Purdue Universiy Purdue e-pubs Inernaial Refrigerai and Air Cdiiing Cference School of Mechanical Engineering 24 Analysis he Operaing Characerisics of a Household Dehumidifier Hg Qi Li Beijing Universiy of Technology Li Wen Jin Xi'an Jiaog Universiy Follow his and addiial works a: hp://docs.lib.purdue.edu/iracc Li, Hg Qi and Jin, Li Wen, "Analysis he Operaing Characerisics of a Household Dehumidifier" (24). Inernaial Refrigerai and Air Cdiiing Cference. Paper 676. hp://docs.lib.purdue.edu/iracc/676 This documen has been made available hrough Purdue e-pubs, a service of he Purdue Universiy Libraries. Please cac epubs@purdue.edu for addiial informai. Complee proceedings may be acquired in prin and CD-ROM direcly from he Ray W. Herrick Laboraories a hps://engineering.purdue.edu/ Herrick/Evens/orderli.hml
ANALYSIS ON THE OPERATING CHARACTERISTICS OF A HOUSEHOLD DEHUMIDIFIER Hgqi Li, Liwen Jin 2 Beijing Universiy of Technology, Deparmen of Refrigerai,, Pingleyuan, Chaoyang Disric, Beijing, P. R. China, 22 Tel: 86--673963, Fax: 86--847856, E-mail: lihgqi@bju.edu.cn 2 Xi an Jiaog Universiy, Deparmen of Air-cdiiing, 8, Xianning Road, Xi an, Shannxi, P. R. China,749 Tel: 86-29-266873, E-mail: gxjin@xju.edu.cn ABSTRACT R36, Page Aiming a he energy saving and he improvemen of dehumidifying effecs, he working process of a household dehumidifier is analyzed, including he changes of evaporaing emperaures and cdensing emperaures, compressi raio, he sae parameers of he air, he hea ransfer during he working cycle, ec. The influences of hese facors he operaing characerisics are also presened, which give a heory basis o exploi he fuzzy crolled dehumidifier.. INTRODUCTION Humidiy is e of he mos imporan facors of comfor, especially in he souhern China where he relaive humidiy of air is usually greaer han 8%. High humidiy grealy influences he life and he work of people. A dehumidifier can reduce he room humidiy o a proper range so ha he food and he clohing can be kep in a relaive lg ime, he household elecric appliances will have lg life and peoples are able o say in a comforable area. Therefore, humidifiers are more and more used in modern families as air-cdiiing ools o reduce he room humidiy. Wih he developmen of echnologies and he improvemen of people s living sandard, fuzzy crolled echnology ha is compleed by singlechip is more and more widely used in domesic elecric appliances. Wheher in performance, funci and ecomizing energy or oher aspecs, hey all show powerful compeii abiliy in marke. In comparis, he household dehumidifier, which is sill crolled by radiial -off mode, no ly goes agains energy saving, bu also he compressor has o sarup cinually and he room humidiy waves grealy. In order o achieve saisfying dehumidificai effec, i s necessary o adop proper inelligen crolling mehod o make he dehumidifier has beer performance and creae more comforable indoor envirmen. I is said ha in he overall marke of air-cdiier in Japan, he frequency cversi machines using fuzzy crol has reached a level of 56.% in 994, 8.8% in 997, 9.% in 999. The developmen foreground of fuzzy crolled air-cdiier shows ha i has very imporan applicai value o sudy fuzzy crol echnology in dehumidifiers. Therefore, a fuzzy crol sysem of a household dehumidifier was fully analyzed in heory and researched in experimen. The working process and he characerisic of a dehumidifier were analyzed. The resuls will provide some references o he design of is fuzzy croller. 2. THE PROCESS OF DEHUMIDIFICATION Dehumidificai is a process of reducing moisure cen of air. A presen, here are four dehumidifying mehods. They are aerai dehumidifying, absorbing dehumidifying, adsorpi dehumidifying, and cooling dehumidifying. Household dehumidifier belgs o he regi of cooling dehumidifying. I uilizes refrigerai principle o make we air cool o below he dew emperaure poin, hen hea i afer removing he cdensae waer, finally i becomes dry air as requesed. This kind of dehumidifying process can be realized by refrigerai sysem and fan uni (Figure Inernaial Refrigerai and Air Cdiiing Cference a Purdue, July 2-5, 24
