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CO2 Pipe Sizing

Three main lines — MT suction, LT suction, and liquid. Drag evaporators into position, connect one line to another, and click any line to set its length.
R744 · MT -8.0 °C · LT -32.0 °C · 26.50 kW

System Data

Evaporating temperature for each circuit and liquid receiver temperature. The liquid temperature also sets the flash-gas share, and with it the mass flow.
Useful superheat at evaporator outlet. Real vapor is less dense than saturated vapor — suction lines are sized based on the lower density.
Installed compressor capacity for comparison with connected evaporator loads. Enter compressor power for total heat rejection.
Directly controls pipe sizing — higher velocity yields smaller pipes.
Pipes that can't meet this pressure are marked ⛔ and never chosen automatically. The defaults follow common EN 378 reference values (32°C ambient) for a transcritical booster system — CO2's standstill pressure often governs more than the running temperature. Adjust for your own system.
Advanced – Margins & Pipe Material
Increases the length in the pressure-drop calculation — shows up in the Δp column in the “Main Headers”/“Evaporator Branch Connections” tables below, not in the pipe size or the cards at the top.
Adds ordering margin to the Bill of Materials and Insulation Schedule totals only (offcuts, mistakes) — a different thing from “Extra length for fittings” above, and does not affect the pipe size or Δp.
High-strength copper alloy for CO2, 120 bar. Standard choice for subcritical and transcritical systems. Only made from 3/8″ to 2-1/8″ — larger sizes need a different pipe material.
What pressure each size can take: see the table at the bottom ↓
Pipes smaller than this threshold will never be selected automatically.
Select insulation specification (thickness/color) per header for the material bill.

Evaporators

Name Circuit kW
A pipe stub contains only ONE line (liquid, MT, or LT). An MT/LT stub can be converted to an evaporator by assigning a kW load.
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Project

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Options · Units

Changes how values are shown and entered — all calculations still run in the background as before.

Flow Diagram

MT Suction Liquid LT Suction Tapped off another pipe
100% 100%
Drag an evaporator freely to position it. Click any pipe segment to set its length or manually override size.
Click here, then on a pipe run, to place a power meter.
— m Size:
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Bill of Materials

Lengths include the waste allowance above, but not the fitting length allowance (which only affects Δp).

Insulation Schedule

Insulation types are selected under 'Advanced' in System Data.

Evaporator Branch Connections

"Own Load" is the evaporator's own kW value. "Load in Pipe" is the accumulated flow through that specific pipe — larger if other evaporators/pipes are tapped onto it. Never sum the "Load in Pipe" columns for a grand total; use "Own Load" instead.

