A frozen carbonated beverage (FCB) is a mixture of flavored sugar syrup, carbon dioxide, and water that is frozen by a custom machine creating The back-end of an FCB machine is very similar to a regular soda fountain. Concentrated flavor syrups are mixed with filtered water, then carbonated. This mixture is then injected into a cylinder surrounded by freezer coils. The mixture freezes to the wall of the cylinder, then is scraped off by a rotating dasher, which also keeps the mixture uniformly mixed. FCB machines will often freeze to a temperature well below the freezing point of water, but the combination of pressure (up to 40 p.s.i.), sugar, and the constant stirring prevent the mass from freezing solid.
FCB machines are distinctive from other slush drink machines in that they require a pressure chamber and a carbon dioxide source. Many modern non-carbonated slush machines use a spiral-shaped plastic dasher to scrape crystals off a freezing cylinder, often integrated into a clear hopper. This product is often much 'wetter' than a true FCB. On the other hand, non-carbonated machines are much simpler and less expensive, and so they are more common. A non-carbonated slush machine can be purchased for well under $2000, and rented for less than $100/day or from £65.00 per day in the UK for a single head machine. By comparison, FCB machines often must have a dedicated service staff.