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1. A power-saving vacuum pump system, comprising: an input valve at an input end of a the power-saving vacuum pump system for receiving a gas mixture of saturated water vapor and non-condensed air from a condenser of a power plant; a first root vacuum pump connected to the input valve, receiving the gas mixture from the input valve and then compressing the gas mixture, and then transferring the compressed gas mixture out; a second root vacuum pump connected to the first root vacuum pump, receiving the gas mixture from the first root vacuum pump and then compressing the gas mixture, and then transferring the compressed gas mixture out; a third root vacuum pump connected to the second root vacuum pump, receiving the gas mixture from the second root vacuum pump and then compressing the gas mixture, and then transferring the compressed gas mixture out; wherein each of the first root vacuum pump, the second root vacuum pump, and the third root vacuum pump comprises a casing having an inlet and an outlet; an interior of the casing being formed with a vacuum chamber and two bearing chambers at two sides of the vacuum chamber; the vacuum chamber being connected to the inlet and the outlet; a driving shaft being installed within the casing and penetrating through the vacuum chamber and the two bearing chambers; one end of the driving shaft passing out of a right wall of the casing; a blade set being installed within the vacuum chamber and arranged around the driving shaft; wherein by rotation of the blade set, the gas mixture enters the vacuum chamber and is compressed; wherein inner connection walls between the vacuum chamber and the two-bearing chambers comprise respective bearings arranged around the driving shaft; wherein an opening is formed in the right wall of the casing and comprises another bearing arranged around the driving shaft; wherein the bearings support the driving shaft and completely seal spaces between the driving shaft and the inner walls of the casing so that the vacuum chamber is completely isolated from the two bearing chambers, liquid in the two bearing chambers cannot permeate into the vacuum chamber, and the gas mixture in the vacuum chamber cannot enter into the bearing chambers; wherein, in operation, the vacuum chamber of the first root vacuum pump receives the gas mixture from the input valve and liquid within the bearing chambers cannot drain out of the casing; wherein the inlet of the second root vacuum pump is serially connected to the outlet of the first root vacuum pump and the inlet of the third root vacuum pump is serially connected to the outlet of the second root vacuum pump; wherein each of the first root vacuum pump, the second root vacuum pump, and the third root vacuum pump is capable of withstanding an inlet pressure of 5000 Pa to 30000 Pa under a condition that the condenser is in a vacuum state and a pressure difference larger than 5000 Pa; wherein each of the first root vacuum pump, the second root vacuum pump, and the third root vacuum pump operates at temperatures higher than 130° C.
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