Any fan can blow cold air, you just need a cooling coil to cool the air. If the conveyor is at 20C and you want it close to freezing, I don't think air is the best option.
How is it being cleaned? If the conveyor is being washed with water, why not add a final 'cooling rinse' before you blow dry it? If you use filtered or RO water, you won't/shouldn't have a hygiene problem. If you really want, run the conveyor under a UV light to kill germs just before you deposit food product on it.