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. The shape is not arbitrary — it is nine physical limits Look at any vendor envelope and you see a rectangle with selectively trimmed corners. That shape is the intersection of about nine independent physical constraints, each one a curve in TE/TC space. Walk inward from any direction and you hit a different limit first — which is exactly why the corners are trimmed the way they are. The nine constraints, and the edge each one carves: Limit Symbol What it is Edge it carves ------------ Discharge-gas temperature Td,max Oil cokes and valves fail above 130 °C (HFC), 110 °C (hydrocarbon), 180 °C (CO2 transcritical) upper-left corner Compression-ratio ceiling Πmax Leakage past valves/rotors and bearing load grow too fast above it upper-left curve Compression-ratio floor Πmin Below 1.3 the discharge gas is too cold to drag oil back to…