Horn & Whistle Magazine: Source for Horns, Whistles, Sirens, Collecting signal devices and Related Information
Compressed
Air Receivers
The following article appeared in issue #110 of Horn & Whistle. We have reproduced it here for your convenience. Whether or not you are a subscriber, If you are involved with our hobby you probably use compressed air for sounding your horns and/or whistles. Using the right receiver for any compressed air application is critical; your life can depend on it. We're showing you this article as a service from H&W. If you do any activity at all that requires having and operating a compressed air system, read the following.
If you are an active horn and/or whistle collector and have a compressed air system for sounding your prized possessions, then potentially you do have a dangerous device on your property. I refer of course to the receiver or tank which is associated with your air compressor. [Past H&W articles have mentioned this topic, but it is so important that it is worth repeating periodically, particularly for the benefit of our new subscribers who may not be aware of its implications or potential danger.] ed.
We need receivers or air tanks mainly because most of the air compressors that we are likely to have in either home workshops or even some industrial locations do not supply air as quickly as a signal device, particularly a whistle, consumes it. Also, most of us use reciprocating or piston type compressors, and the air output from these consists of a series of puffs with each compression stroke of the final compression stage’s cylinder. In order to smooth out the airflow, and in particular to be able to provide an intermittent much higher flow rate for a certain limited time interval, we follow the air compressor with a vessel, usually made from steel, which serves as a storage container for the compressed air. It is true that industrial centrifugal compressors deliver a steady output, and big industrial centrifugals can sustain even a large whistle indefinitely, but a big multi-HP centrifugal compressor in a home workshop setting is not possible. Most of us do our horn and whistle tooting in our home shops instead of at one of our major events where we are fortunate enough to have steam for our whistles and we typically do it with home workshop air systems.
There are many types of vessels that might hold air under pressure, but compressed air service has several very, very important considerations that mandate special design factors which we must observe when we need to use a vessel or tank to contain compressed air. The terms receiver and tank are often used interchangeably. The term receiver is the preferred term for a tank which immediately follows the output of an air compressor and “receives” the compressed air. In many cases, the compressor and its driving motor mount on a steel plate which is welded to the top of the receiver and makes the entire system fairly compact.
“But I have this great
propane tank that I use (or refrigerant gas cylinder, or some other hollow,
steel vessel) so I am all set. Been using it for a long time without incident.”
STOP THIS FOOLISHNESS RIGHT NOW! Be aware of this!
Compressed air receivers are typically holding pressure which varies all
the time when in use. The compressor pumps air into the receiver. The pressure
increases. Then the pressure switch reaches its preset pressure limit and
shuts off the compressor. We continue to use compressed air and the receiver
pressure drops. When the pressure drops below the pressure switch’s
lower limit, it turns on the compressor, and the receiver pressure starts
to increase, unless there is a bigger demand than the compressor can keep
up with in which case the pressure will drop further. But eventually the
demand stops and the compressor continues to run until the receiver pressure
is restored and the pressure switch turns the compressor off. And so it
goes with a receiver; the pressure generally rises and falls repeatedly.
This action puts a cyclical mechanical stress on the receiver. It’s
somewhat like bending a piece of metal back and forth until eventually it
weakens and breaks. Springs, of course can flex repeatedly without breaking
(usually) because they are designed to flex.
The point is that the repeated pressure cycling in a receiver causes repeated stressing of the receiver and many gas tanks and other vessels are designed to be filled once and emptied slowly. When you subject them to repeated pressurizations and depressurizations, you are potentially stressing
them in a way for which they were not designed, and thereby little-by-little weakening them.
Another big issue concerning air receivers is temperature. Ever notice how hot the air is when it leaves a compressor? Where’d all that heat come from? It was in the air all the time. It is the latent heat which becomes in effect “concentrated” when you compress the air. Compressing air doesn’t add heat to the air, but it significantly raises its temperature by concentrating the latent heat already in the air. You can find the physics of adiabatic compression in textbooks on the subject if you’d like it explained in much better detail. The point of our concern is that air coming out of a compressor is hot. I have seen small, single-stage home workshop compressors in which the copper tubing leaving the compressor is actually discolored from the heat of the air which passes through.
In most home and small workshop air systems such as we are likely to use, especially where the compressor mounts on top of the receiver, the air going into the receiver is quite hot although better compressors usually send the air through a length of finned tubing which is known as an aftercooler and which dissipates some of the heat of compression to the surrounding air. Many better compressors have the spokes of the compressor flywheel angled in such a way that they act like fan blades and cause cool ambient air to pass over the compressor and the finned copper tubing. Bigger compressors have water-jacketed cylinders, but these are unlikely in a small shop installation.
At any rate, not only does a compressed air system like this cyclically vary the receiver pressure, but also subjects it to cyclical thermal stresses as well. Air receivers are engineered and built with these factors taken into consideration. Propane and refrigerant cylinders are not, because there is no need for them to be. It’s easy to get an old propane or refrigerant cylinder, and it’s very tempting to say, “this will work fine.” But this is very unwise thinking. It is taking a huge risk. In using these other types of vessels on compressed air systems, you are asking a vessel to hold up under service which is much rougher than it was designed to withstand.
Still another very, very important consideration in a compressed air system is moisture. Our normal atmosphere contains water vapor as one of its many constituents. When we compress air, we take a given volume of air and stuff it into a much smaller space which is why it develops pressure in the first place. The water vapor concentration in the air in the receiver likewise increases along with the temperature. Typically, the water vapor will condense on cooler surfaces such as the inside walls of the receiver. It then drips and runs to the bottom of the receiver where it pools. This is why all air receivers have drain valves on the bottom, so that you can drain off this water on a regular basis. Propane and refrigerant cylinders have no drains because they don’t need them.
If you open an air receiver’s drain valve with pressure in the tank, the draining will occur much faster than if you do it with the tank depressurized. Be careful when you do this-avoid getting your eyes splashed. You will also note another interesting phenomenon when you do this. The water will appear very cloudy. This is because the water has absorbed some of the air, just as beer absorbs carbon dioxide when subjected to CO2 under pressure in the carbonation process. The cloudiness is really thousands of very small air bubbles in the liquid. Now, what happens to beer when you depressurize it? Easy, just open a beer bottle and observe. The beer fizzes and little droplets initially spray out. The same thing happens to the water that collects in an air receiver when you draw off compressed air and the receiver pressure drops. The water fizzes a little, releasing tiny droplets into the air to get carried
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