1、2009 ASHRAE 689ABSTRACTIn this paper, the experimental testing of a run-around membrane energy exchanger (RAMEE) is considered and data are compared to numerical simulations. The effects on the performance of the system due to different exchanger sizes, liquid and airflow rates, external heat gains/
2、losses and desic-cant concentrations are considered in detail. Also studied is the transient response of the system during both initial start-up and due to changes in the outdoor air conditions. INTRODUCTIONAir-to-air energy exchangers are becoming more widely used as the world begins to focus more
3、on sustainability and energy conservation. Air-to-air energy exchangers not only reduce energy consumption throughout the life of the building, but also have the capability of providing these savings at little or no added capital costs due to the reduced size of heating and cooling equipment (Faucho
4、ux et al., 2007; Asiedu et al., 2004; and Asiedu et al., 2005). There are many devices currently commercially available that are capable of transferring energy between the supply and exhaust air ducts of a building, each with their unique advan-tages and disadvantages (ASHRAE, 2004; Besant and Simon
5、-son, 2003). The existing air-to-air energy exchangers can be divided into two groups based on whether the exchanger is capable of heat and moisture transfer (e.g., energy wheels (Simonson, 2007) or permeable plate energy exchangers (Zhang and Niu, 2002), or is restricted to heat transfer only. As w
6、ell, the exchangers can be split into two additional cate-gories of ones that require the supply and exhaust ducts to be adjacent, and those that can be located remotely from each other (Larson, 2006). Ethylene glycol coupled run-around heat exchangers (Johnson et al., 1995; and Fan et al., 2005) ar
7、e examples of exchangers that transfer heat between remote supply and exhaust airstreams.The ideal energy exchanger is one that can transfer both heat and moisture because during hot and humid conditions such an exchanger is capable of transferring up to four times as much energy as an exchanger tha
8、t can transfer sensible heat only. It would be beneficial if the exchanger can transfer heat and moisture between remote supply and exhaust airstreams, as this may minimize the ducting required and reduces contaminant transfer from one airstream to the other. This is very important for applications
9、such as hospitals, laboratories, and manufacturing facilities, where slight cross contamination can cause serious health effects (Zhang et al., 2008). The abil-ity to implement remote exhaust and supply airstreams would allow the exchanger to be applied in retrofit applications with minimal costly c
10、hanges in ducting. This retrofit market is large due to the slow building replacement rate of 2 to 3% per year.Currently, there is only one type of commercial system available that transfers both heat and moisture between remote supply and exhaust airstreams. This type of system is based on the twin
11、-tower enthalpy recovery loop, and has not been a very popular choice since its inception in the 1980s due to several disadvantages caused by using a direct liquid-air contact system for energy transfer (ASHRAE, 2004). The first disadvantage is that the energy transfer is achieved by direct contact
12、between the supply air and the desiccant solu-tion (Ali et al., 2004; Mesquita et al., 2006; and Park et al., 1994). Although the direct contact allows for high moisture transfer rates, it also results in a small fraction of the desiccant being transported downstream by the air through the supply du
13、cts. This can result in corrosion problems and poor indoor Experimental Measurements of a Run-Around Membrane Energy Exchanger (RAMEE) with Comparison to a Numerical ModelBlake Erb Mehran Seyed AhmadiStudent Member ASHRAECarey J. Simonson, PhD, PEng Robert W. Besant, PEngMember ASHRAE Fellow/Life Me
14、mber ASHRAEBlake Erb is a masters candidate, Carey J. Simonson is a professor, and Robert W. Besant is professor emeritus in the Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, SK, Canada. Mehran Seyed Ahmadi is a doctoral candidate in the Depart-ment of Material Science
15、 and Engineering at the University of Toronto, Canada.LO-09-065 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmissi
16、on in either print or digital form is not permitted without ASHRAEs prior written permission.690 ASHRAE Transactionsair quality. Demister pads, which can be used to reduce this transfer, are not 100% effective and add to the cost of the system. The second main disadvantage of the open twin-tower sys
17、tem is that the desiccant is gravity fed through the airstream. This allows for only a small range of desiccant flow rates to be controlled, resulting in little control over the energy transfer rate. Even when higher desiccant flow rates are achieved, another disadvantage becomes evident. As the des
