Development and testing of a multi-type air conditioner without using AC inverters

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Abstract

This paper reports the results from the development and performance testing of a cost effective, energy efficient, multi-type air conditioner that connected five indoor units (evaporators) to one outdoor unit (condenser) with a digital scroll compressor. Instead of using inverter technology, which has a potential risk of harmonic current emissions, this study used a digital scroll compressor, which provided the variable refrigerant volumes. The measured results for this innovative design showed: (1) The relationship between the degree of opening of the electronic expansion valves (s) and the compressor output ratio (%) could be represented by regression functions, which formed the basic parameters of the system control. (2) The developed system provided true zoning capability because it could run indoor units under part load conditions, therefore wasting little energy. The power consumption of the developed system was reduced from 100% to 25% when the full load was reduced to a partial load of 17%, saving more than 75% of the work required using a conventional un-loading method. (3) The developed system has a broader range of capacity output (from 17% to 100%) than that of an AC variable frequency control system (from 48% to 104%). The developed system cost 20% less than a comparable system with an AC inverter.

Introduction

Multi-type air conditioners have been employed in small and medium sized buildings recently due to their several benefits over a conventional air conditioning system, such as the window unit type and a duct type system. Their advantages include easier system maintenance, diversification of facility use and, most importantly, precise capacity control, which not only conserves energy but also provides the possibility of an intelligent building control. Multi-type air conditioners have been popular in Asia and much of Europe for many years, and they are gaining popularity in the US recently, primarily in retrofit situations in which ductwork cannot be installed. However, some people think that multi-type systems have high initial costs and harmonic current emissions (hmc) because they use inverter technology, which in some situation, may distort the electric sine wave and emit electronic noise into the electric system, damaging the electronic equipment. Accordingly, the development of a multi-type air conditioner without inverters and, consequently with a reduced cost, is an important goal of manufacturers of air conditioners. Little literature addresses the multi-type air conditioner. Fujita et al. [1] examined the capacity and refrigerant flow control for a multi-type air conditioner that used a twin rotary compressor with two indoor units and EEVs (electronic expansion values). They also reported some means to improve thermal comfort and save energy. Okuzawa [2] reported the optimization of the variation in superheating of a multi-type air conditioner with a horizontally installed twin rotary compressor, which included four to six indoor units. Several papers addressed the performance of an air-conditioner with a variable speed compressor [3], [4], [5]. However, none of them elucidated the relationship between the performance of the electronic expansion valve (EEV) and the compressor output. Recently, Park et al. [6] conducted a simulation analysis for a multi-type inverter air conditioner with a linear electronic expansion valve and a variable speed compressor. A further study reported by Choi and Kim [7] focused on the capacity modulation for an inverter driven, multi-type air-conditioner having two indoor units. They suggest that maximum cooling capacity can be achieved when the superheat is maintained at 4 °C. As far as the author is aware, no paper has been published regarding a multi-type air conditioner without the inverter technology. In view of the above issues, this study utilizes a variable refrigerant volume scroll compressor for the multi-type air conditioning system. The compressor is able to operate at 10–100% capacity and at relatively low cost without using the inverter. Fig. 1 depicts the capacity control of the screw compressor by on/off switching of the solenoid valves to change the position of a static scroll, thus providing variable refrigerant output. Fig. 2 shows the tested relationship of the compressor output ratio to frequency.

Section snippets

System development and setup of the test rig

Key works related to the development and setting up of the test rig of the multi-type air conditioner are discussed below.

Degree of opening of EEVs and the compressor output ratio

The most important task in developing the system was to identify the relationship between the degree of opening of the EEVs and the compressor output ratio (%). The relationship between the degree of opening of the EEV and the output capacity (%) of a compressor for a particular indoor unit capacity was determined. The outdoor unit has a nominal load of 12,000 kcal/h. The indoor unit has six capacity settings, 2000, 2500, 3150, 3550, 4500 and 5000 kcal/h. An indoor unit with a capacity of 3150

Conclusions

The variable capacity scroll type compressor was successfully applied to the multi-type air conditioner without using inverter technology. Key works related to the development and setting up of the test rig of the multi-type air conditioner are discussed. The developed system with one outdoor unit and the outdoor unit can handle up to five indoor units. This work established a technique for controlling the electronic expansion valve as the load varies, following laboratory tests with various

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