The transfer of heat from a source to a receiver by radiant energy is radiation. The sun transfers its energy to the earth by radiation. A fire in a fireplace is another example of radiation. The fire in the fireplace heats the air in the room and by convection heats up the room. At the same time, when you stand within line of sight of the fireplace, the radiant energy coming from the flame of the fire itself makes you feel warmer than when you are shielded from the line of sight of the flame. Heat is being transferred both by convection and by radiation from the fireplace.

Most heat transfer processes in field gas processing use a conduction or convection transfer process or some combination of the two. Radiant energy from a direct flame is very rarely used. However, radiant energy is important in calculating the heat given off by a flare. A production facility must be designed to relieve pressure should an abnormal pressure situation develop. Many times this is done by burning the gas in an atmospheric flare. One of the criteria for determining the height and location of a flare is to make sure that radiant energy from the flare is within allowable ranges. Determining the radiation levels from a burning flare is not covered in this text. API Recommended Practice 521, Guide for Pressure
Relief and Depressuring Systems provides a detailed description for flare system sizing and radiation calculation.

Some gas processes use direct fired furnaces. Process fluid flows inside tubes that are exposed to a direct fire. In this case radiant energy is important. Furnaces are not as common as other devices used in production facilities because of the potential fire hazard they represent. Therefore, they are not discussed in this volume.

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