What Type of Heat Exchanger Is Used With a Cascade System?

Coolants

In cascade systems, the high-temperature circuit, that is, the first stage, uses a coolant with a high boiling point. Most common for the initial stage is ammonia or propylene, also common in single-stage home-refrigeration systems. For the second stage, or low-temperature circuit, a coolant with a very low boiling point is used. Carbon dioxide is the most common second-stage coolant because it maintains good pressure at very low temperatures and is easily and cheaply available as an industrial by-product.

Several different types of heat exchangers are used in cascade systems. A shell-and-tube system -- which floods a sealed shell with liquid, while liquid of another temperature is piped through it -- was initially common, but is bulky. Plate-and-shell heat exchangers and microchannel heat exchangers dramatically reduce the bulk, but are significantly more expensive. In a compact deep-freezing unit, smaller is better, and the more expensive systems are often found in the smallest, high-tech units.

The most common refrigerants, ammonia and carbon dioxide, form chemical reactions resulting in a solid preciptate if they come in direct contact with each other. Keep in mind that in a standard cascade cooling system, the heat exchanger acts simultaneously as an evaporator for the initial stage and condenser for the second, deeper freezing stage. Piping must be designed to withstand extreme cold temperatures without developing leaks. Technicians must regularly monitor for leaks in the piping and keep the system rigorously maintained.

Cascade cooling systems are commonly used throughout the food industry in transportation, warehouses and supermarkets. An ammonia-carbon dioxide system can routinely freeze foods at temperatures of -55˚ C. This helps keep foods frozen as they are transported by boat or rail, even if they are briefly exposed to outside temperatures. Cascade systems are also used in the production of liquefied natural gas and for cooling in petrochemical plants.