Recent Advances in Energy, Environment, Economics and Technological Innovation
Testing method of materials for enthalpy wheels
TOMÁŠ VÍT, PETR NOVOTNÝ, NGUYEN VAN VU, VÁCLAV DVOŘÁK Department of Power Engineering Equipment Technical University of Liberec Studentská 2, 461 17 Liberec CZECH REPUBLIC [email protected] www.kez.tul.cz
Abstract: The present paper provides information about a new method for testing the material properties of the fill materials for enthalpy wheels. Current methods of testing, where it is necessary to manufacture an entire regenerator, are very expensive and time consuming. There is an increasing demand for the development of a methodology that would facilitate a rapid and relatively accurate comparison of different materials used by producers of heat exchangers. The article gives a description of the test facility and the theoretical basis and methodology for processing the results. It also shows an example of experimental results obtained for samples of different materials.
Key-Words: Enthalpy exchanger, material research, enthalpy wheel, zeolit.
1 Introduction necessary to reduce formaldehyde emissions and The issue of indoor environment quality is cigarette smoke, radon emissions and to eliminate becoming increasingly important with the the leakage of asbestos and to strictly comply with development of energy efficiency methods in the requirements on the storage of volatile substances construction of new commercial and residential outside of living spaces. Similarly, it is possible to buildings. At the moment, these savings are mainly reduce the number of microorganisms by controlling achieved by improving the air-tightness of buildings indoor humidity. and thermal insulation. In cases where it is not possible to reduce the Various key studies (e.g. [1, 2, 3]) show that the emission of pollutants, it is necessary to provide degree of contamination of the indoor environment some form of passive or active ventilation. Passive ventilation is the exchange of air through windows with undesirable substances (CO2, CO, butyric acid or doors with respect to the action of natural forces. CH3CH2CH2-COOH, NO2, volatile organic compounds - VOCs, dust particles and fibers, However, passive ventilation has been reduced to a microorganisms such as fungi and mold spores, minimum to save energy in modern, energy- bacteria and viruses) is several times higher than efficient buildings. outdoors. This fact, coupled with the fact that people In contrast, active ventilation systems work spend approximately 90% of their time inside continuously on the basis of well-defined buildings, points to the necessity of high-quality parameters. The amount of air pollutants is inversely ventilation. Adverse indoor conditions often lead to proportional to the intensity of the ventilation in the development of so-called Sick Building actively ventilated buildings. Syndrome (SBS) in people who live and work in The requirements of new hygienic standards for air-tight and thermally insulated buildings. the amount of incoming fresh air are rising SBS is a serious problem. Professional studies alongside the development of modern technologies [4, 5] conducted in the U.S. quantified the financial for the construction of energy-saving and energy- loss due to poor indoor air quality to be USD 60 passive buildings [1]. An air exchange value of 20 billion a year. Similar studies conducted in the UK cubic feet per minute/person, is recommended to estimated annual losses of up to USD 400,000 for improve indoor climate conditions (compared to 5 office buildings with 2,500 workers. cfm/person, which is considered the industry There are two basic approaches for maintaining standard). It is expected that demands on the amount acceptable conditions in an indoor environment. of air to be exchanged will increase even more in The first is to control the emission of pollutants the near future. into the environment. The control of emissions is It is necessary to install ventilation systems that not always possible or appropriate. However, it is include heat recovery equipment to meet the requirements for the exchange of air as well as
ISBN: 978-960-474-343-8 34 Recent Advances in Energy, Environment, Economics and Technological Innovation
demands for the lowest possible energy modified (oxidized) aluminum coatings, kraft paper, consumption. different types of hydrophilic polymer coatings or Heat recovery systems can be divided into two various molecular sieve coatings such as zeolite. main groups, with respect to the form of transmitted During the development phase of a new energy. The first group includes systems that use exchanger, it is necessary to run a number of only sensible heat. The second group consists of different tests. An entire enthalpy wheel (as shown systems which use both sensible and latent heat in Fig. 2) must be used for the tests. This process is (enthalpy systems). Enthalpy systems can process very expensive when testing new materials for up to three times more energy than systems enthalpy wheels. operating with sensible heat only. An example of thermodynamic processes in heat and enthalpy exchangers is shown in Fig. 1. Today's market offers a relatively wide range of design variants (rotary regenerative heat exchangers, plate heat exchangers, heat pipes, etc.) for the ventilation of buildings using heat recovery. Information regarding research in the field of enthalpy plate exchangers can be found, for example, in [6, 7]. The main disadvantage of sensible heat exchange systems is a reduction in the relative humidity of the indoor environment during the winter season. This fact creates a need for Fig. 2 Enthalpy wheel produced by Kastt additional air humidification, which involves a very energy intensive process. The recommended relative To reduce the costs of enthalpy wheel humidity of interiors should be between 40% and development, this paper proposes a new method for 60%. It is almost impossible to exceed the lower testing and comparing different materials for their limit when ventilation without humidification is suitability. used.
