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Heat and Mass Transfer of a Countercurrent Flow Energy Recovery Ventilator (ERV) by Roy Pastor An Engineering Project Submitted to the Graduate Faculty of Rensselaer Polytechnic Institute in Partial Fulfillment of the Requirements for the degree of MASTER OF ENGINEERING Major Subject: MECHANICAL ENGINEERING Approved: _________________________________________ Norberto Lemcoff, Primary Project Adviser _________________________________________ Ernesto Gutierrez-Miravete, Secondary Project Adviser Rensselaer Polytechnic Institute Hartford, Connecticut December, 2010 (For Graduation January 2011) i © Copyright 2010 by Roy Pastor All Rights Reserved ii CONTENTS LIST OF TABLES ............................................................................................................. v LIST OF FIGURES .......................................................................................................... vi ACKNOWLEDGMENT ................................................................................................. vii NOMENCLATURE ....................................................................................................... viii ABSTRACT ..................................................................................................................... ix 1. INTRODUCTION ....................................................................................................... 1 1.1 Background ........................................................................................................ 1 1.2 Previous Work.................................................................................................... 2 1.3 Problem Description........................................................................................... 3 2. METHODOLOGY ...................................................................................................... 5 2.1 Physical Model ................................................................................................... 5 2.2 Mathematical Model .......................................................................................... 5 2.2.1 Heat and Mass Transfer ......................................................................... 5 2.2.2 Boundary Conditions ............................................................................. 7 2.2.3 Heat Transfer Effectiveness ................................................................... 8 3. FINITE ELEMENT MODEL ...................................................................................... 9 3.1 ERV Dimensions and Parameters ...................................................................... 9 3.2 Fluid Dynamics ................................................................................................ 10 3.3 Heat Transfer .................................................................................................... 11 3.4 Convection and Diffusion ................................................................................ 11 3.5 Meshing ............................................................................................................ 11 4. RESULTS .................................................................................................................. 13 4.1 Problem Scenarios ............................................................................................ 13 4.2 Summer and Winter Conditions with Equal Supply and Exhaust Flow .......... 13 4.2.1 Summer Conditions.............................................................................. 13 4.2.2 Winter Conditions ................................................................................ 16 iii 5. CONCLUSION.......................................................................................................... 17 6. REFERENCES .......................................................................................................... 18 7. APPENDIX A ............................................................................................................ 19 7.1 Sensible and Latent Effectiveness Calculation ................................................ 19 7.1.1 Summer Conditions.............................................................................. 19 7.1.2 Winter Conditions ................................................................................ 19 iv LIST OF TABLES Table 1. ERV Basic Dimensions ....................................................................................... 9 Table 2. Inlet Properties and Parameters ........................................................................... 9 Table 3. Outlet Properties and Parameters ........................................................................ 