 Thermodynamics 4
From what has been said so far in Thermodynamics 1-3, it is clear that the relationship between pressure and temperature, as well as the transition between states of matter, is very important for the design of an air
conditioning system. We can now focus on the transition between 'liquid' and 'gaseous' (evaporation) and back (condensation). It should be clear that the energy released during evaporation can, in principle, be recovered
during condensation.
The image above you see again the last diagram from Thermodynamics 3. It also shows, in dashed lines, the area we'll be focusing on from now on. This occurs above an absolute pressure of 1 bar, because we don't
want to create a vacuum in the air conditioning system. Furthermore, we're only dealing with the one change of state outlined above.
This is the vapor pressure curve of R134a, which begins at a much lower temperature and ends roughly where that of water begins. This may clarify once again why water, although used for cooling and heating, is
completely unsuitable as a refrigerant. We'll now leave water behind and consider the vapor pressure of the refrigerant only.
Here, the gaseous region on the right is shown in a lighter color than the liquid region on the left. In the image below, you can see essentially the same thing. This low-pressure gauge additionally displays the saturation
temperature values from 0 to 10 bar. The high-pressure gauge is similar, but for pressures up to, say, 30 bar.
What you saw above as the vapor pressure curve in the p-t diagram now appears below as the p-h diagram. Again, the regions are divided into liquid on the left, wet vapor in the white field in the middle, and superheated
vapor on the right. If heat were added here at constant pressure, it would result in a horizontal line starting at the purely liquid refrigerant, which would then partially evaporate after transitioning to the middle field and be
completely evaporated by the time it reaches the right field.
The critical point is at the very top of the U-shaped curve. At higher pressures than those assumed there, the refrigerant immediately changes from a liquid to a gaseous state and vice versa. But this range isn't of interest to
us for the technical solution of an air conditioning system. We're asking ourselves what pressure changes we can use to bring refrigerant from 10°C or even less to a temperature of, say, 85°C.
And here's the solution: Number 1 indicates the state of the gaseous refrigerant, which is captured by the compressor and reaches a temperature of 85°C during compression. In this case, a (high) pressure of 16 bar is
reached, which is quite realistic, perhaps even slightly lower. It then passes through the condenser in front of the cooler, and the temperature of the superheated vapor initially drops by 25°C and then reaches the
condensation limit.
After that, the temperature stops dropping because all the heat to be released into the ambient air is now generated by the condensation of the vapor. Only when this has completely occurred and the left part of the curve is
reached does the temperature drop another 10°C. The curve between 3 and 4 can be explained by the throttle. The pressure drops almost suddenly. The refrigerant is now in the low-pressure section.
The 3 bar assumed here is also taken from practical experience, but may be lower depending on the vehicle, for example. At this pressure and cooled to 10°C, the coolant passes through the evaporator. Beforehand,
some of the refrigerant had already evaporated; now it's the rest's turn. It's not the coolness of the refrigerant that has the enormous effect in the interior heat exchanger, but rather the renewed change in state. At corner 1 is
ensured that the compressor draws in vaporous refrigerant even at lower temperatures.
The right part of the curve is also called the dew line. It's important that point 1 always lies slightly away from this line in the gaseous region. After all, the cooling capacity required of the evaporator varies. If it's too low, liquid
components could escape. This could cause problems not only with compression but also the washing away of oil residues on the cylinder track(s), and thus insufficient lubrication.
Optimal compressor safety is usually ensured by controlled overheating. This can be achieved, for example, by routing and extending the line between the evaporator and compressor through areas with interior
temperature or even engine temperature. Additionally, a controllable expansion valve can achieve a few additional degrees. Low- and high-pressure lines also run side by side, which allows a defined temperature
compensation, for example.
Just as there is deliberate overheating, there is also deliberate cooling. This ensures that, on the side opposite the compressor, all the refrigerant after the condenser has actually turned into liquid. We've already seen with
water that its vapor takes up well over a thousand times more space. Therefore, to pass through the expansion valve quickly and effectively, all the refrigerant must be liquid.
And how do you do that? Remember the horizontal line at 16 bar in the diagram? From 2 to 3, the pressure first passes through the steam and then the wet steam range. Now, the only thing that really matters is how far the
horizontal line extends into the liquid range. The further it extends, the safer you can be against steam formation.
Here's an example of subcooling. Imagine that the refrigerant is forced through various partition plates inside this condenser into the path marked in red and blue. The condenser is designed so that all condensation takes
place in the area of the red line, and cooling only continues along the blue line.
| Red | Vapor inlet | Approx. 80°C |
| Red | Condensation | 80-50°C |
| Blue | Liquid | Approx. 50°C |
| Blue | Subcooling | 50-47°C |
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