Superplasticidad y FSP en aleaciones de aluminio

You should note that the term vacuum is used to denote any pressure below atmospheric pressure.

We also speak of a deep vacuum, which is a vacuum well below atmospheric pressure. A shallow vacuum on the other hand, is a vacuum which is only slightly below atmospheric pressure.

Figure 16 is a simple illustration of a two stage vacuum deaerator. You will probably notice that it looks similar to the gas stripping deaerator.

However, it does have a number of different features. The first thing we should note is that there are two separate sections of packing. The upper section operates under a shallow vacuum. The lower section operates under a deeper vacuum.

This, then, is a two stage vacuum deaerator. Single stage and three stage deaerators have been used but two stages are the most common.

In this vessel, the sea water from the process heat exchangers enters the deaerator through a sparge pipe and flows downwards over the upper section of packing.

Oxygen and water vapour are sucked out of the vessel via an overhead vapour line, creating a shallow vacuum in the vessel.

Figure 16

When the water reaches the bottom of the upper section of packing, it flows through a set of seal chimneys. These operate in a similar manner to the

‘U' bend under a sink. The water has to flow upwards and over a weir before it can reach the lower section of packing. The pressure difference between the two sections is maintained by the height of the chimney weirs.

A simplified sketch of a seal chimney is shown alongside the deaerator in Figure 16.

After the sea water leaves the seal chimneys it is re-distributed and flows downwards over the lower section of packing. Oxygen and water vapour are again sucked out of the vessel via a vapour line which goes to the deep vacuum section.

The deaerated water leaves the lower section of packing, falls into the bottom of the column, and leaves via the deaerated water outlet.

Vacuum deaerators are very efficient. Most two stage vacuum deaerators can achieve an oxygen concentration as low as 0.1 to 0.15 ppm (parts per million) in the deaerated sea water outlet.

We have seen how a vacuum deaerator works. Now let’s take at look at two items of equipment which may be used to create the vacuum in the deaerator.

The most common method of creating this vacuum is by the use of a liquid ring compressor. These are also called vacuum pumps, which is the term I shall use.

Figure 17 is a simplified drawing of a vacuum pump.

(An actual pump would look very different from this.

However, the illustration is intended to explain the principles of operation.)

If you look at the first part of Figure 17 you will see that I have drawn a shaft, fitted with four vanes, rotating inside an empty casing. I have positioned the shaft and vanes central to the casing.

In the second drawing of Figure 17, I have shown a water inlet and outlet which introduces service water into the casing at the near end and removes it from the far end. The spinning action of the shaft and the vanes imparts a centrifugal force to the water. The water is thrown against the inside wall of the casing due to this force.

The water forced against the casing in this way is the liquid ring which gives the compressor its name. The central part of the casing is filled with air.

Now take a look at Figure 18 and see if you can detect the difference in the position of the shaft and vanes.

You will notice that I have drawn the shaft and vanes offset from the centre of the casing. Service water is again introduced into the casing and again forced outwards to form the liquid ring.

You should notice that:

• although the shaft and vanes are offset the water still forms a uniform liquid ring against the casing

• the offset shaft and the uniform liquid ring combine to form different sized void spaces

The differing sizes of these void spaces means that the pressure in each will change as the shaft and vanes rotate. The pressure will:

• decrease when the void space increases • increase when the void space decreases If we control the flow of air into the unit so that it enters a void space at the low pressure side and leaves at the high pressure side, we have created a liquid ring compressor. The compressor will in fact suck the air into the low pressure side and create a vacuum.

In real vacuum pumps, slide valves control the air inlet and outlet.

Vacuum pumps are only capable of creating a limited amount of vacuum. Therefore, in order to create the deeper vacuum required in the second stage of deaeration, the vacuum pump is augmented by an ejector. This piece of equipment is also called an eductor or venturi.

Figure 19, on the next page, shows how such a unit works.

The discharge side of the ejector is connected to the suction of the vacuum pump. A line connects the nozzle of the unit to the deep vacuum section of the deaerator.

As the vacuum pump draws atmospheric air through the nozzle of the ejector the air speeds up. This creates an area of very low pressure at the nozzle which in turn pulls a deep vacuum on the deaerator.

You should by now have a good idea of how oxygen is removed from the injection water.

Check your understanding now by having a go at Test Yourself 7.