ASHRAE AB-10-015-2010 Crystallization Limits of LiCl-Water and MgCl2-Water Salt Solutions as Operating Liquid Desiccant in the RAMEE System.pdf

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1、494 ASHRAE TransactionsABSTRACTAvoiding crystallization in the exchangers is the main challenge in the operation of a Run-Around Membrane Energy Exchanger (RAMEE) system. In the present study, crystalliza-tion risk of two binary salt solutions (LiCl-Water and MgCl2-Water) was investigated in the RAM

2、EE system as a novel air-to-air energy recovery device. The effect of different climatic conditions and design parameters of the system on the risk of crystallization of these two salt solutions was also studied in detail. It is shown that in very dry outdoor conditions the MgCl2-Water solution may

3、crystallize in the RAMEE system while the LiCl-Water solution operates without crystallization issues in almost all climatic conditions. A MgCl2solution, as a cost effective alternative for a LiCl solution, may still be used in normal operating humidities.INTRODUCTIONMaintaining comfortable indoor a

4、ir with high Indoor Air Quality (IAQ) is the primary task for HVAC engineers. Venti-lating buildings with fresh, outdoor air is one way to create a pleasant environment for the occupants. Recent researches (Bornehag et al. 2005; Fanger 2006) show the effect of venti-lation on improving the health an

5、d productivity of office work-ers. The importance of ventilation can also be seen in the significant increase in the minimum recommended ventilation rates for commercial buildings in ASHRAE Standard 62.1 from1981 to 2004 (ASHRAE 2004). Increasing oil prices and consequently rising energy costs for c

6、onditioning buildings has drawn attention to energy recovery technologies to reduce the additional costs imposed by ventilation. Air-to-air energy recovery systems use exhaust air energy to precondition the supply air and can significantly reduce the HVAC life-cycle costs in buildings (Fauchoux et a

7、l. 2007 and 2009; Asiedu et al. 2005). Several air-to-air energy recovery devices are currently used to precondition outdoor ventilation air. These systems can be divided into two categories: (1) devices that only recover sensible energy such as plate heat exchangers, heat pipes and run-around heat

8、exchangers, and (2) systems with the ability to transfer both heat (i.e. sensible energy) and mois-ture (i.e. latent energy) between the supply and exhaust air streams. Ideal energy recovery systems are the ones with the ability to recover moisture as well as heat as these systems provide more energ

9、y savings and better indoor conditions (Erb et al. 2009; Fauchoux et al. 2007 and 2009).Desiccants are used in air-to-air heat and moisture recov-ery systems to remove moisture from humid ventilation air (Simonson 2007; Ali et al. 2004). These systems can be cate-gorized into two major groups accord

10、ing to the type of desic-cant they use. (1) Devices such as energy or dehumidification wheels utilize solid desiccants (e.g. Silica gel or LiCl) to dehu-midify/humidify air streams (Stabat and Marchio 2009; Simonson and Besant 1999); and (2) systems such as twin-tower enthalpy recovery loops use liq

11、uid desiccants (e.g. aque-ous solution of LiCl) as the working fluid. In the first group of energy exchangers (energy wheels) supply and exhaust air ducts need to be adjacent since a practical system to transmit solid adsorbents between remote supply and exhaust exchang-ers has not been developed (L

12、i et al. 2009a and b). Adjacent supply and exhaust ducts can be sometimes problematic. For instance in hospitals and laboratories where exhaust and supply air ducts need to be separated to eliminate cross contamination. Crystallization Limits of LiCl-Water and MgCl2-Water Salt Solutions as Operating

13、 Liquid Desiccant in the RAMEE SystemMohammad Afshin Carey J. Simonson, PhD, PEng Robert W. BesantStudent Member ASHRAE Member ASHRAE Fellow/Life Member ASHRAEMohammad Afshin is a masters candidate, Carey J. Simonson is a professor, and Robert W. Besant is professor emeritus in the Department of Mec

14、hanical Engineering, the University of Saskatchewan, Saskatoon, SK, Canada. AB-10-0152010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, dist

15、ribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.2010 ASHRAE 495While solid desiccant based systems need to have adja-cent exhaust and supply exchangers, liquid desiccant can be easily pumped between the two exchangers. Another probl

16、em associated with solid desiccant systems is that the power required to move air through the regenerative part is quite large. This problem can also be reduced by using liquid desic-cants (Kettleborough and Waugaman 1995). Several liquid desiccant air conditioners (LDACs) have been designed and app

