AC system on the airliner

[sticky_beaver]

Does anybody know exact structure of cabin pressurization and air delivery system on arliners?

According to data about Boeing... it uses engine bleed air, taken between low (150°C) and high pressure (300°C) compressor stage.
Bleed air is used to power pneumatic system that deliver brake power, heat for deicing wing's and engine's leading edges, fuel heating, fuel and water tanks pressurization and of course cabin pressurization. Bleed air from APU that is conected to the main pneumatic system also starts the main engines.

According to information about B787 it won't use engine bleed air (due to the danger of oil wapors in the airstream) but rather AC electric powered pneumatic motor that will deliver pressure to the cabin. Someone also sais that this will incrise overall fuel efficiency.

Now I was reading on Wikipedia about air cycle machine...

"An air cycle machine (ACM) is the refrigeration unit of the environmental control system (http://en.wikipedia.org/wiki/Environmental_control_system) used in pressurized (http://en.wikipedia.org/wiki/Pressurized_cabin)turbine (http://en.wikipedia.org/wiki/Turbine)-powered aircraft (http://en.wikipedia.org/wiki/Aircraft). Normally an aircraft has two or three of these machines arranged in a system called a "pack". The usual compression, cooling and expansion seen in any refrigeration (http://en.wikipedia.org/wiki/Refrigeration) cycle is accomplished in the ACM by a centrifugal compressor (http://en.wikipedia.org/wiki/Centrifugal_compressor), two air-to-air heat exchangers (http://en.wikipedia.org/wiki/Heat_exchanger) and an expansion turbine (http://en.wikipedia.org/wiki/Expansion_turbine). Bleed air (http://en.wikipedia.org/wiki/Bleed_air) from the engines, an auxiliary power unit, or a ground source, which can be in excess of 150°C and at a pressure of perhaps 32psi [1] (http://en.wikipedia.org/wiki/Air_cycle_machine#cite_note-0), is directed into a primary heat exchanger. Outside air at ambient temperature and pressure is used as the coolant in this air-to-air heat exchanger. Once the hot air has been cooled, it is then compressed by the centrifugal compressor. This compression heats the air (the maximum air temperature at this point is about 250°C) and it is sent to the secondary heat exchanger, which again uses outside air as the coolant.
At this point, the temperature of the compressed cooled air is somewhat greater than the ambient temperature of the outside air. The compressed, cooled air then travels through the expansion turbine which extracts work from the air as it expands, cooling it to below ambient temperature (down to -20°C or -30°C). It is possible for the ACM to produce air cooled to less than 0°C even when outside air temperature is high (as might be experienced with the aircraft stationary on the ground in a hot climate).[2] (http://en.wikipedia.org/wiki/Air_cycle_machine#cite_note-1)
The work extracted by the expansion turbine is transmitted by a shaft to spin the air pack's centrifugal compressor and an inlet fan which draws in the external air for the heat exchangers during ground running; ram air is used in flight. The power for the air conditioning pack is obtained by the reduction of the pressure of the incoming bleed air relative to that of the cooled air exiting the system. After the air has been cooled down, water vapor in the air condenses, forming fog or high humidity. To get rid of this, the air exiting the expansion turbine is passed through a water separator, which uses centrifugal force to throw the water particles into a coalescer bag that absorbs the moisture.
The air can now be combined in a mixing chamber with a small amount of uncooled engine bleed air. This warms the air to a desired temperature, and then the air is vented into the cabin or to electronic equipment."

...but this reading has no sense for me. I can say that this configuration will be applied on plane that doesn't use engine bleed air. Planes that do, don't have secondary compressor and heat exchange between as it takes pressure air from the engine itself via light air colectors and pipes. And because we have to maintain fuel temp above -37°C (normal operation altitude OAT is -56), and we also have to maintain wings and engines leading edges above water freezing point (I say 0° C), we have a huge heat consumer - so I don't see the point of any extra heat exchanger.

I also can't emagine the AC electric consumption of heating the wings and fuel withought of engine bleed air. I can just predict that above 50kW.

The other thing is that I don't beliewe that you can extract mechanical power from hot air. Car's supercharger (turbine and compresor stage on the same shaft) doesn't use temperature but pressure - meaning it doesn't lower EGT but it only ads backpressure.

