Experimental Analysis of a Heat Pump Dryer with an External Desiccant Wheel Dryer

processes

Article Experimental Analysis of a Heat Pump Dryer with an External Desiccant Wheel Dryer

Kai-Shing Yang 1, Khalid Hamid 2, Shih-Kuo Wu 3, Uzair Sajjad 2 and Chi-Chuan Wang 2,*

1 Department of Refrigeration, Air Conditioning and Energy Engineering, National Chin-Yi University of Technology, Taichung 411, Taiwan; [email protected] 2 Department of Mechanical Engineering, National Yang Ming Chiao Tung University, Hsinchu 300, Taiwan; [email protected] (K.H.); [email protected] (U.S.) 3 Green Energy & Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-3-571-2121 (ext. 55105); Fax: +886-3-572-0634

Abstract: This study examines the performance of three heat pump dryers: the original reference design, a modified drying chamber, and an external desiccant wheel design. Unlike most existing studies that normally adopt organic products as the drying materials, in this study we used moist sodium polyacrylate (Orbeez) as the drying material for consistent characterization of the heat pump performance. R-134a was adopted as the refrigerant for the heat pump system. The experiments were performed subject to different weights of Orbeez (drying material) at a constant volumetric flow rate of 100 m3/h. During experimentation, different parameters like the coefficient of performance

(COPHP), drying rate, heat transfer rate by the condenser, moisture extraction rate, and specific moisture extraction rate were calculated. The average COP , mass transfer rate, heat transfer HP rate, MER, and SMER of the system were calculated as 3.9, 0.30 kg/s, 0.56 kW, 0.495 kg/h, and 1.614 kg/kWh, respectively. The maximum COP for the refrigeration system was achieved at lower Citation: Yang, K.-S.; Hamid, K.; Wu, S.-K.; Sajjad, U.; Wang, C.-C. test loads with the desiccant wheel. The moisture extraction rate for a lower test loading was higher Experimental Analysis of a Heat than that for a higher test load due to the higher penetration of drying air at the lower test load, Pump Dryer with an External although the maximum test load showed the maximum relative humidity at the dryer outlet. The Desiccant Wheel Dryer. Processes desiccant wheel showed good performance in terms of moisture extraction rate and COPHP, but it 2021, 9, 1216. https://doi.org/ showed poor performance in terms of the specific moisture extraction rate due to the high power 10.3390/pr9071216 consumption (around 2.6 kW) of the desiccant dehumidifier. The moisture extraction rate (MER) for all designs increased to a maximum value, followed by consistent decline. However, the maximum Academic Editor: Dariusz Dziki MER for the desiccant design exceeded those for the other designs.

Received: 15 June 2021 Keywords: Orbeez; heat pump dryer; desiccant wheel; coefﬁcient of performance Accepted: 13 July 2021 Published: 15 July 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional afﬁl- Drying is widely used in polymer, food, pharmaceutical, mineral, and other indus- iations. trial processes. However, current commercially available dryers mainly adopt hot air to complete the drying process, and most of the traditional hot air dryers use direct electric heating to raise the inlet air temperature to evaporate the moisture contents of the dry matter. The drying process is strongly associated with ambient conditions in terms of humidity. This is an especially essential consideration in tropical or subtropical countries Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. where the humidity is comparatively high. For example, the average relative humidity in This article is an open access article Taiwan is greater than 70% throughout the year, thereby imposing difﬁculties in drying distributed under the terms and systems and consuming more energy in the heating process. conditions of the Creative Commons An alternative drying process is via heat pump, which transports heat energy from a Attribution (CC BY) license (https:// low temperature source to a high temperature source with supplied work. It can offer much creativecommons.org/licenses/by/ larger thermal energy at the expense of much lower input work. Heat pumps have potential 4.0/). applications in heating, ventilation, and air conditioning, water heating, district heating,

Processes 2021, 9, 1216. https://doi.org/10.3390/pr9071216 https://www.mdpi.com/journal/processes Processes 2021, 9, 1216 2 of 17

and industrial heating, and heat pump dryers (HPDs) are energy efficient when compared to conventional heater dryers. Conventional dryers possess numerous limitations, such as non-homogeneous product quality caused by under/over-drying due to inadequate or long-time exposure of the product. Besides this, the lower contact efficiency between the product being dried and the drying medium is also a big issue. Furthermore, over-drying may result in surface hardening of the drying product. The aforementioned problems cause very poor drying and increase operational costs, which is especially likely with traditional drying. Hence, many efforts have been made in the literature to resolve these limitations of conventional drying methods. These efforts include improving the product quality and overcoming the operational problems. Most of the conventional drying methods (either by convection or direct heating) employ fossil fuels as the power source and are thereby accompanied by greenhouse gas emissions and pollution, which are a prime concern. Biomass and renewable energy may be incorporated to tackle this problem, but again, concerns regarding better preservation and drying product quality may prevail. This is because the quality and the cost of the drying product are directly influenced by the drying method and the operational conditions. Another important point to be considered is reliable and consistent product quality when using the same drying method. In tackling the aforementioned problems, the HPD system has been proven a good solution. The HPD system has several advantages. Firstly, it can be used for effective heat recovery. Secondly, it offers a wide range of operational conditions (humidity, air temperature) that help to achieve better quality. Thirdly, the drying process can be performed at relatively low temperature to preserve the quality of the drying product. This is because the HPD systems employ dehumidified and low-temperature air as the drying medium. A short summary of experimental investigations on HPD systems for the drying of different products can be found in Table1. From Table1, an HPD is applicable for the drying of various products, including vegetables, fruits, etc., and studies evaluated the HPD system in terms of different performance metrics of the drying process, such as weight reduction, moisture extraction rate, specific moisture extraction rate, coefficient of performance, and heat transfer rate for different time durations. However, the foregoing studies mainly focused on different organic products, which are normally quite sensitive to drying conditions. Hence, one of the main objectives of this study is to adopt a non- organic drying material for examination to characterize the more general performance of a heat pump dryer. Moreover, limited investigations can be found in the literature regarding the development and assessment of new HPD system designs. Especially for humid environments, some systems should be integrated with an HPD to pre-condition (pre-dehumidify and pre-heat) the air before it enters the dryer. Thus, the present study examines the applicability of adding a desiccant rotary wheel along with the HPD. In order to increase the drying efficiency of the traditional heat pump dryer and pre-condition the humid air before it enters the dryer, in this study, we developed an adsorption dehumidification device at the entrance of the traditional heat pump dryer, namely, a desiccant rotary wheel, to reduce the inlet humidity and use the energy released by the adsorption process to increase the inlet temperature. Subsequently, the applicability of this design is elaborated in detail. Experiments were performed to evaluate the drying performance on Orbeez material (sodium polyacrylate) over a period of eight hours. Moreover, experiments were conducted with a modified drying chamber to achieve better drying by improving the air distribution and reducing the air resistance. For each case, various performance parameters such as the weight reduction, moisture extraction rate, specific moisture extraction rate, coefficient of performance, and heat transfer rate were calculated for a constant flow rate. Processes 2021, 9, 1216 3 of 17

Table 1. A summary of experimental investigations on HPD systems for drying.

