Refrigeration cycle and refrigerant

What is refrigeration cycle?

AC and centrifugal chillers produce cool water and air by using refrigeration and cycles refrigerants. We ill discuss the refrigeration cycle of an AC, also the relationship between pressure and changes in the state of substances.

State changes and heat

1. When we wipe our body with alcohol, we feel cold, right? This is due to following principles.
2. At atmospheric pressure alcohol evaporation is low.
3. Temperature of body is higher than alcohol evaporation temperature.
4. Alcohol evaporates at body temperature, when it touches the body.
5. The body feels cold as it loses heat (temperature).

The principle of cooling with alcohol

Liquid has the property of removing ambient heat (Heat of vaporization), when it evaporates (vaporizes). Same principle is used when cooling of refrigerators using refrigerants. When alcohol comes into contact with the body, it is released to the atmosphere, but in refrigerators, this cycle is repeated in a closed container with refrigerant, The evaporated alcohol (refrigerant) returns to liquid alcohol again and then again to a gas, this cycle is called as “refrigeration cycle”.

Sensible heat

There are two categories of heat: Sensible heat and latent heat

The amount of heat required to “warm” and object by raising water temperature from 20⁰ C to 100⁰ c is called “Sensible heat”. Object temperature changes but not the “state”. The amount of heat required to change the state of 100⁰ C water into 100⁰ C steam is called “latent heat”. The object “state” changes from “liquid” to “gas”. Following are the five categories of the latent heat.

1. Heat of fusion: The amount of heat required for a solid to become a liquid.
2. Heat of solidification: The amount of heat required to turn a liquid into a solid.
3. Heat of evaporation: The amount of heat required for a liquid to become a gas.
4. Condensation heat: The amount of heat required for a gas to become liquid.
5. Sublimation heat: Heat require for a solid to become a gas and a gas to become solid.

Figure: State change of substance

 

Pressure and material changes

Water boiling point is 100⁰ C, but at the top it is 88⁰ C due to low atmospheric pressure. On high mountain cooking rice leaves a core due to low boiling point and is cooked raw. When water is boiled using a pressure cooker on the ground, boiling point rises a per pressure. Pressure cooker cooking is fast because boiling point of water increases to 100⁰ C or higher due to increase internal pressure raises, enabling cooking at high temperatures, Centrifugal chiller and home AC use this relationship between state of matter and pressure to create cool air and cold water by changing the refrigerant pressure inside the machine. Compressor is use order to crate this pressure difference.

Refrigeration cycles for turbo chillers, home air conditioner, etc.

AC and centrifugal chillers use the state of a substance (refrigerant) and pressure to circulate refrigerant inside the chamber. There are four process for this refrigeration and state change process. Let’s compare it to the cooling operation of a home AC.

1. Condensing process: Outside air or ambient air is used to condense the refrigerant which is at high-pressure and high temperature.
2. Expansion stroke: High-pressure liquid refrigerant’s pressure is decreased.
3. Evaporation process: Applying heat taken from indoor air to the refrigerant to evaporate the liquid refrigerant and create a cool wind
4. Compression process: Evaporated refrigerant gas is compressed into high temperature, high pressure refrigerant gas.

Refrigerant pressure changes in compression (pressure increases) and expansion (pressure decreases), while refrigerant state changes in condensation (gas to liquid) and evaporation (liquid to gas). But the heating a home air conditioner process is different.

Why is refrigerant (Freon gas etc.) used?

Relation between pressure and state change can be created easily. At the largest point pressure difference created by compressor.  Early 1900s ammonia wee used as refrigerant, but due to its toxic nature alternative refrigerant like CFC-12 were invented.

Can water or alcohol be used as a refrigerant?

Machines like AC and centrifugal chiller are made of metal (Iron or copper). If wate comes in contact with air and metal, meal oxidizes. It is also very difficult to create a pressure difference in the compressor using water.  That is the reason water is not suitable as refrigerant, in case of alcohol, there is risk of fire (Ignition). Chlorofluorocarbaons are used considering safety aspects lin not igniting, not damaging the machine and causing no harm to the human body.  It is also now more environment friendly.

*Ammonia is still used as a refrigerant with due consideration of safety.