| Hydraulic energy recovery device for application in the reverse osmosis desalination system can be clasified into two categories by their working principle, the centrifugal type and the positive displacement type. The centrifugal type, also named hydraulic turbine type has an efficiency between 30% and 50%. The efficiency of the positive displacement type which is more used in desalination plants now is as high as 90% or above. It has become a research focus of this field. The valve-controlled energy recovery device (VC-ERD) designed in our research project is of the positive displacement type. It uses valves and cylinders to accomplish the energy exchange process. The flow diagram to test the VC-ERD was given for the first time. Corresponding experimental apparatus with valve controlling system and data acquisition system was set up. Two cyclinders made of organic glass with a length of 1000mm, internal diameter of 50mm were included in the experimental diagram. A high pressure centrifugal pump and a low pressure centrifugal pump were used to supply water from two corresponding tanks. The programmable logic controller (PLC) combining the photoelectric switches was used to control the movements of the valves sequentially. The controlling schemes of the manual operation, the single-cylinder operation, the double-cylinder operation, and two single-cylinder coupling operation were developed respectively. Dynamic characteristics of the unit under different controlling schemes were measured and analyzed for the first time. The experimental results indicate that there always exists colluding problem between the high pressure (HP) concentrate and the depressed concentrate, whether it is under single-cylinder or double-cylinder operation with the standing time remaining zero seconds. In our experiments, the colluding problem can be dealt with successfully when standing time of the process is 1.5 seconds. At this time, for the single-cylinder operation, the flow and the pressure curves of the HP concentrate and the low pressure (LP) seawater vary like a rectangular wave. The flow rate variation of the pressurized seawater is similar to that of the HP concentrate, while its pressure keeps constant mostly. For the double-cylinder operation, the flow curves of the HP concentrate, the LP seawater and the pressurized seawater present a downward variation periodically, while their pressure curves show an upward variation periodically for the former two and a horizontal linear variation for the later. Comparing to the double-cylinder operation, the changing frequencies of the flow and the pressure curves of the HP concentrate, the LP seawater and the pressurized seawater reduce to about a half under the two single-cylinder coupling operation. Pressure curves of the cylinder end show a rectangular wave variation under the three controlling schemes mentioned above. Studies on the energy transfer efficiency of the unit under the experimental conditions indicate that the volumetric efficiency of the VC-ERD keeps 100% mostly, the pressure of the concentrate has a positive effect and that of the seawater a negative effect on the hydraulic efficiency. The hydraulic efficiency will be 90% when the concentrate pressure gets to 0.9 MPa. It is believed that the energy transfer efficiency is sure to be over 90% when the VC-ERD is used in seawater or brackish water reverse osmosis desalination processes. Other aspects such as the slip distance of the piston, the cyclic frequency and the pressure differential distributions of the process were studied and discussed at the same time. The method to simulate the mixing processes in the cylinder between the high pressure concentrate and the seawater feed was established for the first time. The mixing length in the cylinder which is the basic data for the design of the fluid piston, and its influencing factors were studied and analyzed through the fluid dynamis analog calculation. It has been found that the mixing length can be added by increasing the diameter ratio or the concentrat...
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