In the operation, maintenance and manufacturing process of evaporators, "heat treatment" is a high-frequency topic, but many people have misunderstandings about it — for example, they think that the entire evaporator must undergo heat treatment. In fact, this is not the case. The heat treatment of evaporators should be carried out according to different parts and situations: some must be fully heat-treated, some can be partially heat-treated, and others cannot be heat-treated at all!

Today, combining practical experience, we will thoroughly explain the core logic, classification and precautions of evaporator heat treatment. Whether it is for manufacturing or operation and maintenance, it can help you avoid pitfalls and take fewer detours～

First, understand: Why is heat treatment necessary for evaporators?

First, let's go back to the most basic question: Most of the evaporators we make work in a strong alkali environment. Why are they prone to "caustic embrittlement" (that is, equipment cracking and corrosion)?

The core reason is: during the manufacturing process of the evaporator, a certain amount of stress will be generated, and this stress will cause "stress corrosion". If these stresses are not removed, the equipment will be easily corroded and cracked after long-term operation in a strong alkali environment, eventually leading to leakage, which affects production safety and equipment service life.

Heat treatment, in essence, is to eliminate the stress generated during the manufacturing process of the equipment, avoid caustic embrittlement from the root, and ensure the stable operation of the evaporator.

Key Point: Three Types of Evaporator Parts, Greatly Different Heat Treatment Methods

Many people think that "the entire evaporator must be heat-treated", but this statement is not comprehensive. Combined with the structure of the evaporator and the working environment (alkali concentration, temperature), we can divide the parts that need treatment into three categories, and targeted treatment is more efficient and cost-saving.

Category 1: Must be Fully Heat-Treated (Core Key Parts)

These parts are in direct contact with high-concentration lye or high-temperature alkali vapor, which are high-risk areas for caustic embrittlement. They must be fully heat-treated to completely eliminate stress. Moreover, full heat treatment has strict requirements — it must be carried out in a professional heat treatment furnace, and the effect of on-site treatment with heating bags is much worse.

Why must a heat treatment furnace be used? Because the temperature in the furnace is uniform, the temperature inside and outside the equipment is consistent, which can comprehensively and uniformly eliminate the stress of the components in a short time, fundamentally eliminate caustic embrittlement, and avoid equipment leakage.

Specifically including these parts:

? Upper and lower heads of the heating chamber of 1-4 effect evaporators (especially the upper section of the upper head of the first effect heating chamber, where the feed port is located, and the lower section is responsible for vapor-liquid separation, where the liquid stays and is in contact with the material throughout the process);

? Lower section of the separation chamber (the area in direct contact with the material);

? Primary and secondary flash evaporators (parts in contact with the material);

? Connecting pipes of 1-3 effects (some lye or high-concentration high-temperature alkali vapor passes through here, with high temperature and high corrosion risk).

Only when these parts are fully heat-treated can the stability of the core components of the evaporator be maximized and leakage hazards be avoided.

Category 2: No Need for Full Heat Treatment (Low-Risk Parts)

The occurrence of caustic embrittlement and stress corrosion is proportional to the concentration and temperature of the lye — when the temperature is within 80℃ and the concentration of the lye is low, the equipment will hardly undergo caustic embrittlement. For such parts, there is no need for full heat treatment, so as to avoid over-treatment and increase costs.

In addition, the overall heat treatment of large evaporators (such as those with a diameter of 13-14 meters and a length of tens of meters) is extremely difficult. Ordinary large-scale heat treatment furnaces cannot accommodate them at all. Even if the wrapping heating method is used, it can only be regarded as partial heat treatment, which cannot achieve the effect of full heat treatment, so it is unnecessary to take extra trouble.

Specifically including these parts:

? Upper part of the separation chamber of 5-7 effects;

? Parts in 3-4 effects that only come into contact with low-temperature and low-concentration steam (not in contact with high-concentration lye, and the temperature is lower than 80℃).

Category 3: Cannot be Fully Heat-Treated (Easily Damaged Parts)

This type of part is relatively special. Not only is there no need for full heat treatment, but once it is done, it will damage the equipment and lead to equipment scrapping. The core is the heating section of the heating chamber (the area between the upper tube sheet and the lower tube sheet, that is, the part where the heating tubes are inserted).

Why can't it be done? There are two core reasons:

1. The heating tubes of the evaporator are expanded (not welded), and expansion connection itself is a kind of stress connection. If the heating chamber with inserted tubes is put into the furnace for full heat treatment, after the stress is eliminated, the expanded tubes will all loosen and cannot be fixed normally;

2. If full heat treatment is performed without inserting the tubes, the holes drilled in the tube sheet will be deformed (even if properly controlled, there will be a small amount of deformation). The diameter gap between the heating tube and the hole drilled in the tube sheet does not exceed 1 mm. Once deformed, the tube cannot be inserted, and even the baffle will be deformed, and the entire heating chamber will be completely scrapped.

Moreover, from the perspective of the use scenario, in the heating section of the heating chamber, the material only passes through the inside of the tube, and the shell side only contacts steam (the first effect contacts raw steam, and the second and subsequent effects contact the steam from the previous effect), so caustic embrittlement will not occur at all. Therefore, it can neither be done nor is there a need for full heat treatment.

Summary: Evaporator Heat Treatment, Grasp the Core and Avoid Blindness

In fact, the core principle of evaporator heat treatment is very simple: look at the contact medium (whether it is in contact with high-concentration lye), look at the working temperature (whether it exceeds 80℃), and look at the component structure (whether it is a vulnerable expanded part).

Simply sorted out:

? High-concentration, high-temperature contact parts → must be fully heat-treated (carried out in a furnace);

? Low-concentration, low-temperature contact parts → no need for full heat treatment;

? Expanded heating section parts → cannot be fully heat-treated.

Understanding these three types of situations can avoid the cost waste caused by "blind overall treatment" and prevent equipment hazards caused by "missing key parts", making the evaporator more stable and durable～