Forced circulation type boiler has no or very few gas phases in the circulating media due to the overpressure while the media is flowing. The boiling is mainly achieved in a flash tank connected to the heat exchanger when the pressure is released. This type of heat exchanger allows more compact size compared to thermosyphon boiler. It does require a pump which will use electric energy, and which may be high capital cost. Such pump uses special material of construction with media highly corrosive at high temperature. This type of boiler is less exposed to mechanical and thermal stress than thermosyphon boiler (see hereafter).
Boiling process is mainly achieved with or without generation of gas phase in the heat exchanger.
Thermosyphon type boiler will have gas phase developing progressively into the channels of the heat exchangers. No pump is required as the driving force for circulating media is the difference of density between cold and hot media. The ratio between gas and liquid phase need to be estimated to avoid erosion due to high velocity of gas. Pressure drop is also a key point to allow an efficient circulation.
Thermosyphon model is more exposed to stress due to expansion and contraction of change of phase area. This will create vibrations and micro thermal shock. The gradient of temperature can change quickly and from one place to another place into the equipment due to the high energy released from the latent heat of the media.
Boiler usually use steam on shell side. Hot oil as heating media is also possible.
GT take all these parameters in consideration to design the safer equipment with the highest reliability.
Key parameters for design
Velocity of media into channels of graphite
Fouling coefficient of media
Pressure drop for thermosyphon
Common heat transfer coefficient achieved
|Conditions||Typical HTC (W/m2.K)|
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