Moisture diffusion measurement and evaluation for porous membranes used in enthalpy exchangers

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2nd International Conference on Energy and Power, ICEP2018, 13–15 December 2018, Sydney, Australia 2nd International Conference on Energy and Power, ICEP2018, 13–15 December 2018, Sydney, Australia

Moisture diffusion measurement and evaluation for porous The diffusion 15th International Symposium onand District Heating and for Cooling Moisture measurement evaluation porous membranes used in enthalpy exchangers membranes used in enthalpy exchangers AssessingAhmed the feasibility usingMa thea,*,heat K. Albdoora,bof , Zhenjun Paul demand-outdoor Coopera a,b a, a AhmedBuildings K. Albdoor , Zhenjun Ma *, Paul Cooper temperatureSustainable function for a long-term district heat demand forecast Research Centre, University of Wollongong, 2522, NSW, Australia a

Department of Mechanical Techniques, Al-Nasiriyah Technical Institute, Southern Technical University, 64001, Thi-Qar, Iraq a Sustainable Buildings Research Centre, University of Wollongong, 2522, NSW, Australia a,b,c a a b c c b Department of Mechanical Techniques, Al-Nasiriyah Technical Institute, Southern Technical University, 64001, Thi-Qar, Iraq b

I. Andrić

*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre

a

IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Abstract

Abstract

Air-to-air membrane enthalpy exchangers using semi-permeable membranes are widely used in building ventilation systems to pre-condition the supply air byexchangers exchangingusing energy with the exhaust air stream.are Moisture is one ventilation of the mostsystems important Air-to-air membrane enthalpy semi-permeable membranes widely diffusivity used in building to properties of porous membranes. This property has a significant influence on the design and performance of membrane enthalpy pre-condition the supply air by exchanging energy with the exhaust air stream. Moisture diffusivity is one of the most important Abstract exchangers. this membranes. study, moisture resistances of five porousonmembranes measured, and the effectsenthalpy of the properties of In porous Thisdiffusion property has a significant influence the design were and performance of membrane membrane pore size on the moisture diffusivity were evaluated under different test conditions. The five porous membranes tested exchangers. In this study, moisture diffusion resistances five porous membranes were measured, and the of the District heating networks are commonly addressed in theofliterature as one of the most effective solutions for effects decreasing the included: i) two PVDF (Polyvinylidene difluoride) membranes with mean pore diameters of 0.22 µm and 0.45 µm respectively, membrane pore size on the moisture diffusivity were evaluated under different test conditions. The five porous membranes tested greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat ii)sales. two Nylon withclimate 0.1 µmconditions and 0.45 µm pore sizes with respectively, anddiameters iii) aheat PESof (Polyethersulfone) membrane with a included: i) two PVDF (Polyvinylidene difluoride) membranes mean pore 0.22 µm µm respectively, Due tomembranes the changed and building renovation policies, demand in and the 0.45 future could decrease, 0.1 µm pore size. A theoretical model to predict the effectiveness of a crossflow membrane enthalpy exchanger was alsoa ii)prolonging two Nylonthe membranes 0.1period. µm and 0.45 µm pore sizes respectively, and iii) a PES (Polyethersulfone) membrane with investmentwith return developed with respect to the latent heat transfer. The experimental results showed that the PVDF membrane with a mean pore Theµm main scope thistheoretical paper is tomodel assessto thepredict feasibility using the heat – outdoor temperature function for heat demand 0.1 pore size.of A the of effectiveness of demand a crossflow membrane enthalpy exchanger was also diameter 0.45district µm outperformed others in Lisbon terms of(Portugal), the moisture diffusivity. from the theoretical model the forecast.ofThe of Alvalade, located in was used as aThe caseresults study. Themembrane district iswith consisted ofpore 665 developed with respect to the latentthe heat transfer. The experimental results showed that the PVDF a meanof enthalpy exchanger agreed well with those from the experiments reported in the literature. The latent effectiveness was found to buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district diameter of 0.45 µm outperformed the others in terms of the moisture diffusivity. The results from the theoretical model of the renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were be insensitive to the outdoor air conditions. enthalpy exchanger agreed well with those from the experiments reported in the literature. The latent effectiveness was found to with from air a dynamic heat demand model, previously developed and validated by the authors. becompared insensitive to results the outdoor conditions. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications ©(the 2019 TheinAuthors. by Elsevier Ltd.20% for all weather scenarios considered). However, after introducing renovation error annual Published demand was lower than 2018 This is an open access articleincreased under the theupCC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) scenarios, the error value to the weather and renovation scenarios combination considered). This is an open access article under CC BY-NC-ND license on (https://creativecommons.org/licenses/by-nc-nd/4.0/) © 2018 The Authors. Published byresponsibility Elsevier 59.5% Ltd. of (depending Selection and peer-review under the scientific scientific committee the 2nd Conference on The value of slope coefficient increased on average within the range of of 3.8% 8% per decade, that corresponds the Selection and peer-review under responsibility of the committee of the up 2ndtoInternational International Conference on Energy Energytoand and This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Power, ICEP2018. decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and Power, ICEP2018. Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Energy and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the Power, ICEP2018. coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * Corresponding author. Cooling. E-mail address: [email protected] (Z Ma)