Product Moisture Moisture W (kW) Drying SMER Dried Product C DT RH MER kg /h Initial Content Content (Q ) w Time (kg /kW.h) Ref. EV (◦C) (%) (MC%/h) w Mass (kg) Initial Final (kW) (hr or min) (COP) 30 55 3.1–4.48 Vegetable seeds [1] 0.2 30 (N/S) 6 (N/S) No No 2 55 50–80 (4.2–6.5) Paddy [2] 1200 30–35 (w.b.) 13 (w.b.) 4.2 42, 26 26,14 8–15.9 15–16 h 2.0 0.5–1 25, 30, (32.9– Potato slices [3] No 4 kgw/kg (2–3.52) DED ~240 min No kgw/kg 40 44) 40.6 16.2 30 to 81 h Mushroom [4] 1 203 (d.b) 12 (d.b.) No DED No 28.4 28.5 (total) Cranberry and potato −13 to 1.19–2.73 No 84 (w.b.) 15 (w.b.) No 85 to 15 No 2 h [5] −10 (4) Chopped alfalfa [6] 3.6 70 (w.b.) 10 (w.b.) 0.424 30–40 No 0.288 4 (batch) 0.5–1.02 40, Tomato [7] No 23 (d.b.) 0.1 (d.b.) 2.2–2.6 10 to 15 DED ~1000 min (2.56–2.68) 45, 50 62.7–89.6 5.1–10.8 Specialty crops [8] 11.6–20.7 No 30 & 35 No DED 3.3–120 h 0.06–0.61 (w.b.) (w.b.) Jew’s mallow, 40, 45, spearmint, and 2.5–7.5 81–83 (w.b.) 6 (w.b.) No No DED 4.75 to 6.35 h DED 50, 55 parsley [9] Ginger [10] 0.1 No No No 45 10 DED 8 h No 40, 800 to 900 120 to 200 Ginger [11] 0.2 12 (d.b.) No 50, DED No (d.b.) min 60 Shredded radish [12] 200 95 16 15 (CD) 40 No 6.3 25 h 1.5 (3–4) 30 40 Green sweet peppers 0.025 1453 (d.b.) ~10.7 (d.b.) No 35 27 DED 16, 25 & 36 h 0.55 to 1.1 [13] 40 19 270 to 390 Olive leaves [14] No 69.55 (N/S) 5.17 (N/S) No 45–55 No No No min Grain [15] ~1000 21.3 12.5 14.6 ~69 No 103.6 ~1 h ~4.38 Saffron [16] 0.5 80 (w.b.) 10 (w.b.) DED 40, 60 No No ~0.5 h 0.5–1.1 20, 35, 50, Ivy gourd leaves [17] 0.035 17 (d.b.) 4 (d.b.) 0.25 No DED 1 to 2 h (1.2) 55, 60

2. Equipment and Methods In the current study, we develop a heat pump dryer (HPD) to assess the performance of a closed-loop heat pump dryer system. In total, three designs were made and tested, including an original reference design, a modified drying chamber with better airflow distribution and less flow resistance, and a desiccant wheel on top of the modified design. Moreover, the assessment was extended to examine various parameters in a transient state. In the current study, the refrigerant used for the HPD was R-134a. A single external blower was used to circulate the air in a closed loop for eight hours. The experiments were performed on different weights of Orbeez (drying material) at a constant volumetric flow rate of 100 m3/h (0.5 m/s velocity). During experimentation, different parameters like the COPHP, drying rate, heat transfer rate by the condenser, moisture extraction rate, and specific moisture extraction rate were calculated. Figure1 illustrates the schematic of the heat pump dryer. The experimental setup was developed in order to validate or verify the system performance of the HPD. In addition to the four basic components of refrigeration—compressors, condensers, expansion valves, and evaporators—the system included an adsorption wheel (Dehutech, DA/DT-450, Täby, Sweden, Total Power Connection 3.5 kW) acting as an external dehumidifier, heat exchang- ers, electronic scales, and a drying chamber. Drying air is heated by the condenser of the HPD, and the low-relative-humidity air flows into the drying chamber via centrifugal fan. Subsequently, the hot and dry air absorbs moisture from the wet material, yielding highly humid air at the outlet of the dryer chamber. Afterwards, the humid air enters the evaporator to remove its moisture content. Temperature and humidity sensors were installed at the inlet and exit of the test box, and the measurement temperature and humidity value were transmitted to the data extractor via a signal transmitter, with a temperature range of −100 ◦C to 200 ◦C, an accuracy of 0.1 ◦C, a humidity range of 0% to 100%, and an accuracy of 0.8%. Processes 2021, 9, x FOR PEER REVIEW 4 of 18

Processes 2021, 9, x FOR PEER REVIEW 4 of 18

enters the evaporator to remove its moisture content. Temperature and humidity sensors were installed at the inlet and exit of the test box, and the measurement temperature and enters the evaporator to remove its moisture content. Temperature and humidity sensors humidity value were transmitted to the data extractor via a signal transmitter, with a tem- were installed at the inlet and exit of the test box, and the measurement temperature and peraturehumidity range value of were −100 transmitted °C to 200 °C, to thean accuracy data extractor of 0.1 via °C, a asignal humidity transmitter, range of with 0% a to tem- 100%, Processes 2021, 9, 1216 andperature an accuracy range of of − 1000.8%. °C to 200 °C, an accuracy of 0.1 °C, a humidity range4 of of 170% to 100%, and an accuracy of 0.8%.

Figure 1. Heat pump dryer with an external dehumidification system. Figure 1. HeatFigure pump 1. dryer Heat pump with an dryer external with dehumidiﬁcationan external dehumidification system. system.

Then, a ﬂowmeterThen,Then, (SCHMIDTaa flowmeterflowmeter Technology (SCHMIDT SS30.302, TechnologyTechnology Georgen, SS30.302, SS30.302, Germany) Georgen, Georgen, with Germany) aGermany) mea- with with a a 3 suring rangemeasuring frommeasuring 1.5 to range 417range m from3from/h of 1.5 20 to◦C 417 and mm 1013.253/h/h ofof 2020 hPa, °C °C andaccuracy: and 1013.25 1013.25± 3%hPa, hPa, of accuracy:m.v. accuracy: +0.3% ±3% ±3% of m.v.of m.v. of full scale,+0.3% was+0.3% employed of of full full scale, scale, to measure waswas employed the changes toto measuremeasure in air speedthe the changes changes in the in circulated in air air speed speed airﬂow in inthe the circulated circulated and the staticairflowairflow pressure and and of the the the static static airstream pressurepressure lifted of thethe by airstreaairstrea the blower;mm lifted lifted then by by the a the desiccant blower; blower; then wheel then a desiccant a was desiccant wheel wheel installed to furtherwaswas installed installed lower the toto further humidityfurther lower of the the airstream, humidityhumidity andof of the the the airstream, airstream, low-humidity and and the airthe low-humidity enters low-humidity air air the condenserentersenters to lower the the condenser condenser the relative toto humiditylower the relative torelative complete humi humi theditydity airto to complete cycle. complete The the the voltages air air cycle. cycle. and The The voltages voltages currents of theandand compressor currents currents ofof and thethe blower compressorcompressor were and measuredand blowerblower using were were ameasured measured power meter using using (Arch a powera power Meter meter meter (Arch (Arch CorporationMeter PA310,Meter Corporation Corporation measurement PA310,PA310, range measurement from 0 to 200A, range range accuracy from from 0 0to 0.5%. to 200A, 200A, accuracy accuracy 0.5%. 0.5%.

2.1. Drying Material2.1.2.1. Drying Drying Material Material Orbeez or sodiumOrbeezOrbeez polyacrylate or or sodium sodium (orpolyacrylate Acrylic Polymer (or(or Acrylic Acrylic Salt Polymer sodium)Polymer Salt was Salt sodium) used sodium) as was a dryingwas used used as aas dry- a dry- material as iting caning material material absorb 200~300as as it it cancan absorb timesabsorb its 200~300 mass in timestimes water. its its mass Sodiummass in in water. polyacrylatewater. Sodium Sodium ispolyacrylate usedpolyacrylate in is used is used many products,inin many suchmany products, as products, items for suchsuch baby asas anditems feminine forfor babybaby use, and and surgical feminine feminine sponges, use, use, surgical surgical fuel, sponges, cables, sponges, etc. fuel, fuel, cables, cables, Dry and wetetc. Orbeezetc. Dry Dry areand and shown wetwet OrbeezOrbeez below are in Figureshown2 .belowbelow The diameter in in Figure Figure of 2. 2. theThe The Orbeez diameter diameter increased of ofthe the Orbeez Orbeez inin- from 2.75 mmcreasedcreased to 15.5 mmfrom from after 2.75 2.75 absorbing mmmm toto 15.5 water, mm and afterafter its absorbingabsorbing porosity waswater, water, 47.64% and and atits its theporosity porosity beginning was was 47.64% 47.64% at at the beginning of the test. of the test. the beginning of the test.

