Seems like a perfect afternoon to talk about some cooling…and in the spirit of sustainability, how about some evaporative cooling!
If you live in a hot and dry climate you have to be familiar with the ubiquitous rattling desert cooler (or swamp cooler as known in some parts). Absence of good design (both engineering and aesthetics) sometimes make them undesirable or unusable but evaporative cooling is a serious affair and goes beyond this simple application. Cooling by evaporation is not a new science; nature perfected it in the form of perspiration. We sweat when our body needs to cool down and the process of evaporation lowers the temperature.
So how do we benefit from evaporative cooling? Let's start with the basics.
What is evaporative cooling?
In layman's language, when water (or any liquid) evaporates, it cools. It happens because when water turns into vapor, it needs energy (heat) that is drawn from its surroundings. So water evaporating in an earthen pot will cool the water in the pot, the pot itself and the air surrounding it.
Examples of evaporative cooling.
Sweating on a hot day: Sweat evaporates and cools our body.
Water in a clay pot: In the good old days, before the big fat refrigerators, we had earthen pots to store water. You can still find them in a lot of households in India. Earthen pots are porous and when water seeps through the pores and evaporates. The result is cool water without using any artificial form of energy. If you have not experienced a cool tall glass of water from a 'match' on a hot summer day, you should.
Desert (or swamp) coolers: desert coolers provide low cost and low energy cooling. They are most effective in hot and dry climates. The simplest form of desert coolers have a water storage tank with pads made of wood shavings or vetiver (khus-khus) that are kept moist with the help of a small pump. When a fan mounted in the front of the cooler cabinet draws air through these wet pads, the process of evaporation cools the air. Modern variations of desert coolers use thicker pads made from treated paper (celdek). The biggest disadvantage of desert coolers is the rise in indoor humidity levels. There are variations available (indirect evaporative or two stage cooling) to address the humidity issue.
Cooling towers: Primary purpose of cooling towers is to cool the water itself in industrial applications.
Mist cooling: In mist cooling, tiny water droplets (>10 microns) are sprayed through nozzles directly in the air. When these tiny droplets evaporate, they absorb heat from the air that results in cooling. High humidity is a concern if ventilation is not good but these systems work well in outdoor settings. Imagine walking by a fountain on a hot day.
There is a lot to discuss on the subject of evaporative cooling and I will discuss a few topics in more detail in the coming days.