# Use only a powerful fan without a compressor?

vmars316
Hello & Thanks,
Is it possible to use only a powerful fan
without a compressor to make very cold air ?
Thanks

Gold Member
2022 Award
Hello & Thanks,
Is it possible to use only a powerful fan
without a compressor to make very cold air ?
Thanks
What do you think? Why would the air coming out of the fan be colder than the air going into the fan?

russ_watters
vmars316
What do you think? Why would the air coming out of the fan be colder than the air going into the fan?
Yes, obviously,
But I am thinking of a fan blowing into a funnel shaped thing
where the speed of the air increases as it goes thru
the very narrowest end of funnel shaped thing .

And if the funnel was made of iron
I am wondering where on that funnel would be the coldest point ?
Where would the coldest point begin (before it spread) ?

I am trying to get a mental picture of what is happening and where .
Is there a diagram somewhere showing this ?
Thanks

Gold Member
2022 Award
Yes, obviously,
Obviously WHAT?

But I am thinking of a fan blowing into a funnel shaped thing
where the speed of the air increases as it goes thru
the very narrowest end of funnel shaped thing .

And if the funnel was made of iron
I am wondering where on that funnel would be the coldest point ?
why would any point be colder than any other point?

jartsa
But I am thinking of a fan blowing into a funnel shaped thing
where the speed of the air increases as it goes thru
the very narrowest end of funnel shaped thing .

And if the funnel was made of iron
I am wondering where on that funnel would be the coldest point ?
Where would the coldest point begin (before it spread) ?

If air inside vacuum cleaner's hose has mass 0.01 kg and its speed is 10 m/s, then
the air has kinetic energy 0.5 J.

That air's thermal energy is reduced by the amount of its kinetic energy, 0.5 J. That means that the air in the hose is just a little bit cooler than the air in the room where the vacuum cleaner is.

If air inside vacuum cleaner's hose has mass 0.01 kg and its speed is 10 m/s, then
the air has kinetic energy 0.5 J.

That air's thermal energy is reduced by the amount of its kinetic energy, 0.5 J. That means that the air in the hose is just a little bit cooler than the air in the room where the vacuum cleaner is.

This is just wrong. Center of mass energy is independent of thermal energy. Do you think astronauts get colder and colder as they accelerate into space?

Homework Helper
Gold Member
And i think you will find vigorously stirring a fluid (eg with fan blades) heats it up.

russ_watters
vmars316
Obviously WHAT?

why would any point be colder than any other point?
Because the cold has a starting point .
Suppose the funnel is made of a stack of iron rings .
Each ring having a smaller diameter than the ring above it .
And in between each ring is a ring of an insulating material
keeping the cold from spreading .

Again , where does the cold begin ?
Anybody ?
Thanks

jartsa
This is just wrong. Center of mass energy is independent of thermal energy. Do you think astronauts get colder and colder as they accelerate into space?

We don't want to use compressor, for some reason. Well luckily we have compressed air all around us.

We just need to create a place we the compressed air can expand into. Then the compressed air does work and cools.

Doesn't air cool when it rushes into a vacuum cleaner hose?

Mentor
Hello & Thanks,
Is it possible to use only a powerful fan
without a compressor to make very cold air ?
Thanks
This sort of question has come up before. First, a "fan" and a "compressor" are similar devices physically, having the same basic purpose and in many cases operating principle; to move air by increasing its pressure. The terminology difference is typically just based on how much pressure they can provide.

Next, a fan or compressor itself does not cool the air, it heats it. A fan, with a low pressure rise, heats the air due to its inefficiency. A compressor heats it due to the compression itself.
But I am thinking of a fan blowing into a funnel shaped thing
where the speed of the air increases as it goes thru
the very narrowest end of funnel shaped thing .

And if the funnel was made of iron
I am wondering where on that funnel would be the coldest point ?
Where would the coldest point begin (before it spread) ?
[separate post]
Because the cold has a starting point .
Suppose the funnel is made of a stack of iron rings .
Each ring having a smaller diameter than the ring above it .
And in between each ring is a ring of an insulating material
keeping the cold from spreading .

Again , where does the cold begin ?
I really don't have any idea what you mean by this. What does it mean for the cold to "begin"? Some effect has to make the air colder - what effect do you think that is?

