An internally-cooled parallel-plate membrane-based liquid desiccant dehumidifier (IMLDD) with cooling tubes inside the solution channels has been used for air dehumidification. The concept is similar to a three-fluid heat and mass exchanger containing processing air, liquid desiccant, and refrigerant streams. The processing air and the liquid desiccant streams flow in the neighboring channels, which are formed by the plate membranes. A row of the cooling tubes are installed in each solution channel to take away the absorption heat generated in the solution by absorbing the water vapor transferred from the processing air across the membranes into the solution. A lumped parameter model is established in a unit cell containing an air channel, a plate membrane, a solution channel, and a row of the cooling tubes, to study the heat and mass transports in the IMLDD. An analytical solution of the normalized governing equations is obtained based on the dimensionless parameters, which have their specific physical meanings. Sensible cooling effectiveness, dehumidification effectiveness, dehumidification rate, energy transfer rate of the air stream, and the ratio of the sensible heat transfer rate of the water stream to the energy transfer rate of the air stream are then calculated. Only a few operating conditions of inlet water temperatures and sensible heat capacity ratios from previous studies have been employed to validate the model. However more validations would be conducted in future under more operating conditions. It can be found that the performances of the IMLDD are largely improved when the water inlet temperature is decreased to keep the solution temperature always in a relatively small level. The water inlet temperature is recommended to be set as about 20 degrees C, which is somewhat smaller than the solution inlet temperature. (C) 2017 Elsevier Ltd. All rights reserved.