It's Just a Lot of Hot Air
September 1, 1999
The essence of most direct-fired convection ovens is the heating of air. Air enters the oven, mixes with the fuel and supports its combustion, and the products of combustion heat the work load. I've explored various aspects of this in the past. This time, I'll look at the relationship between air-fuel ratios and oven temperatures.
For any fuel, there's a fixed quantity of air required to burn it completely. Known as the stoichiometric air-fuel ratio, it varies with the chemical makeup of the fuel. For natural gas, it ranges from around 9.5 to 10.5 ft3 of air per cubic foot of gas, yielding between 950 and 1,100 BTUs of heat. Values of 10 to 1 and 1,000 BTU commonly are used for estimating purposes. Combustion produces a mixture of gases -- the products of combustion -- containing about 10% CO2, 20% water vapor and 70% nitrogen. The heat of combustion is absorbed by these gases, raising them to 3,200oF or higher. This is the flame temperature.
The flame temperature is far too high to be tolerated by the oven or its load, so it's reduced to more useful ranges by blending the super-hot products of combustion with excess air to lower the average air temperature. It's just like blending cold water with hot so you won't get scalded in the shower or bathtub. The blend temperature is often called the hot mix temperature.
The more excess air you add, the cooler the mixture becomes, allowing you to regulate the temperature of the gases entering the oven. In other words, you change the air-fuel ratio to change the temperatures. It can be done several ways, but a common method is to hold the airflow constant and vary the gas. As the gas flow decreases, the air-gas ratio increases and the temperature of the blended gases drops. Increase the gas flow, and the opposite happens.
For any fuel, this relationship between air-fuel ratio and mixture temperature can be tabulated or graphed. So-called hot mix charts can be found in several combustion manuals -- they're a quick, handy way of estimating how much excess air is needed to create a certain combustion product temperature. Most of these graphs have rectangular coordinates, however, and the temperature vs. ratio curve isn't linear. It drops steeply at first, but flattens out at air-natural gas ratios above about 60 to 1. Consequently, the charts are fairly easy to read down to temperatures of about 1,000oF. At lower temperatures -- the area of most interest in oven and dryer operations -- the curve becomes so flat that it's difficult to estimate ratios or temperatures accurately.
Plot the same data on a log-log graph, however, and you have a tool that's useful for the lower temperatures (figure 1). It incorporates one other modification -- the temperature rise of the gases is plotted, instead of the outgoing temperature. This allows you to use the chart for incoming air temperatures other than 60oF (16oC), 70oF (21oC) or whatever else was assumed in creating the chart. This is needed for accuracy at low oven temperatures. Say you wanted to know what air-gas ratio will produce a hot mix temperature of 180oF (82oC). If the incoming air is 60oF, you need a 120oF rise, which works out to a ratio of around 350 to 1. On the other hand, if it's a 100oF (38oC) summer day, you need a temperature increase of only 80oF, and a ratio of about 650 to 1 will be required.
This chart was plotted for natural gas with a heating value of 1,000 BTU/ft3 and a stoichiometric ratio of 9.4 to 1. If the properties of the gas are different, the ratios will be affected slightly, but for most natural gases, only by 1 or 2%. This chart also assumes bone-dry air. Air containing moisture will absorb more heat, so the air-gas ratio has to be decreased to hold the desired temperature rise. For saturated air (100% relative humidity), deduct about 5% from the ratio, and you'll be close.