Figure 2. Dry and wet Orbeez (as a drying material). Figure 2. Dry and wetFigure Orbeez 2. Dry (as aand drying wet material).Orbeez (as a drying material).

2.2. Design of the Drying Chambers In the current research work, the drying chamber was modified to improve the system efficiency and drying rate for the material subject to different test loads under a constant flow rate. Figure3 shows the original drying chamber where the drying air enters through the small blower from the corner side. There are only few holes in the second bucket for passing airflow to facilitate drying. For more efficient drying, a new drying chamber was developed to enhance the drying rate, to decrease air resistance, to consume less auxiliary power, and to increase the effective surface area for higher heat and mass transfer between the hot air and wet product. A schematic of the modified design is shown in Figure4. ProcessesProcesses 2021 2021, 9, ,9 x, xFOR FOR PEER PEER REVIEW REVIEW 5 5of of 18 18

2.2.2.2. Design Design of of the the Drying Drying Chambers Chambers InIn the the current current research research work, work, the the drying drying chamber chamber was was modified modified to to improve improve the the sys- sys- temtem efficiency efficiency and and drying drying rate rate for for the the material material subject subject to to different different test test loads loads under under a a con- con- stantstant flow flow rate. rate. FigureFigure 3 3 shows shows the the original original drying drying chamber chamber where where the the drying drying air air enters enters through through the the smallsmall blower blower from from the the corner corner side. side. There There are are only only few few holes holes in in the the second second bucket bucket for for pass- pass- inging airflow airflow to to facilitate facilitate drying. drying. For For more more ef efficientficient drying, drying, a a new new drying drying chamber chamber was was de- de- velopedveloped to to enhance enhance the the drying drying rate, rate, to to decrease decrease air air resistance, resistance, to to consume consume less less auxiliary auxiliary Processes 2021, 9, 1216 5 of 17 power,power, and and to to increase increase the the effective effective surface surface area area for for higher higher heat heat and and mass mass transfer transfer between between thethe hot hot air air and and wet wet product. product. A A schematic schematic of of the the modified modified design design is is shown shown in in Figure Figure 4. 4.

FigureFigureFigure 3. 3.3. Heat HeatHeat pump pumppump drying dryingdrying chamber. chamber.chamber.

FigureFigureFigure 4. 4.4. Schematic SchematicSchematic of ofof the thethe modified modiﬁedmodified drying dryingdrying chamber. chamber.chamber.

2.3.2.3.2.3. Data DataData Analysis AnalysisAnalysis 2.3.1.2.3.1.2.3.1. Coefficient CoefﬁcientCoefficient of ofof Performance PerformancePerformance TheTheThe coefficient coefﬁcientcoefficient of ofof performance performanceperformance ( ( COP(COP) )of of of an an an HP HP HP is isis used usedused in inin a aa drying dryingdrying process processprocess to toto calculate calculatecalculate thethethe energy energyenergy consumption consumptionconsumption of ofof the thethe system. system.system. The TheThe heat heatheat energy energyenergy supplied suppliedsupplied in inin the thethe condenser condensercondenser was waswas evaluatedevaluatedevaluated using usingusing Equation EquationEquation (1). (1).(1). The TheThe analysis analysisanalysis wa waswass divided divideddivided into intointo two twotwo parts: parts:parts: heat heatheat transfer transfertransfer and andand operating cost. operatingoperating cost. cost. . Qcd COP = 𝑄𝑄 (1) 𝐶𝑂𝑃𝐶𝑂𝑃W = = comp (1)(1) 𝑊𝑊 . . Q = m Cp (Tco − T ) (2) 𝑄𝑄cd=𝑚=𝑚 ia 𝐶𝑝𝐶𝑝air(𝑇(𝑇−𝑇−𝑇ci )) (2)(2) . . 𝑚m𝑚ia ==𝜌=𝜌ρia·Vi.𝑉.𝑉 (3)(3)(3)

Here,. Qcd = heat transfer rate by condenser (kW); . mia = mass flow rate of dry air (kg/s); Cpair = specific heat of dry air (kJ/kg K); ◦ Tco = condenser outlet temperature ( C); ◦ Tci = condenser inlet temperature ( C); 3 ρia = density of air (kg/m ); . 3 Vi = volumetric flow rate (m /h). Processes 2021, 9, 1216 6 of 17

2.3.2. Moisture Content (Wet Basis %) The moisture content (MC) for moist Orbeez can be calculated on a wet basis (w.b) as

Ww MCw.b (%) = × 100 (4) Wp

where Ww = weight of water in material; Wp = total weight of material. The moisture ratio was determined by the following equation

M − M M = e (5) Mo − Me The drying rate (DR) was calculated from the change in moisture content for sodium polyacrylate that occurred in each consecutive time interval by using the equation

(M − M ) DR = t+dt t (6) dt where DR = drying rate (kgwater/kg drysolid). The relative humidity was measured at four points during heat pump drying: two at the dryer inlet and two at the evaporator outlet and condenser inlet. The amount of water removed per hour is called the moisture extraction rate or water removal rate (MER). The MER represents the effectiveness of a dryer in terms of water removal. It can be calculated from the humidity ratios by using the temperature and relative humidity at the dryer inlet and outlet: . MER = mda(ωdout − ωdin) (7) where MER = moisture extraction rate (kg/h); . kg mda = mass ﬂow rate of dry air s ; kg ωdout = Humidity ratio at dryer oulet ; kgDA kg ωdin = Humidity ratio at dryer inlet . kgDA The term SMER is related to the power consumption and is determined using the following equation: . m (ωd − ωd ) SMER = da out in (8) Wcomp + Wblower where SMER = moisture extraction rate (kg/kWh); kg ωdout = humidity ratio at outlet ; kgDA kg ωdin = humidity ratio at inlet kg ; . DA mda = mass ﬂow rate (kg/s); Wcomp = compressor output power (kWh); Wblower = blower output power (kWh). The experiment would have been affected by the environment or the accuracy of the parameters, so the uncertainty of the mean values of measured and calculated parameters was calculated and presented in Table2. Processes 2021, 9, x FOR PEER REVIEW 7 of 18

𝑚 = mass flow rate (kg/s); 𝑊 = compressor output power (kWh); 𝑊 = blower output power (kWh). The experiment would have been affected by the environment or the accuracy of the parameters, so the uncertainty of the mean values of measured and calculated parameters Processes 2021, 9, 1216 7 of 17 was calculated and presented in Table 2.

Table 2. Uncertainty of mean values of measured and calculated parameters. Table 2. Uncertainty of mean values of measured and calculated parameters. Blower power consumption Wb ±0.3 Compressor power consumption Wc ±0.056 Blower power consumption Wb ±0.3 Heat transfer rate atCompressor condenser power consumption Qcd Wc kW ±±0.7360.056 Moisture extractionHeat rate transfer rate at condenser MER Qcd kg/hrkW ±0.736±0.45 ± Specific moisture extractionMoisture rate extraction rate SMER MER kg/kWhkg/h ±0.3360.45 Specific moisture extraction rate SMER kg/kWh ±0.336 . Air mass flow rate Air mass flow rate 𝑚 m kg/skg/s Coefficient of performanceCoefficient of heat of performance pump of heat pumpCOP COP - - ±±0.4580.458 Moisture content Moisture content MC MC g water/gg water/g wet wetmaterial material ± ±0.0240.024

3.3. ResultsResults andand DiscussionDiscussion FigureFigure5 5illustrates illustrates the the drying drying cycle cycle in in a a psychrometric psychrometric chart chart for for the the airstream airstream for for a a typicaltypical heatheat pump pump dyer dyer and and the the heat heat pump pump dryer dryer with with an an integrated integrated desiccant desiccant wheel. wheel. In In FigureFigure5 ,5, the the drying drying air air across across the the desiccant desiccant wheel wheel is denoted is denoted 5 → 1,5→ and1, and state state 1→2 1 represents→2 repre- sensiblesents sensible heating heating at the condenserat the condenser while thewhile humidity the humidity ratio is ratio constant. is constant. The state The 2 →state3 is 2 for→3 theis for airstream the airstream in the drying in the chamber drying chamber to facilitate to thefacilitate drying the process. drying State process. 3→4→ State5 represents 3→4→5 therepresents sensible the cooling sensible and cooling dehumidification and dehumidif of theication humid of air the at humid the evaporator. air at the evaporator.