Main Headers

Show calculation

Calculation per pipe

PipeMass flow [kg/s]Density [kg/m³]Inner diameter [mm]Velocity [m/s]RefLength (+ allowance) [m]Δp [kPa]Δp per metre [Pa/m]Pipe type, PS
#1 1/2" · Liquid: Refrigeration unit → Cold Room 10.1043972.411.21.09103,0000.01856 (+1.8)7.44954K65 · PS 60 bar
#2 1/2" · Liquid: Cold Room 1 → Freezer 10.07272972.411.20.75971,9000.01986 (+1.8)3.87496K65 · PS 60 bar
#3 3/8" · Liquid: Freezer 1 → Cold Room 20.05316972.48.40.98670,1000.02016 (+1.8)8.841,130K65 · PS 60 bar
#4 3/8" · Liquid: Cold Room 2 → Freezer 20.02948972.48.40.54738,9000.02266 (+1.8)3.06392K65 · PS 60 bar
#5 3/8" · Liquid: Freezer 2 → MT Display Case0.01579972.48.40.29320,8000.0266 (+1.8)1.01129K65 · PS 60 bar
#6 1/2" · MT Suction: Refrigeration unit → Cold Room 10.0710370.2611.210.3584,0000.01466 (+1.8)37.74,830K65 · PS 52 bar
#7 3/8" · MT Suction: Cold Room 1 → Cold Room 20.0394670.268.410.1433,0000.01556 (+1.8)526,670K65 · PS 52 bar
#8 3/8" · MT Suction: Cold Room 2 → MT Display Case0.0157970.268.44.05173,0000.01736 (+1.8)9.291,190K65 · PS 52 bar
#9 1/2" · LT Suction: Refrigeration unit → Freezer 10.0332632.811.210.3310,0000.01576 (+1.8)192,430K65 · PS 30 bar
#10 3/8" · LT Suction: Freezer 1 → Freezer 20.0136932.88.47.53170,0000.01746 (+1.8)151,920K65 · PS 30 bar
#11 3/8" · Line to Cold Room 1 · Liquid0.03157972.48.40.58641,6000.02235 (+1.5)2.88443K65 · PS 60 bar
#12 3/8" · Line to Cold Room 1 · MT Suction0.0315770.268.48.11346,0000.01595 (+1.5)28.44,370K65 · PS 52 bar
#13 3/8" · Line to Cold Room 2 · Liquid0.02368972.48.40.43931,2000.02375 (+1.5)1.72265K65 · PS 60 bar
#14 3/8" · Line to Cold Room 2 · MT Suction0.0236870.268.46.08260,0000.01645 (+1.5)16.52,540K65 · PS 52 bar
#15 3/8" · Line to MT Display Case · Liquid0.01579972.48.40.29320,8000.0265 (+1.5)0.84129K65 · PS 60 bar
#16 3/8" · Line to MT Display Case · MT Suction0.0157970.268.44.05173,0000.01735 (+1.5)7.741,190K65 · PS 52 bar
#17 3/8" · Line to Freezer 1 · Liquid0.01956972.48.40.36325,8000.02475 (+1.5)1.23189K65 · PS 60 bar
#18 3/8" · Line to Freezer 1 · LT Suction0.0195632.88.410.8243,0000.01655 (+1.5)24.33,740K65 · PS 30 bar
#19 3/8" · Line to Freezer 2 · Liquid0.01369972.48.40.25418,1000.02695 (+1.5)0.654101K65 · PS 60 bar
#20 3/8" · Line to Freezer 2 · LT Suction0.0136932.88.47.53170,0000.01745 (+1.5)12.51,920K65 · PS 30 bar

Basis of the calculation

Method
Each pipe is sized for the smallest size that keeps the velocity at or below the target and is rated for its PS; the pressure drop is calculated with Darcy–Weisbach.
Equations
  • ṁ = Q / (h″(t₀) − h′(t_liq))
  • v = ṁ / (ρ · π·d²/4)
  • Re = ρ·v·d / μ
  • f = 0.25 / [log₁₀(ε/(3.7·d) + 5.74/Re^0.9)]² (Re ≥ 2300); f = 64/Re (Re < 2300)
  • Δp = f · (L/d) · ρ·v²/2, L = length × (1 + allowance %)
Friction factor
Swamee–Jain (explicit form of Colebrook–White)
Fluid data
CoolProp, Span–Wagner EOS (R744) 8.0.0
Standards
EN 378-2 (PS) · ASTM B280 / EN 12735-1 (outside diameters, walls) · ASME B31.5 (P = 2·S·t/D, S = 41 MPa annealed) · EN 14276 (K65, manufacturer 120 bar)
Notes
  • Mass flow from the cooling capacity and the enthalpy difference between saturated vapour at the evaporating temperature and liquid at the liquid temperature; suction lines with the given superheat.
  • Static height differences and the pressure drop of valves and fittings beyond the allowance are not included.

