Why is there a under pressure effect after opening of a refrigerator door?

Slickepot

When opening a refrigerator door, the cold air inside mix with the warmer air outside. Directly after closing the door, it is noticably harder to open it again. Does anyone know how these temperature changes affect the pressure difference and exactly why?

Thank you.

harrylin

Obviously the refrigerator cools down the warm air and that creates an under pressure; however, it's a bit surprising how fast the effect is (perhaps there is something else that we don't think of).

See: http://en.wikipedia.org/wiki/Gay-Lus...emperature_law
The pressure change in the fridge when the air cools down is P2-P1, and this is equal to the pressure difference between the fridge and the outside.

Slickepot

Yes true, but I got hooked up on the fact that the air inside the fridge before opening the door is alot cooler than the outside, but the door is easy to open.
Therefore my idea is that when you shut the door (usually with a slam) the door is "tighter" and preserves the mass, volume variables for a few seconds before the door starts to leak again and evening out the pressure difference.

harrylin

OK I misunderstood your question. If you wait a while, air will leak from the outside to the inside which reduces the pressure difference. That leaking takes time and only becomes important when the under pressure becomes important, so I think that there is no need to "slam" the door to make it "tighter".

However, you remind me now of an unanswered question that I have never solved. Why is a fridge still a bit difficult to open after it has been non-hermetically but rather well closed for a long time?

Jasso

I believe that the sealing strip on most refrigerators are magnetic to help hold the seal and keep the door closed. That magnet would require a bit extra pull to get the door open.

But also, the effect of the door needing to push the air in front of it out of the way isn't negligible; especially since until the seal breaks, pulling on the door creates a pressure difference between the inside and outside while the seal stretches.

Slickepot

Could it be because the fridge is working to cool down the air inside and because of the fact that it is rather well-closed there has to be a certain pressure difference before air can slip in.

harrylin

Ah yes that makes sense! If the strip is magnetic, it will keep the seal closed when pulling a little on the door, so that perhaps most of the required pulling force will still be from the pressure difference and not from the magnet.

harrylin

As I hinted at in my earlier reply, with common leaks there is no "offset" pressure before leakage starts. Instead, the leakage is greater when the pressure difference is greater.

Yuri B.

Before you opened the door for the first time, the pressure inside the fridge had already been equalized with the ambience (there was no pressure difference between P inside and the P outside of it).

As the door was opened and closed, warm air entered the fridge space, quickly cooled down, became more dense and with lowered pressure. Pressure differential appeared which has not yet the time to equalize with the outside (due to good insulation).

Shortly after that you opened the door for the second time, and now have felt the slight negative pressure inside "holding" it.

haruspex

I believe humidity can affect it. The cooling of the air after the door closes will cause water to condense, dropping its contribution to the total pressure.

A.T.

I subscribe to this theory. The entire door with the soft compressible seal behaves like a suction cup.

sophiecentaur

I couldn't make up my mind what you meant by "under-pressure". If you are referring to the 'sucking' noise the fridge may make when you open it, I think that could be because of the door seal that compresses when it closes - pulled shut by the magnetic strip. When you start to pull the door open, this seal stretches (like an accordion) until the magnet lets go. It involves a significant change in the internal volume of the fridge (seal stretches by several mm compared with the depth of the cabinet - say 500mm, giving a few percent change in volume) hence the internal pressure and air will rush in to equate the pressure once the seal breaks. The same thing (in the opposite sense) won't happen when the door is closed because the magnet only grips when it is almost touching the frame when closing.
The seal is not perfect so there will not be a constant pressure difference between inside and outside and any heating / cooling of air cannot be fast because air doesn't mix quickly enough.

anillatoo

I don't understand how could pressure decrease after the fridge was closed? The volume of the fridge remains the same, so if cold air is entering inside the fridge, the pressure should increase I think?

Whovian

Cold air is not entering the fridge. The amount of air in the fridge is constant, but fridges work by ... um ... "pumping" the heat out of the interior. Note the quotations around "pumping."

Now the Ideal Gas Law states that if the volume and amount of gas in a container is constant and the temperature decreases, the pressure also necessarily decreases. Does this help?

anillatoo

Alright, we are assuming the gas in the fridge is ideal. Ok, it does help.

Whovian

True, not all gases are ideal, but they're pretty close.

sophiecentaur

Air is lousy at heating up and cooling down. It will just not do it quickly. This effect would be noticeable in a sealed cylinder and piston after a long time. The fridge is not sealed and air will flow to equalise the pressure before there is any detectable difference in the force on the door.
As I said in my earlier post, the effect has to involve the magnetic strip and bungee seal.
You don't even need to have a fridge to verify what I say - just a magnet and a piece of steel. The steel sticks easily to the magnet when you hold them together. The high force only kicks in when the gap is small and you hear the klunk but don't feel an exceptional 'pull' on contact. But, to separate the two is much more subjective effort.
Yes, I know the force is the same but, if there is any slop / resilience in the system, the feel is different for the two directions.