Size copper ACR suction, liquid and discharge lines from duty and operating temperatures. Velocity, Darcy–Weisbach pressure drop and saturation-temperature penalty for R410A, R32, R134a, R404A, R22 & R407C — with oil-return checks and a full size-by-size table. Updated live.
About the Refrigeration Pipe Sizer
This tool sizes the three copper refrigerant lines of a direct-expansion system — suction, liquid and discharge — from the duty, the operating temperatures and the equivalent length of the run. For each line it recommends the smallest standard ACR tube that meets the industry sizing criteria, and shows the velocity, pressure drop and capacity penalty for every size from 1/4″ to 4-1/8″ so the trade-offs are visible, not hidden.
Refrigerant line sizing matters more than most piping decisions. An oversized suction line quietly strands compressor oil in the pipework; an undersized one throws away capacity — roughly 2–3% of duty for every extra kelvin of saturation-temperature penalty — and raises energy use for the life of the plant. The calculator makes both failure modes explicit for HVAC&R engineers, contractors and students.
Sizing criteria used
| Suction line | Saturation-temperature penalty ≤ 1.0 K over the equivalent length; velocity ≤ 15 m/s for noise and erosion; sizes below ≈ 4 m/s are flagged for oil return (vertical risers need more — often one size smaller than the horizontal run, or a double riser on systems that unload). |
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| Discharge line | Saturation-temperature penalty ≤ 1.0 K; velocity ≤ 18 m/s; low-velocity selections flagged for oil carry-over. |
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| Liquid line | Velocity ≤ 1.5 m/s and friction ΔP ≤ 75 kPa so subcooled liquid reaches the expansion valve without flashing. Static lift (≈ 10–12 kPa per metre of rise) must be added separately and offset by condenser subcooling. |
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Methodology & formulas
Mass flow: ṁ = Q ÷ RE (refrigerating effect at SST / SCT, kJ/kg)
Velocity: V = ṁ ÷ (ρ × A)
Pressure drop: ΔP = f × (L/D) × ρV²/2 (Darcy–Weisbach, f = 0.3164/Re0.25)
Sat.-temp penalty: ΔT = ΔP ÷ (dP/dT) at the line's saturation temperature
Recommended size = smallest tube meeting all criteria above
Vapour density is taken at the saturated suction temperature for the suction line and at discharge conditions for the hot-gas line; liquid density is taken at the condensing temperature. Refrigerant saturation pressures, densities and refrigerating effects are curve-fitted over −40 °C to +10 °C evaporating and 30–50 °C condensing.
Input parameters explained
| Refrigerant | R410A, R32, R134a, R404A, R22 or R407C. The refrigerant sets density, pressure–temperature slope and refrigerating effect — which is why an R32 system needs smaller lines than an R134a system of the same duty. |
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| Cooling capacity | Evaporator duty in kW or TR (1 TR = 3.517 kW). Mass flow — and therefore every result — scales directly with it. |
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| Evaporating temp (SST) | Saturated suction temperature: about +5 °C for comfort AC, −5 to −10 °C for medium-temp and −25 to −40 °C for low-temp refrigeration. Lower SST means thinner gas, higher volume flow and larger suction lines. |
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| Condensing temp (SCT) | Saturated condensing temperature — typically 45–50 °C for air-cooled plant in hot climates. Sets liquid density, discharge conditions and the refrigerating effect. |
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| Equivalent length | Actual run plus fitting allowances, per line. A quick early estimate is actual length × 1.2–1.5; detailed designs sum each fitting's published equivalent length. |
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Worked example
35 kW (≈ 10 TR) R410A system · +5 °C evaporating · 45 °C condensing · 30 m equivalent length:
Suction line → 1-3/8″ OD8.1 m/s · ΔT 0.38 K
Liquid line → 3/4″ OD1.07 m/s · ΔP 15 kPa
Discharge line → 7/8″ OD6.9 m/s · ΔT 0.59 K
Note: 1-1/8″ suction would cost ≈ 1.0 K≈ 2–3% capacity
Assumptions & limitations
- Refrigerant properties are curve-fitted approximations; ASHRAE tables or REFPROP-based software should confirm final selections.
- Friction uses the smooth-tube Darcy–Weisbach correlation with 0 K subcooling and typical superheat; oil circulating in the refrigerant is not modelled.
- Vertical risers, double risers, pump-down piping, hot-gas bypass and long liquid lifts need the dedicated checks described in the ASHRAE Refrigeration Handbook.