* Corresponding author. E-mail address: [email protected] (Z Ma) Keywords:©Heat Forecast; Climatebychange 1876-6102 2018demand; The Authors. Published Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Energy and Power, ICEP2018. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Energy and Power, ICEP2018. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Energy and Power, ICEP2018. 10.1016/j.egypro.2019.02.198

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Ahmed K. Albdoor et al. / Energy Procedia 160 (2019) 499–506 Albdoor et al. / Energy Procedia 00 (2018) 000–000

Keywords: enthalpy exchangers; porous membranes; moisture diffusivity

1. Introduction Air-to-air energy recovery ventilators (ERVs) are widely used to reduce the energy required for conditioning the air in building applications [1]. This type of technology transfers valuable sensible and latent energy in the exhaust air stream to the supply air stream. Using ERVs as part of building heating, ventilation, and air-conditioning (HVAC) systems can save up to 65 % of energy consumed in processing the fresh air, and improve indoor air quality [2]. Membrane enthalpy exchangers (MEEs) use semi-permeable membranes that separate exhaust and supply air streams and allow for both heat and moisture transfer between the two air streams. Compared to other energy recovery devices (e.g. enthalpy wheels and runaround membrane enthalpy exchangers), air-to-air MEEs offer advantages such as low cross-contamination rate, simplicity, high effectiveness and less embodied energy [3]. Membrane properties can significantly influence the design and performance of MEEs. Moisture diffusivity is one of the most important properties of the membranes [4]. Niu and Zhang [5] and Min and Su [4] found that membrane moisture permeability can significantly affect both the latent effectiveness and total effectiveness (i.e. enthalpy effectiveness). On the other hand, the moisture diffusion resistance is not constant and it varies with changes in temperature and humidity [6]. This can result in significant performance variations of the MEE under various operating conditions. To evaluate these variations, a set of experiments are often required in order to test the membrane moisture diffusion under a wide range of operating conditions. Typically, membranes are classified into dense and porous types according to the mean pore size. The pore size of ‘dense’ membranes is commonly of the order of 0.1 nm, while that of ‘porous’ membranes is of the order of 0.1 μm [7]. Although both types of membranes have been used in the MEE fabrication, they have completely different moisture transfer mechanisms. In the majority of the previous studies, the moisture diffusivity of the membrane was considered either a constant value [8, 9] obtained from a single experimental test or a variable value [10]. However, only a few studies investigated the influence of the operating conditions on the permeability of the porous membranes, and their impact on the performance of the MEE. In this study, the influence of the membrane pore size on the moisture diffusion resistances of five different porous membranes was measured using a wet cup test method under different test conditions. The moisture diffusivity of the tested membranes was then calculated. The performance of a crossflow MEE using these membranes was then evaluated using a theoretical model to calculate its latent effectiveness. Nomenclature J Mass flux G Weight change T Time during which weight change occurred Ac Test cup area A Total transfer area Dm Moisture diffusivity Rm Moisture diffusion resistance rm Volumetric moisture diffusion resistance K Convective mass transfer coefficient 𝑚𝑚𝑚𝑚̇𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 Minimum mass flow rate Dv Diffusivity of water vapour in air Dh Hydraulic diameter W Absolute humidity Um Total mass transfer coefficient

kg/m2.s kg s

Acronyms Le Lewis number NTUm Number of mass transfer units

m2 m2 m2/s m2.s/kg s/m m/s kg/s m2/s m kg/kg m/s

Nu Sh

Nusselt number Sherwood number

Greek letters εl Latent effectiveness Ρ density subscripts A Air E Exhaust S Supply

kg/m3



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2. Experiments 2.1. Materials and test matrix Five porous membranes with different pore sizes were chosen for the test. The membrane samples were supplied by Tianshan Precision Filter Material Co., Ltd. (Hangzhou, China), and their properties are presented in Table 1. The tests were conducted under different humidity and temperature conditions as listed in Table 2. Table 1 Properties of the five membranes tested Porous membrane