9 Table 4. Membrane Properties and Parameters .............................................................. 10 Table 5. Elements Spacing of the ERV .......................................................................... 11 v LIST OF FIGURES Figure 1. Schematic of a Cross-Flow Membrane ERV .................................................... 3 Figure 2. Schematic of a Quasi-Counter Flow Membrane ERV ...................................... 3 Figure 3. Schematic of a Countercurrent Flow Membrane ERV ..................................... 4 Figure 4. Schematic of a Concurrent Flow Membrane ERV............................................ 4 Figure 5. Pictorial Description of the Mathematical Model ............................................. 7 Figure 6. Countercurrent Flow Sensible and Latent Effectiveness ................................ 14 Figure 7. Concurrent Flow Sensible and Latent Effectiveness ....................................... 14 vi ACKNOWLEDGMENT Type the text of your acknowledgment here. vii NOMENCLATURE C water concentration in membrane (kg/m3) Cw water vapor density in the air, kg/m3 cp specific heat, J / kg K D diffusivity, m2/s d channel height or membrane spacing, m h convective heat transfer coefficient, m/s k convective mass transfer coefficient, m/s L channel length m air flow rate, kg/s mw moisture flow rate through the membrane, kg / m2s q heat flux, W/m2 Q total heat transfer rate, W T temperature, C or K U overall heat transfer coefficient, W / m2 K v air velocity in the core, m/s x,y,z coordinates Greek Symbols Thermal conductivity, W / m K Density, kg/m3 Subscripts a air e exhaust L latent s supply S sensible tot total w water vapor viii ABSTRACT The purpose of this project is to evaluate the effectiveness of an energy recovery ventilator (ERV). The ERV that will be evaluated will have a countercurrent and concurrent flow configuration. ix 1. INTRODUCTION 1.1 Background In the recent years, there is a high demand to conserve energy. Therefore, there is a push in many engineering systems to use less energy, while maintaining the same functions and exceeding performance required by previously designed system. This is especially true for heating, ventilating, and air conditioning (HVAC) system, which is required to provide comfort and quality air for occupants in buildings or offices, within reasonable installation, operation, and maintenance cost. A traditional HVAC system will typically consists of coils, fans, heaters, ducts, and filters. The purpose of the coil is to reduce the air temperature and allow the incoming air to condensate for dehumidification. The fan is the driving force to allow the conditioned air to flow through the duct in buildings or offices. To control the thermal comfort in the space a heater will be use, to heat the cool air that passed through the cooling coil. The ducts are used distribute conditioned air to various location of a building. To provide quality air, a filter will be used to prevent airborne bacteria, dust, or odors that may be found from outside air to be distributed in conditioned spaces. Using a traditional HVAC system for buildings and requiring higher volume of outside air for heating and cooling, the ventilation system will have to grow to meet the demands by newer buildings. This may be accomplish by using a larger coil, fans, or/and heaters. Larger ventilation systems will increases operating and system equipment cost. Therefore, using a larger system is not a viable solution for conserving energy and meeting system demands. Additionally, one of the major costs for ventilation system is the need to dehumidify the incoming air from outside environment. Dehumidification is costly, because outside air must first past through a cooling coil where it is cooled below the saturation temperature of the air to allow condensation. The cool air must then be reheated, since the conditioned space requires a higher temperature to meet the proper thermal comfort (21°C, 30-60% Relative humidity (Reference (a)) typically required. Therefore, by reducing the need to use a cooling coil and heaters, the energy and maintenance cost required to maintain a HVAC system may be reduced. 