An Analytical Approach to Optimize the Performance of the Bypass for Two-Way Control Valves in Chilled Water Central Air-Conditioning Systems

Abstract

Primary-secondary pumping system is widely utilized in Heating, Ventilating, and Air Conditioning industry. This type of system has an energy saving potential over the constant speed primary pumping system. However, the system uses a surplus amount of energy that can be saved, the low delta T syndrome and low pumping efficiency are the main energy wasting reasons. The implementation of a proper design and novel control strategy can eliminate these two problems, knowing that the design of the bypass (Decoupler Bridge) is still a topic of debate. Thus, the focus in this thesis will be enhancing the design and control of the bypass and developing an energy efficient control for the secondary loop including the pumps and the two way control valves. Calculating the bypass pipe diameter and its control strategy are often estimated according to the designer experience, rather than a code of practice, the goal for this thesis is setting a guide for the bypass design. An analytical model is used to find the most efficient design, where the model simulates the water distribution loop, pumps, chillers and control valves.

Pressure drop in the system is categorized into three types; speed dependent, pipes, and controlling pressure drops. Pumps are modelled by curve fitting to real pump performance curves. Control valves are modelled with their non-linear flow characteristics. While the control scheme is applied as the signal given to the variable frequency drive on the secondary pump. The model is built to simulate chilled water system.

Results are acquired from several schemes, having different number of load terminals, each having its separate control valve, each system has its constant speed primary pumps, where one pump is installed for each chiller, and a variable speed secondary pumps set are chosen according to each system. Also the results are validated with an experimental model. The various configurations for both bypass and secondary pump control are implemented on this model, in order to demonstrate the energy savings, and also testing the performance of each system showing its effect on the heat loads. The results show that some modifications to the design can save a considerable amount of energy compared to the conventional design. As a conclusion a design guideline is suggested in this thesis for an energy efficient bypass pipe.

Key words: Primary-Secondary; Bypass; Two way control valve; Decoupler-bridge; Air Conditioning

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