Trouble with a milk vat - what on earth is this valve?

[The Red Krawler]

Images: http://imgur.com/a/oMSU0

A customer of ours bought this "package" of a second-hand milk vat and new condensing unit (labelled R404a but scribbled out with a pen and R134a hand-written over the top) to be used to chill water. It was supposed to be an all inlcusive kit, so we cleaned it up and assembled it probably 12'ish months ago using nothing but what they gave us. It has mostly worked over autumn/winter/spring, but now that summer is here (southern hemisphere ) it is very inefficient.

They are using 316L/hour of water that enters at +31°C that they would like to leave at +2°C which according to my heatload calculating (in)ability comes in at around 26,000 Btu/hour.

The condensing unit fitted is rated at 31,110 Btu/hour (desired conditions, actually closer to 45,000 Btu/hour in the current operating conditions) which means it should have sufficient capacity.

The issue I have is very low TD between SST and fluid temperature (read: 3°C) and very low superheat (read: 1.5°C system superheat). It makes me think the expansion devices are not providing sufficient pressure drop - but there seems to be no adjustment? The bolt on the bottom of the valve is seized absolutely solid.

Anyone with milk vat experience able to provide any thoughts on what the balls that valve is, and what sort of operating conditions I should expect to find?