t (°C ) t (°C ) φ= 1 φ= 1 2 Mass transfer theory
) ) Thermodynamics processes that occur in an A A gD D k kg kJ/ J/ ( (k enthalpy wheel are typical examples of unsteady x x + + y 1 1 h y h mass and thermal diffusion. When we consider the b b hydrophilic coating through which H2O molecules diffuse, we can write the species conservation x a x a equation in the form:
x (kg/kgDA ) x (kg/kgDA) 2OH jnm , (1) Fig. 1 Thermodynamic processes in a heat exchanger 2 2OHOH (left) and an enthalpy exchanger (right). The enthalpy where is the partial density of H2O, m is efficiency can be expressed as a/x, temperature efficiency 2OH 2OH as b/y. the mass fraction of H2O, n denotes the total mass flux in the coating, and j is the diffusion mass A solution for the challenges mentioned above is 2OH to use enthalpy exchangers. Both regenerative and flux of H2O. recuperative enthalpy exchangers are available on For low mass transfer rates, we can the market nowadays. In the case of recuperative consider m 1. This means that only diffusion 2OH exchangers, there is a wide range of materials – contributes significantly to the mass flux, and Eq. from simple paper through to chemically modified (1) could be written as: cellulose and special hydrophilic membranes and composites – to choose from. 2OH j OH . (2) In the case of regenerative exchangers, enthalpy 2 wheels of various designs dominant the market. When we denote the diffusion coefficient of the There are only a few materials used as a H2O in the hydrophilic coating as D , with use of 2OH humidity absorbent in enthalpy wheels. Most Fick's law in the form: applications use modified silica gel, chemically
ISBN: 978-960-474-343-8 35 Recent Advances in Energy, Environment, Economics and Technological Innovation
ALMENO 2590 device (manufactured by OH mDj OHOH (3) 2 2 2 ALHBORN) (12). and considering that D and the density will 2OH The dry and humidified airflows were separately and sequentially delivered to the enthalpy not vary much with small m OH , we can write Eq. 2 exchanger’s inlet every 30 seconds by manually (2) as: switching two ball valves (9d and 9h). A m 2OH 2 mD , (4) measurement for each sample was taken at three 2 2OHOH volumetric velocities: 400, 800, and 1600 l/h. Each Eq. (4) is often called the mass diffusion equation. volumetric velocity lasted for 15 minutes, meaning The value of the Biot number for mass transfer in there were 30 switches between the dry airflow and the hydrophilic coating is much larger than unity, the humidified airflow. even for thin coatings (with a range of m). The The average area of the tested material was 2 time-dependent mass fraction profile in the coating 200 cm . This area was large enough to achieve should be found in the form: meaningful results with low uncertainty.
