6.2 Ef fi cient Equipment for Liquid Desiccant
6.2.1 Experimental Test of a Internally Cooled/Heated
6.2.1.1 Structure of the Internally Cooled/Heated Dehumidifier/Regenerator
In the adiabatic gas–liquid contactors, the release and absorption of the latent heat obviously changes the desiccant temperature during humidification/regeneration, leading to poor mass transfer performance. To solve the problem, a plate-fin heat exchanger (PFHE) was designed [16]. The structure of the PFHE is shown in Fig.6.14a, b. A unit of PFHE consists of seven parallel water channels and six solution/air channels intermingled with each other. All channels are 298 mm in length. The water channel is 3 mm in width, while the solution/air channel is 12 mm in width. In addition, the height of the PFHE unit is 100 mm. In order to provide more contacting area for desiccant solution and air, between two neigh- boring plates, there are three layers offins with the distance of 4 mm side-by-side intercrossing.
As depicted in Fig.6.14c, the internally cooled dehumidifier or internally heated regenerator is composed by six identical units of PFHE stacking up along the vertical direction. For the convenience of installation, two neighboring PFHEs have opposite water inlet sides. Therefore, three PFHEs have their inlet/outlet on one side and the remaining PFHEs have their inlet/outlet on the other side. The desiccant solution from the top of the dehumidifier/regenerator is distributed evenly over the fins andflows countercurrently relative to the air from the bottom. For dehumidi- fication, cooling water entering the water channelsflows horizontally and removes
Fig. 6.14 Structures of PFHE and dehumidifier/regenerator: a top view of the plate-fin heat exchanger; b schematic diagram of the plate-fin heat exchanger; c structure of the whole dehumidifier/regenerator [16]
vaporization heat to maintain low solution temperature during the dehumidification process, producing high mass transfer potential. For regeneration, heating water is transported into the water channels and provides vaporization heat to maintain high solution temperature as well as high mass transfer potential. If no water is used, the dehumidifier/regenerator becomes adiabatic.
6.2.1.2 Experimental Test and Performance Comparison
In order to test the performance of the designed internally cooled/heated dehumidifier/regenerator, pertinent experiments were conducted using lithium chloride aqueous solution as liquid desiccant. A comparative study was also con- ducted between the adiabatic and the internally cooled/heated dehumidifier/regenerator. The humidity ratio difference is taken as a performance index. On the one hand, the humidity ratio difference can represent moisture removal/evaporation rate with the constant air flow rate. On the other hand, it reflects the processed air outlet humidity ratio, which is critical for dehumidification requirement in comfort air-conditioning systems. In this section, the humidity ratio difference for dehumidification is defined as:
Dwaẳwaiwao ð6:28ị And the humidity ratio difference for regeneration is defined as:
Dwaẳwaowai ð6:29ị As known to us, the air velocity has a remarkable impact on the heat and mass transfer coefficient [13,14]. Besides, the solution temperature significantly affects the partial vapor pressure at the solution surface, which determines the mass transfer potential in some degree. Thus, the effects of air velocity and solution temperature on the performance index were investigated. In addition, the effect of cooling water temperature was also discussed for internally cooled dehumidifier.
As to dehumidification, inlet conditions of air, solution and cooling water are shown in Table6.5.
Figure6.15shows humidity ratio difference under different air velocities for the internally cooled and adiabatic dehumidifier, respectively. When the air velocity increases, the humidity ratio differencefirstly increases linearly and afterward stays
Table 6.5 Inlet parameters for the internally cooled dehumidifier during experiments Variable va(m/s) Ms(kg/s) Mw(kg/s) ta(°C) ts(°C) tw(°C) wa(g/kg) Xs
va 6.49–3 0.1036 0.151 30.9 25.3 24 10.6 0.386
ts 2.84 0.1036 0.151 30.5 20.4–36.6 22 13.4 0.377
2.84 0.1036 0.151 30.9 23.6–28.9 22 12.6 0.388
tw 3.36 0.1036 0.151 30.3 27.0 19–25 12.6 0.387
almost constant. Both the internally cooled dehumidifier and the adiabatic dehu- midifier have the same trend but with the different magnitude of humidity ratio difference. The humidity ratio difference of the internally cooled dehumidifier is 0.5 g/kg larger than that of the adiabatic dehumidifier on average. Compared with the adiabatic dehumidifier, performance of the internally cooled dehumidifier is improved by about 25%. This is because the cooling water effectively carries away the vaporization heat and keeps the solution temperature low, producing higher overall vapor pressure difference between air and desiccant in internally cooled dehumidifier. The appeared significant increase in the humidity ratio difference can be attributed to the mass transfer coefficient, which increases more quickly than the airflow rate under higher air velocity. The reason for the almost constant trend later is that the increase in the mass transfer coefficient is close to the increase in the air flow rate when the air velocity is high enough. If the air velocity increases further, the air flow rate may increase more rapidly than the mass transfer coefficient, resulting in the drop of humidity ratio difference.
