Air Conditioning Systems

One of the most difficult jobs for Concorde was keeping everyone cool (not to mention keeping them alive). But why should this all be such a big deal? Well, although the still air temperature outside is around -56°C, the kinetic heating due of supersonic flight heats up this air dramatically, to well over 100°C. And here we are, sat in this narrow aluminium tube those heats up to an average of about 90°C, with the temperature on the nose on the aircraft anything up to 127°C. And to make matters worse, each of us chucks out about 10 Watts of heat when we are sober, and up to around 150 Watts when we’ve been at the Champagne. ‘So what’, you might say, ‘let’s just cool this place down then’. Well that’s MUCH easier said than done, the air that we are going to use to cool us down is taken from the final stage of the high pressure compressor in the engine, and at Mach 2 this is around 550°C!!, To keep our cabin comfortable (you are NEVER cold in Concorde at Mach 2) you need to pump the air into the cabin at around FREEZING POINT. We need FOUR independent air conditioning systems, a 747 only has three, and a 777 only two!!! Not only do we have to pump air through our cabin at a very low temperature, we also need to pump in enough MASS of air to cool the place down. (180 POUNDS of this freezing air every minute!!).

Even more difficult is the fact that for freezing air to enter the cabin it needs to leave the air conditioning systems, which are mounted in the wings, at around MINUS 25°C, due to the fact that the hot wing surface warms up quite a lot the air in the ducting before it can reach the cabin. So we are talking about taking air at 550°C and dropping its temperature by about FIVE HUNDRED AND SEVENTY DEGREES CENTIGRADE. To do this cooling we have three ‘heat exchangers’ (just think turbocharger intercoolers and you are on the right track). Two of these exchangers are mounted directly above the engines, and use just ram air as the cooling medium (bearing in mind that this ram air itself is anything up to 127°C) and the third is mounted in the wing and uses aircraft FUEL to do the cooling. (But the average temperature of the fuel is around 60°C at Mach 2). So hang on a minute, we need to get air down to about -25°C, but our coldest bit of the cooling stuff (the fuel) is still 60°C, so how the **** are we going to get down to -25°C . Well, we obviously need to cheat a bit, and to do this we use a thing called a Cold Air Unit (CAU), which works something like a turbocharger in reverse. (These are used in subsonics too, but FAR less work to do and for only part of the time. The Boeing terminology for a CAU is Air Cycle Machine, but the principal is still the same). Without going into ‘techno-gobbledygook’ mode, it works something like this: The air whizzes into the compressor and as the compressor squeezes up the air heat energy is obviously added and the air warms up, just like when you rub your hands together. (I know, ‘I THOUGHT WE WERE TRYING TO COOL THINGS DOWN…’, but bear with me). Now comes the clever bit, because the air is warmer it can get cooled down more (more headroom if you like) when it passes through the secondary and fuel heat exchangers, and when it passes finally passes through the turbine the pressure of the air drops. Now the same amount of heat energy that was added to the air when the air was compressed is consequently REMOVED, as work is done by the air to rotate the turbine shaft, and hence the compressor, giving us our astoundingly high temperature figures. What we are REALLY doing is converting mechanical energy to heat energy and vice versa Promise, it will all make sense soon, honest.

We are going to look at what is known as ‘A Mach 2, ISA+5 Day’. Don’t faint, all this just means that the outside (still) air temperature is 5°C warmer than an ISA (International Standard Atmosphere) day. ISA (above about 36,000 feet) is -56.5°C, so +5 gives us a temperature of -51.5°C. OK so far? At ISA, the TOTAL temperature (the kinetic heating bit) is around 116°C, but ISA+5, 127°C. (These are also the temperatures on the nose of the aircraft, and as ALL our readers know, 127°C is the limit for Concorde).