Pressure rating per size

Allowable pressure (bar) for every size and pipe type, with wall thickness. Green is enough for all three stems' design pressure (MT suction 52, LT suction 30, liquid 60 bar). Amber is enough only for the stems named in the cell. Red ⛔ is enough for none of them. – means the pipe type is not made in that size.
SizeStandard Copper (Cu-DHP) Reinforced Copper (Cu-DHP) K65 Copper (CuFe2P)
selected pipe type
1/4" 6.4 mm98 bar
0.76 mm		123 bar
0.95 mm		–
3/8" 9.5 mm70 bar
0.81 mm		88 bar
1.02 mm		120 bar
0.56 mm
1/2" 12.7 mm53 bar MT, LT
0.81 mm		66 bar
1.02 mm		120 bar
0.75 mm
5/8" 15.9 mm46 bar LT
0.89 mm		57 bar MT, LT
1.11 mm		120 bar
0.93 mm
3/4" 19.0 mm38 bar LT
0.89 mm		48 bar LT
1.11 mm		120 bar
1.19 mm
7/8" 22.2 mm42 bar LT
1.14 mm		53 bar MT, LT
1.43 mm		120 bar
1.38 mm
1-1/8" 28.6 mm36 bar LT
1.27 mm		46 bar LT
1.59 mm		120 bar
1.78 mm
1-3/8" 34.9 mm33 bar LT
1.40 mm		41 bar LT
1.75 mm		120 bar
2.17 mm
1-5/8" 41.3 mm30 bar LT
1.52 mm		38 bar LT
1.91 mm		120 bar
2.56 mm
2-1/8" 54.0 mm27 bar ⛔
1.78 mm		34 bar LT
2.22 mm		120 bar
3.35 mm
2-5/8" 66.7 mm25 bar ⛔
2.03 mm		31 bar LT
2.54 mm		–
3-1/8" 79.4 mm24 bar ⛔
2.29 mm		30 bar ⛔
2.86 mm		–
3-5/8" 92.1 mm23 bar ⛔
2.54 mm		28 bar ⛔
3.18 mm		–
4-1/8" 104.8 mm22 bar ⛔
2.79 mm		27 bar ⛔
3.49 mm		–
5-1/8" 130.2 mm20 bar ⛔
3.18 mm		25 bar ⛔
3.97 mm		–
6-1/8" 155.6 mm19 bar ⛔
3.56 mm		23 bar ⛔
4.45 mm		–

Standard Copper (Cu-DHP): Common refrigerant pipe (ACR). Only the smallest sizes handle CO2's pressure — see the ⛔ mark against your design pressure.

Reinforced Copper (Cu-DHP): Same material but about 25% heavier wall, for a higher allowable pressure than standard copper.

K65 Copper (CuFe2P): High-strength copper alloy for CO2, 120 bar. Standard choice for subcritical and transcritical systems. Only made from 3/8″ to 2-1/8″ — larger sizes need a different pipe material.

Source and method: standard and reinforced copper by the thin-wall formula P = 2·S·t/D with S = 41 MPa (6,000 psi), the allowable stress of annealed copper tube up to 38 °C. Brazing anneals the tube at every joint, so brazed systems are rated on the annealed value (Copper Tube Handbook, ASME B31.5); the hard-drawn value of about 69 MPa applies only to flare or compression joints. K65: the manufacturer's (Wieland) stated 120 bar for the CuFe2P alloy to EN 14276, 3/8″–2-1/8″. Always check against your supplier's data before installation.

Other countries

The pressure ratings above follow European practice (EN 378, EN 14276, EN 13480-3; copper walls to ASTM B280/B88). In other markets refrigerant piping is designed and approved to these codes instead. This calculator has not been checked against them – use the code that applies where the system is installed.

