Cheers Gary.
It makes a bit of sense now.

The chiller had a large approach temp of 9-10°C, but it also had a low superheat of 2°C.
This meant the shell+ tube evap was not producing anough heat transfer which concluded was due to the concentration of glycol which the customer could not tell us and i didnt have the correct tool.

What i dont understand is - if you have low subcool it means the liquid is not staying in the condenser long enough to cool, would this also mean you have a low head pressure? But if you have inverter fans controlling head pressure then this should be o.k. resulting in normal subcool?

I think i,m starting to confuse myself again.

I totally understand. Suction superheat and discharge superheat but somehow cannot get to grips with subcool 100%.

Anyone shed some light?