SWIMMING POOLS - Methods of Heating
Heating swimming pools poses additional problems due to the levels of Chlorides and Bromides often added to kill germs & bacteria in the water.
Chlorine and Bromine both attack metals, including copper, iron and stainless steel. Chlorine attacks metals more aggresive than Bromine.
Copper is very susceptible to attack and should not be used in pipework for treated water. Instead, plastic pipework should always be used. Also, copper cylinders and calorifiers should not be used where the treated swimming pool water is in contact with copper - a copper cylinder may be used providing a heat exchanger is used to keep it seperate from the treated water.
Nearly all boilers designed for use on swimming pools are fitted with copper heat exchangers. The boiler suppliers make the point that it is very important that swimming pool water is tested at least 2 to 3 times a week. Some boilers, typically oil fired, will use stainless steel heat exchangers, providing added resistance to attack.
Stainless steel plate heat exchangers are most commonly brazed together using copper brazing alloys. If a copper brazed plate heat exchanger is used on treated water then the copper is open to attack and may result in the failure of the heat exchanger. An alternative is to use Nickel brazed plate heat exchangers, all-stainless steel heat exchangers, which are far more resistant, and should be perfectly good in most instances.
The stainless steel used in plate heat exchangers is also open to attack from Chlorine and Bromine, however levels of less than 0.5 PPM (parts per million) combined with a pH greater than 7.5 should avoid any problems.
The pH level of a liquid is a measure of its acidity. A pH of 7 indicates neutral, a pH less than 7 indicates acid (with acidity increasing as the pH decreases), and a pH greater than 7 indicates alkaline (with alkalinity increasing as the pH increases). If the treated water has a pH less than 7, i.e. it is acidic, then the levels of metal corrosion greatly increase.
One must also take into account that the average pH level of the water in the pool may be 7, however the actual pH level in the water may vary slightly throughout. It is good practice therefore to ensure that the pH level of the swimming pool does not drop below an average pH of 7.5. This needs to be checked and maintained by the pool owner.
Litmus paper is a very cheap and easy way of testing the pH level. The paper changes colour to indicate the pH level. Once the pH is known, then additives can be easily added to adjust the pH. Alternatively, automatic systems are available.
It is important that any additives added into a swimming pool are iether added into the body of the pool, or are injected into the heating circuit downstream of the boiler or heat exchanger. If the additives are added into the heating circuit upstream of the boiler then the concentrations of additives in the boiler will be higher than average and may cause problems.
Metal corrosion from Chlorine and Bromine also increases with temperature, and temperature levels should not be allowed to reach 50°C. Although swimming pools are never heated to this temperature, the water in contact with the heating surface, such as the plates in a heat exchanger, will reach higher temperatures. Measures should therefore be taken to keep the surface temperature within the heat exchanger as low as possible. This is done by choosing a heat exchanger capable of transferring the entire boiler output at temperatures below 50°C.
A swimming pool is very comfortable at a temperature of 30°C. If the boiler heat source is not to rise above 50°C then we have a 20°C temperature difference to drive the heat transfer. The temperature difference can be kept low by keeping the flow rates through the heat exchanger as high as possible. Once the pump duties and the size of the boiler are roughly known, then it is simply a matter of choosing the appropriate heat exchanger.
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