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Similarity of coupled heat and mass transfer between air–water and air–liquid desiccant direct-contact systems
Building and Environment 44 (2009) 2501–2509
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Similarity of coupled heat and mass transfer between air–water and air–liquid desiccant direct-contact systems Xiao-Hua Liu a, *, Zhen Li b, Yi Jiang a a b
Department of Building Science, School of Architecture, Tsinghua University, Beijing 100084, PR China Department of Engineering Mechanics, Tsinghua University, Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 December 2008 Received in revised form 12 April 2009 Accepted 30 April 2009
Packed-bed heat and mass transfer devices are widely used in air-conditioning systems, such as cooling tower, evaporative cooler of air–water direct-contact devices, dehumidifier and regenerator of air–liquid desiccant direct-contact devices. Similarities of heat and mass transfer characteristics between air–water and air–liquid desiccant devices are considered and investigated in this paper. Same reachable handling region of outlet air can be obtained for both air–water and air–liquid desiccant devices, which is among three boundary lines, isenthalpic line of inlet air, iso-relative humidity line of inlet fluid (water or desiccant), and the connecting line of inlet statuses of air and fluid. Inlet conditions of air and fluid affect heat and mass transfer characteristics to some extent, so that a zonal method is proposed only according to the relative statuses of inlet air to inlet fluid. Four zones, dehumidification zones A, D and regeneration zones B, C, are divided for air-desiccant direct-contact devices. The first three zones A, B and C are divided for air–water direct-contact devices, with the same zonal properties as those of air-desiccant devices. In order to obtain better humidification performance, fluid should be heated (in zone C) rather than air (in zone B). And fluid should be cooled (in zone A) rather than air (in zone D) to obtain better dehumidification performance. Counter-flow pattern should be applied for best mass transfer performance in the same conditions within the recommended zone A or C, while parallel-flow pattern is the best in zone B or D. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Heat and mass transfer Liquid desiccant/water Reachable handling region Zonal method Flow pattern Air-conditioning
1. Introduction The task of air-conditioning systems is to create comfortable indoor environment, including required indoor temperature and humidity ratio (or relative humidity). Usually outdoor air will be cooled and dehumidified in summer, heated and humidified in winter before being supplied into occupied spaces. The air-conditioning systems are composed of various heat and mass transfer devices. Water and liquid desiccant are two liquids commonly used in air-handling processes, which contact humid air directly. Heat and mass transfer occur between air and water (or liquid desiccant). Air–water direct-contact devices [1,2] include cooling tower, evaporative cooler, humidifier, etc. Liquid desiccant air-conditioning systems have developed quickly in recent years [3–5], for the reason that they can be driven by low grade heat, such as solar energy and waste heat. Commonly used liquid desiccants are the mixture of salt and water, for instance, lithium bromide (LiBr) aqueous solution, lithium chloride (LiCl) aqueous solution and calcium chloride (CaCl2)
* Corresponding author. Tel.: þ86 10 6277 3772; fax: þ86 10 6277 0544. E-mail address: [email protected] (X.-H. Liu). 0360-1323/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2009.04.013
aqueous solution. Dehumidifier and regenerator are the most important components in liquid desiccant air-conditioning systems. Air will be dehumidified by contacting concentrated liquid desiccant in the dehumidifier. And in the regenerator air will be humidified and desiccant will be concentrated. Heat and mass transfer characteristics between air and water (or liquid desiccant) essentially determine the performance of the packed-bed air-handling devices. Vapor pressure difference between air and fluid (water or liquid desiccant) is the driving force of mass transfer, and it should be noticed that the fluid’s vapor pressure will be greatly influenced by its temperature. Meanwhile temperature difference between air and fluid is the driving force of heat transfer, which is affected by the released/absorbed vaporization latent heat along with the mass transfer process. Therefore, heat transfer process and mass transfer process within packed-bed devices influence each other and should not be considered separately. The performances of air–water [1,2,6–9] or air-desiccant [3–5,10–13] direct-contact devices have been widely investigated separately through numerical analysis and experimental tests. Performance influencing factors have been obtained, such as flow rates of air and water-desiccant, packing volume and sizes, and optimization methods have been proposed. Questions appear in the
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
2. Properties of liquid desiccant and water Air-handling processes in air–water and air–liquid desiccant direct-contact devices are usually conducted in standard atmospheric pressure, which is regarded as a constant in this paper. The following two equations will be satisfied when humid air is in equilibrium with water or liquid desiccant, where subscripts a, w and s stand for humid air, water and liquid desiccant respectively.
Air—water : ta ¼ tw ;
Air-desiccant : ta ¼ ts
Air—water : pa ¼ pw ;
x. An increase in desiccant temperature and a decrease in desiccant concentration at the same time result in an increase in desiccant vapor pressure. As indicated in Fig. 2, status of liquid desiccant, which is from the crystal line of desiccant to the saturated line of air, can reach a larger area in the psychrometric chart compared with water. The isoconcentration line of liquid desiccant almost coincides with the iso-relative humidity line of air, which means that liquid desiccant at constant concentration and water both lay on an isorelative humidity line of air. The status of desiccant is near the saturated line of air, when desiccant concentration is low enough. The variances of desiccant concentration within packed-bed dehumidifier/regenerator during the experiments [4,5,10,13] are so small, as indicated in Table 1, that they are usually omitted and the
a Temperature / ºC
further study of such kind of air-handling devices. What are the similarities between air–water and air-desiccant direct-contact devices? Stevens et al. gave [14] analytical solutions of outlet conditions of air and liquid desiccant in packed-bed dehumidifier/ regenerator, using the NTU method which was widely used in the analysis of air–water system. Ren [15] gave the analytical solutions of both air–water and air-desiccant direct-contact systems, and got similar solutions for the two systems. Previous studies provide a useful way to investigate the similarities of these two kinds of direct-contact devices. This paper will focus on the comparison of air–water direct-contact system and air-desiccant system, in order to find out the similarities between the two systems, upon which heat and mass transfer processes will be analyzed and optimized. As indicated in a former study [12], in air-desiccant devices, heat and mass transfer characteristics can be determined to some extent only according to the relative statuses of inlet air to inlet desiccant. Maybe similar conclusion can be drawn for air–water system, which will benefit the design of air-handling systems with better mass transfer performance.
55
Crystal 50 line =60%
Air-desiccant : Pa ¼ Ps
(2)
=40%
40 35
20%
30%
45
40% 50% 60%
40
80% ϕ =100%
35 30
60% 80% =100% Isoconcentration line Iso-
0
0.01
0.02
0.03
b
Crystal line
50
=50%
40%
=20%
40 35
20%
40%
60%
30
80%
=100% Isoconcentration line
25
Iso-
0.01
0.02
0.03
0.04
0
0.01
0.02
0.03
0.04
0.05
Humidity ratio / (kg/kg) 55 Crystal line
=50% 40%
45
30% =20%
40 35
20% 40%
30
60% 80% =100% Isoconcentration line Iso-
0.05
line
20
Equivalent water status 0
0.05
30%
45
20 20
0.04
55
25 25
line
Humidity ratio / (kg/kg)
Temperature / ºC
Temperature / ºC
20%
40%
30
50 ϕ =10%
45%
20
c
55
50%
45
(1)
Eq. (2) can be rewritten as ua ¼ ue , since humidity ratio is coincident with vapor pressure ðu ¼ 0:622 p=ðB pÞÞ, where ue is the equivalent humidity ratio of water/desiccant with humid air. Therefore, status of water or liquid desiccant can be expressed in the air psychrometric chart with the equilibrium conditions. As indicated in Fig. 1, status of water coincides with saturated line of humid air (4a ¼ 100%). Surface pressure of liquid desiccant is about the same to vapor pressure, due to the large boiling temperature difference between salt and water. For instance, boiling point of salt, such as LiBr, LiCl and CaCl2, is as high as 1200 C, while boiling point of water is only 100 C at standard atmospheric pressure. Surface vapor pressure of liquid desiccant ps depends on its temperature ts and concentration
50
55%
25
Temperature / ºC
2502
0
0.01
0.02
0.03
line 0.04
0.05
Humidity ratio / (kg/kg)
Humidity ratio / (kg/kg) Fig. 1. Status of water shown in the air psychrometric chart.
Fig. 2. Status of commonly used liquid desiccants shown in the air psychrometric chart: (a) LiBr aqueous solution; (b) LiCl aqueous solution; and (c) CaCl2 aqueous solution.
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
2503
Table 1 Parameters of air and liquid desiccant in the dehumidification/regeneration experiments.
Patnaik et al. [4] Fumo et al. [5] Yin et al. [13] Patnaik et al. [4] Fumo et al. [5] Lof et al. [10] Yin et al. [13]
Dehumidification/regeneration
Liquid desiccant
R
ta,in/ C
ua,in/(g/kg)
ts,in/ C
xin/%
NTUm
Dx /%
Dehumidification Dehumidification Dehumidification Regeneration Regeneration Regeneration Regeneration
LiBr LiCl LiCl LiBr LiCl LiCl LiCl
1.4–3.1 0.1–0.3 0.7 0.9–1.2 0.1–0.2 1.7–2.2 1.4
28–39 30–40 28–35 48–77 29–40 81–109 30–36
12–23 14–22 11–19 5–11 14–21 11–18 14–20
23–32 25–35 25–30 40–60 60–70 34–38 55–80
44–59 33–35 38–40 57–61 32–35 8–27 20–30
0.2–1.0 1.0–1.3 – 0.2–0.8 2.4–4.2 0.6–1.0 0.1–0.4
<0.7 <0.2 <0.2 <0.4 <0.8 <0.4 <1.1
a
Air outlet
b
Fluid inlet
Packing
Air inlet
Air inlet
Fluid outlet Fig. 3. Packed-bed heat and mass transfer devices for air–water or air–liquid desiccant direct-contact systems: (a) device; and (b) packing.
Packed-bed is a commonly used type of heat and mass transfer device for air–water and air–liquid desiccant direct-contact systems, as shown in Fig. 3. Large flow rate of water or desiccant is usually chosen for two purposes, 1) fully wet the packing, and 2) enlarge heat capacity of liquid flow. Therefore, flow rate variance of water or liquid desiccant within packed-bed devices can be usually omitted. Heat and mass transfer models for air–water or airdesiccant have been derived by many researchers. And the model for parallel-flow pattern [9,14,15] is listed here. The energy conservation equation between air and fluid (water or liquid desiccant) is shown below, where subscript f stands for water (w) in air–water system or liquid desiccant (s) in air-desiccant system.