FigureFigure 5. 5.Psychrometric Psychrometric representation representation of of drying drying air air paths paths of of both both systems. systems.

InIn thisthis section,section, wewe discussed discussed variousvarious aspectsaspects suchsuch asas thethe variationvariation inin the the drying drying air air temperature,temperature, variationvariation in the the relative relative humidity humidity at at the the dryer dryer inlet, inlet, comparative comparative RH% RH% at the at thedryer dryer outlet, outlet, moisture moisture content content (%wet (%wet basis), basis), weight weight reduction, reduction, moisture moisture extraction extraction rate, rate,specific specific moisture moisture extraction extraction rate, rate, heat heat transf transferer rate, rate, and andcoefficient coefficient of performance of performance for fordif- different HPD system designs including the original design, a modified drying chamber, ferent HPD system designs including the original design, a modified drying chamber, and and an HPD with a desiccant dehumidifier with 4 kg or 7 kg of drying material. an HPD with a desiccant dehumidifier with 4 kg or 7 kg of drying material. 3.1. Variation in the Drying Air Temperature at the Inlet of the Drying Chamber Figure6 shows the variation in the drying air temperature with time for different designs with variable test loads. The results are provided only for the maximum and minimum test loads under transient conditions and a constant volumetric flowrate of 100 m3/h. During the operation, the temperature gradually increased and later showed asymptotic behavior to reach a nearly steady state for all three designs (original, modified,

and with a desiccant wheel (DW)) subject to 4 kg or 7 kg of drying material. For the original design, a steady-state temperature of 50 ◦C for both 4 kg and 7 kg was observed with an average relative humidity of 25% at the dryer inlet. Processes 2021, 9, x FOR PEER REVIEW 8 of 18

3.1. Variation in the Drying Air Temperature at the Inlet of the Drying Chamber Figure 6 shows the variation in the drying air temperature with time for different designs with variable test loads. The results are provided only for the maximum and minimum test loads under transient conditions and a constant volumetric flowrate of 100 m3/h. During the operation, the temperature gradually increased and later showed asymptotic behavior to reach a nearly steady state for all three designs (original, modified, and with a desiccant wheel (DW)) subject to 4 kg or 7 kg of drying material. For the orig- Processes 2021, 9, 1216 inal design, a steady-state temperature of 50 °C for both 4 kg and 7 kg was observed 8with of 17 an average relative humidity of 25% at the dryer inlet.

FigureFigure 6. 6. VariationVariation in in the the drying drying air air te temperaturemperature at the dryer inlet. Also, Figure6 shows that the temperature increment for a test load of 4 kg with a Also, Figure 6 shows that the temperature increment for a test load of 4 kg with a modified drying chamber reached steady state earlier than the original design. This is modified drying chamber reached steady state earlier than the original design. This is be- because the air resistance was decreased and a better airflow distribution into the drying cause the air resistance was decreased and a better airflow distribution into the drying chamber prevailed. For the case with a desiccant wheel, slightly higher RH% was observed chamber prevailed. For the case with a desiccant wheel, slightly higher RH% was ob- (1–2%), the average temperature across the condenser was 51~54 ◦C, and the time to reach served (1–2%), the average temperature across the condenser was 51~54 °C, and the time steady state was even shorter. The temperature profile at the dryer inlet for the 4 kg test to reach steady state was even shorter. The temperature profile at the dryer inlet for the 4 load with the desiccant wheel was slightly higher than those for the other cases. This is kgsomewhat test load expected with the duedesiccant to the generatedwheel was adsorption slightly higher heat accompanyingthan those for the desiccant.other cases. This isFigure somewhat7 indicates expected the relative due to humiditythe generated at the adsorption inlet of the dryingheat accompanying chamber for differentthe desiccant.designs with different test loads. Upon operation, the relative humidity decreased appreciably initiallyFigure and 7 approachedindicates the a ratherrelative low humidity RH accordingly. at the inlet Figure of the7 indicates drying chamber that the respectivefor differ- entinitial designs values with of thedifferent relative test humidity loads. Upon of the operation, drying air the at relative the inlet humidity of the drying decreased chamber ap- preciablywere about initially 57.1%, 80%,and approached and 60% for thea rather three designs.low RH However,accordingly. the Figure final RHs 7 indicates of the original that theand respective modified designsinitial values for 4 kg of and the 7relative kg test loadshumidity reached of the average drying steady air at relative the inlet humidity of the dryinglevels of chamber 24% and were 27%, about respectively. 57.1%, On80%, the and other 60% hand, for the upon three introducing designs. However, the desiccant the wheel final RHsdehumidifier, of the original the average and modified relative humidity designs for was 4 observedkg and 7 to kg plunge test loads further reached to only average around Processes 2021, 9, x FOR PEER REVIEW 9 of 18 steady1~2%. Therelative relative humidity humidity leve atls the of drying 24% and chamber 27%, inletrespectively. was maintained On the below other 2.5%hand, with upon the introducingdesiccant wheel the dehumidifierdesiccant wheel during dehumidifier, the eight hours the experimentation.average relative humidity was observed to plunge further to only around 1~2%. The relative humidity at the drying chamber inlet was maintained below 2.5% with the desiccant wheel dehumidifier during the eight hours experimentation.

FigureFigure 7. 7. VariationVariation in in the the relative relative humidity at the dryer inlet for all three cases.

3.2. Comparison of the RH at the Dryer Outlet Figure 8 indicates the relative humidity levels at the dryer outlet for variable test loads on three different designs. The drying rates shown in the figure can be categorized into transient, constant, and falling rates. At the beginning, the original and modified designs show the maximum RH at the outlet of the drying chamber. For the test load of 7 kg, the air resistance of the bucket was much higher for the original design as compared to the others, and the material possessed a large water content, thereby resulting in a rather low dehumidifying process and a high relative humidity, accordingly. With the introduction of the modified drying chamber, a better heat and mass transfer process occurred; consequently, the RH at the outlet dropped consistently from the initial peak toward a lower value in a steady manner. This was further made clear with the introduction of the DW dehumidifier, where an even lower RH prevailed for the much lower inlet RH at the inlet of the drying chamber, as depicted in Figure 7.

Figure 8. Variation in the relative humidity at the dryer outlet.

3.3. Moisture Content (% Wet Basis) Figures 9 and 10 illustrate the effect of test loads (4 kg and 7 kg with initial moisture contents (wet basis) of 98% and 99%, respectively) on the drying process of the heat pump dryer subject to an air velocity of 0.5 m/s under transient conditions. Before the experiment, the dry weight of the material was calculated, and it was then immersed in water

Processes 2021, 9, x FOR PEER REVIEW 9 of 18

Processes 2021, 9, 1216 9 of 17 Figure 7. Variation in the relative humidity at the dryer inlet for all three cases.

3.2. Comparison of the RH at the Dryer Outlet 3.2. Comparison of the RH at the Dryer Outlet Figure 8 indicates the relative humidity levels at the dryer outlet for variable test Figure8 indicates the relative humidity levels at the dryer outlet for variable test loads on three different designs. The drying rates shown in the figure can be categorized loads on three different designs. The drying rates shown in the figure can be categorized intointo transient, transient, constant, constant, and and falling falling rates. rates. At Atthe the beginning, beginning, the theoriginal original and andmodified modified de- signsdesigns show show the themaximum maximum RH RH at the at the outlet outlet of the of the drying drying chamber. chamber. For For the the test test load load of of7 kg,7 kg, the the air air resistance resistance of ofthe the bucket bucket was was much much higher higher for for the the original original design design as as compared compared toto the the others, others, and and the the material material possessed possessed a larg a largee water water content, content, thereby thereby resulting resulting in a inra- a therrather low low dehumidifying dehumidifying process process and and a high a high relative relative humidity, humidity, accordingly. accordingly. With With the in- the troductionintroduction of the of the modified modified drying drying chamber, chamber, a better a better heat heat and andmass mass transfer transfer process process occurred;occurred; consequently, consequently, the theRH RH at the at theoutlet outlet dropped dropped consistently consistently from from the theinitial initial peak peak to- wardtoward a lower a lower value value in ina steady a steady manner. manner. This This was was further further made made clear clear with with the the introduction introduction ofof the the DW DW dehumidifier, dehumidifier, where anan eveneven lowerlower RHRH prevailed prevailed for for the the much much lower lower inlet inlet RH RH at atthe the inlet inlet of of the the drying drying chamber, chamber, as as depicted depicted in in Figure Figure7. 7.

