Experimental performance of bubble column humidifier and dehumidifier under varying pressure

The performance of bubble column humidifier and dehumidifier is investigated experimentally under sub-atmospheric pressures for the humidifier and elevated pressures for the dehumidifier. The bubble columns examined are of 10 cm diameter and 25 cm height. They are equipped with internal cooling and heating coils and the air is sparged through perforated plates at its bottom. The pressure tested in the humidifier ranges from 7 psia (0.48 bar) to 15 psia (1.03 bar), while the pressure tested in the dehumidifier ranges from 15 psia (1.03 bar) to 30 psia (2.07 bar). In addition, the water level in the bubble columns and the superficial velocity were varied in the range of 5-7 cm and 2-20 cm/s respectively. The performance was evaluated by measuring the total heat transfer rate and the effectiveness. Moreover, the experimental values of the mass transfer coefficient are presented as a function of the superficial velocity and pressure. The results show that operating the bubble column humidifier at sub-atmospheric pressure enhances the total heat transfer by about 35% and the effectiveness by about 7.1%. However, operating the dehumidifier at elevated pressures, results in higher heat transfer rate by about 27% but lower effectiveness by about 3.2%. It was demonstrated that the liquid height in the column has no significant effect on the performance however; the superficial velocity increases both the heat transfer and effectiveness for the humidifier and dehumidifier. Moreover, a modified effectiveness-number of transfer units (ε-NTU) model for counter flow cooling tower was found to agree well with the experimental results of the effectiveness and NTU. The ε-NTU model is an effective approach to analyze the performance of bubble column humidifier and dehumidifier.