The Ideal Gas Law and the Steam Dome

The ideal gas law describes the approximate behavior of gases at temperatures that are high relative to the critical temperature and at pressures that are low relative to the critical pressure.  This figure shows the approximate region where the ideal gas law applies best. 

Normally, one would use steam tables to determine properties close to the saturation region, and only use the ideal gas approximation far away, as shown in the figure. Today we’ll look at the error introduced by the ideal gas approximation as we get close to the saturation region.

Using Pipe Curve Models

Last post we demonstrated a parabolic model for the pressure-flow relationship in a pipe, and showed a more robust way to represent the flow-pressure relationship.  In this post, we’ll discuss the application of both models.

Inverting the Pipe Curve

Fluid flow in a pipe, described by the pressure drop as a function of flow rate, is commonly modeled with a parabola: ΔP=a*Q² + b*Q + c.  However, it is sometimes very useful to have the inverse relationship: Q(ΔP).  In this post we’ll talk about a convenient and accurate way to model that inverse.

Mary, Mary, How does your dewpoint vary?

Today we are modifying the old nursery rhyme which inquires about Mary's garden  to consider typical fluctuations in dewpoint temperature.  Since the dewpoint depends on the total atmospheric pressure, and that changes from day-to-day, the dewpoint will change (for a given amount of moisture in the air) as the barometric pressure changes. We'll also look at how dewpoint might change with altitude.

Heat from Breath, part 2

In the last post we considered heat lost through breathing and determined that breathing air at 35 °F results in a heat loss of about 2 Watts for every liter/min of air flow.  Of course, this depends strongly on the temperature and relative humidity of the ambient air.  In this post, we’ll consider both the effect of the condition of the ambient air, and also how much of the heat is latent (due to moisture) and how much is sensible (due to temperature).

Heat from Breath

In an earlier post, we looked at how much water is lost through breathing.  Today, we’ll consider the amount of body heat lost through breathing.  Of course, depending on surroundings, air movement, and  how one is dressed, a lot of (or a little) heat might be lost through the skin, but we’re just considering the part that goes out with your breath.

Buoyancy

I used to read to my kids from a popular children's book which featured a curious monkey who got into trouble by grabbing a large bunch of helium balloons from a balloon salesman.  According to the storyline, the monkey was carried out over the city hanging from the balloons.  Now, I'm sure the illustrator for the book was deeply interested in thermodynamics (isn't everyone?) but may have been in too much hurry to calculate the appropriate number of balloons.  Or maybe artistic aesthetics overruled thermodynamic accuracy.  In any case, today we'll go back and look at the situation in more depth.