Lumped Capacitance Revisited

Lumped capacitance is a very useful heat transfer approximation that we mentioned originally here,  and explored in more depth with extensions here and here.  Today, we’ll look at another extension of the basic lumped capacitance approach.

Refrigerant 718

News flash! A major spill of refrigerant R718 occurred on the Interstate last night!  The refrigerant soaked the highway and surrounding ground.  Luckily, in a few hours it dried up with no effect other than helping nearby plants and small animals.  Because, of course, refrigerant R718 is the designation for pure water.  Today we’ll consider advantages and disadvantages of using water as a refrigerant.

The Ideal Gas Law and the Superheated Region

Last post we talked about using the ideal gas law on the right side of the steam dome and found that it could be used as a decent approximation for very low pressures, even when the vapor is about to start condensing into liquid.  Today we’ll look at the accuracy of the ideal gas approximation out in the superheated region.

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.