in. ByFigure opening 1. By and opening closing and the closing air duct the valve, air duct the valve, style ofthe its style hot ofair its cycle hot air can cycle be changed. can be changed.

12 6 1010 7 8 11 12 44 6 7 8 55 1313

Drying 22 1111 chamberchamber

Ⅰ Ⅱ Ⅲ Duct valve TemperatureTemperature andand humidityhumidity sensorsensor Temperature sensor Quality sensor

Air flow sensor Compressor Expansion valve Fan

Condenser Evaporator Electric heaters Humidifier Note : In the figure , 1–13 are the air valves number, Ⅰ–Ⅲ are the heat pumps number.

Figure 1. Heat pump dryer structure diagram. Figure 1. He a t pump dryer structure diagram. 2.1. Heat Pump System 2.1. Heat Pump System The heat pump system is composed of three groups of air source vapor compression heat pump. BecauseThe theheat frequency pump system conversion is composed technology of hasthree a limited groups capacity of air forsource compressor vapor compression power, researchers heat pump.can start Because and stopthe frequency the heat pump conversion units technology according tohas the a actuallimited workingcapacity conditionsfor compressor of the power, HPD. researchersCompressor can power start is and shown stop inthe Table heat1 .pump units according to the actual working conditions of the HPD. Compressor power is shown in Table 1. Table 1. Electric power. Table 1. Ele ctric powe r. Equipment Power Number Equipment Pow er N u mber Main fan 0.8 kw 1 Auxiliary fanMain fan 0.30.8 kw kw 1 1 CompressorsAuxiliary fan 0.60.3 kw kw 1 3 Electric heaterCompressors 10.6 kw kw 3 1 Electric heater 1 kw 1 For the heat pump system, the refrigerants selection for the heat pump is quite important. This is becauseFor the it not heat only pump affects system, the performance the refrigerants greatly selection but also for restrictsthe heat the pump application is quite important. of an HPD, This as it is because it not only affects the performance greatly but also restricts the application of an HPD, as Sustainability 2018, 10, 3216 4 of 17 Sustainability 2018, 10, x FOR PEER REVIEW 4 o f 17 hasit has to to satisfy satisfy the the international international standard standard of of ozone ozone depression depression potential potential (ODP). (ODP). R407CR407C isis aa newnew type of environmentally friendlyfriendly heatheat pump medium with anan ODPODP of 0, which can effectivelyeffectively protectprotect thethe ozone layer.layer. R407C R407C has has very very similar similar characteristics characteristics and performance and performance to R22 whichto R22 makes which it amakes long-term it a alternativelong-term alternative to R22. R134a to R22. has goodR134a thermodynamic has good thermodynamic properties with properties an ODP with value an of ODP 0. The value ambient of 0. temperatureThe ambient of temperature group 1, 2, and of 3group heat pumps1, 2, and (see 3 Figure heat 1pumps) gradually (see increased.Figure 1) Differentgradually refrigerants increased. areDifferent suitable refrigerants for different are ambient suitable temperatures. for different Consideringambient temperatures. the cost and Considering feasibility ofthe heat cos pumpt and development,feasibility of theheat working pump mediumdevelopment, of the the three working groups ofmedium heat pump of the systems three were groups refrigerants of heat R407C,pump R22,systems and were R134a, refrigerants respectively. R407C, R22, and R134a, respectively.

2.2.2.2. Air Duct Layout

The stylestyle ofof the the hot hot air air cycle cycle can can be achievedbe achieved by adjusting by adjusting the air the valves. air valves. According According to the degreeto the ofdegree ventilation of ventilation with the with surroundings, the surroundings, the HPD canthe beHPD divided can be into divided open, semi-open, into open, andsemi closed-open, types. and Exhaustclosed types. gas exiting Exhaust the gas drying exiting chamber the drying often chamber contains often excess contains heat, and excess the heat, outflow and gas the temperature outflow gas istemperature generally higher is generally than the higher ambient than temperature.the ambient temperature. Therefore, it isTherefore, usually necessaryit is usually to passnecessary the gas to flowingpass the out gas of flowing the drying out chamberof the drying through chamber the evaporator through the of the evaporator heat pump of system.the heat However, pump system. if the materialHowever, to if be the dried material contains to be more dried dust, contains the dust more will dust, be the mixed dust into will the be exhaustmixed into gas. the When exhaust flowing gas. throughWhen flowing the evaporator, through the dustevaporator, will adhere the dust to the will evaporator adhere to surface, the evaporator affecting itssurface, heat exchange. affecting its In thisheat case, exchange. dust should In this be avoidedcase, dust for should dry exhaust be avoided gas passing for throughdry exhaust the evaporator. gas passing The through proportion the ofevaporator. the bypassed The airproportion in the closed of the circulation bypassed air can in be the adjusted closed circula by adjustingtion can the be degree adjusted of theby adjusting opening andthe degree closing of of the the opening air duct valves.and closing The fans’ of the power air duct levels valves. are shown The fans’ in Table power1. The levels main are fan shown is a fan in closeTable to 1. theThe drying main fan chamber. is a fan close to the drying chamber.

2.3. Data Acquisition and Control System 2.3. Data Acquisition and Control System The hardware of the data acquisition and control system is mainly composed of executive The hardware of the data acquisition and control system is mainly composed of executive components such as industrial personal computer, Programmable Logic Controller (PLC), sensors, components such as industrial personal computer, Programmable Logic Controller (PLC), sensors, and inverters. The main hardware conﬁguration is shown in Table2 below. Programmable Logic and inverters. The main hardware configuration is shown in Table 2 below. Programmable Logic Controller control programming is completed using STEP7 V4.0 and the conﬁguration software is Win Controller control programming is completed using STEP7 V4.0 and the configuration software is CC (Siemens, Berlin, Germany). Win CC does not have a PPI driver, so it cannot directly communicate Win CC (Siemens, Berlin, Germany). Win CC does not have a PPI driver, so it cannot directly with the S7-200 serial port. However, Win CC is driven by the Object Linking and Embedding for communicate with the S7-200 serial port. However, Win CC is driven by the Object Linking and Process Control (OPC) server. Therefore, communication between Win CC and S7-200 can be realized Embedding for Process Control (OPC) server. Therefore, communication between Win CC and through OPC. PC Access is used as the OPC server. The data acquisition and control process is shown S7-200 can be realized through OPC. PC Access is used as the OPC server. The data acquisition and in Figure2. control process is shown in Figure 2.

PPI Computer CPU Expansion module

Sensor system Inverter

Temperature Motor Humidity Quality Air flow

FigureFigure 2.2. HeHeat at pump pump drying drying de device vice data data acquisition acquisition and control processproce ss diagram. diagram.