_ a dha þ m _ f dhf ¼ 0 m
(3)
The overall heat transfer and moisture transfer between air and fluid are given in Eqs. (4) and (5) respectively, with the definitions of Le and NTUm shown in Eq. (6).
40
a
Temperature / ºC
3. Heat and mass transfer model and main performance influencing factors
a
a
a
3
a
4
2
a
5
1
30
a
6
20
w a
R =2 7
R =1 R =0.1
10
0
10
20
30
40
Humidity ratio / (g/kg)
b
40 a
Temperature / ºC
desiccant concentration is assumed as a constant within the devices [14–16]. Therefore, liquid desiccant will locate on an isorelative humidity line in air-desiccant direct-contact devices, and water will always locate in 100% iso-relative humidity line in air– water direct-contact devices.
a
a
3
a
4
2
a
5
1
30 a
6
20
w a
R =2 7
R =1 R =0.1
NTU dha NTUm m ¼ $Le$cp;m tf ta þ $r$ðue ua Þ dx H H NTUm Le 1 ¼ ðhe ha Þ þ r 1 ðue ua Þ H Le
10 0
(4)
10
20
30
40
Humidity ratio / (g/kg) Fig. 4. Effect of R, flow pattern and inlet status on the conditions of outlet air in air–water direct-contact device: (a) parallel-flow pattern; and (b) counter-flow pattern.
2504
a
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
a a
Temperature / ºC
water and liquid desiccant (ue and he in Eqs. (4) and (5)), as indicated in Figs. 1 and 2. The Le number is regarded as 1 for air–water and also air–liquid desiccant system [1,16]. Similar solutions can then be obtained. Analytical solutions [16] of enthalpy efficiency 3h and moisture efficiency 3m for both air–water and air-desiccant directcontact systems in parallel-flow configuration can be obtained:
40 a
4
3
a
R =2 2
R =1
5
a
30 a
a
s
6
1
R =0.1
*
3h ¼ 20 a
a 7
a
8
a
a
9
5
10
15
20
40
Temperature / ºC
a
a
4
a
2
R =1
5
a
30 a
a
s
6
20 a
a 7
a
8
a
a 9
R =0.1
1
12
11
=1 00 %
10
10 0
5
10
15
20
Humidity ratio / (g/kg) Fig. 5. Effect of R, flow pattern and inlet status on the conditions of outlet air in air–liquid desiccant direct-contact device using LiBr as liquid desiccant: (a) parallel-flow pattern; and (b) counter-flow pattern.
dua NTUm ¼ ðue ua Þ dx H Le ¼
(5)
a am A ; NTUm ¼ _a am cp;m m
*
ua;in ua;out ua;in ua;m m* m* þ 1 eNTUm þ eNTUm ðm þ1Þ ¼ 3h þ $ ua;in ue;in ua;in ue;in m* þ 1
(8)
_a cp;e m dhe m* ¼ R$ where cp;e ¼ ; R ¼ _f m cp;f dtf
R =2 3
(7)
Heat capacity ratio of air to fluid m* shown in Eqs. (7) and (8) is defined as:
Humidity ratio / (g/kg)
a
¼ f1 m* ; NTUm
3m ¼
10
¼ f2 m* ; NTUm
0
þ1Þ
11
10
b
ha;out ha;in 1 eNTUm ðm ¼ he;in ha;in m* þ 1
12
(6)
The heat and mass transfer model is the same for air–water and air–liquid desiccant direct-contact devices. The differences between these two kinds of devices are the equilibrium characteristics of
(9)
As indicated in Eqs. (7) and (8), two main performance influencing factors are heat capacity ratio of air to liquid m* and mass transfer unit NTUm. Apart from being influenced by the physical properties of air and fluid, heat capacity ratio m* also depends on mass flow rate ratio of air to fluid R. Figs. 4 and 5 show the effects of R, flow pattern and conditions of inlet air and fluid on the heat and mass transfer performances, when NTUm is taken as 1. The detailed information concerning the conditions of inlet and outlet air and fluid are listed in Tables 2 and 3 for air–water and air-desiccant devices respectively. The points of inlet air a1 and a7 are on isorelative humidity line of inlet fluid (4a,in ¼ 4e,in), the humidity ratios of points a3 and a9 are equal to the equivalent humidity ratio of inlet fluid (ua,in ¼ ue,in), the enthalpies of points a5 and a11 are the same as the equivalent enthalpy of inlet fluid (ha,in ¼ he,in), and the temperatures of points a6 and a12 are the same as that of inlet fluid (ta,in ¼ tw,in or ta,in ¼ ts,in). As indicated by Figs. 4 and 5 and Tables 2 and 3, the relative positions of inlet air to inlet fluid do affect the heat and mass transfer characteristics to some extent. Counter-flow configuration is not always the best flow pattern for various inlet air conditions. Counterflow configuration exhibits worse mass transfer performance than parallel-flow pattern or cross-flow pattern in the same operating conditions, when inlet air a3 contacts water w, inlet air a3, a4, a9 or a10 contacts liquid desiccant s. The similarities of these operating conditions are that the combined heat and mass transfer direction (denoted by enthalpy difference Dh ¼ ha he) is opposite to the mass
Table 2 Inlet and outlet conditions of air and water through the packed-bed devices as shown in Fig. 4 (water inlet temperature tw,in ¼ 25 C). Inlet Air inlet
ta,in/ C ua,in/ (g/kg)
a1
30.5
27.9
a2
34.1
25.1
a3
36.0
20.1
a4
34.8
16.1
a5
30.8
12.5
a6
25.0
11.1
a7
18.2
13.0
Flow pattern
Outlet (R ¼ 0.1) ta,out/ C
ua,out/
27.2 27.2 28.6 28.5 29.2 29.2 28.7 28.7 27.1 27.1 24.9 24.9 22.3 22.4
23.2 23.1 22.1 22.1 20.2 20.1 18.6 18.6 17.1 17.2 16.5 16.6 17.2 17.3
Outlet (R ¼ 1) ta,out/ C ua,out/ (g/kg)
tw,out/ C
ta,out/ C
ua,out/ tw,out
25.4 25.4 25.3 25.3 25.2 25.2 25.0 25.0 24.8 24.8 24.7 24.7 24.6 24.6
28.2 27.9 29.5 29.1 29.7 29.5 28.7 28.7 26.6 26.8 23.9 24.3 21.3 21.7
27.6 28.0 27.3 27.6 26.2 26.4 25.0 25.0 23.6 23.4 22.6 22.3 22.4 22.0
28.9 28.5 30.0 29.7 30.0 29.8 28.7 28.7 26.2 26.4 23.3 23.7 20.6 21.0
25.5 25.0 24.1 23.7 21.2 20.9 18.6 18.6 16.0 16.3 14.7 15.2 15.3 15.8
(g/kg) Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter
Outlet (R ¼ 2)
tw,out/ C
24.6 24.1 23.3 22.9 20.8 20.5 18.6 18.6 16.5 16.7 15.4 15.9 16.0 16.5
Zone Dehumidification/ Mass transfer Humidification performance
(g/kg) 28.7 29.5 28.3 29.0 26.7 27.1 25.0 25.0 22.9 22.5 21.4 20.7 21.1 20.2
A
Dehumidification
Parallel < Counter
A
Dehumidification
Parallel < Counter
B
Humidification
Parallel > Counter
–
Humidification
Parallel ¼ Counter
C
Humidification
Parallel < Counter
C
Humidification
Parallel < Counter
C
Humidification
Parallel < Counter
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
2505
Table 3 Inlet and outlet conditions of air and liquid desiccant through the packed-bed devices as shown in Fig. 5 (Desiccant inlet ts,in ¼ 25.0 C, and xin ¼ 45.0%). Inlet Air ta,in/ C ua,in / (g/kg) inlet a1
30.5
11.7
a2
33.6
10.3
a3
35.0
8.3
a4
34.3
6.8
a5
32.1
5.5
a6
25.0
4.3
a7
18.0
5.5
a8
15.7
6.8
a9
15.0
8.3
a10
15.7
9.8
a11
18.0
11.2
a12
25.0
12.3
Flow Outlet (R ¼ 0.1) pattern ta,out/ C ua,out/ ts,out/ C xout/% (g/kg)
ta,out/ C ua,out/ ts,out/ C (g/kg)
xout/%
ta,out/ C ua,out/ ts,out/ C xout/% (g/kg)
Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter
28.2 27.9 29.4 29.0 29.6 29.3 29.0 28.8 27.7 27.7 24.2 24.4 21.2 21.6 20.4 20.8 20.4 20.7 21.0 21.2 22.3 22.3 25.8 25.6
44.9 44.9 45.0 45.0 45.0 45.0 45.1 45.1 45.1 45.1 45.1 45.1 45.1 45.1 45.0 45.0 45.0 45.0 45.0 45.0 44.9 44.9 44.9 44.9
28.9 28.5 30.1 29.7 30.1 29.8 29.3 29.1 27.7 27.7 23.7 24.0 20.4 20.9 19.7 20.1 19.9 20.2 20.7 20.9 22.3 22.3 26.3 26.0
27.2 27.2 28.4 28.4 28.9 28.8 28.6 28.5 27.7 27.7 24.9 24.9 22.2 22.3 21.4 21.4 21.1 21.2 21.4 21.5 22.3 22.3 25.1 25.1
9.7 9.6 9.2 9.1 8.4 8.4 7.8 7.8 7.3 7.3 6.8 6.8 7.2 7.2 7.7 7.7 8.3 8.3 8.9 8.9 9.4 9.4 9.9 9.9
25.4 25.4 25.4 25.4 25.3 25.3 25.2 25.2 25.0 25.0 24.7 24.7 24.6 24.6 24.7 24.7 24.7 24.7 24.9 24.9 25.0 25.0 25.3 25.3
45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0
Outlet (R ¼ 1)
10.3 10.0 9.7 9.5 8.8 8.6 8.0 7.9 7.3 7.3 6.4 6.5 6.6 6.8 7.2 7.4 7.9 8.0 8.7 8.7 9.4 9.4 10.3 10.2
Outlet (R ¼ 2)
27.6 28.0 27.5 27.9 26.9 27.2 26.1 26.2 25.0 25.0 23.1 22.8 22.3 21.9 22.5 22.2 23.1 22.8 23.9 23.8 25.0 25.0 26.8 27.1
transfer direction (denoted by moisture difference Du ¼ ua ue). Various flow patterns will behave the same mass transfer performance, when inlet air locates on the equivalent isenthalpic line of inlet fluid (ha,in ¼ he,in). Moreover, counter-flow pattern performs best mass transfer property, when the combined heat and mass transfer direction is the same as the mass transfer direction, for instance, inlet air a1, a2, a5, a6 or a7 contacts inlet water w. Take inlet air condition a3 in Fig. 4(a) or 5(a) as an example to clearly investigate the influencing characteristics of the heat transfer process and the mass transfer process within air–water or air-desiccant direct-contact devices. The vapor pressures (or humidity ratios) of inlet air and inlet fluid are the same, which means there is no driving force for mass transfer at the entrance. However, water or liquid desiccant is heated along the process by hot air, and hence the vapor pressure of fluid increases, which generates the mass transfer driving force. The air (point a3) will be humidified although the mass transfer driving force at the entrance is zero. In this condition, the mass transfer performance essentially depends on the promotion of heat transfer to mass transfer process. Parallel-flow pattern benefits to this promotion effect that is why parallel-flow configuration behaves better humidification performance than that of counter-flow pattern.
