Thursday, June 14, 2012


Before I write anything else about anything else, there needs to be a clearing up of confusion.

Just recently there have been a few misconceptions posted here and there. 

For instance, there is a person on ‘E News’ who has taken to using the expression ‘Ell-Beez’. This is usually in conjunction with the revelation that a certain celebrity (who is, invariably, completely unknown to me!) has gained, or lost, a few ‘Ell-Beez’.

Let us all, with one accord as distinguished Jet Engine Professionals, put these people into the light of knowledge.

The plural of ‘lb’, the accepted abbreviation for ‘pound weight’, is ‘lb’

It comes from the Latin ‘Libra’ meaning ‘pound’; the plural of this is ‘Librae’. Observe – no ‘s’.

Ergo, said ‘celebrity’ has gained, or lost, a few ‘Ell-Bee’. Not, perhaps, quite so catchy but much more accurate. It also avoids pitying looks from the intelligentsia who watch the show.

Now heat.

Heat and temperature are not the same thing.

Heat is a form of energy. We get ours from the Sun. Exclusively. Try very hard not to tell me that we can get heat from burning coal or wood. The energy that gave us both these fuels came from the Sun so they are ‘second-hand’ solar heat.

Cold is not energy. Cold is an absence of heat in the same way as there is no such thing as ‘dark’. Dark is an absence of light (which is another form of heat energy.

Temperature is a measurement of how much heat is present in any particular volume of… well… anything.

Let’s take an example.

10 lb of dry air at ISA (International Standard Atmosphere – 14.7 psi at 15°C) takes up the space of a sphere approximately 6.28’ in diameter.

If we pass this volume of air through the compressor of a jet engine, the compressor will make it smaller. We shall say that the compressor gives a CR (Compression Ratio) of 10:1.

Ten to one? Time for lunch – see you later.

Ah! Good food.

Where were we? Oh, yes. We are going to reduce the volume of air by a factor of ten.

It will now be 0.628’ in diameter or, if we convert that, 7.536”. Sit in the hand just nicely; just under an octave

Of course, you would not want it sitting in your hand because we have also compressed the heat in it by a factor of ten.

15°C now becomes 150°C – considerably more than the boiling point of water (at ISA).

It is exactly the same amount of HEAT but it is now occupying a smaller VOLUME of, in this case, AIR.

Of course, this is ignoring the work done on it to compress it. Since we are less than efficient, the work done to do the compressing will be mostly wasted in heat energy transferred into our bundle of gas and so the heat within our sphere will be much greater than it was at the start.

In other words, the temperature will be much higher. Much.

An axial flow compressor giving a CR of about 11:1 will give us an exit gas temperature of around 550°C. If you yearn to know how that feels then go to a friend who longs to kill him/herself with noxious gases. Ask them to light the cigarette and request that they suck long and hard on it.

When the tip glows brightly that will equate to around 550°C. Try to avoid the temptation to put your finger on it.

The HEAT in the cigarette end will be quite small but the TEMPERATURE will be very high.

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