Ammonia Water Resistant Centrifugal Fan

Overview

Ammonia fan selection starts with two questions: is the medium aqueous ammonia or dry ammonia gas, and does the stream contain chlorides or oxygen. Ammonia attacks copper selectively. Copper and copper alloys form soluble ammine complexes, and stressed areas are prone to ammonia stress corrosion cracking. The first rule for this duty is therefore: no copper alloys.

Material Selection by Medium State

Aqueous ammonia and dry ammonia gas behave very differently. Dry ammonia is relatively mild to carbon steel, while ammonia water attacks carbon steel noticeably when oxygen is present or temperature rises.

Location	Material / process	Basis
Impeller and casing	304 or 316L stainless steel (GB/T 4237)	Common for ammonia water; 316L where chlorides are present
Chloride bearing or mixed media	Duplex 2205 (GB/T 20878)	Better chloride pitting resistance
Non-metallic option	PP, PVC or FRP	Avoids metallic corrosion at ambient temperature
Fasteners and shaft	Stainless steel only; no copper, brass or galvanised parts	Ammine complexing and zinc attack
Dry ammonia gas	Carbon steel with oxygen and residual stress controlled	Dry ammonia is milder, but stress corrosion cracking must be prevented

Stainless steel is not immune. Ammonia water is relatively mild to 304 and 316L, but when chlorides are also present, pitting and crevice corrosion still occur. Such duties require the grade to be reviewed against GB/T 20878, or duplex steel to be used.

Structural and Process Measures

Measure	Practice	Purpose
Weld passivation	Pickling and passivation of stainless welds	Restores weld corrosion resistance
Shaft seal	Mechanical seal, or packing with leak drainage	Limits ammonia escape
Drainage	Low point drain in the casing	Prevents ammonia water accumulating
Avoid dead corners	No liquid traps in the flow path or base	Reduces crevice corrosion
Earthing	Equipotential bonding per GB 12158	Ammonia is flammable; static control
Enclosure rating	IP grade selected per GB/T 4208	Outdoor and washdown protection

Applications

Typical duties include ammonia recovery in synthetic ammonia and fertiliser plants, ammonia water tank breathing, ammonia area ventilation for SCR and ammonia based desulphurisation, refrigeration plant ammonia discharge, and pharmaceutical or chemical ammonia dosing exhaust.

Medium temperature in these duties is usually low. The corrosion risk comes from ammonia concentration, water content and chloride content rather than temperature, so the concentration range, chloride content and oxygen content should be mandatory entries in the specification.

Ammonia is toxic and flammable, with an autoignition temperature around 630 to 650 C and an explosive range of roughly 15% to 28%. Ventilation and explosion protection must be confirmed against the site hazardous area classification. This page covers material selection only and does not replace an explosion protection design.

Acceptance and Verification

Standard	Title	Purpose
HG/T 20679	Design code for external anti-corrosion of chemical equipment and piping	Anti-corrosion design basis
GB/T 4237	Hot rolled stainless steel plate, sheet and strip	Stainless base material acceptance
GB/T 20878	Stainless and heat-resisting steels - Designation and chemical composition	Grade and composition confirmation
GB/T 1236	Industrial fans - Performance testing using standardized airways	Air flow, pressure and efficiency test
GB/T 9239.1 / ISO 21940-11	Mechanical vibration - Balance quality requirements for rigid rotors	Impeller balancing, typically G6.3

Fan size, air flow, pressure and motor power are calculated for the actual duty. Please provide ammonia concentration, medium temperature, chloride content, oxygen content and available space.

FAQ

Why can't copper alloys be used with ammonia water?

Ammonia forms soluble ammine complexes with copper, so brass and bronze keep dissolving, and residual stress leads to ammonia stress corrosion cracking that often appears without visible thinning. Impellers, fasteners and instrument fittings must not be copper alloys, and galvanised parts are equally unsuitable in ammonia service.

Which material should an ammonia water fan use?

At ambient temperature, 304 or 316L stainless steel is common; where chlorides are present, 316L or duplex 2205 is preferred. PP, PVC or FRP can be used where metal is a concern or concentration fluctuates. Dry ammonia gas allows carbon steel, provided oxygen and residual stress are controlled.

What else matters in ammonia duty?

Ammonia is toxic and flammable. Shaft sealing must control leakage, the casing needs a low point drain, and earthing follows GB 12158. Whether explosion protection is required depends on the site hazardous area classification and cannot be decided from corrosion alone.

Related pages: Anti-corrosion fan range  Hydrochloric acid resistant fan  Product range