Air dehumidification by membrane with cold water for manned spacecraft environmental control

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1 Air dehumidifiction by membrne with cold wter for mnned spcecrft environmentl control Shng Yonghong,, Wng Jing, Bi Ynqing,Jin Ybin,Pei yifei, Yun Weixing 2 nd Yng Bo 2 Beijing Institute of Spcecrft Environmentl Engineering, Beijing, Chin 2 School of Aeronutic Science nd Engineering, Beihng University,Beijing, Chin Abstrct. The trditionl condenstion dehumidifiction method requires dditionl gs-liquid seprtion nd wter recovery process in the mnned spcecrft humidity control system, which would increse weight nd complexity of systems. A new membrne dehumidifiction with cold wter is proposed, which uses wter vpor prtil pressure difference to promote wter vpor trnsmembrne mss trnsfer for dehumidifiction. The permebility of the membrne ws mesured nd the experimentl results gree well with the theoreticl clcultions. Bsed on the simultion of dehumidifiction process of cold wter-membrne, the influence of module structure nd working condition on dehumidifiction performnce ws nlyzed, which provided reference for the design of membrne module construct. It cn be seen from the simultion nd experiments tht the cold wter-membrne dehumidifiction cn effectively reduce the therml lod of the mnned spcecrft. Induction Humidity control is tkes n importnt role in Environmentl Control nd Life Support System for mnned spce missions. An stronut will relese lmost.5 kg/d moisture by metbolism[]. High humidity will not only influence the therml comfort nd helth, but lso is hrmful to electronic equipment, which my cuse mny security problems[ ].An ctive environmentl control method of condensing dehumidifiction hs been reserched nd utilized in mny mnned spcecrft such s Spce Shuttle, ISS nd Shenzhou[3]. Wrm nd humid ir is cooled below dew point by Condensing Het Exchnger(CHX) with humidity nd temperture decresing. The condenste wter is bsorbed by hydrophilic coting on the surfce of CHX fins nd is seprted from ir-wter mixture by mechnicl rotry seprtor under microgrvity environment. But CHX requires much energy consumption to cool nd rehet ir before send bck to cbin, nd to run ir/wter seprtor. Moreover, the relibility nd lifespn of the movement ssembly my limit the system performnce[4]. 2 Membrne dehumidifiction Membrne dehumidifiction technology is bsed on the principle tht wter vpor cn trnsfer through membrne pores by prtil pressure difference of wter vpor. A novel cold wter nd Corresponding uthor: The Authors, published by EDP Sciences. This is n open ccess rticle distributed under the terms of the Cretive Commons Attribution License 4.0 (

2 membrne ssembly for ir dehumidifiction is reserched. The membrne dehumidifier is typicl shell-nd-tube construct s shown in Figure. A bundle of hollow fibre membrnes is pcked in the shell with termintions epoxy seled to crete the tube nd shell side. The wrm nd humid ir enters entrnce nd flows t the shell side, nd the wter vpor permetes through membrne pores into the tube side, becuse cold wter flows in the tube side bring condensing wter wy to relize ir dehumidifiction. The wter t the tube side cnnot lek out from pores becuse of the hydrophobic nture of membrne. There re no moving prts or electronics within the system. Wter out Membrne Air in Air out Wter in Figure. The hollow fibre membrne ssembly Spcesuit Wter Membrne Evportor hve been developed by NASA to provide het rejection cpbility in Mrs[5]. Bundles of Nfion membrne re ssembled inside vcuum shell s humidity control subssembly for the Boeing CST-00 Strliner spcecrft[6].a semi-permeble membrne mteril is used to remove wter vpor from humid ir. Vrious mterils such s Polyvinylidene Fluoride(PVDF), Polytetrfluoroethylene(PTFE) nd Cellulose Acette(CA) were used nd compred. 3 Methodology Dehumidifiction process is combintion of het nd mss trnsfer. Het trnsfers serilly through the therml boundry lyer t the ir-side, nd then conducted by membrnes nd trnsfers through the therml boundry lyer t the wter-side. The convective trnsport boundry lyer theory is pplicble to the mss trnsfer s well. The mss trnsfer resistnce of wter-side is considered to be negligible, only the mss trnsfer resistnce of ir-side should be tken into ccount. 3. Membrne diffusion A kind of hydrophilic-hydrophobic membrne ws dopted in this reserch. The inner lyer of membrne is PVDF porous structure of s hydrophobic support. The outer lyer of membrne is thin hydrophilic PVA with dense structure. The prmeters of mteril re listed in Tble. Tble. Physicl prmeters of mteril Lyer Prmeter Symbol Unit Vlue Thickness δ μm 0 Inner Averge pore size d p μm 0.5 Porosity ε Tortured τ - 3 Outer Thickness δ 2 μm 40 Diffusion coefficient D ws m 2 /s

