How to calculate the pressure at the inlet of the tower bottom pump
Thread Content
The pressure in the vacuum distillation column is 100 mmHg. The liquid level in the column bottom is 5 meters above the pump inlet, and its density is almost the same as that of water. Is the column pressure absolute pressure?
Pressure at the liquid surface + Pressure generated by the liquid level – Pipeline losses
Is the pressure generated by the liquid level, p=ρgh, gauge pressure? 100 mmHg equals 13.3 kPa. The degree of vacuum is equal to atmospheric pressure minus absolute pressure: 100 – 13.3 = 86.7 kPa. The inlet pressure of the pump at the bottom of the tower is approximately -86.7 + 50 = -36.7 kPa. Ignoring losses, is this calculation correct?
Atmospheric pressure: 101.3, that’s the value to use
Why does it alternate between positive and negative values? If you calculate it based on absolute pressure, then use absolute pressure consistently.
Based on the conditions provided by the moderator, the pressure inside the tower is 100 MMHG; this value is likely measured using a U-tube manometer. Therefore, the actual pressure inside the tower is equal to atmospheric pressure minus 100 MMHG, which gives 760 – 100 = 660 MMHG. There is a height difference of 5 meters from the liquid level in the tower bottom to the pump inlet; since the density is similar to that of water, this can be considered as equivalent to 5 meters of water column, or 380 MMHG. Finally, the pressure at the pump inlet is the sum of the pressure inside the tower and the pressure resulting from the height difference of the liquid level, That is, 660 + 380 = 1040 MMHG, which is approximately 1.37 bara.