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. 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. Because $h4$ is determined by the condensing pressure and the subcooling (via the isenthalpic expansion), and $h1$ by the evaporating pressure and the superheat, $q0$ responds to: - Lower evaporating temperature → lower $h1$ (slightly), lower $h4$ (less), net effect usually slightly decreasing $q0$ - Higher condensing temperature → higher $h4$ (throttling from higher pressure), clearly reducing $q0$ - More subcooling → lower $h4$ → larger $q0$ (see also the superheat and subcooling guide) - Refrigerant choice → different refrigerants have fundamentally different $q0$ values at the same temperatures Mass flow rate The mass flow rate is determined by the compressor. A reciprocating compressor of swept volume $V{sw}$ running at speed $n$ delivers: $\dot{m} = \frac{V{sw} \cdot n \cdot \lambdav}{v1}$ where $\lambdav$ is the volumetric efficiency (typically 0.65–0.90) and $v1$ is the specific volume of the suction gas (m³/kg). As evaporating…