- For packaged split and VRF equipment the manufacturer's line-sizing tables and maximum lengths are binding and take precedence.
- This is a preliminary-design estimate — verify against project specifications and the equipment supplier's data before construction.
Frequently asked questions
How do I size a refrigerant suction line?
Suction lines are sized against two limits at once: the pressure drop over the run should cost no more than about 1 K of saturation temperature (roughly a 1 K penalty costs ~2% of system capacity and raises power), and the gas velocity must stay high enough to carry oil back to the compressor. Enter the refrigerant, duty, evaporating and condensing temperatures and the equivalent length, and the calculator returns the smallest copper OD that satisfies both, with the velocity and ΔT shown so you can see the margin.
Why is suction line sizing always a compromise?
A larger pipe reduces pressure drop and protects capacity, but it also slows the gas down — and below roughly 4 m/s in horizontal runs (higher in vertical risers) oil no longer travels with the refrigerant and can log in the line or starve the compressor. A smaller pipe returns oil reliably but wastes capacity through pressure drop. The recommended size is the smallest pipe that keeps ΔT within the design limit, which naturally keeps velocity healthy; the tool flags any size where velocity falls below the oil-return threshold.
What velocity should refrigerant lines run at?
Typical design ranges are 4–15 m/s for suction lines (7–15 m/s in vertical risers), 5–18 m/s for discharge lines, and 0.5–1.5 m/s for liquid lines. Vapour lines have a minimum for oil return and a maximum for noise, erosion and pressure drop; liquid lines have no oil-return minimum but are kept slow to limit pressure drop, noise and liquid hammer at solenoid valves.
What does the saturation temperature penalty (ΔT) mean?
Pressure drop in a vapour line is equivalent to lowering the saturation temperature the compressor sees. Dividing the line pressure drop by the refrigerant's pressure–temperature slope (dP/dT) converts kPa into kelvin. Industry practice sizes suction and discharge lines for about 1 K total penalty each — every extra kelvin of suction penalty costs roughly 2–3% capacity and increases compressor power, which is why ΔT, not raw kPa, is the sizing criterion.
What is equivalent length and how do I estimate it?
Equivalent length is the actual pipe run plus an allowance for the extra friction of elbows, tees, valves and other fittings expressed as metres of straight pipe. A quick early-design rule is actual length × 1.2–1.5 for a typical run; for a detailed check, add each fitting's published equivalent length (a long-radius elbow is roughly 0.5–1 m depending on size). Using plain measured length understates the pressure drop.
Why does the liquid line have a pressure-drop limit instead of a ΔT limit?
The danger in a liquid line is flashing: if friction plus static lift drops the pressure below the liquid's saturation pressure, vapour bubbles form and the expansion valve loses capacity and hunts. The calculator limits liquid-line friction to about 75 kPa and velocity to 1.5 m/s; remember that every metre of upward lift adds roughly 10–12 kPa of static loss on top, which is offset by subcooling at the condenser.
Should I just use the pipe sizes from the manufacturer's catalogue?
For packaged split, VRF and close-coupled systems — yes, the manufacturer's line-sizing tables are binding, because they account for oil charge, compressor limits and accessory pressure drops specific to that unit, and warranty usually depends on following them. This calculator is most valuable for built-up DX and refrigeration systems, for checking a catalogue selection at unusual lengths or temperatures, and for understanding why a given size was chosen.
Do vertical suction or discharge risers need a different size?
Often, yes. Oil must be dragged up a riser by gas shear, which needs a higher minimum velocity than a horizontal run — so a riser is frequently one size smaller than the horizontal line it serves. On systems that unload to low part-load, a single riser sized for full load may drop below oil-entrainment velocity, which is when a double riser with an oil trap is used. The tool flags low-velocity selections so you can check riser behaviour deliberately.
Which refrigerants does the calculator cover, and how accurate is it?
R410A, R32, R134a, R404A, R22 and R407C, from −40 °C to +10 °C evaporating and 30–50 °C condensing. Mass flow comes from the refrigerating effect at your conditions and pressure drop from the Darcy–Weisbach equation with smooth-tube friction; properties are curve-fitted, so results are preliminary-design accuracy. Verify final selections against ASHRAE Refrigeration Handbook tables or the equipment manufacturer's data, particularly for low-temperature and long-line applications.
Last updated: 2026-07-02 · Preliminary design only — verify against ASHRAE data, manufacturer line-sizing tables and local code.