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Table 2. Data acquisition and control system of the main hardware configuration. Table 2. Data acquisition and control system of the main hardware configuration. Equipment Manufacturer Type Number ProgrammableEquipment Logic Controller (PLC) ManufacturerSiemens S7-200 (CPU224XP) Type Number1 ProgrammablePLC Exte Logic nsion Controller Module (PLC) SiemensSiemens EM231 S7-200(Thermal (CPU224XP) resistance type) 11 PLC ExtensionExte nsion Module Module SiemensSiemens EM231 (ThermalEM231 resistance(AI4) type) 12 PLC ExtensionExte nsion Module Module SiemensSiemens EM232 EM231 (AO4) (AI4) 21 PLC Extension Module Siemens EM232 (AO4) 1 PLC Exte nsion Module Siemens EM235 (AI4, AO1) 1 PLC Extension Module Siemens EM235 (AI4, AO1) 1 InverterInverter SiemensSiemens MM440 MM440 44 Industrial Computer Compute r YanhuaYanhua Ipc610l Ipc610l 11 TemperatureTemperature and and Humidity Humidity Sensor Sensor BeijingBe ijing Jiuchunjian Jiuchunjian JCJ200W JCJ200W 55 TemperatureTemperature Sensor Sensor ShenzhenShenzhen RBD RBD PT100 PT100 44 Air flowflow SensorSe nsor E+EE+E EE65 11 Solid State Relay Omron G3N 1 Solid State Re lay Omron G3N 1 Mass Sensor Tecsis EX301B 1 SignalMass Amplifier Sensor InterfaceTe csis EX301B SGA/A 11 Signal Amplifie r Interface SGA/A 1

The sensor transmitstransmits the the temperature, temperature, humidity, humidity, air air flow flow velocity, velocity, and and mass mass signals signals collected collected from fromthe site the to site the to PLC the expansion PLC expansion module. module. After theAfter A/D the conversion A/D conversion of the of Analogy the Analogy Input Input (AI) port (AI) of port the ofexpansion the expansion module, module, the analog the analog signal signal is converted is converted into a digitalinto a digital signal signal and stored and stored in the inputin the imageinput imageregister. register. The Central The Central Processing Processing Unit (CPU)Unit (CPU converts) converts the digital the digital quantity quantity into theinto actual the actual value value and andstores stores it in it the in variablethe variable memory. memory. The The data data is transmitted is transmitted to the to upperthe upper Industrial Industrial Computer Computer via thevia thePoint Point to Point to Point Interface Interface (PPI) (PPI communication.) communication. PC AccessPC Access onthe on IPCthe willIPC will obtain obtain data data from from the PLC the PLCdata data registers registers as the as OPC the server. OPC server. The configuration The configuration software software Win CC Win will readCC will data read from data PC Accessfrom PC as Accessan OPC as client an OPC and client display and it ondisplay the monitor it on the screen monitor in real screen time. in real time. Traditional PID ( (proportion-integral-derivative)proportion-integral-derivative) controlcontrol hashas thethe advantagesadvantages of having a simple algorithm, goodgood robustness,robustness, and and high high reliability, reliability, and itand has beenit has widely been used widely in industrial used in productionindustrial processes.production The processes. HPD realized The HPD closed-loop realized control closed- ofloop the control temperature of the and temperature air flow at and the entranceair flow ofat the entrancedrying chamber. of the drying The air chamber. flow and temperatureThe air flow signals and temperature in the drying signals chamber in collectedthe drying by chamber the data collectedacquisition by system the data are acquisition used as feedback system are signals. used Theas feedback control algorithmsignals. The adopted control PID algorithm control, adopted and the PIDkey parameterscontrol, and in thethe controlkey par processameters were in determinedthe control throughprocess thewere PID determined adaptive and through tuning functionthe PID adaptiveof STEP7 and V4.0. tuning PID control function process of STEP7 diagram V4.0. is PID as shown control in process Figure diagram3. is as shown in Figure 3. The specific specific control control process is as follows. The The flow flow rate rate determined by by the air flow flow will cause a disturbance toto the the temperature. temperature. The The control control strategy strategy is to control is to thecontrol air flow thefirst, air thenflow thefirst, temperature. then the Thetemperature. control process The control of the process air flow of isthe as air follows. flow isThe as follows PLC’s. expansion The PLC’s moduleexpansion outputs module the outputs analog thesignal analog to control signalthe to control frequency the converter,frequency andconverter, the frequency and the offrequency the blower of the drive blower motor drive is controlled motor is controlledby the frequency by the converter.frequencyThen, converter. the blower Then, speedthe blower is controlled, speed is and controlled, the air flow and is the controlled air flow and is controlledadjusted through and adjust the blowered through speed. the The blower process speed. of temperature The process control of temperature is similar control to the control is similar of airto theflow control and also of air uses flow frequency-converting and also uses frequency control.-converting control. The temperature control process is asas follows.follows. When the temperature of the drying chamber differs greatly from the set temperature, the researchers start or stop the compressors and the electric heater accordingaccording to to the the actual actual situation. situation. When When the drying the drying chamber chamber temperature temperature is stable, is it isstable, controlled it is controlledby PLC. Programmable by PLC. Programmable Logic Controller Logic Controller controls the controls frequency the frequency converter. conv Theerter. frequency The frequency converter convertercontrols the controls compressor the compressor drive motor drive frequency motor thenfrequency controls then the controls motor rotation the motor speed rotation and adjustsspeed and the adjustsheat pump the heat heating pump amount heating to achieveamount the to achieve purpose the of controllingpurpose of thecontrolling temperature. the temperature.

PID Controlled D/A Actuator Controller object

A/D Sensor

FigureFigure 3. 3.PIDPID (proportion (proportion-integral-derivative)-inte gral-de rivative ) control control process diagram. diagram.

