Because temperature is relative to applications, we need to understand what temperature really means.

For instance, if we are walking outside in the garden on a chilly day, we will consider 16°C to feel cold.

However, if we are handed a cold drink that is at 16°C, we will consider that to be warm.

Therefore, what we determine to be hot, warm or cold is subjective to the application.

But there is more to this

Because nothing is truly cold unless it reaches −273.15°C (or absolute Zero), it will always have a “heat” relative to its temperature, application or medium.

Effectively, the temperature does not determine how hot or cold something is, instead, what it refers to is the speed of motion of atoms.

Because atoms move around a lot, they generate heat. At absolute zero, atoms cease to move, so this means they stop moving around, which means they cease to generate heat.

So even a can of Cola in the middle of Siberia sitting at -18°C still has atoms moving, therefore it’s still generating heat.

Because of this, the only way we could determine what temperature meant was to complicate it even further by using science.

Think about this:

A small piece of copper weighing a few grams kept at 727°C will not contain as much heat as a 5Kg piece of copper kept at 140°C.

Although 727°C is considerably hotter than 140°C, the intensity of heat is greater in the 5Kg piece of copper.

Scientists then took this observation and invented a scale called Fahrenheit, only later did they introduce the Celsius Scale for civilized human beings to understand.

So today we have degrees to understand temperature which are added to thermometers to determine degrees of heat at how fast atoms are moving.

One fun way to think of a thermometer is that it’s a speedometer for atoms!

Understanding “dry” and “wet” cold

Humidity plays an important role in preserving foods. This is evident when foods can sometimes spoil faster in one fridge and last longer in another, even when the temperatures are the same.

The cause of this is the level of humidity in the cabinet during cooling.

Parameters such as the amount of moisture entering the fridge either due to the environment, fridge door seals, or even the Specific Temperature of the evaporator will affect the dry/cold conditions.

This principle can be understood when certain locations seem colder than others even when the temperatures are the same. Certain climates will have a dry cold, which has a more penetrating feel, while wet climates will have a different “feel” of cold.

The application of understanding dry versus wet cold needs to be understood in refrigerators.

A well-balanced refrigerator will have a “balanced” cold, whereas an air conditioner will have a “dry” cold. Although the principles of refrigerators and air conditioners are similar, Specific Temperatures can alter the types of cold.

For instance, a butchery will use a Swamp-Filter to cool down their shop, which is just water sprayed over a filter which keeps the room cool by a blowing fan. This produces a cold. They use this because a normal air-conditioner will dry out the meat as the air-conditioner extracts moisture from the room.

Hence why butcheries seem to feel warmer at 16°C as opposed to a Gym using an air conditioner at the same temperature. One is wet, the other is dry.

This also explains why your eyes may seem a little dry when sitting in an air-conditioned room all day.

To help you think this further, a “wet” fridge will spoil food faster than a “dry” cold fridge.

Why? Because of the level of moisture that is gathered on the evaporator circulates around the fridge and causes condensation to form on fresh produce which can cause a “wet rot”, and even promote bacteria growth in the subsurface of water droplets.

Excessive moisture accumulated in a refrigerator is a tell-tale sign that the fridge is producing a “wet cold”, and therefore will cause food to spoil faster. This moisture is usually caused by excessive opening and closing of fridge doors or bad seals. The specific Temperature of the refrigerator (usually on TXVs) can also alter this cold effect.

Heat Transfers

Understanding that even when a fridge is cold, it is still considered heat. In this case, we will understand any temperature conductance or variation as heat.

Heat can be transferred in three ways.

Radiation

This is heat transfer through heat rays. Think of the sun. The sun uses heat rays to heat up the earth. Another application where we understand heat radiation is when we look at a road on a very warm day, the objects in the distance seem to shimmer. This is because the road is emitting heat radiation which affects the light rays between our eyes and the objects we are looking at.

Certain coloured objects will also affect the absorption of heat. For instance, dark objects will absorb heat more effectively than brighter objects. This is why ovens are usually black inside and fridges are white.

In refrigeration, food and beverages will radiate their heat, which affects cooling. Badly packed fridges can affect the efficiency of the fridge as well as the preservation of foods which can affect the cyclic radiation of heat.

As certain food items emit different intensities of heat, they need to be placed in certain areas. This is why vegetables are usually located at the bottom of the fridge as they are generally cooler and radiate less heat than a cooked steak would. For this reason, this is why fresh produce remains freshest at the bottom of the fridge as it’s not interrupted by the heat radiation of other food items.

Conduction

Heat conduction is heat transferring between two mediums by touching one another. A pot on a stove, for instance, is an example of conduction where heat is transferred from the hot plate to the pot via conduction.

A steel rod placed in a fire is another example of conduction where it will eventually become hot from the end and gradually heat up across the rod via conduction.

Conduction in refrigeration is considered when heat is transferred from the evaporator to the passing refrigerant. This conduction must be understood when re-gassing a fridge unit to the correct pressure/volume which will be explained if the coming chapters.

Convection

This is heat transfer from one medium to another by means of air or liquid flow. In refrigeration, this application can be understood when using fans to cool refrigerant gas in the condenser or when using fans to assist with cooling in an evaporator.