Flour Milling Flow Sheet Design: Break, Reduction, Sifting and Purification

A flow sheet is a diagram of decisions. Where you break the wheat, how hard, what you sift out, and what you send back determine your extraction, your ash, your power bill and your product range — all before a single machine is ordered.

This article explains the reasoning behind the flow sheets we publish, so that a buyer can evaluate a competing quotation on engineering rather than on machine count.

The notation

Symbol	Passage	Function
B	Break	Opens the wheat, scrapes endosperm off bran
M	Reduction	Grinds released endosperm into flour
S	Scratch / sizing	Separates coarse middlings into size fractions
T	Tailing	Recovers flour from the coarsest reject material

So 4B5M1S1T is four break passages, five reduction passages, one scratch and one tailing. The numbers are passages, not machines. A single passage at 100 t/d may be served by four roll stands running in parallel.

Two flow sheets with identical capacity can have radically different capital cost and radically different extraction. That difference is the entire subject of this article.

What a break passage does

Wheat is not a homogeneous particle. It is a kernel with a starchy endosperm core wrapped in several bran layers. The first break’s job is not to make flour — it is to open the kernel and release endosperm in chunks large enough to be graded.

This is why break rolls run a large differential (typically 2.5:1) with fluted rolls. The fast roll holds the kernel while the slow roll shears. The result is a clean split rather than a crush.

From the first break you get roughly:

• 5–8% flour (fine enough to pass)
• 55–60% middlings of various sizes
• 30–35% bran with adherent endosperm

Only the flour is finished. The middlings go to sizing, then to reduction. The bran goes to the second break.

Why each break matters

Every additional break passage scrapes additional endosperm off bran that the previous passage could not reach. Diminishing returns set in, but the returns are money.

Breaks	Extraction ceiling	Bran residual starch
2 breaks	68–70%	12–15%
3 breaks	72–74%	9–11%
4 breaks	76–78%	6–8%
5 breaks	78–80%	4–6%

“Bran residual starch” is the flour you paid to mill and then sold as animal feed at a 70% discount. At 100 t/d, the difference between 11% and 7% residual starch in bran is roughly 1 tonne of flour per day — at USD 400/t that is USD 400/day, or USD 132,000 a year.

A 4th break passage costs perhaps USD 30,000–45,000 installed. It pays back in four months.

This is why we push buyers toward one more break than they think they need. It is the highest-return capital in the plant.

Why reduction passages are different

Reduction rolls take clean endosperm and reduce it to flour. They run smooth rolls at a small differential (2.5:1 down to 1.25:1 as the passage number rises) because the objective is attrition, not cutting.

A high differential on a reduction passage cuts endosperm particles, generating heat and fine bran that then has to be sifted out. A low differential crushes and rubs, producing a controlled particle size distribution with less bran contamination.

Passage	Typical differential	Roll type
1B–4B	2.5 : 1	Fluted
1S–2S (scratch)	1.5–2.0 : 1	Fluted, fine
1M–2M	2.0 : 1	Smooth
3M–4M	1.5 : 1	Smooth
5M+	1.25 : 1	Smooth

Ordering the wrong differential for a position is a common and expensive mistake. A 2.5:1 roll in a 4M position will overheat the stock and raise ash. A 1.25:1 roll in a 2B position will not open the wheat at all.

The sifter is usually the constraint

Roll stands grind. Sifters classify. Most flow sheets are limited by classification, not by grinding — because sifting area is expensive and bulky, while roll capacity is relatively cheap to add.

The working rule:

Sifting area (m²) = daily capacity (t/d) ÷ 0.8

Capacity	Sifter area	Typical configuration
20 t/d	25 m²	2 × twin-bin (5 m²/bin)
30 t/d	38 m²	2 × twin-bin, or 1 × FSFG4×24
60 t/d	75 m²	2 × FSFG6×24
100 t/d	125 m²	FSFG8×24 + FSFG6×24
200 t/d	250 m²	4–5 high square units
300 t/d	375 m²	7–8 high square units

Two caveats. First, the rule assumes a standard 4B5M flow sheet; more passages need proportionally more area. Second, sifting area must be distributed by stream, not just totalled — a plant with 125 m² concentrated on one machine cannot grade five streams independently.

Where the purifier belongs

A purifier takes middlings from the break sifters and separates them by density rather than size. Pure endosperm is lighter and more angular, so it floats on the air stream and travels further; bran-contaminated particles of the same size settle and are rejected.

The consequences are large:

• Ash reduction of 0.08–0.15 points on the purified stream
• 5–8 points more patent extraction at the same ash specification
• The ability to produce genuinely low-ash flour rather than merely well-sifted flour

But a purifier needs feed. Below four break passages there is not enough middlings to keep it loaded, and the machine runs half-empty, consuming air and attention for nothing. That is why 40 t/d is the practical threshold for a purifier — it is also the point at which the flow sheet typically reaches 4 breaks.

Scratch and tailing passages

Scratch (S) passages handle coarse middlings that are too big for reduction but too valuable to discard. A scratch roll reduces them to a size the sifter can handle, sending the clean portion to reduction and the contaminated portion to tailing.

Tailing (T) recovers flour from the coarsest, most bran-contaminated reject stream. Without a tailing passage, that material leaves as bran — with a significant amount of good flour still attached. A tailing passage typically recovers 0.5–1.5% of mill intake.

Neither is glamorous. Both are pure margin.

Impact detachers: the invisible passage

Smooth reduction rolls produce flat flakes. Flakes blind sifter screens and lock flour inside their structure, so the sifter rejects material that is actually finished flour.

An impact detacher throws the stock against pins at ~2,900 rpm, breaking flakes apart without grinding bran finer. The effect on sifting efficiency is immediate and measurable.

Fit one after each group of smooth reduction mills. It costs 3–5.5 kW and typically lifts sifting throughput by 8–15% on the affected streams.

Comparing two flow sheets

Consider two quotations for a 60 t/d plant:

	Option A	Option B
Flow sheet	3B4M1S	4B5M1S1T
Roll stands	9	12
High square sifters	1 × FSFG6×24	2 × FSFG6×24
Purifier	None	1 bank
Installed power	118 kW	150 kW
Extraction	74%	78%
Patent ash	0.68%	0.52%

Option A looks 21% cheaper on capital. Now run the numbers.

At 60 t/d of wheat and 74% extraction, Option A produces 44.4 t of flour per day. Option B produces 46.8 t. The 2.4 t/day difference, at USD 400/t, is USD 960/day — USD 316,800 a year at 330 operating days.

The extra capital is roughly USD 90,000. It pays back in about three and a half months, and then continues paying for twenty years.

Add the price premium for 0.52% ash versus 0.68% ash — typically 3–6% on the ex-mill price — and the gap widens further.

This is the comparison that matters, and it is almost never presented in a quotation.

The flow sheet decision, in order

1. Target extraction and grade count. This sets the minimum passage count and whether a purifier is required.
2. Break count. One more than the minimum, almost always.
3. Reduction count. Two to three more than the break count for a standard flow sheet.
4. Sifter area. Capacity ÷ 0.8, distributed by stream.
5. Purifier requirement. Yes at 4 breaks and 40 t/d and above.
6. Detachers. One per smooth-roll group.
7. Power and building. Follows from all of the above.

Ask for the flow sheet, not the price

Any supplier can quote a machine list. A flow sheet with the passages named, the differentials specified, the sifter area calculated and the extraction target stated is a document you can hold someone to.

If a quotation does not include one, that is the answer to your question about that supplier.