A freeze dryer that includes a product chamber configured to accommodate products to be freeze-dried; a condenser connected to the product chamber and configured to trap water during a freeze-drying process; a product chamber cooling circuit configured to cool the product chamber, the product chamber cooling circuit having a first heat transfer fluid; a condenser cooling circuit configured to cool the condenser, the condenser cooling circuit having a second heat transfer fluid and being separate from the product chamber cooling circuit; a first additional cooling circuit having carbon dioxide or ammonia or liquid nitrogen as refrigerant; and a first heat exchanger configured to transfer heat between the condenser cooling circuit and the first additional cooling circuit.