Why Does A Propane Torch Get Cold When Used? Updated, With Refrigerators

I was fixing a leaky water pipe in my garage yesterday, and so I had occasion to use a propane torch to heat up the pipe and melt the solder into the joint. It’s not something I do often, but I do remember learning from my father (who was experienced in plumbing, and was somewhat a jack-of-all-trades) how to do these kinds of repairs. The pipe was horizontal, and so it was a bit of a trick to get the solder to flow horizontally into the joint, when it would rather have fallen onto the ground below. But I found that by not getting the pipe too hot, I could get most of the solder to solidify by the time it reached the bottom of the pipe, and I hoped that this would suffice to seal the joint.

I let the pipe cool, turned the water back on, and was pleased to see that the leak was plugged. Yay me!

My father-in-law is fond of telling a story of how he once tried to fix a water pipe in an emergency during the night, but just could not do so, because there was no way to drain the water pipe. No matter how long he tried, he just could not heat the pipe hot enough to melt the solder. In the morning, he managed to figure out a way to drain the pipe, after which he plugged the leak immediately. This is a great illustration of the high heat capacity of water, isn’t it?

In fixing the pipe yesterday, I couldn’t help but notice that the torch seemed to get colder in my hand. I was momentarily puzzled by this, thinking it might be just my imagination (holding a cold object for a while does make the hand feel cold, and it was a cold day). But after thinking about it, I figured out the reason.

And I remembered a corroborating observation: Last summer we were running the barbecue for a very long time on a hot day, when the propane tank became exhausted. I removed the “empty” tank, and noticed that it felt quite cold to the touch, and replaced it with a full tank that was as warm as the air.

Why does using a propane tank make it get cold?

Update: Commenter William has come up with the explanation for why the propane tank gets cold. He makes a nice connection with rubbing alcohol feeling cold on our skin, which happens for the same reason: evaporation.

When water in a cup evaporates, the molecules that leave the cup must overcome the attractive forces from nearby molecules in the cup. This means an evaporating molecule must absorb some energy, so as to increase its potential energy, even if its kinetic energy does not increase. Where does this energy come from? From the kinetic energy of the nearby molecules; because the average kinetic energy of the water molecules in the cup is a measure of its temperature, the temperature of the water in the cup decreases when water evaporates.

Why don’t we notice that the temperature of the water in the cup decreases? Because the evaporation process is slow, and because the water in the cup also continually absorbs energy from the air (air molecules collide with the water and the outside of the cup), so that its temperature remains more or less constant.

The same happens when you exit a shower. The water on your skin begins to evaporate; to do so, it must absorb energy from the surrounding water that is still on your skin. The cooled remaining water on your skin absorbs energy from your skin; it is this flow of energy from our skin to the water on our skin that makes us feel cold. Once we dry our skin, then evaporation stops, and energy no longer flows out of our skin at an abnormally high rate (our body temperature is higher than the temperature of the air, so there is always some flow of energy from our bodies to the air), and we no longer feel cold.

We can summarize this discussion by saying that evaporation cools the remaining liquid. It is perhaps surprising that boiling does the same! Boiling is just like evaporation, except that the conversion from liquid to gas proceeds at a greater rate. Why don’t we notice that boiling cools the liquid in the pot? Because the stove’s heating element is pumping energy into the pot (and its liquid) like crazy.

If you were careful and measured the temperature in the liquid in the pot as it was heated to boiling (this is a standard experiment in many high-school physics or chemistry classes; search for “heating and cooling curves” if you want to learn more), you would notice that the temperature of the liquid gradually increases until the point of boiling, at which time the temperature remains constant. Before boiling, most of the input energy goes to increase the kinetic energy of the liquid molecules (of course the evaporation rate increases, so some of the energy goes to increase the potential energy of the evaporated molecules). At the boiling point, the average kinetic energy of the remaining liquid in the pot stays constant, as all of the input energy goes to increase the potential energy of the evaporated molecules.

Inside the propane tank, as the propane escapes the tank and is burned, the pressure in the tank decreases, which stimulates boiling of the liquid propane in the tank. Just as the water boiling in the pot on the stove tends to cool the remaining water in the pot, the boiling propane tends to cool the remaining liquid propane in the tank. The difference is that the energy from the surrounding air flows into the propane tank at a much slower rate than the energy flowing into the water pot from the heating element. This allows us to notice that the propane tank cools. However, if we wait a little while once the propane tank is shut off, then enough energy will flow into the tank from the surrounding air, and the temperature of the tank will return to the temperature of the surrounding air.

Boiling is an example of what is called a phase transition; follow the links to learn more.

A similar idea is used in the operation of a refrigerator or air conditioner. A suitable fluid is compressed, which increases its temperature, as it flows through a thin pipe outside the back of the refrigerator. This cools the liquid, and when it reaches the end of the pipe and passes inside the refrigerator, it flows through a nozzle which expands the fluid, rapidly turning it into a gas. This cools the fluid considerably, so that it is colder than the contents of the refrigerator. As the fluid flows through a pipe inside the refrigerator, it absorbs energy from the refrigerator. When it reaches the end of the inside part of its cycle, and passes outside the refrigerator, the fluid is compressed again, which turns it back into a liquid, and increases its temperature. The fluid flows in the pipe outside the refrigerator, giving off energy to the surrounding room, and the cycle continues.

The cycle described in the previous paragraph transfers thermal energy from a colder place to a warmer place; this is unnatural, and would not happen without the clever process described. And of course, there is a cost: it takes energy from an external source to run the pump and compressor that circulates the fluid. See the laws of thermodynamics, particularly the second law of thermodynamics, and the Carnot cycle to learn more.

If the point of the refrigeration process described above is not to cool the inside of the refrigerator, but to warm a home, then the location of the pipes is changed, but the process is essentially the same. See heat pump to learn more.

(This post first appeared at my other (now deleted) blog, and was transferred to this blog on 22 January 2021.)