Air Conditioning Overview

by Koert DuBois 2015

To make a decision on the practicality of air conditioning, you really need to get a math/physics expert to create a chart that determines a desirable (goal) temperature (mostly based on humidity) and then compute how much cooling energy is required to achieve the goal temperature in different situations (temperature, humidity, sunshine being the main variables) and then how much cooling energy can be provided by the fuel tank.

The first line of defense is always ventilation, so either open windows or a fan. The next line of defense is reflectivity and insulation. Reflectix over the windows, compartmentalizing the coach interior, insulation from hot chassis and coach body parts, or perhaps reflective mylar awnings and roof cover are all ways to reflect heat.

A fan will lower the perceived temperature by at least two degrees, significantly more under the right conditions. If you have moisture on your clothes or body, the evaporation of the moisture causes for an even greater perceived temperature drop. (“Perceived” because a fan only lowers body temperature, not room temperature.) If you can introduce moisture into dry moving air (as with a mister or evaporative cooling system) the actual temperature drop can be as much as 20 to 40°!

If initial temperature drop isn’t enough, you’re out of luck. Evaporative cooling is a one-shot deal; you can’t recirculate the now-humid air and repeatedly cool it using evaporation. Evaporative cooling needs a constant supply of warm dry air and a quick way to exhaust the cool humid air. Opposing windows is usually the easiest way – in one window and then right out another.

When the difference between the goal temperature and the actual temperature is great, and the humidity is high, chilled air becomes the next best alternative. (Although, in the middle ground, a combination air chiller/evaporative cooler, works very well.)

The simplest way to chill air is to blow it across a cool surface, like a fan behind a bowl of ice. Unlike evaporative cooling, chilled air can be recirculated, that is, it can be cooled and re-cooled until it drops to the goal temperature.

As the air conditioner cools the air, the interior air is being heated by sunlight radiation, conduction, the occupants’ body heat, and cooking. On a very hot day, it’s not unusual to reach equilibrium at a temperature as high as 85° to 90° or so, which with high humidity doesn’t feel cool at all.

Even though air conditioners are simply cooling the air, humidity is still a huge factor. First, humid air feels hotter. Secondly, when an air conditioner tries to cool humid air, much of the energy goes into condensing the water vapor (humidity) into liquid water. That’s why an air conditioner doesn’t feel as cool when it’s first turned on. Not only is it dealing with very warm air, it also has to spend energy removing humidity before it can get on to its real job – cooling the air.

In very humid heat, a de-humidifier can be just as important as an air chiller. (To make things even worse, this liquid water builds up and insulates the warm air from the cool condenser. And, ironically, a larger air conditioner, which cools the air rapidly and then cycles off, removes less moisture than a small air conditioner which runs continuously.)

Even with unlimited electrical power, cooling isn’t always as simple as turning on the air conditioner. When electricity is limited, the other variables become even more important.

For us, electrical hookups aren’t part of the equation, and we’re usually in low humidity heat (or, more frequently, you just leave when it gets too hot) so you tend to see our generator/air-conditioner as more of an emergency tool than as a daily appliance.

Here’s a start – this chart shows maximum temperature drop achievable by an evaporative cooler.