I always wondered why the "Vac" was allowed to be included in the title for the RTHA. I think the reason they went went the title, for the screw, was that it , like the centrifugal, had an orfice set and an economizer cycle. To the original poster, are you familiar with discharge superheat? forget suction superheat for this one and charge by approach on the chiller barrel and discharge superheat on the screw.