18、iccant solution flow rate increases, it fills more of the pores in the contact media, causing the pressure drop on the air side to increase. Therefore, operating conditions that require high desiccant flow rates would require higher fan power to maintain adequate ventilation rates. The twin-tower lo
19、op is also difficult to configure in a counter flow arrange-ment unless the ducts are arranged to deliver the supply and exhaust air in a vertical upward direction. Therefore, a cross flow arrangement is mostly used, which provides lower performance. A novel design of a run-around membrane energy ex
20、changer (RAMEE) system has been proposed to eliminate all of these disadvantages (Fan et al., 2006). The RAMEE system uses an aqueous salt solution to transfer both heat and moisture between two remote liquid-to-air membrane energy exchangers (LAMEE). The exchangers are constructed using membranes t
21、hat are permeable to water vapor, but imperme-able to liquid water, such as some polytetrafluoroethylene (PTFE), polypropylene and polyethylene membranes. Numerical results by Larson et al. (2006) show that an effec-tiveness of over 70% is possible from a cross flow configura-tion, by choosing the p
22、roper membrane and exchanger size. Previous studies on the RAMEE system (Fan et al., 2006 and Larson et al., 2007) have been based on numerical simu-lations. Thus, the main purpose of this paper is to present experimental data for a RAMEE system and to identify the impact of several RAMEE design and
23、 operation parameters on performance of the RAMEE system. These parameters include the LAMEE physical parameters, as well as the air and desiccant flow rates. A prototype design of a complete RAMEE system is tested and compared to the numerical model of Seyed Ahmadi et al. (2008a).RAMEE PROTOTYPE DE
24、SIGNThe RAMEE system consists of two separate LAMEEs; one located in the supply air duct entering the building, and the other located in the exhaust air duct leaving the building. Both exchangers are coupled by a continuous desiccant loop, which transfers heat and moisture between the two LAMEEs. Tw
25、o small centrifugal pumps drive the liquid desiccant flow, and are along with the external fans the only mechanical power needed to operate the system. Small desiccant storage tanks are placed in each desiccant line to allow room for volume changes as the desiccant losses or gains moisture. A schema
26、tic of the RAMEE system is shown in Figure 1.Each LAMEE is 0.1 m (3.9”) wide, 0.6 m (23.6”) long and 0.4 m (15.7”) high and is designed to be cross flow in config-uration, so that the air and desiccant flow perpendicular to each other. The cross flow configuration was chosen for simplicity. In futur
27、e studies a counter flow design will be investigated. A small desiccant reservoir is located on both top and bottom of each LAMEE to help provide proper flow distri-bution entering and leaving. Figure 2 shows the LAMEE design.Each LAMEE consists of a series of desiccant panels, each of which is asse
28、mbled separately and then combined with an outer casing to produce the exchanger. The membrane panels retain the desiccant solution inside a channel, but allow heat and water vapor to be transferred into/from the air that flows past the outside of these panels. The prototype LAMEEs consisted of 10 p
29、anels, each of which provides a 1.70 mm (0.067”) wide desiccant channel and a 4.76 mm (3/16”) wide airflow channel. However, this design allows for the easy modification to fit any size of existing ducting, by simply changing the number of panels. A single LAMEE panel is shown in Figure 3. The main
30、component of each panel is the semi-permeable membrane, which allows for vapor transmission, but not liquid water because of surface tension. There are many different semi-permeable membranes available including ones made of polyethylene and polypropylene. Larson et al. (2006) found that a polypropy
31、lene composite may be a good choice for use in a LAMEE. This composite membrane was a two-layer lami-nate consisting of a thin polypropylene membrane bonded to a polypropylene non-woven fabric. The fabric layer provides structural support and is permeable to both liquid and vapor water. The thin pol
32、ypropylene membrane is permeable to water vapor, but impermeable to liquid water (except at very high pressures above the operating pressures of a RAMEE system). The mean thickness of the chosen membrane was 0.5 mm (0.020”).In this design, the polypropylene material is wrapped around a small pore pl
33、astic-fiberglass screen, producing an envelope. The membrane side of the material is located on the inside where the liquid desiccant will flow. The membrane is glued to itself, which provides a sealed seam in the envelope. The small pore screen that is wrapped inside of the membrane is made of soft
34、 woven plastic strands which provide a consis-tent channel (1.70 mm (0.067”) for the desiccant to flow through. The screen also distributes the desiccant flow more evenly in the channels, ensuring desiccant contact with the membrane wall. A metal screen is placed on the outside of each panel in orde
