Expansion valve adjustment

hvac2010

Is best to adjust an expansion valve under a heavy load or medium load or low load or does it even matter?

nicktech

why are you adjusting the TXV?

hvac2010

I am working on a Amish milk tank it is a dual expansion valve system running at 22 degrees super heat on both expansion valves with milk temp down to 55 degrees and i was trying to decide if i should leave it alone or try and get the super heat down more the milk does come down to 35 degrees with in acceptable time limts the more efficent these systems run the less diesel fuel is used but for now the customer is happy with way the system is running what do you think I should do?

nicktech

leave the TXV alone!! the superheat adjustment is not meant for that. if you super heat is too high then its an indication that the evap is starved. try installing a sight glass just before the TXV in the liquid line to see if your completely a liquid. the target milk temp is 35 deg, what is you desired evap temp? what refrig are ya using?

hvac2010

I am using r22 the sight glass is 2 feet away from the txvs and is not full and never does get full even under heavy loads now as far as what the cold walls/evaporator temp is supose to be for the bulk tank I dont know I will try and find out. What I can tell you is that I am running a head pressure of 250 psig and a suction pressure of 50psig my condenser subcooling is 8 degrees this data was taken when the system was under a medium load and the ambient temp was 75 degrees.

nicktech

the sight glass needs to be filled with liquid, if not you are not delivering the full volume of liquid refrigerant the TEV needs then the evap cant do it's job. check the TEV's superheat, if its more than 12 deg then the valve is max'd out and more charge is required. At 50 psi, r22 has an SRT of about 26deg which is a 9 deg delta T from the target milk temp. try to shoot for a lower evap temp to increase the TD to about 15 deg.

hvac2010

I went back out to the Amish farm and added more refrigerent to the system until the sight glass was clear and that brought the super heat down to 10 degrees on both TXVs the customer said when he milks the cows tonight he would call me and let me know if the system gets down to temp faster.

nicktech

awesome, now that the evap have the right amount of refrig. it can pull more latent heat out and bring the temp. lower. next step is to see if it has a leak somewhere.

nicktech

well....? what happened? how is it working now?

hvac2010

The milk tank works the same it takes about 45 min to cool the milk down. I think the system is working as good as it is going to the customer is happy with the way it works he said its never cooled milk any faster than 45 min since the system was installed.