Mathematical Modeling for Absorption Chiller with Heat Exchanger

Abstract

Heat exchangers form a vital part of absorption chillers. Majority of research works utilize
LMTD (Log Mean Temperature Difference) methodfor their analysis anddevelopmentof absorption
chillers. However, this method has some limitations. The fluid inlet and outlet temperatures
must be known or readily determined from the energy balance equations. The value of log mean
temperature difference for the heat exchanger may then be determined. Practically, the outlet
temperatures are unknown for the design of absorption chillers. If only the inlet temperatures are
known, use of the LMTD method requires an iterative procedure. This research work proposed
an alternative approach, termed the effectiveness-NTU method. The study results show that cold
fluid heat capacity rate is always higher than hot fluid heat capacity rate because of 3 factors.
Firstly, the mass flow rate of solution leaving from absorber is always higher than the mass
flow rate of solution leaving from the generator. Secondary, the specific heat of LiBr solution is
a function of solution concentration and the concentration of solution leaving from absorber is
always lower than the concentration of solution leaving from the generator. Thirdly, the fluid
heat capacity rate is the product of mass flow rate and specific heat of LiBr solution. This effect
to the constraint of designed temperature outlets and also the performance of heat exchanger
used in absorption chiller.