the econimizer pulls flash gas off of the liquid to lower subcooling, that gas goes into the motor and then into the screw at a different point than the suction gas. that pressure will be higher than suction. the electronic expansion valve is inside the econimizer. also a float, there is a small line that feed gas pressure under the float so it can float. the float seperates the liquid line pressure to the econimizer pressure so the liquid is cooled for better subcooling. you may need to look at a cut away picture for this to make sence.
Sometimes the float is prone to stick, If i remember right you should only need 3 lbs of force to move the float. On air cooled models you will need to remove the freon to inspect the float due to your bubbler circuit. Their is no service valve on the bubbler circuit on the air cooled models, unless the newest version comes with a valve now.
i started up a couple of 300 tons last week and the new unit did not have this style of economizer. it used a plate heat exchanger and a expansion valve that is out where you can see it. it was on a 30gxr machine