Calculators
HVAC School has some handy calculators to help with various HVAC/R field tasks, including recovery tank fill and more.
Superheat & Delta T
Calculate target superheat for fixed metering device (piston) systems and air split on 400 CFM per ton A/C
To learn more about Superheat and Delta T go HERE
Voltage Imbalance Calculator
Calculate three-phase voltage imbalance
To learn more about Voltage Imbalance go HERE
Nitrogen Pressure Calculator
To learn more about Nitrogen Pressure go HERE
Under Load Capacitor Test
Test capacitors while the system is running
Air Density
Accurately calculate air density based on local altitude and humidity
Recovery Tank Fill
Calculate total tank and refrigerant weights
Additional Charge Weight Calculator
Enter the length in Feet ADDITIONAL from the standard charge the system comes in. Many systems come charged for a 15' lineset but this is not universal. Make sure you follow the manufacturer's guides before using this calculator.
Nitrogen Purge & Tank Calculator
Purge first: send nitrogen through the tubing at a higher rate to push air out before brazing. Then flow: keep a small amount of nitrogen moving through the tubing, usually 2–5 SCFH, while you braze and let the joint cool. This helps stop black scale from forming inside the copper.
SCFH means standard cubic feet per hour—how much nitrogen passes through in an hour. Use this calculator to see how long a purge may take, what percent of your cylinder it uses, and how many purges like it one cylinder can supply.
Pressure pushes the gas. Flow is how much gas moves. A PSI reading alone cannot tell you the flow or purge time. Use PURGE on a purge/flow regulator. With a standard T-handle regulator, enter the flow if you know it; you can still estimate cylinder use without it.
How to purge, then flow while brazing
- Give the air a way out. Connect dry nitrogen at one end and leave the other end open. Make sure gas passes through every branch. A closed-off branch may still hold air.
- Purge as fast as your setup safely allows. Use the PURGE setting on a purge/flow regulator. For large tubing, a standard nitrogen regulator and a hose that does not limit flow may work faster. Keep the outlet clear and stay within the pressure limits of all parts. The highest PSI number on the regulator is not a purge setting to aim for.
- Set up the low flow. If you used a standard regulator, shut off the nitrogen and release hose pressure before adding a ¼-inch nitrogen flowmeter or flow control. Set the pressure required by that tool. Do not leave it at the high-pressure purge setting. For example, DiversiTech’s GAU225-PURGE allows no more than 50 PSI.
- Turn the flow down to 2–5 SCFH. Only for large-diameter pipe during low-flow brazing, such as brazing branches: after the initial purge, you can cover the far pipe end with masking tape and punch a small vent hole at the top. Never use this taped-hole outlet for the fast initial purge. Keep tape away from heat, leave the hole clear, and make sure gas escapes without pressure building. For smaller tubing, use the available service ports or Schrader fittings with the valve cores removed from the nitrogen path and keep the exit open.
- Keep flowing while brazing and cooling. Some BRAZE settings cover 3–6 CFH; use a flowmeter if you need to check the exact rate. Never close the outlet while gas is flowing. Purge again if air gets back in.
Work where there is fresh air: nitrogen can push out the air you need to breathe. Follow the equipment maker’s brazing instructions. The timer cannot tell you whether all air is gone.
Your estimate
“This purge” includes all tubing and repeats you entered. The two purge results below do not include the extra gas used while brazing.
| Tubing | Length | Times purged | Gas for purge | Cylinder used | Purge time |
|---|
Compare cylinder sizes
| Cylinder size | Cylinder used for this purge | Purges like this per cylinder | Full cylinders for this job |
|---|
Time assumes one regulator purging sections one after another. It does not include setup, cylinder changes, or time spent flowing while brazing. Opening several branches does not mean each gets the full flow.
How the numbers work and where they come from
First, we find the space inside each tube from its inside size and length. We multiply that by the pipe volumes per purge and the number of times you purge it. Adding the rows gives the gas needed for “this purge.”
The percent used is purge gas divided by the gas in one full cylinder, times 100. More than 100% means more than one cylinder’s worth of gas. To count purges per cylinder, we first subtract the gas you want to leave in it, then divide by the gas for this purge. We count only complete purges.
If you enter brazing hours, we add hours × flow rate to the job’s gas use. We divide the total by the gas you plan to use from each cylinder and round up to full cylinders. Changing the purge flow changes the time, not the gas needed for the same purge.
For the math: tube space (cu. ft.) = π × inside diameter² (inches) × length (feet) ÷ 576. Purge time in seconds = purge gas ÷ flow in SCFH × 3,600.
Tube sizes come from CDA Table 14.2e. Hard and soft copper can have different inside sizes. Choose “Other inside size” if yours differs. This estimate is for tubing open to the air, near room temperature. It does not apply if pressure builds up in the tubing. Add extra gas for hoses, fittings, equipment, leaks, or sections not listed. These numbers help you plan; they do not check whether all air is gone.
Sources: CDA copper tube sizes; Western VN-500; Victor EDGE 2.0; DiversiTech regulators and flowmeters; HVAC School: flowing nitrogen while brazing.
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