Screw machine

Greetings fellow Fridge heads. There are almost too many of these forums out here now. Anyway consider this scenario.
Screw chiller, water cooled, 500 ton using R-22. Setpoint 40 degree F chilling plain old water. Machine running along with little to no load and nominal cooler flow, also cool condenser entering temp. say 65-68 degree F. Flow control on outlet of condenser water to maintain at least 160 psi. Large load comes on suddenly, 85-90 degree water entering cooler with vastly improved cooler flow. What might happen to the compressor?

Mike Hopkins

Hi, Mike, nice to hear from you. R22 screw running on water chilling with low condening temp. This only a guess, possible oil failure due to low diff pressure from the rapidily increasing suction presssure, withthe head not increasing just as quickly due to the cold condenser.
Regards. Andy

Hi, guys, flood back is possible, especially if the plant has ran near cutout temp with the evaporator nice and cold. A large or should, I say larger than normal amount of system charge will be in the evaporator leading to a very quick boiling action there and to little or no suction superheat. I have seen this happen the way I got around it was to load the screw very slowly, but this has other effects, it may lead to the loaded overtaking the capacity of the plat leading to high peak loading and an un exceptable rise in product temp.
Regards. Andy.

I have noticed when these machines are running at part to no load there is indeed, according to the sightglass, a higher refrigerant level in the cooler. Sightglass is usually covered or above, one cannot determine where it would be if the glass is clear. The scenario we have taken to is that the machine was running at low load, possibly having a low temp. cutout or two.
Then some valves were opened allowing 90+ water to enter the loop with increased flow. When the load hit it indeed caused a floodback to such a degree that the oil was diluted and the screw was fed a mixture of oil and refrigerant to the bearings, causing failure. Also this may have happened in the past as well, and the repeated exposure of the bearings to refrigerant diluted oil may have a time accumulated effect and this was the final blow. Another scenario, unrelated is that the motor may have excessive runout causing thrust to be excess. I don't buy into this theory as yet due to, being an open drive there would be seal problems, which there have been none. One things for sure, the screw locked down for one reason or another :(

Mike Hopkins