HEAT AND MASS TRANSFER CONSIDERATIONS IN THE DESIGN OF A COILED ABSORBER FOR AMMONIA-WATER REFRIGERATION SYSTEM
The performance of the absorber significantly affect the overall efficiency of any absorption refrigeration system. One of the major reasons for the poor design of the absorber for ammonia-water refrigeration systems is the neglect of mass transfer resistances in the vapour or liquid phase in favour of only heat transfer considerations. Heat removal is critical to the absorption process, however, the ammonia gas is constantly absorbed by the liquid ammonia-water solution. This means that two-phase mass and heat transfer operations take place during the absorption process. Therefore, this implies that, for any proper absorber design, both heat and mass transfer considerations are paramount. Accurate absorber design is further predicated on accurate properties correlations. The correlations required for the absorber design include liquid and vapour enthalpies, saturated vapour pressure, specific heat capacity of water and ammonia vapour, liquid solution dynamic viscosity, liquid solution density, surface tension, liquid solution thermal conductivity, liquid-liquid diffusivity and vapour-liquid diffusivity. Procedures for calculating these parameters have been presented in this paper and their accuracy with previous studies compared. Results showed relatively low percentage deviations and so can be applied for the design and simulation of the absorber. Designs of the absorber for various cooling capacities have been carried in this paper. The variation of ammonia vapour temperatures, liquid ammonia-water solution temperatures, cooling water temperatures, and flow rates of ammonia-water solution and ammonia vapour across the absorber is investigated for the four cooling capacities. For refrigeration systems with cooling capacities of 1 kW, 2 kW, 3 kW and 4 kW, the calculated lengths for a water-cooled coiled absorber are 4.484 m, 8.1467 m, 12.50459 m and 14.78329 m respectively. The calculated overall heat transfer coefficients for the absorption process for the four cooling capacities (1 kW, 2 kW, 3 kW and 4 kW) were 0.4427, 0.4376, 0.4208 and 0.4309 kWm-2K-1 while the mass transfer coefficients were 3.721 x10-9, 4.206 x10-9, 4.556 x10-9and 4.856 x10-9 m2s-1 respectively. Concurrent flow is considered in this study, but the procedure can also be applied to countercurrent flow.
KEYWORDS: Absorption Refrigeration, Ammonia Water Heat and Mass Transfer, Design