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3. Experiment 3. Experiment 3.1. Experimental Materials 3.1. Experimental Materials The fresh garlic was harvested in Shouguang, Shandong Province, according to Chinese The fresh garlic was harvested in Shouguang, Shandong Province, according to Chinese national national standard GB5009.3-2016. The wet basis moisture content of garlic was 66.714%. Fresh garlic standard GB5009.3-2016. The wet basis moisture content of garlic was 66.714%. Fresh garlic was peeled was peeled and sliced into thin slices, according to Yan’s study [20]. The garlic slice thickness was set and sliced into thin slices, according to Yan’s study [20]. The garlic slice thickness was set to 3 mm. to 3 mm. 3.2. Experimental Setup 3.2. Experimental Setup The HPD described above is shown in Figure4. The air duct valves are divided into a total of six The HPD described above is shown in Figure 4.. TheThe airair ductduct valvesvalves areare divideddivided intointo aa total of six grades (0–5), where 0 is fully closed and 5 is fully open, as shown in Figure5. The style of the hot air grades (0–5), where 0 is fully closed and 5 is fully open, as shown in Figure 5.. TheThe stylestyle ofof thethe hothot airair cycle in the heat pump drying process is changed by adjusting the air duct valves. The heat pump cycle in the heat pump drying process is changed by adjusting the air duct valves. The heat pump operating state parameters are shown in Table3. operating state parameters are shown in Table 3.. Table 3. Heat pump operating state parameters. Table 3.. Heat pump operating state parameters.. Type Heat Pump 1 Heat Pump 2 Heat Pump 3 Type Heat Pump 1 Heat Pump 2 Heat Pump 3 Evaporating temperature 0 ◦C 0 ◦C 0 ◦C EvaporatingEvaporating pressure temperature 500 kPa0 °C 4980 kPa °C 0 °C 293 kPa CondensingEvaporating temperature pressure 55 ◦C500 kPa 498 55 ◦ CkPa 293 kPa 55 ◦C CondensingCondensing pressure temperature 2200 kPa55 °C 218155 kPa °C 55 °C 1491 kPa Condensing pressure 2200 kPa 2181 kPa 1491 kPa

Drying Heat pump chamber Control system Industrial Personal Computer

FigureFigure 4 4... MultiMulti-functional-functional he heat at pump pump drying drying te test st device.de vice .

FigureFigure 5 5... AirAir valve valve. . Fresh garlic slices with a mass of 15 kg were spread on a drying tray with ﬁne holes in a drying chamberFresh which garlic as slices shown with in a Figure mass6 of (1 15× kg1 ×were0.6 spread m). The on mass a drying sensor tray on thewith drying fine holes plate in can a drying weigh chathe m quality b er which of the as garlic shown slice. in Figure Therefore, 6 (1 × garlic 1 × 0.6 slices m).. TheThe can mass be weighed sensor inon thethe dryingdrying processplate can without weigh thetaking quality them of out the of garlic the drying slice. Therefore, chamber, reducing garlic slices the effect can be of weighed heat loss in on the the drying experimental process results. without taking them out of the drying chamber, reducing the effect of heat loss on the experimental results. Sustainability 2018, 10, 3216 7 of 17 Sustainability 2018, 10, x FOR PEER REVIEW 7 o f 17

FigureFigure 6 6.. DryingDrying chamber. chamber.

3.3. Experimental Ambient Conditions

In order to study thethe meanmean annualannual efﬁciencyefficiency and thethe performanceperformance under cold ambient temperatures, three kinds of typical climaticclimatic conditions inin NanjingNanjing werewere chosenchosen asas experimentalexperimental ◦ ◦ ambient conditionsconditions in thethe experiment:experiment: JanuaryJanuary (T1(T1 == 2.4 °C,C, RH1 = 76%), July (T2 = 27.8 °C,C, RH2 = ◦ 81%), and the yearly average temperature andand relativerelative humidityhumidity (T3 (T3 == 15.415.4 °C,C, RH3 = 76%).

3.4. Experimental Cases 3.4. Experimental Cases Adjustment of the air duct valves and the style of the hot air cycle selected are shown in Table4. Adjustment of the air duct valves and the style of the hot air cycle selected are shown in Table 4. The semi-open heat pump drying systems were grouped according to the indraft of fresh air volume The semi-open heat pump drying systems were grouped according to the indraft of fresh air volume into the environment. For closed heat pump drying systems, they were divided into six groups into the environment. For closed heat pump drying systems, they were divided into six groups according to hot air bypass rate (BAR), as follows. according to hot air bypass rate (BAR), as follows.

ArAr QuantityQuantity Bypass BARBAR= = (1(1)) Air Quantity Air QuantityFlowing ThroughBypass the Evaporator Flowing Through the Evaporator Table 4. The style of the hot air cycle. Table 4. The style of the hot air cycle. Number The Style of the Hot Air Cycle Regulation Number The Style of the Hot Air Cycle Regulation Case 1 Open type Open air valves 1, 3, 5, 7, 9, and 12 CaseCase 2 1Semi-open Open type type 1 Open air valves Open air1, 3 valves (60%), 1, 5, 3, 7, 5, 7,9 (60%), 9, and 12and 12 Case 2 Semi-open type 1 Open air valves 1, 3 (60%), 5, 7, 9 (60%), and 12 Case 3 Semi-open type 2 Open air valves 1,3 (20%), 5, 7, 9 (20%), and 12 Case 3 Semi-open type 2 Open air valves 1,3 (20%), 5, 7, 9 (20%), and 12 CaseCase 4 4Closed Closed type BAR type BAR= 0 = 0Open Open air valves air valves 2, 5, 2, 7, 5, 7,and and 12 12 CaseCase 5 5Closed Closed type BAR type BAR= 0.2= 0.2Open Open air valves air valves 1 (20%), 1 (20%), 2, 5, 2, 7, 5, 7,8 (20%), 8 (20%), and and 12 12 CaseCase 6 6Closed Closed type BAR type BAR= 0.4= 0.4Open Open air valves air valves 1 (40%), 1 (40%), 2, 5, 2, 7, 5, 7,8 (40%), 8 (40%), and and 12 12 Case 7 Closed type BAR = 0.6 Open air valves 1 (60%), 2, 5, 7, 8 (60%), and 12 Case 7 Closed type BAR = 0.6 Open air valves 1 (60%), 2, 5, 7, 8 (60%), and 12 Case 8 Closed type BAR = 0.8 Open air valves 1 (80%), 2, 5, 7, 8 (80%), and 12 CaseCase 8 9Closed Closedtype BAR type = BAR 0.8 = 1Open air valves Open air1 (80%), valves 2, 1, 5, 2, 7, 5, 7,8 (80%), 8, and 12and 12 Case 9 Note:Closed In the type experiment, BAR = air1 valves not mentionedOpen in air the valves table were 1, closed.2, 5, 7, 8, and 12 Note : In the experiment, air valves not mentioned in the table were closed.