1 1.2 Previous Work To reduce the energy consumption of ventilation system, research in areas such as air-toair energy recovery ventilator (ERV) or enthalpy exchanger has been accomplished. The ERV allows ventilation system to reduce energy consumption because it uses conditioned air that is normally exhausted out of the buildings, to either heat or cool (sensible heat) and humidify or dehumidify (latent heat) incoming air taken from outside. Therefore, this allows the ERV to be used during all the seasons. The moisture and heat transfer is possible because the porous membrane located between the conditioned and supplied air, allows the heat and moisture to pass through the membrane. The cost of a ventilation system will also be reduced, because an ERV does not have the complexity generally found in rotary dehumidifiers or cooling coils. The simplified design of the ERV also reduces the maintenance cost, because it does not have any moving parts that can wear over time and only routine cleaning is required. Therefore the ERV allows the ventilation system demands to grow, while maintaining air quality required by buildings and offices mandated by state and local codes such ASHRAE, but does not increase the energy consumption of a ventilation system. The most typical ERV design generally found in the market are the cross flow design, due to its simplified design, and the ease of duct sealing required for ERV systems. A depiction of a cross flow ERV design is shown below in Figure 1 (Reference (b)). Due to the popularity of cross flow ERV systems, Zhang, L.Z. and Jiang, Y. (Reference (b)(b)) analyzed the heat and mass transfer of ERV through the use of numerical analysis and conducting a test of a commercial product in a test lab. Min, Jingchun and Su, Ming (Reference (c)) analyzed the performance of ERV by changing the membrane spacing and thickness of the ventilator through numerical computation. 2 Figure 1. Schematic of a Cross-Flow Membrane ERV Another type of ERV design that has been in research is a quasi-counter flow design ERV. A schematic of a quasi-counter flow design is shown in Figure 2 below (Reference (d)). Due to the lack of research in countercurrent flow ERV design, Zhang, Li (Reference (d)) conducted a research of an ERV with a quasi-counter flow design, because a countercurrent flow membrane ERV has a much higher effectiveness than a cross flow design. Figure 2. Schematic of a Quasi-Counter Flow Membrane ERV 1.3 Problem Description Based on previous research of ERV systems, it was determined that countercurrent and concurrent flows ERV have not been evaluated greatly. Therefore, in this paper the effectiveness of countercurrent and concurrent flows will be evaluated and compared to each other. This paper will not focus on the complexity of creating a countercurrent or concurrent flow membrane ERV or the cost required to create it. It is assumed that a countercurrent and concurrent flows ERV will be feasible. A countercurrent flow membrane ERV is when the exhaust and supply air, flows in opposite direction, as shown below in Figure 3. 3 d Exhaust Air d Porous Membrane d Supply Air L Figure 3. Schematic of a Countercurrent Flow Membrane ERV A concurrent flow membrane ERV is when the exhaust and supply air, flows in the same direction, as shown below in Figure 4: d Exhaust Air Porous Membrane d d Supply Air L Figure 4. Schematic of a Concurrent Flow Membrane ERV One study that will be conducted in this project is to evaluate the impact of ERV’s performance as the airflow speeds through the supply and exhaust are varied. The ERV will also be evaluated when the mass flow rate through the exhaust duct is reduce, while the supply duct mass flow rate remains the same. This study will be conducted because in most ventilation system, the exhaust flow dispelled to the outside environment is generally lower than the supply flow from the outside environment, because some of the conditioned air is redirected back in the space or to the coil. The ERV will also be evaluated when the mass flow rate through the supply and exhaust is held constant; however, the opening of the exhaust duct is reduced. All the studies mentioned above will be evaluated for both summer and winter conditions. To model the ERV’s performance, the countercurrent and concurrent flow will be analyze in COMSOL. 4 2. METHODOLOGY 2.1 Physical Model A typical membrane-based ERV with a countercurrent or concurrent flows are shown in Figures 3 and 4, respectively. The ERV design that will be analyzed is a core that contains alternate layers of membranes to separate and seal the exhaust and supply airstream passages. As described above, a countercurrent flow ERV is designed such that the exhaust and supply airstream flows in opposite direction, while a concurrent flow ERV is designed that the exhaust and supply airstream flows in the same direction. As the exhaust and supply air flows through the membrane, the airstream will exchange heat and moisture through the membrane. Since the ERV has a symmetric design, the domain that will be evaluated will contain only half of the volume of the supply and exhaust airstream and a membrane as shown in Figure 3 for countercurrent flow ERV and Figure 4 for concurrent flow ERV. 2.2 Mathematical Model Based on the physical model