3 Density ρ s kg/m 3 2 The verge pore size of the support lyer is 0.5μm, which is close to the verge moleculr free pth (0.09μm). According to the diffusion model, the diffusion mechnism of wter vpor in the support lyer includes Knusson diffusion, viscous flow nd moleculr diffusion[7,8]. Where C , v v 20., v 2.7; R is the gs constnt; M v is the molr mss of wter vpor; M is the molr mss of ir; μ is the kinetic viscosity of wter vpor; p nd T re the verge pressure nd temperture in the pores. D d pε DK 3τ 8RT πm 2 d pεp DP 32τμ C T.75 v p 2 v 3 3 ( v + v ) M v M v + () (2) (3) According to the diffusion resistnce model, the diffusion coefficient of the support lyer is D vm DP DK + D + (4) v The diffusion coefficient of wter vpor in the outer lyer is clculted by the equivlent of the diffusion coefficient of wter, where ρ is ir density; ψ is prtition coefficient equls to 5. s (5) Dvm2 Dws So the totl diffusion coefficient of wter vpor is δ + δ2 (6) Dvm δ Dvm + δ2 Dvm2 According to bove, the diffusion coefficient of wter vpor through membrne is m 2 /s. 3.2 Convective het nd mss trnsfer According to the similrity theory, the mss trnsfer cn be compred with the convective het trnsfer. The reltion of the mss trnsfer number Sh nd the convective het trnsfer number Nu will - 3 stisfy with the Chilton-Colburn correltion[9], Sh NuLe,Where Le is the Lewis number, Pr Le ; Sc Pr is the Prndtl number, μc p Pr ; Sc is the Schmidt number, μ ; ρ, c Sc p, λ nd μ re respectively λ ρd v the density, specific het cpcity, therml conductivity, nd dynmic viscosity of humid ir. 3.3 Control eqution The sensible het trnsfer will occur between humid ir nd cold wter, nd the temperture of humid ir will be decresed. While the moisture content of humid ir will decrese becuse of trnsmembrne mss trnsfer of wter vpor. Cold wter bsorbs the sensible het from wrm humid ir, while the ltent het will be produced in the process of moisture bsorption, so the temperture of cold wter will increse. dq rdj m c dt (7) The rte of het nd mss trnsfer will be clculted s follows, where h t nd k t re the totl het nd mss trnsfer coefficient. dq h t t da (8) w pw t - w w 3

4 dj k - da (9) t w 3.4 Membrne permebility experiments In order to determine the diffusion coefficient of wter vpor through membrne by experiment, shell-nd-tube membrne ssembly ws mde, which ws filled with six hollow fibre membrne tubes nd with the totl membrne re of 2. The micro-flow test bed ws built, including circulting wter nd gs loop to provide 0mL/min wter flow nd 0L/min ir flow. The experimentl diffusion coefficient of wter vpor in membrne ws m 2 /s. The experiment results were consistent with the theoreticl model result of m 2 /s, verifying the relibility of the theoreticl model. 4 Dehumidifiction performnce nlyses 4. Flow form The performnce nlysis of the counter-flow or cross-flow of the shell-nd-tube het exchngers is crried out. The convective mss trnsfer resistnce of the counter flower is lrger thn tht of the cross-flow t the sme Reynolds number s shown in Figure 2. However, the het trnsfer resistnce of counter-flower is lso lrger thn cross-flow s shown in Figure 3. When the Reynolds number is 300, the het trnsfer resistnce of counter-flow (0.0457m 2 K/W) is 6 times of cross-flow ( m 2 K/W) nd the mss trnsfer resistnce of counter-flow (s/m) is.8 times of cross flow (44.8s/m). In order to prevent the dewing on fibre outer surfce s result of the fster het trnsfer rte thn the mss trnsfer, the form of counter-flow nd the cylindricl structure of shell re dopted. This configurtion is lso more fvourble for the flow uniformity Totl mss trnsfer resistnce (s/m) Cross Counter Totl het trnsfer resistnce (m 2 K/W) Cross Counter Re d Figure 2. Comprison of mss trnsfer resistnce Figure 3. Comprison of het trnsfer resistnce Re d 4.2 Assembly structure design The membrne ssembly ws designed for the quntity of membrne tubes nd the inner dimeter of shell with the trget dehumidifiction rte of 400g/h. To increse the quntity of tubes cn enlrge the mss trnsfer re nd reduce the shell side circultion re, so tht the convective mss trnsfer coefficient of shell side will increse, nd the dehumidifiction rte will increse too. To reduce the shell inner dimeter cn increse the convective mss trnsfer coefficient of the shell-side, thereby the dehumidifiction rte will increse. Improve the mss trnsfer performnce will enhnce the het trnsfer performnce t the sme time, which will led to the reltive humidity of the outlet ir increse nd the risk of wter condenstion on the tube surfce. According to the typicl operting conditions of the mnned spcecrft humidify control requirements, the inlet ir temperture is 26 C nd the reltive humidity is 60%, the inlet cold wter temperture is 5 C, the flow rte of ir nd cold wter re both nd the tube length is 300 mm. The design rnge of the membrne tube quntity nd shell inner dimeter is clculted s shown in 4

