DX suction, discharge, and liquid line sizing in ACR copper.
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⚠ Preliminary sizing only. Verify against the equipment manufacturer's line-sizing data and local codes. Riser oil return is checked by a velocity rule of thumb only — confirm risers with the OEM.
System Parameters
Recommended Line Sizes
Suction Line
—
Velocity—
Friction loss—
Mass flow—
Discharge Line
—
Velocity—
Friction loss—
Mass flow—
Liquid Line
—
Velocity—
Friction loss—
Mass flow—
Capacity Table — All Line Sizes
OD Size
ID (in.)
Suction cap. (tons, 2 °F)
Suc. vel. at load (fpm)
Discharge cap. (tons, 1 °F)
Liquid cap. (tons, 1 °F)
Inputs, results, and method — as an Excel workbook or plain CSV.
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
This refrigerant pipe sizer recommends suction, discharge, and liquid line sizes for a direct-expansion (DX) system. Choose the refrigerant — R-410A, R-454B, R-32, R-134a, R-407C, or legacy R-22 — and enter the system tonnage, evaporating and condensing temperatures, superheat, subcooling, and the equivalent length of the run. The tool computes the mass flow from the refrigerating effect, finds each line’s friction loss as a saturation-temperature penalty, and selects the smallest ACR copper tube within the ASHRAE line-sizing limits, then checks velocity for oil return.
Each line has a different job: the suction and discharge (vapor) lines must hold pressure drop down without going so slow that compressor oil stops returning, while the liquid line must stay subcooled to the metering device. The capacity table shows what every tube size carries at your conditions, so you can trade one size up or down with the pressure-drop and oil-return consequences in view.
Formula & Method
Mass flow: ṁ (lb/h) = tons × 12,000 ÷ (h1 – h4)
Velocity: v (fpm) = ṁ ÷ 60 ÷ ρline ÷ A where A = π · (ID ÷ 24)²
Friction: ΔP = f · (L ÷ D) · ρv² ÷ 2gc (Darcy–Weisbach, Colebrook f); ΔT = ΔP ÷ (dPsat/dT)
Size: smallest tube with ΔT ≤ 2 °F suction, 1 °F discharge, 1 °F liquid over L, and v ≤ 4,000 / 3,500 / 300 fpm
h1 is the suction vapor leaving the evaporator (saturated suction temperature plus superheat) and h4 = h3 the liquid leaving the condenser (bubble point minus subcooling); 12,000 BTU/h is one ton, and the same mass flow passes all three lines. Densities, enthalpies, viscosities, and the saturation slope dP/dT come from the CoolProp reference equations of state — the REFPROP-equivalent data ASHRAE Handbook—Fundamentals Ch. 30 tabulates — interpolated at your temperatures (within about 0.5%). For the blends R-407C and R-454B, the entered temperatures are dew points and the liquid uses the bubble point. Discharge gas density assumes compression at 70% isentropic efficiency. Friction uses drawn-copper roughness (0.000005 ft) and ASTM B280 ACR tube IDs (hard-drawn straight lengths; annealed coils have slightly thinner walls). The 2 / 1 / 1 °F limits are the basis of the ASHRAE Refrigeration Handbook line-capacity tables (per 100 ft), applied here to your equivalent length; manufacturers may allow a larger liquid-line loss when subcooling covers it. Oil return uses rule-of-thumb minimums of about 500 fpm horizontal and 1,000 fpm in risers, checked at full load; staged and inverter equipment must also be checked at minimum capacity, and the handbook’s riser tables govern large risers. When a smaller liquid tube would stay within the subcooling, the liquid card shows that OEM-style option.
Frequently Asked Questions
How are refrigerant line sizes determined?
By pressure drop, then checked by velocity. From the capacity and the refrigerating effect (h1 − h4) the tool finds the mass flow, then computes each ACR copper tube's friction loss with the Darcy-Weisbach equation and converts it to a change in saturation temperature. It picks the smallest tube that holds the loss to 2 °F on the suction line and 1 °F on the discharge and liquid lines over your equivalent length (the basis of the ASHRAE line-capacity tables) and stays under 4,000, 3,500, and 300 fpm respectively. The vapor-line velocity is then checked for oil return.
Why does suction line velocity matter for oil return?
Refrigerant oil circulates with the gas and must be swept back to the compressor. If suction or discharge velocity is too low, oil pools in the line and especially in vertical risers, starving the compressor. A common rule of thumb is at least about 500 fpm in horizontal runs pitched toward the compressor and about 1,000 fpm in risers, with large risers needing more; the ASHRAE Refrigeration Handbook tabulates minimum capacities for oil entrainment by riser size. The tool checks velocity at full load, says how far the load can drop before a riser falls below about 1,000 fpm, and suggests a riser size when needed; check staged or inverter equipment at its minimum capacity.
How does pressure drop relate to capacity loss?
In refrigerant lines, pressure drop is expressed as an equivalent change in saturation temperature, often called a saturation-temperature penalty. A larger drop in the suction line lowers the compressor suction pressure and cuts capacity and efficiency; a drop in the liquid line can cause flashing before the metering device if it exceeds the subcooling. ASHRAE line-capacity tables are built on 2 °F per 100 ft for suction lines and 1 °F per 100 ft for discharge and liquid lines; this tool applies those limits to your equivalent length.
Should I trust these sizes for final design?
Treat them as preliminary. The refrigerant properties come from reference equations of state, but the method is a single-line friction calculation: it does not include accessories such as filter-driers and valves, liquid-line lift, or double-riser design. Final selection should use the equipment manufacturer's line-sizing data for the actual equivalent length and lift, and ASHRAE Refrigeration Handbook guidance. R-32 and R-454B are A2L refrigerants, so charge limits per ASHRAE 15 (or 15.2 for residential) and UL 60335-2-40 also apply.
Connect the calculation to the rest of the project.
AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.
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An AI co-pilot that drafts changes — you approve each one
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