3.5. Experimental Procedures Every experimentexperiment was was performed performed under under a constant a constant drying drying temperature temperature of 50 ◦ofC and50 °C an and air velocity an air ofvelocity 1 m/s. of Under 1 m/s. the Under three selected the three ambient selected conditions, ambient every conditions, experimental every case experimental was performed case threewas timesperformed according three totimes the experimentalaccording to the procedure. experimental procedure. For allall experiments, experiments, the the drying drying system system was runwas for run about for 20about min to20 obtain min steady-stateto obtain steady conditions,-state andconditions, when the and drying when chamber the drying temperature chamber reachedtemperature 50 ◦C, reached 15 kg of 50 fresh °C, garlic15 kg slicesof fresh were ga putrlic inslices the dryingwere put chamber. in the drying When thechamber. moisture When content the moisture of the garlic content slices of reached the garlic 10% slices w.b., reached the garlic 10% slices w.b., were the takengarlic outslices of were the drying taken out chamber. of the drying Flow chart chamber. of experimental Flow chart of operating experimental procedures operating is as procedures shown in Figureis as shown7. in Figure 7. Sustainability 2018, 10, 3216 8 of 17 Sustainability 2018, 10, x FOR PEER REVIEW 8 o f 17

Step 1 Run for 20 minutes to Indoor air Run Indoor Run obtain steady-state conditioner humidifier ambient conditions

Step 2 Run Run for 20 minutes to Air valves Air fan Run Compressor obtain steady-state drying Set according to chamber temperature Table 4

Step 3 Put into Monitoring and recording Drying Material system operation using chamber Industrial Personal Computer

Step 4 Indoor air conditioner stop stop stop The experiment Compressor Air fan was terminated Indoor humidifier

FigureFigure 7 7.. FlowFlow c charthart of of experimental experimental operatingoperating procedures. procedures.

3.6.3.6. EvaluationEvaluation Parameters Parameters

TheThe HPDHPD should input thethe energyenergy andand extractextract thethe waterwater forfor thethe material.material. ToTo evaluateevaluate thethe systemsystem efficiencyefficiency ofof anan HPD,HPD, SMERSMER waswas alwaysalways used.used. For this study, thisthis waswas defineddefined asas thethe ratioratio ofof waterwater extractedextracted fromfrom materialmaterial toto thethe totaltotal energyenergy consumptionconsumption (including(including thethe electricelectric energyenergy consumedconsumed byby compressors,compressors, electric electric heater, heater, and and fans) fans) in in whole whole drying drying process, process, as as follows. follows. The The Dehumidification Dehumidification QuantityQuantity SMERSMER== ((2)2) ElectricElectric Energy Energy Consumption Consumption total total wherewhere SMER SMER is is measuredmeasured in in kg/(kWh). kg/(kWh). ForFor HPDHPD evaluation, evaluation, in in addition addition to consideringto considering energy energy consumption, consumption, the efficiency the efficiency of the dryingof the processdrying isprocess also an is important also an important indicator. indicator. In this study, In this MER study, was used MER to was evaluate used theto evaluate drying efficiency the dryin ofg HPD,efficiency as follows. of HPD, as follows. TheThe Dehumidification Dehumidification QuantityQuantity MERMER== ((3)3) TimeTimeUsed Used wherewhere “Time “Time Used” Used” refersrefers to to the the time time thethe material material is is placed placed inin thethe dryingdrying chamberchamber until until the the dryingdrying isis completed,completed, andand MERMER is is measuredmeasured in in kg/h.kg/h. 4. Results and Discussions 4. Results and Discussions 4.1. Experimental Results 4.1. Experimental Results Comparing the data in the table, it is easy to find that the higher the degree of closure of HPD, the smallerComparing the influence the data of external in the environmentaltable, it is easy factors. to find Whenthat the the higher hot air the circulation degree of mode closure of theof HPD, HPD isthe the smaller open type, the influence the performance of external of the environmental HPD is greatly factors. affected When by ambientthe hot air conditions. circulation When mode the of hotthe airHP circulation D is the open mode type, of the the performance HPD is the semi-open of the HPD type, is greatly the performance affected by of ambient the HPD conditions. is affected When both bythe fresh hot air indraftcirculation rate andmode the of ambient the HPD conditions. is the semi When-open the type, hot air the circulation performance mode of of the the HPD isis theaffected closed both type, by the fresh performance air indraft ofrate the and HPD the is ambient greatly affectedconditions. by theWhe BAR.n the Therefore, hot air circulation the discussion mode of the HPD is the closed type, the performance of the HPD is greatly affected by the BAR. Therefore, Sustainability 2018, 10, 3216 9 of 17 and analysis of the three hot air circulation methods of HPD are presented separately in the following sections. Experimental Results is as shown in Table5.

Table 5. Experimental results.

Ambient Condition 1 Ambient Condition 2 Ambient Condition 3 Number (T = 2.4 ◦C, RH = 76%) (T = 15.4 ◦C, RH = 76%) (T = 27.8 ◦C, RH = 81%) MER SMER MER SMER MER SMER Case 1 2.215 ± 0.003 d 1.028 ± 0.002 f 2.093 ± 0.001 f 1.031 ± 0.003 e 1.927 ± 0.001 h 1.037 ± 0.003 e Case 2 2.200 ± 0.001 d 1.120 ± 0.002 b 2.065 ± 0.003 c 1.118 ± 0.003 bc 2.067 ± 0.003 f 1.122 ± 0.002 bc Case 3 2.087 ± 0.002 b 1.109 ± 0.002 c 2.113 ± 0.003 d 1.108 ± 0.002 cd 2.113 ± 0.002 d 1.115 ± 0.002 c Case 4 2.152 ± 0.001 c 1.093 ± 0.002 e 2.190 ± 0.001 a 1.101 ± 0.001 d 2.189 ± 0.002 a 1.095 ± 0.004 d Case 5 2.139 ± 0.001 c 1.102 ± 0.001 d 2.146 ± 0.001 b 1.103 ± 0.003 d 2.144 ± 0.002 b 1.109 ± 0.001 c Case 6 2.121 ± 0.002 c 1.130 ± 0.002 a 2.128 ± 0.002 e 1.134 ± 0.004 a 2.128 ± 0.002 c 1.136 ± 0.001 a Case 7 2.083 ± 0.001 ab 1.121 ± 0.002 b 2.082 ± 0.003 g 1.122 ± 0.002 b 2.083 ± 0.003 e 1.125 ± 0.002 b Case 8 2.067 ±0.003 ab 1.109 ± 0.002 c 2.072 ± 0.001 h 1.113 ± 0.003 c 2.073 ± 0.003 f 1.113 ± 0.004 c Case 9 2.058 ± 0.002 a 1.107 ± 0.002 cd 2.050 ± 0.002 i 1.107 ± 0.002 cd 2.053 ± 0.002 g 1.111 ± 0.004 c Note: Mean ± standard error. Different shoulder letters in the same column indicate a signiﬁcant p < 0.05 difference, as below. Where T is temperature, RH is relative humidity, MER is moisture extraction rate, SMER is speciﬁc moisture extraction rate.

