How Mechanical Refrigeration Works
Do you understand the principle of mechanical refrigeration? It's the same on a vehicle as in your kitchen. Refrigerant compound, a gaseous substance pressurized into a liquid, carries heat away from the cargo in this way:
• A compressor pressurizes and heats refrigerant, and moves it through lines to an expansion valve, which lowers pressure and causes the refrigerant to cool. This is the key action in the refrigeration process.
• The cold, low-pressure refrigerant moves through an evaporator, a finned heat exchanger in the truck or trailer body. The cold exchanger contacts warm air drawn in from the compartment; the air is cooled and blown across the load.
• The refrigerant, carrying heat from the load, moves out of the evaporator up and to the condenser — the radiator-like heat exchanger in front of the reefer unit — where the heat is taken out.
• Refrigerant moves back into the compressor. The process continues, stops and resumes according to temperatures sensed in the trailer or truck body.
Heating the load in cold weather is done by keeping the compressed refrigerant hot so it warms the circulating air in the trailer or truck body. Or electric heating strips are placed in the walls of the body. Some commodities needn't be cooled but must be kept from freezing. For these, there are heaters — usually powered by diesel fuel or propane.
Most reefer units built today use Refrigerant 404A, a compound that contains no fluorocarbons to harm the Earth's protective ozone layer. Some use R-134a, the same "environmentally friendly" compound that's in your vehicle air conditioner. R-404A is a greenhouse gas, though, and can contribute to global warming if vented into the atmosphere. It must be carefully captured by technicians when they work on the units.
