Condensation Stage in Air Conditioners? Heard It Somewhere…
Condensation is the next stage in air conditioners operating cycle, after compression. In other words, this is a side branch to compression page.
This stage is coupled with compression, for two reasons,
- to change the refrigerant’s state from high pressure gas to high pressure liquid
- to avoid having very large compressor to compress refrigerant beyond critical point – i.e. change gas to liquid without going through liquid-vapour phase. In other words, ensuring air conditioners are designed properly
|Although compression alone may change the state of refrigerant from vapour to liquid, it is deemed unpractical for air conditioners’ design. You’ll only waste your electricity bills, and you might not get cool air in the end. So, condensation is required.|
As explained in the fundamentals page, condensation happens mainly in a heat exchanger, through a heat exchange process. That heat exchanger is known as condenser.
Sneak peek into the basics:
Every atom and molecule of matters has a specific internal energy, as explained in compression page. And due to this specific internal energy, different matter exists in different state (solid, liquid, or vapour) at a given pressure and temperature.
Solids will have atoms or molecules packed together very closely, hence the movement only involves rotation about each atom or molecule’s axis.
Liquid however, will have a close formation of molecules or atoms. The movement is much more flexible than solids, but still restricted to short distances. Liquid has higher molecule or atomic energy compared to solid, but lower energy compared to vapour.
Vapour has all the freedom in this world, to move in all direction possible, with large distances between each atoms or molecules, high speed and random in motion. It has the highest molecule or atomic energy between solid and liquid.
Our interest is in vapour and liquid phases, and the phase in between. The phase in between? Keep reading.
Condensation wishes to introduce itself:
|Condensation is defined as the state, when a vapour starts to change phase into liquid state, as a result of temperature drop.|
It starts when a superheated vapour reaches its saturation point.
We have to bear in mind that condensation does not occur at a singular temperature. It occurs at different ranges of temperature. Reason being, the pressure of the vapour itself.
Compression page has explained that increasing the pressure will reduce the distance between the vapour molecules. Thus, vapour molecules will have lower net energy.
Decreasing the pressure will have a reverse effect, where the distance between the molecules will increase, and net molecular energy will increase.
So, if a gas or vapour has a higher net energy, more energy needs to be removed to reduce the distance between the molecules.
Similarly, lower net energy of vapour requires less energy removal for molecules distance to close.
What’s with all the molecule distance and energy removal?
As explained earlier, the molecule distance is a characteristic to different states of a matter. Solid, liquid, or vapour.
Energy can be in terms of work, or heat. And when condensation is in the picture, energy removal is in terms of heat.
Hence, as the pressure of the vapour increases, the heat removal required is smaller to condense it. This means that condensation of the matter starts to happen at a higher saturation (or boiling) temperature. This is good, as any temperature lower than the saturation temperature, means we will have a condensed liquid.
The flip side occurs for low vapour pressure.
Let us consider Refrigerant 12 as an example.
Referring to Rogers’ and Mayhew’s “Thermodynamic and Transport Properties of Fluids”:
- the saturation temperature at 1 bar is about -30 oC (-22 oF)
- whereas the saturation temperature at 9.6 bar is about 40 oC (104 oF)
This means that we have to have ambient air at -30 oC (-22 oF) for condensation of the refrigerant to occur at ambient pressure.
Well, if we have that kind of ambient temperature, we wouldn’t need cooling anymore don’t we?
You see, the ambient temperature for the compressed refrigerant does not have to be very low for condensation to occur. In fact the saturation temperature is higher than most ambient temperatures during hot summer. Hence heat exchange occurs from the refrigerant, to ambient air.
This is why we have to couple condensation with compression in air conditioner systems.
Bring the pressure up, close the gap between the gases, and remove the heat from the refrigerant to close the gap even more – liquid refrigerant we have!
Short meeting with vapour and liquid, and the phase in between:
This meeting occurs when heat is continually removed from it.
If all of the matter is in vapour form, then we call it superheated vapour, as there is no condensation.
Once the temperature drops to saturation line, we will have a mix of vapour and liquid. This, is the phase in between.
|At this time, temperature will not drop. This is due to energy consumption of the atoms or molecules to bond and change phase from vapour, to liquid.|
Temperature will not decrease until all vapour within an enclosed space, turn into liquid.
Once in liquid form, temperature will start to decrease again, until it meets freezing point. But that’s a different story.
Condensation wishes to conclude with real life example:
- Order a glass of iced lemon tea
- Watch condensation of water vapour happening on the glass
That’s about it! Next to come – expansion
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