10.7 10.4 10.1 9.9 9.1 8.9 8.1 8.1 7.2 7.2 6.1 6.3 6.3 6.5 6.9 7.1 7.7 7.8 8.5 8.6 9.4 9.4 10.6 10.4
28.7 29.5 28.7 29.5 27.8 28.3 26.6 26.9 25.1 25.1 22.2 21.6 21.0 20.1 21.3 20.5 22.1 21.5 23.4 23.1 24.9 24.9 27.6 28.2
44.9 44.9 45.0 45.0 45.1 45.1 45.1 45.1 45.2 45.2 45.2 45.2 45.1 45.1 45.0 45.0 44.9 45.0 44.9 44.9 44.8 44.8 44.8 44.8
A
Dehumidification Parallel < Counter
A
Dehumidification Parallel < Counter
B
Regeneration
Parallel > Counter
B
Regeneration
Parallel > Counter
–
Regeneration
Parallel ¼ Counter
C
Regeneration
Parallel < Counter
C
Regeneration
Parallel < Counter
C
Regeneration
Parallel < Counter
D
Dehumidification Parallel > Counter
D
Dehumidification Parallel > Counter
–
Dehumidification Parallel ¼ Counter
A
Dehumidification Parallel < Counter
a ain (3)
win
(1) p
(2)
ain (ta,in, ωa,in, ha,in) win (tw,in, ωe,in, he,in) p
(ta,m, ωa,m, ha,in)
b p
4. Similarity of reachable handling region and zonal method
(1) ain
(2)
4.1. Reachable handling region Heat transfer driving force Dt between air and water/desiccant, and mass transfer driving force Dp (or Du) are coupled and will influence each other. As indicated by a former study [12] on airdesiccant direct-contact devices, the combined heat and mass transfer driving forces can be decoupled as relative humidity difference D4 and enthalpy difference Dh. Conditions of outlet air in packed-bed air-desiccant direct-contact devices are within a triangle area in the psychrometric chart as shown in Fig. 6, which is composed of three boundary lines: (1) isenthalpic line of inlet air, (2) isoconcentration line (or iso-relative humidity line) of inlet fluid, and (3) connecting line of the statuses of inlet air and inlet fluid. Same
Zone Dehumidification/ Mass transfer Regeneration performance
sin
(3)
ain (ta,in, ωa,in, ha,in) sin (ts,in, ωe,in, he,in) p (ta,m, ωa,m, ha,in)
Fig. 6. Reachable handling regions of outlet air: (a) air–water direct-contact system; and (b) air–liquid desiccant direct-contact system.
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
25 a in
20 15
p
a out
10 w out w in
5 0
0
5
10
c
b
25 20
a in
15
a out
10
Humidity ratio / (g/kg)
p w out
w in
5 0
15
Temperature / ºC
Temperature / ºC
a
Temperature / ºC
2506
0
5
10
50
40 a in
30
w out p
20
10
15
w in a out
0
10
20
30
40
Humidity ratio / (g/kg)
Humidity ratio / (g/kg)
Fig. 7. The changes of air and water conditions during the experiments: (a) dehumidification by Ge et al. [7]; (b) humidification by Ge et al. [7]; and (c) humidification by Lucas et al. [6].
p
40 s out
a out
s in
30
a in
c
80
s out
60
a out p
40
90 a in
s in
Temperature / ºC
b
50
Temperature / ºC
Temperature / ºC
a
70
50
a in
20
0
5
10 15 20 25
Humidity ratio / (g/kg)
20
0
p
a out s in s out
20
40
60
30
80
Humidity ratio / (g/kg)
0
10
20
30
40
Humidity ratio / (g/kg)
Fig. 8. The changes of air and desiccant conditions during the experiments: (a) dehumidification by Fumo et al. [5]; (b) regeneration by Fumo et al. [5]; and (c) regeneration by Lof et al. [10].
reachable handling region as air-desiccant direct-contact devices can be obtained for air–water direct-contact devices, since the heat and mass transfer models are the same for the two systems. The boundary lines (1) and (2) are the same as the uncoupled heat and mass transfer driving forces. The physical meaning of boundary line (3) is that the air outlet conditions will locate on this line when water/desiccant flow rate is essentially large comparing with the air flow rate. Point p shown in Fig. 6 is the intersection point of isenthalpic line of inlet air and iso-relative humidity line of inlet fluid. The inlet and outlet conditions of air–water and air-desiccant direct-contact devices during the experiments are shown in the triangle reachable handling region, as indicated in Figs. 7 and 8, consulting Table 4 for specific data. The reachable maximal/ minimal parameters of outlet air can be obviously obtained from the figures. For example, in the humidification process of air–water direct-contact devices, the maximal humidity ratio of outlet air is the same as equivalent humidity ratio of inlet water (win) in the experiments by Lucas et al. [6] as shown in Fig. 7(c). The maximal humidity ratio of outlet air is the humidity ratio of point p in the experiments by Ge et al. [7] as shown in Fig. 7(b).
4.2. Handling zonal method Conditions of inlet air and inlet water/desiccant do affect heat and mass transfer characteristics from the results of Figs. 4–8 and Tables 2–4. Only according to the relative statuses of inlet air to inlet water/desiccant, four zones A–D [12] are divided in the psychrometric chart for air-desiccant direct-contact system as shown in Fig. 9(a), including two dehumidification zones A and D and two regeneration zones (or humidification zones) B and C. In zones A and C, mass transfer direction (driving force Du) is the same as the combined heat and mass transfer direction (driving force Dh). Counter-flow pattern shows the highest mass transfer performance, and parallel-flow is the poorest in the same operating condition. In zones B and D, mass transfer direction is opposite to that of combined heat and mass transfer. Mass transfer performance of counter-flow pattern is the poorest while parallel-flow is the best in the same condition, which is rather different with the result in zones A and C.
Table 4 Inlet and outlet conditions of air and fluid through the packed-bed devices as shown in Figs. 7 and 8. Figure
Fig. Fig. Fig. Fig. Fig. Fig.
7(a) 7(b) 7(c) 8(a) 8(b) 8(c)
Source
Ge et al. [7] Ge et al. [7] Lucas et al. [6] Fumo et al. [5] Fumo et al. [5] Lof et al. [10]
Fluid
Water Water Water Desiccant Desiccant Desiccant
Inlet conditions
Outlet conditions
ta,in/ C
ua,in/(g/kg) tw,in or ts,in/ C xin/% ta,out/ C ua,out/(g/kg) tw,out or ts,out/ C xout/%
19.4 20.2 29.1 30.1 30.3 81.0
8.3 8.3 13.7 18.0 18.2 11.6
6.2 6.9 35.2 30.1 65.4 36.0
d d d 34.6 34.4 27.2
12.2 15.1 30.2 31.3 57.6 40.0
7.6 9.3 21.0 10.4 50.2 20.3
8.0 10.3 29.0 32.5 57.0 37.0
– – – 34.5 34.9 27.6
Zone
Dehumidification/Humidification
A B C A B C
Dehumidification Humidification Humidification Dehumidification Humidification Humidification
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
40
Zone B
(2)
Temperature / ºC
(1)
Regeneration
30
Zone A
Zone C
Dehumidification
s
Regeneration 20
Zone D Dehumidification 10 0
5
10
15
20
Humidity ratio / (g/kg)
b
40
Temperature / ºC
(2)
(1)
Zone B Humidification
5. Utilization of the reachable region and handling zones
Zone A Dehumidification
30
Zone C Humidification
20
w
10 0
10
The dividing line of zones B and C (and zones A and D) is the equivalent isenthalpic line of inlet desiccant. However, the dividing line of zones A and B (and zones C and D) can hardly be determined only with the statuses of inlet air and inlet desiccant, since the influencing factors include flow pattern, flow rate, heat and mass transfer coefficients, etc. Fortunately, the dividing line of zones A and B is between isoconcentration line (or iso-relative humidity line) and equivalent iso-humidity ratio line of inlet desiccant. The common air-handling processes using liquid desiccant are shown in Fig. 8, including dehumidification processes lay in zone A, hot air driven regeneration processes in zone B and hot desiccant driven regeneration processes in zone C. And there is barely dehumidification process in zone D in available literature. Similar to the divided zones for air-desiccant direct-contact system, zones for air–water system can then be derived, as shown in Fig. 9(b). Three zones (A–C) are derived for air–water system, with the same zonal characteristics as the zones in air-desiccant system. The common air–water direct-contact processes are shown in Fig. 7.
