What Is a Float & Thermostatic Steam Trap?
A Float & Thermostatic (F&T) steam trap combines two mechanisms in one body: a ball float that controls condensate discharge based on liquid level, and a thermostatic air vent that automatically removes air and non-condensable gases. This combination makes F&T traps the most versatile and widely used steam trap type for process heating applications.
PureSys India supplies F&T steam traps from GESTRA and Spirax Sarco. Browse our float & thermostatic steam traps range.
How Float & Thermostatic Traps Work
The Float Mechanism
Inside the trap body, a hollow stainless steel ball float rises and falls with the condensate level:
- As condensate enters the trap, the float rises
- The float is linked to a valve — as it rises, it opens the discharge valve proportionally
- Condensate flows out through the seat at steam pressure
- As condensate level drops, the float falls and the valve closes
- The discharge is continuous and proportional — not on/off like a thermodynamic trap
This means the trap passes condensate immediately as it forms — there is no backup of condensate, and heat transfer surfaces drain instantly.
The Thermostatic Air Vent
A small thermostatic capsule (filled with a liquid that boils below steam temperature) is fitted in the upper part of the trap body:
- When air or CO₂ (which is colder than steam) is present, the capsule contracts and the air vent opens — releasing air rapidly
- When steam reaches the capsule, the liquid inside vaporises, expanding the capsule and closing the air vent
- This provides automatic air venting on every start-up and whenever non-condensable gases accumulate
GESTRA UNA Series — Ball Float Traps
GESTRA’s UNA series are ball float steam traps available in SIMPLEX (level-dependent control only) and DUPLEX (with integral thermostatic air vent) variants. Key models include:
- UNA 14/16: Compact screwed traps for light to medium loads
- UNA 23/25: Medium-duty flanged float traps, up to 16 bar
- UNA 45/46: Heavy-duty flanged float traps for high-capacity process applications
Spirax Sarco FT Range
Spirax Sarco’s FT series are the most widely used F&T traps in India:
- FT14: Compact screwed F&T, up to 14 bar — general purpose process heating
- FT37/FT43/FT44: Mid-range float traps for heat exchangers and unit heaters
- FT47: Heavy-duty float trap with stainless steel body for demanding process applications
Where to Use Float & Thermostatic Steam Traps
F&T traps are the preferred choice wherever continuous, immediate condensate discharge is required — which is most process heating applications:
✅ Ideal Applications
- Shell-and-tube heat exchangers: Continuous condensate discharge prevents condensate flooding the heat transfer surface (“stall” condition)
- Steam-heated tanks and vessels: Continuous drainage maintains maximum heating surface area
- Unit heaters and air heaters: Excellent air venting ensures rapid warm-up
- Autoclaves and sterilisers: Rapid air removal via thermostatic vent ensures correct sterilisation temperature is reached quickly
- Jacketed vessels and reactors: Continuous drainage prevents condensate backup and maintains uniform jacket temperature
- Hospital and pharmaceutical steam heating: Continuous, reliable operation critical
- Dryers and drying cylinders: In paper, textile, and food industries — continuous drainage essential for dryer efficiency
❌ Where F&T Traps Are Not the Best Choice
- Steam main draining: Thermodynamic or bimetallic traps better suited — F&T traps are larger and more expensive for this application
- Steam tracing: Bimetallic traps preferred for outdoor tracer applications
- Very high pressures (>32 bar): Float mechanism has pressure limitations — check manufacturer ratings
- Superheated steam: Thermostatic air vent capsule may be damaged by excessive superheat — bimetallic preferred
Key Selection Criteria
| Parameter | Guidance |
|---|---|
| Maximum operating pressure | Select a trap rated above your maximum steam pressure |
| Condensate load (kg/h) | Size the trap for the maximum condensate load at minimum differential pressure |
| Back pressure | F&T traps work against back pressure — check Kv at actual ΔP |
| Start-up load | F&T traps handle high start-up loads well (proportional valve opening) |
| Stall condition | Check for stall — if back pressure > steam pressure at low load, use a pump-trap |
Installation Tips
- Always install with an upstream Y-strainer to protect the float valve seat
- Install in horizontal pipework — the float relies on gravity
- Ensure the trap is installed right-way up (arrow on body indicates flow direction)
- Install with upstream and downstream isolation valves and a test point
- Ensure the trap outlet is not submerged — back pressure must be less than inlet pressure
PureSys India Float Trap Supply
Browse our full range of float & thermostatic steam traps, all steam traps, or contact our engineers for application-specific selection and sizing.