And the final question... does airliner AC system maintaining cabin temperature by delivering new warm air to the cabin and releasing old one via cabin air pressure valve or does it use heat exchanger?

I personaly think that there is higher energy heat demand to maintain cabin temperature between 20-24°C than oxygen demand - changing the CO2 rich air. Meaning that it will be waste of air pressure - equal energy if you will maintain tempreature thus constantly applying new warm air.

[barit1]

Let's start with this one: The other thing is that I don't beliewe that you can extract mechanical power from hot air. Car's supercharger (turbine and compresor stage on the same shaft) doesn't use temperature but pressure - meaning it doesn't lower EGT but it only ads backpressure.

Any turbine uses temperature and pressure together to extract energy from an air or gas stream. Can't have one without the other; it's the natural adiabatic property (http://en.wikipedia.org/wiki/Adiabatic_process) of air. A jet engine with multiple turbine stages may drop the gas temperature 200-300C per stage, at least in the HP turbine.

This basic thermodynamic process is the big part of how an air cycle machine works, as well as any other turbine.

[flyboyike]

I'm not entirely sure I understand what specifically our friend is asking. He seems to be under the impression that wings and cowls A/I is always on and that the air for that comes from the same source as the ECS air. He also seems to think that there is a bulk fuel heating system present. I can't speak for the 787, but on the airplane I fly, none of the above is the case. The fuel is heated by the FOHEs near the engines, not in the tanks. I gather from his post that he's thinking that the ACMs do a lot more than they actually do.

[SNS3Guppy]

SB, air conditioning and pressurization in an airliner are tied together for the simple reason that the bleed air coming from the engine is far too hot to be used to pressurize the cabin without first being treated, or conditioned.

Before bleed air can be admitted to the aircraft pneumatic system at all, it must be precooled to a value that won't cause system damage. Typically this is around 350 degrees F. The exact values vary, and aren't particularly important. It's worth knowing that the raw bleed air can be well over six hundred degrees F...which is hot enough to do structural damage as well as harm or kill occupants of the aircraft...so it's pretreated and cooled with a "precooler" which acts as a heat exchanger, or air radiator, before the bleed air is sent to the aircraft pneumatic system.

Once in the aircraft pneumatic system, pressurized air is used for a number of functions which you already identified...these include pressurizing the airplane, heating the interior, actuating things such as leading edge flaps or devices, pressurizing hydraulic resorvoirs and potable water, anti-ice and de-ice, fuel heat, etc.

Pressurization works simply enough; pressurized air is allowed into the "pressure vessle" (the part of the aircraft that's to be pressurized) until a desired value is reached, then the remaining pressure is vented out of the airplane through "outflow valves." The outflow valves open and close as required, usually automatically, to keep the cabin differential pressure constant, or as established by the pilots or flight engineer. The outflow valves also have automatic protective features attached to them through the pressurization system, such as limiting the rate of cabin climb or descent, and altitude limiting (running fully closed to prevent cabin air loss when cabin altitude reaches a particular value---when it climbs too high).

In order to make the pressurized bleed air acceptable for use in the cabin, it has to be conditioned, and this is done through air conditioning packs. Packs usually involve air cycle machines which use expansion turbines and heat exchangers to extract heat energy from the bleed air. The amount of heat energy extracted is is determined by how much airflow is allowed to go through the air cycle machine turbines, and how much is allowed to go around them. Additionally, the use of more than one extraction turbine gives a more precise means of controlling this temperature.

The output from the packs comes in a fairly narrow range. The packs cool the air, and if the temperature is to be increased, then "trim air" from the hot side of the pack is used. Trim air is air that hasn't been through the conditioning process yet; it's hot air that can be mixed with the cooled air to precisely set the desired temperature.

According to information about B787 it won't use engine bleed air (due to the danger of oil wapors in the airstream) but rather AC electric powered pneumatic motor that will deliver pressure to the cabin. Someone also sais that this will incrise overall fuel efficiency.