USA

  • ASME B31.5 – Refrigeration Piping and Heat Transfer Components: pipe design, design pressure and test pressure
  • ANSI/ASHRAE Standard 15 – Safety Standard for Refrigeration Systems: safety requirements for refrigerating systems
  • ASTM B280 – Seamless Copper Tube for Air Conditioning and Refrigeration Field Service: copper tube

Canada

  • CSA B52:23 – Mechanical refrigeration code: refrigeration code; refers to ASME B31.5 for piping

Australia and New Zealand

  • AS/NZS 5149 – Refrigerating systems and heat pumps — Safety and environmental requirements: safety requirements for refrigerating systems
  • AS 4041 – Pressure piping: pipe design, design pressure and test pressure
  • AS/NZS 1571 – Copper — Seamless tubes for air-conditioning and refrigeration: copper tube

CO2 pipe sizing – frequently asked questions

How do you size CO2 (R744) refrigeration pipes?

The calculator works out the mass flow from each evaporator's cooling capacity and the CO2 properties at the evaporating and liquid temperatures, then picks the smallest tube size that keeps the velocity below your target. Pressure drop is calculated with Darcy–Weisbach and also shown as the equivalent temperature drop (K).

Calculate in the tool above ↑CO2 (R744) pipe sizing – steel →

What velocity should I use for CO2 suction and liquid lines?

The defaults are 12 m/s in the MT and LT suction lines and 1.2 m/s in the liquid line. A higher velocity gives smaller pipes but a larger pressure drop, and suction risers need enough velocity to carry the oil back to the compressors. Every value can be changed.

Calculate in the tool above ↑

Why K65 copper tube for CO2?

CO2 systems run at far higher pressures than HFC systems. K65 (CuFe2P alloy, EN 14276) is rated for 120 bar in sizes 3/8" to 2-1/8" and is the default here. Standard ACR copper has a much lower rating in the larger sizes; a size that cannot take the design pressure you enter is marked ⛔ and never selected automatically.

Calculate in the tool above ↑CO2 (R744) pipe sizing – steel →

Which design pressure (PS) should I enter?

The defaults – 52 bar MT suction, 30 bar LT suction and 60 bar liquid – are common starting values for a transcritical CO2 booster system based on EN 378-2. Always use the PS of your own system according to EN 378 and the PED.

Calculate in the tool above ↑Relief valve & discharge line (EN 13136 / EN ISO 24664) →

Does it work for transcritical CO2 booster systems?

Yes. On the low-pressure side it sizes MT suction, LT suction and the liquid line from the receiver to the evaporators, where CO2 is subcritical (−50 to +10 °C). Tick “Line to gas cooler” to size the high-pressure side as well: the discharge line to the gas cooler and the return line from it (75–140 bar, CO2 data from CoolProp). The pressure ratings are calculated at room temperature, so check the discharge line's rating at its actual temperature.

Calculate in the tool above ↑

Is the calculator free, and what do I get?

It is free and needs no login. You get tube size, velocity and pressure drop for every pipe section, a materials list per size, insulation, and a flow diagram that prints on A4 to A1 and can be saved as a project file.

Calculate in the tool above ↑

Assumptions: Saturated liquid line, subcritical CO2 (R744) between −50 and +10 °C. Pipe sizes are nominal ACR inch sizes. Verify wall thickness and design pressure against EN 378/PED before installation. Mass flow is based on the net refrigerating effect, h_g at the evaporating temperature minus h_f at the liquid temperature, so flash gas from expansion is included. Superheat enthalpy and pump overfeed are not counted.

Disclaimer: This tool does not guarantee correct results. Always verify pipe wall thickness, pressure rating, and other values manually before installation.

Help – how to use the calculator

The calculator sizes the pipes in a CO2 (R744) refrigeration system: MT suction, LT suction and liquid. Enter the system data, build the plant in the flow diagram, and read off pipe sizes, pressure drops and the bill of materials. The load of each evaporator determines the flow in every pipe run, and pipes are chosen so that the velocity does not exceed the target velocity.

