Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)23

Small and High-Efficiency
3-HP DC Twin Rotary Compressor
SHIGEKI MIURA*1

IKUO ESAKI*2

MAKOTO OGAWA*2

TETSUO KANIE*3

HAJIME SATO*4

TAICHI TATEISHI*4

Mitsubishi Heavy Industries Ltd. (MHI) previously developed a small and highly efficient DC
twin rotary compressor for R410A refrigerant, for use in 4 to 6-HP air conditioners for commercialuse. This paper describes the company’s new 3-HP DC twin rotary compressor, which wasdesigned to be even smaller and more efficient.

|1. Introduction
MHI is continuously improving the efficiency of compressors for air conditioners to saveenergy and reduce annual energy consumption.1 In recent years, there is increasing demand forsmaller outdoor units of the air conditioning system to save resources and allow for easierinstallation. In response to this demand, one technology trend within the industry involves
expanding the capacity of compressors that are smaller than conventional ones. MHI recently
developed a 3-HP DC twin rotary compressor for R410A refrigerant; this compressor is smallerand more efficient than competing products. This paper provides an overview of this newcompressor.

|2. Features of the MHI Twin Rotary Compressor
2.1

Smaller size

Figure 1 compares the external dimensions of MHI's newly developed 3-HP twin rotarycompressor with competitors. MHI's new compressor has an external diameter smaller than thecompetitors’ equivalent DC twin rotary compressor, resulting in a smaller size and lighter weight,
which enhances ease of installation in the outdoor unit and saves resources.

Figure 1

Comparison of external dimensions*1

Engineering Manager, Air-Conditioner Designing & Engineering Department,*2

Air-Conditioner Designing & Engineering Department, Air-Conditioning & Refrigeration Systems Headquarters*3Research Manager, Nagoya Research & Development Center, Technical & Innovation Headquarters*4

Nagoya Research & Development Center, Technical & Innovation Headquarters

Air-Conditioning & Refrigeration Systems Headquarters

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)24

2.2

Increased efficiency

Figure 2 shows the internal structure of MHI's newly developed 3-HP twin rotarycompressor.

Figure 2 Internal structure of the twin rotary compressor

With regard to the compression mechanism, the crankshaft diameter has been reduced from20 dia. in previous 3-HP compressors to 16 dia., resulting in less mechanical loss at the bearings.
The upper and lower cylinder widths have been reduced by approximately half overall, as
compared to previous single rotary compressors, and the bores have been expanded, reducing
leakage loss and increasing capacity.
In previous MHI rotary compressors, the compression mechanism was fixed to thecompressor housing by spot-welding the upper cylinder. Reduction of the width and increase of thebore of this cylinder meant it would lose its rigidity, which could result in thermal deformation due
to welding, leading to negative effects with regard to compressor efficiency and reliability.
Therefore, MHI has adopted a new structure in which the outer circumference of the upper bearing
is welded onto the housing. Figure 3 shows an example FEM analysis of cylinder deformation due
to welding. Deformations of the blade holding groove and the cylinder bore were observed in theprevious structure in which the cylinder is welded, but no cylinder deformation appears in the newstructure in which the upper bearing is welded. Therefore, the new process eliminates thepossibility of reduced efficiency and reliability.

Figure 3

Example FEM analysis of the cylinderMitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)
25

With regard to the motor, which is strongly related to compressor efficiency, MHI has
adopted a concentrated winding DC motor with a rare earth permanent magnet, which is more
powerful and efficient than the original distributed winding DC motor with a rare earth permanentmagnet. To improve the Annual Performance Factor (APF), a detailed magnetic field analysis wasexecuted to facilitate the design of a more efficient low speed section in particular; this improved
efficiency by approximately 4%, as compared to the original distributed winding DC motor withequivalent power.
Figure 4 compares efficiency between a concentrated winding motor and a distributed
winding motor.

Figure 4 Comparison of motor efficiency

Through these technologies, the new compressor is 4% more efficient than the MHI twinrotary compressor equipped with a distributed winding motor, as shown in Figure 5, enabling
greater efficiency than those of competitors.

Figure 5 Comparison of compressor efficiency

2.3Improved reliability

Although a reduced crankshaft diameter can effectively reduce mechanical loss, it is also
more likely to cause abnormal wear, such as abnormal bearing wear and seizure, as a result of
increased shaft deformation. As a countermeasure, MHI has adopted a highly rigid shaft structure(Figure 6), similar to previously-developed 6-HP compressors. In particular, the cross-section of
the intermediate shaft between upper and lower crank pin is shaped such that it minimizes
deformation in the direction in which the maximum reactive force of refrigerant gas compressionacts.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)26

Figure 6

Highly rigid crankshaft

Refrigerant oil plays an important role by lubricating sliding parts and sealing thecompression mechanism. It circulates in the refrigeration cycle together with a certain ratio ofrefrigerant, which is an operating fluid. The weight percentage of the oil circulating in the cycle
against the sum of the oil and refrigerant is called the oil circulation ratio, which increases inproportion to the refrigerant mass flow rate and the compressor revolution speed. If refrigerant oil
flows out to the refrigeration cycle, it hampers heat transfer in the heat exchanger, reducing cycle
efficiency and reduces the retained oil level inside the compressor, thereby decreasing its ability tolubricate the sliding parts of the compressor.
MHI has therefore adopted a structure in which the oil is separated in the chamber above themotor to control increases in the oil circulation rate. The mixture of refrigerant and oil mist, whichmoves up the refrigerant gas path connecting the chambers above and below the motor, flows outof the compressor through the bottom of the discharge pipe located at the upper center of thecompressor. The oil separation plate (Figure 7) is installed on the upper part of the motor rotor,
closest to the bottom of the discharge pipe. Table 1 shows the effectiveness of the oil separation
plate: it reduces the maximum oil circulation ratio to less than half that without an oil separation
plate.
Table 1 Effectiveness ofthe oil separation plate onoil circulation ratio
Rate (-)
No oil separation plate

1.00

Equipped with oil separation plate

0.46@120 rps maximum cooling condition

Figure 7Structure of oil separation plate|3. Conclusion
The 3-HP DC twin rotary compressor for R410A refrigerant, developed by MHI, has
successfully achieved the smallest size and highest efficiency in its class due to combination of theexisting MHI technologies (from the development of the 6-HP compressor) and new technologies.
This newly-developed compressor is scheduled to be mass-produced by MHI's foreign
related company.

Reference
1.

Fujita et al., The Smallest and Most Lightweight High-Efficiency 6-HP DC Twin Rotary Compressor inthe Industry, Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 45 No. 2 (2008) pp.19-22