Refrigerated trucks are a wonderful thing. They’re why ice cream is even possible as a summer snack. However, refrigeration is also a counterintuitive thing. Why? Well, because you can’t actually “make cold”. So, how exactly do some pipes and fans seem to do just that? This article will try to answer that question in simple terms. Not too much science! Promise.
The answer is all to do with a simple insight: the air inside the fridge or freezer van has had its heat moved outside. This is done by using a special fluid called refrigerant. Through controlling how it turns from liquid to gas and back, you can make it absorb heat from inside the refrigerated space. Then you circulate it outside where it loses that heat to the atmosphere. Let’s look closer.
Insulating For Cold
First, to create a cold environment, you have to have an airtight space that’s insulated to stop temperature outside affecting the temperature inside. In refrigerator trucks, this insulation is usually made of multiple layers where thin panels sandwich thicker layers of low-density material similar to styrofoam.
Pressure, Gas And Absorption
Now, a little science: technically, heat is a measure of how fast molecules of something are vibrating at a microscopic level. Less vibration = less energy = less temperature. While it is simple to add more energy to something (you can warm your hands up just by rubbing them together), it’s much harder to remove heat. Refrigeration gets around this problem by using refrigerant to absorb the heat inside the van and then pumping it through pipes that are exposed to the outside so the external environment can absorb it in turn.
A Three-Stage Process
There are three main mechanical components in the refrigeration cycle mentioned above. And moving between them all is the refrigerant. This special fluid has a very low boiling point, often around 4 degrees C (water boils at 100 degrees C). So yes, when refrigerant boils it’s cold to the touch. Keep that in mind as we look at how the basic cycle works.
1. Condenser – Sending Cold Higher-Density Refrigerant To The Evaporator
The condenser is the complex pipework surrounded by thin metal fins you’ll see on the exterior of the refrigeration unit. The condenser is fed from the compressor with refrigerant in its gaseous state. As the gas arrives in the condenser pipes, it’s exposed to the temperature difference between its own temperature and the temperature of the outside atmosphere. Because temperatures always try to balance, the outside atmosphere quickly sucks the excess heat out of the fluid. As the fluid cools, it becomes a liquid (which has higher density than a gas) and moves onto the next stage…
1. Evaporator – Sending Hot Low-Density Gas To The Compressor
This is the part of the cooling system inside the fridge: the bit most people think of as “making cold”. The refrigerant arrives in the evaporator in its liquid state (i.e. below 4 degrees). As the air inside the space flows over the evaporator coils and fins, it tries to heat the fluid up by passing energy through the metal. As the refrigerant picks up heat, it boils (at 4 degrees) and expands inside the pipes, turning into gas. This gas, still trapped in the pipes, then moves to the third part…
1. Compressor – Sending Cool High-Pressure Refrigerant Gas To The Condenser
The compressor – mounted outside the cool compartment – can be thought of as the heat absorber. The refrigerant gas is drawn into the compressor and is, you guessed it, pressurised. Essentially, the gas is pumped into a space that causes its molecules to cram together. This process slows the vibration of the gas molecules thus releasing the heat energy. The gas now heads to the condenser to become a liquid and go around the circuit again.
That’s Pretty Cool
When installed on an insulated truck, the three components can quickly move heat from the inside to the outside. The main factors in how cold the inside can get are the insulation’s quality, the power of the components and the refrigerant’s formulation. Ramp everything up and the refrigerator becomes a freezer.
However cold you want things, the essential function is the same right across the cold chain where chilled warehouses pack refrigerated shipping containers and refrigerated trucks. And it’s the cold chain we have to thank for transporting anything that would perish if it heated up. This could be meat, cut flowers, medical supplies and, yes, ice cream.