Salt Chlorinators : – for swimming pool applications
Producer of chlorine, or more precisely Sodium Hypochlorite,( liquid chlorine ) from a salt solution relies on the application of a small DC voltage across two special conductors ( electrodes ) immersed in that mild salt solution.
This voltage causes an electric current (Amps) to flow from the positive electrode (Anode) through the salt solution to the negative electrode (Cathode). In doing so the salt in the solution is changed into Sodium Hypochlorite – i.e. Liquid Chlorine.
Several other products are also formed during this process, the most significant being Hydrogen gas and Calcium carbonate ( scale ). The Sodium hypochlorite and the Hydrogen gas are taken back to the pool by the water flow, while the scale attaches itself to the Cathode.
The basic chemical equation for this process is:
Salt + Water + Electric Current = Sodium Hypochlorite + Hydrogen Gas
2NaCL + H2 0 + e = Na CL + H2
The reaction is valid for water temperatures in the range 5 degrees Celsius to 30 degrees Celsius. Below 5 degrees some sodium chlorate may be formed and above 30 degrees some sodium perchlorate may be formed. These two insoluble substances form part of the scale which builds up on the negative ( titanium ) electrode in the cell.
Other base elements such as calcium, magnesium, copper, iron and a variety of positive metallic ions will plate out or scale the negative electrode.
The Salt Cell:
The salt cell known as the Electrolytic Cell, where the chlorine is produced, consists of a plastic casing containing the electrodes. These are usually constructed from the highly corrosion resistant material Titanium. The Anode being coated with one of a number of special materials. In the 70’s it was Platinum. This was later followed by Platinum-Iridium electroplate and today 90% of coatings contain baked coating of the Platinum metals group, but mainly Ruthenium Oxide Iridium and Carbon are now used. The composition of this coating plays a major role in both the Chlorine producing efficiency of the cell and its life expectancy.
The cell coating can be destroyed by the following;
- Fluoride above 1ppm – In water with a pH below 7.0, fluoride will cause rapid premature loss of coating.
- High Acid Levels – Water with a pH less than 6.0
- High levels of Manganese – Manganese at 50 parts per billion needs a higher voltage potential due to the deposition of manganese dioxide, resulting in loss of efficiency and reduced coating life.
- High Copper & Tin Concentrations – Normal Copper / Tin algaecides in the pool have little effect. However, placing copper / tin algaecides blocks in the skimmer box to dissolve, can provide the very high concentration needed to break down the coating.
- Water Temp Above 70 degrees Celsius – High water temperature may also dramatically shorten coating life. This can occur when water flow through the cell us reduced as a result of diverting water through a spa, waterfall, heater, etc. In such cases the cell may get very little water and overheat.
- Scale – Calcium carbonate scale, if allowed to cover or touch the coated electrode, it will block the release of sodium hypochlorite and chlorine gas will form. Chlorine gas will dissolve all the platinum metals and will completely destroy the electrodes.
The Power Pack: – also called the control box
The Power Pack has a multi-purpose role to perform. It’s prime functions are:
- To reduce and convert the 240 Volt ac power to around 8 Volts DC, ( direct current – depending on which model you have installed ) this is the required current to produce sodium hypochlorite. The power pack must deliver enough voltage and current to make enough chlorine to do the job. Most units use between 6.2 Volts dc and 9.2 Volts ac and operate between 10 and 30 Amps.
- To control the operation of the cell – i.e., provide switching, water flow monitoring, gas sensing, etc.
- To Provide a means of interconnecting the pool pump by providing a 240 Volt ac outlet socket and to house auxiliary equipment such as an automatic time switch, underwater pool light power supply etc!.
As a general guide 20 bathers need 1kg of chlorine and most domestic swimming pools need a minimum of 80 grams hypochlorite equivalent each day with 8 hours of filtration, to remain clean, clear and properly disinfected. One can vary the amount of chlorine and vary the amount of filtration, but one without the other will not clear the pool.
The smaller the transformer, the higher the operating temperature within the power pack. A 160 VA transformer when operating at near 20 Amps and 8 Vac tends to operate at about 130 degrees Celsius.
Higher temperatures mean greater wear and tear on cables / wires and other circuitry components which then become more prone to failure.
Selection of a Suitable Unit :
There are many good, well designed salt chlorinators available today and there are a number of factors which affect the selection of a salt chlorinator, apart from personal preferences. These are based on the chlorine requirements for similar sized pools and will include the following:
- Sizes of the pool or spa & location – Larger pools need larger output capacity salt chlorinators, in some instances 2 or more chlorinators need to be installed in parallel to produce the required amount of chlorine demand per hour for that pool.
- Bathing load – heavier loads consume more chlorine
- Size of the filtration system – poor water flow will require longer running times per day.
- Summer water temperatures – high temperatures and strong sunlight cause faster loss of free chlorine
- Water chemistry – this needs to be maintained at the recommended levels 24/7. Cyanuric acid / Stabiliser / Sun screen levels are important and help reduce chlorine consumption during the summer months. Poor chemical balance will affect chlorine requirements just as much as a manually dosed pool. Lack of proper cell and filter maintenance will also affect chlorine production.
- Do you have a cover for the pool? this is a very important factor to take into account when correctly sizing a salt chlorinator.
