Operating envelopes, end to end: from a vendor PDF to a physics-generated polygon for every brand
By DigiEntropy Engineering · 2026-03-28 · 13 min read
An operating envelope is the TE/TC region where a compressor is qualified to run — and manufacturers publish it as a picture, not data. This walks the whole pipeline: the nine physical limits that shape the envelope, two ways to turn a vendor chart into a polygon, and how we now generate envelopes from physics for 15 brands (about 42,000 of them) when no chart exists.
About this post — Authored by an AI assistant using DigiEntropy's polynomial corpus, the universal compressor predictor, and the project's chart-generation tooling. Charts are produced by Python scripts that read the same database the live site queries; tables and formulas are pulled from the same engineering modules. Findings reflect the project's current dataset and methodology — send corrections or deeper questions to admin@digientropy.com.
An operating envelope is the region on a TE/TC chart where a compressor is qualified to run. TE is the evaporating (suction) saturation temperature on the x-axis; TC is the condensing (discharge) saturation temperature on the y-axis. Inside the polygon, the manufacturer warrants the compressor. Outside it, you are on your own — the motor overheats, the discharge gas cokes the oil, the bearings starve, or the head pressure runs past the vessel rating.
Every selection tool needs this polygon. Benchmark, Cold Room, Selection, and F-Gas Retrofit all have to answer one question before they show a capacity or a COP: is this operating point even allowed? A capacity number at an out-of-bounds point is worse than no number — it is a confident wrong answer.
The catch is that manufacturers publish the envelope as a picture. A PDF or PNG chart — a polygon drawn on a TE/TC grid — with no machine-readable vertices behind it. To use it, you have to turn the picture into numbers.
This post walks the whole pipeline: what shapes the envelope (it is not arbitrary), the two ways we turn a chart into a polygon, how we calibrate the physics against real vendor charts, and how we now generate the envelope from physics for brands that never published one — about 42,000 envelopes across 15 brands, all queryable.
41,938 - envelopes in the shipped seed set - across 19 files and 15 brands
The shipped seed set is 41,938 envelopes across 19 files and 15 brands: four digitised from real vendor data (Bitzer, Dorin, Frascold, Snowkey) and the rest generated from spec.
9 - physical limits carve the shape - each one a curve in TE/TC space
That shape is the intersection of about nine independent physical constraints, each one a curve in TE/TC space.
0.95 - median IoU, fitted to the image - 51 hand-scraped Bitzer envelopes
Across 51 hand-scraped Bitzer envelopes the median IoU is 0.95. The physics reproduces the published outline almost exactly.
0.60 - median IoU, from spec alone - 274 held-out models
On 274 held-out models the median IoU is 0.60 (reciprocating 0.65, screw 0.50).
1,420 - fitted envelope records - across 974 models
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