Baffles, vapour cooled

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-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. 

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. 

A drawback of oil diffusion pumps is the so-called back-steaming. It is the flow of a small quantity of oil vapour towards the inlet of the pump. A water-cooled baffle like that shown in Fig. 1.13 can be put above the inlet. Baffles are made up of arrays of optically dense fins cooled by a continuous water flow. A baffle always reduces the pumping speed. 

The gases are (hen saturated in an internally baffled wash tank, followed by a pressure drop de-mister on the exit of the tank. The water in the tank is cooled externally by recirculation through an air-cooled exchanger. The cooled gases are then passed through a condensing coil to remove more water vapour and entrained droplets. 

The Standard Messo multistage vacuum crystallizer Figure 8.52) provides a number of cooling stages in one vessel. The horizontal cylinder is divided into several compartments by vertical baffles that permit underflow of magma from one section to another but isolate the vapour spaces. Each vapour space is kept at its operating pressure by a thermocompressor, which discharges to a barometric condenser. 

We can now begin to see how the 300 K radiation heat flow into LNG can be reduced by a large factor of 500-1000 by using a set of low emissivity baffles, or an equivalent multi-layer aluminium, suspended deck, each cooled by the cold vapour to a temperature of 150 K or less, above the liquid surface. 

We now know that the cooling of the baffles by the rising cold vapour takes place via an efficient heat transfer process, because the natural convective heat transfer for the vapour is greatly enhanced (perhaps by as much as tenfold or more) in the vertical temperature gradient above the liquid. 

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