Drawing No. EH–TH–037 // Thermal Engineering & HVAC
Refrigeration Cycle Simulator
Reviewed August 2026
Explore how evaporating and condensing temperatures set the theoretical ceiling on refrigeration efficiency, and how an assumed fraction-of-Carnot performance compares to that ideal benchmark.
What problem does this solve?
The temperature lift between evaporation and condensation sets a thermodynamic upper bound on refrigeration COP. This teaching model calculates the reversed-Carnot ceiling, then applies a user-selected fraction of that ceiling as a performance assumption; it does not calculate a refrigerant-specific vapor-compression cycle.
Inputs
Results
Why real cycles fall short of Carnot
A real vapor-compression cycle never reaches the reversed-Carnot COP because of several unavoidable irreversibilities.
| Loss mechanism | Effect |
|---|---|
| Compressor inefficiency | Real compression is not isentropic; friction and gas dynamics add extra work beyond the ideal minimum |
| Throttling (expansion valve) | The expansion process is irreversible, unlike Carnot's reversible expansion with work recovery |
| Superheat & subcooling | Real cycles run some superheat at the compressor inlet and subcooling at the condenser outlet, both adding practical margin at a small efficiency cost |
| Heat exchanger ΔT | Evaporator and condenser need a finite temperature difference to transfer heat at all, which is exactly what widens the temperature lift beyond the space/sink temperatures |
Background
COPCarnot = Tevap / (Tcond − Tevap), with both temperatures in absolute units (Kelvin). This is the theoretical maximum coefficient of performance for any refrigeration cycle operating between these two temperatures — no real cycle can exceed it.
Percent-of-Carnot is useful as a normalization benchmark because it compares performance against the thermodynamic ceiling at the same temperature lift. It is not a universal prediction of a real unit: refrigerant choice, compressor efficiency, pressure drops, superheat/subcooling and heat-exchanger approaches all matter. Treat the entered percentage as an assumption or calibration parameter.
Compressor work W = Qevap / COP. By energy balance, heat rejected at the condenser Qcond = Qevap + W — the condenser always has to reject more heat than the evaporator absorbs, by exactly the compressor work added.
Frequently asked questions
Practical questions about inputs, assumptions and interpretation.
The Carnot formula shows COP falls as (Tcond − Tevap) grows. Physically, a bigger gap between where heat is absorbed and rejected means the compressor has to do more work to "pump" the same amount of heat across that gap — which is why raising evaporator temperature or lowering condensing temperature (e.g. with cooler outdoor air) improves efficiency.
It's a reasonable mid-range estimate for a well-maintained, correctly charged vapor-compression system. Poorly maintained, badly sized, or older equipment often runs closer to 35–40% of Carnot, while modern high-efficiency variable-speed systems can approach 55–65% under favorable conditions.
An exact cycle simulation needs pressure-enthalpy data specific to the refrigerant (R-410A, R-32, R-134a, CO₂, etc.), which varies significantly between refrigerants and isn't something a general-purpose educational calculator can respresent accurately without a full property database. This tool intentionally stays at the temperature-lift level, which is refrigerant-independent and still captures the main efficiency driver.