I have a McQuay with two 10 ton Copeland scrolls on a VAV system that has me stumped? The ambient was 90deg F. Head 225 psi, suction 57psi. At the coil suction line was running 42deg F for a superheat of about 9 or 10. CFM was running about 6000 (a little low for 20 ton) I think that's why the coil was so cold. So this is where it stumps me... the site glass at the TXV was clear except for about every 1.5 minutes it would get really foamy for 15 seconds. So I went out to the outside unit and checked the superheat and subcooling to find the superheat to be less at the end of a 90'(aprox) line set. Suction line was running 34deg F??!(HOW?) The installing contractor also installed a site glass at the condensing unit and the same thing was happening there..site glass was clear but every 1.5 minutes it would foam up for about 15 seconds.
I thought that the TXV was hunting but the pipe temp at the coil was rock steady at 42deg F and at the unit the pressures were rock steady as well as the 34deg F at the suction line at the unit.
The subcooling at the condensing unit was 15deg F and was rock steady too.
How could I be getting intermittent foam at both site glasses without a TXV hunting??? I could see if the valve was opening and using all the liquid and then shutting and stacking it up again. But the valve didn't seem to be hunting. I still don't know how it can get colder on the suction line as it gets out to the condensing unit????? Most of the insulated line was running through a mechanical room that was 78deg F. I was using the same Cooper SH66 therm inside and out for temps.
Anyone have any thoughts???