20
30
40
Humidity ratio / (g/kg) Fig. 9. Handling zones of heat and mass transfer processes in: (a) air–liquid desiccant; and (b) air–water direct-contact devices.
a
b
Fluid inlet (w1 or s1)
Air inlet (a2)
(w2 or s2) Heater
Fluid inlet (w1 or s1)
Air inlet
Air outlet
(a1)
The reachable region and handling zones can be utilized in the optimization of air-handling processes. Heating or cooling capacity can be given to air side or liquid side. Which mode can yield better mass transfer performance and which kind of flow pattern can realize the best mass transfer performance? Two cases with humidification purpose are taken as examples shown in Fig. 10 to answer the above questions. Heater is adopted to heat inlet air in mode 1 and to heat inlet fluid in mode 2. Fig. 11 gives the handling processes of the two modes in air– water direct-contact systems. The heat and mass transfer process of mode 1 is in zone B and mode 2 in zone C. Conditions of inlet air (point a1) are 1 kg/s, 20 C and 8 g/kg, and conditions of inlet water (point w1) are 0.5 kg/s and 20 C. The power of the heater in Fig. 10 is
Air outlet (a2)
(a1)
(a3) Packing
Heater
Fluid outlet (w3 or s3)
Packing
Fluid outlet (w2 or s2)
Fig. 10. Humidification of the processed air: (a) mode 1: heat inlet air; and (b) mode 2: heat inlet water/desiccant.
a
50 a2
Temperature / ºC
40
a 3 (Parallel)
30 a 3 (Counter) a1
20
w1
p w 2 (Counter) w 2 (Parallel)
10
b
40
Temperature / ºC
a
2507
30
a 2 (Counter)
w2
a 2 (Parallel) w 3 (Parallel)
a1
20
w1
w 3 (Counter)
p 10
a out 0
0
5
10
15
20
Humidity ratio / (g/kg)
25
0
0
5
10
15
20
25
30
35
Humidity ratio / (g/kg)
Fig. 11. Humidification performance of air–water system: (a) mode 1: heat inlet air; and (b) mode 2: heat inlet water.
2508
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
Table 5 Inlet and outlet conditions of air and fluid through the packed-bed devices as shown in Figs. 11 and 12. Figure
Fluid
Mode Flow pattern Inlet conditions
Outlet conditions
Variance of air humidity ratio /(g/kg)
ta,in/ C ua,in/(g/kg) tw,in or ts,in/ C xin/% ta,out/ C ua,out/(g/kg) tw,out or ts,out/ C xout/% Water Water Water Water Desiccant Desiccant Desiccant Desiccant
1 1 2 2 1 1 2 2
Parallel Counter Parallel Counter Parallel Counter Parallel Counter
Temperature / ºC
a
47.7 47.7 20.0 20.0 47.7 47.7 20.0 20.0
8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0
20.0 20.0 33.4 33.4 20.0 20.0 43.0 43.0
– – – – 40.0 40.0 40.0 40.0
b
50 a2
a 3 (Counter) p
30
s 2 (Counter) a1
s1
s 2 (Parallel)
10 0
13.0 12.8 15.6 17.9 10.4 10.0 13.0 14.4
21.9 22.2 22.6 20.2 26.8 28.2 27.5 23.6
– – – – 40.2 40.1 40.4 40.5
5.0 4.8 7.6 9.9 2.4 2.0 5.0 6.4
50
a 3 (Parallel)
40
20
31.3 31.0 23.1 24.2 33.8 33.1 26.4 27.5
Temperature / ºC
Fig. 11(a) Fig. 11(a) Fig. 11(b) Fig. 11(b) Fig. 12(a) Fig. 12(a) Fig. 12(b) Fig. 12(b)
40
s2 s 3 (Parallel)
30
s 3 (Counter) a1
20
p
a 2 (Counter)
s1
a 2 (Parallel)
10
0
5
10
15
20
25
Humidity ratio / (g/kg)
0
0
5
10
15
20
25
30
35
Humidity ratio / (g/kg)
Fig. 12. Humidification or regeneration performance of air–liquid desiccant system: (a) mode 1: heat inlet air; and (b) mode 2: heat inlet liquid desiccant.
28 kW, and NTUm value of the packed-bed humidifier is 1. The humidification performances of the two modes are shown in Table 5. The reachable maximal humidity ratio of outlet air is 17.8 g/kg (point p) for mode 1 and 33.1 g/kg (point w2) for mode 2. The actual humidity ratio of outlet air in parallel-flow packed-bed humidifier is 13.0 g/kg for mode 1 and 15.6 g/kg for mode 2, and the value changes to 12.8 g/kg and 17.9 g/kg in counter-flow humidifier. In mode 1, parallel-flow packed-bed device shows better mass transfer performance than that of counter-flow device. However, in mode 2, counter-flow packed-bed device shows better mass transfer performance than that of parallel-flow configuration, with about 30% higher humidification capacity. Mode 2 with higher temperature of inlet water into packed-bed device (in zone C) has much higher humidification performance, and the humidification capacity is about twice as much as that of mode 1 with higher temperature of inlet air (in zone B). In air-desiccant direct-contact system, similar conclusion can be drawn as air–water system. The air-handling processes in mode 1 (heat the air, in zone B) and mode 2 (heat the desiccant, in zone C) are shown in Fig. 12, with the detailed conditions indicated in Table 5. Heating the desiccant (mode 2) has much higher humidification or regeneration capacity, 2–3 times as much as that of mode 1. In mode 2, humidification capacity of counter-flow packed-bed regenerator is 28% higher than that of parallel-flow configuration. To sum up, in order to gain better humidification performance, fluid should be heated (in zone C) rather than air (in zone B). Counter-flow packed-bed configuration is recommended in the heating fluid mode (in zone C), comparing with other flow patterns. In the dehumidification process of air-desiccant direct-contact system, similar conclusion can be drawn: liquid desiccant should be cooled (in zone A) rather than air (in zone D) to obtain better mass transfer performance, and counter-flow configuration is recommended in zone A.
6. Conclusions Packed-bed heat and mass transfer devices of air–water and air– liquid desiccant direct-contact systems have been widely utilized in air-conditioning systems. Coupled heat and mass transfer characteristics of air–water and air-desiccant direct-contact devices, especially the similarities between the two kinds of devices, are investigated. And the main conclusions are: 1) The statuses of water and liquid desiccant through packed-bed devices can be regarded as locating on a constant iso-relative humidity line. Similar heat and mass transfer model and similar analytical solutions can be obtained for air–water and airdesiccant system. 2) Reachable handling region of air–fluid (water or liquid desiccant) direct-contact device is among the three boundary lines, (1) isenthalpic line of inlet air, (2) iso-relative humidity line of inlet fluid, and (3) the connecting line of inlet statuses of air and fluid. 3) Apart from being influenced by flow pattern, heat and mass transfer coefficients and flow rates of air and fluid, the heat and mass transfer characteristics will be affected by the conditions of inlet air and inlet fluid to some extent. 4) Only according to the relative statuses of inlet air to inlet fluid, the psychrometric chart are divided into four handling zones A–D for air-desiccant direct-contact processes and three handling zones A–C for air–water direct-contact processes. Zones A and D are dehumidification zones, and zones B and C are humidification zones (or regeneration zones). In zone A or C, counter-flow pattern has best mass transfer performance in the same condition. While in zone B or D, parallel-flow pattern performs best. 5) In order to obtain better humidification performance, fluid should be heated (in zone C) rather than air (in zone B). And
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
fluid should be cooled (in zone A) rather than air (in zone D) to obtain better dehumidification performance. Acknowledgements The research described in this paper was supported by Chinese 11th Five-year Plan project (No.2006BAJ01A08) and National Natural Science Foundation of China (No.50778094). Nomenclature
A B cp cp,m H h Le _ m m* NTUm p R r t
heat and mass transfer area (m2) atmospheric pressure (kPa) specific heat (kJ/kg C) specific heat of humid air (kJ/kg C) height of packed-bed heat and mass transfer device (m) enthalpy (kJ/kg) Lewis number (dimensionless) mass flow rate (kg/s) heat capacity ratio of air to fluid (water or liquid desiccant) (dimensionless) number of transfer unit (dimensionless) vapor pressure (kPa) mass flow rate ratio of air to fluid (dimensionless) water vaporization latent heat (kJ/kg) temperature ( C)
Greek symbols a heat transfer coefficient (kW/m2 C) am mass transfer coefficient (kg/m2 s) 3h enthalpy efficiency (%) 3m moisture efficiency (%) 4 relative humidity (%) x concentration (mass ratio of desiccant to solution) of liquid desiccant (%) u humidity ratio (kg/kg) Subscripts a air a,m intersection point of air inlet isenthalpic line and fluid inlet iso-relative humidity line shown in the psychrometric chart e air in equilibrium with fluid (ta ¼ tf, and pa ¼ pf)
f in s out w
2509
fluid (water or liquid desiccant) inlet liquid desiccant outlet water
References [1] Zhao RY, Fan CY, Xue DH, Qian YM. Air conditioning. 3rd ed. Beijing: Architectural & Industrial Press; 2003. [2] Lu YQ. Design handbook of heating, ventilation and air-conditioning. 2nd ed. Beijing: Architectural & Industrial Press; 2008. [3] Zhang LZ. Dehumidification technology. Beijing: Chemical & Industrial Press; 2005. [4] Patnaik S, Lenz TG, Lof GOG. Performance studies for an experimental solar open-cycle liquid desiccant air dehumidification system. Solar Energy 1990;44(3):123–35. [5] Fumo N, Goswami DY. Study of an aqueous lithium chloride desiccant system: air dehumidification and desiccant regeneration. Solar Energy 2002;72(4):351–61. [6] Lucas M, Martinez PJ, Viedma A. Comparative experimental drift study between a dry and adiabatic fluid cooler and a cooling tower. International Journal of Refrigeration 2008;31(7):1169–75. [7] Ge KS, Wang Q, Ren DF. Determination of air–water mass transfer coefficient and drop of pressure in wet curtain. Journal of China Agriculture University 1999;4(2):82–3. [8] Muangnoi T, Asvapoositkul W, Wongwises S. Effects of inlet relative humidity and inlet temperature on the performance of counterflow wet cooling tower based on exergy analysis. Energy Conversion and Management 2008;49(10):2795–800. [9] Song YZ. Model calculation of air water counter flow heat and mass transfer and comparison with experimental data. Journal of Chemical and Industrial Engineering 2005;56(6):999–1003. [10] Lof GOG, Lenz TS, Rao S. Coefficients of heat and mass transfer in a packed bed suitable for solar regeneration of aqueous lithium chloride solutions. Journal of Solar Energy Engineering 1984;106(4):387–92. [11] Dai YJ, Sumathy K. Theoretical study on a cross-flow direct evaporative cooler using honeycomb paper as packing material. Applied Thermal Engineering 2002;22(13):1417–30. [12] Liu XH, Jiang Y. Coupled heat and mass transfer characteristic in packed bed dehumidifier/regenerator using liquid desiccant. Energy Conversion and Management 2008;49(6):1357–66. [13] Yin YG, Zhang XS, Chen ZQ. Experimental study on dehumidifier and regenerator of liquid desiccant cooling air conditioning system. Building and Environment 2007;42(7):2505–11. [14] Stevens DI, Braun JE, Klein SA. An effectiveness model of liquid-desiccant system heat/mass exchangers. Solar Energy 1989;42(6):449–55. [15] Ren CQ. Corrections to the simple effectiveness-NTU method for counterflow cooling towers and packed bed liquid desiccant–air contact systems. International Journal of Heat and Mass Transfer 2008;51(1–2):237–45. [16] Liu XH, Jiang Y, Xia JJ, Chang XM. Analytical solutions of coupled heat and mass transfer processes in liquid desiccant air dehumidifier/regenerator. Energy Conversion and Management 2007;48(7):2221–32.
