The invention is based on the device indicated and described in NO 305525 and the international application no. N098/000888 belonging to the same applicant. It should be understood that the presentation of this patent publication or international application should form part of the presentation of the invention and consequently in the following description of the invention, only those components of the international application's device which are directly connected with the invention need to be described.

The object of the invention is to improve the above-mentioned device in order to achieve an optimal operation thereof.

According to the invention this object is achieved by providing between the tank and the compressor an additional heat exchanger which is similarly supplied with coolant from the cooling plant, and which cools, but does not condense the boil-off flowing from the tank to the compressor, the coolant being added to the additional heat exchanger via a controllable regulator valve, by providing between the additional heat exchanger and the compressor a temperature sensor, which senses the temperature of the boil-off, and by the regulator valve being controlled by a control device which is supplied with signals from the temperature sensor for controlling the flow of coolant to the additional heat exchanger so that the boil-off from this heat exchanger to the compressor is held at a constant temperature.

The invention will now be described in greater detail with reference to the drawing, which illustrates an embodiment of a device according to the invention.

The single figure is a connection diagram schematically illustrating an embodiment of a plant for storage or transport of LNG with a device according to the invention.

As illustrated in the figure, the plant comprises a tank 2 containing LNG 4 whose surface is indicated by reference numeral 6. The tank space above this surface 6 communicates via a pipeline 8 with the inlet of a first compressor 12 driven by a motor 14.

The outlet of the compressor 12 communicates via a pipeline 16 with the inlet of a first passage 18 of a first cooler or heat exchanger 20. In the heat exchanger a

second passage 24 is provided, whose inlet is supplied with a refrigerated coolant from a cooling plant 26 constructed in a standard per se manner via a pipeline 30, and whose outlet returns the heated coolant to the cooling plant 26 via a pipe 28.

The cooling plant 26 comprises a second compressor 58 driven by a motor 60. On the downstream side of the second compressor 58 an intercooler 62 is provided on whose downstream side is mounted a third compressor 64.

The inlet of the second compressor 58 communicates with the outlet of a first passage 66 of a second cooler or heat exchanger 68 via a pipeline 70.

The outlet of the third compressor 64 communicates with the inlet of a second passage 72 of the second heat exchanger 68 via an intercooler 48 and a pipeline 73, and the outlet of the second passage. 72 of the second heat exchanger 68 communicates with the inlet of an expansion turbine 74, whose outlet is connected with the inlet of the second passage 24 of the first heat exchanger 20 via the pipeline 30. The third compressor 64 is driven by the expansion turbine 74.

According to the invention boil-off flows from the tank 2 to the first compressor 12 via a first passage 111 of an additional or third cooler or heat exchanger 110 and a pipeline 113 which is provided between this heat exchanger 110 and the first compressor 12.

A pipeline 100 is connected to the outlet of the second heat exchanger 68 and is connected to the inlet of a second passage 112 of the additional heat exchanger 110 via a regulator valve 104 and a pipeline 108.

The outlet of the second passage 112 of the additional heat exchanger 110 is connected via a pipeline 114 with the inlet of the second compressor 58 and/or via a pipeline 116, which is indicated by a dotted line, with the pipeline 28 connecting the second passage 24 of the first heat exchanger 20 with the first passage 66 of the second heat exchanger 68.

For sensing the temperature of the boil-off in the inlet of the first compressor 12, on the pipeline 113 between the additional heat exchanger 110 and the first compressor 12 there is mounted a temperature sensor 120, which is arranged to provide signals concerning the value of this temperature to a control device 12. The control device 121 in turn is arranged to provide signals to the regulator valve 104 for controlling the flow of coolant from the cooling plant 26 to the second passage 112 of the additional heat exchanger 110 in order thereby to attempt to keep the temperature of the boil-off in the pipeline 113 constant.

It will be appreciated that the temperature sensor 120 may be arranged to transmit electrical signals to the control device 121 which in turn can transmit electrical. signals to the regulator valve 104, but the temperature sensor and the control device may instead, e. g. , be mechanical and arranged for mechanical control of the regulator valve.

Even though the temperature of the boil-off flowing into the pipeline 8 may vary much, it is therefore ensured by means of the invention that the boil-off at the inlet of the first compressor 12 is kept at a stable, low value, thus ensuring optimal operating conditions for the plant.