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I went to a grocery store on a hot day that was very well air-conditioned, and I noticed as I went through the open entrance that there seemed to be a very powerful downward air current right at the doorway. After crossing the invisible threshold, the temperature immediately dropped a good 15 degrees or so.

How does this process work?

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This is called an air curtain or air door and it actually keeps flying insects from being able to enter the store. It also helps trap the colder air inside.

Edit: see the link for how it works.

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    $\begingroup$ I think the author is looking for why it works, not just that it exists and what it does. That may be in your link but the pertinent information should be put into your answer here. If you edit it to explain why it works, ping me and I'll remove the downvote. $\endgroup$ – tpg2114 Jun 29 '13 at 1:15
  • $\begingroup$ @tpg2114 All he has to do is read my link. Should I have regurgitated it just to keep from getting down-voted? $\endgroup$ – user6972 Jun 29 '13 at 1:18
  • $\begingroup$ Actually I liked the answer: i.e. that there is a specific design aim for the downdraught and that the downdraught is not just some needful, unavoidable consequence of the need to air conditioning a room. Now I recall: our interferometers at work are in clean rooms with a similar device - although here I'm guessing that the aircon designer aims for a slightly higher pressure in the whole of the interferometer's alcove so as to keep dust out. $\endgroup$ – WetSavannaAnimal Jun 29 '13 at 1:41
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    $\begingroup$ @user6972 It's well-established that simply posting a link to an answer without adding, expanding, or contributing original content of your own is not a great answer. Links change, websites change, any number of things can happen, so it is preferred to have answers self-contained. $\endgroup$ – tpg2114 Jun 29 '13 at 2:08
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    $\begingroup$ @tpg2114 I guess I'll leave it to a show down to see if SE or Wikipedia lasts longer. I think once the OP knows the name of the "mysterious unknown process" they can ask more in depth questions or research it themselves -- even if the link dies. Feel free to go into the fluid thermodynamics of air curtains, but I don't think that the OP is really that interested or asking for that depth of knowledge. It is fun watching the votes go up and down though. $\endgroup$ – user6972 Jun 29 '13 at 3:16
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Most shopping malls have this kind of air door that blast you with high velocity air flow when you enter. As you might have noticed, shopping malls are always the cleanest places of them all. And I am not just talking about cleanliness of the floor area but the entire atmosphere in a mall. The reason they keep it clean is obvious. The method which they use to keep it clean is the answer to your question. The cleaning process in a shopping mall is modelled on the same process which is used in an IC fabrication facility, only that the constraints are less stringent. The pressure inside the mall is relatively higher than outside. Because of this airflow is always outward. The air comes from the ceilings & is collected at the floor. There is something called ACH(Air Changes Per Hour). In a fabrication facility, its about 100ACH. If you ever happen to enter a Fab facility, you always have to go through a series of doors, in each you are blasted with high velocity clean air to remove dust particles on your clothes & body. The constraint is less stringent in malls, purpose is same i.e. to aid the system set up in the mall to keep it clean. Also it acts as an air door since it would be irritating for a customer to have an actual door.

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  • $\begingroup$ Welcome to physics.SE. Please use plain English, it helps readability $\endgroup$ – Tobias Kienzler Jun 29 '13 at 7:17
  • $\begingroup$ @TobiasKienzler newbie mistake i guess. Will keep in mind next time $\endgroup$ – KharoBangdo Jun 29 '13 at 7:33
  • $\begingroup$ +1 for regurgitating what I couldn't. You did forget to mention about the thermal differences which was the original question ;-) $\endgroup$ – user6972 Jul 1 '13 at 1:29
  • $\begingroup$ The thermal difference is nothing as complicated as rocket science. Its the same effect that you feel when you enter an AC room. The only difference is that an actual door is replace by an air door for convenience. The colder air is separated from normal air. Hence thermal difference $\endgroup$ – KharoBangdo Jul 1 '13 at 4:07

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