Comparison of four flow paths for evaporators: co-current, counter-current, parallel flow, and mixed flow.

In evaporation processes, process layout often determines operational success more than heat exchange area. For the same triple-effect evaporator, co-current flow saves pumps, counter-current flow increases concentration, parallel flow reduces scale buildup, and mixed flow offers a balance of advantages. This article uses 800 words to thoroughly explain the mechanisms, energy consumption, applicable scenarios, and failure modes of these four process flows, allowing process engineers to complete the initial selection within 5 minutes.

I.Definitions and Diagrams

1.Co-current Flow: Feed liquid and secondary steam flow in the same direction: First effect → Second effect → Third effect, concentration increases with each effect, temperature decreases with each effect.

2.Counter-current Flow: Feed liquid flows in the opposite direction: Third effect ← Second effect ← First effect, the highest concentration is also the highest temperature.

3.Parallel Flow: Feed enters each effect simultaneously, each effect concentrates independently, and secondary steam remains in series.

4.Mixed Flow: Combining the above three flow types, common configurations include a two-stage "co-current + counter-current" flow, or "first effect co-current + second effect parallel flow".

II.Comparison of core parameters (taking 10 t·h⁻¹ sodium chloride solution, triple-effect, evaporation rate of 8 t·h⁻¹ as an example)

III. Mechanism-Level Disassembly

1.Temperature-Concentration Coupling: Co-current flow at low temperature and high concentration is prone to reduced evaporation at the final effect due to boiling point rise (BPR); counter-current flow reserves the highest temperature for the highest concentration, which can further increase the discharge concentration by 4-6%, but the high temperature and high salt content is precisely an accelerator for scaling.

2.Power Consumption: Co-current flow utilizes the pressure difference between effects for self-flow, requiring the fewest pumps; counter-current flow requires large-flow pumps between effects, with 30% higher shaft power, but maintains better heat transfer temperature difference and saves 15% of the area.

3.Scaling and Flushing: Co-current flow has the smallest concentration gradient per effect, with low supersaturation of CaSO₄, SiO₂, etc., which can extend the cleaning cycle from 7 days to 20 days; counter-current flow must be cleaned online with seed crystals or EDTA, otherwise pipe blockage may occur within 48 hours.

4.Heat-Sensitive Products: The co-current final effect has the lowest temperature (48–55℃), suitable for heat-sensitive materials such as Vitamin C and amino acids; the counter-current first effect has a temperature of 95–110℃, but proteins are prone to denaturation, requiring a residence time of <3 minutes.

IV. Failure Modes and Engineering Countermeasures

1.Co-current "Concentration Backflush": Due to excessive BPR in the final effect, the evaporation rate decreases, manifesting as increased pump current and vacuum in the first effect. Countermeasure: Add an MVR rising film "concentrator" to the final effect or introduce 0.8MPa live steam for reheating.

2.Counter-current "Pump Cavitation": High-temperature, high-concentration liquid flashes into the low-temperature effect, causing a sudden drop in pump inlet pressure. Countermeasure: Select a side channel for the inter-effect pump or add a 0.5m liquid column injection head.

3.Horizontal "Steam Short Circuit": Each effect has independent vapor-liquid balance; if the first effect suddenly depressurizes, secondary steam will backflow. Countermeasure: Add double check valves to the steam pipe and create a 0.3m liquid seal.

4.Mixed-flow "Control Coupling" The two-stage flow exhibits strong coupling between three variables: concentration, temperature, and vacuum. The DCS must employ a feedforward-feedback composite control system; otherwise, "concentration oscillation" will occur, leading to centrifuge blockage.

V.Selection Quick Reference Table

VI.Conclusion

There is no "best" process, only a "more suitable" one. Flow-coordinated flow is like an economy car—easy and cost-effective; counter-flow is like a performance sports car—high limits but requires meticulous maintenance; parallel flow is like a heavy-duty truck—resistant to scaling and robust; mixed flow is like a hybrid powertrain—maximizing energy consumption but resulting in a complex system. By inputting five parameters—raw material concentration, viscosity, scaling tendency, steam price, and electricity price—and referring to the above table and relevant design software during selection, the evaporator can find a more suitable balance between energy consumption, investment, and operating cycle.