What is an exhaust valve in an air dryer?

An exhaust valve in a compressed air dryer is a quick-acting 2-way valve designed to vent pressurised air or regeneration gas to atmosphere during the purge or depressurisation cycle. In twin-tower desiccant dryers (Heatless Regenerative type), one tower is drying while the other regenerates — the exhaust valve opens to vent the regenerating tower to atmosphere, driving off moisture from the desiccant bed. The valve must be capable of high-frequency cycling (thousands of cycles per day) and should open very quickly to produce the expansion pulse needed for regeneration.

What is the difference between a CI and a bronze exhaust valve for air dryers?

Cast iron (CI) exhaust valves per ASTM A126 are used for larger sizes (DN50 and above) where bronze would be expensive and heavy. CI Class 125 per ASME B16.1 is compatible with compressed air service at moderate pressures (up to approximately 10 bar at ambient temperature). Bronze exhaust valves are typical for smaller sizes (DN15–DN40) and offer slightly better corrosion resistance in humid or wet air service. For standard instrument air or compressed utility air, CI Class 125 with CA15 stainless trim is the standard engineering choice at DN80 (3 inch).

How do I order a replacement exhaust valve for a Hankison or Ingersoll Rand dryer?

Provide the following to get a cross-reference quote: OEM model number (from dryer nameplate or MRO documentation), nominal size (typically 3 inch / DN80), connection type (flanged Class 125 or screwed), body material (CI), trim material (CA15 disc), and the OEM part number if available. Supreme Valves India can cross-reference Keckley, Watts, Forbes Marshall, and ITT Goulds exhaust valves and supply equivalent or OEM-compatible replacements with test certificates.

Exhaust Valve — Cast Iron Class 125 Flanged for Compressed Air Dryer Service

Technical guide · Supreme Valves India · 2026

What is an Exhaust Valve?

An exhaust valve is a 2-way (on/off) valve specifically designed to rapidly vent compressed air, gas, or regeneration flow to atmosphere. In compressed air systems, exhaust valves are most commonly found in twin-tower desiccant (heatless regenerative) dryers, where they perform the depressurisation and purge cycle of the regenerating tower.

The defining characteristic of an exhaust valve for dryer service is its high-cycle duty: these valves typically cycle thousands of times per day, every day of the year. Valve types commonly used include poppet valves (piston or mushroom disc type) and specialty globe-pattern exhaust valves with hardened trim. The valve must open quickly — within milliseconds — and close positively without chatter or wire-draw erosion on the seat.

Unlike standard isolation valves, which may cycle only a few times per year, exhaust valves in dryer applications accumulate millions of cycles over a 5–10 year service period. This demands durable trim materials and robust seating geometry.

Typical Application — Hankison / Ingersoll Rand / Atlas Copco Dryers

Twin-tower heatless regenerative desiccant dryers — produced by manufacturers including Hankison (SPX Flow), Ingersoll Rand, Atlas Copco (ZD series), Beko Technologies, and domestic Indian brands — all use exhaust valves to control the purge cycle. In a typical 10-minute cycle:

Tower A dries incoming compressed air (adsorption); Tower B regenerates
The exhaust valve on Tower B opens to vent the tower from line pressure to atmosphere
A controlled purge flow of dry air passes through Tower B, stripping moisture from the desiccant
After regeneration, the exhaust valve closes, Tower B repressurises, and the towers switch

Exhaust valves on larger dryers (airflow above 500 Nm³/hr) are typically flanged — Class 125 (DN80 / 3 inch) or larger. Smaller dryers use screwed NPT or BSP connections in bronze body valves.

Body Material — CI ASTM A126 Grade B, Class 125

Cast iron to ASTM A126 Grade B is the standard body material for low-to-medium pressure (up to approximately 10 bar) compressed air and gas service valves in sizes DN50 and above. Key properties:

ASTM A126 Grade B: Higher tensile strength than Grade A (224 MPa vs 138 MPa), suitable for pressure-containing parts
ASME B16.1 Class 125: Flanged end dimensions for CI valves; 125 pound class rated at 125 PSI (8.6 bar) saturated steam, but higher allowable pressure for air and water at ambient temperature
Flange face: Flat face (FF) per ASME B16.1 — mates with CI piping flanges; do not bolt against raised-face (RF) steel flanges without a full-face gasket or spacer
Working pressure in air service: Typically 10–12 bar at ambient temperature when body is sound and flanged joints properly made up

Trim Material — CA15 (13% Chromium Stainless Steel)

CA15 is a martensitic stainless steel casting alloy (13% Cr, balance Fe) equivalent to wrought AISI 410. For exhaust valve trim (disc, seat ring), CA15 provides:

Significantly better erosion resistance than CI or bronze — important in the high-velocity air purge pulse
Corrosion resistance adequate for compressed air containing condensate
Hardness (typically 22–28 HRC) that resists wire-draw and surface damage from repeated seat contact

For very aggressive applications (wet air with H2S trace, or chemical plant compressed air), consider upgrading to SS316 (CF8M) disc or a full SS316 body globe exhaust valve.

Key Sizes and Configurations

The most common exhaust valve size for industrial compressed air dryers in India is DN80 (3 inch / NPS 3), flanged Class 125 per ASME B16.1. Face-to-face per ASME B16.10 for globe pattern (globe exhaust valve): approximately 305 mm for DN80. Poppet-type exhaust valves may have shorter face-to-face as they are purpose-designed, not per B16.10.

Other common sizes: DN40 (1.5"), DN50 (2"), DN100 (4") for larger dryer stations. Screwed NPT versions in DN15–DN25 for small package dryers.

Differences from Standard Globe, Gate, and Check Valves

Exhaust valves differ from standard process valves in several important ways:

Poppet geometry: Many exhaust valves use a guided poppet disc rather than a threaded globe plug — this allows faster, more repeatable opening without stem rotation
High cycle rating: Standard globe valves are rated for infrequent operation; exhaust valves are designed for millions of cycles with hardened seating surfaces
Actuator compatibility: Exhaust valves in automated dryers are operated by solenoid pilot or pneumatic actuator (diaphragm or piston), not by handwheel
Exhaust port: Some exhaust valve designs have a dedicated exhaust (silencer) connection to reduce the noise of the purge pulse to atmosphere

OEM Cross-Reference and Equivalent Supply

When replacing an OEM exhaust valve on a Hankison, Ingersoll Rand, or Forbes Marshall dryer, the following cross-references are commonly encountered:

Keckley Company: CI globe-style exhaust valves, Class 125, ASTM A126
Watts Industries: CI body exhaust/pressure relief type valves
Forbes Marshall: Indian-supplied globe and poppet exhaust valves widely used on domestic dryer OEMs
ITT Goulds / Engineered Valves: Specialty exhaust valves for process compressed gas service

Supreme Valves India can evaluate OEM part numbers and supply compatible exhaust valves with equivalent or superior materials, complete with dimensional check reports and hydrostatic test certificates.

Ordering Checklist

Body material: CI ASTM A126 Grade B
Disc/trim material: CA15 or SS316
Nominal size: DN80 (3") / or specify DN40, DN50, DN100
End connection: Flanged Class 125 FF (ASME B16.1) or screwed NPT/BSP
Pressure rating: confirm working pressure of dryer circuit (typically 7–10 bar g)
Actuator: manual handwheel (for maintenance isolation) or pneumatic/solenoid pilot (for automatic cycling)
Cycle rate: state estimated cycles/hour or cycles/day if known — affects trim specification
OEM reference: provide if replacing existing valve

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