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Nitrogen cycle

MAN B&W ME-C series for LNG Carriers. Nitrogen cycle

The cryogenic temperature inside the cold box is produced by means of a nitrogen compression-expansion cycle, shown in Fig. 3. Nitrogen gas at a pressure of 13.5 bar is compressed to 57 bar in a 3-stage centrifugal compressor. The gas is cooled by water (seawater or indirect) after each stage. After the last cooler, the gas is led to “warm” part of the cryogenic heat exchanger where it is pre-cooled to about -110oC and then expanded to a pressure of 14.5 bar in the expander. The gas leaves the expander at about -163oC and is then introduced into the “cold” part of the cryogenic heat exchanger where it cools and reliquefies the boil-off gas to LNG.

Fig. 3. N2 compressor/expander

The nitrogen then continues through the “warm” part of the cryogenic heat exchanger before it is returned to the suction side of the 3-stage compressor.
The N2-compressor/expander unit is a three-stage integrated gear centrifugal compressor with one expander stage. The unit has a gear with 4 pinions where each of the 4 wheels is coupled to a separate pinion. The result is that the expander work goes directly into the gearbox and relieves the electric motor.
The advantages of this solution are:

More compact designReduced costImproved control of the refrigerationReduced power consumption.
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