Taking a stab in the dark, I'm going to guess that you think the Venturi effect, which lowers pressure, will cause cooling. It could, but this would be very limited and unless you had a heat exchanger after the fan to get the air temperature back down to ambient, it would still end up warmer than it started.

Now, if you had a supersonic nozzle, the expansion after the throat would significantly cool the air. But that takes a pretty high pressure to accomplish.

PaulK2
vmars316
We just need to create a place we the compressed air can expand into. Then the compressed air does work and cools.
This sounds right
But :
When I use a can of bug spray ,
the can in my hand gets cold .
I don't know if the spay is cold or not (too toxic to check it out) .
But the hole in the nozzle is much smaller that the tube leading to it .
So there is work being done there also as air is released into the atmosphere .

It sounds like you are saying then that :
There are two sources of cold .
1) because the compressed air is expanding (doing work) into the
narrow tube , that goes from the can to the nozzle ,
the potential energy of the compressed air spends itself into kenetic energy ,
with an end product of velocity and cold .
2) because the compressed air is expanding into the atmosphere
ie., doing work , it also has end product of velocity and cold .

Could you explain a little further ?
And what is the name of the law that says
When compressed air does work it cools ?
What's going on there exactly ?

Mentor
This sounds right
But :
When I use a can of bug spray ,
the can in my hand gets cold .
I don't know if the spay is cold or not (too toxic to check it out) .
But the hole in the nozzle is much smaller that the tube leading to it .
So there is work being done there also as air is released into the atmosphere...

What's going on there exactly ?
The can gets cold because it contains a chemical liquid that boils at below room temperature and a certain pressure. So when you spray it, the chemical boils and that absorbs heat, making the can cold.

jartsa
This sounds right
But :
When I use a can of bug spray ,
the can in my hand gets cold .
I don't know if the spay is cold or not (too toxic to check it out) .
But the hole in the nozzle is much smaller that the tube leading to it .
So there is work being done there also as air is released into the atmosphere .

As mentioned by @russ_watters , the can contains some liquid that becomes gas.

But what if we let out half of the contents of a compressed air can? The can gets cool, right?

So it must be so that the gas in the can is doing work as it is expanding. Only work there is to do is to push the gas that is going out, push it from the back, so to speak.

Now it happens to be so that I forgot that the air in the room is losing energy as it is doing work by pushing the air that is going into the hose of the vacuum cleaner.

So the calculation was not right.

But anyway, we can still make the air in the hose about as cool as we want by increasing the power of the fan.

Mentor
As mentioned by @russ_watters , the can contains some liquid that becomes gas.

But what if we let out half of the contents of a compressed air can? The can gets cool, right?

So it must be so that the gas in the can is doing work as it is expanding. Only work there is to do is to push the gas that is going out, push it from the back, so to speak.
What do you mean by "compressed air can"? Do you mean the small cans that you use to blow dust off computer keyboards and other electronics? Those use a liquid hydrocarbon too...

jartsa
What do you mean by "compressed air can"? Do you mean the small cans that you use to blow dust off computer keyboards and other electronics? Those use a liquid hydrocarbon too...

Oh. I thought those contained compressed air.

Well any air container that actually contains air should cool as air is let out of them. Not as much as those liquid containing cans, but some amount.

vmars316
The can gets cold because it contains a chemical liquid that boils at below room temperature and a certain pressure. So when you spray it, the chemical boils and that absorbs heat, making the can cold.
So you are saying that the insect spray contains freon ?
Why would it contain freon , instead of just compressed air ?
Thanks

Mentor
So you are saying that the insect spray contains freon ?
Why would it contain freon...
Not freon. The MSDS for Raid, for example, says it contains 10-30% isobutane, some propane and a couple of other hydrocarbons.
https://marketing.msdsonline.com/library/PMI/PMI854.pdf

Isobutane boils at 11F at atmospheric pressure. Propane at -44F.
.instead of just compressed air ?
Because liquids are denser than gases so you can fit more in the can.

vmars316, anorlunda and Asymptotic
jartsa
And what is the name of the law that says
When compressed air does work it cools ?

Well the law of conservation of energy is a law that always applies.

Then there are the Bernoulli's laws for incompressible flow, compressible flow, steady flow and unsteady flow.

And these laws: https://en.wikipedia.org/wiki/Gas_laws

Maybe you can pick a suitable law from those.

vmars316 and anorlunda
vmars316
Thanks All , Great help !