# Capillary tube under atmospheric pressure

I found that capillary tubes in refrigeration system are tubes with very small diameter and very long length. Pressure drops down suddenly due to very small diameter of the capillary and length. The smaller the diameter and longer the length of the capillary, more the drop in pressure.

I am not from mechanical stream, I am just trying to learn.

If capillary tube inlet is $$1$$ atm pressure, can we achieve dropping the pressure to the perfect vacuum?

If anything wrong correct me?

• You might be interested in this video youtu.be/BickMFHAZR0 Sep 20, 2022 at 15:17

Your picture of this phenomenon is in error, as follows.

capillary tubes can lift liquids up against gravity because of the interaction between the liquid molecules and the molecules that form the walls of the tube.

If there is no liquid in contact with the tube, there is no pressure difference along the length of the tube.

• Thanks for your answer, but i forgot to mention that the capillary tube inlet is 1 atm pressure fluid and can we attain negative pressure in the outlet of the tube? (Consider the fluid is liquid) Jun 25, 2019 at 0:44
• the negative pressure exists behind the fluid-air meniscus. Not sure what you mean here. Jun 25, 2019 at 3:46
• I think i miscommunicated in the wrong way. I have a doubt regarding refrigration capillary tube? Whether we can achieve negative pressure all the way up to perfect vacuum? Jun 25, 2019 at 5:07

For a water-filled glass tube in air at standard laboratory conditions, γ = 0.0728 N/m at 20 °C, ρ = 1000 kg/m^3, and g = 9.81 m/s^2. For these values, the height of the water column is.

h=(1.48*10^-5)/r m

https://en.wikipedia.org/wiki/Capillary_action#Height_of_a_meniscus

The "vacuum height" for a column of water is of 10.3 meters so with a capillary tube with a radius of 0.00144 mm you could reach 10.3 meters.

I don't know if there is a liquid that has much more capillary force than water. For this answer i will write about water, but this answer might not be true for other fluids, instead you may get a high vacuum or no vacuum at all.

There is a simple approximation for the mean free path, mfp, of particles in air at room temperature (T = 25 C) given by.

mfp (cm) = (5*10^-3)/p (torr)

https://www.davidpace.com/mean-free-path-of-air-2/

With a pressure of 17.36 torr the mean free path will be at 0.000288 cm, which means that air wont move easily through the extremely thin tube and the decompression will stop.

That is why large diameter tubes are used in very high vacuum systems.

This is a high vacuum hose. The atmospheric pressure is of 760 torr and 17.36 torr is a very low vacuum. So depending on what vacuum means for you, it is possible to achieve a vacuum or not.

• This question is related to Refrigeration system. Capillary tubes are used to drop Pressure (i.e High to Low). Say, the High Pressure is at 10 atm pressure capillary tube drop to 3 atm Pressure. Now, if we take 1atm as High Pressure will it drop to 0.1 atm Pressure is the question Jun 27, 2019 at 15:35

The capillary action of liquid in a tube, is due to tensile force due to the wetting (of a liquid) at the liquid's surface. Lacking a surface, a gas has no such applied force.

The capillary in a refrigeration apparatus has a different action entirely, acting as a porous plug. Any gas molecule can and will diffuse toward the low-pressure end of such a tube, but as it moves, it hits the capillary walls, and sheds momentum in doing so. This drag force is appreciable only because the narrow passage is mainly very close to the wall, so such collisions are likely.

So, far from creating a vacuum, a capillary tube will move high pressure gas slowly into a lower pressure region (like, from the outlet of a pump to the inlet of the same pump) accomplishing a temperature change as it does so.

The refrigerator uses the temperature change to accomplish its main task. In doing so, it degrades the "vacuum" (which the pump must re-generate).