Any time that bleed air is taken from a turbine engine during operation (save for special bleeds designed for engine operation), the engine's fuel efficiency suffers. If the engine is working hard to move a certain amount of air through the engine and one is taking air away from the engine, then the engine must work harder, or the engine is less efficient. The use of bleed air for anti-ice reduces engine efficiency. The engine runs hotter and may produce less power (thrust). Using electric compressors in lieu of air cycle machines may or may not have benefit, given that the method and size and application are important factors in the equation. You appear to have made up your own mind on that, however, so further discussion is probably unwarranted.

...but this reading has no sense for me. I can say that this configuration will be applied on plane that doesn't use engine bleed air. Planes that do, don't have secondary compressor and heat exchange between as it takes pressure air from the engine itself via light air colectors and pipes. And because we have to maintain fuel temp above -37°C (normal operation altitude OAT is -56), and we also have to maintain wings and engines leading edges above water freezing point (I say 0° C), we have a huge heat consumer - so I don't see the point of any extra heat exchanger.


Why can you say that this configuration (air cycle machine) will be "applied on a plane that doesn't use engine bleed air?"

You state that airplanes using bleed air don't have secondary compressors and heat exchangers...but they do. Multiple heat exchangers, in some cases..and not all of them involve expansion turbines.

What is a "light air collector?"

You appear to further be saying that because you have a need to elevate fuel temperature, you have no need for a heat exchanger.

Fuel temperature is another topic entirely, and numerous methods of affecting fuel temperature are in use. To suggest that it should be the sole means of cooling the cabin air is a bit of a stretch, particularly when considering aircraft that operate in the desert. Further, the outside air temperature isn't necessarily relevant, depending on the fuel system in use, and depending on the fuel tank, and tank location and dimensions in use, too. Additionally, fuel freezing points vary with the type of fuel in use; your temperatures aren't accurate save for a limited range of fuels.

Why do you feel you have to maintain wings and engine leading edges above the freezing point of water? Engine, nacelle, and wing anti-ice is applicable at some times and not at others, and can be quite damaging if used all the time in some applications...to say nothing of the reduction in engine efficiency. Sometimes it's needed and warranted, sometimes it's not. Additionally, compressibility affects the local temperature differently to affect ice formation, and must be considered.

The other thing is that I don't beliewe that you can extract mechanical power from hot air. Car's supercharger (turbine and compresor stage on the same shaft) doesn't use temperature but pressure - meaning it doesn't lower EGT but it only ads backpressure.


Energy is traded between one form and another all the time. Chemical energy for heat energy for mechanical energy and so on. It's how powerplants operate, how we work our engines, how we do nearly everything in the aircraft and in flight. We trade one form of energy for another. We trade burning fuel and spinning turbines for thrust, we use compressed air to pressurize, to move devices, to do all sorts of things. Most certainly you can extract mechanical energy from hot air.

Your understanding of a supercharger is somewhat lacking. You may be thinking or a turbocharger, rather than a supercharger (turbocharger is driven by exhaust gasses, while a supercharger is driven by the engine). You also express a lack of understanding regarding exhaust gas temperatures and both piston and turbine theory.

When you state a "supercharger" adds backpressure, you're talking about a unit in the exhaust stream, which is of course, a turbocharger. A turbocharger extracts energy from the exhaust stream, and uses this energy to turn a turbine wheel. This turbine wheel is mechanically linked, often by a direct shaft, to a compressor wheel in the induction side of the engine. This is entirely unrelated and irrelevant to the function of an air cycle machine, because among other things, the induction air in the ACM isn't being heated by combustion, with more energy being added between induction and exhuast.

You're confusing systems and trying to apply principles from one to the other...but you misunderstand both systems and consequently it's not making so much sense to you.

As far as EGT...yes, it can be affected by both backpressure and by scavenging (tuned exhaust), but it's entirely irrelevant to the function of an ACM, and EGT is a function of combustion, which is directly affected by induction pressure...again, no bearing on a discussion of the ACM. You're clouding the issue by trying to make an example out of an unrelated topic.

You appear to be upset, or displeased with what you've read about the Boeing 787 product. You have that right. However, you appear to be asking a question on the one hand, and on the other attempting to make a point or rally agaisnt Boeing. Which is it? Are you looking to gain an understanding of aircraft pressurization systems, or are you looking to complain about Boeing?

I imagine either one is probably just fine, but you should probably be clear as to what it is that you're trying to accomplish.

