Do air brake cylinders get hot, cold, or neither?

You are talking about air brakes on trucks and diaphragm used on equipment. The only heat in an air system is at the air compressor and that comes out of the compressor at some where around 300 degrees give or take. Most all systems go to a reservoir / supply tank. The supply tank is then the distribution center. Then from there any number of foot, hand and electoral valves can be used.

This little piston is not so little as you think. Most in use are 10 to 12 inches across with push force up to 100,000 ft lbs. Per square. May not be that high it all depends on how much air presser is being applied. Most diaphragms range in size from 4 to 12 inches in use on trucks. To answer your question . After the compressed air leaves the compressor it cools down to ambient temperature. Air when moved in lines become colder than the ambient resting temperature. At the cylinder it is some 30 degrees colder as it enters the chamber it can warms some. Because the chamber is warmer and that is the only reason I can think of why it warms up. But this is short lived as it soon turn cold again as the incoming cold air cools the metal and rubber down. When the air is released the exhausting air temperature drops to as much as a minus degree temperature. The minus is below 0' and there again only monetarily. Take in to account wind chill factors for air movement.

I do not understand what you are trying to say about a smaller volume getting smaller.? In this case where you are putting a volume of air in a confined space. Your volume of air get larger as in a 10 inch chamber with diaphragm. Then depending on the amount of travel on the cylinder. I read somewhere about air volumes. I think in this case you would have 5,000 to 25,000 square feet of air in the cylinder. I do not think this is a smaller volume getting smaller. As for air temperatures , air being compressed starting at say 6 to 1 compression builds heat the more the compression ratio the hotter the air will become at 14 to 1 if you inject a fuel oil you will have an explosion. In a engine cylinder you combustion being converted to power.

Yes the air will get hot only under a compression ratio factor greater the 15 lbs per square inch or more.

The probability of my explanation answering your question seams small as I do not know Boyle's law

Hey Luther,

what is Compression & Expansion?

Compression means by some external mean you are increasing the Pressure of air within a Fixed area.

Expansion means a given amount of steam will naturally (or by any process)attempt to expand into a space; means the the gases are expanding to occupy everywhere.

ok, coming to ur Q :- i am working from last 3 Years practically on Brake System. First the Air is feed through Compressor to Drying & Distribution Unit (DDU), this Unit is set upto 8 Bar Pressure, when Pressure gets to Increase, it releases the excess air & gives the feed back signal to Compressor. The air coming through compressor is aboout 110 -120 Deg Celcius, to reduce the Temperature a 15 to 20 Feet metal pipe is used to reduce its temperature (Convection) upto 80-90 Deg Celcius. This 8 Bar air is stored into the Reservior (Air Tanks 25-30 Liters depend upon System requirement). Through Reservior it goes to Relay Valves & then to Brake Chambers Or Actuatour. When Brakes are applied the compressed air (through Reservior) goes into the Actuator, pressing the Diaphragm; actuating the Drum Brakes. When the Brakes are released, the Compressed air gets released through the Relay Valves; causing the Brake Actuator gets to normal atmosperic pressure. As the Air Pressure is 7-8 Bar, the Temperature of Brake Actuator goes to 50-60 Deg Celcius. but due to convection it comes down to normal. I have not yet faced any Teperature Issue of Brake Actuator due to air only other than Brake Drum Friction.

Hope you get ur answer.

Say whaaat?

Air temps will change volumes somewhat , but the big issue with air brakes come from condensing water at pressure and then cooling the air. This can cause ice in places you definitely don't want it as the outside temperature drops. Experienced truckers get very particular about brake blowdowns in cold weather. They also use a type of antifreeze in very cold climates to keep their brakes working on the trailers. The long lines on them cool the air quickly, even under pressure, making evaporative cooling much more serious when the pressure is released. Ten ferr?

Simply put, no.

Certainly all friction brake surfaces get hot, but the pneumatic or hydraulic actuation plays no part in this

One may argue the effects of compression, etc. but its really not enough to consider in this application.

Any pressure change will produce a thermal delta when the volume is held constant. Moisture condensation is the primary concern. As the line air under pressure begins to fill a low pressure cylinder, the pressure drop lowers the air temp. Heat transfer from a metal cylinder would instantly equalize the temperature. When the piston or diaphram moves, the increased volume creates cooling. As the cylinder pressure builds, the air warms. When the air is exhausted, it will cool. Just like the cool stream from an air hose or tire valve stem. Ice formation is the practical consideration from these effects, so additives that lower the freezing point of internally condensed water are often added to systems not having an air dryer. Steve

There is a large spring inside the air brake servo that actuates the brakes. The air pressure releases the brakes. this is a safety feature the prevents loss of brakes. have you ever seen skid marks from trucks on the highway? That is because the system lost pressure and the driver has to mechanically disingage the brakes to continue. Aire does not take the place of brake fluid in the barake drum. The servo spring actuates a cam inside the drum camming the brake shoes out against the drum surface. As the servos are far away from the brake system they do not get hot from braking or use.

