Air Compressor: Newsletter Challenge (06/06/06)

I think that is what you need. It all revolves around pressure, volume, and differential pressure as you move air from one chamber to the next.

The cold air is easy, since you get that when you open the valve to the tank as a high pressure gas escapes into a lower pressure environment. The temperature of the gas drops as per the PV=nRT equation.

However, I am not clear how to raise the temperature of the exhaust gas. You need to compress the gas to increase its temperature. Normally this happens when the compressor starts running by compressing the air in the room into the tank (low pressure to high). Maybe if you drain the tank and start the compressor or disconnect the output of the compressor and you should get warm air directly from the compressor when it runs.

If the air is allowed to vent into a much smaller volume the pressure would raise in this smaller volume very rapidly and according to the law PV=nRT, the temperature would rise. I saw this all the time when using high flow dispensing equipment to fill natural gas fuel cylinders on vehicles. Once the temperature of the gas and tank settles to room temperature, the pressure would be relative to the volume filled based upon PV=nRT. If you wanted to fill the smaller tank to a specific pressure, you could calculate the elevated pressure to fill the tank and compensate for the desired "settled" pressure at room temperature. Basic physics. M Hyatt Technical Consultants Denver, CO.

Expansion will cool the air a little, but for really cold air make it do work, such as routing it through an air tool. No air tools? Then fill a bucket half full of something (pecans?)and insert hose in the bucket such that the air will move the objects (ie-do work on the objects). The air will be cooler than just expansion alone.

Make a vortex tube

Yes, a vortex tube is known technology that will split the compressed air into a hot and a cold stream. It is used to provide spot cooling to do such things as dry the compressed air stream (by cooling to condense out water), or cooling suited up workers in environments such as paint booths.

Yep. Our machine shop has been using vortex tubes for years to blow cold air on cutting tools and stuff they're cutting.

And I'm sure that everyone has a vortex tube lurking around near the air compressor, if you can find a friend to help you clean out your garage, just so you can impress him (her) with a bit of magic.

Cold air is straightforward as already described by anonymous hero. If you can use a long enough tube, the fast-moving cold air will warm towards ambient. Slowing the air by using a widening taper will warm the air further, to become warmer than ambient. Of course, this doesn't strictly meet the specification of "no external power", as the air stream is being heated by the surrounding air. Mixing ambient air using the ventura effect and then allowing that to slow would work as well, but suffers from the same problem. I suppose we could wind metal tube round the compressor, so that at least some of required heat was taken from the compressor itself...

I have a laser diamond cutter that uses a compressor which blows directly on the diamond. I need to keep the compressed air at room temperature or above. Is there any way, including adding some type of equipment, to control air temperature?

Use a Hilsch tube.

Did you mean Ranque-Hilsch vortex tube. http://en.wikipedia.org/wiki/Vortex_tube

Yes. The thing that Wikipedia left out is that the Germans found Ranque's notes as they overran France. Hilsch did further work and improved it, but he didn't invent anything. There also was a guy named Von Brimen or something--he was a movie double for Edward G. Robinson--who claimed 40 or so years ago that he had perfected it and had a high efficiency/low pressure design. He offered it to Detroit for auto air conditioning but never reached an agreement.

A momentary compression of the air is enough to heat it. A conical oil funnel and a rag may work just fine. Place the large end of the funnel at the compressor outlet seal it the best you can with the rag and open the valve. Just as long as the air at the compressor outlet is traveling faster than the air leaving the small end of the funnel you'll have hot air! Kids! Don't try this at home without an adult present. You may shoot your eye out!

I used to work in a gas station - you know the place where "they" filled your car and did everything.

To show cold air, let some out at him from a distance (greater than 2 mm).

To show hot air, press the nozzle on his hand and let air out as it presses on his hand to escape it will heat up and feel warm (because it is) to him.

Games you can play with the air hose.

If you want to do something dangerous like this - use your own hand.

IIRC one of the plant engineers at the armory in Harper's Ferry was perplexed over the fact that whenever an air supply line ran to a T fitting with different sized outlet pipes the smaller oulet pipe would always feel warmer than the larger outlet pipe whenever the machines attached to each of the outlet pipes were drawing air at the same time. Anyway he tinkered with the idea for a while and may have patented a tool or device. At any rate the patent would have expired by 1900. In the 1970's you could buy a tool called a choke tester that was simply an improved version. Attached to shop air, one end would blow hot enough air to open a carburetor bimetal choke spring or conversely blow cold enough air to close the choke back up. You could open and close seat heater thermostats, chill an interior temperature thermistor enough to inhibit A/C function, heat/cold stress test small electronic components in difficult to otherwise access locations without worrying about collateral damage from a heat gun, for example. All in all, a pretty useful concept.

How about applying air pressure to the "small" end of a double acting air cylinder, the ambient air in the other end will heat up upon compression.

With the air compressor off, simply open drain valve and the compressed air will be cooler.Due to the build-up of condensation in the bottom of the tank. Whenever air is compressed,it will cause this condensation build up. Open an air hose connected to the outlet and the air will be warmer. When releasing the air thru the outlet ( by use of an air hose), this will not collect the condensation at the bottom of the tank, therfore the air will be warmer.

This device can separate gas into two streams, one hot and one cold, by spinning it quickly. Cold air comes from the middle, and hot gas from the outside. This site has a picture and explanation http://www.visi.com/~darus/hilsch/