I personaly think that there is higher energy heat demand to maintain cabin temperature between 20-24°C than oxygen demand - changing the CO2 rich air. Meaning that it will be waste of air pressure - equal energy if you will maintain tempreature thus constantly applying new warm air.


I have absolutely no idea what you're trying to accomplish with this paragraph. Are you asking a question or trying to explain a new theory? Changing CO2 rich air? What CO2 rich air?

What is a waste of air presure?

Are you trying to say that pressurizing the cabin isn't necessary, so long as air temperature is maintained?

You understand that the entire cabin air supply is constantly being replenished, and every few minutes the entire cabin air is completely replaced is a constant, ongoing proces...correct? Air is constantly being pumped in, constantly let out, and it's conditioned on an ongoing basis to maintain a comfortable and safe temperature.

[Old Fella]

If all you want to know is what your post asks, then this is a simple answer. Bleed air, taken from the Bleed Air Manifold is passed through the Air Conditioner Pack Valve (or Valves) and initially passes through a Primary Heat Exchanger (Air -to - Air) before passing through a Secondary Heat Exchanger (Air-to-Air) where it by-passes the ACM before entering a Plenum, or Conditioned Air Manifold, from where it is distributed to the various "Zones" of the aircraft. Under conditions of high demand for Cold Air the air is directed through the Air Cycle Machine where the extraction of energy by the turbine in the ACM causes the air temperature to drop. This air is then directed into the Conditioned Air Manifold and distributed as previously described. Depending on requirement in individual Zones, bleed air is added to each individual zone outlet to set the desired temperature in that Zone. (Zone temperatures can vary greatly depending on passenger numbers within the Zone) As the demand for Cold air is reduced, e.g. as the aircraft climbs to colder ambient air conditions, first the ACM will eventually be "by-passed" and put into what is virtually an "idle" circuit. As the demand for Cool air reduces even more the Inlet and Exhaust doors on the Ram Air ducts, which allow air flow over the heat exchangers, will move towards the "Closed" position to reduce the cooling effect of the heat exchangers. (Also reduces drag and therefore helps reduce fuel consumption to a small extent) The Inlet can fully close, but not the Exhaust. Pressurisation is simply a function of controlling the "rate" at which the Conditioned Air, introduced into the pressurised hull, is allowed to escape via Outflow Valves resulting in the aircraft becoming pressurised. To guard against structural damage due to "over-pressurisation" Safety Valves are fitted to relieve the pressure if the Max Differential is exceeded. Hope this explains what you originally asked.

[james ozzie]

double posted - deleted

[Old Fella]

It's OK James Ozzie. ChristiaanJ is located in France and is interested only in Concorde. His "go" at Sticky_Beaver probably comes naturally if he is living in his country of birth.

[ChristiaanJ]

Hi Old Fella and James Ozzie,

You'll find Dutchmen often do tend to be fairly blunt, even when not behind a computer and even when not living in their country of birth...

As to the "nom de plume", nearly everyone here has one. Keeps some of the spam out of the mailbox.

Sans rancune!

CJ

[Old Fella]

The people of Dutch origin with whom I have worked, including one of whom I consider a great mate, are nowhere near as "blunt" as yourself ChristiaanJ. Maybe the question from Sticky_Beaver was not all that flash, however he/she did not deserve to be answered so "bluntly".

[SNS3Guppy]

Gentlemen, there's a reason this is the tech form, not the ethnic forum. Perhaps staying on point might be more professional, and appropriate.

[Old Fella]

SNS3Guppy. Point taken regarding the objectives of this forum, however it is also inappropriate to "put down" people who ask a question in the manner in which ChristiaanJ initially did.

[SNS3Guppy]

I think that's been fairly thoroughly covered already. Don't you? Perhaps any further discussion on that topic might best be handled by private messages.

[sticky_beaver]

Great answears from SNS3Guppy and Old Fella despite my question was not raise up good, I see it now after I have read it. Also I see I have mixed a lot of stuff, but please forgive me for that, as I am not the proffesional, but I am interested in planes since I am playing the Microsoft flight simulator, and I wish to know more about aeronautics and techology of a plane.

I now see that pneumatic system on an airliner isn't that connected as I thought. Based on my understanding of your answers I see that deicing and fuel heating is really separate system from environmental control system.