17、lied in residential and industrial appli-cations (Mei and Dai 2008; Lowenstein and Novosel 1995; Kettleborough et al. 1993). In most of these systems, the supply and exhaust air is in direct contact with the liquid desiccant (Ali et al. 2004; Mesquita et al. 2006). Although direct contact between th

18、e air and liquid flow enhances the moisture transfer between the exhaust and supply air streams, evaporation of the desiccant solution into the air may reduce the indoor air quality and cause corrosion of metallic parts (Kettleborough et al. 1993).In a newly proposed air-to-air energy recovery syste

19、m, liquid desiccant is pumped in a closed loop between two semi-permeable membrane energy exchangers (Figure 1(a). Semi-permeable membranes allow water vapor to transfer through the membrane but prevent the liquid from passing through (Larson et al. 2006). Therefore in the RAMEE system, the problem

20、of direct contact between the liquid and air flow is solved. Also the liquid desiccant allows moisture and heat to be transferred between remotely located exhaust and supply exchangers (Seyed-Ahmadi et al. 2009a; Erb et al. 2009).The RAMEE system is comprised of two Liquid-to-Air Membrane Energy Exc

21、hangers (LAMEEs), shown in Figure 1(b), in which the liquid desiccant is in contact with air streams separated by a membrane. The liquid desiccant is pumped from the exhaust storage tank (see Figure 1(a) to the supply exchanger. During summer operating conditions, the desiccant in the supply exchang

22、er absorbs heat and moisture from the hot, humid outside air and leaves the supply exchanger as a warm and dilute solution. This warm and dilute solution flows to the supply storage tank where it is pumped to the exhaust exchanger. In the exhaust exchanger, the solution loses heat and moisture to th

23、e cool and dry indoor air that is exhausted from the building. Therefore the solution is regenerated (cooled and concentrated) in the Figure 1 Schematic of (a) a Run-Around Membrane Energy Exchanger (RAMEE) system and (b) Liquid to Air Membrane Energy Exchanger (LAMEE).2010, American Society of Heat

24、ing, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission

25、.496 ASHRAE Transactionsexhaust exchanger. A similar process happens during the winter with the difference that outside air is dry and cold, so the liquid desiccant will lose moisture and heat in supply exchanger and gain moisture and heat in the exhaust exchanger. As with most LDACs, the RAMEE syst

26、em uses a mixture of salt and water as the liquid desiccant. The main problem associated with the use of salt solutions in LDACs is the crys-tallization of salt in the system. At any specific salt solution temperature, there is a maximum salt concentration known as the saturation concentration, Cs.

27、When the concentration of salt exceeds this maximum amount, the solution becomes supersaturated and starts to crystallize on nucleation surfaces such as salt particles and other solid surfaces.In the RAMEE system, crystallization is most likely to occur on the surface of membranes in the LAMEEs wher

28、e moisture is being transferred from the salt solution to the air stream. Formation of salt crystals on the membrane may obstruct the diffusion of water vapor through the membrane pores resulting in a reduction of the moisture transfer rate. In addition, the accumulation of crystals increases the me

29、mbrane heat and mass resistance. In extreme cases, the growth of salt crystals may block the liquid passage in some regions of the exchangers.Liao and Radermacher (2007) investigated crystallization control strategies in an air-cooled absorption chiller and noted that high ambient temperatures may t

30、rigger crystallization in the system. Several researchers (Izquierdo et al. 2004; Kim and Infante Ferreira 2009) have investigated crystallization issues in solar absorption systems. Izquierdo et al. (2004) have concluded that crystallization occurs if the absorption temper-ature is higher than 50 C

31、.In this paper, the operating factors that affect crystalliza-tion of two binary salt solutions (MgCl2-Water and LiCl-Water) in the RAMEE system are investigated. LiCl solutions are now widely used in the HVAC industry, but recent increase in the price of Lithium based salts has forced the industry

32、to seek for more cost effective alternatives where practical. In this study, MgCl2-Water solution is investigated as an alterna-tive for LiCl solutions.PROPERTIES OF LICL AND MGCL2SOLUTIONSEquilibrium Vapor PressureDesiccants are materials with a particular affinity for water. LiCl, for example, as

33、a common commercial liquid desiccant can absorb water by more than 1000% on a dry mass basis (ASHRAE 2004). Water absorption capacity of liquid desiccants is a result of their ability to lower the vapor pressure (and consequently humidity ratio) at their interface with air. The equilibrium humidity

34、ratio of air in contact with a liquid desiccant is lower than that of pure water at the same temper-ature as shown in Figure 2 which contains the equilibrium air properties in contact with pure water and different concentra-tions of LiCl-water and MgCl2-water salt solutions. The air humidity ratio (

35、or surface humidity ratio of the liquid desic-cant) increases as the solution temperature increases and decreases as concentration increases. For example, Figure 2 shows that when a 30% MgCl2solution is heated from 20 C to 30 C the surface humidity ratio increases from 7.9 g/kg (0.0079 lb/lb) to 14.

