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.
17 % - capacity lost - Bitzer 44CES-12Y, R404A, −12 / 42
Capacity falls 17 %, COP falls 12 %. That gap is real cooling the system will not deliver — and it is invisible in a tool that assumes zero ΔP.
12 % - COP lost - same compressor, same pressure drops
The System Simulator's job is to put the piping back. Once it does, the same compressor on R404A at −12 / 42 gives up about 17 % of its capacity and 12 % of its COP to a half-bar suction drop and a half-bar discharge drop.
0.5 bar - suction drop in the example - pulls the suction saturation to −15.7 °C
A 0.5 bar suction drop pulls the suction saturation to −15.7 °C; a 0.6 bar discharge rise pushes it to 43.3 °C.
7.1 K - per bar at −12 °C - the same bar at 42 °C: only 2.2 K
At −12 °C, one bar of pressure moves the saturation temperature 7.1 K. At 42 °C, the same bar moves it only 2.2 K. Suction-side ΔP is the one to attack.
0.1–0.2 - bar on a well-sized suction line - past 0.5 bar costs a sixth of the capacity
A well-sized suction line keeps the drop near 0.1–0.2 bar; a long, undersized, or oil-logged riser can push it past 0.5 bar — and there goes a sixth of the capacity.
The cycle is built backwards from two approach temperatures
Every state point in the loop is anchored by two heat-exchanger approach temperatures, one per tab:
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
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