Forced circulation boiler

A forced circulation boiler is a boiler where a pump is used to circulate water inside the boiler. This differs from a natural circulation boiler which relies on current density to circulate water inside the boiler. In some forced circulation boilers, the water is circulated at twenty times the rate of evaporation.^[1]

In water tube boilers, the way the water is recirculated inside the boiler before becoming steam can be described as either natural circulation or forced circulation. In a water tube boiler, the water is recirculated inside until the vapor pressure of the water overcomes the vapor pressure inside the stream drum and becomes saturated steam. The forced circulation boiler begins the same as a natural circulation boiler, at the feed water pump. Water is introduced into the steam drum and is circulated around the boiler, leaving only as steam. What makes the forced circulation boiler different is its use of a secondary pump that circulates water through the boiler. The secondary pump takes the feed water going to the boiler and raises the pressure of the water going in. In a natural circulation boilers, the circulation of water is dependent on the differential pressures caused by the change of density in the water as it is heated. That is to say that as the water is heated and starts turning to steam, the density decreases sending the hottest water and steam to the top of the furnace tubes.

In contrast to the natural circulation boiler, the forced circulation boiler uses a water circulation pump to force that flow instead of waiting for the differential to form. Because of this, the generation tubes of a forced circulation boiler are able to be oriented in whatever way is required by space constraints. Water is taken from the drum and forced through the steel tubes.^[2] In this way it is able to produce steam much faster than that of a natural circulation boiler.