| name | evaporator-design |
| description | Evaporator design — single-effect/multiple-effect, steam economy, boiling point rise, Dühring's rule, NTU-effectiveness, fouling factors, falling film vs. forced circulation, ASTM/TEMA standards, concentration ratio. |
| metadata | {"priority":7,"promptSignals":{"phrases":["evaporator design","evaporation","multiple effect evaporator","steam economy","falling film evaporator","boiling point rise"],"minScore":3}} |
Evaporator Design — Complete Skill
Evaporator Types
Forced Circulation: pump circulates liquid through heat exchanger tubes; boiling occurs in flash vessel; high velocity prevents scaling; for salting/scaling solutions
Falling Film: liquid falls as thin film on inside of vertical tubes; high h; no submergence pressure; gentle heat (sensitive products); scale-prone without good distribution
Rising Film (Long Tube Vertical): liquid enters bottom; climbs as film; vapor-lift; simple; moderate scaling risk
Plate Evaporator: compact; low liquid holdup; easy CIP; for heat-sensitive food/pharma
Wiped Film (Thin Film): mechanical wiper creates thin film; minimum residence time; for viscous, heat-sensitive products
Mass and Energy Balance
Single-Effect Evaporator
Feed: F [kg/hr] at concentration x_F; Temperature T_F
Product: L [kg/hr] at x_L; Vapor: V [kg/hr] as steam
Material balance:
F = L + V (total mass)
F × x_F = L × x_L (solute balance; solute non-volatile)
V = F × (1 - x_F/x_L) = F × (x_L - x_F) / x_L
Energy balance:
Q = V × λ_vapor + L × h_L - F × h_F [kJ/hr]
Q ≈ V × λ + F × c_p_L × (T_boil - T_F) [approximate; enthalpy of feed heating + evaporation]
Steam consumption:
S = Q / λ_steam [kg/hr; λ_steam = latent heat of steam at steam pressure]
Steam economy: E = V / S [kg vapor evaporated per kg steam; single effect: E ≈ 0.8–0.95]
Multiple-Effect Evaporator
N-effect forward feed:
V₁ steam from effect 1 → heating medium of effect 2; V₂ from effect 2 → effect 3...
Steam economy (N effects, forward feed, approximate):
E_N ≈ N × η_e [η_e = boiling point rise correction; 0.85–0.95 per effect]
3-effect: E ≈ 2.5–2.8; 5-effect: E ≈ 4.0–4.5
Optimal number of effects (capital vs. operating):
Add effects as long as annual steam savings > annualized capital cost of additional effect
Typically 3–5 effects for large installations
Boiling Point Rise (BPR)
BPR: dissolved solutes raise boiling point above pure water value at same pressure
BPR = T_solution_boil - T_water_boil at same P
Dühring's Rule:
T_solution_boil = m × T_water + c [linear relationship; m, c from tabulated Dühring lines for each solute]
Plot: T_solution vs. T_water → straight lines for each concentration → Dühring chart
Effect on driving force:
ΔT_effective = T_steam - T_boiling_solution = (T_steam - T_water_at_P) - BPR = ΔT_available - BPR
BPR reduces effective ΔT → less evaporation for same steam
Common BPR values (at 1 atm):
NaOH 10%: BPR ≈ 2°C; NaOH 50%: BPR ≈ 25°C
NaCl 5%: BPR ≈ 0.5°C; NaCl 25% (saturation): BPR ≈ 5°C
Sugar 60°Brix: BPR ≈ 1.5°C; 80°Brix: BPR ≈ 8°C
Heat Transfer Coefficient
Overall U (shell-and-tube evaporator):
1/U = 1/h_steam + R_fo + R_wall + R_fi + 1/h_boiling
h_steam = 5000–15,000 W/(m²·K) (film condensation of steam)
h_boiling: depends on evaporator type and flow regime
Boiling h correlation (forced circulation, tube interior):
h_boiling = h_convective + h_nucleate [Chen correlation]
h_convective: Dittus-Boelter for single-phase liquid (Dittus-Boelter); quality modifier
h_nucleate: Forster-Zuber correlation
Typical overall U values:
| Evaporator type | U [W/(m²·K)] |
|---|
| Falling film (thin watery) | 2000–4000 |
| Forced circulation (thin solution) | 1500–3000 |
| Forced circulation (viscous) | 500–1500 |
| Natural circulation (watery) | 1000–3000 |
| Wiped film (viscous) | 200–600 |
Fouling factors (TEMA standards):
R_fo = 0.0001 m²·K/W (clean steam condensate); R_fi = 0.0001–0.002 (process side; depends on fluid)
Scaling solutions (CaCO₃, CaSO₄): R_fi = 0.0002–0.001 after operation → design with margin
Area Calculation
Heat transfer area:
A = Q / (U × ΔT_eff) [m²; ΔT_eff = T_steam - T_boil - BPR]
Multiple effects — design ΔT distribution:
Equal ΔT assumption: ΔT per effect = ΔT_total / N [crude; more accurate: equal area per effect]
ΔT_total = T_steam_entering - T_condensate_last_effect - Σ(BPR_each_effect)
Sequence (forward vs. backward vs. mixed feed):
Forward feed: same direction as steam; simplest; feed dilute → concentrated; less pumping; lower final T
Backward feed: opposite; final effect at highest T → lower final product viscosity; more pumping
Evaporation Rate and Concentration Ratio
Concentration ratio: CR = x_L / x_F = F / L [feed/product ratio]
CR = 10 means: 10 kg feed gives 1 kg product + 9 kg vapor
Water evaporation duty:
V = F × (1 - 1/CR) × (x_F / x_F) [simplification for dilute feeds]
V = F × (1 - x_F/x_L)
Vapor flow in each effect (forward feed):
V_n ≈ V_total / N [approximately; more precise with enthalpy balance per effect]
Standards
| Standard | Scope |
|---|
| TEMA | Tube heat exchanger standards (evaporator shell-and-tube) |
| ASME Section VIII | Pressure vessel code (evaporator body) |
| 3-A Sanitary Standards | Food and beverage evaporators hygiene |
| ASTM E741 | Tracer tests for industrial processes |
| AIChE Handbook | Evaporation chapter for correlations |
Output
Provide: evaporator type (falling film/forced circulation/rising film), feed composition x_F [%] and product x_L [%], feed F [kg/hr] and product L [kg/hr], vapor V [kg/hr], number of effects N, steam pressure [bar] and temperature [°C], boiling point rise BPR [°C] per effect, effective ΔT [°C] per effect, steam economy E [kg/kg], steam consumption S [kg/hr], overall U [W/(m²·K)], heat transfer area A [m²] per effect, fouling factor R_fi [m²·K/W], CIP/fouling cleaning frequency, concentration ratio CR, and applicable standard (TEMA, ASME VIII, 3-A Sanitary Standards for food).
