A Study On The Operating Characteristics Of A Marine Variable-air-volume Air-conditioning System And Its Supply Air Control

The variable-air-volume air-conditioning(VAV A/C)system is believed to be significantly energy-saving and find more and more application in buildings. However the application of such A/C systems on ships is still at the initial stage. In this thesis the research about marine VAV A/C systems was carried out, The features of marine VAV A/C systems were analyzed firstly. On this basis, an experimental rig was built. The in-depth theoretical analysis and experimental study on the operating characteristics and supply air control of a marine VAV A/C system were then conducted.In the marine VAV A/C system, a direct expansion (DX) evaporator is used as cooling coil. The VAV air-distribution system is directly connected with the refrigerating unit, without intermediate chilled water system. System capacity is relatively small and the number of VAV terminals in the system is few. Hence the regulation and variation of damper opening in terminals are comparatively frequent, and the system load and total supply air flow rate have a large varying range. The controls of system's air-side and refrigerating-unit side directly interfere with each other. In this system a tube-in-tube type condenser with sea water cooling is adopted.The marine VAV A/C experimental rig is made up of a VAV air-conditioning system, and a measuring and acquisition system. The VAV air-conditioning system consists of a VAV air handling system equipped with VAV terminals and a direct expansion refrigerating unit which evaporator is directly used for air cooling and dehumidifying. The measuring and acquisition system is composed of data acquisition instrument, various sensors and a central data-processing computer, etc.Using this experimental rig, an experimental study was carried out to firstly investigate the operating characteristics for the marine direct expansion VAV A/C system. The system's cooling-down performance and its responses to step-changes of the cabin setting air temperatures and inner cooling load were examined. The changes of such parameters as cabin temperature, VAV terminals'opening, supply air temperature, fan operating frequency, evaporating temperature, condensing temperature, compressor operating frequency were analyzed. The performance of VAV terminals, variable-frequency fan and compressor was studied. The experiment results showed that the marine VAV A/C system could achieve good and independent control for air temperatures of air-conditioning cabins. A comparative study for the marine VAV A/C system working with two different supply air pressure/flow rate control methods was made. One method was that the supply air static pressure/ flow rate in the system was regulated by a variable-frequency fan, the other by a static pressure control unit. The cabin temperatures, other various parameters respectively for air handling side and refrigeration side, especially the system's power consumption were compared in detail. It can be concluded from the comparative study that both methods of supply air static pressure control could achieve desirable control results. The marine VAV A/C systems equipped with a variable-frequency fan to control supply air static pressure is more energy-saving than that with a static pressure control unit.