The diagram above shows the general layout of our system, and to make it all work here are some figures, but don’t worry too much about all the valves and stuff. (If you WANT to know anything of course, just ask).
Our air leaves the engine at 550°C, and after passing through the various valves enter the Primary Heat Exchanger (A), remembering that our’ cooling’ air is 127°C. After passing through the exchanger the temperature of our air has already fallen by 380°C, down to 170°C. (Still bloody warm though). Our next port of call is the compressor of the CAU (C), when as the air passes though it’s temperature INCREASES by 100°C, up to 270°C. (For now ignore the branch of ducting and valve (E) below the CAU). After passing though the Secondary Heat Exchanger we lose another 100°C and are back down to 170°C again! We next get to the Fuel Exchanger (E) where our we now drop all the way down to 80°C, thanks to our nice ‘cold’ 60°C fuel temperature. And finally we pass through the turbine of the CAU, remembering that this is what is keeping the thing rotating, where our temperature drops 105°C ALL the way to -2f°C. As the air passes up along the wing ducting it will enter the cabin at around ZERO °C. Without this system, as mechanically complex as it is, Concorde could NEVER had carried passengers at Mach 2, and it worked well, but only just!
More info to come on this, after you’ve all had a chance to digest it all. (I DO hope I don’t cause any indigestion though).

KEEPING UP THE PRESSURE:

Pressurising any aircraft is a ‘fairly’ simple task of controlling how much air we pee out!
Putting it simpler (?). At Mach 2 cruise we’ve already established that we are pouring in about 180 lbs. of air every minute. To MAINTAIN the cabin pressure that we have, we need to leak away the same 180 lbs. of air every minute, if we need to INCREASE the pressure in the cabin we REDUCE the leakage, if we need to DECREASE the pressure in the cabin we INCREASE the leakage, simples
The simplified diagram below shows the whole picture, as far as the flows go.

You can see the engine bleeds on the left, the ‘HP AIR GROUND SUPPLY’ is there air from the ground start truck, used for starting the engines. (But also to check the operation of the air conditioning system on the ground). It also shows the bleeds coming off the engines, into the four air conditioning systems, known as ‘Groups’. (Subsonic systems are referred to as ‘Packs’, where all of the main systems, in terms of heat exchangers etc., are placed neatly together, usually in a fuselage bay, but in the case of Concorde and her limited space available, the individual components are kind of ‘splurged’ around the engine bays and wings. On the extreme right can be seen the four DISCHARGE VALVES, one FWD. and one AFT DISCHARGE VALVE per pressurisation system. (There are two independent systems, 1 on the left of the aeroplane and 2 on the right). One system is usually selected for the ‘outbound’ leg, and the other for the return leg. All four valves however are always fully open on the ground, but after take-off one pair (the non-selected system) fully close, and the other pair are controlled (modulated) in position to control the cabin pressure.
Our conditioned ‘chilled out’ air flows into the three aircraft zones, the fwd. and aft cabins and the flight deck, each of course with independent temperature control from the flight engineer’s panel. This air, in the case of the cabin is pumped in at passenger shoulder level and extracted at roof level. This is then pumped forwards by the three SUPPLY FANS (that run continuously throughout flight) and fed into the electronic racks, both on the flight deck and below floor. THIS air, after passing through all the electronics is then directly vented overboard; via one of two FWD. DISCHARGE VALVES (these are located just to the rear of the nose undercarriage). When the aircraft is on the ground, or not yet pressurised, two FWD. EXTRACT FANS direct this air into the D/Vs, automatically switching off when the aircraft pressurises. Air is also freely vented from the cabin at floor level, into the under-floor area, where it travels rearwards towards on of two AFT DISCHARGE VALVES, located underneath the rear fuselage, just about in line with the elevons. Air is also forced into the under-floor area via REAR ELECTRONIC RACKING EXTRACT FANS, which suck the air from the electronic racks located either side of the rear galley. (These racks contain eight of the thirteen air intake ‘boxes’, the four reheat amplifiers, both ‘black box’ accident recorders, both HF radio transceivers, as well as the two ADF (Automatic Direction Finding) receivers and some other ‘minor’ stuff).

Concorde cabin operated at an extremely high differential pressure of 10.7 PSIG. (That’s a difference in pressure between the cabin and outside). This would give us a cabin altitude of only SIX THOUSAND FEET, right up to our passenger ceiling of 60,000’, where a jumbo cabin, flying up to twenty thousand feet below us has a cabin altitude of EIGHT THOUSAND FEET, due to its far lower operating differential pressure.
(Our aeroplane is twice as high as the jumbo, but our cabin is two thousand feet LOWER… NICE.