Description

Pore size (μm)

Thickness (μm)

PVDF45 Nylon45 PVDF22 Nylon10 PES10

Polyvinylidene difluoride Nylon Polyvinylidene difluoride Nylon Polyethersulfone

0.45 0.45 0.22 0.1 0.1

100 100 100 100 100

Table 2 Test matrix Test cases

1

2

3

4

5

6

7

8

9

Temperature (ºC) Relative humidity (%)

27.5 80

30.0 80

32.5 80

27.5 50

30.0 50

32.5 50

27.5 30

30.0 30

32.5 30

2.2. Moisture diffusion resistance measurement The moisture diffusivity of the membranes used in enthalpy exchangers has a crucial role in influencing the latent effectiveness and total effectiveness. The water vapour flux transferred through the membrane is inversely proportional to the moisture diffusion resistance (MDR). There are various devices and methods available to measure the moisture permeability of the membranes [11]. In this study, the wet cup method specified in the standard of ASTM E96 [12] was used as an experimental approach as this method offers a low cost, simplicity and high accuracy [13]. As shown in Fig. 1, the test apparatus consisted of a 90 mm diameter PVC cup and distilled water, and the test sample was mounted on the rim of the cup by covering with a gasket and flange to hold its position. The cup assembly was placed on an electronic weighing scale with a resolution of 0.01 g, and connected to the data logger and tested in an environmental chamber, which was capable of maintaining the temperature and relative humidity at specified set points with uncertainties of ± 0.1 oC and 1.0 %, respectively. A controlled vapour pressure gradient was created for driving the moisture transfer from the high humidity region inside the cup of 100 % to the environmental chamber where the humidity and temperature were changed according to the test conditions. Once the test reached the steady state, the constant water vapour transmission rate (J) was then calculated according to Eq. (1). 𝐺𝐺𝐺𝐺

𝐽𝐽𝐽𝐽 = 𝑡𝑡𝑡𝑡 𝐴𝐴𝐴𝐴

𝑐𝑐𝑐𝑐

(1)

The corrections due to the still air and surface resistance were performed according to those specified in the standard [12]. The water vapour transmission rate through the membrane can also be expressed as Eq. (2) [14]. The volumetric moisture diffusion resistance can then be calculated by Eq. (3).

𝐽𝐽𝐽𝐽 =

𝑤𝑤𝑤𝑤𝑠𝑠𝑠𝑠 −𝑤𝑤𝑤𝑤𝑒𝑒𝑒𝑒 𝑅𝑅𝑅𝑅𝑚𝑚𝑚𝑚

(2)

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(3)

𝑟𝑟𝑟𝑟𝑚𝑚𝑚𝑚 = 𝑅𝑅𝑅𝑅𝑚𝑚𝑚𝑚 𝜌𝜌𝜌𝜌𝑎𝑎𝑎𝑎

Fig. 1 Wet cup test apparatus.

3. Latent effectiveness model of the MEE The latent effectiveness of the MEE was theoretically calculated to evaluate the performance of the MEE. Moisture diffusivity of the porous membranes was required to calculate the latent effectiveness. The correlation between the moisture diffusivity and the operating conditions could be obtained from the experimental results. For cross flow with unmixed streams, the latent effectiveness can be calculated with ε-NTU method as shown in Eq. (4) [14], in which the number of the mass transfer unit is defined by Eq. (5). 0.78 �−1 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒�−𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑚𝑚𝑚𝑚

𝜀𝜀𝜀𝜀𝑙𝑙𝑙𝑙 = 1 − 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 � 𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑚𝑚𝑚𝑚 =

−0.22 𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑚𝑚𝑚𝑚

𝜌𝜌𝜌𝜌𝑎𝑎𝑎𝑎 𝑁𝑁𝑁𝑁𝑚𝑚𝑚𝑚𝐴𝐴𝐴𝐴

(4)

�

(5)

𝑚𝑚𝑚𝑚̇𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚

The overall mass transfer coefficient is the only unknown parameter needed to obtain the latent effectiveness, which can be obtained from Eq. (6) [14]. The membrane moisture transfer resistance, which is the middle term in the parentheses of Eq. (6), is dependent on the type of the membrane used. This term cannot be neglected as it accounts for more than half of the overall mass transfer coefficient [14]. 𝑁𝑁𝑁𝑁𝑚𝑚𝑚𝑚 𝐴𝐴𝐴𝐴 = �

