Evaporator Steam Economy
Evaporator Steam Economy
Evaporators concentrate a dilute liquid — fruit juice, black liquor in a pulp mill, seawater — by boiling off water using steam as the heat source, and because steam is expensive to generate, plants track "steam economy": how many kilograms of water they manage to evaporate per kilogram of steam they spend. A single-effect evaporator can never economize much beyond 1:1, which is why multi-effect and vapor-recompression evaporators — reusing the vapor from one stage as the heat source for the next — were developed; steam economy is exactly the number used to justify the extra capital cost of adding effects.
The steam economy is SE = m_vapor/m_steam, the mass of vapor produced divided by the mass of steam consumed. where m_vapor is the mass flow rate of vapor evaporated from the process liquid and m_steam is the mass flow rate of heating steam supplied.
Inputs
m_vap := 4500 kg/hr=1.25 kg/s
m_steam := 5000 kg/hr=1.39 kg/s
Dividing the vapor generated by the steam spent gives the steam economy, the standard efficiency figure evaporator plants are compared and priced by.
SE_ratio := m_vap/m_steam=0.9
Results
A steam economy of 0.9 is consistent with a well-run single-effect evaporator, which typically tops out close to 1.0 because nearly all the latent heat in the steam goes into vaporizing water minus some sensible heating and losses. If a much higher economy — say 3 or 4 — were required by the process economics, the answer is not to tune this single unit further but to add evaporation effects or mechanical vapor recompression, since a single effect is thermodynamically capped near unity. Economy below about 0.8 usually signals excessive heat losses or sub-cooled feed robbing latent heat from the steam.
Assumptions & limitations
- Single-effect evaporator (no vapor reuse between effects).
- Feed entering close to its boiling point (little sensible heating needed).
- No boiling point elevation correction applied.
- Steady-state operation with negligible heat losses to ambient.