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How to Use the Flash Tank Calculator
Enter the condensing, intermediate and evaporating temperatures as dew points, in that order from high to low. The intermediate temperature is the tank's saturation temperature, which you can get from a two-stage calculation, and it must sit between the other two.
Capacity in kW sets the mass flow through the evaporator, and so the flash gas flow and the vessel size. Sub-cooling is how far the liquid leaving the condenser is below its bubble point, and it lowers the flash fraction.
Enter the maximum vapour velocity your vessel standard or manufacturer allows, the height-to-diameter ratio, and the operating liquid level. The calculator applies a 20% margin to the diameter and rounds up to the next standard size.
The diameter and height are a starting point. Compare them with the vessels a manufacturer offers, check the velocity status and the liquid volume, and use the high-stage mass flow to size the high-stage compressor.
Flash tank design for low-temperature refrigeration
A flash tank sits between the condenser and the evaporators of a two-stage system. It does two jobs at once: it separates the gas formed when the condenser liquid is expanded to the intermediate pressure, and it leaves the remaining liquid at the tank's saturation temperature, colder than the liquid that entered. The evaporators then receive liquid with more cooling capacity per kilogram, and the flash gas is compressed from the intermediate pressure and not from suction.
How the flash tank model works
The flash fraction is x = (h3 - hf) / (hg - hf), where h3 is the enthalpy of the liquid entering the tank and hf and hg are the saturated liquid and vapour enthalpies at the tank pressure. The calculator takes h3 as the saturated liquid enthalpy at the condenser's bubble point less your sub-cooling. For R-404A at 40°C condensing, -5°C intermediate, -35°C evaporating and 5 K of sub-cooling, that gives 34.5% flash gas. The liquid leaving the tank is saturated at -5°C, so the refrigerating effect is 154.1 kJ/kg against 95.2 kJ/kg for the same liquid expanded directly, a gain of 61.9%. That gain is before the cost of compressing the flash gas in the high stage. The model treats the vessel as a flash gas separator, with the low-stage discharge joining the flash gas in the high-stage suction line outside the vessel. If your low-stage discharge enters the tank, as in an open intercooler, the vapour load is larger than the flash gas alone. The two-stage compression calculator is the better starting point for that layout.
Refrigerant data and accuracy
Properties come from fits to CoolProp data, which uses high-accuracy equations of state of the kind maintained in the NIST REFPROP database. Saturation pressure agrees within 1% over -50 to 60°C, and enthalpies within about 0.6 kJ/kg. In spot checks against direct property calls, the flash fraction agreed within 0.001 for the HFC and HFO blends and within 0.006 for CO2. Temperatures are dew points, and the liquid is taken at its bubble point at the same pressure. For R-448A the glide at -5°C is 6.2 K, and for R-449A it is 5.8 K, so bubble and dew enthalpies differ and the calculator handles them separately.
Sizing the vessel
The vessel diameter comes from the flash gas volume flow and the vapour velocity you allow: D = sqrt(4 V / (pi u)). In the example the flash gas is 616 kg/h, or 6.6 L/s at the tank's vapour specific volume, and at 0.07 m/s the minimum diameter is 346 mm. A 20% margin gives 416 mm, which rounds up to a 450 mm standard vessel. At a height-to-diameter ratio of 2 it is 900 mm tall and holds 57 L at 40% level. The velocity limit is your input, and 0.07 m/s is a starting value and not a code limit. The velocity that avoids carry-over depends on the vessel internals and on the liquid and vapour densities, so take it from your vessel standard or manufacturer. Pressure ratings are often quoted in psi, and the pressure converter handles the conversion to bar.
Choosing the refrigerant
R-404A and R-507A are the older high-GWP refrigerants in this list, and R-448A and R-449A are the lower-GWP blends commonly used as replacements. At the same conditions R-448A flashes 28.1% against 34.5% for R-404A, and its refrigerating effect is 198.2 kJ/kg against 154.1 kJ/kg, so less mass circulates. That outweighs its less dense vapour, and the vessel for 50 kW is 400 mm against 450 mm. R-22 is included for servicing existing plant only. Use the refrigerant GWP calculator to compare the climate impact, and the refrigerant regulation checker to see what is permitted for your equipment type.
Flash gas in CO2 systems
In a CO2 transcritical system the vessel after the gas cooler collects a large share of flash gas. The EPA's GreenChill webinar on transcritical CO2 systems shows about 40% of the fluid leaving the gas cooler as gas after expansion without parallel compression, and notes that flash gas compressors run at a lower compression ratio and discharge temperature. This calculator covers subcritical R-744 up to 26°C, so it suits a cascade or low-temperature stage and not gas-cooler operation. At 20°C condensing, -10°C in the tank, -35°C evaporating and 3 K of sub-cooling it gives 26.9% flash gas. The refrigerant comparison calculator shows how CO2 operating pressures compare with the HFCs.
Frequently Asked Questions
Flash fraction is the share of the refrigerant mass that vaporises when liquid from the condenser expands to the intermediate pressure. It comes from an enthalpy balance: the enthalpy of the entering liquid minus the saturated liquid enthalpy at the tank pressure, divided by the saturated vapour enthalpy minus the saturated liquid enthalpy at that pressure. For R-404A at 40°C condensing, -5°C intermediate, -35°C evaporating and 5 K of sub-cooling, the calculator gives 34.5% flash gas. That gas would do no cooling in the evaporator, so the tank separates it at intermediate pressure and sends it to the high stage, which compresses it from a higher suction pressure than the low stage uses. The liquid left in the tank is at the tank temperature, which raises the refrigerating effect.
Follow the vessel manufacturer's procedure and start with both stages at low load. Bring the system to its operating pressures, then watch the liquid level in the sight glass until the float valve holds it steady at the design level, which this calculator takes as 40% of the vessel height unless you change it. A level that keeps rising means the float is not discharging enough, and wet vapour can then reach the high-stage suction. A level that keeps falling starves the liquid line to the evaporator expansion valves. Once the level is stable at design conditions, compare the high-stage suction temperature with the saturation temperature at the intermediate pressure, and log the intermediate pressure and both stage discharge temperatures as a baseline for later service calls.
At full load the vapour rises through the vessel faster than the design velocity, so liquid droplets are carried out with the flash gas into the high-stage suction. Wet suction gas washes oil off cylinder walls and valves, dilutes the crankcase oil and, in severe cases, causes liquid slugging that damages valves or connecting rods. The calculator flags the vessel when the actual vapour velocity exceeds your limit, and it marks it as too small when the required diameter is beyond the largest standard size listed. Undersizing is most likely after an expansion or a compressor upgrade, so check the vessel against the real installed flow and not only the original design.
A single stage at these conditions has to compress across the whole pressure range. For R-404A at 40°C condensing and -35°C evaporating the pressure ratio is 11.1, which lowers volumetric efficiency and raises discharge temperature. With a tank at -5°C the ratio splits into 3.12 for the low stage and 3.57 for the high stage, and the flash gas is compressed from the intermediate pressure and not from suction. The liquid sent to the evaporator is also colder, so each kilogram circulated absorbs more heat: 154.1 kJ/kg against 95.2 kJ/kg without the tank. Plants that run year-round, such as cold stores and distribution warehouses, benefit most from that saving, although the extra high-stage work has to be set against it.