For maximum effectiveness in steam systems, steam traps should have operating characteristics which
closely match the requirements of the applications for which they are used. A trap which holds back condensate
until it is subcooled and some of the sensible heat has been utilized is unsuitable where the need is to get
maximum output from an exchanger by discharging condensate as soon as it forms. Equally, a trap discharging
condensate at steam temperature can exacerbate flash steam problems in cases where surplus heat exchange area
exists and a subcooling trap might be more suitable.
In all cases, undersized traps simply cannot drain condensate from the steam equipment at the required
rate, while oversized traps which cost more will usually wear faster and begin leaking expensive steam.
This emphasizes the need for carefully selecting trap sizes that are properly engineered for maximum
system efficiency. And, of course, the ability of a trap to cope with varying loads and to discharge noncondensible
gases is often important. The recommended procedure is to first select the trap type which has performance
capabilities that satisfy specific application needs, and then to choose a size which handles the condensate
load without any unnecessary excess capacity. The Selection Guide, Table 1, is not comprehensive but
helps in many applications where no unusual operating conditions or severe corrosion problems exist. Choosing
the correct trap size then implies estimating the steam consumption rate, which of course equals the condensate
load. Sometimes the load has already been measured, or the rated output of the steam equipment is known or can
be obtained from the original manufacturer. In other cases, an estimate must be made and a Table o f Load Formulas
will help although it, too, cannot be comprehensive. After making the best possible estimate of the
load, a safety factor is applied. This allows for any inaccuracies in the estimating, for increased condensation rates at start-up, and for lower than anticipated pressure differentials across the trap