Let’s take a PH Diagram.
Pressure on the Y axis
Enthalpy on X axis
In a two-phase fluid I have a bell shape, like this,
This is a curve that has a superior limit which is called Critical temperature/pressure that ideally divides 2 phases. The points on the left along the curve is the saturated liquid. The points even more at the left are subcooled liquid. The points at the right are called saturated vapour. The ones even more to the right are gas.
The ones over the bell are hypercritical. In this zone it is uncertain whether it is gas or liquid. You can call it gas, it has gas characteristics, but the thermophysical properties along the curves over the critical point vary very violently.
Let’s see what happens, let’s suppose that we have a pressure (the horizontal line that I traced), and the fluid is a pure fluid, like ammonia or water…water vapour. These are pure fluid, which have a bell shape.
What is the QUALITY? See the points where I put 0 and 1.
The quality tells me how much vapour I have inside of the bell. So, if here it’s all liquid, saturated liquid, the quality will be 0.
If here is all saturated vapour, the quality will be 1.
If I put myself in the point marked with an X , if I make the difference between the segment from 0 to X and all the segment 0 to 1, I can derive the quality, for example quality 0,1 or 0,2 or 0,3 or 0,4.
Let’s see what happens in a refrigerant cycle.
The refrigerant cycle works on 2 pressures, a higher pressure and a lower pressure, in the higher pressure I have the condenser, in the lower pressure I have the evaporator. Than the functional form of the object can be an air cooled or water cooled condenser or evaporator etc a shell and tube, never mind the object, but one has to condense, the other has to evaporate.
What does condensing mean? The cycle is composed by an evaporator, a compressor and a condenser and a thermostatic valve.
The compressor takes the fluid from lower pressure to higher pressure, so in the graphic, going from pressure line A to pressure line B.
The thermostatic valve takes it back from higher to lower, so from B to A, look the graph.
Practically what happens?
We have seen that over here we have a condenser, a condenser can have a defined surface, bigger or smaller. Condensing means taking from gas to liquid phase, right?
So the condenser takes the gas coming from the compressor, because I have a small overheating (see thick line), then it goes in the diagonal (isohentropic) and the system goes out to point C, where I have same pressure but the temperatures are different, because if you see the temperature curves are wavy like in the drawing, see the waves? Ok.
So in point C , I have same pressure but very different temperature.
So if here I have a temperature corresponding to the condensing of 45 °C (remember the pure fluid condenses at constant temperature, there can be a slight difference due to the pressure drops, but if we don’t consider pressure drops, temperature is constant. If fluid is not pure, temperature is not constant because there’s a glide, like R410A and so on..).
So, what does it mean? That I enter the compressor at 45 °C but coming out of the compressor I can even have 80 °C, so I have 80 °C and from 80°C I have to arrive at 45°C, and let’s suppose I only condense, so when I condense I go down along the thermostatic valve, If I go down directly, I don’t enter the liquid zone, but I enter directly inside the bell, see hereunder the red line, it goes down the bell. So when I go in this pressure I still find myself with a quality still a bit high.
And the more the pressure is low, the more the bell gets larger, the more the quality increases!
Usually though, the condensing coil does a subcooling, so it has a surface that allows a subcooling, so it shift slightly here (say 40°C), see the light blue line. So quality will be lower!
What can I do to lower the quality? I have to INCREASE SUBCOOLING.
This, on pure fluids. Ammonia…water.
Now let’s see the refrigerants. Let’s take R134a.
It does not have a bell shape! It’s a particular shape, see here. I have liquid on one side, vapour on the other. Let’s see what happens.
If I go to the pressures lined by line A and B, since it has a strange shape, if I don’t subcool a lot, and I go down directly from point C to D, I find myself with almost 50% of quality!
In order to have a good quality, I need to have a good subcooling, like the one on point E, allowing me to go down to point H1.
WHY THE SOFTWARE IS GIVING WARNING?
Let’s explain why. We have to introduce a new thing which is called Latent Heat, which is the difference of hentalpy (see hentalpy axis) between the H2 point which is saturated vapour, and the entry point H1.
So the latent heat LH is equal to H2 – H1.
LH = H2 – H1
Why latent heat is so important? Because the Capacity exchanged by this system (Q), is equal to the refrigerant mass (m) multiplied by the latent heat (LH).
Q = m * LH
If I want to get the refrigerant mass, it will be the exchanged capacity (Q) divided by latent heat (LH).
If I have a big quality like 0,3, this means I have a very small piece of latent heat. So the bigger the quality, the smaller the latent heat.
So the smaller the LH, with same Capacity (Q), the bigger is the refrigerant mass (m).
So I am not taking advantage of the equipment at the fullest, my aim is having a quality so that I can take advantage at the fullest of my evaporator. I need to have the correct capacity, trying to have less refrigerant mass as possible, so having less refrigerant means having a bigger LH.
If I want a smaller mass (m), I need to have a bigger latent heat (LH). This is why we suggest not going over 0,3, BUT it is a warning, a suggestion, not a limit. Then, there is a limit, where over 0,65 the software tells you that you are having a mixture that is all liquid, and is not correct, the equations used by coils will not be more attendable, also because in literature or test NO equation goes BEYOND 0,65.
If you don’t want to put subcooling, our suggestion is to input the quality.
Coils is a simulator made by equations and formula and procedures that have a validity in the field of experimental data that hundreds of researchers have analyzed. And nobody has done tests with qualities beyond 0,6, nobody in the world. So the uncertainty of the equations may be very high, with high risk of error. This is why we give a warning…the ideal would be entering with a title of 0,2 or 0,25. This is the ideal.