Hi
I have been building my own ice bath chillers.
A standard ice bath is usually has a target temperature between 5C (41F) to 12C (53F)
From my first design I figured that as long as my Gas evaporator temp (R404a) was at least 10C (18F) below my target water temperature and it was above freezing that 0c that it should get to that temperature.
In practice what I found was that the rate of cooling would start to slow down at around the 7C (45F) mark and progressively get loose efficiency as I approached 5C (41F) and then to get from 5C to 4c would take too long. The load (current) would drop right of and the suction and discharge pressure would also drop right off.
I figured (after a time) that this is probably due to the evaporator coil frosting up and loosing heat transfer.
My first action was to raise the flow rate of my pump to see if a higher flow rate might assist in the slowing down of the frosting process.
I did seem to do this and I did a few tests at various flow rates to confirm.
the end result is that I could get lower, quicker, however the temp was getting to 4C and still slowing right down.
This then led me to the idea of a defrost cycle. At first i explored the idea of Hot gas bypass. A little research led me to a danfoss representative saying that hot gas bypass isn’t so viable for a small system as the system would run out of hot gas vey quickly and would only be good for short blasts of hot gas.
There is a schematic in this clip that I screen shot and have included it in the attachments. There is one component is being used that I would like to understand better. It can be seen between the evaporator outlet and compressor inlet. Its what I now know to be a AEV or constant back pressure valve.
I have a few questions about this valve, my initial search found it shown being used in a few different ways. I understand that it can be used in combination with a txv to limit the minimum pressure that the suction pressure can drop too. I haven’t yet fully wrapped my head around it… below in the attachments I have included a few diagrams of how this arrangement was demonstrate. what I observed in my tests is that once the evaporator started to frost up and go into a low load state the the suction pressure would drop (20 psi) and subsequently the evaporator temperature would drop really low (-25C or – 13F). I imagine having the temperature drop so low below the water temperature would only make the problem of frozen coils worse. If I could use the AEV constant pressure valve to limit the suction pressure then I’m thinking that this could stop help the situation with frosting. If for example my absolute lowest target temperature is 2C then I might limit the suction pressure so that it can go below 47 psi (-10c).
Id like to get clear whether this is a viable option before further exploring a defrost function such as hot gas bypass or using an electric heater.
Below there are 2 diagrams, One from the danfoss clip and another schematic from a company that is selling AEVs
Id really like to know if anyone has any advice on this valve. Is it something that can help, if so, what would be the best arrangement for it.