Heat pumps in a cold plant: interlock it or chase alarms

We added two industrial heat pumps to scavenge compressor waste heat and preheat process water. The slideware promised COP 3+ and “set and forget.” What I got was a winter of HP-low-pressure alarms and operators cursing defrost.

Root causes: oil carryover fouled the plate HX in weeks; glycol was a bad mix and slushed at -18 C; dry-cooler fan short-cycling packed snow into the coil. Well, there’s your problem. Fixes: real coalescer and DP across the HX to trigger CIP; a snow mode with minimum fan speed and heat-traced drains; verify glycol with a refractometer weekly; add a buffer tank and a 3-way bypass so defrost doesn’t starve the header.

Most important: interlock it. I added permissives to shed to a small electric boiler before low-temp trips cascade, and harmonics alarms so VFDs don’t face-plant the loop pumps. The HMI now tells ops what will happen before it does. COP is 2.4 in February, but uptime is 98%. I’ll take that.

Cold-climate folks: what are your must-have instruments and interlocks on heat pump or waste-heat retrofits? Anyone have a defrost strategy that doesn’t spam the fault log?