36、7 g/kg (0.0147 lb/lb) dry air. The effect of increasing concentration on humidity ratio at the surface of LiCl-water and water-MgCl2-water solutions is also illus-trated in Figure 2.From a different perspective, equilibrium relative humid-ity of air in contact with liquid desiccant at a given concen

37、tra-tion approximately follows constant relative humidity lines on a psychrometric chart. Higher concentrations give lower rela-tive humidities, therefore the minimum equilibrium relative humidity achievable at the surface of a desiccant solution is the one at the saturation concentration of the sol

38、ution. The satu-ration concentration of the salt solutions increases slightly as the temperature increases. In this research, the equilibrium relative humidity at the solution-air interface of the saturated salt solutions is calculated using correlations of Greenspan (1977). Saturated LiCl and MgCl2

39、solutions may reduce the equilibrium relative humidity of air down to 11% and 33% respectively at 25 C (77 F).When the humidity ratio of air surrounding the liquid desic-cant is lower than that of a saturated solution at the same temper-ature, excessive evaporation of water from the liquid desiccant

40、 may result in the crystallization of salt in the solution.Other PropertiesSalt solution is used in the RAMEE system as the coupling medium to transfer heat and moisture between the supply and exhaust air streams. Therefore thermal and physi-cal properties of the salt solution are required in order

41、to numerically simulate the operation of the RAMEE system. Properties of the salt solutions at different concentrations are determined using empirical correlations by Zaytsev and Aseyev (1992). Safety of the salt solution used in the system is also an important factor since the RAMEE system is suppo

42、sed to operate in residential and commercial buildings. Both salt solutions investigated in this research are safe if used in the form of liquid (MSDS 1993 and 2002).NUMERICAL MODELA two-dimensional steady-state numerical model origi-nally developed by Fan et al. (2006) is modified and used in this

43、research to find the humidity ratio and temperature distri-bution of air and liquid desiccant in two LAMEEs coupled in a RAMEE system. The model solves equations of continuity, momentum, heat and mass transfer numerically in the LAMEEs using the finite difference method. Major assumptions made in th

44、is model are as follows: (1) the fluid flow in the channels is laminar and fully developed, (2) heat and mass transfer only occur perpendicular to the membrane, (3) properties of membrane do not change with temperature and humidity, (4) the RAMEE is operating under 2010, American Society of Heating,

45、 Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.201

46、0 ASHRAE 497steady-state conditions (5) and the supply and exhaust heat exchangers are identical (Vali et al. 2009).The governing equations based on the above assumptions are two equations describing the conservation of energy and two equations describing the conservation of mass (one pair of equati

47、ons for the liquid stream and another for the air streams) in each LAMEE shown in Figure 1(b).Conservation of energy:Liquid side:(1)Air Side:(2)Conservation of mass:Liquid side:(3)Air side:(4)Where:(5)(6)The humidity ratio of the liquid desiccant at the membrane surface ( - see Figure 3) in the abov

48、e equa-tions is a function of temperature and concentration of the liquid desiccant and is defined analytically by Cisternas and Lam (1991):(7)In order to solve the above equations for known air and solution inlet boundary condition two additional equations describing the mass balance at the interfa

49、ce of the salt solution and membrane are required. These are:(8)(9)The solution to equations (1) to (9) provides the bulk temperature and humidity ratio of the air and solution as well as temperature and equilibrium humidity ratio at the interface Figure 2 Constant concentration lines of (a) MgCl2 and (b) LiCl solutions (Greenspan 1977; Cisternas and Lam 1991).2x0UmmSolcpSol- WAirWSol,mem()hfg2x0UhmSolcpSol- TAirTSol()+TSoly-=2y0UhmAircpg- TAirTSol()TAirx-=2Umx0mSalt- WAi