Refrigeration
Restarting a Two-Stage Ammonia Plant After a Shutdown: The Order We Follow
Shutting a refrigeration plant down is easy. Bringing it back up in the right order, without tripping it again, is the part worth writing down.
I run an underground freezer. The refrigeration side is a two-stage ammonia plant: booster compressors on the low side, high-stage compressors above them, and a brine loop carrying the cold out to 34 refrigeration units in the rooms. Most days nobody touches the start buttons. The controls run it. But the plant does come down, for planned shutdowns and for power outages, and every time it does, someone has to bring it back.
Our first planned shutdown was in February 2025. The restart taught us a couple of things the paper procedure didn’t spell out. Here’s the order we follow, with those lessons folded in.
Why the order matters
In a two-stage plant, the boosters pull from the cold side and push into the intermediate stage. The high-stage machines take it from there and push it up to the condensers. If the boosters run with nothing above them carrying the load away, pressures climb in the middle and the safeties do what they’re built to do: shut you down. So the plant comes back in pairs, a little at a time, with time for pressures to settle in between.
Shutting down
- Turn the booster switch off first.
- Shut off one high side, then one booster.
- Repeat step 2 until everything is off.
Restarting
1. Brine pumps first. Reset the cold brine pump, then the warm one. The brine loop is how the cold gets to the rooms, and the plant has nothing to work against until it’s moving.
2. Reset the oil sensors on each compressor. After a shutdown, the oil safeties need to be cleared before a machine will run.
3. Reset each compressor, then bring them on in pairs, in hand. Our start sheet brings on a high-stage machine, then a booster, and then stops to let pressures stabilize. Then the next high stage and the next booster, and stabilize again. Then the last booster, and stabilize again. Don’t rush the “let pressures stabilize” lines. They’re the whole procedure.
4. Two lessons from February 2025. Make sure the king valve is open before you start the screw. And start the boosters unloaded, so they come on easy instead of pulling hard on a system that hasn’t settled yet. Neither one is on our written start sheet. We learned both on that first restart, which is why they’re here.
5. Switch each compressor to Auto, one at a time. Go through Off on the way from Hand to Auto, and do one machine at a time so you can see each one take over before you move to the next.
6. Tower fans. If a tower fan shows red, reset it. If it’s green, it starts on its own.
7. The freezer units last. Reset the evaporator switches and the fan breakers out on the units. By then the plant is stable and ready to carry the load.
Done right, it’s boring. In July 2025 we had a power outage, and the restart was clean. That’s the goal: a restart nobody remembers.
When it won’t come back
Our sheet has four things to check, and they cover most of what we’ve seen:
- The motor starter. Reset it at the starter before you go looking for anything fancier.
- Head pressure too high or too low. Check the VFD and reset it.
- Suction pressure too low. Find out if the compressor is in alarm or just off. They look similar from across the room.
- The two-stage lock. If a chiller or vertical vessel is too full, the plant won’t run. That’s a safety doing its job, and you have to find out why the level is high before you fight it.
How to write your own restart sheet
- Write it for the person who’s never done it. The restart usually happens after an outage, at a bad hour, and maybe not by the person who wrote the sheet.
- Put the waits on the sheet. “Let pressures stabilize” looks like filler. It’s the step that keeps you from tripping the plant again.
- Add the lesson the day you learn it. Our king-valve and unloaded-booster lessons came from one restart. Lessons that stay in someone’s head get learned twice.
- Keep a short “won’t start” list at the bottom. When something goes wrong at 2 a.m., four checks beat a manual.
- Watch your room temperatures after. The restart isn’t done when the compressors run. It’s done when every room is back where it belongs.
That last check is the one our temperature log is for. If you want a starting point, get the free cold storage temperature log template through the signup box below.
Related: The Checklist That Keeps Our Underground Freezer Running, on the daily checks that catch trouble before a shutdown, and The Day From Hell, on what a bad week in a heat wave looks like.
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