- Heat Transfer Technology - Technical Questions and Answers - Technical Q & A Database

· Unlock · Lock 15 Jul
Tween
How to convert PPMw (by weight) to PPMv(Volume) for H2S & moisture(H2O) in gas mixture?
Hide Comments

Submitted Chris Haslego, Nov 21 2011 11:21 AM | Last updated Nov 21 2011 01:29 PM

Category:	Heat Transfer Technology
Question:	Can large temperature differences in vaporizers cause operational problems?
Keywords:	shell,tube,heat,transfer,exchanger,vaporizer,temperature,difference,boiling,large
Answer:	Large temperature differences in heat exchangers where liquid is vaporized are a warning flag. When the temperature differences reach a certain value, the cooler liquid can no longer reach the heating surface because of a vapor film. This is called film boiling. In this condition, the heat transfer deteriorates because of the lower thermal conductivity of the vapor. If a design analysis shows that the temperature difference is close to causing film boiling, the vaporizer should be started with the boiling side full of relatively cooler liquid. This way, you don't start off flashing the liquid. The liquid is slowly heated up to a more stable condition. If the vaporizer is steam heated, the steam pressure should be reduced which will reduce the temperature difference. With steam heating, take a close look at the design if the MTD is over 90 ⁰F This is close to the critical temperature difference where film boiling will start.