So if you allow (and I hope others won't get mad) I' ll wish to question (discous) some further into the topic.

I also made a quick schematics (picture says more than a 1000 words) of how I now understand the ECS on an airliner and wish that you either confirm or correct it. Also please show where is the connection for other pneumatics system - braking, water&fuel tanks pressurization, deicing, hydraulic dempand pump (B747)...
And why is there a need for a centrifugal compresor, why bleed air isn't extracted from high pressure compressor stage in an engine - sure there is more than 32 psi of pressure? Than it will be cooled down and depressurised via expansion turbine?

BTW are objects marked red on the picture below air vents for air to air heat exchangers?
http://www4.slikomat.com/08/1222/slt-untitl.jpg

simple ECS schematics
http://www4.slikomat.com/08/1222/pwl-untitl.jpg

The pressure and temperature data of air at different stages is according to wikipedia. I aslo found some equations that describes the connection of temperature and pressure of air during the compression / decompression. Here they are;
http://www4.slikomat.com/08/1222/ssh-untitl.jpg

These equations should describe air during adiabatic compression and decomression in constant air mass, so we could use it for calculating the temperature and pressure of air durring compression in a piston compressor, but as far as I read this is not so in a radial compressor. Also I don't think that we have constant air mass because we don't have close cylce. But equation that describes the heat power losses over the fuselage walls should be correct. And that was my consideration earlier in the topic. Is it really necesary to maintain inside temperature with an open loop system - energy transsfer between inside and outside of a fuselage isn't done through the heat exchangers but rather directly - warm or cold air is constantly flowing throug fuselage.
Personaly I thinh that safe level of CO2 in cockpit and air pressure could be maintained with little amount of fresh air introduction, but maintaining the desired temperature could be done throug air to air heat exchanger - (similar as in central heating system for houses). Meaning that hot pressurised bleed air directly from engine would warm or cold air directly from ACM would cool the inside of a fuselage through a separate heat exchanger. Well I don't say this will do any better, maybe this extra fuselage heat exchangers and pipes would raise the weight of a plane too much, maybe efficient cooling with cool air and a heat exchanger wouln't even be possible because of a low air density.

But one is for certain, any heat taken from the engine is a loss of energy, so why cargo planes, that don't even need lots of air refreshing don't have this kind of system where bleed air is warming the fuselage via heat exchanger? Maintaining a safe air pressure and CO2 % in the fuselage can be done with a very low air stream from ACM. In this kind of configuration pressurised hot bleed air that will heat up the fuselage will be returned to the engine - to a lower stage compressor so pressure difference will be obtained and less energy will be lost.

And just the last think on my mind, also earlier mentioned in my thread.
Could instead of air to air heat exchangers, cooling of bleed air will be done by liquid to air heat exchanger with fuel as liquid. As I know jet fuel is certified for operating down to -37°C. 30-36,000 feet above is -56°C so is fuel really heated just prior it is injected into the combustion chamber? If the bleed air will be cooled down trough wing's leading edges, (doing usefull job) and fuel, the energy taken away from engine will be renewed throug fuel that is sooner or later injected to the engine. Aslo at plane stationary on the ground with hot outside temperatures, bleed air could be cooled down to temp od a fuel and when decompressed throug expansion turbine it will be cool enough to make its job.

[Swedish Steve]

why is there a need for a centrifugal compresor, why bleed air isn't extracted from high pressure compressor stage in an engine - sure there is more than 32 psi of pressure? Than it will be cooled down and depressurised via expansion turbine?
The compressor drives the turbine. the two are connected and it is called an ACM (air cycle machine). Il the B787 this is electrically driven.

BTW are objects marked red on the picture below air vents for air to air heat exchangers?
Yes they are the ram air intakes for the heat exchangers. The air that goes in there passes through the heat exchangers and then goes overboard. It does not enter the cabin. In the cruise this is enough cooling for the bleed air and the ACMs are not really used at all.

[barit1]

The compressor drives the turbine.

Yeah, right... :rolleyes:

[ChristiaanJ]

barit1,
When the whole thingmajig is windmilling, that's true, actually.