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Similarity of coupled heat and mass transfer between air–water and air–liquid desiccant direct-contact systems Xiao-Hua Liu a, *, Zhen Li b, Yi Jiang a a b
Department of Building Science, School of Architecture, Tsinghua University, Beijing 100084, PR China Department of Engineering Mechanics, Tsinghua University, Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 December 2008 Received in revised form 12 April 2009 Accepted 30 April 2009
Packed-bed heat and mass transfer devices are widely used in air-conditioning systems, such as cooling tower, evaporative cooler of air–water direct-contact devices, dehumidifier and regenerator of air–liquid desiccant direct-contact devices. Similarities of heat and mass transfer characteristics between air–water and air–liquid desiccant devices are considered and investigated in this paper. Same reachable handling region of outlet air can be obtained for both air–water and air–liquid desiccant devices, which is among three boundary lines, isenthalpic line of inlet air, iso-relative humidity line of inlet fluid (water or desiccant), and the connecting line of inlet statuses of air and fluid. Inlet conditions of air and fluid affect heat and mass transfer characteristics to some extent, so that a zonal method is proposed only according to the relative statuses of inlet air to inlet fluid. Four zones, dehumidification zones A, D and regeneration zones B, C, are divided for air-desiccant direct-contact devices. The first three zones A, B and C are divided for air–water direct-contact devices, with the same zonal properties as those of air-desiccant devices. In order to obtain better humidification performance, fluid should be heated (in zone C) rather than air (in zone B). And fluid should be cooled (in zone A) rather than air (in zone D) to obtain better dehumidification performance. Counter-flow pattern should be applied for best mass transfer performance in the same conditions within the recommended zone A or C, while parallel-flow pattern is the best in zone B or D. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Heat and mass transfer Liquid desiccant/water Reachable handling region Zonal method Flow pattern Air-conditioning
1. Introduction The task of air-conditioning systems is to create comfortable indoor environment, including required indoor temperature and humidity ratio (or relative humidity). Usually outdoor air will be cooled and dehumidified in summer, heated and humidified in winter before being supplied into occupied spaces. The air-conditioning systems are composed of various heat and mass transfer devices. Water and liquid desiccant are two liquids commonly used in air-handling processes, which contact humid air directly. Heat and mass transfer occur between air and water (or liquid desiccant). Air–water direct-contact devices [1,2] include cooling tower, evaporative cooler, humidifier, etc. Liquid desiccant air-conditioning systems have developed quickly in recent years [3–5], for the reason that they can be driven by low grade heat, such as solar energy and waste heat. Commonly used liquid desiccants are the mixture of salt and water, for instance, lithium bromide (LiBr) aqueous solution, lithium chloride (LiCl) aqueous solution and calcium chloride (CaCl2)
* Corresponding author. Tel.: þ86 10 6277 3772; fax: þ86 10 6277 0544. E-mail address: [email protected] (X.-H. Liu). 0360-1323/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2009.04.013
aqueous solution. Dehumidifier and regenerator are the most important components in liquid desiccant air-conditioning systems. Air will be dehumidified by contacting concentrated liquid desiccant in the dehumidifier. And in the regenerator air will be humidified and desiccant will be concentrated. Heat and mass transfer characteristics between air and water (or liquid desiccant) essentially determine the performance of the packed-bed air-handling devices. Vapor pressure difference between air and fluid (water or liquid desiccant) is the driving force of mass transfer, and it should be noticed that the fluid’s vapor pressure will be greatly influenced by its temperature. Meanwhile temperature difference between air and fluid is the driving force of heat transfer, which is affected by the released/absorbed vaporization latent heat along with the mass transfer process. Therefore, heat transfer process and mass transfer process within packed-bed devices influence each other and should not be considered separately. The performances of air–water [1,2,6–9] or air-desiccant [3–5,10–13] direct-contact devices have been widely investigated separately through numerical analysis and experimental tests. Performance influencing factors have been obtained, such as flow rates of air and water-desiccant, packing volume and sizes, and optimization methods have been proposed. Questions appear in the
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
2. Properties of liquid desiccant and water Air-handling processes in air–water and air–liquid desiccant direct-contact devices are usually conducted in standard atmospheric pressure, which is regarded as a constant in this paper. The following two equations will be satisfied when humid air is in equilibrium with water or liquid desiccant, where subscripts a, w and s stand for humid air, water and liquid desiccant respectively.
Air—water : ta ¼ tw ;
Air-desiccant : ta ¼ ts
Air—water : pa ¼ pw ;
x. An increase in desiccant temperature and a decrease in desiccant concentration at the same time result in an increase in desiccant vapor pressure. As indicated in Fig. 2, status of liquid desiccant, which is from the crystal line of desiccant to the saturated line of air, can reach a larger area in the psychrometric chart compared with water. The isoconcentration line of liquid desiccant almost coincides with the iso-relative humidity line of air, which means that liquid desiccant at constant concentration and water both lay on an isorelative humidity line of air. The status of desiccant is near the saturated line of air, when desiccant concentration is low enough. The variances of desiccant concentration within packed-bed dehumidifier/regenerator during the experiments [4,5,10,13] are so small, as indicated in Table 1, that they are usually omitted and the
a Temperature / ºC
further study of such kind of air-handling devices. What are the similarities between air–water and air-desiccant direct-contact devices? Stevens et al. gave [14] analytical solutions of outlet conditions of air and liquid desiccant in packed-bed dehumidifier/ regenerator, using the NTU method which was widely used in the analysis of air–water system. Ren [15] gave the analytical solutions of both air–water and air-desiccant direct-contact systems, and got similar solutions for the two systems. Previous studies provide a useful way to investigate the similarities of these two kinds of direct-contact devices. This paper will focus on the comparison of air–water direct-contact system and air-desiccant system, in order to find out the similarities between the two systems, upon which heat and mass transfer processes will be analyzed and optimized. As indicated in a former study [12], in air-desiccant devices, heat and mass transfer characteristics can be determined to some extent only according to the relative statuses of inlet air to inlet desiccant. Maybe similar conclusion can be drawn for air–water system, which will benefit the design of air-handling systems with better mass transfer performance.
55
Crystal 50 line =60%
Air-desiccant : Pa ¼ Ps
(2)
=40%
40 35
20%
30%
45
40% 50% 60%
40
80% ϕ =100%
35 30
60% 80% =100% Isoconcentration line Iso-
0
0.01
0.02
0.03
b
Crystal line
50
=50%
40%
=20%
40 35
20%
40%
60%
30
80%
=100% Isoconcentration line
25
Iso-
0.01
0.02
0.03
0.04
0
0.01
0.02
0.03
0.04
0.05
Humidity ratio / (kg/kg) 55 Crystal line
=50% 40%
45
30% =20%
40 35
20% 40%
30
60% 80% =100% Isoconcentration line Iso-
0.05
line
20
Equivalent water status 0
0.05
30%
45
20 20
0.04
55
25 25
line
Humidity ratio / (kg/kg)
Temperature / ºC
Temperature / ºC
20%
40%
30
50 ϕ =10%
45%
20
c
55
50%
45
(1)
Eq. (2) can be rewritten as ua ¼ ue , since humidity ratio is coincident with vapor pressure ðu ¼ 0:622 p=ðB pÞÞ, where ue is the equivalent humidity ratio of water/desiccant with humid air. Therefore, status of water or liquid desiccant can be expressed in the air psychrometric chart with the equilibrium conditions. As indicated in Fig. 1, status of water coincides with saturated line of humid air (4a ¼ 100%). Surface pressure of liquid desiccant is about the same to vapor pressure, due to the large boiling temperature difference between salt and water. For instance, boiling point of salt, such as LiBr, LiCl and CaCl2, is as high as 1200 C, while boiling point of water is only 100 C at standard atmospheric pressure. Surface vapor pressure of liquid desiccant ps depends on its temperature ts and concentration
50
55%
25
Temperature / ºC
2502
0
0.01
0.02
0.03
line 0.04
0.05
Humidity ratio / (kg/kg)
Humidity ratio / (kg/kg) Fig. 1. Status of water shown in the air psychrometric chart.
Fig. 2. Status of commonly used liquid desiccants shown in the air psychrometric chart: (a) LiBr aqueous solution; (b) LiCl aqueous solution; and (c) CaCl2 aqueous solution.
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
2503
Table 1 Parameters of air and liquid desiccant in the dehumidification/regeneration experiments.