4.2. Theory In the equations that follow, the following symbols with subscripts and superscripts are used, as follows.

Symbol Name Units SMER Specific moisture extraction rate kg/(kW·h) MER Moisture extraction rate kg/h COP Coefficient of performance - cp Specific heat at constant pressure kJ/(kg·K) T Temperature K RH Relative humidity - d Absolute humidity kg water vapor/kg dry air DR Drying rate g/(g·h) rs Evaporative latent heat of water kJ/kg h Enthalpy kJ/kg m Mass flow rate kg/s QL Cooling capacity of heat pump kW QH Heating capacity of heat pump kW TM Torque of the compressor N·m n Rotation rate of the compressor r/min W Power consumption kW x Fresh air entering rate - Subscript Name dr drying chamber evap evaporator cond condenser ref refrigerant air circulated air in inlet out outlet comp compressor HPD heat pump dryer eh electric heater Sustainability 2018, 10, 3216 10 of 17

A HPD without an electric heater consists of a heat pump system and drying chamber. In the ideal model, the process of heating hot air in the drying chamber can be regarded as a hot air adiabatic cooling process, as follows.

cp(Tair,dr,in − Tair,dr,out) = (dair,dr,out − dair,dr,in)rs (4)

However, in practice, the hot air enthalpy decreases when hot air ﬂows through the drying chamber, as follows.

∆hair,dr = cp(Tair,dr,in − Tair,dr,out) − rs(dair,dr,out − dair,dr,in) (5)

For materials, there is the following formula.

M − M + DR = t t ∆t (6) ∆t where t1 and t2 are the drying times (in h), and Mt and Mt+∆t are the moisture contents (in db) at times t and (t + ∆t), respectively. In the drying chamber, the lost moisture of the material migrates to circulated air. The following formula can be obtained according to the law of conservation of mass.

DR m (d − d ) = G (7) air,dr air,dr,out air,dr,in 3, 600, 000 where G is the mass of the absolute drying material. There are two heat exchange processes with the heat pump system during the hot air cycle, as follows. QL = mair,evap hair,evap,in − hair,evap,out = mref,evap href,evap,out − href,evap,in (8)

QH = mair,cond(hair,cond,out − hair,cond,in) = mref,cond(href,cond,in − href,cond,out) (9) The enthalpy value of circulated air is calculated as follows.

hair = cp,airT + rsd (10)

cp,air = 1.01 + 1.84d (11) For the heat pump system, there is the following formula.

T n W = Q − Q = M (12) comp H L 9550 By adjusting the motor frequency, the compressor speed and the compressor power can be controlled. The heating coefﬁcient of performance (COP) formula is as follows.

Q COP = H (13) Wcomp

4.3. Discussion and Analysis of the Closed Type HPD The enthalpy–humidity diagram of circulated air is used to analyze and discuss the closed type HPD. As shown in Figure8, the air ﬂowed through 1-2(1’-2’) in the drying chamber. It was expected that point 2 was delegated the outlet parameter from the mainly drying chamber, as shown in Formula (4).∆h1−2 is enthalpy loss of circulated air when the hot air passes through the drying chamber. When Sustainability 2018, 10, 3216 11 of 17 the BAR was zero, the air ﬂowed through 2-3-4 (2’-3’-4’) in the evaporator. The heat absorbed from Sustainabilitycondenser 2018 through, 10, x 4-1(4’-1’).FOR PEER REVIEW 11 o f 17 h 1' 1 2'

6' 3 3' 5' 4' 4

d FigureFigure 8 8.. EnthalpyEnthalpy–humidity–humidity diagram diagram of of a close closed d typetype HPD.HPD.

According to to the the second second law of thermodynamics, thermodynamics, heat will not flow flow spontaneously from a cold object toto a hota hot one, one, so W compso Wis indispensable is indispensable for HPD. Accordingfor HPD. toAccording the first law to ofthe thermodynamics, first law of for a closed type HPD, the formula is as follows. thermodynamics, for a closed comptype HPD, the formula is as follows. W + W = m h + Q (14) Wcomp + Weh = mair∆hair,dr, + ∆QHPD (14) where Q is HPD’s heat changecomp value pereh unitair time,∆ air indr kJ/s.∆ HPD where ∆QHPD is HPD’s heat change value per unit time, in kJ/s. When HPD is in a steady-state, W is greater than m h , . Many researchers use the When∆ HPD HPD is in a steady-state, W is greater than m ∆h . Many researchers use the auxiliary condenser to remove excess heatcomp from the system. Asair we allair,dr know, this will waste a lot of auxiliary condenser to remove excess heatcomp from the system. Asair we∆ allair dr know, this will waste a lot of energy. For a clos ed type HPD, if the inlet of the drying chamber maintains a constant temperature, energy. For a closed type HPD, if the inlet of the drying chamber maintains a constant temperature, Q should be small to save energy. The compressor power can be controlled by adjusting the ∆QHPD should be small to save energy. The compressor power can be controlled by adjusting the frequencyHPD converter, so it is achievable to decrease Q . ∆frequency converter, so it is achievable to decrease ∆QHPD. It is known that if W decrease, Q will decrease.HPD As shown in Figure 5, 1-2-3-4 is the cycle It is known that if Wcomp decrease, Q will decrease.∆ As shown in Figure5, 1-2-3-4 is the cycle of circulated air when compressor frequencyL is high and 1’-2’-3’-4’ is the cycle of circulated air when of circulated air when compressorcomp frequencyL is high and 1’-2’-3’-4’ is the cycle of circulated air when compressor frequency is low. In the experiment, we found that drying rate (DR) will decrease when compressor frequency is low. In the experiment, we found that drying rate (DR) will decrease when the air humidity at the inlet of the drying chamber increases. This is consistent with Hao-Yu’s the air humidity at the inlet of the drying chamber increases. This is consistent with Hao-Yu’s research research conclusions [21]. According to Formula (7), d > d . This increases the time for conclusions [21]. According to Formula (7), ∆d2−1 > ∆d20−10 . This increases the time for drying the drying the material to the specified moisture content and the electric′ ′ energy consumed by the fan. material to the specified moisture content and the electric∆ 2 energy−1 ∆ consumed2 −1 by the fan. This, in turn, This, in turn, affects the MER and SMER values of HPD. affects the MER and SMER values of HPD. As we know from experimental data, when the hot air circulation mode of the HPD is the closed As we know from experimental data, when the hot air circulation mode of the HPD is the closed type, the performance of the HPD is greatly affected by the BAR. As shown in Figure 5, if the system type, the performance of the HPD is greatly affected by the BAR. As shown in Figure5, if the system operated at different BARs, air flow from the main drying chamber divided into two paths. One was operated at different BARs, air flow from the main drying chamber divided into two paths. One 2’-3’-5’ which flowed through the evaporator and extracts the moisture from the air. The other path was 2’-3’-5’ which flowed through the evaporator and extracts the moisture from the air. The other flow s through the air bypass duct. The two airs mixed at point 6’. The heat absorbed from the path flows through the air bypass duct. The two airs mixed at point 6’. The heat absorbed from condenser from 6’ to 1’. Thus, the HPD with air bypass duct could decrease the heat absorption as the condenser from 6’ to 1’. Thus, the HPD with air bypass duct could decrease the heat absorption follows. as follows. ( ) QQL == mmair,dr, (hh20 − hh60 ) == cp,air, (T20 − T60))++rrs((d 20 −d 60)) (15(15)) ′ ′ ′ ′ ′ ′ Q +LW =airmdr 2 (h 6 h ) p=airc 2 (T 6 T )s+ r2(d 6 d ) QH + Weh = mair,dr, (h−10 − h60 ) = cp,air, (−T10 − T60 ) + rs(−d10 − d60 ) (16(16)) ′ ′ ′ ′ ′ ′ H eh air dr 1 6 p air 1Q 6 s 1 6 W − = Q Q = Q−H − (17) Wcomp = QH − QL = (17) COPCOPH comp H L After HPD enters steady-state conditions, the −COP of the heat pump system tends to be stable. Compared with the HPD without an air bypass duct, the heating capacity of the HPD with an air bypass duct is reduced, and the compressor work is reduced as follows.

