Vapour cooling

A road tanker was loaded with l-chloro-2,3-epoxypropane and then driven 250 miles overnight to the delivery point. On arrival, the contents were found to have self heated (undoubtedly from polymerisation initiated by some unknown contaminant) to the boiling point (115°C at ambient pressure) and soon afterwards the relief valve lifted and discharged large volumes of vapour. Cooling with water sprays eventually restored thermal control over the remaining tanker contents [1]. The material is incompatible with strong acids, caustic alkalies, zinc, aluminium, aluminium chloride or iron(III) chloride, all of which catalyse exothermic polymerisation [2],... 

Inner vapour cooled shield Middle vapour cooled shield... 

Ferric selenide or iron sesqui-selenide, Fe2Se3, was obtained by Little 2 by heating iron to redness in selenium vapour, cooling, and fusing the product with excess of selenium under a layer of borax. It also results when hydrogen selenide is passed over ferric oxide at red heat.3 It is a grey crystalline powder. 

Heat transferred from vapour = Cooling water flow =... 

Stop the reactions in the unit—which can generate heat, pressurise the contents, or generate toxic/inflammable vapours. Cooling operation may not be stopped (if it will not choke the nozzles due to solidification) well before opening it for maintenance. 

Evaporate to dryness on a steam bath (caution flammable vapour). Cool and dissolve the residue in 3 ml 9 1 chloroform-butanol-l-ol. 

After combustion heat the solution to boiling, add 5 ml of 50 per cent sodium formate solution and again boil for a short time. Draw air through the flask to remove chlorine vapours, cool and dilute to about 100 ml with water. Add 1 g of potassium iodide, 25 ml of 6N sulphuric acid and 1 drop of 0 5N ammonium molybdate and titrate immediately with 0 02N sodium thiosulphate. Carry out a blank determination under the same conditions. 

The ratio of available sensible heat/latent heat varies from 0.79 for methane, down to 0.25 for propane. In general, if the ratio is less flian about 0.25, the vapour cooling effect becomes insignificant, and this is so for propane, the higher hydrocarbons and LPGs. 

The wall boundary layer flow in turn ensures good cooling of the walls by the cold vapour. This vapour cooling effect is a bonus, since it can be used to absorb, as B heat in-flows, part or all of the conducted heat flows down the walls which would otherwise enter the liquid as A heat in-flows. 

The heat transfer in the steep vertical temperature gradient in the vapour space is also greatly enhanced, and this aids the efficiency of the vapour cooling effect... 

Design system to convert A heat in-flows into B in-flows by using vapour cooled baffles, vapour cooling of necks and vapour space walls, and pipework entering liquid. 

Vapour cooled radiation baffles or a suspended deck in the cold vapour space. 

Vapour cooling of the unwetted tank wall to reduce conduction into the liquid. 

Vapour cooled multi-shields to reduce thermal conduction through the wall and... 

Vapour-Cooled Radiation Baffles and Suspended Decks... 

The first systematic studies on the use of vapour-cooled baffles were carried out in the early 1960s at Southampton University, and were reported in 1965 at an HR conference in Grenoble, France by Lynam et al. [2]. Their paper describes experimental... 

The disc baffles work in the following way. The downward radiation heat flow from the top of the tank or container is partially absorbed and partially reflected back. The baffles are in turn cooled by increasing the enthalpy of the cold vapour. For each baffle, the radiative heating is balanced by the vapour cooling. In this way, with a series of baffles, the ambient radiation heat flow is almost completely stopped from entering the liquid and contributing to the liquid evaporation. 

In very large LNG tanks, the vapour cooled baffle system is called a suspended deck, which has revolutionised and simplified their design and has, at the same time, significantly reduced construction costs. See Fig. 3.1. 

The use of vapour cooled baffles was the subject of several patents back in 1965, but since the technique is so easy to apply, the general use of baffle systems has spread very quickly into all areas of cryogenics, regardless of patents. 

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. 

Fig. 3.1 Vapour cooled suspended deck insulation in upper section of large LNG tank at 1 bar...

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. 

Ambient temperature radiation can funnel down neck tubes and pipelines by internal specular reflection, without significant diminution, directly into liquid baths. Even if the tubes are vapour cooled, the radiation is not absorbed at the walls of the tubes, during internal reflection. To reduce radiation funnelling, the inner surfaces must therefore be rough so as to promote diffuse reflection at the relevant infra-red wavelengths. It is also advisable to use radiation baffles and traps, in all neck tubes and lines entering a cryogenic system. 

Ahead of the liquid front, 2-phase flow occurs as the inner wall of the pipeline, inner diameter D, is cooled down to the liquid temperature then there is single phase cold vapour cooling the pipeline to a point about lOOOD further down, where the temperature difference between wall and vapour has decreased to zero. The region from this warm point back to the liquid front is the so-called cooldown wave . 

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