HTH Dry Chlorine


The idea of a swimming pool is quite simple in engineering terms. A tank filled with water. A main circulating pump that continuously re-circulates the water, and the means to heat and chemically treat it. Simply a water treatment plant! We will first of all follow this water through the physical operation of the circulation system and following that look at how it is chemically controlled. The tank of water usually has a deep end and a shallow end. The deep end is the obvious place to put the pool outlets or where the water, having been swum in (and polluted) by the bather, will leave the tank for replenishment and treatment. The shallow end is where the water returns to the pool after receiving that treatment. So what happens to the water between leaving and returning to the pool tank? Water leaving the pool is low in chlorine content though its temperature is little different to when it entered the pool. It contains bacteria and debris introduced by bathers – fluff and fibres from bathing costumes, cosmetics washed from bathers' bodies, amongst other things. Don't worry, the level of chlorine maintained in a well run pool, renders these potentially hazardous products quite harmless almost instantaneously and makes bathing safe for hundreds of bathers per day. We will now follow the path that the water takes through the treatment process, via easiflo1 feeder or easiflo3 feeder. The strainer basket is the first filter the water comes to, it is there to remove all the big particles and trap anything that could cause damage to the main circulating pump impeller – e.g. hair grips and plasters. Children bring hard objects such as coins or marbles to dive for and these may find their way down the grill at the deep end – so the strainer is there to ensure no damage is done to the equipment in the plant room. It is worth noting that an outdoor pool may require the strainer basket to be cleaned several times each day, as leaves and sweet wrappers can easily enter the basket and reduce the water flow dramatically. The circulation pump sucks the water away from the pool and pushes it through the other pieces of equipment as it travels back to the pool. The pump may be a stand alone assembly with a separate strainer basket, but often in smaller pools it is reduced in size due to the volume of water being dealt with and amalgamates to form one unit called a combined pump and strainer. In all situations, the pump impeller is driven by an electric motor, once again a unit that is sized to suit the particular application. It is the size of the pump that will determine the turnover, i.e. the theoretical time that it takes to treat the total contents of the pool. The next major step is filtration. This process involves the removal of suspended particles from the water by passing it through a filtering medium such as sand. Filtration of pool water via sand filters is a part chemical and part physical phenomenon. Simple straining of the water is only part of a complex process and is covered in greater depth within our Pool Professsionals Handbook. The size and number of filters will alter and is determined predominantly by the volume of the water and its anticipated levels of pollution. As the filter cleans the water passing through it, over a period of time it will become saturated and require cleaning, this is a process known as a backwash. Backwashing is simply the removal of accumulated dirt and bacteria from the filter, by reversing the flow of water. In normal operation mode, pool water will enter the filter at the top and flow out at its base. By opening or closing valves in a strict order, the water is introduced through the base of the filter, up through the sand and down to a waste pipe, taking with it the collected dirt. This dirty water can be seen through the waste pipe water sight glass. The water, having left the filter/s, will now travel towards the calorifier (or heat exchanger). This is a series of tubes through which hot water passes from the building boiler system. The pool water passes over these tubes and a transfer of heat occurs from the hot water in the tubes to the pool water outside. The water then reaches the pool, having been strained, filtered, heated. This continuous process along with the chemical process described next provides constantly safe conditions for bathers. Pools, large or small, residential or the largest commercial, will have the same operating system, though the larger pools may have more of the same. As stated above, the process of circulation is ongoing, using almost the same water all of the time. It is for this reason that swimming pool operatives must ensure that there is adequate provision of a disinfectant to kill off incoming pollutants of all types. Chemical treatment When the pressurised water leaves the pool circulation pump a small sample is taken off and passed through what is known as a sample cell. Here the water is analysed electronically for its chlorine content and its pH value and measured against pre determined parameters. When the water is out side of those pre determined parameters the automatic controller to which the probes within the sample cell are connected, sends a signal to either the chlorine or the pH correctant system calling for it to dose until the desired reading is reached. The accuracy of this automatic control unit is paramount in the operation of the volume of water, its chemical control and obviously provision of safe water conditions. See automatic control The treatment process describes how this chemical control works and what it does to the pool water.		Click thumbnails to view animated example

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