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Abstract: A direct-expansion (DX) variable-air-volume (VAV) air-conditioning (A/C) system consists of a VAV air-distribution sub-system and a DX refrigeration plant. This paper reports in detail on a novel capacity controller developed for the DX VAV A/C system to regulate its compressor speed and hence its cooling capacity. The capacity controller consisted of both a numerical calculation algorithm (NCA), which was fundamentally based on the principle of energy balance using a number of real-time measured system’s operating parameters, and a dead-band for decoupling the control actions from both the capacity controller and a conventional PI feedback controller for regulating the opening of an electronic expansion valve (EEV) in the refrigeration plant. To study the feasibility of the capacity controller, an experimental rig for the DX VAV A/C system having two conditioned spaces was built and experimental tests were carried out. The test results showed that using the capacity controller, the cooling capacity of the system’s refrigeration plant can be accurately and continuously regulated and the supply air temperature well maintained at its desired value. The desirable independent zoning-control for space air temperatures can be successfully achieved by the DX VAV A/C system and the control performance for air temperatures in the conditioned space was highly satisfactory. [Copyright &y& Elsevier]

Abstract: The refrigerant mass flow distribution in a two phase fluid network of multi-connected air conditioning units (MAU) is generally uniform in the available literature based on the assumption that the fluid resistance of the pipes connecting the evaporators is ignored. However, such a fluid network of multiple evaporators and connecting pipes is actually a complex two phase direct return system, which leads to refrigerant flow distributary disequilibrium, for which few investigations have been conducted. In this paper, with pipe and local fluid resistance considered, fluid network theory was employed to develop a MAU fluid network model as a series of fluid circuits of components including single phase and two phase pipes, two phase electronic expansive valve (EEV), evaporator and pipes in parallel or series. A particular iterating control algorism was designed to distribute the refrigerant mass flow in proportion to the corresponding fluid resistance. For a given refrigerant fluid network with five indoor units, it is concluded by the model that the closer an indoor unit is to the centre of the fluid network, the less its mass flow deviates from its nominal value, and the units at the greatest distance from the centre of the fluid network are the ones with maximum disequilibrium. Furthermore, for a fluid network with multiple indoor units, an IDDR was proposed to evaluate the refrigerant distributary disequilibrium. [Copyright &y& Elsevier]