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Heat pump unit TECHNICAL REPORTS Hot water tank unit air-conditioners) indicated low wear resistance when CO2 refrigerant was used and failed to sat- isfy the required service life, so we more than doubled the load bearing characteristic and veri- fied that the service life was sufficient. 4.2 High-efficiency water-to-refrigerant heat ex- changer In addition to the compressor, this water heater system features Mitsubishi’s new technology for heat exchangers. In the water-to-refrigerant heat exchanger (also referred to as gas cooler in water heaters) which heats water for hot-water supply to users, we used a newly developed “twist & spiral gas cooler”. As shown in Fig. 3, the heat exchanger uses a twisted pipe with three lines of spiral grooves for a water pipe, which are prepared by a twisting method. A refrigerant pipe (thin pipe) is wound around the twisted pipe and connected to each other by zinc-free soldering for heat conduction. This arrangement increases the contact surfaces on both pipes, thus increasing the heat transfer area. The continuous spiral grooves in the twisted pipe accelerate the turbulence effect in the water and also help reduce pressure loss in the heat exchanger. Furthermore, three pipes are wound around the three spiral grooves in parallel to reduce the pressure loss in the refrigerant. Water pipe (Twisted pipe) Refrigerant pipe Fig. 3 Appearance of water-to-refrigerant heat exchanger Since pressure loss is minimum in both the water pipe and refrigerant pipe, the efficiency of the com- pressor and heat transfer efficiency are improved, and the input to the circulation pump can be reduced. The features of the heat exchanger are described below. Figure 4 compares the heat transfer efficiency be- tween Mitsubishi’s twisted-pipe type heat exchanger and other types of heat exchangers. The figure clearly indicates that the twisted-pipe type has a far higher (more than double) heat transfer efficiency. Fig. 1 Appearance of heat pump water heater ECO CUTE 4.1 High-efficiency rotary compressor The heat pump unit serving as the heat source of the water heater uses a rotary compressor whose performance has been proved in Mitsubishi air-conditioners. The HP water heater is a high-pressure shell type rotary system using a Poki Poki Motor for the first time in the industry; its perform- ance and quality ensure sufficient reliability of the HP water heater. The key technologies of the compressor are as follows. (1) Use of high-efficiency Poki Poki Motor Mitsubishi’s original Poki Poki Motor, with its high coil density and high efficiency, is used in the com- pressor of this water heater. Figure 2 shows the stator of the motor. Fig.2 Appearance of Poki Poki Motor The stator core of the Poki Poki Motor is open for coil winding, so the winding process is not spatially constrained and dead space is minimized. With a 20% higher line volume ratio (the space density of the copper coil) than conventional motors, the motor efficiency is 2% higher. (2) Use of wear-resistant vanes (2) Since CO2 refrigerant requires a very high operat- ing pressure (11.3 MPa), the reliability of sliding parts was expected to be vulnerable, so we coated the surface of sliding parts with super-hard DLC-Si (DLC containing silicone), thus improving reliability. The conventional nitrided vanes (for Mitsubishi Electric ADVANCE December 2007 7PDF Image | Heat Pump with Natural Refrigerants 3041
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