The thermal properties of the process fluid flowing through the fired heater are extremely important to the fired heater designer. These properties not only have a direct affect on the amount of heat transferred, they also are important in predicting the pressure loss and furnace coking rates, etc.

For single phase fluids, liquid or vapor, the properties can normally be assumed to change on a straight line basis from the inlet to the outlet of the heater. Therefore, providing the designer with the properties of the process fluid at the inlet and outlet conditions will normally suffice.

The one exception to this, is the viscosity. And this problem is made even worse when an attempt to extrapolate from two given points, such as the inlet and outlet, to get a value for the process fluid at a higher temperature which may occur due to the film temperature rise in the heat absorbing tubes. The following formula may be used to correct the viscosity using the two given values.

m_{new} = Corrected viscosity, Cp |

m_{in} = Inlet viscosity, Cp |

m_{out} = Outlet viscosity, Cp |

T_{new} = Temperature at new condition, °R |

T_{in} = Temperature at inlet, °R |

T_{out} = Temperature at outlet, °R |

For mixed phase process, obtaining the thermal properties of the fluid at the different points in the fired heater is much more difficult than with the single phase flow. However, for a heater with mixed phase at the inlet, the thermal heat transfer calculations may be performed using a straight line approximation similar to that used with single phase, without much loss in reliability of the results. It should be noted that when a heater has mixed phase at inlet and multiple tube passes, the actual flow conditions in the various passes may not be equal.

For the more normal situation, where the inlet process is a single phase liquid and vaporization begins at some unknown point in the heater, it becomes more difficult to estimate the properties. One way to do this is to set up a grid of the properties based on various pressures and temperatures. This works fairly well, but it is very important to assure that grid points near the dew point and the bubble point are included, if the points are going to be crossed in the heater design.