Figure 8. Variation in the relative humidity at the dryer outlet. Figure 8. Variation in the relative humidity at the dryer outlet. 3.3. Moisture Content (% Wet Basis) 3.3. Moisture Content (% Wet Basis) Figures9 and 10 illustrate the effect of test loads (4 kg and 7 kg with initial moisture contentsFigures (wet 9 basis)and 10 of illustrate 98% and the 99%, effect respectively) of test loads on (4 the kg drying and 7 processkg with of initial the heat moisture pump contentsdryer subject (wet tobasis) an air of velocity98% and of 99%, 0.5 m/srespecti undervely) transient on the conditions.drying process Before of the experiment,heat pump dryerthe dry subject weight to ofan the air material velocity was of 0.5 calculated, m/s under and transient it was then conditions. immersed Before in water the forexperi- three ment,hours. the The dry comparison weight of herethe material is based was on the calc dryulated, weight and and it was wet then weight. immersed In the graph, in water the moisture content of the Orbeez is given as a function of time. After drying, the moisture content or water content was reduced. For 4 kg loading, the modified drying chamber showed better drying performance compared to the original design and the design with a desiccant dehumidifier. This is because the modified drying chamber experiences much lower air resistance, and the drying rate is therefore relatively effective. The graph also illustrates that the moisture decreased from 99% to 97% for the original design, from 99% to 87% for the modified drying chamber, and from 99% to 94% for the DW design. The lowest moisture reduction was observed in the original design. Processes 2021, 9, x FOR PEER REVIEW 10 of 18

Processes 2021, 9, x FOR PEER REVIEW 10 of 18

for three hours. The comparison here is based on the dry weight and wet weight. In the graph, the moisture content of the Orbeez is given as a function of time. After drying, the for three hours. The comparison here is based on the dry weight and wet weight. In the moisture content or water content was reduced. For 4 kg loading, the modified drying graph, the moisture content of the Orbeez is given as a function of time. After drying, the chamber showed better drying performance compared to the original design and the de- moisture content or water content was reduced. For 4 kg loading, the modified drying sign with a desiccant dehumidifier. Processes 2021, 9, 1216 chamber showed better drying performance compared to the original design and the10 ofde- 17 sign with a desiccant dehumidifier.

Figure 9. Variation in the moisture content at the minimum test load. Figure 9. Variation in the moisture content at the minimum test load. Figure 9. Variation in the moisture content at the minimum test load.

99.5

99.099.5

98.599.0

98.098.5

97.598.0

97.097.5 7kg-Old design 97.0 7kg-Modified DC 96.5 7kg-Old design 7kg-DW Moisture content (wet basis %) %) basis content (wet Moisture 7kg-Modified DC 96.096.5 7kg-DW

Moisture content (wet basis %) %) basis content (wet Moisture 0 100 200 300 400 500 600 96.0 0 100 200Time (minutes) 300 400 500 600 FigureFigure 10. 10. VariationVariation in in the the moisture moistureTime content(minutes) content at at the the maximum maximum test test load. load. Figure 10. Variation in the moisture content at the maximum test load. 3.4.This Weight is because Reduction the modified drying chamber experiences much lower air resistance, and theAs drying the drying rate continued,is therefore the relatively weight ofeffective. the Orbeez The material graph also was illustrates gradually that reduced the This is because the modified drying chamber experiences much lower air resistance, moisturedue to the decreased evaporation from or loss99% ofto moisture 97% for fromthe original the surface design, of the from material. 99% Initiallyto 87% sensiblefor the and the drying rate is therefore relatively effective. The graph also illustrates that the modifiedheat was drying transferred chamber, to the and material’s from 99% surface to 94% and for caused the DW the moisturedesign. The to evaporatelowest moisture quickly. moisture decreased from 99% to 97% for the original design, from 99% to 87% for the reductionThe weight was reductions observed of in the the moisturized original design. Orbeez were observed to be (for a starting weight modified drying chamber, and from 99% to 94% for the DW design. The lowest moisture of 4 kg) 1.6 kg, 0.5 kg, and 1 kg in different cases in eight-hours experiments with an reduction was observed in the original design. average mass ﬂowrate of 0.031 kg/s and drying air temperature in the range of 48 ◦C to 54 ◦C (see Figure 11).

Processes 2021, 9, x FOR PEER REVIEW 11 of 18

3.4. Weight Reduction As the drying continued, the weight of the Orbeez material was gradually reduced due to the evaporation or loss of moisture from the surface of the material. Initially sensible heat was transferred to the material’s surface and caused the moisture to evaporate quickly. The weight reductions of the moisturized Orbeez were observed to be (for a starting weight of 4 kg) 1.6 kg, 0.5 kg, and 1 kg in different cases in eight-hours experiments Processes 2021, 9, 1216 with an average mass flowrate of 0.031 kg/s and drying air temperature in the range11 of of 48 17 °C to 54 °C (see Figure 11).

FigureFigure 11. 11. WeightWeight reduction reduction for for the the minimum minimum test test load. load.

NoteNote that that the the flow flow resistance resistance for for the the dry dry air air was was much much higher higher in in the the original original design design becausebecause of of the the irregular irregular drying drying path. path. However, However, during during the the 7 7 kg kg test test load, load, the the results results with with thethe modified modified drying drying chamber chamber were were superior superior to to those those with with the the other other designs, designs, as as shown shown in in Processes 2021, 9, x FOR PEER REVIEWFigure 12. The results with the DW dehumidifier also showed the influence on the12 drying of 18 Figure 12. The results with the DW dehumidifier also showed the influence on the drying rate because the desiccant wheel reduces the relative humidity of the air cycle at the inlet rate because the desiccant wheel reduces the relative humidity of the air cycle at the inlet of the drying chamber. of the drying chamber.

FigureFigure 12. 12. WeightWeight reduction reduction for for the the maximum maximum test test load. load. 3.5. Moisture Extraction Rate 3.5. Moisture Extraction Rate The HPD, along with the drying chamber, was designed to decrease the air resistance The HPD, along with the drying chamber, was designed to decrease the air resistance and auxiliary power and to check the performance and effect of an external dehumidiﬁer andwith auxiliary variable power test loads. and Theto check heat pumpthe performance dryer performance and effect can of bean evaluatedexternal dehumidifier with respect withto the variable capacity test of loads. water The removal heat pump and energy dryer performance effectiveness. can The be moisture evaluated extraction with respect rate toindicates the capacity the amount of water of removal water removed and energy per hour.effectiveness. Figure 13 The indicates moisture the extraction results for rate an indicateseight-hours the drying amount process. of water Initially, removed for all per three hour. cases, Figure the speciﬁc13 indicates moisture the extractionresults for ratean eight-hours drying process. Initially, for all three cases, the specific moisture extraction rate increased toward a plateau value, followed by a steady decline. As seen from Figure 13, the maximum MER values for 4 kg were observed as 0.68 kg/h, 0.82 kg/h, and 1.0 kg/h for the original design, modified drying chamber, and design with the DW dehumidifier, respectively, during the eight-hours experiment. Note that the desiccant wheel design offered the highest initial peak plateau, followed by the modified design and, finally, the original design. As depicted in Figure 7, the relative humidity for the DW design was the lowest among the three designs, resulting in the highest initial MER when compared to the other designs. As time proceeded, the Orbeez close to the edge of the drying chamber shrunk more pronouncedly due to the high mass transfer rate and returned to a much smaller size, like that of the original dry Orbeez, as shown in Figure 14. As a consequence, the very dry and small Orbeez particles packed together more closely. This eventually led to a further mass transfer barrier for the dry air to penetrate into the center to facilitate effective drying. For this reason, the MER of the DW design fell behind that of the modified design after 200 min, while it was about the same as that of the original design after 400 min.

