Cold/Hot Air Isolation Dampers: Construction, Sealing and Actuator Matching

Cold/hot air isolation dampers are the on-off type dampers of the boiler air & flue-gas system: installed in cold-air ducts and hot primary/secondary air ducts, they isolate entire duct sections when mills, air preheaters or other equipment are under maintenance or during fault conditions — they only "fully open / fully close" and do not modulate flow. Two key terms first: by duct cross-section there are rectangular dampers for square ducts and circular dampers for round ducts, with single-blade or double-blade plates (a double-blade pair forms a seal-air chamber in between); they are driven mainly by quarter-turn electric actuators or cylinder-type pneumatic actuators with handwheels, ensuring manual operation on power or air failure. Hot-air media temperature can reach 300-400 °C depending on the air-preheater outlet. Product photos: Shandong Yingkong Intelligent Technology (cgyingkong.com).

Rectangular isolation dampers with cylinder-type actuators and manual handwheels, in the workshop (photo: Shandong Yingkong)

1. Where they are installed and what they isolate

Typical locations in the boiler air & flue-gas system: the hot primary air duct at mill inlets (isolating hot air during mill maintenance), air-preheater inlet/outlet ducts (isolating the preheater), secondary air mains and branches, and cold-air ducts of the pulverizing system. Unlike regulating dampers (any opening for flow control) and fast-closing dampers (trip service), the isolation damper's mission is to seal tight and isolate reliably, so maintenance personnel can work safely downstream and standby equipment can be split off from the running system.

2. Construction types

Dimension	Type	Description
Duct section	Rectangular	For large rectangular ducts; frame + single/double blade
	Circular	For round ducts; flanged frame + rotating blade
Blade	Single blade	Simple; blade edge presses sealing strips against the frame
	Double blade	Cavity between two blades can receive seal air for a higher isolation class
Motion	Rotating	Blade rotates about 90° to open/close (mainstream)
	Sliding gate	Gate slides in/out of the frame; requires side space
Drive	Electric actuator	Quarter-turn; handwheel operation on power failure
	Cylinder pneumatic	Fast two-position stroke; handwheel fitted for manual action on air failure
	Manual	Small sizes or infrequent operation

Large circular isolation damper: double blades with stiffeners, electric actuator via linkage (photo: Shandong Yingkong)

3. Sealing: the key to the isolation class

• Blade-edge sealing: metal clamp plates and flexible sealing strips on the blade edge press against the frame sealing surface when closed;
• Double blade + seal air: seal air is admitted into the cavity between the two blades, forming an "air curtain" that blocks penetration — the same concept as the seal-air system of flue-gas dampers, raising the isolation class from "low leakage" to "near-zero leakage";
• Leakage figures: without seal air, closed leakage is typically assessed at the ≤1%-2% order of magnitude; double-blade dampers with a seal-air system achieve near-zero leakage. Actual values are governed by the project technical agreement.

4. Temperature and materials

Cold-air media runs near ambient; hot-air sections (primary/secondary air after the preheater) reach 300-400 °C. Materials are graded by temperature: plain carbon steel frames (commonly Q235B/Q345) for lower temperatures, heat-resistant grades for hot sections with attention to thermal-expansion allowance — frames and blades of large dampers must accommodate thermal growth, and sealing strips use heat-resistant stainless steel. Duct design pressure is low (usually within ±10 kPa); blade strength is checked against the differential-pressure case.

5. Actuator matching

Damper array: each damper has its own cylinder actuator and manual handwheel (photo: Shandong Yingkong)

• Electric actuator: quarter-turn (90°) with a local handwheel for manual operation on power failure; limit and torque protection fitted, position signals to the DCS. Torque is selected as max differential-pressure torque × 1.25-1.5 safety factor (same basis as regulating dampers);
• Cylinder pneumatic actuator: fast two-position action, robust; cylinder drives carry a handwheel mechanism for local manual action on air failure — suitable where fast isolation or harsh environments are involved;
• Interlocks: isolation dampers usually participate in maintenance interlocks (e.g. manholes may only be opened after the damper reaches its closed position) — the actuator's position feedback is the precondition of the interlock.

Sliding-gate isolation dampers with multi-turn electric actuators and drive screws (photo: Shandong Yingkong)

6. Ordering information checklist

#	Please provide when ordering
1	Duct section: rectangular A×B or circular DN (mm), orientation
2	Media: cold/hot air; design and operating temperature (°C)
3	Design pressure and operating differential (kPa)
4	Isolation class: standard, or seal-air near-zero leakage
5	Blade type: single/double blade; rotating / sliding gate
6	Drive: electric / cylinder / manual; required state on power or air failure
7	Stroking time (s) and operating frequency
8	Control: position feedback, interlock signals, bus/analog
9	Flange interface and sealing-strip material requirements
10	Coating and anti-corrosion requirements (indoor/outdoor)

Summary

• Different jobs: isolation dampers handle on-off isolation (open/close), regulating dampers handle flow control — they appear in pairs in the air & flue-gas system;
• Isolation class = two things: blade type (single/double) plus the seal-air system — a double blade with seal air reaches near-zero leakage;
• Three drive points: quarter-turn electric or cylinder with handwheel (manual on power/air failure), torque sized at max differential-pressure torque × 1.25-1.5, and position feedback as the precondition of maintenance interlocks.

Need cold/hot air isolation dampers, dampers, or matched electric/pneumatic actuators sized and supplied? Submit an RFQ.

Related reading

• Damper Types and Construction: Square and Circular, Manual/Electric/Pneumatic
• Why Flue-Gas Dampers Need a Seal-Air System
• Regulating Dampers: Types, Construction and Actuator Matching
• Expansion Joints in Air & Flue-Gas Ducts: Metallic and Non-metallic