NATURAL CIRCULATION * A monotube once-through boiler consists of a single tube running from feed water inlet to discharge.  Parallel tube once-through boilers have two or more tubes running along side one another from inlet to discharge. *  Cool feed water enters the economizing section where the water is warmed by exhaust gasses departing the boiler. *  The warmed water passes into the generating zone and the burner heated generating tubes fully evaporate the water  into steam. *  The steam is raised above saturation temperature in the superheater zone.  A thermostat at the steam outlet causes a valve to divert from a cooler region of the boiler into the superheater.  This “normalizer” regulates the degree of superheat, allowing the boiler to produce steam of adequate temperature without overheating.    FORCED CIRCULATION MONOTUBE, ONCE-THROUGH BOILER CIRCULATION  REVIEW * Cold feed water enters the steam drum by way of the economizer where it is warmed by  exhaust gasses leaving the boiler. * Warmed feed water entering the steam drum is still cooler than the saturated temperature water in the drum and therefore sinks downwards towards the lower water drum by way of the unheated downcomers. * Water entering the lower drum through the downcomers displaces water in the drum up into the generating tubes.   The tubes are heated by the burner, converting part of the water into steam.  This less dense steam/water mix rises rapidly through the tubes to the steam drum. *  The steam/water mix separates in the steam drum, the steam rising to the upper side of the drum where it is collected and distributed to the engine and other uses; often first passing through a superheater to raise the temperature to a more efficient level.  The water is cooled by feed entering the boiler and descends through the downcomers to repeat the cycle.
*  Cold feed water enters the drum by way of the economizer where it is warmed by  exhaust gasses leaving the boiler. *  The circulating pump draws water from the  drum and forces it through generator coils heated by the burner.   Part of the water is vaporized to steam. *  The steam and water separate in the drum, the steam component rising to the top of the drum where it is collected and flows to the engine by way of the illustrated superheater.  The circulating pump sends the water component back through the generating coils.