Patnaik et al. [4] Fumo et al. [5] Yin et al. [13] Patnaik et al. [4] Fumo et al. [5] Lof et al. [10] Yin et al. [13]
Dehumidification/regeneration
Liquid desiccant
R
ta,in/ C
ua,in/(g/kg)
ts,in/ C
xin/%
NTUm
Dx /%
Dehumidification Dehumidification Dehumidification Regeneration Regeneration Regeneration Regeneration
LiBr LiCl LiCl LiBr LiCl LiCl LiCl
1.4–3.1 0.1–0.3 0.7 0.9–1.2 0.1–0.2 1.7–2.2 1.4
28–39 30–40 28–35 48–77 29–40 81–109 30–36
12–23 14–22 11–19 5–11 14–21 11–18 14–20
23–32 25–35 25–30 40–60 60–70 34–38 55–80
44–59 33–35 38–40 57–61 32–35 8–27 20–30
0.2–1.0 1.0–1.3 – 0.2–0.8 2.4–4.2 0.6–1.0 0.1–0.4
<0.7 <0.2 <0.2 <0.4 <0.8 <0.4 <1.1
a
Air outlet
b
Fluid inlet
Packing
Air inlet
Air inlet
Fluid outlet Fig. 3. Packed-bed heat and mass transfer devices for air–water or air–liquid desiccant direct-contact systems: (a) device; and (b) packing.
Packed-bed is a commonly used type of heat and mass transfer device for air–water and air–liquid desiccant direct-contact systems, as shown in Fig. 3. Large flow rate of water or desiccant is usually chosen for two purposes, 1) fully wet the packing, and 2) enlarge heat capacity of liquid flow. Therefore, flow rate variance of water or liquid desiccant within packed-bed devices can be usually omitted. Heat and mass transfer models for air–water or airdesiccant have been derived by many researchers. And the model for parallel-flow pattern [9,14,15] is listed here. The energy conservation equation between air and fluid (water or liquid desiccant) is shown below, where subscript f stands for water (w) in air–water system or liquid desiccant (s) in air-desiccant system.
_ a dha þ m _ f dhf ¼ 0 m
(3)
The overall heat transfer and moisture transfer between air and fluid are given in Eqs. (4) and (5) respectively, with the definitions of Le and NTUm shown in Eq. (6).
40
a
Temperature / ºC
3. Heat and mass transfer model and main performance influencing factors
a
a
a
3
a
4
2
a
5
1
30
a
6
20
w a
R =2 7
R =1 R =0.1
10
0
10
20
30
40
Humidity ratio / (g/kg)
b
40 a
Temperature / ºC
desiccant concentration is assumed as a constant within the devices [14–16]. Therefore, liquid desiccant will locate on an isorelative humidity line in air-desiccant direct-contact devices, and water will always locate in 100% iso-relative humidity line in air– water direct-contact devices.
a
a
3
a
4
2
a
5
1
30 a
6
20
w a
R =2 7
R =1 R =0.1
NTU dha NTUm m ¼ $Le$cp;m tf ta þ $r$ðue ua Þ dx H H NTUm Le 1 ¼ ðhe ha Þ þ r 1 ðue ua Þ H Le
10 0
(4)
10
20
30
40
Humidity ratio / (g/kg) Fig. 4. Effect of R, flow pattern and inlet status on the conditions of outlet air in air–water direct-contact device: (a) parallel-flow pattern; and (b) counter-flow pattern.
2504
a
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
a a
Temperature / ºC
water and liquid desiccant (ue and he in Eqs. (4) and (5)), as indicated in Figs. 1 and 2. The Le number is regarded as 1 for air–water and also air–liquid desiccant system [1,16]. Similar solutions can then be obtained. Analytical solutions [16] of enthalpy efficiency 3h and moisture efficiency 3m for both air–water and air-desiccant directcontact systems in parallel-flow configuration can be obtained:
40 a
4
3
a
R =2 2
R =1
5
a
30 a
a
s
6
1
R =0.1
*
3h ¼ 20 a
a 7
a
8
a
a
9
5
10
15
20
40
Temperature / ºC
a
a
4
a
2
R =1
5
a
30 a
a
s
6
20 a
a 7
a
8
a
a 9
R =0.1
1
12
11
=1 00 %
10
10 0
5
10
15
20
Humidity ratio / (g/kg) Fig. 5. Effect of R, flow pattern and inlet status on the conditions of outlet air in air–liquid desiccant direct-contact device using LiBr as liquid desiccant: (a) parallel-flow pattern; and (b) counter-flow pattern.
dua NTUm ¼ ðue ua Þ dx H Le ¼
(5)
a am A ; NTUm ¼ _a am cp;m m
*
ua;in ua;out ua;in ua;m m* m* þ 1 eNTUm þ eNTUm ðm þ1Þ ¼ 3h þ $ ua;in ue;in ua;in ue;in m* þ 1
(8)
_a cp;e m dhe m* ¼ R$ where cp;e ¼ ; R ¼ _f m cp;f dtf
R =2 3
(7)
Heat capacity ratio of air to fluid m* shown in Eqs. (7) and (8) is defined as:
Humidity ratio / (g/kg)
a
¼ f1 m* ; NTUm
3m ¼
10
¼ f2 m* ; NTUm
0
þ1Þ
11
10
b
ha;out ha;in 1 eNTUm ðm ¼ he;in ha;in m* þ 1
12
(6)
The heat and mass transfer model is the same for air–water and air–liquid desiccant direct-contact devices. The differences between these two kinds of devices are the equilibrium characteristics of
(9)
As indicated in Eqs. (7) and (8), two main performance influencing factors are heat capacity ratio of air to liquid m* and mass transfer unit NTUm. Apart from being influenced by the physical properties of air and fluid, heat capacity ratio m* also depends on mass flow rate ratio of air to fluid R. Figs. 4 and 5 show the effects of R, flow pattern and conditions of inlet air and fluid on the heat and mass transfer performances, when NTUm is taken as 1. The detailed information concerning the conditions of inlet and outlet air and fluid are listed in Tables 2 and 3 for air–water and air-desiccant devices respectively. The points of inlet air a1 and a7 are on isorelative humidity line of inlet fluid (4a,in ¼ 4e,in), the humidity ratios of points a3 and a9 are equal to the equivalent humidity ratio of inlet fluid (ua,in ¼ ue,in), the enthalpies of points a5 and a11 are the same as the equivalent enthalpy of inlet fluid (ha,in ¼ he,in), and the temperatures of points a6 and a12 are the same as that of inlet fluid (ta,in ¼ tw,in or ta,in ¼ ts,in). As indicated by Figs. 4 and 5 and Tables 2 and 3, the relative positions of inlet air to inlet fluid do affect the heat and mass transfer characteristics to some extent. Counter-flow configuration is not always the best flow pattern for various inlet air conditions. Counterflow configuration exhibits worse mass transfer performance than parallel-flow pattern or cross-flow pattern in the same operating conditions, when inlet air a3 contacts water w, inlet air a3, a4, a9 or a10 contacts liquid desiccant s. The similarities of these operating conditions are that the combined heat and mass transfer direction (denoted by enthalpy difference Dh ¼ ha he) is opposite to the mass
Table 2 Inlet and outlet conditions of air and water through the packed-bed devices as shown in Fig. 4 (water inlet temperature tw,in ¼ 25 C). Inlet Air inlet
ta,in/ C ua,in/ (g/kg)
a1
30.5
27.9
a2
34.1
25.1
a3
36.0
20.1
a4
34.8
16.1
a5
30.8
12.5
a6
25.0
11.1
a7
18.2
13.0
Flow pattern
Outlet (R ¼ 0.1) ta,out/ C
ua,out/
27.2 27.2 28.6 28.5 29.2 29.2 28.7 28.7 27.1 27.1 24.9 24.9 22.3 22.4
23.2 23.1 22.1 22.1 20.2 20.1 18.6 18.6 17.1 17.2 16.5 16.6 17.2 17.3
Outlet (R ¼ 1) ta,out/ C ua,out/ (g/kg)
tw,out/ C
ta,out/ C
ua,out/ tw,out
25.4 25.4 25.3 25.3 25.2 25.2 25.0 25.0 24.8 24.8 24.7 24.7 24.6 24.6
28.2 27.9 29.5 29.1 29.7 29.5 28.7 28.7 26.6 26.8 23.9 24.3 21.3 21.7
27.6 28.0 27.3 27.6 26.2 26.4 25.0 25.0 23.6 23.4 22.6 22.3 22.4 22.0
28.9 28.5 30.0 29.7 30.0 29.8 28.7 28.7 26.2 26.4 23.3 23.7 20.6 21.0
25.5 25.0 24.1 23.7 21.2 20.9 18.6 18.6 16.0 16.3 14.7 15.2 15.3 15.8
(g/kg) Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter
Outlet (R ¼ 2)
tw,out/ C
24.6 24.1 23.3 22.9 20.8 20.5 18.6 18.6 16.5 16.7 15.4 15.9 16.0 16.5
Zone Dehumidification/ Mass transfer Humidification performance
(g/kg) 28.7 29.5 28.3 29.0 26.7 27.1 25.0 25.0 22.9 22.5 21.4 20.7 21.1 20.2
A
Dehumidification
Parallel < Counter
A
Dehumidification
Parallel < Counter
B
Humidification
Parallel > Counter
–
Humidification
Parallel ¼ Counter
C
Humidification
Parallel < Counter
C
Humidification
Parallel < Counter
C
Humidification
Parallel < Counter
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
2505
Table 3 Inlet and outlet conditions of air and liquid desiccant through the packed-bed devices as shown in Fig. 5 (Desiccant inlet ts,in ¼ 25.0 C, and xin ¼ 45.0%). Inlet Air ta,in/ C ua,in / (g/kg) inlet a1
30.5
11.7
a2
33.6
10.3
a3
35.0
8.3
a4
34.3
6.8
a5
32.1
5.5
a6
25.0
4.3
a7
18.0
5.5
a8
15.7
6.8
a9
15.0
8.3
a10
15.7
9.8
a11
18.0
11.2
a12
25.0
12.3
Flow Outlet (R ¼ 0.1) pattern ta,out/ C ua,out/ ts,out/ C xout/% (g/kg)
ta,out/ C ua,out/ ts,out/ C (g/kg)
xout/%
ta,out/ C ua,out/ ts,out/ C xout/% (g/kg)
Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter Parallel Counter
28.2 27.9 29.4 29.0 29.6 29.3 29.0 28.8 27.7 27.7 24.2 24.4 21.2 21.6 20.4 20.8 20.4 20.7 21.0 21.2 22.3 22.3 25.8 25.6
44.9 44.9 45.0 45.0 45.0 45.0 45.1 45.1 45.1 45.1 45.1 45.1 45.1 45.1 45.0 45.0 45.0 45.0 45.0 45.0 44.9 44.9 44.9 44.9
28.9 28.5 30.1 29.7 30.1 29.8 29.3 29.1 27.7 27.7 23.7 24.0 20.4 20.9 19.7 20.1 19.9 20.2 20.7 20.9 22.3 22.3 26.3 26.0
27.2 27.2 28.4 28.4 28.9 28.8 28.6 28.5 27.7 27.7 24.9 24.9 22.2 22.3 21.4 21.4 21.1 21.2 21.4 21.5 22.3 22.3 25.1 25.1
9.7 9.6 9.2 9.1 8.4 8.4 7.8 7.8 7.3 7.3 6.8 6.8 7.2 7.2 7.7 7.7 8.3 8.3 8.9 8.9 9.4 9.4 9.9 9.9
25.4 25.4 25.4 25.4 25.3 25.3 25.2 25.2 25.0 25.0 24.7 24.7 24.6 24.6 24.7 24.7 24.7 24.7 24.9 24.9 25.0 25.0 25.3 25.3
45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0
Outlet (R ¼ 1)
10.3 10.0 9.7 9.5 8.8 8.6 8.0 7.9 7.3 7.3 6.4 6.5 6.6 6.8 7.2 7.4 7.9 8.0 8.7 8.7 9.4 9.4 10.3 10.2
Outlet (R ¼ 2)
27.6 28.0 27.5 27.9 26.9 27.2 26.1 26.2 25.0 25.0 23.1 22.8 22.3 21.9 22.5 22.2 23.1 22.8 23.9 23.8 25.0 25.0 26.8 27.1
transfer direction (denoted by moisture difference Du ¼ ua ue). Various flow patterns will behave the same mass transfer performance, when inlet air locates on the equivalent isenthalpic line of inlet fluid (ha,in ¼ he,in). Moreover, counter-flow pattern performs best mass transfer property, when the combined heat and mass transfer direction is the same as the mass transfer direction, for instance, inlet air a1, a2, a5, a6 or a7 contacts inlet water w. Take inlet air condition a3 in Fig. 4(a) or 5(a) as an example to clearly investigate the influencing characteristics of the heat transfer process and the mass transfer process within air–water or air-desiccant direct-contact devices. The vapor pressures (or humidity ratios) of inlet air and inlet fluid are the same, which means there is no driving force for mass transfer at the entrance. However, water or liquid desiccant is heated along the process by hot air, and hence the vapor pressure of fluid increases, which generates the mass transfer driving force. The air (point a3) will be humidified although the mass transfer driving force at the entrance is zero. In this condition, the mass transfer performance essentially depends on the promotion of heat transfer to mass transfer process. Parallel-flow pattern benefits to this promotion effect that is why parallel-flow configuration behaves better humidification performance than that of counter-flow pattern.
