I was thinking it might be a good idea to set up a basic thread to send the newbies to:
Let's start from the beginning:
Wet your finger and wave it in the air. What you are feeling is a refrigeration effect. When a liquid turns to a vapor it absorbs heat. In this case it is sucking the heat out of your finger.
The opposite is also true. If that vapor then loses that heat, it will turn back into a liquid.
In a refrigeration system, we force a liquid to become a vapor in the evaporator, thus absorbing heat from the refrigerated space.
We then use a compressor to pump that vapor to the condenser.
In the condenser we force that vapor to reject the heat and thus turn back into a liquid so that we can re-use it.
We then meter the liquid back into the evaporator to complete the loop and do it all over again and again and again.
How do we force a liquid to become a vapor?... or a vapor to become a liquid? By manipulating its boiling point.
The boiling point is the temperature at which the liquid turns to vapor when heat is added. It is also the temperature at which a vapor turns to liquid when heat is removed.
Boiling point = saturation temp = evaporating temp = condensing temp
When we think of the boiling point of a liquid it is the boiling point at zero psi pressure. If we increase its pressure we raise its boiling point. If we decrease its pressure we lower its boiling point.
In the evaporator we force liquid to become a vapor by lowering its pressure until its boiling point/evaporating temperature is lower than the air it is trying to cool.
In the condenser we force the vapor to become a liquid by raising its pressure until its boiling point/condensing temperature is higher than the air it is trying to heat.
Different substances have different boiling points at different pressures.
We can tell what the boiling point/saturation temp/evaporating temp/condensing temp is at various pressures for common refrigerants by checking a pressure/temperature chart.
Okay... let's go a step further: Superheat and subcooling.
If we boil off a liquid into vapor and then add heat to that vapor its temperature will rise above the saturation temperature. This is called superheating the vapor. When its temp is 10 degrees above the saturation temperature it is superheated 10 degrees. When its temp is 20 above saturation it has 20 degrees of superheat, etc, etc.
Similarly if we condense a vapor into liquid and then further cool the liquid this is called subcooling. When the temp gets 10 degrees below saturation it has 10 degrees of subcooling. When its temp is 20 degrees below saturation it has 20 degrees of subcooling.
Refrigerant flows very rapidly through the evaporator coil into the suction line. Many people believe that you can't have superheat until the liquid has all turned to vapor, but this is not true. Because of the velocity of the refrigerant flow it is possible to have liquid droplets surrounded by superheated vapor at the outlet of the evaporator... and in fact this is what happens. All of the liquid droplets are gone by the time there is 5-10F/3-5.5K superheat.
We want the superheat at the evaporator outlet to be low enough to ensure that we are fully utilizing the coil, thus maximizing its ability to absorb heat, but we do not want liquid droplets to be sent back to the compressor.
Similarly, it is possible to have vapor bubbles surrounded by subcooled liquid at the outlet of the condenser. All of the vapor bubbles disappear at about 10-15F/5.5-8.5K subcooling.
We want the subcooling to be high enough to ensure that we are sending sufficient liquid to the metering device, but not so high that we are backing up liquid into the condenser, thus reducing its ability to reject heat.
On a cap tube system there is a fixed amount of liquid flowing into the evaporator. When the load is heavy there is warmer air flowing through the coil and thus the liquid is all boiled off long before it reaches the outlet of the coil, thus the superheat is high when the load is heavy. If properly designed and charged, the superheat will be just right when the design temperature (design load) is reached.
Many people believe that a TXV will maintain a fixed superheat, regardless of load. This is just simply not true. When the load is heavy the superheat rises and more liquid is fed to the evaporator. The superheat remains high as long as the load remains high. And again, the superheat is just right when the design temperature (design load) is reached. But the design temp will be reached sooner because of the extra refrigerant feed.
As we see, when the load decreases the superheat decreases... so what happens when the filter gets dirty, or the evap coil... or the blower wheel? Less airflow means less load therefore the superheat drops, even though the refrigerated space may be at design temp.
When the load is high the superheat is high, and when the load is low the superheat is low... even with a TXV.
Everywhere, throughout the system, there are opposing forces balancing against each other, and it can be very difficult to tell which of these forces is out of balance.
And yes, there is more... much much more... but that's enough for now.