View Full Version : Low ambient temperature subcooling

22-01-2017, 10:12 PM
On an air cooled or water cooled chiller does subcooling decrease or increase with colder ambient temperature or colder water at the condenser? And if so please explain why


The Viking
23-01-2017, 01:15 AM
Hi Duty and welcome to...

In an ideal system, no.
However, in real life the answer is a bit more complex.

First we need to understand what sub cooling is and how a colder condenser actually affect the refrigerant inside the system.
Sub cooling is the drop in the refrigerant's temperature after it been fully condensated in to liquid. If you see the condenser as a long tube where you inject superheated refrigerant vapour at one end, as the vapour is pushed down the tube it will cool and droplets of liquid (condensation) will form, after it travelled far enough all the vapour will have transformed in to liquid, any of the pipe's length after this point will cool the liquid down further and this is what we refer to as sub cooling.
Now, what will happen if the temperature outside that tube get colder?
The heat transfer rate will increase and this will lead to the point at which all vapour been turned in to liquid being closer to the inlet, this in turn will leave a longer section of pipe where sub cooling will take place thus giving us a cooler liquid leaving the tube.
Now it is easy to say that you therefore got an increased sub cooling BUT remember that sub cooling is a relationship between temperature and pressure, yes your liquid temperature will be lower but so will your pressure.
In a very basic system it is likely that your sub cooling increase with a drop in ambient temperature but after fan speed controls/flow controls/receivers/and so on, been added to keep the head pressure up in cold weather and ensure smooth running of the system it is not a given that the sub cooling will fluctuate with changes in ambient temperatures.