Great one mate.

Just one question.

Quote Originally Posted by Gary View Post
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.
As you say the superheat will be high when the load is high (thats why they say you should only set the valve at design temp).
scenario, Your client is complaining of high room temperatures, So you check everything and you've got good sub-cooling, everythings is running well, but you have high air on temps and high air off temps and your superheat at the evap oulet is also high as you said.
What actions do you take??? or where do you start to confirm that there is not a problem, it's just high superheat due to pull down. I think most engineers would assume the TEV requires opening to reduce the superheat,

Like with the cappillary system, you arrive on site cabinet temps high, suction superheat high what are you checking for to confirm there is not a fault.

I'm sure you'll have the answer for us and i'm sure i missing something totally obvious.

CB