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?

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dee 1 month ago

Must-haves: suction/discharge P&T so the PLC computes SST/SCT and superheat, coil-face RTDs (min of N), DP across coalescer and plate HX, oil level switch, flow and leaving-brine low-temp cutout, and a live COP tag to flag fouling. Defrost that doesn’t spam: demand defrost on coil-min RTD vs ambient dew point plus rising fan power, stagger units with a lockout, hold minimum head with the fan VFD, and pre-shed to the buffer or electric boiler when dSST/dt turns ugly so you never ride LP trips.

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

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