R36, Page 2 ). The we air goes hrough he fan, ener he dehumidifier, and hen exchange hea wih he evaporaor. Because he surface emperaure of he evaporaor is lower han he dew poin of air, when he air is cooled, waer in he air would be cdensed. The air whose emperaure has dropped discharges from he dehumidifier via emperaure increasing in he cdenser. Humidiy Air Filer Evaporaor Capillary Cdenser Fan Dry Air Compressor Figure The working sysem of a dehumidifier During he working process of he dehumidifier, he sae parameers of air would be changed. When we air goes hrough he evaporaor coil, because he coil emperaure is lower han he dew poin of he we air, he dehumidificai begins. Here he air emperaure drops, waer in he air coagulaes ou, and he moisure cen declines, while he relaive humidiy increases. Wih a fan, he cooled air in evaporaor goes hrough he cdenser, which makes he air emperaure increase o a lile higher han ha i ges ino he evaporaor, here he moisure cen in air is no change, bu he relaive humidiy declines. when he air is discharged from he dehumidifier, is emperaure and humidiy reach a needed cdii. The change of sae parameers in each phase is shown in Figure 2. In his figure, Poin is he unseled air cdii before he evaporaor, poin x is he air cdii before he cdenser. Line -x shows he hea and we exchange when he air goes hrough he evaporaor, poin 2 is he air cdii when i leaves he cdenser. The inerseci poin of he exended line -x and he sauraed line is he cdii of sauraed we air correspding o he average ou-surface emperaure, w,of he evaporaor. i 2 % x x wo X Figure 2 Enhalpy-moisure diagram of dehumidificai process 3. WORKING CHARACTERISTIC ANALYSIS OF THE DEHUMIDIFIER In he real working process of he dehumidifier, when he room emperaure and humidiy are high, he efficiency of dehumidificai is high; while he room emperaure and humidiy are low, he efficiency of dehumidificai is Inernaial Refrigerai and Air Cdiiing Cference a Purdue, July 2-5, 24 X X
R36, Page 3 low. This is he characerisic of cooling dehumidificai. As shown in Figure 3, when he high emperaure air (poin ) is cooled from o, he humidiy drops from x o x; when low emperaure air 2 is cooled from 2 o 2, he moisure cen drops from x 2 o x 2. Even if he air enhalpy difference is he same ( i i 2 ) wih he same cooling effecs, he dehumidifying cen of is lager han 2. This is because ha ly when air is cooled below he dew poin, he dehumidificai is possible. I is more differen for 2 o reach he dew poin han, and i also cosumes more cooling capaciy when i reaches he dew poin. I is known from he hyhergraph, if he moisure cen is low, he saurai curve is plane, hus he dehumidificai efficiency is low. Csidering he operai cdii of he compressor, he evaporaing emperaure a poin 2 is lower han ha in poin, he compressi raio is also larger a poin 2. High-pressure raio causes low volumeric efficiency and low refrigeraing capaciy. So he dehumidificai capaciy decreases, oo. i Relaive moisure % 8% 6% 4% x i2 2 x' x2 x2' Moisure 2' ' 2 We bulb emperaure Figure 3 Hyhergraph of dehumidificai process The dehumidifier wih radiial /off crol has dehumidificai capabiliy loss when i urns or off. Compared wih he sensible hea capaciy, he laen hea capaciy loss is much bigger. When he compressor sops, he fan of he dehumidifier is sill working, he waer which originally coagulaed from he air o he coil, changes o evaporae from he coil o he air; If he fan moor also sops afer he compressor is