mm x 2 2 ,sOHOH erf , (5) 1 OH 0, mm ,sOH 2 D OH 2 2 2 8d 2 10 where m and m are mass fractions of H2O 3 2 ,sOH 2OH 0, 7d 9d HI 9h in surroundings and at the beginning of the process. HO 5 8h The complete derivation can be found in [8]. 4
Using Fick's law (3), and after a derivation of H 7h 0 21 3 (5), we can write the equation for the time- H-humidified airflow -dry airflow ALMENO dependent mass flux from the coated surface to the I-inlet PC 2590 O-outlet 6 surrounding air in the form: 13 12 11 D Fig. 3. 1 - compressor, 2 - dehumidifier, 3 - mmj 2OH . (6) manometer, 4 - air filter, 5 - pressure regulator, 6 - water 2OH 2 , 2OHsOH 0, tank, 7d and 7h - throttle valves, 8d and 8h - rotameters, 9d and 9h - ball valves, 10 - experimental enthalpy exchanger, 11 - experimental membrane, 12 - Almeno 2590, 12.0 - sensor for dry airflow inlet , 12.1 - sensor 3 Experiments for dry airflow outlet , 12.2 - sensor for humidified-inlet 3.1 Experimental setup airflow, 12.3-sensor for humidified-outlet airflow, 13-PC Figure 3 shows the arrangement of this experimental equipment. Dry compressed air from a dehumidifier 3.2 Conditional averaging (2) was passed through an air filter (4) in order to An example of recorded data is presented in Fig. 4. eliminate dust. The airflow’s pressure was adjusted The conditional averaging of the data from to 2 MPa, and then the airflow was divided into two humidity sensors 12.0 - 12.3 during one cycle was parts by using a 3-way junction; one part went carried out by means of a decomposition of the directly to the experimental regenerative exchanger quantity. (10), while the second part was first moistened by The measured quantity had to be decomposed passing through water in the water tank (6) before into individual parts as: = + f + ’, where is being sent to the exchanger. the time-mean humidity, f is the periodic phase- The relative humidity of the humidified airflow locked component, and ’ is the fluctuation resulting from this method varied from 85% to over ~ component. The conditional averaged value of 90%. Both of these volumetric airflows were t measured and adjusted by two rotameters (8d and the humidity should be calculated as: 8h) and two throttles (7d and 7h). A coated ~ 1 N t lim nTc , (7) aluminum membrane (11) was placed within the N N n0 experimental regenerative exchanger. The where T is time interval between two cycles. The exchanger was equipped with four high-precision c switch from the humid to dry air, via ball valves 9d FHAD36R humidity sensors, which were able to and 9h, was set as the beginning of each individual measure temperature, relative humidity, pressure, cycle. etc. at the inlet and outlet of the exchanger. Data from these sensors was recorded to a PC (13) via an
ISBN: 978-960-474-343-8 36 Recent Advances in Energy, Environment, Economics and Technological Innovation
Start of experiment Data for evaluation To quantify the results, two parameters were selected. The first of these was the total moisture 100 50 absorbed by the material (Sum). 95 45 0.0 90 40 85 35 -0.5
80 30 -1.0 75 25 -1.5
70 20 ) -1 rel. humid (%) rel. humid (%) 65 15 DA -2.0
60 10 (g.kg -2.5 Alu x
55 5 PUR -3.0 PVA 50 0 SG -3.5 0 200 400 600 800 Com. Mat Zeolit Y time (sec) -4.0 Zeolit 4A Fig. 4 Example of recorded data. Data from sensors -4.5 12.0 and 12.1 are presented along the right axis, data 0 5 10 15 20 25 30 35 40 (sec) from sensors 12.2 and 12.3 along the left axis. Fig. 6 Comparison of different materials.