10.7 10.4 10.1 9.9 9.1 8.9 8.1 8.1 7.2 7.2 6.1 6.3 6.3 6.5 6.9 7.1 7.7 7.8 8.5 8.6 9.4 9.4 10.6 10.4
28.7 29.5 28.7 29.5 27.8 28.3 26.6 26.9 25.1 25.1 22.2 21.6 21.0 20.1 21.3 20.5 22.1 21.5 23.4 23.1 24.9 24.9 27.6 28.2
44.9 44.9 45.0 45.0 45.1 45.1 45.1 45.1 45.2 45.2 45.2 45.2 45.1 45.1 45.0 45.0 44.9 45.0 44.9 44.9 44.8 44.8 44.8 44.8
A
Dehumidification Parallel < Counter
A
Dehumidification Parallel < Counter
B
Regeneration
Parallel > Counter
B
Regeneration
Parallel > Counter
–
Regeneration
Parallel ¼ Counter
C
Regeneration
Parallel < Counter
C
Regeneration
Parallel < Counter
C
Regeneration
Parallel < Counter
D
Dehumidification Parallel > Counter
D
Dehumidification Parallel > Counter
–
Dehumidification Parallel ¼ Counter
A
Dehumidification Parallel < Counter
a ain (3)
win
(1) p
(2)
ain (ta,in, ωa,in, ha,in) win (tw,in, ωe,in, he,in) p
(ta,m, ωa,m, ha,in)
b p
4. Similarity of reachable handling region and zonal method
(1) ain
(2)
4.1. Reachable handling region Heat transfer driving force Dt between air and water/desiccant, and mass transfer driving force Dp (or Du) are coupled and will influence each other. As indicated by a former study [12] on airdesiccant direct-contact devices, the combined heat and mass transfer driving forces can be decoupled as relative humidity difference D4 and enthalpy difference Dh. Conditions of outlet air in packed-bed air-desiccant direct-contact devices are within a triangle area in the psychrometric chart as shown in Fig. 6, which is composed of three boundary lines: (1) isenthalpic line of inlet air, (2) isoconcentration line (or iso-relative humidity line) of inlet fluid, and (3) connecting line of the statuses of inlet air and inlet fluid. Same
Zone Dehumidification/ Mass transfer Regeneration performance
sin
(3)
ain (ta,in, ωa,in, ha,in) sin (ts,in, ωe,in, he,in) p (ta,m, ωa,m, ha,in)
Fig. 6. Reachable handling regions of outlet air: (a) air–water direct-contact system; and (b) air–liquid desiccant direct-contact system.
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
25 a in
20 15
p
a out
10 w out w in
5 0
0
5
10
c
b
25 20
a in
15
a out
10
Humidity ratio / (g/kg)
p w out
w in
5 0
15
Temperature / ºC
Temperature / ºC
a
Temperature / ºC
2506
0
5
10
50
40 a in
30
w out p
20
10
15
w in a out
0
10
20
30
40
Humidity ratio / (g/kg)
Humidity ratio / (g/kg)
Fig. 7. The changes of air and water conditions during the experiments: (a) dehumidification by Ge et al. [7]; (b) humidification by Ge et al. [7]; and (c) humidification by Lucas et al. [6].
p
40 s out
a out
s in
30
a in
c
80
s out
60
a out p
40
90 a in
s in
Temperature / ºC
b
50
Temperature / ºC
Temperature / ºC
a
70
50
a in
20
0
5
10 15 20 25
Humidity ratio / (g/kg)
20
0
p
a out s in s out
20
40
60
30
80
Humidity ratio / (g/kg)
0
10
20
30
40
Humidity ratio / (g/kg)
Fig. 8. The changes of air and desiccant conditions during the experiments: (a) dehumidification by Fumo et al. [5]; (b) regeneration by Fumo et al. [5]; and (c) regeneration by Lof et al. [10].
reachable handling region as air-desiccant direct-contact devices can be obtained for air–water direct-contact devices, since the heat and mass transfer models are the same for the two systems. The boundary lines (1) and (2) are the same as the uncoupled heat and mass transfer driving forces. The physical meaning of boundary line (3) is that the air outlet conditions will locate on this line when water/desiccant flow rate is essentially large comparing with the air flow rate. Point p shown in Fig. 6 is the intersection point of isenthalpic line of inlet air and iso-relative humidity line of inlet fluid. The inlet and outlet conditions of air–water and air-desiccant direct-contact devices during the experiments are shown in the triangle reachable handling region, as indicated in Figs. 7 and 8, consulting Table 4 for specific data. The reachable maximal/ minimal parameters of outlet air can be obviously obtained from the figures. For example, in the humidification process of air–water direct-contact devices, the maximal humidity ratio of outlet air is the same as equivalent humidity ratio of inlet water (win) in the experiments by Lucas et al. [6] as shown in Fig. 7(c). The maximal humidity ratio of outlet air is the humidity ratio of point p in the experiments by Ge et al. [7] as shown in Fig. 7(b).
4.2. Handling zonal method Conditions of inlet air and inlet water/desiccant do affect heat and mass transfer characteristics from the results of Figs. 4–8 and Tables 2–4. Only according to the relative statuses of inlet air to inlet water/desiccant, four zones A–D [12] are divided in the psychrometric chart for air-desiccant direct-contact system as shown in Fig. 9(a), including two dehumidification zones A and D and two regeneration zones (or humidification zones) B and C. In zones A and C, mass transfer direction (driving force Du) is the same as the combined heat and mass transfer direction (driving force Dh). Counter-flow pattern shows the highest mass transfer performance, and parallel-flow is the poorest in the same operating condition. In zones B and D, mass transfer direction is opposite to that of combined heat and mass transfer. Mass transfer performance of counter-flow pattern is the poorest while parallel-flow is the best in the same condition, which is rather different with the result in zones A and C.
Table 4 Inlet and outlet conditions of air and fluid through the packed-bed devices as shown in Figs. 7 and 8. Figure
Fig. Fig. Fig. Fig. Fig. Fig.
7(a) 7(b) 7(c) 8(a) 8(b) 8(c)
Source
Ge et al. [7] Ge et al. [7] Lucas et al. [6] Fumo et al. [5] Fumo et al. [5] Lof et al. [10]
Fluid
Water Water Water Desiccant Desiccant Desiccant
Inlet conditions
Outlet conditions
ta,in/ C
ua,in/(g/kg) tw,in or ts,in/ C xin/% ta,out/ C ua,out/(g/kg) tw,out or ts,out/ C xout/%
19.4 20.2 29.1 30.1 30.3 81.0
8.3 8.3 13.7 18.0 18.2 11.6
6.2 6.9 35.2 30.1 65.4 36.0
d d d 34.6 34.4 27.2
12.2 15.1 30.2 31.3 57.6 40.0
7.6 9.3 21.0 10.4 50.2 20.3
8.0 10.3 29.0 32.5 57.0 37.0
– – – 34.5 34.9 27.6
Zone
Dehumidification/Humidification
A B C A B C
Dehumidification Humidification Humidification Dehumidification Humidification Humidification
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
40
Zone B
(2)
Temperature / ºC
(1)
Regeneration
30
Zone A
Zone C
Dehumidification
s
Regeneration 20
Zone D Dehumidification 10 0
5
10
15
20
Humidity ratio / (g/kg)
b
40
Temperature / ºC
(2)
(1)
Zone B Humidification
5. Utilization of the reachable region and handling zones
Zone A Dehumidification
30
Zone C Humidification
20
w
10 0
10
The dividing line of zones B and C (and zones A and D) is the equivalent isenthalpic line of inlet desiccant. However, the dividing line of zones A and B (and zones C and D) can hardly be determined only with the statuses of inlet air and inlet desiccant, since the influencing factors include flow pattern, flow rate, heat and mass transfer coefficients, etc. Fortunately, the dividing line of zones A and B is between isoconcentration line (or iso-relative humidity line) and equivalent iso-humidity ratio line of inlet desiccant. The common air-handling processes using liquid desiccant are shown in Fig. 8, including dehumidification processes lay in zone A, hot air driven regeneration processes in zone B and hot desiccant driven regeneration processes in zone C. And there is barely dehumidification process in zone D in available literature. Similar to the divided zones for air-desiccant direct-contact system, zones for air–water system can then be derived, as shown in Fig. 9(b). Three zones (A–C) are derived for air–water system, with the same zonal characteristics as the zones in air-desiccant system. The common air–water direct-contact processes are shown in Fig. 7.
