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Advantages And Disadvantages Of Reciprocating Compressors

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For lower flow rates during holding mode, reciprocating compressors are usually a good fit because of their efficient turndown capabilities. They cater to a wider range of flow rates than any other compressor type.
For large scale terminals, to cater higher BoG flow rate during unloading, centrifugal compressors are appropriate. Multiple (2 or 3) compressors are installed in parallel so that shutdown of any one of them does not hamper the operation. Depending on the tank type, reciprocating compressors can be used if the tank is concrete roof tank. Those tanks generally are operated at higher pressures and therefore, vapour generation is significantly low and hence reciprocating compressors can be used even for unloading mode.
Composition
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The simple and modular design of SCV’s make capital cost of SCV’s much lower than ORV’s. But, operating cost of SCV’s is much higher than ORV’s because of use of natural gas as a fuel. Typically about 1.5% of the vaporized LNG is used up for running SCV’s.
 Shell and tube vaporizers
Shell and tube vaporizers (STV) involve 2 levels of heat transfer, where, first level is heat exchange between LNG and intermediate fluid like propane and the second level is heat exchange between intermediate fluid and sea-water. The surface areas of the heat exchangers are optimized to have maximum heat exchange. The major advantage of STV’s is, they prevent freezing (unlike ORV’s) and reduce fouling risks. But of course, they are costly because of use of intermediate fluid like propane.
 Ambient air vaporizers
The emissions because of SCV’s and unknowns associated with ORV’s has made companies focus on something which is reliable and cheap and renewable and hence, companies are focussing on ambient air vaporizers which use ambient air as a source of heat transfer. The technology of using ambient air for LNG vaporization was there in the market before, but for smaller scales. The recent advancements has made it possible to use it at larger scales as well. The impact of temperature and climate is an important factor for designing ambient air vaporizers. The technology is considered ‘green’ because it has no emissions and makes use of solar heat stored in air as a heat transfer
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