[barit1]

The schematic could be improved with a couple of arrows indicating direction of flow, but it's not too bad. Yes, bleed air from the engines can be a lot higher than 32 psi, but that's the pressure delivered (via regulating valves) into the aircraft pneumatics.

First, it's cooled by the primary heat exchanger, then compressed to 50 psi by the right-hand rotor, which is a compressor, NOT a turbine. As it's compressed, of course it's heated (adiabatically) as well.

Then it's cooled some more by the secondary heat exchanger, and routed to the turbine. There it gives up some pressure and temperature as shaft energy is extracted to drive the compressor.

The air has now cooled to the point it's saturated with moisture, hence the water separator.

Finally some hot air is mixed in to bring it to cabin temperature.

[SNS3Guppy]

SB,

Without getting too far into the theory of an expansion compressor and turbine, perhaps it's best to simplify what's occuring in order to make it easier to understand.

The purpose of the air conditioning pack is to make bleed air safe and acceptable for use pressurizing the cabin...make it safe and tolerable for the cabin occupants. As we discussed before, the bleed air is hot, so the primary function of the pack is to cool the air. What the pack is doing is extracting heat energy.

To do this, initially, a heat exchanger is used. This is the same as a radiator on a car, practically speaking. In a car, hot water is circulated through a radiator, and cooler air blows through the radiator. Heat is exchanged from the hot water, to the air. As a result, the air leaving the radiator is warmer, or hotter than the air entering the radiator, because as it passes through the radiator, heat is transferred or exchanged from the water to the air.

In the airplane, a bleed air heat exchanger does the same thing, except that the radiator doesn't have water passing through it, it's got hot bleed air. Cooler external air from outside the airplane passes through the radiator, and the heat energy from the bleed air is transferred to the external ram air this way in the heat exchanger. The pictures you posted showing the underside of a Boeing 747, show three pack inlet openings which permit ram air from outside the aircraft to flow through heat exchangers for each respective air conditioning pack in the airplane.

You'll also see some square shapes to the rear, or bottom of the picture...these are the pack outlets. Air goes in the areas you circled, goes through the heat exchangers to extract heat from the bleed air in the packs, and then passes out the pack outlet doors.

Think about this heat extraction process a little like extracting water from a wash cloth. Prior to putting the air through the heat exchanger, the air is "squeezed" by compressing it. It's this compressed air that is run through the heat exchanger. By compressing the air, the temperature is raised even more, providing more energy that can be extracted. This is a little like wringing or twisting a rag to get all the water out of it. The bleed air is compressed and then the energy extracted in order to get as much as possible out of it during the heat exchange process.

In fact, the ACM uses a water extractor too, because while it's extracting heat, it's also extracting moisture, and it removes water to prevent the ACM from freezing up. AMC's are so effective that I've seen some aircraft blow snow out of the air conditioning vents during operations in high humidity, and we regularly see thick fog or mist in the airplane under certain conditions on the ground.

In order to control the amount of cooling or extraction that takes place in the heat exchanger, the door position can be varied. Also in order to control the heating and cooling that takes place in the pack, a turbine bypass valve is used which allows bleed air to go through the heat exchanger, then go around the expansion turbine, instead of going through it.

Just like the compressor was used to compress air and heat it up, an expansion turbine is used after the heat exchanger in order to drop air pressure and cool the air even more. Compress air, it heats up, allow it to expand, it cools down, and it's the cooler air that's desired. This is the same process that takes place in any air conditioning system. The big difference between what's going on here, and what goes on in your car is that the car uses an enclosed system with refrigerant that undergoes the same process and the bleed air is undergoing here, and in your car this refrigerant then extracts heat energy from the air that blows on you. In the airplane, the liquid refrigerant is skipped, and the bleed air is run through the same process to condition the air for your use.

You asked about the compressor in the pack air cycle machine. The compressor isn't there to boost pressure for use. We get our air pressure from the engines...it's pressurized by the engine compressors. What's referred to as the compressor in the pack ACM is a device that is used to briefly compress the air in order to extract heat energy, that's all. The bleed air arrives with all the pressure that's required to fulfill it's various functions, such as operate flight controls (leading edge devices, for example), or pressurize the cabin. The pack doesn't boost this pressure; it simply conditions the temperature for cabin use.

