Why Does Compressed Air Get Cold?
While the question itself is rather vague, there are several instances in which one may come into contact with compressed air that could be observed or seen as being cold or to have a cooling effect of some kind. When releasing compressed air, there are many thermodynamic forces at play.
While there are several different reasons that compressed air may feel cold, the most significant contributing factors are the Joule-Thomson Effect, adiabatic expansion, and the mass of air’s velocity along with its own intrinsic ability to absorb heat from the environment via conduction.
In this article, we will take a look at several situations in which compressed air or the container it is stored in can get cold. We will also go over why this occurs and we will talk a bit about the science behind the process.
Different Types of Compressed Air Delivery Systems
Canned Air (Synthetic Compressed Air)
- Low power
- Toxic
- Highly susceptible to The Joule-Thomson Effect
Tanked Air (Traditionally Compressed Air)
- High power
- Non-Toxic
- Minimally susceptible to The Joule-Thomson Effect
Historically, if one were in need of access to compressed air of any kind, they would have to buy an air compressor. In modern times, however, several canned air products have been introduced that contain various gasses that are compressed down so much that they are stored in their liquid state inside the can.
While canned air is an easy, approachable, and very low-cost way to access compressed air, this type of compressed air delivery is not powerful enough to run tools. Even if it was, it would not be economical because compressing air will always be much cheaper than what is essentially a synthetic compressed air alternative.
With that being said, canned air is an extremely useful and practical way of using compressed air for light dusting tasks. Because the two are essentially the same thing at different scales and pressures, we will discuss the use of compressed air in both canned air and tank form.
Inside An Air Compressor Tank
The contents of an air compressor tank is just normal air under high amounts of pressure. When any gas of a higher pressure is released, it expands which drastically increases its pressure. As the pressure of a given gas drops, its temperature also drops. This principle is known as adiabatic expansion.
In practice, however, one would have to rapidly decompress an entire air tank to achieve any observable level of cooling from this effect.
Inside A Can Of Air
Canned air is a little different. This product is usually a mixture of nitrogen and other inert gases that are compressed to anywhere between 40 and 70 psi. Most of these gases are known to displace oxygen in the local atmosphere, so you should always use canned air in well ventilated areas.
These gasses in canned air are compressed to liquid form, so the phenomenon described below apply much more to canned air than traditionally compressed air.
What Happens If You Shake A Can Of Compressed Air?
Most canned air contains refrigerant that is a liquid so long as it’s held under pressure. Canned air is designed to spray out as a gas rather than a liquid, so the valve is specifically designed to capture vapor from the top of the can. So, that means when you shake, turn upside-down, or otherwise flip canned air, you run the risk of getting liquid refrigerant to the valve.
The Joule-Thomson Effect
Any time a gas expands, it requires energy to do so. So, as a gas transitions from a high-pressure state to a low-pressure state, it consumes energy from the surrounding area to do so.
The phenomenon of a gas drawing its energy from the surrounding air is known as the Joule-Thomson effect. As this heat is suddenly drawn into the expanding gas, the air surrounding it drops in temperature.
Why Does An Aerosol Get Cold?
Similar to compressed or canned air, common spray cans and the substances sprayed from the are known to get cold and have cooling properties. This happens as a result of a combination of evaporation and expanding gas. The same processes in canned air apply to aerosol cans and their contents.
Is Compressed Air Hot or Cold?
Although compressed air may feel cool by the time you come into contact with it, the act of compressing air actually heats it up.
In fact, without any form of cooling, an air compressor can heat air to as much as 300 F. So, there is a multi-stage cooling system that cools down the newly heated, compressed air before it’s stored in the tank. This is why the best air compressors are fitted with highly sophisticated multi-stage cooling systems.
Does compressed air freeze?
While the air itself will not freeze, most air compressor systems are designed to operate above 40° Fahrenheit. Air compressors generate condensate, which does freeze. When that happens it can clog or even damage internal components.
It’s All About Thermodynamics
One of the reasons compressed air feels cold is because of the adiabatic cooling process, which is a thermodynamic process. This principle states that a gas at high pressure will cool significantly when that pressure is released. There is a certain amount of energy required to do that, and that energy has to come from somewhere.
This Doesn’t Play A Big Role In Traditionally Compressed Air
This type of cooling effect is much more pronounced in a can of air. This is because canned air is stored as a liquid. The evaporation of the liquid inside the can results in a massive drop in pressure as the substance in the can is converted from a liquid to gas. When this happens, a ton of heat is absorbed from the local environment.
You can, however, exaggerate this effect if you are wanting to satisfy a scientific curiosity. All you have to do is fill your air compressor tank and then rapidly release all of its pressure.
After you do that, feel the side of the air tank. You will notice that it’s very, very cold. This is a result of the Joule-Thomson Effect. Under normal circumstances, however, this kind of situation does not occur when using an air compressor, unless there is a leak.
So, Why Does Compressed Air Get Cold?
If you have made it this far into the article, you have learned that the cooling effect provided by the Joule-Thomson Effect does little in terms of cooling traditionally compressed air. You may be wondering, ‘If that’s the case, then why does compressed air from my air compressor get cold?’
The overwhelming majority of any cooling effect that you will feel as a result of coming into contact with compressed air will be as a result of direct heat conduction. As it turns out, a given mass of air has a certain amount of heat that it is able to absorb. So, a given mass of moving air has a certain level of heat removing ability.
This makes sense if you think about it. It’s how you can sit in front of a fan on a hot day and feel cooler. This works even though the fan itself is not producing any kind of cooling effect. All a fan, or in this case, an air compressor has to do to remove heat from a situation is to blow air towards it.
The simple act of coming into contact with moving air has a drastic impact on a given body’s ability to retain heat.
What is Heat Conduction?
Conduction heat transfer happens when molecular excitement within one material causes direct molecular excitation of another material. In other words, it happens when two materials come into physical contact.
There Is No Such Thing As Coldness
While it may seem counterintuitive, there is really no such thing as coldness. Physically speaking, all things are cold until you make them hot. So, when something is cold, it is returning to a state in which it has simply lost some of the heat that was given to it.
So, when you feel cold air blowing out of your air condition, you are not really feeling cold air. What you are feeling is air that the heat was removed from. It’s not blowing cold air on you, it’s blowing air on you that it removed the heat from.
Conclusion
The question of ‘Why does compressed air get cold’ is quite a broad one. So, considering the fact that there are several cases in which compressed will be perceived as cold, we did our best to cover the most common ones here.
When it comes to canned air, most of the cold that you will feel from it comes from the Joule-Thomson Effect due to adiabatic expansion. When it comes to traditionally compressed air, however, heat conduction is the dominant factor at play.
You may be surprised to realize just how much heat-energy can be moved away from a given object just by blowing a little air on it. In fact, air is such an effective means of moving heat energy via conduction, that fans are the primary cooling component in just about any cooling system.
We hope this article helps you learn a little more about compressed air and why it gets cold. Thanks for reading!
