What this calculator does
A circuit is commissioned with a charge weighed in, and then nobody checks whether that charge is actually right. Three things have to agree: the MASS in the circuit, the state at the evaporator, and the state at the condenser. A charge can be correct and the circuit still be damaging itself — a low superheat is a flooded compressor whatever the weigh-in said.
How to use it
- Enter the total internal circuit volume — every line, both heat exchangers and the receiver — and the refrigerant density at the charge condition.
- Enter the evaporating and condensing temperatures from your own pressure-temperature chart, and the suction and liquid line temperatures measured at the heat exchangers.
- Enter the charge you actually weighed in, and the superheat/subcooling bands you are targeting. Those bands differ between manufacturers and this tool has no default.
- Read the worst issue, not just the numbers. A floodback condition outranks a charge error every time, because it is the one that destroys the compressor.
Governing formula
m = V · ρ → ΔT_evap = T_suction − T_sat(evap) → ΔT_cond = T_sat(cond) − T_liquid Where:
-
V= Total circuit volume [m³] -
ρ= Refrigerant density [kg/m³] -
T_sat= Saturation temperature from your P–T chart [°C]
Derived Equations:
error = (m_actual − m_required) / m_required Worked example — 0.05 m³ circuit at 900 kg/m³, 8 K superheat, 8 K subcooling
The figures below were produced by running this page's own engine with the example inputs (Total circuit volume (m³), Refrigerant density at charge condition (kg/m³), Evaporating (saturation) temperature (°C), Suction temperature at evaporator outlet (°C), Condensing (saturation) temperature (°C), Liquid temperature at condenser outlet (°C), Charge weighed in (kg), Target superheat — minimum (K), Target superheat — maximum (K), Target subcooling — minimum (K), Target subcooling — maximum (K)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Charge the volume holds | 45 kg |
|---|---|
| Charge error | 0 % |
| Charge | ok |
| Superheat | 8 K |
| Superheat check | ok |
| Subcooling | 8 K |
| Subcooling check | ok |
| Charge difference | 0 kg |
| Circuit commissioned correctly | — |
| Most serious problem | |
| What this means | The charge is within 5 % of the volume, superheat is 8.0 K and subcooling is 8.0 K, all inside the target band. This circuit is commissioned correctly. Re-check after a warm day, because a charge that is right on a cool morning can be under-charged by the time the plant is fully loaded. |
- The circuit holds 0.05 × 900 = 45 kg, and 45 kg was weighed in, so the charge is right on.
- Superheat is −2 − (−10) = 8 K, inside the 5–12 K band.
- Subcooling is 40 − 32 = 8 K, inside the 3–10 K band. All three checks pass.
Assumptions & limitations
Engineering calculation. Charge mass from a volume and density balance, with superheat and subcooling checked against the bands YOU supply. Refrigerant properties are entered, never looked up.
- Refrigerant density, saturation temperatures and target superheat/subcooling bands are all USER inputs. This tool holds no refrigerant property table, no pressure-temperature chart and no manufacturer data, and looks nothing up.
- The required charge is V × ρ, which is only a mass balance. It does not account for the fact that the liquid and suction lines hold vapour and liquid respectively, so it ignores the charge split between the two sides of the circuit.
- Superheat and subcooling must be measured AT the evaporator outlet and condenser outlet. Readings taken elsewhere, or taken while the circuit is cycling rather than at steady load, do not mean what they appear to.
- Liquid floodback and flash gas in the liquid line can also come from a failed expansion valve, a mis-sized line, an incorrect accumulator charge or a flooded suction line. A charge correction will not fix any of those.
- No code or manufacturer acceptance check is applied. The target bands are the ones YOU supplied.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- ASHRAE Handbook — Fundamentals, refrigeration and thermophysical properties of refrigerants.
- BS EN 378 — refrigeration systems and safety requirements.