Mitsubishi Heavy Industries, Ltd. has studied the improvement of the dynamic characteristics of the absorption refrigerator, has developed a dynamic simulation program for a double effect absorption refrigerator and has selected the PID control parameters by this program. PID means to combine a proportional, integral and differential action. They have applied this simulation program to the current refrigerator and improved the control characteristics and reliability. This dynamic simulation program is able to calculate the transitional quantity of state, a pressure, temperature, concentration, liquid level in each component of a double effect absorption refrigerator starting, stopping and changing the cooling load. For example, they took place the operating simulation of the parallel flow typed double effect absorption refrigerator in which the lithium bromide aqueous solution flowed from an absorber to both the high and low pressure regenerators as shown in Fig. 7.5. As a result, the calculation values almost agreed with the measured values on the control characteristics of the refrigerator, a refrigerating capacity, a liquid temperature at each part of it and so on. They confirmed that this program could be the good tool for evaluation and improvement of characteristics of these absorption refrigerators. Secondary, they applied this program to the improvement of the control characteristics of the current refrigerator. As a result, they found that the PID control method was the best effective to maintain the outlet cold water temperature constant in spite of any value of the inlet cold water temperature. They confirmed to be able to control the refrigerator better for any load by the optimization of the PID method.
Fig. 7.5 Flow diagram of parallel flow typed double-effect absorption refrigerator
As mentioned above, the absorption refrigerator can be driven by the low temperature heat source such as the exhaust gas of a diesel engine. This dynamic simulation program can improve easily for its operating characteristics to apply to many uses. If a ship has thus improved absorption refrigerator on board, she will get many energies saving.
7.5 Development of Compressors for Alternative Refrigerant R-410A in Refrigerators5), 6)
Now R22 is used as the refrigerant of the room air conditioner. Such HCFC' s (Hydro-Chloro-Fluoro-Carbon) will be phased out at the end of 2020. The following two companies have developed the refrigerators using the alternative refrigerant R410A (R32/R125=50/50 wt%). The R410A belongs to a kind of HFC (Hydro-Fluoro-Carbon) and has 0 of the Ozone Depleting Potential. As the R410A's pressure at a temperature is 1.6 times higher than the R22 at the same temperature, the structure of the R410A refrigerator needs to be strong against the high pressure. R410A is not compatible with the current refrigerating machine oil (the mineral oil) used with R22. It is very important to select the optimum lubricating oil for the refrigerator. Both companies have revolved these problems. They have developed more efficient and reliable refrigerators than the current R22 refrigerator. By these developments, it is expected that the development of the refrigerator using the alternative HFC refrigerant will be accelerated in the section of the marine air conditioning.
The outlines of the technical developments of each company are as follows.
(1) Mitsubishi Heavy Industries. Ltd. has developed the new scroll compressor for the air conditioner “Beaver” using the alternative refrigerant R410A. (Please refer to Fig. 7.6.) The developed scroll compressor for R410A had higher reliability and efficiency than the compressor for R22 by the following technologies. The scroll profile, which had the 0.7 times' inlet volume than for the R22 compressor, was evaluated and optimized. The scroll compressor was equipped with the multiple-discharge-ports in order to prevent the growth of the super compression at lower compression ratio in use than the designed compression ratio. It was adopted the double drive outlet pressure method, which is to extract the middle pressure gas at the same angle to the setting angle of the multiple-discharge-ports, in order to push the moving scroll to the fixed scroll by the minimum force and make air tight so that the gas would not escape.
