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A normal refrigeration cycle uses one coolant, so why does vapor compression cycle of an ice rink you a primary coolant and a secondary coolant?

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The reason is a kind of a politically related technicality.

In fact, some ice rinks used to have a direct cooling by a single coolant. They may have operated at higher temperatures and achieved higher pressures as well as higher efficiencies. See the fourth paragraph of this document.

However, when one looks what the most appropriate coolant is in such systems, he finds out that it's really either ammonia or R-22. The latter causes ozone depletion at 20 times lower rates than other ozone-depleting compounds but even this small amount is unacceptable. On the other hand, R-22 is a 2,000 times more efficient greenhouse gas than CO2 which some people also care about. Due to a combination of building codes, emissions regulations to fight ozone and global warming, and fire regulations, direct coolants ammonia and R-22 are no longer kosher.

There don't seem to be good enough alternatives for direct cooling – which would have similarly favorable thermal properties. That's why one uses a system of two coolants. The primary coolant is a rather harmless compound (which couldn't work as a direct coolant, however) and runs in the tubes beneath the ice rinks and this primary coolant is cooled by a secondary coolant which may be R-22 or ammonia or something even more dramatic because a lower amount of it is needed and it's in a safer environment than on the surface of the rink.

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