evaporator-design

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).