Air Flow and Ice – A Thought Experiment

Imagine a glass of ice sitting on a table.

Now imagine you place a lid on the glass so all the water and ice is contained in the glass.

If the ice and water are well mixed the water and ice will both be at 32°F because the ice is slowly changing state from ice to water which we call melting. Becasue this is happening at atmospheric pressure we can know what temperature this will occur at and the heat being transferred is going toward melting the ice rather than changing the water temperature which we call latent heat.

Let's say the temperature in the room is 75°F. In this scenario, heat leaves the air molecules as they contact the exterior of the glass and heat moves through the glass into the water and ice. Becasue glass is a pretty good insulator this happens pretty slow but this heat still moves from hotter to colder.

This movement of heat from the air to the exterior of the glass transfers THROUGH the glass via conduction.

What happens if we blow air toward the glass? what changes? 

If we move more air over the side of the glass we deliver more air molecules to the glass via convection but it doesn't change the fact that the heat makes it through the walls and into the glass via conduction.

By delivering more air to the glass we warm the outside of the glass more which causes the water melt inside the glass faster, in other words more air over the glass means more heat transfer even though we didn't change the temperature of the water or the air.

This same basic thing happens inside an evaporator and condenser coil, when we increase the flow of air we also increase the transfer of heat through the walls of the copper tubing in the coils. In the condenser more airflow increases the heat rejection out of the refrigerant and in the evaporator more heat is gained.

Because the refrigerant circuit is dynamic (refrigerant moving) and under pressure more or less heat entering or leaving the system impacts the process and changes the pressures of the refrigerant inside.

If we move less air over the condenser the pressure on the high side increases, if we reduce the air over the evaporator coil less heat enters the circuit, and pressures drop.

This is a basic picture for you to consider next time you see high or low system pressures and how coil airflow impacts heat transfer.

A more advanced but similar thought experiment is what would happen if the evaporator coil had no fins.

— Bryan

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