1. System data

  • Enter the evaporating temperature for MT and LT, the liquid line temperature, and the superheat for MT and LT (superheat affects the sizing of the suction lines).
  • Set a separate target velocity for MT suction, LT suction and liquid. A higher target velocity gives smaller pipes; a lower one gives larger pipes and lower pressure drop.
  • The installed rack capacity and compressor power are optional. They are used to compare with the evaporator loads and to show the total heat rejection.
  • Under “Advanced – Margins & Pipe Material”: extra length for fittings (affects only the pressure drop, not the pipe size), pipe type, minimum allowed pipe size, and insulation per header (used in the bill of materials).
  • Pipe type and design pressure together decide which sizes are allowed — ⛔ in the tables means that size doesn't meet the pressure you entered.
  • Tick “Line to gas cooler” to also size the discharge line to the gas cooler: enter the high pressure, discharge temperature, gas cooler outlet temperature, target velocity and PS of the high-pressure side. The line is drawn above the refrigeration unit.

2. Evaporators and pipe stubs

  • Add an evaporator with “+ Add Evaporator” and enter its name, circuit (MT/LT) and load. Use ▲ ▼ to change the order and × to remove a row.
  • A pipe stub is a single line (liquid, MT or LT) that other evaporators can be tapped onto. Give an MT/LT stub a load if it should become an evaporator.

3. The flow diagram

  • Drag an evaporator to move it. Click its name to rename it and its load value to change it. Click [MT]/[LT] to switch circuit (only possible when the evaporator's suction line connects directly to the header).
  • Click a pipe to set its length, lock the length or choose the pipe size manually. A ✎ in the tables shows that the size was chosen manually.
  • In the same box you can choose the pipe type for that line alone, e.g. standard copper for small pipes and K65 for the rest. Each size shows its rating (bar) in that pipe type, and ⛔ means it cannot take the stem's design pressure (PS).
  • Right-click a pipe to add an evaporator or a pipe stub at that spot.
  • The round handle at an evaporator's pipe can be dragged and dropped onto another evaporator's pipe of the same kind to tap into it, or onto empty space to connect straight to the header. The small squares are dragged along the evaporator's edge to change where the pipe enters it.
  • Select an evaporator or pipe and press Delete to remove it. Undo with “Undo” or Ctrl/Cmd+Z.

4. Navigating the drawing

  • Drag the background to pan. Zoom with the buttons, with Ctrl/Cmd + mouse wheel, or by pinching on a trackpad. A− and A+ change the text size in the drawing.
  • “Print Drawing” prints the flow diagram together with a summary.
  • Tick “Force 90° Bends” to route every pipe in clean right angles; untick it to connect and drag pipes freely, however you like.
  • Tick “Force Parallel Lines” so that pipes running directly from a stem connect in line with the evaporator's connection and become one straight line with no bend. The tap point on the stem then follows along when you move the evaporator or its connection.

5. Power meter

  • Click “Add Power Meter” and then a pipe. The meter shows how many kW flow on each side of the point.
  • Drag the dot to measure at another place, drag the box to move the readout, and click × to remove the meter.

6. Reading the results

  • The cards at the top show total cooling load, total mass flow and the pipe size of the three main headers.
  • The tables list, for each evaporator and pipe run, the load, chosen pipe size, velocity and pressure drop (Δp). The program picks the smallest pipe size that stays within the target velocity.
  • ⚠ means the flow needs a larger pipe than the largest size in the table. 🚩 means two pipes or evaporators have the same name.
  • The bill of materials and the insulation list add up pipe lengths per size, without the fitting allowance. Use them as a basis when ordering.

7. Saving and opening

  • “Save” and “Save As…” store the whole plant as a file, and “Open Project” loads it again.
  • In Chrome and Edge, “Save” and autosave write to the file you chose with Save as or Open. Other browsers download a new file with “Save”, and autosave keeps the project in the browser instead.

8. Units

  • Under “Options · Units” you choose the unit for temperature, superheat, pressure, velocity, length, power and mass flow separately. This only changes how values are shown and entered, not the calculations.
Disclaimer: This tool does not guarantee correct results. Always verify pipe wall thickness, pressure rating, and other values manually before installation.