Hanbell is a very good compressor. Humbel uses York on his chillers . Have him and resource more. If you change to Bitzer, you will have to change a lot, including the control system itself. If we are to change, then change the two compressors. Better buy a new chiller. And why such a concentration of low -15 ? Should be 35-40%. Damage to the evaporator of course and at -10 will not, but at a low concentration of ethylene glycol solution more corrosion of steel elements. The compressor could not break because of the low concentration, there is another reason.Do you have ethylene glycol in your compressor? Could be to break the evaporator tubes large flow, but not because of the freezing point of ethylene glycol. Ethylene glycol does not expand like water when frozen.