Dry and Wet Chemical Systems: Kitchen Hoods and Listed Appliance Protection
Dry and Wet Chemical Systems (NFPA 17 and NFPA 17A)
This domain tests pre-engineered chemical systems: kitchen hood wet chemical systems under NFPA 17A and dry chemical systems under NFPA 17. The focus is appliance protection, nozzle placement, actuation, and shutdown interlocks.
Core rules
- Wet chemical systems protect cooking appliances, hoods, ducts, and plenums. Agent quantity is not a room-volume calculation; it comes from the listed appliance and nozzle schedule in the manufacturer's manual. A fryer, range, griddle, or wok may each require a different quantity and nozzle arrangement.
- Every cooking appliance under the hood must be protected, including appliances that can be moved or added later. The hood plenum and duct need coverage where grease can accumulate.
- Automatic actuation usually uses a fusible link or listed electronic detection; a manual pull station must be provided and accessible on the path of egress. Gas or electric fuel shutdown must operate when the system discharges.
- Dry chemical systems use listed nozzles and application rates for the hazard. Sodium bicarbonate, potassium bicarbonate, and monoammonium phosphate agents are not interchangeable; use the agent listed for the fuel and equipment.
- Inspections verify cylinder weight or pressure, nozzle caps, actuation linkage, and shutoff operation.
Worked example
A kitchen has one fryer and one range under a single hood. The listed manual gives 1.5 gal of wet chemical for the fryer and 0.75 gal for the range, for a total of 2.25 gal. The next listed cylinder is 3 gal, so the 3-gal unit is selected. The same manual specifies two nozzles for the fryer and one for the range, plus a plenum nozzle and a duct nozzle. The calculation is not complete until nozzle count, piping, and fuel shutdown are checked against the listing. Adding a third appliance later requires a new evaluation, not just more agent.
Traps that cost marks
- Sizing a kitchen system by hood length or room volume.
- Protecting the hood but omitting the duct or plenum.
- Using dry chemical in a commercial cooking-oil hazard.
- Leaving fuel shutdown untested or relying on a manual valve.