Pressure drop: the penalty selection software hides
By DigiEntropy Engineering · 2026-03-28 · 8 min read
Bitzer's selection software assumes zero pressure drop across the evaporator and condenser. A real loop has four pressures, not two — and once you put the piping back, the same compressor on R404A gives up about 17% capacity and 12% COP. Here is why, with the numbers.
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. Open the System Simulator, drop in a compressor, a condenser, an expansion valve, and an evaporator, and it draws a picture like the one below — every pipe run labelled with temperature, pressure, enthalpy, and mass flow at both ends. One question comes up every time an engineer who lives in selection software first sees this: the numbers do not match what Bitzer's software gives me. This post explains why — and why the simulator is right to disagree. The cycle is built backwards from two approach temperatures Every state point in the loop is anchored by two heat-exchanger approach temperatures. High side. The liquid leaving the condenser sits at $T\text{liquid} = T\text{ambient} + \Delta T\text{cond} - \Delta T\text{SC} = 32 + 10 - 2 = 40\ ^\circ\text{C}$ The condensing temperature itself is $\text{SDT} = 32 + 10 = 42\ ^\circ\text{C}$. For R404A, saturation at 42 °C is 19.0 bar — that is the discharge pressure. Low side. With a −5 °C room and a 7 K evaporator approach, the evaporating temperature is $\text{SST} = -5 - 7 = -12\ ^\circ\text{C}$. Add 5 K of useful superheat and the gas leaving the evaporator is at −7 °C. (If you wrote that as "−15 − 7 + 5", check the room temperature: for the −7 °C result to hold, the room is −5 °C, not −15 °C — and an evaporator can never be warmer than its room.) Saturation at −12 °C is about 4.1 bar. Everything else follows from the saturation curve. Here is the screenshot's cycle, with each pressure checked against CoolProp: Station Pressure Saturation temp Shown temp Note --------------- Compressor suction 4.1 bar −11.4 °C −6.7 °C 4.7 K superheat Evaporator inlet 4.6 bar −8.1 °C two-phase 0.5 bar above suction Compressor discharge 19.1 bar 42.1 °C 63.1 °C superheated gas Condenser outlet 18.5 bar 40.8 °C 39.8 °C 2 K subcooling So the reading of the cycle is correct. The interesting part is the pressure difference inside each heat exchanger — 4.6 versus 4.1 bar across the evaporator, 19.1 versus 18.5 bar across the condenser. Selection software pretends those do not exist. The simulator does not. What a compressor polynomial actually responds to A compressor rating polynomial…