Boiling

Forced circulation boilers have minimal or no gas phases in the circulating fluid because of the high pressure during
flow. Boiling mostly happens in a flash tank connected to the heat exchanger when the pressure is released. These
boilers are more compact than thermosyphon boilers but need a pump that uses electricity, which can be expensive. The
pump must be made of special materials to handle corrosive fluids at high temperatures. Forced circulation boilers are
less exposed to stress than thermosyphon boilers.

In the boiling process, gas phases may or may not be generated in the heat exchanger.

Thermosyphon boilers gradually develop gas phases in the heat exchanger channels. They don’t need a pump because the
difference in density between cold and hot fluids drives circulation. The ratio between gas and liquid phases must be
carefully considered to prevent erosion due to high gas velocity. Efficient circulation also depends on maintaining the
right pressure drop.

Thermosyphon boilers are more prone to stress from the expansion and contraction of phase change areas. This can cause
vibrations and micro thermal shocks. The temperature gradient can change rapidly and vary within the equipment due to
the high energy released from the latent heat of the fluid.

Boilers typically use steam on the shell side, but hot oil can also be used as a heating medium. GT considers all these
factors to design equipment that is safe and highly reliable.

Key parameters for design

• Velocity of media into channels of graphite.
• Fouling coefficient of media.
• Pressure drop for thermosyphon.

Common heat transfer coefficient achieved