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The float-and-thermostatic (F&T) trap, illustrated in Figure 6.11, works on a similar principle. In this case, instead of a bucket, a buoyant float rises and falls in the chamber as condensate enters or is discharged. The float is attached to a valve, similar to the one in

Fig. 6.10 Inverted bucket trap.

Fig. 6.11 Float and thermostatic steam trap.

Fig. 6.12 Thermostatic steam trap.

Fig. 6.11 Float and thermostatic steam trap.

a bucket trap, which opens and closes as the ball rises and falls. Since there is no natural vent in this trap and the ball cannot distinguish between air and steam, which have similar densities, special provision must be made to remove air and other gases from the system. This is usually done by incorporating a small thermostatically actuated valve in the top of the trap. At low temperature, the valve bellows contracts, opening the vent and allowing air to be discharged to the return line. When steam enters the chamber, the bellows expands, sealing the vent. Some float traps are also available without this thermostatic air-vent feature; external provision must then be provided to permit proper air removal from the system. The F&T-type trap permits continuous discharge of condensate, unlike the bucket trap, which is intermittent. This can be an advantage in certain applications.

Figure 6.12 illustrates a thermostatic steam trap. In this trap, a temperature-sensitive bellows expands and

Fig. 6.12 Thermostatic steam trap.

contracts in response to the temperature of the fluid in the chamber surrounding the bellows. When condensate surrounds the bellows, it contracts, opening the drain port. As steam enters the chamber, the elevated temperature causes the bellows to expand and seal the drain. Since air also enters the chamber at a temperature lower than that of steam, the thermostatic trap is naturally self-venting, and it also is a continuous-drain-type trap. The bellows in the trap can be partially filled with a fluid and sealed, such that an internal pressure is produced which counterbalances the external pressure imposed by the steam. This feature makes the bellows-type thermostatic trap somewhat self-compensating for variations in steam pressure. Another type of thermo-static trap, which uses a bimetallic element, is also available. This type of trap is not well suited for applications in which significant variations in steam pressure might be expected, since it is responsive only to temperature changes in the system.

The thermodynamic, or controlled disk steam trap is shown in Figure 6.13. This type of trap is very simple in construction and can be made quite compact and resistant to damage from water hammer. In a ther-modynamic trap, a small disk covers the inlet orifice. Condensate or air, moving at relatively low velocity, lifts the disk off its seat and is passed through to the outlet drain. When steam enters the trap, it passes through at high velocity because of its large volume. As the steam passes through the space between the disk and its seat, it impacts on the walls of the control chamber to produce a rise in pressure. This pressure imbalance between the outside of the disk and the side facing the seat causes it to snap shut, sealing off the chamber and preventing the further passage of steam to the outlet. When condensate again enters the inlet side, the disk lifts off the seat and permits its release.

Fig. 6.13 Disk or thermodynamic steam trap.

Fig. 6.14 Drain orifice.

Fig. 6.13 Disk or thermodynamic steam trap.

An alternative to conventional steam traps, the drain orifice is illustrated in Figure 6.14. This device consists simply of an obstruction to the flow of condensate, similar to the orifice flowmeter described in an earlier section but much smaller. This small hole allows the pressure in the steam system to force condensate to drain continuously into the lower-pressure return system. Obviously, if steam enters, rather than condensate, it will also pass through the orifice and be lost. The strategy of using drain orifices is to select an orifice size that permits condensate to drain at such a rate that live steam seldom enters the system. Even if steam does occasionally pass through, the small size of the orifice limits the steam leakage rate to a value much less than would be lost due to a "stuck-open" malfunction of one of the types of traps discussed above. Drain orifices can be successfully applied in systems that have a well-defined and relatively constant condensate load. They are not suited for use where condensate load may vary widely with operating conditions.

As mentioned above, a number of operating requirements must be taken into consideration in selecting the appropriate trap for a given application. Table 6.9 lists these application considerations and presents one manufacturer's ratings on the performance of the various traps discussed above. In selecting a trap for a given application, assistance from manufacturers' representatives should be obtained, since a great body of experience in actual service has been accumulated over the years.

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