How to

8 June 2016

Refrigerated Shipping: How it Works

So, we know how to store bulk quantities of goods so that they keep cold, but what happens when you inevitably need to move them around? In any other circumstance, you’d simply load them into a lorry cab, a truck or a shipping container and let the engine take care of the rest, but this is different. We’re not talking about goods which vaguely benefit from being cold; we’re talking about goods which become worthless if they aren’t kept cold.

The solution, much like with the warehouses, is to take the regular equipment, and give it the chill factor. Space is the issue with warehouses, but with vehicles, a whole new list of challenges arrives. The biggest one is power, considering that if you plug a truck into the mains and then drive it away, there are a limited number of things that can happen next, and none of them are productive.

Happily, the actual powered part of the refrigeration or freezing system used in road vehicles doesn’t actually require that much power, so it can run off the battery. The most important thing is creating a proper seal. With a truck, the cab is sealed with high-density polymer foam, as well as rubber and polymer sealant around the door. This creates such a reliable heat-tight atmosphere that you only need a relatively basic refrigeration system.

This consists of a compressor, condenser and evaporator. The condenser is filled with coolant fluid which absorbs heat, absorbing it until the surrounding air cools and the liquid turns to gas. That gas is then pulled into the compressor, which pressurises it until it is in a form closely resembling aerosol. The evaporator then pushes external air through that gas, expelling heat from it and turning the coolant back into liquid which is in turn reintroduced into the condenser. This system is so efficient that trucks can be outfitted with both refrigerating and freezing cabs.

The same principle can be applied to larger goods vehicles with external trailers, as the energy cost increases relative to the amount of power available, but what about air and sea transport? Refrigerator ships, or ‘reefers’, have been around for nearly 150 years, firstly just storing goods in a kind of salted ice concoction, then later advancing to a proper refrigeration system. They tend to have a similar condenser-compressor-evaporator structure to the ones found in trucks, but on a larger scale, and often using brine as a refrigerant.

The same technology can also be applied to shipping containers, and when there isn’t power available, a liquid nitrogen cooling system is used. This is known as total loss refrigeration, as it can only be used on short distance trips. The liquid nitrogen slowly evaporates, cooling the container for as long as there is still gas present. The maximum for this is usually around 2 weeks or just over, and they’re most commonly found on trains, whereas on aircraft dry ice systems are sometimes used. A dry ice system is fairly self-explanatory – the lower chamber is full of dry ice, and there are fans to circulate the cold air. These tend to last a maximum of 100 hours, but no plane is going to be in the air for as long as that. 

Callum Davies

Callum is a film school graduate who is now making a name for himself as a journalist and content writer. His vices include flat whites and 90s hip-hop.