Two group experiments were conducted to study the effect of solution temper- ature on the humidity ratio difference for two types of dehumidifiers. As illustrated in Fig.6.16, the humidity ratio difference shows an obvious downward trend with the increase in the solution temperature. The performance of the internally cooled Fig. 6.15 Effect of air
velocity on the humidity ratio difference for
dehumidification [16]
Fig. 6.16 Effect of solution temperature on the humidity ratio difference for dehumidification [16]
dehumidifier is better than that of the adiabatic dehumidifier in terms of the humidity ratio difference. In order to disclose effects of the solution temperature on the humidity ratio difference accurately, the mass transfer coefficient between the processed air and solution is calculated by the following equation according to the literature [15]:
kdẳGaðwaiwaoị
A wð aiweiị ð6:30ị wherekdis different from aforementionedhdbut can also reflect the mass transfer process.
From Fig.6.17, the mass transfer coefficient decreases with the solution tem- perature. This means that the reduction of the humidity ratio difference is because of not only the bigger vapor pressure at the solution surface but also the smaller mass transfer coefficient the high solution temperature brings. The overall solution temperature in the internally cooled dehumidifier is lower than that in the adiabatic dehumidifier. So, it is clear that the humidity ratio difference of the internally cooled dehumidifier is higher than that of the adiabatic dehumidifier.
Since no cooling water is used in the adiabatic dehumidifier, the humidity ratio difference stays constant when the cooling water temperature changes. Under this condition, the adiabatic dehumidifier is not analyzed. Figure6.18presents the effect of cooling water temperature in the internally cooled dehumidifier. As the cooling water temperature varies from 19 to 25 °C, the humidity ratio difference declines.
The overall solution temperature variation accounts for this phenomenon.
As to regeneration, inlet conditions of air, solution and heating water are shown in Table6.6.
Figure6.19shows the effect of the air velocity on the performance of internally heated regenerator and the adiabatic regenerator. The humidity ratio difference of the internally heated regenerator is much higher than that of the adiabatic regen- erator. The average deviation is about 3 g/kg. Compared with the adiabatic dehu- midifier, performance of the internally heated regenerator is improved by about 33%. This can be explained as follows. In the adiabatic regenerator, the vapor pressure at the solution surface is higher than that of the scavenging air. So the
Fig. 6.17 Mass transfer coefficients under different solution temperatures for dehumidification
moisture transfers from the desiccant solution to the air, accompanying the absorption of vaporization heat, which mainly comes from the sensible heat of desiccant solution. Hence, the temperature of the desiccant solution decreases dramatically and regeneration performance decays. However, in the internally heated regenerator, the heating water enters the regenerator and provides the des- iccant solution with sensible heat-by-heat transfer. During regeneration process, the solution can stay high temperature and the regeneration performance can be enhanced accordingly. With the increase in the air velocity, the humidity ratio difference also increases for both internally heated and adiabatic regenerators. This should be explained from two aspects. Thefirst is that the higher air velocity helps to increase the mass transfer coefficient for stronger turbulent fluctuation. The second is that the regeneration air can be carried away rapidly. As a result, the mass transfer potential is maintained relatively high.
Fig. 6.18 Effect of water temperature on the humidity ratio difference for dehumidification [16]
Table 6.6 Inlet parameters for the internally heated regenerator during experiments Variable va(m/s) Ms(kg/s) Mw(kg/s) ta(°C) ts(°C) tw(°C) wa(g/kg) Xs
va 6.68–3.59 0.1 0.14 26.5 70.0 70 16.5 0.380
ts 2.42 0.1 0.14 26.7 62.0–73.0 72 16.3 0.360
Fig. 6.19 Effect of air velocity on the humidity ratio difference for regeneration [16]
The effect of the solution temperature for regeneration is shown in Fig.6.20.
During experiments, the solution temperature varies from 62 to 73 °C by regulating the electric heater with a temperature controller. It can be seen that the humidity ratio difference of the internally heated regenerator is much more than that of the adiabatic regenerator especially for the solution temperature among 65–69 °C.
Moreover, both internally heated and adiabatic regenerators show upward trend in the humidity ratio difference. This may be due to higher vapor pressure difference between liquid desiccant and air with an increase in the solution temperature.
The above analysis experimentally verified the excellent superiority of the internally cooled/heated dehumidifier/regenerator by comparison with the adiabatic dehumidifier/regenerator. The cooling water helps to keep the low temperature of desiccant solution during dehumidification, and the heating water helps to keep the high temperature of desiccant solution during regeneration, which promotes mass transfer. With good dehumidification/regeneration performance, the internally cooled/heated dehumidifier/regenerator can also be made smaller in terms of the size. It indicates that the internally cooled/heated dehumidifier/regenerator is a promising alternative to the adiabatic dehumidifier/regenerator.
For the liquid desiccant air-conditioning system, the desiccant regeneration determines the system economy because the regeneration process consumes much energy. Therefore, the internally heated regenerator was further discussed theoretically.