Dynamic Modelling of Water- Ammonia Absorption Chiller in Gax Configuration
Résumé
In this paper a general model for the dynamic simulation of an absorption chiller is proposed. The softwe has been designed in a modul way allowing the simulation of vious absorption chiller configurations (single or multi-stage). Thermodynamic Properties of the mixture of the working fluids can be calculated by suitable thermodynamic models. Hence the working pair can be changed easily. The model is based on mass and energy balances applied to the vious devices of the machine (evaporator, absorber, valves, etc.) and it includes mass storage. A general Positive Flash model enables the description of all the possible states (sub-cooled, super-heated, biphasic...) of the vious streams which e all considered as multicomponent mixtures. In the positive flash the same set of governing equations is valid for all phase regions so that this model can handle the formation of a new phase or the disappeance of an existing one because of composition, pressure or temperature changes due to the dynamic state. The generator is modeled using the concept of theoretical plates: MESH equations (Mass balance, Equilibrium, Summation, Heat balance) have been written for each stage. Finally a general procedure is developed for the modeling of heat exchangers in order to take into account a local overall heat transfer coefficient according to the local states of the two fluids (sub-cooled, super-heated, evaporating, condensing). Despite the modul structure of the softwe, a global solution strategy has been implemented, using a Ge's method. This general model is then applied to an ammonia/water absorption chiller. The simulated cycle is a GAX (Generator-Absorber heat eXchange) configuration.