Naked Science Forum
Non Life Sciences => Technology => Topic started by: neilep on 03/03/2019 21:56:04
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Dearest Peeps with degrees in Fridgology,
Ewe are so cool (crap joke mode)
As a sheepy I spend all my waking minutes opening and closing fridges, it is quite simply my raisin d'etre.
Look , here's a fridge,
(https://res.cloudinary.com/twenty20/private_images/t_watermark-criss-cross-10/v1495104983000/photosp/5f14fa68-f42c-413c-be12-1e09ebf28684/stock-photo-food-shopping-fresh-shelf-counter-stock-fridge-refrigerator-stocked-fridge-5f14fa68-f42c-413c-be12-1e09ebf28684.jpg)
Fridgey the fridge, being frigid just moments ago !!
After sneaking into my neighbours house at 3am this morning so that I could open and close his fridge a few times I noticed that his fridge was not cooperating at all !!...I just could not re-open the fridge door immediately after closing it. I had to wait a good thirty seconds and this infuriated me. I snuck into my neighbours bedroom and he willingly without protest allowed me to sedate him whilst he was sleeping. He even allowed me to drag him by the feet and down his stairs, and despite is head banging on each step he was so obligingly happy to cooperate whilst I used his head to try and wedge the door open, but it didn't work...so no luck there.
so, Why are some fridge doors so difficult to immediately reopen ?
I have a conference next Tuesday on the opening and closing of fridge doors and so this soupcon of info would be most helpful.
hugs and shmishes mwah mwah mwah !!
Neil
Oh Fridgey Fridge thou art so cold
And if i may say, also quite bold
I just wish you'd give me a helpful token
And bloody well quickly learn to re-open
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I'm no expert here, but perhaps the magnets holding some refrigerator doors shut are excessively strong. Alternatively, perhaps the seal around the closed door becomes air-tight, making opening it akin to trying to pull a suction cup off a pane of glass.
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A long shot: Condensation forms a thin film on the seal which sticks the door shut like a tongue on a flagpole until 30 seconds go by to let the low-humidity from the inside absorb that moisture.
Or maybe the thing has a timer lock to prevent immediate opening again, so of a dietary feature for those who keep opening it in hopes something better will materialize there this time.
My fridge has the opposite problem. Opening the door allows the contents to shift, making it hard to close again once just opened.
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Thank ewe Kryptid & Halc. There is one particular fridge at work which is a real struggle to open after someone just closed it. There's no timer or magnets it's just stubborn. You have to wait a good 20-30 seconds before you can open it again.
Hmmm....... this has got me thinking as to whether the duration of the opening time has something to do with it. I will experiment with this.
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I have noticed this before as well. I suspect it has to do with pressure:
Fridges don't have a perfect airtight seal, but they are pretty good.
Let's say that the fridge interior is kept at 8 °C (281 K), and the air in your kitchen is 24 °C (297 K), so there is a fairly significant difference between the two (otherwise you wouldn't need a fridge!). When you open the door, the air mixes--for the sake of argument let's say that the air inside reaches 22 °C (295 K) by the time you are ready to close it. Then when the door is closed, the mixing no longer happens, and the air inside begins to col rapidly (the walls of the fridge and all the stuff in it is still at 8 °C, and has a larger heat capacity than the newly replaced air. If the air inside then cools down to 10 °C (283 K) from 22 °C (295 K), then the pressure would drops to 96% of atmospheric pressure (283/295). Standard atmospheric pressure is 101.3 kPa, so the difference in pressure between the inside and outside would be about 4 kPa, or 4 Newtons per square meter. Thus if your fridge door has an area of 2.5 m2, you would need to supply 10 Newtons of additional force to open the door (about the same as lifting a 1 kg weight). This isn't much, but it is significantly more than would be required to open the door in the first place. (There may be more at play, as this number does seem low--even increasing the temperature difference within reason only gains a factor of 2. Perhaps condensation of humidity in the air compounds the effect--the vapor pressure of water falls from about 3 kPa at 24 °C to about 1 kPa at 8 °C, so this would add another 50% to the 24-8 difference...)
If , however, instead of trying to open the door again right away, you give it a few minutes, there is enough time for the pressure to fall, and then more air is able to leak in, re-equalizing the pressures.
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Let's say that the fridge interior is kept at 8 °C (281 K), and the air in your kitchen is 24 °C (297 K), so there is a fairly significant difference between the two (otherwise you wouldn't need a fridge!). When you open the door, the air mixes--for the sake of argument let's say that the air inside reaches 22 °C (295 K) by the time you are ready to close it.
Experiment time. It seems pretty unlikely that air would warm up to anywhere near room temperature in a box full of cold items. Worst case is an empty fridge: all air and no objects to cool the incoming warm air.
How full/empty is the fridge? It it hard to open right after just peeking into it for a moment, or only after having the door open a long time? Are you forced to wait 30 seconds, or does sufficient effort get the door open despite the resistance? What do you hear when you force the door open early? Air sucking in (vacuum theory)? The seal un-sticking (flagpole theory)?
It is a known energy saver idea to put a collection of boxes or bottles (empty or full) to occupy unused volume in a refrigerator. This prevents there being a lot of space for the warm air to move in each time it is opened.
Feature theory: Exert nominal force to open the door immediately after closing. It doesn't open, but keep on pulling at this force. 30 seconds later, the door opens willingly. Do you hear anything (like a relay click) just before the door opens? If so, there is a feature that holds the door shut for some reason.
