The evaporator body type in each effect of a multi-effect system (or in a standalone evaporator) must match the physical properties of the liquid being concentrated. This is independent of the number of effects or the feed arrangement (forward/backward/mixed).
Falling Film Evaporator
- Liquid enters a distributor at the top of vertical tubes and flows as a thin film down the heated tube wall.
- Residence time: 5–30 seconds — ideal for heat-sensitive products (fruit juices, pharmaceuticals, milk).
- Requires even liquid distribution — maldistribution causes dry patches and burning.
- Not suitable for: viscous liquids (> 50 cP at operating temperature), scaling liquids, crystallising slurries, or liquids with suspended solids.
- Minimum recirculation ratio typically 2:1 to 4:1 (recirculated flow : evaporated flow) to wet all tubes.
Forced Circulation Evaporator
- A pump circulates liquid through vertical or horizontal tubes at high velocity (2–4 m/s).
- Liquid is heated above its boiling point; flashing occurs in the separator vessel.
- Handles viscous liquids (up to several thousand cP), scaling liquids, and crystallising slurries (salt crystallisers).
- Longer residence time than falling film — not suitable for highly heat-sensitive products without careful temperature control.
- Higher power consumption due to circulation pump — factor into operating cost.
Selection Guide
| Liquid Property | Falling Film | Forced Circulation |
|---|---|---|
| Heat sensitivity | Preferred | Use only with low ΔT |
| Viscosity > 100 cP | Not suitable | Preferred |
| Scaling tendency | Poor | Good — high velocity reduces scaling |
| Crystallisation | Not suitable | Preferred — crystals stay in suspension |
| Suspended solids | Not suitable | Good |
| Food / pharma | Preferred | Rarely used |
| Chemical brines, caustic | Rarely used | Preferred |
RELATED EQUIPMENT FROM OHM ENGINEERS