KEEPING WARM:

Although all our talk was about ‘keeping cool’ we have to remember that Concorde spends SOME of her time flying subsonic, where the seriously sub-zero temperatures outside mean that we have to be able to ‘warm up’ for these parts of the flight.
If you look at our ORIGINAL air conditioning diagram, you will notice a valve (E) termed ‘Temperature Control Valve’. Now this guy allows quantities of HOT air coming off the PRIMARY HEAT EXCHANGER to bypass completely the other stuff that produces our lovely FREEZING air at Mach 2. (In supersonic cruise the TCV is modulated pretty well closed. Our diagram below shows this in a more ‘close up’ view.
The indicators (left) on the diagram on the flight engineer’s panel show CAU inlet, wing duct and zone temperature, as well as mass-flow in KGs/Second. (That’s the gauge in the middle).

MORE AIR:

Something that I could certainly do with! To further build up our air conditioning story, here is a diagram showing just how we got the air pumped around the cabin. The air from the AIRCOND’ SYSTEM came into the cabin via the supply duct and travelled upwards, along the riser ducts marked in GREEN. And down from the panel just above the windows. (The Punka Louvre air would come from the same place).
The air would then get extracted from little slits just inboard of the roof fluorescent tubes and run down the sinker ducts marked in RED down to the recirculation duct, where the air is sucked forwards to the forward electronics racks by the SUPPLY FANS. (You can also see the freely vented air at floor level goes straight down the underfloor zone).
The advantage of the system was just how quickly it could get rid of things like cigarette smoke, in the days when such yuch was allowed on aeroplanes. (There was nothing better than Concorde for this). Also all air entering the cabin was fresh, unlike many aircraft that recirculate some of the cabin air, and there was never any stale odours in the air at all..

MORE, more air:

Just to put the finishing touches on the last ‘air picture’ we can see in the picture below how the passengers got their air. After passing up the riser ducts the GREEN air in that last picture it feeds into the black distribution bars (colloquially known as ‘piccolo tubes’ )where it squirts out of the holes. The punka louvers get their feed from the little pipe connected to the punka louvers.

AND WHERE DOES ALL THAT AIR GO?

We’ve breathed in all this lovely cool, fresh air, but now what? The two SUPPLY FANS (remember, these run for the whole flight) suck the air out of the cabin via the SINKER DUCTS (the ‘red’ air shown in our first diagram) forwards towards the electronic racks where it is used to cool down the majority of the electronics on the aeroplane. (The remainder is housed in those racks between the rear galleys that we mentioned before). The L/H and R/H FWD RACKS of course are the electronic racks either side of the flight deck aisle and the air is forced up at floor level into the space behind the panels. It then gets sucked ‘through’ the electronic boxes into the extract system as we will see in a second. If you’re wondering about some of the other abbreviations, INS is Inertial Navigation System and TRU is Transformer Rectifier Unit. (Changes some of he 200 Volts AC coming off the aircraft generators into 28 Volts DC, but don’t worry too much about that stuff, YET). If the aircraft is NOT yet pressurised the three EXTRACT FANS draw all of this air towards the two FWD DISCHARGE VALVES, only one of which will be open in flight as shown, where it is finally expelled overboard. Once the aircraft is pressurised the fans switch off, and the air is carried out by differential pressure. Cabin pressure is controlled by automatically position the DISCHARGE VALVE, together with its partner at the rear.

More ‘AIRY FAIRY’ STUFF:

To help polish off our Air Conditioning topic today, here is the first of the remaing diagrams still to come, this time one showing the airflow in the rear galley area, where it enters the electronic racks either side. (Remember some important stuff here, EIGHT of the intake ‘boxes’ as well as all four reheat control boxes too). The air exits the racks towards the two DISCHARGE VALVES under the floor (remember only one will be open in flight, together with its opposite number at the front of the aircraft). This extracted air is also freely mixed with the underfloor air, which exits the aircraft in the same manner. With the aircraft not pressurised/on the ground, the LEFT & RIGHT REAR EXTRACT FANS are running to carry the air overboard, but as the aircraft starts to pressurise the fans will automatically switch off. The STBY fan can be manually selected in the case of one of the fans failing. The intake boxes in particular run very hot indeed, and the lack of cooling air will soon render these useless, so for that reason as well as warning lights illuminating on the engineer’s panel for a failure of the cooling air, if we are on the ground a very loud horn will also sound. (This is located in the nose undercarriage bay).