Q = Q = m , (h h ) (18) ′ ′ ∆ L ∆ H air dr 6 − 4 Sustainability 2018, 10, 3216 12 of 17

After HPD enters steady-state conditions, the COP of the heat pump system tends to be stable. Compared with the HPD without an air bypass duct, the heating capacity of the HPD with an air bypass duct is reduced, and the compressor work is reduced as follows.

∆QL = ∆QH = mair,dr(h60 − h40 ) (18)

∆Q Sustainability 2018, 10, x FOR PEER REVIEW ∆W = H 12(19) o f 17 comp COP Q According to the ﬂuid ﬂow continuity equation,W = a closed HPD’s circulated air can be calculated(19) using the equation as follows. COPH comp ∆ According to the fluid flow continuity∆ equation, a closed HPD’s circulated air can be calculated using the equation as follows.m air,dr = mair,evap + mBypass = mair,cond (20)

m , = m , + m = m , (20) Point 6’ is the confluence of mair,evap and mBypass, with the following equation. air dr air evap Bypass air cond Point 6’ is the confluence of m , and m , with the following equation. BAR 1 d 0 = d 0 + d 0 (21) 6air evap+BAR Bypass2 + 1 5 d =1 BAR d +1 BAR d (21) 1 + BAR 1 + BAR ′ ′ ′ 6 BAR 2 1 5 h 0 = BAR h 0 + 1 h 0 (22) 6h =1 + BAR h2 +1 + BAR h5 (22) 1 + BAR 1 + BAR ′ ′ ′ Circulated air at the outlet of the6 drying chamber2 is at a high5 temperature and high humidity, Circulated air at the outlet of the drying chamber is at a high temperature and high humidity, it it needs to flow through the evaporator to reduce its temperature and humidity. As BAR increases, needs to flow through the evaporator to reduce its temperature and humidity. As BAR increases, the the amount of air flowing through the evaporator decreases, and the heat exchange effect of evaporator amount of air flowing through the evaporator decreases, and the heat exchange effect of evaporator will increase. However, the improvement of the heat transfer effect of the evaporator is not unlimited will increase. However, the improvement of the heat transfer effect of the evaporator is not and, as the BAR increases, the flow of air through the evaporator drops to a certain amount, and d5’ unlimited and, as the BAR increases, the flow of air through the evaporator drops to a certain can no longer be reduced. amount, and d5’ can no longer be reduced. The following formula can be derived based on Formula (10). The following formula can be derived based on Formula (10). BARBAR 11 h6h0 = = c(p,airc TT20 ++rsrdd20 )++ (ccp,airTT50 + rsd50 ) ((23)23) 1 +1 +BARBAR , 11++BARBAR , ′ ′ ′ ′ ′ 6 p air 2 s 2 p air 5 s 5 AccordingAccording toto thethe aboveabove analysis,analysis, underunder thethe premisepremise ofof keepingkeeping d60 unchangedunchanged oror slightlyslightly increasing,increasing, increasingincreasing thethe valuevalue ofof h 0 can can improve improve the the HPD HPD performance performance index.′ index. AccordingAccording toto thethe 6 𝑑𝑑6 experimentalexperimental results,results, underunder thethe conditionsconditions′ ofof thisthis paper,paper, thethe bestbest SMER SMER valuevalue isis obtained obtainedwhen when the the ℎ6 BARBAR isis equalequal toto 0.4.0.4. However,However, with with the the rise rise of of BAR, BAR,d d6’6’ willwill alsoalso rise,rise, whichwhich willwill affectaffect thethe HPD’sHPD’s DRDR andand MER,MER, as as shown shown in in Figure Figure9 .9.

FigureFigure 9 9.. ExpeExperimental rime ntal drying kinetickine tic curvescurve s ofof garlic garlic slices. slice s.

4.4. Discussion and Analysis of Open Type and Semi-Open Type HPD

The enthalpy–humidity diagram of circulated air is used to analyze and discuss the open type HPD. As shown in Figure 10, point a (a’) indicates parameters of the outside ambient air. The heat was absorbed from the condenser through a–b (a’–b’), and the circulated air that flowed through b-c (b’-c’) in the drying chamber, as shown in Formula (7). is the enthalpy loss of circulated air when the hot air passes through the drying chamber. The air flowed through c-d-e (c’-d’-e’) in the ∆ℎb−c evaporator and was discharged to the outside environment. Sustainability 2018, 10, 3216 13 of 17

4.4. Discussion and Analysis of Open Type and Semi-Open Type HPD The enthalpy–humidity diagram of circulated air is used to analyze and discuss the open type HPD. As shown in Figure 10, point a (a’) indicates parameters of the outside ambient air. The heat was absorbed from the condenser through a–b (a’–b’), and the circulated air that ﬂowed through b-c (b’-c’) in the drying chamber, as shown in Formula (7).∆hb−c is the enthalpy loss of circulated air when the hot air passes through the drying chamber. The air ﬂowed through c-d-e (c’-d’-e’) in the evaporator andSustainability was discharged 2018, 10, x FOR to the PEER outside REVIEW environment. 13 o f 17 h b' b c'

a’

a d d' e' e

d Figure 10 10.. EnthalpyEnthalpy–humidity–humidity diagram ofof thethe openope n type type HPD. HPD.

