SPLASH 142 June-July 2022

Page 24

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Can pool water "boil"?

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his is the second in our series on the qualities of water. This time, boiling water. Why are we talking about boiling water? Because, as you will see later, pool water can and often does “boil”. The boiling point of water is the temperature at which it changes into a vapour (gaseous state). That temperature is not always 100C – in fact, it is more likely to be some other temperature as the temperature at which water boils is always a factor of temperature and air pressure, and air pressure is less at a higher altitude. At sea level, where the nominal atmospheric press is 101.325 kPa, water will boil at 100C. But at higher elevations it will always boil at a lower temperature. If water under a pressure higher than 101 kPa is heated until it boils it will do so at a temperature in excess of 100C. Water at the top of Mt Everest at its altitude of 8850 metres above sea level has an air pressure of only 33.6C and water boils at 68C. If water is placed in a vacuum such as outer space it immediately boils, vaporises then becomes ice. If fact it would appear to be quite confused as to which state it should be in, but it won’t be a liquid. So theoretically in a vacuum it can be said that water will boil at 0C or more likely at 0.00001C. Place some water in a vacuum and it will very quickly turn into water vapour. Boiling of water is actually a cooling process as the water vapour that is given off takes heat energy away from the water. Evaporation is a similar process that cools the remaining water. The reverse process is called condensation where water vapour can and does re-join a water mass as it cools thus making the water warmer.

pressure at which it will boil. It is the other side of the boiling point chart on this page. This vapour pressure is used in the equation to determine the net positive head suction available (NPSHa) to avoid cavitation.

So what has this got to do with swimming pools?

“Can pool water "boil"? “Every time a pool pump is starving for water, the noise you can hear is ‘cavitation’: water boiling in the pump at low pressure and at a low temperature.”

Saturated vapour pressure

While the boiling point is the temperature at which water will boil, the saturated vapour pressure is the

A pool pump does not suck water, it merely creates a low pressure zone into which air pressure on the pool surface forces water into the pump. The lower the pressure in the pump volute, the more water can be pumped. I say “can be pumped” as more often than not, undersized pipes, bends and other fittings in the pool plumbing system prevent the pump's flow rate from operating at its best. Every time a pool pump is starving for water, the noise you can hear is “cavitation”: water boiling in the pump at low pressure and at a low temperature. Note that constant cavitation can destroy the operating parts of a pump relatively quickly.

What is evaporation?

Molecules of water are in a constant state of agitation. They are attracted to each other, bump into each other and bounce off each other. In doing so they transfer energy one to the other, and when this energy transfer is quite one-sided, molecules of water at the surface can and will escape the water body and thus evaporate. In science speak, for molecules to evaporate, they must be located near the surface, be moving in the right direction and have sufficient kinetic energy to overcome the liquid-phase intermolecular forces. Why is evaporation higher on a hot day? As the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more rapidly at higher temperatures. Other factors: • Evaporation is higher when air is dry (low humidity) and less when air is moist; • Evaporation is higher at times of low pressure; • Evaporation is higher from a larger surface area;

LEFT: Saturated vapour pressure chart

Water will evaporate much quicker when there is heat, low humidity and wind – e.g. in a clothes dryer. n For these articles Cal Stanley draws on his 30 years’ experience and success in pool construction, having run award-winning pool construction company Neptune Pools in Western Australia for three decades. He currently works as a pool consultant and delivers hydraulics courses for the swimming pool industry. Contact: neptunepools@westnet.com.au.

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June/July 2022


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SPLASH 142 June-July 2022 by The Intermedia Group - Issuu