All brands of units should be installed indoors and be protected from the elements, the reasons for this should be obvious. Never mind that the manufacturer states that they can be installed outdoors and exposed to the elements. Take a look at the condition of this outdoor unit within 3 years VS the condition of a unit installed indoors. U/V is a killer.
The unit must be installed EXACTLY as per the manufacturer’s instructions. The salt cell and casing will be installed in the return-to-pool plumbing after the filter and / or Heater so that the water will flow through the cell in the correct direction and that the concentrated Chlorine does not pass through any other equipment on its way to the pool via the return jets. The salt cell must be the last piece of equipment in the water path back to the pool.
During operation, both hydrogen and chlorine gas are generated at the cell, therefore the manufacturers installation instructions MUST be followed to ensure these dangerous gasses are dissipated.
Most units have combined flow detectors / gas sensor built into the cell which automatically causes power to the cell to be interrupted before this condition can occur.
The power pack must be fixed to a wall or post close to the cell, within the distance governed by the length of the lead provided. Adequate ventilation is required .
Check the IP rating, ( Internal Protection / weatherproofing ) and if it is not high enough, the unit must be installed outside the pool zone and 1.5 metres above ground level.
Swimming pool grade salt must be added to the pool and allowed to completely dissolve before operating the chlorinator. Pool water temperature determines how quickly the salt will dissolve, brushing and moving any visible salt around on the pool floor with your pool brush will speed things up if the water is cold.
Accurately calculate the volume of water in the pool and add salt directly into the pool (not the Skimmer box) in the required quantity according to the manufacturers’ recommendations.
If the manufacturer stipulates a low salt level do not exceed that level. New Low salt models operate between 2700ppm to 4500ppm of salt. Recently, models have been introduced that will still produce the correct amount of sanitiser with salt levels as low as 1200ppm. You would not know that this was a salt water pool – amazing!!
Otherwise the salt level on the most common units in the field should be in the range 5000ppm to 6000ppm. Higher salt levels may damage some units.
N.B – For new concrete pools with marble plaster interior finishes it is generally recommended that salt not be added to the pool water until the excess calcium compound in the plaster has leached out and the pH of the water has stabilised. Generally this is about 12 weeks for a hand mixed/applied plaster or 24 weeks for a machine mixed / applied plaster. Do not rush this process!!
Cyanuric acid (chlorine stabilizer) should also be added to all pools which experience strong sunlight in order to minimise loss of chlorine due to sunlight. Recommended level is in the range of 30 to 50ppm.
Testing for Correct Operation:
After the correct amount of salt has been added and allowed to fully dissolve in the pool water, the chlorinator can be switched on. Be sure to observe the manufacturers comments at this stage regarding indicator lights, meters etc. If all of the indicators suggest the unit is working normally, a conclusive check can be made by taking a sample of pool water as it enters the skimmer and doing a chlorine check. Likewise, a sample taken from the return-to-pool outlet should yield a chlorine reading at least 1.0ppm higher than from the skimmer, which will confirm the unit is producing chlorine.If unsure uplift a water sample and get your pool shop to confirm salt level and free chlorine at this time. We prefer our salt water pools to run at 3000 – 5000ppm free chlorine 24/7.
All brands of cells will require cleaning periodically, to remove scale deposits which form on the electrodes during normal operation.
When cleaning is indicated, the cell is usually immersed or filled with a solution of at least 3 parts of hydrochloric acid and 7 parts water until the scale deposits have been removed. Some deposits dissolve in a few minutes while others may take 30 minutes to an hour. Remember the weaker the cleaning solution is the longer it will take to clean the cell plates and sensors. Repeat the process if the effervescence subsides and the cell is still not clean.
NOTE : DO NOT allow electrodes / cell plates to be in contact with acid any longer than necessary as damage to the electrodes coatings can result, reducing the life expectancy of these plates. Consult the manufacturers’ handbook for more specific details.
Other maintenance is usually limited to correcting the salt level and water chemistry balance in the pool – i.e. pH, total alkalinity, water hardness and cyanuric acid, and servicing the filtration system so that adequate water flow is maintained.
Remember, the greater the water flow through the chlorinator cell the higher the chlorine production coupled with a slower rate of scale build up and an increase in the life of the cell electrodes.
Common Problems with salt Chlorinators
Problem Probable Cause
Salt content in water reads low : Low salt concentration.
Chlorine output control turned down.
Low Chlorine readings in pool. Low salt levels
Insufficient daily pump running time.
Chlorine control turned down to low.
Low cyanuric acid levels in pool water
Rapid build-up of scale in cell: High calcium hardness levels in pool water.
( Calcium chloride )
Erratic reading on Salt meter: Poor water flow through the cell.
A loose salt cell cable connection/s
Excessive air in cell due to: Partially Blocked Filter.
Blocked pump strainer Basket.
sticking skimmer weir flap.
pump lid leaking/cracked
auto pool cleaner vac hose cracked/damaged
or plumbing leak on suction side.
Pump not operating: Power not switched on.
Time clock off?
Fuse or circuit breaker tripped.
Faulty “ON” lamp on salty.
Salt meter reads in overload area: Dial control knob settings?
No chlorine output at RTP eyeballs: Power not switched on. Output settings to low Faulty / worn out salt cell or controller problems
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