Unit cooler and condenser selection for cold rooms — the catalog pair
By DigiEntropy Engineering · 2026-02-08 · 6 min read
Cold-room sizing is three components, not one. The compressor is the headline; the unit cooler (evaporator) and condenser set the SST/SDT the compressor actually sees. Pick them wrong and the compressor's nameplate doesn't match reality.
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. Most cold-room write-ups talk about the compressor. The compressor's nameplate is meaningful only if the evaporator and condenser deliver the SST/SDT it was sized for. A 25 kW compressor sized at -10/40 °C, paired with an under-sized evaporator that only achieves -15 °C SST and an over-sized condenser at 45 °C SDT, delivers 18 kW. Worse — that 18 kW is at higher current, lower COP, with a discharge superheat margin that's eaten into the envelope. Picking the unit cooler and the condenser is a quarter of the project. DigiEntropy has dedicated catalogs for both, with cross-brand selection that pairs cleanly with the compressor catalog. The capacity math Both evaporator and condenser publish their capacity at a reference condition: - Unit cooler: capacity at DT (room T − SST). DT typically 7-10 K for cold rooms, lower for sensitive products (5-7 K for cut flowers, deli items). - Condenser: capacity at DT (SDT − ambient). DT typically 10-15 K, set by the ambient/SDT economic trade-off. The pair-sizing rule: pick the evaporator that delivers compressor cooling capacity at the target SST, and the condenser that delivers compressor heat rejection at the target SDT. Reverse-engineer from the compressor's preferred operating point, not from the room's design temperature. The pair-sizing workflow 1. Run Cold Room → get total heat load and compressor candidates at standard SST/SDT. 2. Pick a compressor → note its preferred SST/SDT (typically -10/40 for MT, -32/40 for LT). 3. Go to Unit Cooler → set Qe = compressor Qe, target SST, room T. Pick. 4. Go to Condenser → set Qcond = compressor Qe + P (heat rejection), target SDT, design ambient. Pick. 5. Run System Simulator with all three on the canvas to verify the closed-loop solves at the target SST/SDT. Common sizing failure modes Under-sized evaporator (the most common): you spec the evaporator at the room design DT (say 8 K), but the actual operating DT in summer is 6 K because the compressor is undersized at the high ambient. Evaporator delivers only 75 % of nameplate at 6 K DT, system runs at -15 SST instead of -10, compressor capacity drops to 80 %. Two compounding…