Is it really necesary to maintain inside temperature with an open loop system - energy transsfer between inside and outside of a fuselage isn't done through the heat exchangers but rather directly - warm or cold air is constantly flowing throug fuselage.


It's not necessary to maintain temperature with an open loop system. It's necessary to maintain pressure. We have a continuous supply of bleed air, and this bleed air is too hot to use directly in the cabin. To make it safe for use, we must condition it, and conditioning the air by regulating it's temperature is a key part of pressurizing the cabin. Pressurizing is necessary for comfort, safety, and life support.

Energy transfer IS done by the heat exchangers. In the packs.

But one is for certain, any heat taken from the engine is a loss of energy, so why cargo planes, that don't even need lots of air refreshing don't have this kind of system where bleed air is warming the fuselage via heat exchanger?


Cargo aircraft have the same need for pressurization that any other aircraft requires. Additionally, regulating the temperature in cargo compartments is also important. Our cargo aircraft have carried everything from military equipment to full sized whales to teams of race horses to perishible flowers. Maintaining internal temperatures is important for cargo, personnel inside the airplane (we breathe the same air that passes around the cargo), and it's important for proper operation of aircraft systems.

Bleed air must be cooled, in order to warm the fuselage, else it warms the fuselage too much. Heat exchangers aren't used to warm the fuselage. They're used to cool the air that pressurizes the fuselage.

I get the impression that you think an air cycle machine is unnecessary complexity, but the truth is it's utter simplicity. It requires less energy, and fewer steps, to condition the air than any other type of air conditioning system except perhaps an evaporative cooler.

Could instead of air to air heat exchangers, cooling of bleed air will be done by liquid to air heat exchanger with fuel as liquid.


Of course they could, and some aircraft air conditioning systems use freon or other refrigerant, just like your car. However, for large aircraft, the extra weight and complexity, the need for additional electrical compresors, etc, negate any such value. An ACM is more simple, more reliable, easier to maintain, and can handle the volume of air required for a large cabin much better than a liquid refrigerant air conditioning system. More importantly, it's necessary in order to make the air useable to pressurize the cabin.

Smaller airframes can get away with simply mixing cold ram air with bleed air through flow control systems, to come up with an acceptable cabin temperature. This doesn't work so well in a big airframe with large volumes of airflow. It's much more simple to simply condition the air being used to pressurize the aircraft, and then fine tune it with some warm trim air.

As I know jet fuel is certified for operating down to -37°C. 30-36,000 feet above is -56°C so is fuel really heated just prior it is injected into the combustion chamber?


Various systems are used to warm fuel. Fuel-oil heat exchangers are common. These exchange heat between aircraft fuel sources and engine oil. We use fuel to cool hydraulic fluid, too. We heat fuel at certain times when it's about to enter the fuel control unit on the engine, using bleed air. However, this must be monitored very closely and can't be a continuous process. We have schedules spelled out regarding how frequent and how long we can apply fuel heat.

Numerous types of fuel and fuel formulations exist; not all of them have freezing points at -37 deg C. This represents a range for cut fuels (fuels containing gasoline products as well as kerosine). Fuels such as Jet A have freezing points which are lower, around -54 degrees C.

There are other ways to warm fuel. Transferring fuel can warm it. The quantity of fuel affects temperature; bigger tanks with more quantity and capacity maintain temperature better than smaller, thinner tanks with lower capacity and quantity. A bette system has to be found to cool bleed air for cabin use, however, than running it all the time through fuel, because there are many times when heating the fuel isn't desirable. Accordingly, using air cycle machines with heat exchangers and expansion turbines works very well, and very reliably...the reason they're so widespread in their use. they're light weight, simple, and consistent.

If the bleed air will be cooled down trough wing's leading edges, (doing usefull job) and fuel, the energy taken away from engine will be renewed throug fuel that is sooner or later injected to the engine. Aslo at plane stationary on the ground with hot outside temperatures, bleed air could be cooled down to temp od a fuel and when decompressed throug expansion turbine it will be cool enough to make its job.


I have no idea what you just said.

[sticky_beaver]

SNS3Guppy I don't know are just won't understand me or am I writing some none understandable language!

