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Three Categories of BOG Recovery Devices According to the Reliquefaction Cycle Method

Three Categories of BOG Recovery Devices According to the Reliquefaction Cycle Method

The reliquefaction device contains the various components of the refrigeration system, which is mainly composed of a BOG compressor, a liquefaction condenser, a non-condensable gas separator, a heat exchanger, and an intercooler.


According to IMO regulations, the capacity of the reliquefaction device must be able to maintain the vapor pressure in the cargo tank below the set pressure of the pressure relief valve under extreme working conditions (usually the air temperature is 45°C and the seawater temperature is 32°C).


1. Direct cooling cycle of two-stage BOG compressor


A two-stage BOG compressor is used in this system, which can reduce power consumption and improve efficiency. The intercooler can lower the suction temperature of the second stage compression to facilitate the stable operation of the system.


2. Compression and refrigeration cycle of cascade BOG compressor


The cascade compression and refrigeration cycle is a system composed of a single-stage BOG compressor and an R22 refrigeration compressor, vapor cooler, throttle valve, etc. It can ensure that the system has a proper vapor pressure at a low temperature, and can meet the proper condensation pressure when the vapor is condensed under ambient temperature conditions. From the perspective of ship transportation and system operation, this reliquefaction system can better meet the requirements. In addition, the reliquefaction capacity of the system is not greatly affected by the changes in seawater temperature, and the operating conditions are stable; and since the temperature of the cooling working medium R22 bog recondenser can be lower than 0°C, the reliquefaction efficiency is high.


3. Indirect reliquefaction cycle


It is mainly used on chlorine transport ships and special liquefied gas and chemicals multi-purpose ships. From the perspective of reliquefaction efficiency and system stability, the two-stage compression direct cooling cycle, cascade compression, and refrigeration cycle can better meet the above requirements.


4. Low temperature BOG compressor


As the core component of the system, whether BOG compressor works well or not directly affects the operation of the entire system. BOG compressors produced by oil and gas compressor manufacturers working on LNG tankers generally have an air intake of -80 to -160℃ and a working pressure of 0.4 to 6.4Mpa. Parts in contact with low-temperature gas must use corresponding low-temperature materials, and the system must also be equipped with a temperature controller. 

At the same time, from the consideration of operating economy, BOG compressors are required to have a large refrigeration capacity per unit volume, a low compression ratio, and ensure reliable operation of various components under low temperature conditions. In order to prevent gas leakage at low temperatures, the shaft seal performance of BOG compressors should be good; the drive motor should be installed in the unit's motor room, separated by an airtight cabin and a compressor room. The compressor shaft passes through the airtight compartment and is connected to it by a coupling. The lubricating oil should have good flow properties under low temperature conditions.

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