1. Jacket Heating (Heating of the outer shell)

Heat is transferred to the material via the outer shell of the cylinder; there are no heating elements inside, and the interior cavity is empty.

Characteristics of scale formation

Scale forms only on the inner wall of the cylinder, creating a smooth, continuous surface.

There are no blind spots, obstructions or crevices.

Crystallisation typically occurs as a thin layer adhering to the surface and does not become lodged in complex structures.

Difficulty of cleaning

Large areas of scale can be dislodged simply by spraying with a high-pressure water jet.

During acid washing, the chemical solution circulates freely and makes uniform contact with all scaled surfaces.

Manual cleaning by personnel is also convenient, as there are no obstructions.

Even with severe material crystallisation, blockages or hidden deposits are unlikely to occur.

Overall, this can be understood as: scaling on smooth walls is easy to scrape off and wash away.

II. Coil Heating (Heating pipes coiled inside the vessel)

Heating pipes are densely coiled within the equipment, arranged in multiple layers and loops with small spacing, occupying most of the internal cavity space.

Characteristics of Scale Formation

The surface temperature of the heating coils is relatively higher; materials prone to crystallisation tend to precipitate preferentially on the tube walls and adhere firmly.

The coils are layered one upon another with extremely narrow gaps; once crystallisation occurs, it easily blocks these gaps.

Scale forms a thick, hard layer enveloping the tubes 360°.

Difficulty of Cleaning (The main challenges lie here)

The gaps between coils are too narrow for high-pressure water jets to penetrate.

The innermost tubes cannot be reached at all; only the outer layers can be flushed.

Crystallisation easily turns the coils into ‘solid blocks’.

As soon as the equipment is shut down, crystalline materials immediately fill the gaps, forming a single, hard mass.

Chemical circulation is impeded, making acid washing slow and incomplete.

Once blocked, the chemical solution cannot penetrate, and the internal scale cannot be removed.

Manual access is impossible, and hammering is ineffective

The space is too confined for tools to reach; forceful tapping risks puncturing the coils

The more you clean, the more likely scale is to accumulate

Residual crystals left behind act as nucleation sites, causing faster and harder scaling next time

In summary: it’s like a tangled wire mesh of scale—impossible to scrape out, impossible to flush through, and once blocked, it’s a nightmare.

III. A Straightforward Comparison for Materials Prone to Crystallisation

Jacket heating: Scale forms on smooth inner walls; easy to flush, easy to clean, does not harbour scale, and is unlikely to become blocked.

Coil heating: Scale forms on densely packed tube bundles; difficult to flush, difficult to clean, prone to becoming completely blocked, and extremely laborious to clean.

Provided the material is even slightly prone to crystallisation, over the long term, the cleaning workload for coil heating will be several or even ten times that of jacket heating. Furthermore, incomplete cleaning is more likely to lead to progressively poorer heat transfer.

IV. Summary in a Nutshell

For materials prone to crystallisation, jacket heating is the preferred choice, as scaling occurs only on the smooth inner walls, making cleaning straightforward;

Coil heating features densely packed internal tubes with narrow gaps; once crystallisation occurs, the system becomes completely blocked, making cleaning far more difficult and leaving it prone to residual deposits.

Translated with DeepL.com (free version)