1

𝑘𝑘𝑘𝑘𝑒𝑒𝑒𝑒 𝐴𝐴𝐴𝐴

+

𝛿𝛿𝛿𝛿

𝐷𝐷𝐷𝐷𝑚𝑚𝑚𝑚 𝐴𝐴𝐴𝐴

+

1

𝑘𝑘𝑘𝑘𝑠𝑠𝑠𝑠 𝐴𝐴𝐴𝐴

�

−1

=�

1

𝑘𝑘𝑘𝑘𝑒𝑒𝑒𝑒 𝐴𝐴𝐴𝐴

+

𝑟𝑟𝑟𝑟𝑚𝑚𝑚𝑚 𝐴𝐴𝐴𝐴

+

1

𝑘𝑘𝑘𝑘𝑠𝑠𝑠𝑠 𝐴𝐴𝐴𝐴

�

−1

(6)

The exhaust air and supply air convective mass transfer coefficients were assumed to be identical, and can be calculated by Eq. (7), where the Sherwood number is determined using Eq. (8). The values of the Lewis number (Le) and Nusselt number (Nu) were taken as 0.85 and 7.54, respectively [10, 15].

𝑘𝑘𝑘𝑘 =

𝑆𝑆𝑆𝑆ℎ 𝐷𝐷𝐷𝐷𝑣𝑣𝑣𝑣 𝐷𝐷𝐷𝐷ℎ

𝑆𝑆𝑆𝑆ℎ = 𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝐿𝐿𝐿𝐿𝑒𝑒𝑒𝑒 −1⁄3

(7) (8)



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4. Results and discussion 4.1. Effects of membrane pore size on moisture diffusion resistance

b

65.00 60.00 55.00 50.00 45.00 40.00 PVDF45

35.00 30.00

20

30

Nylon45

40

PVDF22

50

Nylon10

PES10

70

80

60

90

Moisture diffusion resistance (m 2 .s/kg)

a

Moisture diffusion resistance (m 2 .s/kg)

Fig. 2a) provides a comparison of the moisture diffusion resistances of the five membranes under the various relative humidity conditions specified in Table 2 and at a constant temperature of 30 oC (i.e. test cases 2, 5 and 8). The PVDF45 membrane showed the lowest resistance while Nylon10 offered the highest resistance among the five porous membranes tested. The membrane resistances were insensitive to the relative humidity difference across the membrane. The result agreed with the conclusion reported by Ge et al. [6]. Fig. 2b) shows the effect of the temperature on the moisture diffusion resistance of the five membranes. The investigations were performed at different temperatures and the same relative humidity of 50 % in the environmental chamber (i.e. test cases 4-6). The MDR of the membranes slightly increased as the temperature increased. 70.00 65.00 60.00 55.00 50.00 45.00 40.00 PVDF45

35.00 30.00

26

27

Nylon45

28

Relative humidity (%)

PVDF22

29

Nylon10

PES10

31

32

30

33

Temperature (oC)

Fig. 2 Effects of the test conditions on the moisture diffusion resistance a) Humidity influence; and b) Temperature influence.

Moisture diffusion resistance (m 2 .s/kg)

Fig. 3 presents the influence of the pore size on the MDR of the PVDF and Nylon membranes at a constant temperature of 32.5 oC. It is worth noting that the PES membrane used had a single pore size and was therefore not presented in this figure. For the PVDF membrane, the MDR increased from 44.8 to 55.9 m2.s/kg at the relative humidity of 80 % and from 48.7 to 63.1 m2.s/kg at the relative humidity of 30 % when the pore size changed from 0.45 µm to 0.22 µm. The MDR of the Nylon membrane increased from 56.5 to 62.4 at 80 % relative humidity and from 60 to 68.5 at 30 % relative humidity when the pore size was 0.45 µm and 0.1 µm, respectively. 80 RH=80%

70

RH=30%

60 50 40 30 20 10 0

PVDF45

PVDF22

Nylon45

Nylon10

Fig. 3 The effect of the pore size on moisture diffusion resistance at 32.5 oC.

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4.2. Validation of the exchanger latent effectiveness model Fig. 4 presents a comparison of the results from the theoretical model and the experimental results reported in [16] for a particular enthalpy exchanger. The latent effectiveness was calculated for three MEE cores, which have different channel heights and different numbers of the channels as presented in [16] but have the same outer dimensions. It can be seen that an acceptable agreement between the two sets of values can be observed. The maximum deviation between the theoretical and experimental data was 6.5 %. 0.70

Latent effectiveness (%)

0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30

Exp. Core A Calc. Core A

0.25 0.20

100

120

Exp. Core B Calc.Core B 140

160

Exp. Core.C Calc. Core C 180

200

220

Volume flaw rate (m 3 /h)

Fig. 4 Model validation results using the experimental data reported in [16].