Long story short...what are you trying to do ? Lets start with that first.

Air in...air out sounds normal so far...

Good one 55418UND. Let's get to the question.

Thanks for all the response. It sounds like the temperature goes down not up. Since I don't know enough about brakes to ask the right question, here is a more straightforward question.

Forget brakes as such, it's about the physical process of pressure equalization.

Say you have a cylinder with a piston in it. The piston retracts, causing an intake of atmosphere. Just like a normal compressor so far.

Now equalize this volume with a reservoir that is large compared to the cylinder, by opening a large valve so the atmosphere-filled cylinder is raised almost to the pressure of what was in the reservoir, while the reservoir pressure goes down just a little. The reservoir and the cylinder are now at the same pressure.

It seems to me that the small volume of atmosphere, which is now compressed, will go way up in temperature, and the air from the reservoir, having lost a little pressure, will lose a little temperature. But the weight of air losing a little pressure exceeds the weight of the air in the cylinder getting hot, so at some point according to Boyle's Law the two temperatures will equalize the same as the pressures do.

I am asking if this temperature equalization is instantaneous, faster than pressure equalization, or slower.

If you find this question interesting, here's the next one: how much work does it take to push the equalized contents of the cylinder back into the reservoir with the piston, compared to what it would take to push a cylinderful of normal atmosphere into the same reservoir like a compressor?

Luther

First off, the pressure in the cylinders seldom if ever reaches the pressure of the reservoir,so there isn't equalization. The reservoir is quite large compared to the volume of all of the brakes together.The pressure from the brake system (controlled indirectly by the pedal) controls the amount of braking being used to slow the truck. I'm sure there are some temperature changes, but it has to be very minor. It would be possible to reuse this pressure, but air pressure is used for more than just brakes. Don,t forget shifting gears in the differentials. Since the pump is a straightforward design mounted on the engine, reliability and cost are bound to be the major consideration. Simplicity rules. It probably would be more efficient to reuse the air, if this is what you were hoping to do, but at more complexity and almost always at a loss of reliability. Hope this helps.

Dear Luther

I do not think the air temperature changes that much in a cylinder. The cylinder is more likely to get a temperature raise from the type of plunger gasket seal being used. In early days of cylinders leather was used, then a semi hard rubber. Now O-rings made with synthetic rubber, Teflon, ploy plastics and other chemical compounds . The other factor involved is how often the cylinder plunger is actuated, or used. For example if the cylinder plunger is in constant motion there will be a temperature raise. Where the time is random infrequent there will be little or no temperature changes.

Other engineering factors to consider are the diameter size of the cylinder, the length of stroke and last exposure. Exposure meaning is the cylinder in a closed area where there is no air cooling or is the cylinder exposed to the elements, air flow, rain, snow and freezing temps. All could and can have factors on temperatures.

It is possible to exhaust the reservoir supply. In this case the supply hose or pipe is more than 50% greater than the size of the cylinder. It is set up for a fast reaction time exhausting the reservoir. The reservoir is refilled between usages. Example a 10 to 15 inch cylinder, a reservoir 30 to 50% bigger and the hose / pipe of equal size needed for the dump. Typical used in amusement rides.

I found the equations for pressure equalization, which apply whether it's complete equalization or not. It's just Boyle's Law with the two initial PV values added together to equal the final condition. Complete equalization apparently has no net temperature change but depending on whether it's complete or not and what any piston might be doing, there might be a temperature change to account for, in which case it's the combined gas laws, same as Boyle's divided by temperature.

Weird thing, Joule's Law, the one that says if compression or expansion take place with no temperature change, no work has been done. I can't figure out how you can move air from a reservoir, change the pressure of air somewhere else, and say that no work has been done. I will have to study the definition of work.

Thanks again for all the response.

PV = P1V1 + P2V2 Boyle's Law adapted to full equalization or other gas mixture with no temp. change.

PV/T = P1V1/T1 + P2V2/T2 Combined gas law for gas mixture with temp change.

The answer was in an old railroad engineering book.

Luther