off, when he compressor sars up again, a he beginning of is operai (6~9 secd), he waer he evaporaor coil would also evaporae back o he air. When analyzing he dehumidificai performance loss as he compressor or off respecively, i is assumed ha: () The evaporaor coil can cain he waer whose laen hea capaciy is M while he coagulaed waer quaniy he coil is larger han M he redundan waer would drain ou from he coil; (2) On he process of sarup, he laen hea capaciy and he sensible hea capaciy all can be expressed by he firs order ineria sep respse, and he ime csan is he same. Assuming ha during he compressor ceasing, due o he evaporaing, he change of laen hea capaciy is: Qe M q() Qee () Where, Q e is he original evaporaing laen hea change raio. In ime ~,he evaporaing laen hea capaciy is: Q e () M M M e (2) In ~ off, of he compressor ceasing ime, he evaporaing capaciy is: Inernaial Refrigerai and Air Cdiiing Cference a Purdue, July 2-5, 24
R36, Page 4 Q ( ) e off M M off M e (3) During he sarup, assuming ha he waer coagulaing laen hea capaciy in he hea exchanger can be expressed as: B τ () Q ( e ) q where: Q Seady coagulaing laen hea change raio; τ ime csan; B he ime when he waer begins o coagulae during he sarup In during he process of sar-up, he coagulaing capaciy is: B (4) B ( ) ( ) τ τ Q q d Q + τ e e (5) The ime when coagulaing waer begins o flow ou: r b off we τ τ τ we e e e (6) Where, M we Q Qe r Q The dehumidifying cen in each cycle is: q Q ( ) (7) If <, here is no dehumidificai cen here. Supposed ha LHRss is he laen hea raio in seady sae, hen he laen hea raio in par load is: q Ql LHR q + Ql Qs + τ τ e LHR ss (8) + τ τ e Where, q is sensible hea capaciy Q s is sensible hea change raio. From above equai: Where, 36 4C, LHR LHR ss off ( x) C x max 4 36 max C max is he maximum off-imes per uni ime + τ e τ (9) Inernaial Refrigerai and Air Cdiiing Cference a Purdue, July 2-5, 24
R36, Page 5 x is he moisure cen. I is known from above analysis, afer he compressor sops, he amoun of hea exchange would decrease gradually, a he same ime he emperaure of he coil would escalae. When he coil emperaure increases o he dew poin emperaure, he waer would no coagulae any more and he dehumidificai is ended. Wih he coil emperaure increasing, he coagulaed waer he coil begins o evaporae back o he air. If he waer he coil is enough and hese is ly hea exchange beween waer and air, he final emperaure he coil surface would be he we bulb emperaure of he air. If he coil emperaure keeps increasing, he waer he coil would evaporae unceasingly ill all evaporaes and he emperaure of he coil is same o he air. Khaar,M.K. e al (988) go he same cclusi, he moisure coagulaing cen he coil is shown in Figure 4. Cdensed waer Time B OFF ON Figure 4 Waer coagulaing cen he coil 6. CONCLUSIONS The analysis resuls show ha, he dehumidificai efficiency is high a high room emperaure and humidiy; In he process of dehumidifying, csanly sarup or ceasing of he compressor will cause he dehumidificai performance loss. To develop he fuzzy crolled frequency cversi dehumidifier wih he compressor operaing cinuously is he main developing direci. This kind of dehumidifier has some feaures such as: A he beginning of dehumidifying, fuzzy crolled frequency cversi dehumidifier can operae wih high frequency, and drop he room humidiy o he se value quickly. In he normal operaing period, i can sabilize he room humidiy waving wih low operaing frequency. Thus i no ly saves he energy, bu also removes he dehumidifying performance loss a he compressor or off, and improves he dehumidifying effec. REFERENCES Khaar, M. K., 985, Fan cycling effecs air cdiier moisure removal performance in warm humid climaes, Final repor, EPRIEM-4226. Inernaial Refrigerai and Air Cdiiing Cference a Purdue, July 2-5, 24