35、r to provide structural support, and eliminate bulging under pressure. This is important, because bulging of the membrane under pressure reduces the size of the air chan-nels, and decreases performance (Larson et al., 2008). A 12.7 mm (1/2”) square screen with a wire thickness of 1 mm (0.04”) is fou
36、nd to provide adequate support (Larson et al., 2006). Finally, the entire membrane and screen assembly is glued between two aluminum header plates on both the top and bottom which provides for the adequate spacing for both the desiccant and air channels, creating a single membrane panel.ASHRAE Trans
37、actions 691PERFORMANCE TESTING Testing ApparatusThe testing apparatus used to test the RAMEE system was designed to meet ASHRAE Standard 84 (ASHRAE, 2008) and consists of two separate airstreams as shown in Figure 4. One airstream represents the supply air entering a building, and the other airstrea
38、m represents the exhaust air leaving the building. One LAMEE is installed in each airstream, and both are coupled together with a desiccant piping system, to create a complete RAMEE system.The supply air is provided by an environmental condition-ing chamber which can supply air at chosen temperature
39、s between -40C (-40F) and 40C (104F) and humidity values up to 90% RH. Two variable speed 3.73 kW (5 hp) vacuum pumps (or fans), located on each side of the supply LAMEE provide airflow from the conditioning chamber, through the LAMEE and into the laboratory room. Similarly, two variable speed 3.73
40、kW (5 hp) vacuum pumps supply exhaust airflow from the laboratory room, through the LAMEE and into the conditioning chamber. Two vacuum pumps are used in order to minimize the pressure difference between the airstream and the ambient pressure at each exchanger, which reduces membrane deflections and
41、 air leakage. The laboratory room temperature and humidity are kept as close as possible to meet AHRI standard 1060 (AHRI, 2005). All air ducts consists of 101.6 mm (4 inch) diameter round PVC pipe, which are insu-lated using 12.7 mm (1/2”) thick fiberglass insulation to reduce heat loss/gain with t
42、he surroundings. Liquid desiccant is supplied to each exchanger using a 93.2 W (1/8thhp) magnetic drive pump. The flow rate of the desiccant is controlled using a rotometer capable of flow rates between 0.2 and 2.2 US gallons per minute (0.013 L/s and 0.14 L/s). All desiccant piping is insulated by
43、9.53 mm (3/8”) foam insulation to reduce heat loss/gain. Figure 1 Schematic of a run-around membrane energy exchanger (RAMEE) system.Figure 2 The liquid-to-air membrane energy exchanger (LAMEE) prototype.692 ASHRAE TransactionsFigure 3 A single membrane panel and atomic force microscope image of the
44、 polypropylene membrane used in the LAMEE prototype.Figure 4 RAMEE testing apparatus schematic.ASHRAE Transactions 693Measured PropertiesIn order to evaluate the performance of the RAMEE system it is important to measure the properties of the airstream before and after each LAMEE to see the changes
45、that occur. The mass flow rate of air through each RAMEE is measured using an orifice plate located before and after each exchanger as shown in Figure 4. The orifice plates create a pressure drop which is measured with a differential pressure transducer. From this pressure drop, the mass flow rate i
46、s calculated by using the standard orifice equation (1)where= discharge coefficient= diameter of the orifice plate opening= inside pipe diameter= measured pressure drop= density of the air= d/DThe orifices used and the length of each pipe section is designed to follow ISO Standard 5167-1 for proper
47、flow rate measurements using orifice plates (ISO, 1991).airstream temperatures are measured on both sides of each LAMEE using both 24 AWG (0.02”, 0.51 mm diameter) T-type thermocouples (average of 3 thermocouples) and a resistance temperature device (RTD). The RTDs are more accurate, but the thermoc
48、ouples have a faster response for transient situations. Humidity is measured at the inlet and outlet of both exchangers using capacitive humidity sensors. Also important are the desiccant properties before and after each LAMEE. Knowing how the desiccant performs allows for the determination of heat
49、gains/losses from the desiccant piping and pumps. Also, the desiccant conditions provide insight into how the desiccant performs compared to other possible desiccants for different operating conditions. The desiccant temperature entering and leaving each exchanger are measured using 30 AWG (0.01”, 0.25 mm diameter) T-type thermocouples which are placed inside a well to avoid contact with the conductive desiccant. Data acquisition is handled with the use of software which collected and stored the readings of all twenty sensors in 10 second intervals. Calibration and UncertaintyIn ord
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