Thermostatic Steam Trap
General / GeneralDefinition
Thermostatic steam traps operate by sensing the temperature difference between live steam and condensate. As condensate cools below the saturation temperature, a temperature-sensitive element (bimetallic strips or a liquid-filled bellows) opens the valve to discharge the sub-cooled condensate. When hot steam reaches the element, it expands or deflects, closing the valve. This sub-cooling before discharge means thermostatic traps return cooler condensate to the return system, which can extract more heat from the steam — an energy advantage in heating applications.
Working Principle
Bellows-type: A sealed bellows filled with a volatile liquid (alcohol, water, or mineral spirits) expands as it heats. When steam surrounds the bellows, the internal liquid vaporizes, expanding the bellows and closing the valve. As condensate collects and cools, the bellows contracts, opening the valve. Bimetallic-type: Stacked bimetallic strips bend when heated by steam, pulling the valve seat closed. When condensate cools the strips, they straighten and open the valve. Bimetallic traps can be designed with a fixed sub-cool range or adjustable set point, allowing the user to tune the discharge temperature.
Key Components
| Bellows or bimetal element | Temperature sensor that opens/closes the valve |
| Valve seat and disc | Shut-off interface actuated by the thermal element |
| Body | Cast iron, steel, or stainless housing |
| Strainer | Protects the small valve orifice from debris |
| Adjustment screw (some models) | Sets the sub-cool temperature |
| Inlet/outlet connections | Typically threaded or socket-weld |
Types & Variants
| Type | Element | Sub-cool | Discharge | Best For |
|---|---|---|---|---|
| Bellows (balanced pressure) | Liquid-filled bellows | 2–10°C below saturation | Continuous (near) | Low-pressure heating, HVAC coils |
| Bimetallic | Bimetal strips | 5–30°C below saturation | Intermittent | Variable-pressure systems, energy recovery |
| Bimetallic (adjustable) | Adjustable bimetal stack | User-set sub-cool | Intermittent | Process applications needing specific drain temp |
Materials
Cast iron bodies for low-pressure heating service (to 250 psi). Forged steel or stainless steel for higher pressures and corrosive condensate. Bellows material: phosphor-bronze or stainless steel, selected for compatibility with the filling liquid and condensate chemistry. Bimetallic elements: proprietary alloy strips optimized for the deflection-temperature curve. Valve seats: stainless steel, often with a hardened insert for long life.
Applicable Standards
ISO 7841 (steam loss), ISO 7842 (drainage capacity). No specific design standard governs thermostatic traps — manufacturer data sheets define performance. EN 14535 applies to thermostatic radiator valves, which share the same operating principle.
Applications
HVAC heating coils: bellows traps are standard, venting air at startup and discharging sub-cooled condensate. Radiators and convectors: thermostatic radiator valves (a form of bellows trap) control individual room temperature. Steam tracing: bimetallic traps tolerate variable pressure and save energy by discharging cooler condensate. Process preheaters: where sub-cooling condensate recovers additional sensible heat before returning to the boiler. Low-pressure steam systems where the simplicity and low cost of thermostatic traps make them economical for large distribution networks.
Advantages
- Sub-cools condensate before discharge — extracts more heat energy
- Self-venting at startup — handles air and CO₂ automatically
- Compact, lightweight design
- Bimetallic: works across a wide pressure range
- Lower cost than mechanical traps in most sizes
Disadvantages
- Condensate must sub-cool before discharge — ba