described above, several assumptions will be made to assist in the modeling of the countercurrent and concurrent flow ERV: Heat and mass transfer process are in steady state The physical properties of the air fluid and membrane are constant Heat conduction and vapor diffusion in the two air streams are negligible compared to energy transport and vapor convection by bulk flow Water vapor diffusion in the membrane only occurs in the thickness direction Temperature and concentration distribution in the thickness direction in membrane are linear 2.2.1 Heat conductivity and the water diffusivity in the membrane are constant Heat and Mass Transfer The governing heat and mass transfer equation for the ERV is shown below for supply air, exhaust air, and through the membrane: Supply Air: 5 Ts 2hs Ts Tms 0 x (1) Cws 2k s Cws Cwms 0 x (2) s c ps vs d vs d Exhaust Air: Te 2he Te Tme 0 x (3) Cwe 2ke Cwe Cwme 0 x (4) e c pe ve d ve d Membrane: mwc pw Tm 2T 2Tm m 2m m 0 y x y 2 (5) C Cme C Dwm ms y d (6) mw Dwm where is the density, cp is the specific heat, v is the air velocity in the core, d is the channel height or membrane spacing, T is the temperature, h is the convective heat transfer coefficient, Cw is the water vapor density in the air streams, k is the convective mass transfer coefficient, and C is the water concentration in membrane at two surfaces, d is the membrane thickness. The subscript e, m, s, w refer to the exhaust air, membrane, supply air, and water vapor, respectively. Based on the equations above, equations (1) through (4) reflect the changes of the energy carried by the supply and exhaust airstreams along their flow directions, through the heat and mass transfer across the membrane. Equations (1) through (4) will be considered as the boundary conditions for the heat and mass transfer within the membrane (equations (5) and (6)). A pictorial description of the equations describe above is shown below in Figure (5). 6 Heat Convection Desorption Exhaust Air Heat Conduction Heat Convection Porous Membrane Water Diffusion Supply Air Adsorption Figure 5. Pictorial Description of the Mathematical Model 2.2.2 Boundary Conditions The boundary conditions for the ERV based on the assumptions and the equations described for the heat and mass transfer are the following: Supply Air: Ts x 0 Tsi Cws x 0 Cwsi (7) Exhaust Air: Te x 0 Tei Cwe x 0 Cwei (8) Membrane: Tm x x 0, L 0 (9) The boundary conditions for the membrane surface on the supply side: m Tm y y 0 hs Ts Tm mw Lw (10) For the membrane surface of the exhaust side: m Tm y y d he Te Tm mw Lw (11) Lw is the latent heat of vaporization for water, and it is assumed that in equations (10) and (11) that the heat sorption is constant and it is equal to the latent heat of moisture. 7 2.2.3 Heat Transfer Effectiveness The heat transfer effectiveness of the ERV is a way to measure its ability to transfer sensible and latent heat. In order to equate the sensible heat transfer effectiveness, the sensible heat transfer of either the supply or exhaust flow will be divided by the maximum sensible heat transfer possible for this system. The sensible heat transfer effectiveness is shown below: S s c ps vs Tsi Tso ec peve Teo Tei 2 cpv (12) Tsi Tei min For the latent heat transfer effectiveness a similar approach will be used as described for the sensible heat transfer effectiveness, except the latent heat transfer is used in lieu of the sensible heat transfer, as the equation is shown below: L s vs Cwsi Cwso s vs Cweo Cwei 2 v min Cwsi Cwei (13) 8 3. FINITE ELEMENT MODEL 3.1 ERV Dimensions and Parameters Based on the mathematical model described above, a finite element software will be used to model the ability of the ERV to transfer sensible and latent heat. The software that will be used for this analysis is COMSOL. The ERV basic dimensions were taken from Reference (c) and are shown in Table 1. Table 1. ERV Basic Dimensions Length (mm) Height (mm) Membrane Height (mm) 250 2 0.1 The supply and exhaust parameters for both the summer and winter season were found by using the data from Reference (e) for air properties typically found on ERV designs. References (f) and (g) were used to evaluate other parameters required for the finite element model, based on the data provided by Reference (e). The data found in References (e), (f), and (g) are shown in Tables 2 and 3 for supply and exhaust, respectively. Table 2. Supply Parameters for Summer and Winter Seasons Inlet Dry Bulb Temperature (C) Inlet Dry Bulb Temperature (K) Inlet Wet Bulb Tempearture (C) Inlet Dry Bulb Temperature (K) Relative Humidity (%) Pressure (mbar) Density (kg/m^3) Dynamic Viscosity (kg/m*s) Thermal Conductivity (W/m*K) Diffusion (m^2/s) Concentration Air (mol/m^3) Concentration Water (mol/m^3) Summer Winter 35.000 1.700 308.150 274.850 26.000 0.600 299.150 273.750 49.340 82.020 56.280 6.910 1.145 1.284 1.895E-05 1.738E-05 0.026 0.024 2.680E-05 2.120E-05 39.550 44.342 1.085 0.248 Table 3. Exhaust Parameters for Summer and Winter Seasons 9 Exhaust Dry Bulb Temperature (C) Exhaust Dry Bulb Temperature (K) Exhaust Wet Bulb Tempearture (C) Exhaust Wet Bulb Tempearture (K) Relative Humidity (%) Pressure (mbar) Density (kg/m^3) Dynamic Viscosity (kg/m*s) Thermal Conductivity (W/m*K) Diffusion (m^2/s) Concentration Air (mol/m^3) Concentration Water (mol/m^3) Summer Winter 24.000 21.000 297.150 294.150 17.000 14.000 290.150 287.150 49.590 45.866 29.850 24.877 1.188 1.200 1.844E-05 1.830E-05 0.025 0.025 2.484E-05 2.436E-05 41.014 41.432 0.600 0.467 The membrane parameters were determined using the average temperatures of the dry bulb and wet bulb temperatures found for the supply and exhaust data found in Table 3 and 4. To determine the other parameters for the membrane Reference (f) was used. The diffusion and the thermal conductivity through membrane were taken from Reference (d). Table 4. Membrane Properties and Parameters Inlet Dry Bulb Temperature (C) Inlet Dry Bulb Temperature (K) Inlet Wet Bulb Tempearture (C) Inlet Dry Bulb Temperature (K) Density (kg/m^3) Thermal Conductivity (W/m*K) Diffusion (m^2/s) Membrane Concentration (mol/m^3) Diffusion (m^2/s) Air to H20 Summer Winter 29.500 11.350 302.650 284.500 21.500 7.300 294.650 280.450 1.160 1.240 0.130 0.130 8.000E-06 8.000E-06 40.269 42.838 2.680E-05 2.272E-05 3.2 Fluid Dynamics The initial stage of the finite element modeling is to resolve the flow through the ERV for both countercurrent and concurrent flow. The fluid dynamics model that will be selected is the incompressible Naiver-Stokes, steady state model in COMSOL. It will be assumed in this model that the wall of the membrane has a no slip condition, and the outer boundary of the supply and exhaust flow is the system boundary. The fluid dynamics model is used, because it allows the heat transfer and convection and diffusion models to be defined. The other two models may be defined by the fluid dynamics 10 model, because the velocity profile is required in their input to evaluate the heat transfer and convection and diffusion models. 3.3 Heat Transfer For the heat transfer of the ERV, the conduction and convection, steady state model in COMSOL will be used. In this mutliphysics model, COMSOL will solve the heat transfer in the ERV, by providing data such as the temperature, temperature gradient, and heat flux. It will be assumed in this model that the inner boundaries have continuity through the membrane and outer boundary of the supply and exhaust flow has thermal insulation. It will also be assumed that there will be no velocity flow through the membrane. The heat transfer model will be used to calculate the sensible heat effectiveness of the ERV 3.4 Convection and Diffusion In the final stage of the analysis, the convection and diffusion, steady-state model will be selected in COMSOL. This model will be used to determine the ability of the ERV to humidify or dehumidify the air, by defining the diffusion constant for the membrane and air, and the vapor concentration in the air for both supply and exhaust. In this model the inner boundaries will also have continuity through the membrane, and the outer boundary of the supply and exhaust flow will be defined as insulation/symmetry. 3.5 Meshing To mesh the model, the mapped mesh parameters will be used. The mapped mesh parameter is use, because it provides more flexibility and the user has better control in preventing the meshing of the model from exceeding the computer’s memory that can be use by COMSOL. In order to solve the ERV in COMSOL, quadrilateral meshes were used, and were divided to equal spaces as defined below: Table 5. Elements Spacing of the ERV d d L 10 10 200 11 Based on the division above, the ERV will have 6000 elements of quadrilateral meshed in the model. 12 4. RESULTS 4.1 Problem Scenarios In order to evaluate the effectiveness of the ERV, various scenarios will be evaluated. The first scenario will be to evaluate the ERV during the summer and winter condition, when the velocity through the ERV at the inlet and outlet is 1.0 m/s, 1.25 m/s, and 1.5 m/s. The ERV will then be evaluated for both summer and winter conditions when the supply velocity is held constant at 1.5 m/s, while the velocity through the exhaust will be varied from 1.0 m/s, 1.25 m/s, and 1.5 m/s. This scenario is evaluated because in most HVAC systems, exhaust flows are generally recirculated back to the system for reheat, so the supply flow is not always equal to the exhaust flow. For the final analysis, the ERV will also be evaluated for both summer and winter conditions, while the supply airflow is held constant at 1.5 m/s, the mass flow rate through the exhaust flow is held constant, but the height of the exhaust plate is varied to 1.33 x 10-3 m (exhaust flow 2.25 m/s). 