What Does PPI Mean in Foam Ceramic—and How Should It Be Selected?

PPI is one of the first specifications encountered when selecting open-cell foam ceramic.

It stands for:

Pores Per Inch.

A 10 PPI foam ceramic has a relatively coarse cellular network. A 30 or 40 PPI product contains a finer network with more pores over the same linear distance.

At first glance, PPI appears to be a simple fineness rating.

In reality, changing PPI influences several engineering characteristics at the same time:

• pore opening size;
• specific surface area;
• flow resistance;
• filtration behavior;
• catalyst-support area;
• fouling tolerance.

This means the highest PPI is not automatically the best choice.

What Does PPI Actually Describe?

Unlike honeycomb ceramic, foam ceramic does not contain straight parallel channels.

It consists of a three-dimensional network of interconnected ceramic struts and open pores.

PPI provides an approximate indication of how fine that cellular network is.

Lower PPI generally means:

• larger pores;
• fewer cells;
• more open passages.

Higher PPI generally means:

• smaller pores;
• more cells;
• greater geometric complexity.

Because the structure is irregular, PPI should be understood as a characteristic pore density rather than an exact precision count of identical holes.

Why Lower PPI Gives More Open Flow

A coarse foam contains larger connected passages.

Gas or liquid can move through the body with less obstruction.

This is useful where the main requirements include:

• high flow capacity;
• low pressure drop;
• larger particles;
• dirty process streams.

A low-PPI ceramic may therefore be preferable even though its available internal surface is lower.

Why Higher PPI Provides More Internal Structure

As the pore network becomes finer, the number of ceramic struts and pore intersections generally increases.

This can provide more:

• contact area;
• particle interception sites;
• catalyst-support surface.

Higher PPI can therefore be useful for:

• fine filtration;
• catalyst carrier applications;
• adsorption or reaction systems requiring more contact area.

But these benefits come with hydraulic penalties.

PPI and Pressure Drop

A finer pore structure normally creates a more tortuous flow path.

The fluid must repeatedly:

• accelerate;
• change direction;
• pass narrow pore throats.

This increases hydraulic resistance.

Actual pressure drop depends on:

• PPI;
• foam thickness;
• porosity;
• strut thickness;
• fluid velocity;
• fluid density and viscosity.

Therefore a statement such as:

“30 PPI foam ceramic pressure drop = X”

is incomplete without operating conditions.

PPI and Particle Filtration

For filtration, the target particle size matters.

A very coarse foam may allow small particles to pass.

A very fine foam may capture more particles but become blocked quickly.

This creates a familiar filtration trade-off:

capture efficiency vs dirt-holding capacity.

The optimum PPI depends on whether the filter is intended to capture:

• large inclusions;
• medium particles;
• fine contamination.

PPI and Catalyst Carrier Applications

When foam ceramic is used as a catalyst carrier, the objective may shift from filtration toward:

• high contact area;
• good gas distribution;
• low pressure drop;
• strong coating adhesion.

A finer PPI can provide more geometric surface for coating.

However, once catalyst coating is applied, pore openings become smaller.

Therefore the bare-carrier PPI should not be selected without considering the final coating thickness.

Why PPI Alone Does Not Define Porosity

Two foam ceramics can both be labeled 20 PPI yet have different:

• strut thickness;
• pore-window size;
• total open porosity.

One may be highly open and lightweight.

Another may contain thicker ceramic struts and greater density.

PPI therefore describes pore frequency—not total empty volume.

This distinction is essential.

Foam Thickness Matters Too

A 20 mm thick 30 PPI foam and a 100 mm thick 30 PPI foam do not create the same pressure drop or filtration behavior.

The thicker piece provides:

• more interception depth;
• more catalyst volume;
• more hydraulic resistance.

PPI must always be considered together with thickness.

Why Staged PPI Can Be Useful

Some systems use several foam layers with different PPI.

For example:

• coarse upstream layer;
• finer downstream layer.

The coarse layer captures larger material and protects the finer layer from rapid blockage.

This graded approach can extend service life.

However, it should be intentionally engineered rather than randomly stacking different foams.

PPI and Mechanical Strength

Higher PPI does not automatically mean stronger ceramic.

Mechanical strength depends heavily on:

• strut thickness;
• ceramic material;
• manufacturing quality;
• density.

A fine-pore foam can actually become fragile if the struts are extremely thin.

Mechanical data should therefore be checked independently.

What Information Should Be Provided for PPI Selection?

Useful project information includes:

• gas or liquid service;
• flow rate;
• target particle size;
• allowable pressure drop;
• operating temperature;
• chemical environment;
• foam thickness;
• catalyst coating requirement;
• fouling tendency.

Without these data, choosing PPI from a catalogue is only preliminary.

Engineering Takeaway

PPI describes the fineness of the three-dimensional foam structure, but it is not a universal performance ranking.

Lower PPI favors openness and fouling tolerance.

Higher PPI favors greater internal contact and finer interception.