Refrigerators and Cryotherapy

What does a common household refrigerator have in common with a home medical treatment? One answer might be that you get ice cubes out of your freezer to put on your black eye or sprained ankle. There might be other answers surrounding your treatment when the refrigerator tips over on top of you, but in this post we are going to talk about refrigerator operation and cryotherapy.

      Nearly all household refrigerators (and air conditioners) work on the “vapor compression cycle” in which the refrigerant gas passes through a compressor which raises it to a high temperature and pressure.

       The hot, high-pressure gas then passes through a heat exchanger which lets the gas cool and condense into a liquid by releasing heat to the surroundings. (This is the black, serpentine tubing on the back of some older freezers, or the finned tubing underneath many modern refrigerators, or the finned tubing in the outside unit of residential air conditioners.) The cooler (but not yet cold) high pressure liquid is allowed to pass through a valve, an orifice, or a length of capillary tubing. In doing this, it expands without restraint, and the temperature drops a lot resulting in a very cold mixture of liquid and vapor. This mixture is then boiled to a gas by passing through another heat exchanger (this one is usually hidden down inside the refrigerator or AC system). In the process of the very cold refrigerant mixture boiling into a vapor, heat is removed from the cooled space, which is the purpose of the whole system. The cold gas is then directed back to the compressor, and goes around the cycle again.

      You can imagine that the little kits that you can buy at any pharmacy to freeze warts with cryotherapy work like a one-time enactment of part of the vapor compression refrigeration cycle.

      Your kit normally contains a canister of “refrigerant” along with some kind of porous applicator. One popular brand uses a mixture of propane and dimethyl ether. (Don’t smoke while you are treating your warts!) The canister is filled by a compressor at the factory, and is strong enough to contain the high pressure refrigerant which has cooled to room temperature long before you buy it. To use it, you open a valve and let some of the refrigerant mixture expand without restraint. Just like inside your refrigerator, this process causes the refrigerant to become a very cold mixture of liquid and vapor. Of course, the vapor portion just dissipates into the room, but the liquid portion is captured in the applicator. You then hold the wet, very cold, applicator against your wart and freeze the tissue with it. In a relatively short time the remaining liquid boils away and disappears into the room, but in boiling away, if all goes as planned, it will have frozen enough tissue to achieve the desired effect.