What does it mean to mark N.R.D on the side of the air duct entering the explosion-proof area in clean air conditioning?

Noun (abbreviation of noun) This is abbreviation, Chinese: air duct check valve.

FCU: Fan Coil Unit

AHU: air handling device (air handler)

FAU: fresh air unit processing device

HVAC: HVAC.

DCC: dry coil (dry coil)

FFU: Fan filter. Fan filter.

MAU: List of air supply units of fresh air air conditioning box

AHU: aerial hurdle group

HEPA: Efficient and Affordable Air Filter

RAC: Recirculating Air Tank Unit Plan Recursive Air Tank Unit Plan

Clean room clean room

ULPA: ultra-low permeability air filter.

AS: the air shower room in the air shower room

PB: transfer box

CB: Clean workbench of clean workbench

RD: decompression damper

Chestnut: refrigerator

C.d.: condensate pipe

CT: cooling tower

CAV: fresh air volume control box

Electric arc furnace: exhaust fan

exhaust pipe

EAG: air outlet

EAL: Exhaust louver

FAG: new tuyere

Fresh air shutter

FAF: air supply fan

fresh air

Fashion: new air duct

Fire protection: fire damper

HC: heating coil

FP: Fan Coil Unit

HX: heat exchanger

Noun (abbreviation of Noun) R&D personnel: air duct check valve

Public address: treated fresh air

PDA: new air duct (treated fresh air)

PAU: new fan (with processing function)

PAL: fresh air shutter

Reply: Return air

Air return pipe

RAG: air return port