AsAs wewe knowknow fromfrom experimentalexperimental data,data, whenwhen thethe hothot airair circulationcirculation modemode ofof thethe HPDHPD isis openopen type,type, thethe performanceperformance ofof thethe HPDHPD isis greatlygreatly affected affected byby ambientambient conditions.conditions. AsAs shownshown inin FigureFigure 1010,, a-b-c-d-ea-b-c-d-e isis thethe cyclecycle ofof circulatedcirculated airair whenwhen thethe ambientambient temperaturetemperature andand ambientambient humidityhumidity areare low, andand a’-b’-c’-d’-e’a’-b’-c’-d’-e’ is is the the cycle cycle of of circulated circulated air air when when the the ambient ambient temperature temperature and and ambient ambient humidity humidity are high.are high. According According to the to first the lawfirst of law thermodynamics, of thermodynamics, for the for open the type open HPD, type the HPD, formula the formula is as follows. is as follows. Wcomp + Weh = mair(∆hair,dr + hair,HPD,out − hair,HPD,in) (24) W + W = m ( h , + h , , h , , ) (24) Point a shows the inlet air state parameters of the HPD when the HPD works in a high ambient Point a shows the inletcomp air stateeh parametersair ∆ air ofdr the HPDair HPD whenout − theair HPDHPD in works in a high ambient temperature and high ambient humidity, and point a’ shows the inlet air state parameters of HPD temperature and high ambient humidity, and point a’ shows the inlet air state parameters of HPD when the HPD works in a low ambient temperature and low ambient humidity. Ta is greater than Ta’, when the HPD works in a low ambient temperature and low ambient humidity. Ta is greater than so if the lower temperature air is heated to the same temperature, the HPD needs to consume more Ta’, so if the lower temperature air is heated to the same temperature, the HPD needs to consume power for the compressor and electric heater, and this would affect the HPD’s SMER. According to more power for the compressor and electric heater, and this would affect the HPD’s SMER. the experimental results, ∆d > ∆d 0 0 , so a high ambient temperature and high ambient humidity According to the experimentalb−c results,b −c d > d , so a high ambient temperature and high reduce the open type HPD’s MER and material’s DR. ambient humidity reduce the open type HPD’s MER ′ and′ material’s DR. The enthalpy–humidity diagram of circulated∆ b−c ∆ airb − isc used to analyze and discuss the semi-open The enthalpy–humidity diagram of circulated air is used to analyze and discuss the semi-open type HPD. As shown in Figure 11, point F(F’) indicates parameters of outside ambient air, circulated air, type HPD. As shown in Figure 11, point F(F’) indicates parameters of outside ambient air, circulated and outside ambient air mixed at point A (A’). The heat was absorbed from the condenser through A–B air, and outside ambient air mixed at point A (A’). The heat was absorbed from the condenser (A’–B’), and circulated air flowed through B-C (B’-C’) in the drying chamber, as shown in Formula (7). through A–B (A’–B’), and circulated air flowed through B-C (B’-C’) in the drying chamber, as shown ∆h is the enthalpy loss of circulated air when the hot air passes through the drying chamber. The in BFormula−C (7). h is the enthalpy loss of circulated air when the hot air passes through t he air flowed through C-D-E (C’-D’-E’) in the evaporator and then divided two paths. One path mixed drying chamber. The air flowed through C-D-E (C ’-D’-E’) in the evaporator and then divided two ∆ B−C paths. One path mixed with the outside ambient air at point F (F’), and the other path discharged to the outside environment. Point A’s state parameters can be calculated as follows. x 1 d = d + d (25) 1 + x 1 + x A x F 1 E h = h + h (26) 1 + x 1 + x A F E Air Quantity x = (27) Air Quantity Fresh air entering

Still circulated air Sustainability 2018, 10, 3216 14 of 17 with the outside ambient air at point F (F’), and the other path discharged to the outside environment. Point A’s state parameters can be calculated as follows.

x 1 d = d + d (25) A 1 + x F 1 + x E

x 1 h = h + h (26) A 1 + x F 1 + x E

Air Quantity Fresh air entering x = (27) Air Quantity Sustainability 2018, 10, x FOR PEER REVIEW Still circulated air 14 o f 17 h B'

B C'

F’ A’ D D' A E' F E

d FigureFigure 11 11.. EnthalpyEnthalpy–humidity–humidity diagram diagram of the semi-opensemi-open type type HPD. HPD.

As we knowknow from experimentalexperimental data,data, when the hot air circulation mode of the HPD is open type,type, thethe performanceperformance ofof the HPDHPD is greatlygreatly affectedaffected byby ambientambient conditions.conditions. As shown in Figure 11,11, E,E, F-A-B-C-D-EF-A-B-C-D-E isis the cyclecycle ofof circulatedcirculated airair whenwhen thethe ambientambient enthalpyenthalpy andand ambientambient humidityhumidity areare lowerlower thanthan circulatedcirculated air’sair’s enthalpyenthalpy andand humidityhumidity atat the outlet of the evaporator respectively, respectively, andand E’,E’, F’-A’-B’-C’-D’-E’F’-A’-B’-C’-D’-E’ is the the cycle cycle of of circulated circulated air air when when the the ambient ambient entha enthalpylpy and and ambient ambient humidity humidity are arehigher higher than than circulated circulated air’s air’s enthalpy enthalpy and and humidity humidity at atthe the outlet outlet of of the the evaporator, evaporator, respectively. AccordingAccording toto the the ﬁrst first law law of thermodynamics,of thermodynamics, for thefor semi-openthe semi-open type HPD,type theHPD, formula the formula is as follows. is as follows. Wcomp + Weh = mair[∆hair,dr + x(hair,HPD,out − hair,HPD,in)] (28) W + W = m [ h , + x h , , h , , ] (28) Point A shows the inlet air state parameters of the condenser when the HPD works in a high Point A shows the inletcomp air stateeh parametersair ∆ air dr of the� ai condenserr HPD out − whenair HPD thein � HPD works in a high ambientambient temperaturetemperature and and high high ambient ambient humidity, humidity, and and point point A’ A’ shows shows the the inlet inlet air air state state parameters parameters of theof the condenser condenser when when the the HPD HPD works works in in a low a low ambient ambient temperature temperature and and low low ambient ambient humidity. humidity. TA TA is greateris greater than than TA’, TA’, so so ifthe if the lower lower temperature temperature air air is heated is heated to theto the same same temperature, temperature, the the HPD HPD needs needs to consumeto consume more more power power for thefor compressorthe compressor and electricand electric heater, heater, and this and would this would affect theaffect HPD’s the SMER.HPD’s > 0 0 AccordingSMER. According to the experimental to the experimental results, results,∆dB−C d∆dB >−C ,d so a high, so a ambient high ambient temperature temperature and high and ambient humidity reduce the open type HPD’s MER and the material’s DR. high ambient humidity reduce the open type HPD’s MER and′ the′ material’s DR. ∆ B−C ∆ B −C 4.5. Discussion on Energy Consumption and Economic Considerations 4.5. Discussion on Energy Consumption and Economic Considerations As shown in Figure 12, the analysis of the HPD’s electricity consumption through the heat As shown in Figure 12, the analysis of the HPD’s electricity consumption through the heat pump, fan, electric heater, and control system accounted for 50, 39, 10, and 1% of total consumption, pump, fan, electric heater, and control system accounted for 50, 39, 10, and 1% of total consumption, respectively. The fan consumes much power. This is mainly because the HPD uses a lot of duct valves respectively. The fan consumes much power. This is mainly because the HPD uses a lot of duct valves and a complex duct layout to achieve the experimental requirements, and this affects SMER as shown in Formula (2). Thus, this machine needs more fluid mechanics analysis and optimization. Sustainability 2018, 10, 3216 15 of 17 and a complex duct layout to achieve the experimental requirements, and this affects SMER as shown Sustainabilityin Formula 2018 (2)., 10 Thus,, x FOR this PEER machine REVIEW needs more ﬂuid mechanics analysis and optimization. 15 o f 17

