If you're looking at a fire suppression system, or trying to figure out what's already hanging over your hazard, you'll keep running into two words: wet and dry. They aren't two brands of the same thing. They're two different agents built for different fires, and the wrong one over a hazard can leave you without real protection when it counts.
Here's the plain version.
The short answer
Wet chemical is for commercial kitchens. It's made for cooking oil and fat, which is a Class K fire.
Dry chemical is a powder. It goes on a lot of industrial hazards, like paint booths, fuel, dip tanks, generators, and heavy equipment, where the fire is ordinary combustibles, flammable liquids, or electrical.
Plenty of sites have both. A hospital or a plant might have a wet chemical system over the kitchen and a dry chemical system out on the floor. Match the system to the room, not to the name on the sign out front.
How a wet chemical system works
A kitchen hood system sits over the cook line. A cable runs across the hood with fusible links along it. Those are small metal links held together with solder that melts at a set temperature. When a grease fire gets going, the heat melts a link and trips the release.
The wet chemical sprays out through nozzles aimed at each appliance and up into the hood and the duct. At the same time, the gas or power to the cook line shuts off. The agent reacts with the hot grease and turns the top of it into a soapy layer that holds the fire down, and the liquid cools the oil so it doesn't flash back up. The trade calls that reaction saponification, which is a fancy word for making soap.
Kitchen systems today are held to UL 300, a fire test written for the way modern cooking oils burn. Those oils burn hotter and hold their heat longer than the ones older systems were built for. An older dry chemical hood system that isn't UL 300 has to be replaced.
A hood system gets inspected and serviced every six months. Between meals is easiest, but our techs can work while you're cooking. We don't shut you down. If you move a fryer or add an appliance, call us before you cook on it, because the nozzles are aimed at where things used to be.
The cook line also needs a Class K extinguisher. Pull the station first and let the system go, then use the K if there's still fire. The K backs up the hood. It doesn't replace it.
How a dry chemical system works
A dry chemical system holds powder in a tank, either under pressure or with a separate gas cartridge that pushes it out. When it's set off, the powder comes out through nozzles aimed at the hazard. It breaks the chain reaction that keeps the fire going and knocks the flames down fast.
Which powder goes in depends on the hazard. A multipurpose ABC powder covers ordinary combustibles, flammable liquids, and electrical fires. Where fuel and flammable liquids are the main problem, a potassium bicarbonate powder, which a lot of people call Purple K, is common. We match the system to the hazard.
Dry chemical has two downsides. The first is cleanup. Fine powder gets into everything, and the area needs a real cleaning afterward. The second is moisture. The powder has to stay dry. If moisture gets into the tank, the powder can cake up and fail to flow when it's needed, even though the tank looks fine from the outside. That's one reason these systems get inspected and serviced every six months.
Dry chemical is also what goes on a lot of heavy equipment. A vehicle suppression system mounts right on the machine, with nozzles aimed at the engine, the hydraulics, and the other places a fire is likely to start.
Which one do you need?
If you cook with grease under a hood, you need a UL 300 wet chemical system over that line, plus the Class K extinguisher next to it. If you have a paint booth, a fuel area, a dip tank, or a machine that can burn, you're usually looking at dry chemical. If you have a room where powder or water would do more damage than the fire, like a server room or a control room, neither one is the right fit, and that room gets a special hazard system instead.
You don't have to sort that out yourself. We walk the hazard and tell you what fits. If what you already have is fine, we'll say so.
After either one goes off
When a system goes off, everybody leaves. Once it's discharged, that hazard isn't protected until it's recharged. In a kitchen, you can't cook under the hood until it's back in service, and the soapy film has to be cleaned off the appliances first. Call us and we'll get it recharged.
Not sure what you've got?
A lot of people don't know whether the system over their hazard is wet, dry, or old enough to need replacing. That's fine. We'll look at it, tell you what it is and what shape it's in, and put it on the same calendar as your extinguishers. We leave a report. We keep up with the date.
Call 1-800-348-9604.