5 Figure 4. In order to chieve dehumidifiction rte is more thn 400g/h, the resonble quntity of membrne tube nd shell inner dimeter should be designed below the solid line nd bove the dotted line. On the other hnd, the smller shell dimeter, the greter pressure drops on the ir-side s shown in Figure 5. Shell inner dimeter (mm) g/h 00% RH Membrne mount Shell inner dimeter (mm) Membrne mount Figure 4. Design rnge(high-humidity) Figure 5. Air-side pressure drop (high-humidity) Under the operting condition tht the inlet ir temperture is 22.5 C nd the reltive humidity is 50%, the tube length is 400 mm, nd the other condition s bove. The design rnge of tube quntity nd shell inner dimeter is clculted s shown in Figure 6. The quntity of tubes required in the low humidity condition is significntly incresed compred with Figure 4, nd the inner dimeter of the shell is lso incresed. The design rnge under low humidity conditions is significntly nrrower, tht is to ensure dehumidifiction rte, nd the risk of condenstion is incresing. The smller shell dimeter, the greter pressure drops on the ir-side s shown in Figure Shell inner dimeter (mm) g/h 00% RH Membrne mount Figure 6. Design rnge (low-humidity) Shell inner dimeter (mm) Membrne mount Figure 7. Air-side pressure drop (low-humidity) 4.3 Operting condition When the mnned cbin conditions of temperture nd humidity re keep constnt, the dehumidifiction rte is djusted by chnging the ir flow nd temperture of cold wter. The greter the ir flow, the greter the mss trnsfer coefficient on the ir side, the esier moisture content of ir is mintined t higher vlue, nd the greter mss trnsfer potentil difference, both of which will contribute to the dehumidifiction performnce. And on the other hnd, the lower temperture of cold wter cn effectively reduce prtil pressure of wter vpor, thereby improving the dehumidifiction performnce. Figure 8 shows the design results of different dehumidifiction performnce of operting conditions, where the ir temperture nd humidity re mintined t 26 C nd 60%, cold wter flow is In order to mintin the dehumidifiction cpcity, the ir flow needs to be enlrged when the temperture of cold wter increses. Since the reltionship between prtil pressure of wter vpor nd cold wter temperture is non-liner, nd the mgnitude of prtil pressure of wter vpor will increse more nd more with the wter temperture increses, which will result in shrp decrese in mss trnsfer potentil difference, so the increse extent of ir flow will be more nd more. 5

6 From the bove simultion nlysis, the membrne ssembly cn meet the dehumidifiction requirements even the temperture of cold wter rose to 0 C. Compred with condensing dehumidifiction s cold wter operting temperture is 5 C, cold wter-membrne dehumidifiction system hs lower requirements on the cooling cpcity, which hs better thermodynmic properties Air mss flow rte (kg/s) g/h 400 g/h Wter temperture ( C) Figure 8. Operting condition design under different dehumidifiction demnds 4 Results nd discussion A novel cold wter-membrne dehumidifiction method is proposed for mnned spcecrft. The het nd mss trnsfer model ws estblished for the cold wter-membrne dehumidifiction process, nd the membrne permebility ws mesured by experiment. The process of cold wter-membrne dehumidifiction ssembly ws simulted nd the counter-flow form ws determined. The structure nd operting conditions were designed for certin dehumidifiction requirements. Compred with condenstion dehumidifiction, cold wter-membrne dehumidifiction hs dvntges:() chieves wter recovery without dditionl gs-liquid seprtion processes;(2) ssembly consisting of polymer mterils re lighter in weight;(3) hs better thermodynmic properties. References. Hung JR nd Fn HL. Numericl simultion of the therml nd humid environment in mnned spcecrft hbittion cbin. Chinese Spce Sci Technol 2004; 24: Michel Izenson, Weibo Chen. Evportive Cooling nd Dehumidifiction Grment for Portble Life Support Systems. 43rd Interntionl Conference on Environmentl Systems, July4-8, Westheimer D., Tun G. Active Therml Control System Considertions for the Next Genertion of Humn Rted Spce Vehicles. AIAA 0-3 Jnury Dvid W., Json D., Gregory G. Interntionl Spce Sttion Environmentl Control nd Life Support System Sttus for the Prior Yer: nd Interntionl Conference on Environmentl Systems Tony JR, John WS, Grnt B. Performnce of Wter Recircultion Loop Mintennce Device nd Process for the Advnced Spcesuit Wter Membrne Evportor. AIAA, Christie Icomini, Josh Hecht, Jim Hrrell, John Lumpkin. Qulifiction of the Boeing Strliner humidity control subssembly, Tomszewsk M., Gryt M., Morwski A. Mss Trnsfer of HCl nd H 2 O cross the Hydrophobic Membrne During Membrne Distilltion. Journl of Membrne Science, 2000, 66(2): Zhng L.-Z., Hung S.-M., Pei L.-X. Conjugte Het nd Mss Trnsfer in Cross-Flow Hollow Fibre Membrne Contctor for Liquid Desiccnt Air Dehumidifiction. Interntionl Journl of Het nd Mss Trnsfer, 202, 55(25): Zhng L.-Z., Li Z.-X., Zhong T.-S., et l. Flow Mldistribution nd Performnce Deteriortions 6

7 in Cross Flow Hollow Fibre Membrne Module for Air Humidifiction. Journl of Membrne Science, 203, 427: -9 7