Propane torch

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Handheld propane blowlamp (UK)/blowtorch (USA)
Large propane torch used for construction

A propane torch is a tool normally used for the application of flame or heat which uses propane, a hydrocarbon gas for its fuel. Propane is one of a group of by-products of the natural gas and petroleum industries known as liquefied petroleum gas or LPG. Propane and other fuel torches are most commonly used in the manufacturing, construction and metal-working industries.

The term "propane torch" may refer to anything from a small consumer-grade hand-held burner with a small attached tank or an industrial torch which has separately located fuel and pure oxygen tanks to feed the flame with 100% oxygen in return for much higher temperatures. Propane is often the fuel of choice due to its low price, ease of storage and availability. Alternative fuel gases can be harder to store and more dangerous for the user. For example, acetylene needs a porous material mixed with acetone in the tank for safety reasons and cannot be used above a certain pressure and withdrawal rate. Natural gas is a common fuel for household cooking and heating but cannot be stored in liquid form without cryogenic refrigeration.

Small air-only torches normally use the Venturi effect to create a pressure differential which causes air to enter the gas stream through precisely sized inlet holes or intakes, similar to how a car's carburettor works. The fuel and air mix sufficiently, but often imperfectly, in the burner's tube before the flame front is reached. The flame also receives some further oxygen from the air surrounding it.

Oxygen-fed torches use the high pressure of the stored oxygen to push the oxygen into a common tube with the fuel. An air-only torch will burn at around 1,995 °C (3,623 °F), less if heat loss to the surroundings is taken into account. Oxygen-fed torches can be much hotter at up to 2,820 °C (5,110 °F), depending on the fuel-oxygen ratio. These are the theoretical maximum temperatures, in reality they will always be less due to incomplete combustion, heat loss etc.

Handheld propane torch being used to solder copper pipes for residential water mains

Propane torches are frequently employed to solder copper water pipes. They can also be used for some low temperature welding applications, as well as for brazing dissimilar metals together.

In addition to above, a large factor in the temperature of the flame is the percentage of oxygen mixing with the propane. With air/fuel torches, since air contains about 21% oxygen, to obtain the maximum flame temperature with air, a very large ratio of air to fuel must be used. Even glass bead-making torches, which are essentially Bunsen burners with an added air pump, can only achieve temperatures of 1,100 °C (2,010 °F).

With oxygen/propane torches, the air/fuel ratio can be much lower. The stoichiometric equation for full combustion of propane with 100% oxygen is:

Unbalanced:

C3H8 + O2 → H2O + CO2

Balanced:

C3H8 + 5 (O2) → 4 (H2O) + 3 (CO2)

The only products are CO2 and water. This is only true with complete combustion.

To get the ratio, the atomic mass of C, H, and O must be determined.

  • O = 16 g
  • C = 12.01 g
  • H = 1.008 g
  • Propane weighs ((3×12.01) + (8×1.008)) = 44.094 g/mol.
  • Oxygen gas weighs (2×16) = 32 g/mol.

The balanced equation shows to use 1 mole of propane for every 5 moles of oxygen.

(5 × 32 g) / (44.094 g) = 160/44.094 = 3.63 grams of oxygen gas for each gram of propane.

If using an oxygen/propane torch, the oxygen/fuel ratio is 3.63 by mass, 5:1 molar ratio.

For an air/propane torch: 3.63 g O2 × (100 O2)/(21% O2) = 17.28 g air/propane by mass,

23.81 mol O2 / 1 mol propane = 23.81:1 molar air/fuel ratio.

This means that it is much harder to achieve complete combustion with air than with oxygen. Complete combustion:

C3H8 + 5 (O2) → 4 (H2O) + 3 (CO2)

If the propane does not receive enough oxygen, some of the carbon from the propane are left unburned: Examples of incomplete combustion:

C3H8 + 4 (O2) → 4 (H2O) + 2 (CO2) + 1 C

The extra carbon molecule will cause soot and the less oxygen used, the more soot will result. There are other unbalanced ratios where incomplete combustion products such as carbon monoxide (CO) are formed, such as:

6 (C3H8) + 29 (O2) → 24 (H2O) + 16 (CO2) + 2 CO

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