Processes 2021, 9, 1216 12 of 17

increased toward a plateau value, followed by a steady decline. As seen from Figure 13, the maximum MER values for 4 kg were observed as 0.68 kg/h, 0.82 kg/h, and 1.0 kg/h for the original design, modified drying chamber, and design with the DW dehumidifier, respectively, during the eight-hours experiment. Note that the desiccant wheel design offered the highest initial peak plateau, followed by the modified design and, finally, the original design. As depicted in Figure7, the relative humidity for the DW design was the lowest among the three designs, resulting in the highest initial MER when compared to the other designs. As time proceeded, the Orbeez close to the edge of the drying chamber shrunk more pronouncedly due to the high mass transfer rate and returned to a much smaller size, like that of the original dry Orbeez, as shown in Figure 14. As a consequence, the very dry and small Orbeez particles packed together more closely. This eventually led to a further mass transfer barrier for the dry air to penetrate into the center to facilitate Processes 2021, 9, x FOR PEER REVIEWeffective drying. For this reason, the MER of the DW design fell behind that of the modified13 of 18 Processes 2021, 9, x FOR PEER REVIEW 13 of 18 design after 200 min, while it was about the same as that of the original design after 400 min.

1.4 1.4 4kg-Old design 1.2 4kg-Old design 1.2 4kg-Modified drying chamber 4kg-Modified drying chamber 1.0 4kg-DW 1.0 4kg-DW 0.8 0.8 0.6 0.6

MER (kg/hr) (kg/hr) MER 0.4

MER (kg/hr) (kg/hr) MER 0.4 0.2 0.2 0.0 0.0 0 100 200 300 400 500 0 100 200 300 400 500 Time(minutes) Time(minutes) FigureFigure 13. 13. MoistureMoisture extraction extraction rate rate versus versus dryi dryingng time time for for the the minimum minimum test test load. load. Figure 13. Moisture extraction rate versus drying time for the minimum test load.

Figure 14. Images of the drying Orbeez in the drying chamber. The left-hand image shows the original chamber, and the right-hand imageFigure shows 14. the ImagesFigure modified of 14. the chamberImages drying ofwith Orbeez the better drying in theair Orbeez distribution.drying inchamber. the drying The chamber.left-hand image The left-hand shows the image original shows chamber, the and the right-hand imageoriginal shows chamber, the modified and the right-hand chamber with image better shows air thedistribution. modiﬁed chamber with better air distribution. 3.6. Specific Moisture Extraction Rate (SMER) 3.6. Specific Moisture Extraction Rate (SMER) In the drying process, the specific moisture extraction rate represents the efficiency In the drying process, the specific moisture extraction rate represents the efficiency of energy or effectiveness of energy used in the drying process. The term SMER is related to the cost and ofthe energy output or of effectiveness the drying sy ofstem, energy which used indicates in the drying the heat process. energy The absorbed term SMER is related to the cost and the output of the drying system, which indicates the heat energy absorbed and the evaporation of water from the wet Orbeez. Figure 15 shows the SMERs for all and the evaporation of water from the wet Orbeez. Figure 15 shows the SMERs for all three cases with a test load of 4 kg. As observed from the graph, the SMER with a desiccant wheel was ratherthree low cases due with to the a test appreciable load of 4 kg.power As obseconsumptionrved from requirements the graph, the to SMER regen- with a desiccant wheel was rather low due to the appreciable power consumption requirements to regenerate the desiccant wheel. Initially, the SMER gradually increased for all three cases due to significant masserate transfer the desiccant from the wheel. material’s Initially, surface; the SMER it then gradually reached aincr maximumeased for value all three cases due to significant mass transfer from the material’s surface; it then reached a maximum value and declined thereafter. and declined thereafter.

Processes 2021, 9, 1216 13 of 17

3.6. Specific Moisture Extraction Rate (SMER) In the drying process, the specific moisture extraction rate represents the efficiency of energy or effectiveness of energy used in the drying process. The term SMER is related to the cost and the output of the drying system, which indicates the heat energy absorbed and the evaporation of water from the wet Orbeez. Figure 15 shows the SMERs for all three cases with a test load of 4 kg. As observed from the graph, the SMER with a desiccant wheel was rather low due to the appreciable power consumption requirements to regenerate the desiccant wheel. Initially, the SMER gradually increased for all three cases due to Processes 2021, 9, x FOR PEER REVIEW 14 of 18 significant mass transfer from the material’s surface; it then reached a maximum value and declined thereafter.

3 4kg-Old design 4kg-Modified drying chamber 4kg-DW 2

1 SMER (kg/kWh)SMER

0 0 100 200 300 400 500

Time (minutes) FigureFigure 15. 15. SpecificSpeciﬁc moisture moisture extraction extraction rate rate versus versus drying drying time. time.

ForFor a a larger larger test test load load of of 7 7 kg, kg, Figure Figure 1616 alsoalso indicates indicates that that the the specific specific moisture moisture rate rate initiallyinitially increased increased for for all all cases cases and and later later reduced reduced gradually gradually with with drying drying time. time. The The modified modified chamberchamber showedshowed thethe maximum maximum performance performance with with an average an average SMER SMER of around of 2.6around kg/kWh 2.6 kg/kWhbecause because of lower of drying lower drying air resistance air resistan withce 27% with relative 27% relative humidity humidity and 52 and◦C 52 drying °C dry- air ingtemperature air temperature at the dryerat the inlet. dryer Also, inlet. it Also, was foundit was thatfound the that SMERs the SMERs for all cases for all at cases the 7 kgat thetest 7 load kg test were load higher were than higher those than at those the 4 kgat the test 4 load. kg test This load. is somehow This is somehow expected expected because becausemore water more is water evaporated is evaporated from larger from contact larger surfaces. contact surfaces. The SMER The of SMER the DW of dehumidifier the DW de- humidifierwas minimal was because minimal of thebecause high powerof the high draw power associated draw with associated regeneration with regeneration of the desiccant of thewheel desiccant (about wheel 2.6 kW). (about 2.6 kW).

3.7. Variation in the COP and Heat Transfer Rate at the Condenser 3.5 By using the heat transfer rate and compressor power, the COPHP of the heat pump can be3.0 obtained. Again, two test loads of 4 kg and 7 kg were used to check the performance of the modified drying2.5 chamber. The average temperature ranged from 33 ◦C to 34 ◦C and the relative humidity was around 62–66% at the inlet of the condenser; heat transfer rates of 0.55 kW and 0.60 kW,2.0 respectively, were observed for 4 kg and 7 kg loads in Figures 17 and 18, whereas the average COP values of the heat pump cycle were calculated as 3.1 and 3.5, respectively, 1.5 HP under the volumetric ﬂow rate of 100 m3/h. Note that higher COP and capacity prevailed 7kg-Old design for the1.0 larger test loading of 7 kg due to the greater heat transfer rate. 7kg-Modified drying chamber SMER (kg/kWh) (kg/kWh) SMER 0.5 7kg-DW

0.0 0 100 200 300 400 500

Time (minutes) Figure 16. Specific moisture extraction rate versus drying time.

3.7. Variation in the COP and Heat Transfer Rate at the Condenser

By using the heat transfer rate and compressor power, the COPHP of the heat pump can be obtained. Again, two test loads of 4 kg and 7 kg were used to check the performance of the modified drying chamber. The average temperature ranged from 33 °C to 34 °C and the relative humidity was around 62–66% at the inlet of the condenser; heat transfer rates of 0.55 kW and 0.60 kW, respectively, were observed for 4 kg and 7 kg loads in Figures 17

Processes 2021, 9, x FOR PEER REVIEW 14 of 18

3 4kg-Old design 4kg-Modified drying chamber 4kg-DW 2

1 SMER (kg/kWh)SMER

0 0 100 200 300 400 500

Time (minutes) Figure 15. Specific moisture extraction rate versus drying time.