20
30
40
Humidity ratio / (g/kg) Fig. 9. Handling zones of heat and mass transfer processes in: (a) air–liquid desiccant; and (b) air–water direct-contact devices.
a
b
Fluid inlet (w1 or s1)
Air inlet (a2)
(w2 or s2) Heater
Fluid inlet (w1 or s1)
Air inlet
Air outlet
(a1)
The reachable region and handling zones can be utilized in the optimization of air-handling processes. Heating or cooling capacity can be given to air side or liquid side. Which mode can yield better mass transfer performance and which kind of flow pattern can realize the best mass transfer performance? Two cases with humidification purpose are taken as examples shown in Fig. 10 to answer the above questions. Heater is adopted to heat inlet air in mode 1 and to heat inlet fluid in mode 2. Fig. 11 gives the handling processes of the two modes in air– water direct-contact systems. The heat and mass transfer process of mode 1 is in zone B and mode 2 in zone C. Conditions of inlet air (point a1) are 1 kg/s, 20 C and 8 g/kg, and conditions of inlet water (point w1) are 0.5 kg/s and 20 C. The power of the heater in Fig. 10 is
Air outlet (a2)
(a1)
(a3) Packing
Heater
Fluid outlet (w3 or s3)
Packing
Fluid outlet (w2 or s2)
Fig. 10. Humidification of the processed air: (a) mode 1: heat inlet air; and (b) mode 2: heat inlet water/desiccant.
a
50 a2
Temperature / ºC
40
a 3 (Parallel)
30 a 3 (Counter) a1
20
w1
p w 2 (Counter) w 2 (Parallel)
10
b
40
Temperature / ºC
a
2507
30
a 2 (Counter)
w2
a 2 (Parallel) w 3 (Parallel)
a1
20
w1
w 3 (Counter)
p 10
a out 0
0
5
10
15
20
Humidity ratio / (g/kg)
25
0
0
5
10
15
20
25
30
35
Humidity ratio / (g/kg)
Fig. 11. Humidification performance of air–water system: (a) mode 1: heat inlet air; and (b) mode 2: heat inlet water.
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X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
Table 5 Inlet and outlet conditions of air and fluid through the packed-bed devices as shown in Figs. 11 and 12. Figure
Fluid
Mode Flow pattern Inlet conditions
Outlet conditions
Variance of air humidity ratio /(g/kg)
ta,in/ C ua,in/(g/kg) tw,in or ts,in/ C xin/% ta,out/ C ua,out/(g/kg) tw,out or ts,out/ C xout/% Water Water Water Water Desiccant Desiccant Desiccant Desiccant
1 1 2 2 1 1 2 2
Parallel Counter Parallel Counter Parallel Counter Parallel Counter
Temperature / ºC
a
47.7 47.7 20.0 20.0 47.7 47.7 20.0 20.0
8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0
20.0 20.0 33.4 33.4 20.0 20.0 43.0 43.0
– – – – 40.0 40.0 40.0 40.0
b
50 a2
a 3 (Counter) p
30
s 2 (Counter) a1
s1
s 2 (Parallel)
10 0
13.0 12.8 15.6 17.9 10.4 10.0 13.0 14.4
21.9 22.2 22.6 20.2 26.8 28.2 27.5 23.6
– – – – 40.2 40.1 40.4 40.5
5.0 4.8 7.6 9.9 2.4 2.0 5.0 6.4
50
a 3 (Parallel)
40
20
31.3 31.0 23.1 24.2 33.8 33.1 26.4 27.5
Temperature / ºC
Fig. 11(a) Fig. 11(a) Fig. 11(b) Fig. 11(b) Fig. 12(a) Fig. 12(a) Fig. 12(b) Fig. 12(b)
40
s2 s 3 (Parallel)
30
s 3 (Counter) a1
20
p
a 2 (Counter)
s1
a 2 (Parallel)
10
0
5
10
15
20
25
Humidity ratio / (g/kg)
0
0
5
10
15
20
25
30
35
Humidity ratio / (g/kg)
Fig. 12. Humidification or regeneration performance of air–liquid desiccant system: (a) mode 1: heat inlet air; and (b) mode 2: heat inlet liquid desiccant.
28 kW, and NTUm value of the packed-bed humidifier is 1. The humidification performances of the two modes are shown in Table 5. The reachable maximal humidity ratio of outlet air is 17.8 g/kg (point p) for mode 1 and 33.1 g/kg (point w2) for mode 2. The actual humidity ratio of outlet air in parallel-flow packed-bed humidifier is 13.0 g/kg for mode 1 and 15.6 g/kg for mode 2, and the value changes to 12.8 g/kg and 17.9 g/kg in counter-flow humidifier. In mode 1, parallel-flow packed-bed device shows better mass transfer performance than that of counter-flow device. However, in mode 2, counter-flow packed-bed device shows better mass transfer performance than that of parallel-flow configuration, with about 30% higher humidification capacity. Mode 2 with higher temperature of inlet water into packed-bed device (in zone C) has much higher humidification performance, and the humidification capacity is about twice as much as that of mode 1 with higher temperature of inlet air (in zone B). In air-desiccant direct-contact system, similar conclusion can be drawn as air–water system. The air-handling processes in mode 1 (heat the air, in zone B) and mode 2 (heat the desiccant, in zone C) are shown in Fig. 12, with the detailed conditions indicated in Table 5. Heating the desiccant (mode 2) has much higher humidification or regeneration capacity, 2–3 times as much as that of mode 1. In mode 2, humidification capacity of counter-flow packed-bed regenerator is 28% higher than that of parallel-flow configuration. To sum up, in order to gain better humidification performance, fluid should be heated (in zone C) rather than air (in zone B). Counter-flow packed-bed configuration is recommended in the heating fluid mode (in zone C), comparing with other flow patterns. In the dehumidification process of air-desiccant direct-contact system, similar conclusion can be drawn: liquid desiccant should be cooled (in zone A) rather than air (in zone D) to obtain better mass transfer performance, and counter-flow configuration is recommended in zone A.
6. Conclusions Packed-bed heat and mass transfer devices of air–water and air– liquid desiccant direct-contact systems have been widely utilized in air-conditioning systems. Coupled heat and mass transfer characteristics of air–water and air-desiccant direct-contact devices, especially the similarities between the two kinds of devices, are investigated. And the main conclusions are: 1) The statuses of water and liquid desiccant through packed-bed devices can be regarded as locating on a constant iso-relative humidity line. Similar heat and mass transfer model and similar analytical solutions can be obtained for air–water and airdesiccant system. 2) Reachable handling region of air–fluid (water or liquid desiccant) direct-contact device is among the three boundary lines, (1) isenthalpic line of inlet air, (2) iso-relative humidity line of inlet fluid, and (3) the connecting line of inlet statuses of air and fluid. 3) Apart from being influenced by flow pattern, heat and mass transfer coefficients and flow rates of air and fluid, the heat and mass transfer characteristics will be affected by the conditions of inlet air and inlet fluid to some extent. 4) Only according to the relative statuses of inlet air to inlet fluid, the psychrometric chart are divided into four handling zones A–D for air-desiccant direct-contact processes and three handling zones A–C for air–water direct-contact processes. Zones A and D are dehumidification zones, and zones B and C are humidification zones (or regeneration zones). In zone A or C, counter-flow pattern has best mass transfer performance in the same condition. While in zone B or D, parallel-flow pattern performs best. 5) In order to obtain better humidification performance, fluid should be heated (in zone C) rather than air (in zone B). And
X.-H. Liu et al. / Building and Environment 44 (2009) 2501–2509
fluid should be cooled (in zone A) rather than air (in zone D) to obtain better dehumidification performance. Acknowledgements The research described in this paper was supported by Chinese 11th Five-year Plan project (No.2006BAJ01A08) and National Natural Science Foundation of China (No.50778094). Nomenclature
A B cp cp,m H h Le _ m m* NTUm p R r t
heat and mass transfer area (m2) atmospheric pressure (kPa) specific heat (kJ/kg C) specific heat of humid air (kJ/kg C) height of packed-bed heat and mass transfer device (m) enthalpy (kJ/kg) Lewis number (dimensionless) mass flow rate (kg/s) heat capacity ratio of air to fluid (water or liquid desiccant) (dimensionless) number of transfer unit (dimensionless) vapor pressure (kPa) mass flow rate ratio of air to fluid (dimensionless) water vaporization latent heat (kJ/kg) temperature ( C)
Greek symbols a heat transfer coefficient (kW/m2 C) am mass transfer coefficient (kg/m2 s) 3h enthalpy efficiency (%) 3m moisture efficiency (%) 4 relative humidity (%) x concentration (mass ratio of desiccant to solution) of liquid desiccant (%) u humidity ratio (kg/kg) Subscripts a air a,m intersection point of air inlet isenthalpic line and fluid inlet iso-relative humidity line shown in the psychrometric chart e air in equilibrium with fluid (ta ¼ tf, and pa ¼ pf)
f in s out w
2509
fluid (water or liquid desiccant) inlet liquid desiccant outlet water
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