Heat Transfer in the Kitchen

When I tell people (and by 'people', I mean 'non-engineers') that my field of specialization is heat transfer, I usually get some incredulous response on the theme of, "There is a whole field where someone would study nothing but heat transfer?" with the implied sub-text of "sane someone" accompanied by lots of extra question marks.  I have to admit that in everyday experience, most people don't have to think quantitatively about heat transfer.  However, almost everybody connects (non-quantitatively) with heat transfer in the kitchen, so in this post we'll explore heat transfer in cooking.

An Alternate View of Condensation

Usually when we think of condensation, we think of the droplets of water that form on a cold glass on a humid day, or perhaps we think of dew making the grass wet on a summer morning, or maybe the fog that forms on the bathroom mirror after you have a hot shower.  In those familiar cases of condensation, the water vapor is mixed in with dry air in a very dilute mixture.  For example, at 75 deg F, 60% relative humidity, the water vapor comprises only about 1.1% of the total moist air, by mass.  Even at 75 deg F, 100% relative humidity, (at which point the vapor is about to start condensing to liquid) the water vapor is only about 1.8% of the mixture.  So, we’re accustomed to condensation of water from a very dilute mixture of water vapor in air.  In this blog, we’ll consider the condensation of pure water vapor, and see some surprising forces.

Pressure Cookers at Altitude

Water boils at 212 deg F at sea level, of course, but the boiling temperature of pure water is a pretty strong function of the ambient pressure.   This figure shows the boiling temperature of water at different pressures.  Atmospheric pressure at sea level is around 14.7 psi, and that point is marked on the figure with a red circle. The standard operating pressure for pressure cookers is 15 lbs which means 15 psi above atmospheric pressure.  That point is marked with a green triangle in the figure.  You can see that the boiling temperature for water is around 250 deg F inside a pressure cooker operating at sea level.