100 50 This is proportional to the area above the curve. 95 0 1 2 45 90 40 The second parameter was based on fitting a 85 35 theoretically derived dependence 80 30 21 OH axj (8) 75 25 2 70 20 in the measured data. rel. humid(%) 65 15 rel. humid(%) 60 10 0.0 55 5 50 0 -0.5
400 420 440 460 480 500 -1.0 time (sec) Sum -1.5 21 )
-1 ax
Fig. 5 Example of an experimental cycle. The time DA -2.0 a 1.42
interval from 0 to 1 represents the charging of the (g.kg -2.5 x experimental exchanger. Time interval 1 to 2 represents -3.0 the discharging phase. Fitted curve -3.5 Measured data -4.0 -4.5 4 Results 0 5 10 15 20 25 30 35 40 A typical example of the experimental cycle is (sec) shown in Fig. 5. At time 0 the 9h ball valve is open Fig. 7 Method of evaluation of the results and the 9d ball valve is closed. The time interval from 0 to 1 represents the charging of the Parameter a is proportional to the square root of experimental exchanger. At time 1 the 9h is closed diffusivity of the selected coating in this equation. and 9d is open. The time interval 1 to 2 represents However, it is not possible to derive the exact value the discharging phase. of diffusivity due to the influence of other The results for materials listed in Table 1 are parameters. The meaning of the parameters (Sum, a) presented in this article. is illustrated in Figure 7. The results were evaluated separately for both The results are summarized in the Table 1. charging and discharging phases. The results for the discharging phase only are presented in this paper. 5 Conclusion Recorded data were processed using conditional The presented method and experimental setup for sampling as described in section 3.2. Subsequently, determining the properties of materials for enthalpy the results were converted from relative to specific exchangers provides a good idea about the humidity. An isothermal process was assumed. suitability of materials for given applications. The main experimental results are presented in The method allows a variety of tests to be Fig. 6. Each of the curves represents an average of performed that would otherwise be very 30 cycles. expensive and time-consuming.
ISBN: 978-960-474-343-8 37 Recent Advances in Energy, Environment, Economics and Technological Innovation
[4] Seppanen, O., W. J. Fisk and Q. H. Le, Table 1 Summary of the results Ventilation and performance in office work, Sum a Indoor Air 16(1), pp 28-36, 2006 Coating Label -1 -1 1/2 [5] American Lung Association, the American g·kgDA ·sec g·kgDA ·sec Medical Association, the U.S. Consumer Oxidized Aluminium Alu 21.8 -14.2 Product Safety Commission, and the EPA, Indoor Air Pollution: An Introduction for Polyurethane film PUR 29.1 -17.5 Health Professionals, Diane Publishing, 1994
Polyvinyl-alcohol film PVA 47.8 -22.1 [6] Novotny, P., Nguyen, V., Measurement of Moisture Transport in the Membrane–Based Silica Gel SG 51.1 -28.4 Enthalpy Exchanger, Experimental Fluid Material from Mechanics 2012, Hradec Kralove, pp. 532-535, commercially available Com. Mat. 53.2 -30.3 enthalpy wheel 2012
Zeolit Y Zeolit Y 67.9 -38.4 [7] Vestfalova, M. Evaluation of material properties determining the moisture transfer, Zeolit 4A Zeolit 4A 76.1 -42.1 Experimental Fluid Mechanics 2012, Hradec
Kralove, pp.762-765 [8] Lienhard, J.H., Lienhard J.H., A heat transfer
The method also allows the use of very small textbook, Philogiston Press, 2003 samples, which further reduces the cost of the tests. The accuracy and repeatability of the method are sufficient to provide an idea about the suitability of the selected material. Another advantage of the method is the ability to quickly and easily change parameters, such as the velocity, temperature, and humidity of the incoming air.
Acknowledgements The authors are grateful for the financial support provided by the Czech Technological Agency under the project TACR TA01020313.
References: [1] Dhital, P., Besant, R. and Schoenau, G.J., Integrating run-around heat exchanger systems into the design of large office buildings, ASHRAE Transactions 101(2).,pp 979-991, 1995. [2] ASHRAE, 2009 ASHRAE Handbook - Fundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc, 2010. [3] Wargocki, P., J. Sundell, W. Bischof, G. Brundrett, P. O. Fanger, F. Gyntelberg, S. O. Hanssen, P. Harrison, A. Pickering, O. Seppanen and P. Wouters, Ventilation and health in non-industrial indoor environments: report from a European multidisciplinary scientific consensus meeting (EUROVEN), Indoor Air, 12(2), pp 113-128, 2002
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