Development of a small-scale absorption cooled water chiller

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Wei, T.Y., 2000. Development of a small-scale absorption cooled water chiller. PhD, Nottingham Trent University.

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Abstract

Legislation to improve environment control has recently led to a number of analytical studies to characterise absorption cooling systems (ACS). ACS play an important role in reducing CFCs and CO2 since it works on natural fluids and can be driven by waste heat or solar energy. However its current performance is low and the system characteristics are, still not well understood. This work aims to provide a better fundamental understanding of existing knowledge on this cooling system technology with the view to improving its performance and establishing the feasibility of developing a small scale unit for the domestic market. The dual pressure water-lithium bromide (H2O-LiBr) absorption system has been investigated in detail.

The outcome of this work has led to the development of a user friendly software named ABCON, which comprises features of data-logging from a custom built experimental rig, First and Second Laws of Thermodynamic analysis, and components design. The software is a significant improvement over its predecessors especially the monitoring and control of the experimental system integrated with an effective design tool not currently available in the commercial market.

Mathematical models for exergetic analysis based on the Second Law of thermodynamics which have been omitted in most of the published literatures were developed and studied as part of this work. Optimum generator temperatures for establishing maximum exergetic coefficient of performance (ECOP) have been presented in a novel carpet plot form. Two new equations derived for maximum ECOP at optimum generator temperature is conveniently stored in a control system to achieve optimized conditions during operation. For instance, when the condenser and absorber temperatures are equal at 35 °C, and the evaporator temperature is at 8°C, the optimum ECOP will be 0.209 assuming the generator temperature is maintained at 72.5 °C. Analytical and numerical models for different types of absorbers were studied and a coil absorber model has been proposed. The model includes three transport mechanisms namely falling film on the tube, droplets fall and the droplet formation which were not included in previous published literature. The re-circulation spray which can help reduce the absorber load and enhance the absorption rate has been incorporated into the model. The counter-current and parallel absorbers with two coil diameters have been compared with variation of mass concentration, coolant and solution flow rate and number of droplet sites.

An absorption rig was built using borosilicate glass so that the process mechanism can be observed conveniently. It was designed to be flexible and used as a benchmark for comparison in studying and developing new cycles and absorbent/refrigerant combinations in future work. The experimental results show the importance of the effects of the inlet solution conditions on the absorber performance. The changes of heat and mass transfer coefficient, absorber load and mass absorption rate with inlet solution temperatures are reported. The results have been validated against the theoretical model and shown to be in good agreement to the trends predicted for different conditions. Crucial factors for achieving good system performance such as accurate control of the pressure and inlet solution temperature have been identified through experiments. It is recommended that the evaporator and absorber should be combined in future designs. The effects of the non-condensable gas and additives such as 2-ethyl-1-hexanol to the overall performance have been recommended for future work.

The design and development of the rig with viewing and other component interchangebility facilities together with the empirical data generated for a small scale ACS will be an invaluable asset to future development of prototype systems thus making an important contribution to the current debate on environmental control.

Item Type:

Thesis

Creators:

Wei, T.Y.

Date:

2000

ISBN:

9781369313154

Identifiers: