Tower Overhead Pressure with Precondensers

In no precondenser designs, pressures can be lower than the vapor pressure of water at the condensing temperature. Lower pressure has both advantages and costs  

Less furnace coil and stripping steam required 

Pressure is controlled at a constant value (vs. varying with cooling media temperature) 

---

Low flash-zone temperature. Have the instrument mechanic check the furnace outlet thermocouple. The optimum tower top temperature for a vacuum tower equipped with a precondenser is usually not the minimum temjjerature. As the tower top temperature is raised, heavy naphtha boiling-range materials are flashed overhead into the precondenser. Acting as an absorption oil, they absorb a portion of the light hydrocarbons that would otherwise overload the jets. However, getting the vacuum tower top too hot can overload the precondensers. By field trials, find the tower top temperature (usually 230°F to 280°F), that minimizes flash-zone pressure. 

---