1. This slide rule was used for calculations related to burner efficiency. The combustion process is governed by a Stoichiometric Curve, that is the curve on which all the carbon (C) is converted to (CO₂), all hydrogen (H) to (H₂O) and all sulphur (S) to (SO₂). On one side of the curve there is excess air and on the other insufficient air for full combustion. It is preferable that the burner be operating with excess air as this means that full combustion can take place.

2. The rule was produced for Shell/BP who produce fuel oil (as well as a range of petroleum products).

3. A copy of the instructions from the rule (provided on an insignificant looking piece of card) are given below.

4. The first graph, for a particular fuel oil, shows the relationship between combustion temperature, stack loss and temperature. For this particular oil a CO₂ level of 8% and temperature of 600 °F are associated with a loss of about 23%. Using the slide rules the value of the net loss is determined as 22%.

5. The second graph for the relationship of excess air and CO₂ shows that for an excess percentage of 60% the percentage of CO₂ is almost exactly 10% for a fuel oil and the oxygen level is almost 8%. The equivalent from the back of the slide are10.2% and 7.8%, again very close.

Scales
Face: Gross Loss %, CO₂ %, °F, °C, C, D, Net Loss %
Reverse of slide: Excess Air %, CO₂ %, Oxygen %

To Determine Stack Loss—Gross or Net
Set hairline on cursor against CO₂ reading, move slide until the stack temperature* coincides with the CO₂ reading set. Then move the cursor until the hairline registers with the engraved arrows on the slide and read loss, gross or net.

Temperature Conversion
Using cursor read directly from the slide.

Calculations
Use the logarithmic C & D scales.

Determination of Excess Air % and Oxygen % in the Flue Gases
Reverse the rule and set % CO₂ - read % excess air and % oxygen direct.

*Actual stack temperature as measured with no adjustments.

                                  Combustion curve