Carbon Freezing Chamber
Carbon freezing chambers are used for the process of storing Tibanna gas in carbonite
to preserve it while it is being transported over long distances. A number of such
chambers are constantly in use in Bespin's Cloud City.
In the freezing process, the gas is pumped into the chamber where it is mixed with molten carbonite, then flash-frozen to cool the carbonite into a solid block. The gas is released later at its destination or at a processing center.
Darth Vader had other ideas, though. Vader's idea was to use the Chamber to freeze Luke Skywalker while he was to be transported to the Emperor. Vader knew that Skywalker would not be easy to capture and transport to the Emperor if he was in a conscious state; as he grew powerful with the force.
For this purpose, Vader wanted to test if the Chamber was safe to transport life. Therefore, he tries it on Han Solo before handing him over to Boba Fett to be taken to Jabba the Hutt. Luckily, for Captain Solo, it works.