Low-Temperature Evaporation Circulation Pumps: Oversizing and Undersizing

The Hidden Operational Drawbacks That Are Easily Overlooked

I. Undersized Circulation Pumps (Insufficient Flow Rate / Head)

1. Flow within the pipe transitions to laminar flow, causing a significant drop in the heat transfer coefficient

Insufficient flow rate → Low flow velocity within the pipe, causing the flow to change from turbulent to laminar;

A stationary viscous layer forms on the pipe wall, causing heat transfer resistance to surge; evaporation efficiency declines imperceptibly, and the cause is difficult to pinpoint.

2. Localised overheating, flow distortion and stagnation on the heat exchange surface, doubling the rate of scaling

Low flow rate and weak flushing result in slow material renewal in dead zones and on the pipe walls;

High-salt, silica-containing and heavy-metal-laden wastewater is highly prone to stagnating on the walls, leading to supersaturated precipitation and localised preferential scaling; the longer the system is used, the faster it becomes blocked.

3. Concentration limit is locked; unable to increase concentration

Low flow rate → unable to move highly viscous material; as concentration rises, the flow stalls, causing pressure build-up in the piping and severe flow deviation;

Forced to reduce the concentration ratio, resulting in concentration efficiency falling short of design specifications.

4. Pumps are prone to cavitation and run dry at the slightest fluctuation in operating conditions

Insufficient circulation volume means even minor fluctuations in the evaporation chamber liquid level cause excessive negative pressure at the pump inlet;

Latent cavitation occurs: whilst noise levels remain low, the impeller is gradually eroded, leading to a year-on-year decline in head.

5. Uneven temperature and concentration distribution, with frequent oscillations in automatic control

Low flow rates cause temperature and concentration stratification within the tank, rendering measurement points inaccurate in reflecting actual operating conditions;

Temperature control, level and concentration loops experience prolonged overshoot and oscillation, resulting in high energy consumption and unstable operating conditions.

II. Circulation Pump Sizing Too Large (Excess Flow Rate / Head)

1. Excessive flow velocity: pipeline erosion and wear + vortex-induced air entrainment

Excessive flow rate → flow velocity within the pipeline exceeds standards, accelerating erosion and wear at bends, welds and heat exchanger inlets;

Simultaneously, excessive velocity generates vortices that draw air into the material, significantly exacerbating foaming, resulting in excessive foam and high entrainment.

2. System resistance is overloaded, leading to continuous waste of unnecessary energy

Oversized pumps operate year-round at inefficient operating points, with valves throttling and building up pressure;

A significant amount of power is wasted on pipeline resistance and throttling, resulting in markedly high power consumption per unit of steam generated, and a substantial difference in electricity costs over the long term.

3. Excessive shear forces force emulsified oils and surfactants into more stable, fine bubbles

High flow rates and high shear forces cause emulsified oils and surfactants in the wastewater to be sheared into finer, more stable foam;

This makes defoaming more difficult, causes demisters to clog more easily, and simultaneously increases mist entrainment.

4. Severe liquid level turbulence and alternating wet/dry zones accelerate scaling

High circulation rates and violent tank turbulence lead to greater fluctuations in liquid level;

The frequency of wet-dry alternation at the liquid-air interface increases, causing salts to crystallise more rapidly as they air-dry, forming stubborn scale rings.

5. Disturbances to negative pressure conditions lead to greater fluctuations in vacuum

Violent turbulence and splashing at the liquid surface within the tank disturb the vapour space;

turbulent secondary vapour flow disrupts gas-liquid separation, causing frequent minor fluctuations in vacuum levels and making it difficult to maintain steady-state conditions.

6. High-concentration conditions are actually more difficult to operate

With an oversized pump and high flow rates, low concentrations pose no problem;

however, once the material becomes viscous, the high flow rate causes a sharp increase in pipeline resistance, leading to a surge in motor load, overloading and overheating—making the system less stable than when using a moderately sized pump.