The inside film coefficient needed for the thermal calculations may be estimated by several different methods. The API RP530, Appendix C provides the following methods,

For liquid flow with Rh_{l} = Heat transfer coefficient, liquid phase, Btu/hr-ft^{2}-°F |

k = Thermal conductivity, Btu/hr-ft-°F |

d_{i} = Inside diameter of tube, ft |

m_{b} = Absolute viscosity at bulk temperature, lb/ft-hr |

m_{w} = Absolute viscosity at wall temperature, lb/ft-hr |

h_{v} = Heat transfer coefficient, vapor phase, Btu/hr-ft^{2}-°F |

T_{b} = Bulk temperature of vapor, °R |

T_{w} = Wall Temperature of vapor, °R |

G = Mass flow of fluid, lb/hr-ft^{2} |

C_{p} = Heat capacity of fluid at bulk temperature, Btu/lb-°F |

h_{tp} = Heat transfer coefficient, two-phase, Btu/hr-ft^{2}-°F |

W_{l} = Weight fraction of liquid |

W_{v} = Weight fraction of vapor |

It should be stressed at this time, that there are many ways to calculate the inside heat transfer coefficient, and a lot of care should be taken in the procedure selected for use in heater design. Other methods, such as HTRI, Maxwell, Dittus-Boelzer, or others may be more appropriate for a particular heater design.