My car's power steering does that. All my other cars had belt-driven power steering, meaning if the engine is turning, I have brakes and steering, even if I have the ignition off. Not this new car, which has belt brakes but electronic power steering. If I turn the ignition off, I have steering for 3 more seconds, and I hear this little click and the steering wheel abruptly gets a lot heavier.
I do these sorts of experiments as part of test drives. I learn far more tooling slowly around a parking lot than most people find out by driving normally on the streets. Knowing what works and what doesn't when the ignition fails may heavily influence my desire to purchase a car. I drove my parent's Ford Torino off the road once due to a stall turning at an intersection. Impossible to steer or stop.
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There is a concertina-like arrangement on my fridge door, behind the magnets.
- I suspect that when you close the door, the concertina compresses more than usual, with the last of the air escaping under the last magnet to make contact.
- Once the escaping air stops, the last magnet makes contact, blocking the return of air. If you try to reopen the door immediately, you have to move the entire fridge door outwards against 1 atmosphere of pressure, before the magnets feel enough tension to start to release.
- However, I understand that the "frost-free" aspect of the fridge means there is a path for air that will allow the air pressure to slowly equalise, making it easier to open at a later time.
A test of this theory: Does the difficulty of moving the door increase with distance the door has moved?
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Such an interesting question. I always thought this was because the momentum of the closing fridge door squeezed out air at the moment of closing, leaving behind a lower pressure inside...
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Im gonna go for condensation of water
Warm air condensate in the fridge creating a vacuum, drill a hole through the door.
It is more likely though it is on the on the seal and frame as it would probably take longer for air to fill the void. It could just be a slow release suction on the seal or it could be a bit of condensation too. Water condenses from warm air onto cold surface when the door is opened. Upon closing of the door the seal is formed. The water continues to condensate out around the seal creating a vacuum until it works air around it to compensate.
I believe manufacturers already have thought of the condensing water air trick for the box, i think there is a pressure release tube, yours may be blocked .
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Thank ewe ALL for your very informed and interesting answers. I will be back at work on Thursday and will scrutinise the fridge door, much to the disgruntlement of my colleagues I'm sure. ;)
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A survey of student residences shows that typical fridge doors are encrusted with dried egg and rancid milk. Albumin tempera has been used to fix pigments to concrete walls since Roman times, and caesin glue held the Mosquito fighter-bomber together. Therefore it takes either 2000 years of atmospheric erosion or a 20 mm cannon shell to open some refrigerator doors. Vegan airframe constructors use lentil curry.
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I do not mean to be pushy but wheres this update. Is it the high pressure ice formation on the seal senario ?
A survey of student residences shows that typical fridge doors are encrusted with dried egg and rancid milk. Albumin tempera has been used to fix pigments to concrete walls since Roman times, and caesin glue held the Mosquito fighter-bomber together. Therefore it takes either 2000 years of atmospheric erosion or a 20 mm cannon shell to open some refrigerator doors. Vegan airframe constructors use lentil curry.
What about the rain? Stickiest stuff i know is noodle paste, dries rock hard and is difficult to hydrate.
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Simply i feel now technology is very smart and such issues are only with old Most of news fridges and good enough in quality to handle such issues...
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I drove my parent's Ford Torino off the road once due to a stall turning at an intersection. Impossible to steer or stop.
I've always been sceptical of accounts like this.
I had a Capri which developed an intermittent fault on the brake servo so that you didn't know whether you were going to get assist until you put your foot on the pedal. It made me jump sometimes, but it was no problem. Just adjust foot pressure accordingly. I continued driving it for a while, until it was convenient to take it to the garage.
When I bought a Cortina with PAS I tried turning off the engine just to see what it was like unassisted. No problem.
The power steering on my Honda Accord once failed suddenly when the crank pulley broke in the middle of a busy gyratory system during the rush hour. It wasn't any problem, I just shoved the wheel harder. When I bought the Accord (with cruise control) I kept getting told stories of how people had been swept along the motorway unable to stop after their cruise controls failed. There was never any explanation of why they would allow that to happen rather than put their foot on the clutch and take it out of gear.
Systems are called fail-safe because they're safe when they fail.
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If you have a throttle jam open for any reason DO NOT put the clutch out as the engine will go to full revs if it has a rev limiter or over full revs if there is no limiter and quite possibly self destruct !
stay in gear , cut the ignition bearing in mind you will lose power steering and power assisted braking and brake hard bearing in mind what effect you will have on other traffic.
DONT PANIC.
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The door of the freezer is jammed because the first time we open it, the cold air goes down. Standing barefoot in front of an open freezer, you will surely notice that the toes have become cold - cold, dense air sweeps over them. It makes room at the top of the freezer for warmer room air.
When we close the freezer, a new portion of air is cooled and compressed, creating a partial vacuum, from which the door closes tightly as if the ground in since the outside air pressure is higher than the inside.
If the day is exceptionally warm, the temperature difference inside the freezer and in the room where it is located will be even more noticeable. In this case, you have to push the sealing gasket on the door with your fingers to let in some air in the freezer and equalize the pressure - after that, the door will be easier to open.
The partial vacuum created inside the freezer quickly disappears as the gasket on the door is not tight, and the pressure is gradually leveled due to air leaks.
If you have not noticed that the door is difficult to open, it's time to check the sealing gasket. Perhaps it gives a lot of cold air leakage. As a result, the freezer has to work with overload, which is harmful to the environment and your wallet.
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interesting
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Curious about this too. I've noticed it to new fridges. Thanks to your insights.