FigureFigure 12 12.. HPD’sHPD’s e leelectricity ctricity consumption consumption percentage. percentage.

Global garlicgarlic production production is approximatelyis approximately 25 million 25 million tons annually, tons annually, and China’s and garlic China production’s garlic productionis about 10 millionis about tons. 10 million In order tons. to improve In order productto improve quality, product dehydrated quality, garlic dehydrated slices have garlic a largerslices havemarket. a larger At present, market. drying At present, dehydrated drying garlic dehydrated slices in China garlic generally slices in usingChina hot generally air circulation using drying.hot air circulationThe energy drying. consumption The energy per unit consumption product of the per hot unit air product circulation of the dryer hot is air 2.8 circulation (kw·h)/kg. dryer In practical is 2.8 (kwproduction,·h)/kg. In after practical reducing production, the energy after consumption reducing of the the fan,energy energy consumption consumption of perthe unitfan, productenergy consumptionof HPD can be per reduced unit product to 1 (kw of· h)/kg.HPD can This be is reduced because to traditional 1 (kw∙h)/kg. hot T airhis circulationis because traditional drying mainly hot airuses circulation electric heaters drying as mainly heat sources. uses electric A large heaters amount as of heat heat sources. energy inA thelarge air amount flowing of out heat of theenergy drying in thechamber air flowing is wasted. out This of heatthe drying can be recoveredchamber usingis wasted. the evaporator This heat in can the HPD.be recovered using the evaporatorThe traditional in the HPD hot. air circulation drying model is as follows. The traditional hot air circulation drying model is as follows. Weh = mair(hair,dr,in − hair,HPD,in) (29) W = m (h , , h , , ) (29) The formula for open type HPD model is Formula (24), that for the semi-open type HPD is The formula for open type HPDeh modelair isair Formuladr in air(24),HPD thatin for the semi-open type HPD is Formula (28), and that for the closed type is Formula (14).− Formula (28), and that for the closed type is Formula (14). 5. Conclusions 5. Conclusions In this paper, a multifunctional air source heat pump dryer was designed, and an experimental investigationIn this paper, on drying a multifunctional performance air for source this dryer heat with pump diffident dryer styleswas designed, of hot air and cycle an was experimental conducted investigationby drying 3 mmon drying garlic chipsperformance in three for typical this conditionsdryer with of diffi ambientdent styles temperature of hot andair cycle humidity. was conductedThe enthalpy–humidity by drying 3 mm diagram garlic of chips circulated in three air wastypical used conditions to analyze ofthe ambient drying temperature rate and energy and humidity.consumption The of enthalpy the HPD’s–humidity different diagram hot air circulationof circulated modes air was in used different to analyze ambient the temperature drying rate and energyhumidity consumption conditions. Theof followingthe HPD’s conclusions different couldhot air be reached.circulation modes in different ambient temperature and humidity conditions. The following conclusions could be reached. 1. The open type HPD is more affected by the environmental temperature and humidity conditions. 1. The open type HPD is more affected by the environmental temperature and humidity In summer, the high temperature and humidity of the ambient air makes the MER smaller and conditions. In summer, the high temperature and humidity of the ambient air makes the MER makes SMER larger. However, in winter, the low temperature and low humidity of the ambient smaller and makes SMER larger. However, in winter, the low temperature and low humidity air makes MER larger and makes SMER smaller. of the ambient air makes MER larger and makes SMER smaller. 2. The semi-open type HPD is affected by the combined effect of ambient temperature and humidity 2. The semi-open type HPD is affected by the combined effect of ambient temperature and conditions and the proportion of fresh air in the environment. Under different conditions of humidity conditions and the proportion of fresh air in the environment. Under different ambient temperature and humidity, changing the ratio of the indraft fresh air by adjusting the conditions of ambient temperature and humidity, changing the ratio of the indraft fresh air by duct valves can significantly improve the performance of the system. adjusting the duct valves can significantly improve the performance of the system. 3. The closed type HPD is less affected by ambient temperature and humidity conditions and is 3. The closed type HPD is less affected by ambient temperature and humidity conditions and is greatly affected by the bypass air rate. When the BAR is 0.4, the HPD’s SMER is maximal. As the greatly affected by the bypass air rate. When the BAR is 0.4, the HPD’s SMER is maximal. As BAR increases, MER decreases. the BAR increases, MER decreases. 4. Through a thermodynamic analysis of the HPD, it is very easy to find that low humidity inlet air in the condenser can improve drying rate. A high enthalpy of the condenser inlet air can reduce energy consumption, and low humidity and high enthalpy of the condenser inlet air Sustainability 2018, 10, 3216 16 of 17

4. Through a thermodynamic analysis of the HPD, it is very easy to ﬁnd that low humidity inlet air in the condenser can improve drying rate. A high enthalpy of the condenser inlet air can reduce energy consumption, and low humidity and high enthalpy of the condenser inlet air can be obtained by adjusting the HPD hot air circulation method in different ambient temperature and humidity conditions.

Author Contributions: H.L. and K.C. conceived and designed the experiments; J.L. performed the experiments; H.L. and K.Y. analyzed the data; R.F. contributed analysis tools; H.L. and S.A.S. wrote the paper. Funding: The authors gratefully acknowledge the financial support from the Jiangsu Agricultural Science and Technology Innovation Fund [CX (17)1003]. Conflicts of Interest: The authors declare no conflict of interest.

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