Refrigerating effect and mass flow: the engine behind cooling capacity
By DigiEntropy Engineering · 2026-01-25 · 9 min read
Cooling capacity is the product of two quantities: how much refrigerant flows per second, and how much heat each kilogram absorbs in the evaporator. Understanding both reveals why the same compressor can deliver radically different capacities depending on refrigerant, operating conditions, and pipe sizing.
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.
The fundamental capacity equation
Every refrigeration system delivers cooling by circulating refrigerant in a closed loop. The total cooling power — usually called the evaporator capacity $\dot{Q}e$ — depends on two quantities:
$\dot{Q}e = \dot{m} \cdot q0$
where: - $\dot{m}$ is the mass flow rate of refrigerant (kg/s), set by the compressor - $q0$ is the specific refrigerating effect (kJ/kg), set by the operating conditions and refrigerant
Both terms matter, and both respond differently to changes in temperature, refrigerant, and system design. Understanding their individual behaviour explains why two nominally "equal" compressors can deliver very different capacities when the refrigerant is changed.
50 kW - rated capacity - at −10 °C / 40 °C
For example: a compressor rated at 50 kW at −10 °C / 40 °C might deliver only 32 kW at −20 °C / 40 °C.
32 kW - the same compressor, colder - at −20 °C / 40 °C
The system must deliver the required cooling at the worst-case evaporating condition, not just the catalogue nominal point.
36 % - capacity drop - almost entirely from suction specific volume
The 36 % drop is almost entirely due to the increase in suction specific volume at the lower evaporating pressure. The refrigerating effect barely changes across that range.
0.65–0.90 - volumetric efficiency - typical, reciprocating compressor
The volumetric efficiency of a reciprocating compressor is typically 0.65–0.90.
R717 - highest volumetric refrigerating effect - among common refrigerants
R717 (ammonia) has by far the highest volumetric refrigerating effect among common refrigerants, which is why ammonia systems use much smaller compressors relative to their cooling output than HFC systems.
Specific refrigerating effect
The specific refrigerating effect is the enthalpy difference across the evaporator:
$q0 = h1 - h4$
where $h1$ is the specific enthalpy at the compressor inlet (superheated suction vapour) and $h4$ is the specific enthalpy at the evaporator inlet (after expansion). On the pressure-enthalpy diagram, $q0$ is the horizontal span of the evaporation line — how far the refrigerant travels along the enthalpy axis while absorbing heat from the space.
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