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Abstract
Entrainment of liquid desiccant droplets into the airstream and flow maldistribution are two challenges for liquid desiccant packed beds. Liquid-to-air membrane energy exchangers are novel liquid desiccant exchangers that have the potential to overcome these challenges by using membranes to separate the air and solution flows but have higher heat and moisture transfer resistances. As a new contribution of this study, the heat mass transfer performance of these two exchangers with the same volume and operating condition are compared at two conditions: (1) with the same pressure drop on the air side and (2) with the same total heat/mass transfer area. Results show that liquid-to-air membrane energy exchanger gets up to 13 and 20% higher latent and total effectiveness respectively than the packed bed at the same air pressure drop. Reversely, the packed bed achieves up to 16% higher mass transfer performance than the liquid-to-air membrane energy exchanger with the same heat/mass transfer area. The flow maldistribution and its influences in the packed bed and liquid-to-air membrane energy exchanger are also discussed. Finally, the impact of membrane on the heat mass transfer in the liquid-to-air membrane energy exchanger is evaluated. The membrane accounts for 6∼12% and 26∼43% of the overall heat and mass transfer resistance, respectively, depending on the width of the air channel.
Nomenclature
| A | = | heat and mass transfer area or surface area of membrane (m2) |
| Across | = | cross-section area of energy exchanger (m2) |
| C | = | heat capacity rate (kW/K) or solution concentration [kg (LiCl)/kg (solution)] |
| Cr* | = | ratio of heat capacity rates |
| cp | = | specific heat capacity(kJ/ kg K) |
| ET | = | slope of equilibrium humidity to temperature of solution (kg/kg K) |
| h | = | specific enthalpy (kJ/kg) |
| H* | = | operating condition factor |
| hc | = | convective heat transfer coefficient (kW/m2 K) |
| hm | = | mass transfer coefficient (kg/m2 s) |
| k | = | thermal conductivity (kW/m K) |
| km | = | moisture conductivity of membrane (kg/m s) |
| L | = | length of energy exchanger (m) |
| Le | = | Lewis number |
| m | = | mass flow rate (kg/s) |
| MRR | = | moisture removal rate (g/s) |
| NTU | = | number of heat transfer units |
| NTUm | = | number of mass transfer units |
| T | = | temperature (°C) |
| U | = | overall heat transfer coefficient (kW/m2 K) |
| Um | = | overall mass transfer coefficient (kg/m2 s) |
| v | = | surface area per unit volume of packing (m2/m3) |
| V | = | volume of energy exchanger (m3) |
| W | = | humidity ratio (kg/kg) |
Greek symbols
| δ | = | thickness of membrane or width of channel |
| λ | = | vaporization latent heat (kJ/kg) |
| ϵ | = | effectiveness |
Subscripts
| air | = | airflow |
| in | = | inlet |
| Lat | = | latent |
| out | = | outlet |
| Sen | = | sensible |
| sol | = | solution flow |
| Tot | = | total |