For a larger test load of 7 kg, Figure 16 also indicates that the specific moisture rate initially increased for all cases and later reduced gradually with drying time. The modified chamber showed the maximum performance with an average SMER of around 2.6 kg/kWh because of lower drying air resistance with 27% relative humidity and 52 °C drying air temperature at the dryer inlet. Also, it was found that the SMERs for all cases at the 7 kg test load were higher than those at the 4 kg test load. This is somehow expected Processes 2021, 9, 1216 because more water is evaporated from larger contact surfaces. The SMER of the DW14 de- of 17 humidifier was minimal because of the high power draw associated with regeneration of the desiccant wheel (about 2.6 kW).

3.5

3.0

2.5

2.0

1.5 7kg-Old design 1.0 Processes 2021, 9, x FOR PEER REVIEW 7kg-Modified drying chamber 15 of 18 (kg/kWh) SMER Processes 2021, 9, x FOR PEER REVIEW 0.5 7kg-DW 15 of 18

0.0 and 18, whereas the average COPHP values of the heat pump cycle were calculated as 3.1 0 100 200 300 400 5003 and 3.5,18, whereasrespectively, the average under the COP volumetricHP values offlow the rate heat of pump 100 m cycle/hr. Notewere thatcalculated higher as COP 3.1 and capacity3.5, respectively, prevailed under for the the larger volumetric test loading flow rate of 7 of kg 100 due m to3/hr. the Note greater that heat higher transfer COP Time (minutes) rate.and capacity prevailed for the larger test loading of 7 kg due to the greater heat transfer rate.FigureFigure 16. 16. SpecificSpeciﬁc moisture moisture extraction extraction rate rate versus versus drying drying time. time. 66 0.8 3.7. Variation in the COP and Heat Transfer Rate at the Condenser0.8 66 0.8 By5 5using the heat transfer rate and compressor power, the COPHP of the heat pump can be obtained. 55 0.6 Again, two test loads of 4 kg and 7 kg were used to check the performance of the 44 0.6 modified drying chamber. The average temperature ranged from 33 °C to 34 °C and the

HP 44 HP relative 3humidity was around 62–66% at the inlet of the0.4 condenser; heat transfer rates of 3 0.4 (kW) HP (kW) HP

0.55 kW and 0.60 kW, respectively, were observed for 4 kg andcd 7 kg loads in Figures 17 COP 3 0.4

3 (kW) COP 0.4

7kg-Q (kW) (kW) 22 cd Q cd COP COP 7kg 7kg7kg-Q - COP-COP(kW) 22 cd HP 0.2 Q 4kg-Q7kg -COPcd(kW) 11 7kg - COPHP 0.2 4kg-Q4kg-COP(kW) 11 4kg - COPcd HP 0 4kg-COP 0.0 0 4kg - COPHP 0.0 00 0 100 200 300 400 500 500 600 6000.0 0 100 Time Time200 (minutes) (minutes) 300 400 500 500 600 600 TimeTime (minutes) (minutes) Figure 17. Variation in COPHP and the sensible heat transfer rate at the condenser for the old de- Figure 17. Variation in COP and the sensible heat transfer rate at the condenser for the old design. sign.Figure 17. Variation in COPHPHP and the sensible heat transfer rate at the condenser for the old design. 6 6 0.80.8 6 6 0.80.8 5 5 5 5 0.60.6 4 4 0.60.6

HP 4

3 3 0.40.4 (kW) (kW ) HP HP cd (kW)

COP 3 0.40.4

COP 3 7kg7kg- -COP COP HP (kW ) 2 Q 2 cd COP

COP 4kg - COP 4kg7kg7kg- -COP COP HP 2 2 0.20.2 Q 4kg7kg4kg - QCOP COPcd (kW) 1 1 HP 0.20.2 4kg7kg - Qcd (kW) 1 1 4kg - Qcd (kW) 0 0 0.00.0 0 100 200 300 400 500 600 0 0 0 100 200 300 400 500 6000.00.0 0 100 100 Time 200Time (minutes) 300300(minutes) 400 500 600 TimeTime (minutes) (minutes) Figure 18. Variation in COPHP and the sensible heat transfer rate at the condenser for the modiﬁed Figure 18. Variation in COPHP and the sensible heat transfer rate at the condenser for the modified drying chamber. dryingFigure chamber.18. Variation in COPHP and the sensible heat transfer rate at the condenser for the modified drying chamber. 4. Conclusions 4. ConclusionsIn the current study, we developed a heat pump dryer (HPD) to assess the perfor- manceIn ofthe a currentclosed-loop study, heat we pumpdeveloped dryer a system,heat pump and dryera modified (HPD) drying to assess chamber the perfor- was designedmance of toa closed-loopoffer a lower heat airflow pump resistance dryer system, to facilitate and a effectivemodified drying. drying Onchamber top of wasthe modifieddesigned design,to offer a a desiccant lower airflow wheel resistancewas installe tod facilitate to further effective lower the drying. humidity On oftop the of dry- the ingmodified air, and design, further a desiccant examination wheel regarding was installe its applicabilityd to further lower was conducted.the humidity Unlike of the most dry- ofing the air, existing and further studies, examination which normally regarding adopt its organic applicability products was as conducted. the drying materials,Unlike most in thisof the study existing we studies,used moist which sodium normally polyacrylate adopt organic (Orbeez) products as the as drying the drying material materials, for con- in sistentthis study characterization we used moist of thesodium heat pumppolyacrylate performance. (Orbeez) R-134a as the was drying adopted material as the for refrig- con- erantsistent for characterization the heat pump of system.the heat The pump experiments performance. were R-134a performed was adopted subject as to the different refrig- weightserant for of the Orbeez heat pump(drying system. material) The at experiments a constant volumetric were performed flow rate subject of 100 to m different3/h. The weights of Orbeez (drying material) at a constant volumetric flow rate of 100 m3/h. The

Processes 2021, 9, 1216 15 of 17

4. Conclusions In the current study, we developed a heat pump dryer (HPD) to assess the performance of a closed-loop heat pump dryer system, and a modified drying chamber was designed to offer a lower airflow resistance to facilitate effective drying. On top of the modified design, a desiccant wheel was installed to further lower the humidity of the drying air, and further examination regarding its applicability was conducted. Unlike most of the existing studies, which normally adopt organic products as the drying materials, in this study we used moist sodium polyacrylate (Orbeez) as the drying material for consistent characterization of the heat pump performance. R-134a was adopted as the refrigerant for the heat pump system. The experiments were performed subject to different weights of Orbeez (drying material) at a constant volumetric flow rate of 100 m3/h. The test loads of 4 kg to 7 kg of Orbeez were tested in the drying chamber. Based on the foregoing discussion, the following conclusions are drawn:

1. The average COPHP, mass transfer rate, heat transfer rate, MER, and SMER of the system were calculated as 3.9, 0.30 kg/s, 0.56 kW, 0.495 kg/h, and 1.614 kg/kWh. 2. The moisture extraction rate for the lower test load was higher than that for the higher test load due to the higher penetration of drying air at the lower test load, although the maximum test load showed the maximum relative humidity at the dryer outlet. 3. The maximum MERs for 4 kg were observed as 0.68 kg/h, 0.82 kg/h, and 1.0 kg/h for the old design, modified drying chamber, and design with the external dehumidifier, respectively, during eight-hours experiments. The addition of the desiccant wheel showed good performance in terms of the moisture extraction rate and coefficient of performance, while it showed poor performance in terms of the specific moisture extraction rate due to the required high-power regeneration (around 2.6 kW) of the desiccant dehumidifier. 4. The moisture extraction rate (MER) for all designs increased to a maximum value, followed by a consistent decline. The maximum MER for the desiccant design exceeded those for the other designs. However, the MER for the desiccant dehumidifier design decreased faster and may become lower than those of the other designs as time pro- ceeds further due to the close packing of dry Orbeez at the edge of the drying material. 5. In the future, further validation and more elaboration on reductions in the heat and mass transfer processes of the drying materials should be carried out, and the blockage phenomenon of the drying materials at the outer edge should be quantitatively studied. More effective drying chamber designs should be studied through numerical examinations (CFD) and verified through experiments.