Three basic things that environment control system have to provide in the cabin:
1. safe pressure 750-1000mbar
2. safe temperature 18-26°C
3. safe CO2 or O2 level (minimum O2 level that human can still breathe is 17,5%)
My point was and I can back it up with some basic data, that fulfiling all three basic task of ECS could be less energy greedy if the close loop heating system will be used and open loop system for fresh air delivery.
Meaning that rough bleed air (hot and pressurized), will heat the cabin via air to air heat exchangers layed down the cabin flor, then still pressurised will return to the engine and small amount of fresh air will be deliverd from ACM via ceiling vents.
Cooling will aslo be done throug ACM, (not really efficient way, because energy is wasted) but because of simlicity and fact that plane doesn't operate much on the ground.
Same is applied in the BUS or a train where hot engine water is heating the inside of a bus, fresh air is deliverd separatly via ceiling vents, cooling on the other hand is one more separate subsystem - buses usualy have evaporator with fan on the back of the roof and airstream is also layed to the ceilling vents.

http://www4.slikomat.com/08/1223/ic-untitl.jpg

Data from this table shows how much air is needed to maintain safe O2 level in the cabin and how much more is needed to warm the cabin durring flight 36.000ft above.

http://www4.slikomat.com/08/1223/plo-untitl.jpg

If you cool the bleed air prior it is introduced to the cabin energy is wasted and if you heat the big volume of fuselage with mixed air from ACM and engine that you will directly introduced to the cabin (can't be very hot) you need much higher mass of air because heat was wasted in heat exchangers.

wiki link to ECS (usful data) Environmental Control System - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Environmental_control_system)

[SNS3Guppy]

I have to go take a flight, and will review all that later...but it appears you're here to ask a question, but to sell a point...on a subject you really don't understand.

If that's indeed the case, we've probably gone as far as we need to go on this subject.

Sounds like with Wikipedia, you've got it all figured out. Now go sell it to all the major manufacturers and you've made your fortune. Good luck.

[barit1]

sticky_beaver, I'm looking at your revised schematic and see you are planning to return bleed air to the engine, after it's been through the cabin.

For me, an old-old engine guy, this presumes that (1) the engine can do something useful with the extra air, and that (2) there's an appropriate place in the engine flowpath to reintroduce this air to the engine cycle. I'm always looking for cycle improvements, and your proposal deserves a fair look.

First of all, the overflow air from the cabin is at about 10-11 psia (equivalent to 8000 ft cabin pressure altitude, approx. - refinement of the numbers welcome!). There's only one place in the engine cycle where this flow could be introduced without backflow - and that's right at the engine fan inlet.

But this reclaimed air is much warmer than the -30C TAT found at cruise, and thus it's of lower density - and the engine would like all the inlet air density it can find, to operate at its best.

Sorry, sticky_beaver, I don't see a practical improvement to the status quo. :uhoh:

[ITCZ]

as I am not the proffesional, but I am interested in planes since I am playing the Microsoft flight simulator, and I wish to know more about aeronautics and techology of a plane.
Perhaps that is the source of the problem, and the angst.

Tech Log used to be a valuable stopping points for professional pilots to look and see what is being discussed re: the finer points of our profession. A good first port-of-call for those tricky questions, or the I-wonder-how-that-came-to-be musings after a day's flying.

It is understandable that there are a number of flight simmers and high schoolers that have an interest in aviation and would like to know more.

But, enthusiasts, what reaction would you expect if you walked into a truck driver's bar and started asking 'where does the air come from for the air brakes?'

Would it not be more appropriate to place your questions in the Spectators Balcony forum, populated by genteel and helpful pilots inclined to answer such questions.

Moderators?

[john_tullamarine]

First, my apologies for taking so long to edit out some inappropriate comments - been away for several days .. Christmas comes but once a year, as they say.

Second, we probably ought not to have an aversion to questions asked by non-pilots (provided that the questions have some relevance to flying) .. on the basis that the best way to test our own knowledge is to try and explain something to someone endeavouring to come to grips with a problem.

Underlying suggestions of elitism are best left out altogether.

Third, if a question doesn't hold the interest of the group the thread will rapidly descend into the ooze of the archives ... hence there is usually no practical need for moderators to worry too much about whether questions are "good" or "bad".