Floating Ball Blankets

There are two simple alternatives to vapour cooled baffles which have been tested, namely plastic foam plugs and floating ball blankets, but both are not so effective. 

In the original work on vapour cooled baffles published in 1965 [2], expanded polystyrene or polyurethane foam plugs were demonstrated to be as effective as horizontal baffles and this finding led directly to the widespread use of foam plugs. However, subsequent work, pubUshed in 1969 [3], demonstrated clearly that foam plugs become unreliable and ineffective insulators after continuous exposure to boil-off gas over a few days. 

In other words, when the low thermal conductivity, high molecular weight, foam gas wifliin the plastic foam is replaced by diffusion by high thermal conductivity, low molecular weight methane gas, the foam plug becomes ineffective, behaving as a thermal short circuit in the vapour space. 

This fliermal short circuiting effect can be easily detected by simply removing the foam plug, when the system will have a lower boil-off rate  

If a set of horizontal, low emissivity metal baffles replaces the foam plugs, a significantly lower boil-off rate will result This finding is cost effective although it seems to have been forgotten. 

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NOTE Many closed-loop cooling water systems are not, in fact, closed, as they contain open-top, return-water tanks. This provides a continuous source of oxygen from the air into the system, with the result that the risk of corrosion greatly increases. Adequate attention should be paid to countering this risk, perhaps by an improved corrosion-inhibitor treatment or a floating blanket or plastic balls in the water tank (which help to minimize the ingress of air). 

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