4.3. Effects of operating conditions on latent effectiveness of the MEE The latent effectiveness under different operating conditions was calculated to evaluate the performance of the MEE. The dimensions of a crossflow MEE as listed in Table 3 were used in this analysis. The calculation was carried out based on an air indoor temperature and relative humidity of 25 ºC and 47 %, respectively. Table 3 Specifications of the crossflow MEE [15]. MEE specifications

Value

Membrane length Membrane width Channel height Number of channels of each stream Mass flow rate

0.25 m 0.25 m 2 mm 180 0.1 kg/s

The moisture diffusivity through the membranes was calculated based on the experimental data for all membranes tested and the results are presented in Fig. 5a). The diffusivity of each membrane was slightly affected by the test conditions. The PVDF45 membrane yielded the maximum average diffusivity (i.e. 1.9 x10-6 m2/s), while the Nylon10 membrane offered the lowest average diffusivity (i.e. 1.41 x10-6 m2/s). In order to extend the results of the experimental tests to a wider range of the operating conditions, the correlations between the moisture diffusivity and the driving force (i.e. humidity difference) were obtained from the PVDF45 and PVDF22 membrane test cases. Fig. 5b) presents the test results and the correlations for the two membranes. The moisture diffusivity of the membranes decreased almost linearly with increasing humidity difference.



Ahmed K. Albdoor et al. / Energy Procedia 160 (2019) 499–506 Albdoor et al. / Energy Procedia 00 (2018) 000–000

b

2.10E-06 1.90E-06

Moisture diffusivity (m 2 /s)

Moisture diffusivity (m 2 /s)

a

1.70E-06 1.50E-06 1.30E-06 1.10E-06 9.00E-07

PVDF45

Nylon45

PVDF22

Nylon10

PES10

2.50E-06

2

4

6

8

10

Dm = 2.05x10 -6 - 1.08x10 -5 (ws - we)

2.00E-06 1.50E-06

Dm = 1.82x10 -6 - 1.85x10 -5 (ws - we) 1.00E-06 PVDF22 Linear (PVDF22)

5.00E-07 0.00E+00

0

505 7

0

0.005

0.01

PVDF45 Linear (PVDF45) 0.015

0.02

0.025

Humidity difference (ws -we ) (kg/kg)

Test No.

Fig. 5 a) Moisture diffusivity of the tested membranes, and b) The correlations of the moisture diffusivity and the moisture difference of the PVDF45 and PVDF22 membranes.

Fig. 6 shows the variation of the latent effectiveness with changes of outdoor relative humidity at two different temperatures for the PVDF45 and PVDF22 membranes. The effectiveness was calculated using the moisture diffusivity obtained from the correlations generated in Fig. 5b). The constant average effectiveness was also plotted. As expected the performance of the PVDF45 membrane was better than the PVDF22 membrane. The outdoor temperature almost did not influence the latent effectiveness, and the effectiveness of the two outdoor temperatures overlapped for each membrane. The outdoor relative humidity had a small impact on the latent effectiveness. The latent effectiveness decreased slightly from 67 % to 66.4 % for the PVDF45 membrane, and from 65.8 % to 64.5 % for the PVDF22 membrane when the outdoor humidity increased from 45 % to 90 %.

Latent effectiveness (%)

0.680 0.670 0.660 0.650 0.640 PVDF45 Variable T30 PVDF22 variable T 30 PVDF45 variable T35

0.630 0.620

40

50

60

PVDF45 constant PVDF22 constant PVDF22 variable T35 70

80

90

100

Outdoor relative humidity (%)

Fig. 6 Latent effectiveness under different operating conditions.

5. Conclusion Moisture diffusion resistance is a very important property of the membranes used in membrane enthalpy exchangers. The moisture diffusion resistances of five different porous membranes were measured using a wet cup method. The influences of the pore size on the diffusive resistance at different test conditions were evaluated. A theoretical model of the latent effectiveness of a crossflow membrane enthalpy exchanger was also presented. The results showed that the test conditions and the pore size slightly affected the moisture diffusion resistance. The PVDF45 membrane showed the lowest diffusive resistance, while the Nylon10 offered the highest diffusive resistance. The latent effectiveness was almost not affected by the outdoor temperature while it was slightly affected by the outdoor humidity. The effectiveness decreased from 67 % to 66.4 % for the PVDF45 membrane, and from 65.8 % to 64.5 % for the PVDF22 membrane when the relative humidity increased from 45 % to 90 %.

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