Enhancement of Boiling

Enhancement of boiling generally follows single phase convective augmentation added to which bubbles generated tend to be divided by the wire into smaller bubbles increasing their surface area and benefiting from their turbulent mixing behaviour. Reducing wall temperature prevents the onset of  nucleate boiling. Exchanger area in sub-cooled region can be shortened, releasing more area for evaporation.

Where total vaporisation is required at high heat fluxes, enhancement is often applied to:

  • Control film boiling
  • Mitigate mist carry-over
  • Enhance fluid distribution
  • Enhance wall wetting
  • Improve heat transfer in the superheated region

Benefits

  • Increasing convective boiling rate
  • Shortening sub-cooled length
  • Improving temperature distribution
  • Use of low temperature driving forces

Falling Film Evaporators

Enhancement mechanisms at the wall are similar to condensation, mixing / boundary layer interruption but with the elements reversed from that preferred in single phase use. In addition the hiTRAN elements effectively distribute fluid evenly over the whole tube surface compensating for any mal-distribution that may occur at the inlet.

Benefits

  • Increased film heat transfer co-efficient
  • Improved liquid distribution
  • Concentration gradient minimised in boundary layer
  • Shorter sub-cooled length
  • Improved evaporation of viscous liquids
  • Reduced wall temperature for equal duty
  • Control of hold-up and residence time

 

 

Vaporisers

Vapourisers

Adding to the beneficial enhancement effects in boiling, hiTRAN Matrix Elements can be effectively used to de-entrain droplets formed towards the outlet of total vaporisers. The occurrence of droplets usually occurs under high heat flux / high throughput conditions.

Benefits

  • Controlling film boiling
  • Mitigating mist carry-over
  • Enhancing fluid distribution
  • Enhancing wall wetting
  • Enhancing convection in sub-cooled region