A Cold Snap, Part 2

In the last post, we talked about the ground temperature during a cold snap.  Today, we’ll look at the same situation from two different viewpoints.

A Cold Snap

Following a series of relatively warm days, we had a sudden (over a period of a few hours) drop in temperature and things then stayed very cold for several days.  This led me to wonder how fast the ground would cool under such circumstances.

For the purposes of getting a simple, quick feel for the effect of the air temperature change, we’ll look at a somewhat simplified version of the actual events.  Of course this result won’t exactly match what happened in real life, but it will provide a reasonable estimate.

Altitude vs Temperature

People who ski, hike, run, or do other vigorous exercise at a higher altitude than usual typically feel some effect of the elevation.  It is common to feel difficulty getting enough breath.  Conversely, sometimes people who travel down from a higher altitude feel like they can run faster and longer than usual.  The cause, of course is that the lower density air at higher altitude provides less oxygen.  At a given altitude, though, the density of air also varies with the temperature.  In this post, we’ll look at the size of that effect.

Water from Air

We’ve talked in other posts about how water will condense out of air when it is cooled past the dewpoint temperature.  So I got to wondering how much it would cost to produce water that way.  In this post we’ll look at a very rough estimate of the minimum cost.

Finite Quench Revisited

We looked earlier at the solution for quenching an object in a finite bath—that is, where the bath is small enough (relative to the object being quenched) that the bath temperature rises while the object’s temperature goes down.  As you’d expect, eventually the object and the bath arrive at the same equilibrium temperature.  Today, we’ll look at getting the quenched object to two specific temperatures at two different times.

Sizes of Power

We've looked before at approximate sizes of heat transfer coefficients and heat fluxes, and in this post we'll extend that to look at approximate comparative sizes of power—both generation and use.  This chart shows sizes from the power generated by the largest hydroelectric plant in the world to the power available from a tiny hearing aid battery.  Note that the scale is logarithmic: each line represents a factor of 10 increase over the preceding line.  The scale is shown in both Watts, which is a common unit of power for electricity, and in horsepower which is a common unit of power for engines.

Potential for Evaporative Precoolers

In the last post we described a possible configuration for an evaporative precooler to lower total air conditioning operating costs at the expense of some additional capital costs (ducts, heat exchanger, evaporator) and perhaps a little additional maintenance costs on those items.   Of course, in order to make a decision on the payoff time, you have to know your climate conditions, your equipment costs, your operating costs, and the potential savings from the addition of the evaporative cooler.  In this post, we’ll provide a fuller description of the potential benefit you might be able to expect from an evaporative cooler for given inlet conditions.