4.2 Summer and Winter Conditions with Equal Supply and Exhaust Flow 4.2.1 Summer Conditions Through the used of COMSOL and the methods described in section 3, the sensible and latent effectiveness of the ERV were evaluated for the summer conditions using the data from Tables 1-4. The effectiveness of the ERV was calculated using the equations (12) and (13). A summary of the results for the summer condition is shown below: Table 6. Sensible and Latent Effectiveness for Summer Conditions Speed (m/s) 1 1.25 1.5 Countercurrent Concurrent Countercurrent Concurrent S S L L 0.605 0.474 0.609 0.478 0.553 0.451 0.555 0.456 0.509 0.427 0.511 0.433 13 Using the data from Table 6, the sensible and latent effectiveness for the countercurrent and concurrent flows are shown below. Countercurrent Flow Sensible and Latent Effectiveness 0.620 0.600 0.580 Sensible Latent 0.560 0.540 0.520 0.500 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 Speed (m/s) Figure 6. Countercurrent Flow Sensible and Latent Effectiveness Concurrent Flow Sensible and Latent Effectiveness 0.480 0.470 0.460 Sensible Latent 0.450 0.440 0.430 0.420 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 Speed (m/s) Figure 7. Concurrent Flow Sensible and Latent Effectiveness Figures 6 and 7 showed that the latent effectiveness of the ERV for both countercurrent and concurrent flow is slightly better than the sensible effectiveness. This result shown above differs from Reference (d), because Reference (d) showed that the sensible effectiveness of the ERV is better than the latent effectiveness. 14 The data from Table 6 will also be used to plot the sensible effectiveness of the countercurrent and concurrent flow. The same plot will also be made for the latent effectiveness. Sensible Effectiveness 0.600 0.580 0.560 0.540 Countercurrent Concurrent 0.520 0.500 0.480 0.460 0.440 0.420 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 Speed (m/s) Figure 8. Sensible Effectiveness for Countercurrent and Concurrent Flows Latent Effectiveness 0.600 0.580 0.560 0.540 Countercurrent Concurrent 0.520 0.500 0.480 0.460 0.440 0.420 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 Speed (m/s) Figure 9. Latent Effectiveness for Countercurrent and Concurrent Flows Figures 8 and 9, shows that for both sensible and latent effectiveness the countercurrent flow ERV is more effective than the concurrent flow configuration, which is the expected results based on the configuration evaluated. The countercurrent flow is more effective because the highest and lowest temperature of the fluid are in opposite 15 direction, which allows the greatest heat transfer as the fluid flows from the inlet to the outlet of the ERV. The full details of the calculation of the sensible and latent effectiveness may be found in Appendix A of the report. 4.2.2 Winter Conditions 16 5. CONCLUSION 17 6. REFERENCES (a) “Thermal Comfort”, Wikipedia, September 26, 2010 http://en.wikipedia.org/wiki/Thermal_comfort, Web. November 11, 2010 (b) Zhang, L.Z.; Jiang, Y., Heat and mass transfer in a membrane-based energy recovery ventilator, Elsevier Ltd., Journal of Membrane Science (1999) 29-38 (c) Min, Jingchun; Su, Ming, Performance analysis of a membrane-based energy recovery ventilator: Effects of membrane spacing and thickness on the ventilator performance, Elsevier Ltd., Applied Thermal Engineering 30 (2010) 991-997 (d) Zhang, Li-Zhi, Heat and mass transfer in quasi-counter flow membrane-based total heat exchanger, Elsevier Ltd., International Journal of Heat and Mass Transfer 53 (2010) 5478-548 (e) 2005 Standard for Performance Rating of Air-to-Air Exchangers for Energy Recovery Ventilation, Air Conditioning, Heating, and Refrigeration Institute (AHRI), 2005, ANSI/AHRI Standard 1060 (f) Cengel, Yunus, Heat and Mass Transfer A Practical Approach, Third Edition, McGraw-Hill Companies, New York, 2007, Pages 782 and 860 (g) “Psychometric Calculations”, Sugar Engineers, http://www.sugartech.co.za/psychro/index.php, Web. November 16, 2010 18 7. APPENDIX A 7.1 Sensible and Latent Effectiveness Calculation 7.1.1 Summer Conditions In order to calculate the sensible and latent effectiveness, the boundary integration of COMSOL was used to aid in the calculation of the average temperature and concentration at the supply and exhaust, inlet and outlet boundary. The average temperature and concentration were calculated using the following equations, respectively. d Tave T u dy 0 (A1) d u dy 0 d Cave C u dy 0 (A2) d u dy 0 7.1.2 Winter Conditions 19