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.

An Evaporative Pre-cooler

We talked in an earlier post about evaporative cooling and how you can use it to cool off air by simply evaporating water.  Evaporative coolers work best in dry climates, and are much cheaper to operate than vapor compression systems since there is no electricity-hungry compressor.  Unfortunately, they result in a large increase in the relative humidity of the air which may render it unacceptable for some indoor air applications.  In this post we’ll talk about how evaporative coolers are used in combination with other air conditioning equipment to lower the overall cost of providing conditioned air.

Fundamentals of Thermal Resistance

The Thermal Resistance Analogy
Thermal resistance is a convenient way of analyzing some heat transfer problems using an electrical analogy in order to make complicated systems easier to visualize and analyze.  It is based on an analogy with Ohm’s law which is:

In Ohm’s law for electricity, “V” is the voltage which drives a current of magnitude “I”.  The amount of current that flows for a given voltage is proportional to the resistance (R_elec).  For an electrical conductor, the resistance depends on the material properties (copper tends to have a lower resistance than wood, for example) and the physical configuration (thick short wires have less resistance than long thin wires).

Error using constant specific heat

In the last post we looked at picking a value to use for c_p when we make the constant specific heat approximation.  In this post, we’ll look more closely at the error involved in that approximation in order to have a feel for when it is appropriate, or not, to use it.

Which cp to use?

In an earlier post we talked about the variation of specific heat with temperature.  Today we’ll explore that variation in more depth and consider the choice of temperature at which to evaluate specific heat.

Water Lost Through Breathing, Part 2

In the last post we talked about the water loss from breathing and used the assumptions of exhaled air being at 92 deg F and 90% relative humidity, and inhaled air close to freezing, to develop a constant to estimate the water loss per hour for any breathing rate. This time, we’ll look at the effect of temperature and relative humidity and elevation on that constant.

Water Lost Through Breathing

Have you ever wondered how much water you lose by breathing? Do you think that it is more or less than that lost through sweating? Most of the time when we breathe, we are exhaling moister air than we breathe in. There might be a few exceptions, but normally our bodies are losing a little bit of moisture with every breath. In this blog, we’ll try to quantify that moisture loss, at least in a ballpark way. 

Tire Heating on Landing, Part 2

In the last post, we talked about the moments of sliding/rolling motion when a tire first contacts a surface with a mismatched velocity.  Using the assumptions and simplifications outlined in that post, today we will make estimates of heat generation at the tire/runway interface, and temperature profiles inside the tire material based on a hypothetical airplane landing scenario.