When it comes to the key to energy saving of multi-effect evaporators, many people have a question in mind: "Since a 7-effect evaporator saves more steam than a 6-effect one, is it because the outlet temperature of the 7-effect evaporator is lower?"

But today, I want to make it clear to everyone — this claim is actually incorrect.

Regarding the logic behind steam saving and outlet temperature control of multi-effect evaporators, there are still many easily confused points. Today, we will clarify them in detail with practical cases for everyone, so as to avoid detours in subsequent equipment selection or production.

Misconception 1: Is it true that the more effects an evaporator has, the lower its outlet temperature?

Many people draw a direct equivalence between "number of effects" and "outlet temperature," thinking that as the number of effects increases, the outlet temperature will drop. In fact, the key influencing factor is not the number of effects at all, but rather the vacuum degree of the last effect.

Take practical scenarios for example:

Whether it is a single-effect, double-effect, 7-effect, or 8-effect evaporator, as long as the vacuum degree of their last effect remains consistent, the final outlet temperature will basically not have significant differences.

Take single-effect evaporators in the alumina industry for example: their steam-water ratio is usually above 1.0; while the steam-water ratio of 8-effect evaporators can be reduced to below 0.2. On the surface, the number of effects differs by 7 and the steam-water ratio also decreases significantly, but as long as the vacuum degree of their last effect is the same, the outlet temperatures of the two remain similar.

So stop focusing on the "number of effects" to judge the (outlet) temperature — the vacuum degree of the last effect is actually the core factor that determines the outlet temperature.

Misconception 2: Is steam saving achieved by lowering the outlet temperature?

Since the outlet temperature is not determined by the number of effects, then how exactly does a multi-effect evaporator achieve steam saving?

The answer lies in the number of times secondary steam is used.

The single-effect evaporator has a simple working principle: after live steam heats the material, the generated secondary steam is directly discharged into the atmospheric condenser — which means it only uses the steam once.

Double-effect evaporators, however, work differently — they collect the secondary steam generated in the first place and reuse it to heat the material in the second effect, which is equivalent to using the steam "twice".

By analogy, a 7-effect evaporator can utilize steam 7 times, and the 8-effect one can utilize it 8 times.

To put it simply, the core of steam saving for a multi-effect evaporator is to make steam "work more", rather than to "lower the outlet temperature". Temperature is only one of the results, not the cause of steam saving.

With the same number of effects: the outlet temperature can be either high or low

Some people may ask: Since the vacuum degree of the last effect determines the temperature, is the temperature of the same multi-effect evaporator fixed then?

Actually, it isn’t. Even for evaporators with the same number of effects and handling the same material, we can still make the outlet temperature "either high or low" by adjusting the process.

For example, the common "two-stage method", "three-stage method", or adjusting the material flow direction — whether it is co-current flow (where material and steam flow in the same direction) or counter-current flow (where they flow in opposite directions) — can all change the final outlet temperature.

Moreover, temperature adjustment can also be combined with the steam-water ratio: For example, the reason why a 7-effect evaporator we have worked on was able to achieve a steam-water ratio of 0.16 (a relatively low level in the industry) is that we further lowered the outlet temperature by adopting a co-current flow process, thereby further optimizing the steam-water ratio.

This also indicates that the design of multi-effect evaporators is not a "one-size-fits-all" approach, but rather can be flexibly adjusted according to requirements.

For enterprises selecting evaporators: Don’t just focus on "temperature" — take the "big picture" into account instead.

Finally, I would like to say to fellow business colleagues: When selecting evaporators, don’t just fix your attention on "whether the outlet temperature is low" or "whether the steam-water ratio is small"; instead, you should pay more attention to the actual conditions of your own factories — especially whether you have waste heat resources.

Take alumina enterprises for example:

After the evaporated mother liquor is discharged, it subsequently needs to undergo desilication and dissolution — and these processes all require temperature elevation.

If a factory has waste heat (such as in steel-aluminum integrated enterprises, where steel plants generate a large amount of waste heat), then the lower the temperature of the evaporated mother liquor, the better — there is no need to burn additional live steam for the subsequent temperature elevation, as this can be directly addressed using waste heat, and the overall energy-saving effect of the entire factory will be even better.

However, if a factory has no waste heat and temperature elevation can only rely on live steam, then making the temperature of the evaporated mother liquor too low is instead not cost-effective: Although the steam-water ratio of the evaporation station is reduced, more live steam will be consumed for subsequent temperature elevation, and in the final analysis, the comprehensive energy consumption of the entire factory may even increase.

Therefore, in many cases, the calculation and design of evaporators is not about "pursuing the lowest temperature", but rather "matching the factory's conditions" — we design specifically based on the temperature manufacturers need and the resources they have. In recent years, steel-aluminum integrated enterprises have been more willing to lower the mother liquor temperature, precisely because they can fully utilize the waste heat from steel plants. This is exactly the principle of "adjusting measures to local conditions".

conclusion

Actually, when it comes to multi-effect evaporators, the key is to clarify three relationships:

1. The outlet temperature is not determined by the number of effects, but by the vacuum degree of the last effect;

2. Steam saving is not achieved by lowering the temperature, but by increasing the utilization of secondary steam;

3. Whether the temperature is high or low is not equivalent to "the lower the carbon emissions, the better"; it must be evaluated based on a factory's waste heat resources from a holistic perspective.

   Going forward, if those of you encounter similar questions during equipment selection or production processes, you might as well think from these three perspectives—and you may just find the answer.