Defining what “cold” means

Users may question the temperature of their laboratory refrigerator or ultralow temperature (ULT) freezer. 

Although it is correct that freezers are cold, according to physics, “cold” results from a lack of heat. The less heat present in an environment, the colder it will be.

Defining what “cold” means

Image Credit: inTEST Thermal Solutions

A refrigerator is an environment with very little heat, and a ULT freezer has considerably less heat.

Absolute zero is the temperature in an environment with no heat (0 Kelvin or –273.15°C). This is the coldest temperature possible. At this point, there is no movement of atoms and molecules, and they exhibit zero kinetic energy.

The correct approach to understanding how refrigerators function is detailed below.

Removing heat from the system

Refrigerants are substances used in ULT freezers and refrigerators to cool the interior. Refrigerants help eliminate heat from objects inside the refrigerator, making them cold.

Simultaneously, refrigerants help gain heat in the process of cooling their environment. They are not able to maintain heat transfer.

For continuous cooling to occur, the warm refrigerant requires a method to cool down again. Here, a combination of chemistry and physics comes into play.

Endothermic reactions

Exothermic and endothermic chemical processes are the two fundamental types in thermodynamics. Exothermic reactions discharge heat as they occur. Lighting a match is one example of this. A hot flame is produced when a match is struck on a rough surface and burns. This reaction helps release heat into the environment.

On the contrary, endothermic reactions are the opposite. They help absorb heat from the environment as they happen. This is how a ULT freezer’s interior is cooled using refrigerants.

Evaporate, condense, repeat

Refrigerants lack the potential to absorb heat from their environment forever. They will eventually become warm and stop being able to absorb heat.

As a result, refrigerants undergo an evaporation and condensation cycle. Refrigerants in liquid form are known to be highly effective in absorbing heat, during which they evaporate and get converted into gas. As evaporation is an endothermic process, it cools the refrigerator cabinet’s interior.

Refrigerants must be converted back into liquid after becoming gaseous to keep the cooling system working. On the other hand, condensation releases heat into the environment since it is exothermic. The condensing refrigerants help radiate heat into the surrounding air, which is why the top and rear of a refrigerator always feel warm.

Repeating this cycle of evaporation and condensation allows refrigerants to keep the ULT freezer at the proper temperature. The refrigerant generally transfers heat from the freezer’s inside to the exterior.

The loss of heat, which is further transferred to the surroundings by the refrigerant, causes the “cold” inside the freezer.

About inTEST Thermal Solutions

inTEST Thermal Solutions (iTS) specializes in the design and manufacture of precision temperature control systems. Our breadth of products and in-house engineering capabilities allow us to be a single-partner-solution for thermal test, process cooling, and cold storage applications. We are recognized globally for our expertise in precise temperature control and simulation of extreme thermal environments, from -185 to +500 °C, with rapid transitions or long dwell times. The iTS family includes four product brands: Temptronic, Sigma Systems, Thermonics, and North Sciences. inTEST Thermal Solutions is a wholly owned subsidiary of inTEST Corp, Mount Laurel, New Jersey, USA.

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Last updated: Feb 16, 2023 at 6:38 AM


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