Guide to Troubleshooting and Solving Common Evaporator Faults

Q1: The evaporator’s heat transfer efficiency has dropped sharply. What could be the causes? How to troubleshoot it?

A1: First, let’s talk about what the fault looks like: the evaporation rate decreases significantly, steam consumption increases instead, and the temperature difference in the system is also abnormal.

The most likely causes are these:

First and foremost, scaling on heat exchange tubes — this accounts for over 60% of faults. Crystals or salts deposit on the tubes, directly reducing the heat transfer coefficient. In severe cases, it can drop by 50% to 80%.
Then there’s steam leakage: the seals of valves or flanges fail, causing some steam to escape and waste heat energy.
Another issue is reduced circulation pump efficiency: impeller wear, cavitation, or shaft seal leakage leads to insufficient solution flow.
Finally, insufficient vacuum: blockage in the condenser or failure of the vacuum pump raises the boiling point of the solution, which naturally affects heat transfer.

For troubleshooting, follow these steps:
First, conduct a heat balance test: compare the actual heat transfer rate with the design value and calculate how much the heat transfer coefficient has decreased.
Next, measure the pressure difference between the inlet and outlet of the circulation pipeline to check if the pump is losing power.
Then, use a vacuum gauge to inspect the system’s tightness in sections and look for air leaks.
Finally, insert an endoscope through the inspection hole to check the thickness and distribution of scale on the inner wall of heat exchange tubes.

Q2: The circulation pump frequently suffers from cavitation or abnormal vibration. How to solve this?

A2: This fault is easy to spot at a glance: the pump makes extremely loud noise, the vibration exceeds the standard (amplitude over 0.1 mm counts), and there are honeycomb - shaped pits on the impeller surface.

Why does this happen? Mainly due to these points:

Insufficient inlet pressure: either the liquid level in the circulation tank is too low, or the pipeline resistance is too high (e.g., a clogged filter).
Gas in the medium: the supersaturated solution releases gas, or air leaks in through the seal.
Improper installation: the pump and motor are misaligned (deviation over 0.05 mm), or the base is not rigid enough.

Here are the solutions:

First, optimize the NPSH (Net Positive Suction Head): raise the liquid level in the circulation tank to ensure a static head of at least 1.5 m. You can also increase the diameter of the inlet pipe or shorten the pipeline length to reduce friction loss.
Then, modify the structure: use a double - suction impeller or add an inducer to enhance cavitation resistance. Install a gas separator, such as a cyclonic degassing device, to remove gas from the medium.
Daily maintenance is also essential: regularly calibrate the alignment of the pump and motor with a laser alignment tool, keeping the accuracy within ±0.02 mm. Check the impeller wear every quarter — the maximum allowable wear thickness is less than 2 mm; if it exceeds this, the impeller must be replaced.

Q3: What are the typical signs when the evaporator’s vacuum system fails? How to repair it?

A3: When the vacuum system fails, the most obvious sign is unstable vacuum degree — for example, the design value is - 0.08 MPa, but the actual value can only reach - 0.05 MPa, and the secondary steam cannot condense.

There are three root causes:

Clogged condenser: either scaling on the cooling water side or crystal accumulation on the steam side (substances like CaCO₃ and SiO₂ are particularly prone to causing blockages).
Vacuum pump failure: contaminated working fluid in the liquid ring pump or worn rotary vanes.
System leakage: aging flange gaskets or failed sight glass seals.

For repairs, follow this process:

First, conduct a segmented pressure retention test: use a helium mass spectrometer leak detector to locate leak points (the allowable leak rate should be less than 1×10⁻⁵ mbar·L/s).
Then, clean the condenser:
For chemical cleaning, circulate 5% citric acid to clean the cooling water pipes (control pH between 3 - 4 and temperature at 60℃).
For mechanical cleaning, use a high - pressure water gun (with pressure ≥100 bar) to flush the steam side.
Finally, maintain the vacuum pump:
Replace the working fluid (15# mechanical oil or special vacuum pump oil is recommended).
Check the rotary vane gap (the standard value is 0.05 - 0.08 mm).

Q4: The crystal particle size is uneven (either too fine or agglomerated). Is this related to the evaporator’s operating parameters? How to adjust them?

A4: First, let’s talk about the impact of this problem: if the crystals are too fine (＜50 μm), solid - liquid separation becomes extremely difficult, and a lot of mother liquor is carried away. If they agglomerate (＞500 μm), the discharge port is easily blocked, and the product purity decreases.

In fact, it is most closely related to these key parameters:

First, supersaturation (σ): if it’s too high (σ＞1.5), explosive nucleation occurs, resulting in fine crystals; if it’s too low (σ＜1.1), crystal growth dominates, making agglomeration easy.
Then, circulation flow rate: when the flow rate is less than 1.5 m/s, crystals settle, leading to local supersaturation.
There’s also residence time: if it’s too long (＞30 min), secondary agglomeration may occur.

For adjustment, you can do the following:

Install an online concentration monitoring device, such as a densitometer or conductivity meter, to view real - time data and adjust the feed rate accordingly.
Adopt gradient cooling: control the evaporator chamber temperature in stages (e.g., 80℃→65℃→50℃) to prevent sudden explosive nucleation.
You can also add seed crystals: add 20 - 100 μm seed crystals with an addition amount controlled at 0.5% - 1% (w/w) to guide the directional growth of crystals.

Q5: The evaporator often leaks, whether at welds or flanges. How to completely solve this?

A5: First, let’s talk about the areas prone to leakage: the welds of the heating chamber shell, which often crack due to thermal stress fatigue; the flanges at the pump inlet and outlet, where gaskets loosen due to vibration; and the sealing surfaces of sight glasses and manholes, where sealing materials are prone to leakage as they age.

To completely solve this problem, you need to start from these aspects:

First, upgrade materials: replace the shell with 316L stainless steel or titanium (replacement is a must when the Cl⁻ concentration in the solution exceeds 500 ppm); replace flange gaskets with PTFE - coated graphite gaskets, which can withstand a high temperature of 250℃ and a pressure of 1.6 MPa.
Then, optimize the structure: use double - sided welding for welds, and conduct radiographic testing after welding (in line with the NB/T 47013 standard); add an expansion joint with a compensation amount of at least 10 mm to relieve thermal stress.
Finally, install intelligent monitoring equipment: install a vibration sensor (with a frequency range of 10 - 1000 Hz) to provide early warning of loose flanges; use an infrared thermal imager to check the temperature field of the shell — if the temperature difference exceeds 15℃, it is highly likely that there is a leak, allowing for timely detection.

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