Author Contributions: Conceptualization, K.-S.Y. and K.H.; methodology, K.H. and U.S.; writing— original draft preparation, K.-S.Y. and K.H.; writing—review and editing, C.-C.W.; supervision, S.-K.W. and C.-C.W.; project administration, C.-C.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Bureau of Energy of the Ministry of Economic Affairs under the contracts of 110-E0209 and by the Ministry of Science Technology of Taiwan under grant numbers: MOST 109-2628-E-167-001-MY3 and MOST 108-2221-E-009-037-MY3. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available on request due to restrictions. Acknowledgments: The authors gratefully acknowledge the financial support from the Bureau of Energy of the Ministry of Economic Affairs under the contracts of 110-E0209 and by the Ministry of Science Technology of Taiwan under grant numbers: MOST 109-2628-E-167-001-MY3 and MOST 108-2221-E-009-037-MY3. Conflicts of Interest: The authors declare no conflict of interest. Processes 2021, 9, 1216 16 of 17

Nomenclature

RH relative humidity (%) SMER specific moisture extraction rate (kg/kWh) Qcd heat transfer rate by condenser (kW) . mia mass flow rate of dry air (kg/s) Cpair specific heat of dry air (kJ/kg K) ◦ Tco condenser outlet temperature ( C) ◦ Tci condenser inlet temperature ( C) 3 ρia density of air (kg/m ) . 3 Vi volumetric flow rate (m /h) Ww weight of water in material Wp total weight of material DR drying rate (kgwater/kg drysolid) MER moisture extraction rate (kg/h) . mda mass flow rate of dry air (kg/s) ωdout humidity radio at dryer outlet (kg/kgDA) ωdin humidity ratio at dryer inlet (kg/kgDA)) ωdout humidity ratio at outlet (kg/kgDA) ωdin humidity ratio at inlet (kg/kgDA) . mda mass flow rate (kg/s) Wcomp compressor output power (kWh) Wblower blower output power (kWh) T temperature of refrigerant (K) TR regeneration temperature (K) t temperature of air (K) Twb wet-bulb temperature (K) u velocity (m/s) V volume (m3) 3 Qva volumetric air flow rate (m /s) 3 va specific volume of air (m /kg dry air) 3 vr1 specific volume of refrigerant vapor at suction (m /kg) 3 vW specific molar volume (m /mol) ω humidity ratio of air (kg water/kg dry air) ∆T temperature drop (K) ∆Tc cold air temperature reduction (K) ηis isentropic efficiency (%) COP coefficient of performance COPHP COP of the heat pump cp specific heat at constant pressure (kJ/kg K) cpa specific heat of dry air (kJ/kg K) cpam specific heat of moist air (kJ/kg K) cpf specific heat of condensate refrigerant film (kJ/kg K) DBT dry bulb temperature (◦C) DPT dew point temperature (◦C) DW desiccant wheel Pcomp compressor power requirement (kW) Pfan fan power requirement (kW) h specific enthalpy of refrigerant (kJ/kg) ha specific enthalpy of air (kJ/kg dry air) hfg specific latent heat of vaporization of refrigerant (kJ/kg) ◦ hfg specific latent heat of vaporization of water at reference temperature of 0 C (kJ/kg) hg specific enthalpy per unit mass of saturated vapors (kJ/kg) HP heat pump HPD heat pump dryer HPDW heat pump desiccant wheel hv specific enthalpy of water vapors (kJ/kg) Processes 2021, 9, 1216 17 of 17

CD thermal capacity of the heat pump’s condenser DED deductible value based on data provided (drying curves, etc.) No no Information or data not provided N/S not speciﬁed QEV evaporator thermal capacity (kW)

References

1. Li, M.; Ma, Y.; Gong, W.; Su, W. Analysis of CO2 Transcritical Cycle Heat Pump Dryers. Dry. Technol. 2009, 27, 548–554. [CrossRef] 2. Zhang, J.; Wu, Y. Experimental Study on Drying High Moisture Content Paddy by Super-Conducting Heat Pump Dryer. In Proceedings of the 5th Asia-Pacific Drying Conference, Hong Kong, China, 13–15 August 2007; pp. 385–390. [CrossRef] 3. Ho, J.C.; Chou, S.K.; Mujumdar, A.S.; Hawlader, M.N.A.; Chua, K.J. An optimisation framework for drying of heat-sensitive products. Appl. Therm. Eng. 2001, 21, 1779–1798. [CrossRef] 4. Chin, S.K.; Law, C.L. Product Quality and Drying Characteristics of Intermittent Heat Pump Drying of Ganoderma tsugaeMurrill. Dry. Technol. 2010, 28, 1457–1465. [CrossRef] 5. Alves-Filho, O. Combined innovative heat pump drying technologies and new cold extrusion techniques for production of instant foods. Dry. Technol. 2002, 20, 1541–1557. [CrossRef] 6. Adapa, P.K.; Schoenau, G.J.; Sokhansanj, S. Performance study of a heat pump dryer system for specialty crops—Part 2: Model verification. Int. J. Energy Res. 2002, 26, 1021–1033. [CrossRef] 7. Queiroz, R.; Gabas, A.L.; Telis, V.R.N. Drying Kinetics of Tomato by Using Electric Resistance and Heat Pump Dryers. Dry. Technol. 2004, 22, 1603–1620. [CrossRef] 8. Adapa, P.K.; Schoenau, G.J. Re-circulating heat pump assisted continuous bed drying and energy analysis. Int. J. Energy Res. 2005, 29, 961–972. [CrossRef] 9. Fatouh, M.; Metwally, M.N.; Helali, A.B.; Shedid, M.H. Herbs drying using a heat pump dryer. Energy Convers. Manag. 2006, 47, 2629–2643. [CrossRef] 10. Hawlader, M.N.A.; Perera, C.O.; Tian, M. Comparison of the Retention of 6-Gingerol in Drying of Ginger Under Modified Atmosphere Heat Pump Drying and other Drying Methods. Dry. Technol. 2006, 24, 51–56. [CrossRef] 11. Phoungchandang, S.; Nongsang, S.; Sanchai, P. The Development of Ginger Drying Using Tray Drying, Heat Pump–Dehumidified Drying, and Mixed-Mode Solar Drying. Dry. Technol. 2009, 27, 1123–1131. [CrossRef] 12. Lee, K.H.; Kim, O.J. Experimental Study on the Energy Efficiency and Drying Performance of the Batch-Type Heat Pump Dryer. In Proceedings of the 5th Asia-Pacific Drying Conference, Hong Kong, China, 13–15 August 2007; pp. 428–433. [CrossRef] 13. Pal, U.S.; Khan, M.K.; Mohanty, S.N. Heat Pump Drying of Green Sweet Pepper. Dry. Technol. 2008, 26, 1584–1590. [CrossRef] 14. Erbay, Z.; Icier, F. Optimization of Drying of Olive Leaves in a Pilot-Scale Heat Pump Dryer. Dry. Technol. 2009, 27, 416–427. [CrossRef] 15. Xiang, F.; Wang, L.; Yue, X.-F. Exergy Analysis and Experimental Study of a Vehicle-Mounted Heat Pump–Assisted Fluidization Drying System Driven by a Diesel Generator. Dry. Technol. 2011, 29, 1313–1324. [CrossRef] 16. Mortezapour, H.; Ghobadian, B.; Minaei, S.; Khoshtaghaza, M.H. Saffron Drying with a Heat Pump–Assisted Hybrid Photovoltaic– Thermal Solar Dryer. Dry. Technol. 2012, 30, 560–566. [CrossRef] 17. Potisate, Y.; Phoungchandang, S. Chlorophyll Retention and Drying Characteristics of Ivy Gourd Leaf (Coccinia grandis Voigt) Using Tray and Heat Pump–Assisted Dehumidified Air Drying. Dry. Technol. 2010, 28, 786–797. [CrossRef]