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Float and Thermostatic Trap
Float and Thermostatic Trap Operating Principles
A float and thermostatic trap is a mechanical trap that operates on both density and temperature principles. Float and thermostatic traps provide high air-venting capacity and are suitable for both industrial and HVAC applications.
How It Works
 | Step 1 Air, steam and water enter the trap near the top. The float rises on the surface of the condensate, opening the valve. As long as the air and steam mixture above the condensate surface stays below steam temperature, the bellows charge remains a liquid and the valve remains open. | |
 | Step 2 As steam accumulates in the trap, it heats the charge in the thermostatic valve until it vaporizes, expanding the bellows or wafer and closing the valve. Note that even though the steam is trapped by the condensate level and the thermostatic valve, condensate is still draining at the same rate that it enters the trap. | |
 | Step 3 If enough air enters the trap to cause the temperature in the gas space to fall below steam temperature, the thermostatic valve will open to vent it. If the condensate load from the heat exchanger falls sufficiently, the float valve closes until the condensate level rises again. In this way steam is trapped in the exchanger regardless of the air or condensate load to the trap. | |
Armstrong F and T Trap Features
 
| 1. | Float can collapse under recurring instances of freezing and water hammer. |
| 2. | Thermostatic bellows can be damaged by freezing and water hammer. Thermostatic trap fails open or closed. High placement of valve reduces chance of condensate blocking air flow. Valve wide open at start-up. Vents non-condensables under low-pressure conditions. Continuous drainage. There is a small amount of live steam loss through the thermostatic element every time it opens. | |
| 3. | Stainless steel is corrosion-resistant. | |
| 4. | Float valve typically fails closed. | |
| 5. | Condensate valve is water-sealed, which prevents steam loss. Condensate is discharged close to steam temperature. Float valve typically fails closed. | |
| 6. | Back pressure reduces trap's capacity. F & T traps are available for use with a wide range of pipe sizes, capacities and pressures. |
