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Duplex softeners

Duplex softeners function similarly to simplex units, except that they consist of two resin columns where one is in service and the second is on standby. The flow of water to service is metered and when the first column reaches exhaustion, the control valve automatically switches the service flow to the second column and then regenerates the exhausted first column. It is then put on standby until required again for service. In this way a duplex softener is capable of giving a continuous 24 hour supply of softened water. Duplex softeners are therefore ideal for sites with large variations in water demand or for applications where a continuous and uninterrupted supply of soft water is essential.

It is normal practice to size a duplex unit on the basis that each column regenerates once each day. Due to the ability to change columns only as required, it is possible to minimise salt usage.


Duplex control valves are sized and specified according to the required flow rate for the application. The inlet and outlet connection ports typically range between 1” to 3” and, in standard configurations, can achieve flow rates of up to 40m3 per hour. Higher flow rates can be achieved using ‘side mounted’ valves.


The size of any water softener is governed by the amount of exchange capacity required for the application. The capacity of each ion exchange bed is a function or the volume and water passed through it and the hardness of the incoming water. The chart below shows the capacity of each standard size duplex unit we supply, assuming that each resin column is regenerated once per day. It is possible periodically to allow for higher capacities than shown (up to two regenerations per column per day) top accommodate exceptional demand periods. The plant should not be sized in this way for routine use as there would be a danger of a loss of quality of the softened water. The figures shown assume the feed water has a hardness of 300mg/ltr (21° Clarke). These volumes can be adjusted on a pro-rata basis for different levels of hardness.

Another important criteria to consider is the continuous flow rate required. This affects the size of the valve that can be used, and sometimes the size of the resin column since the water needs to have a minimum contact time with the resin to achieve full softening. Short term higher peak flows can be tolerated, but this will result in a low level of hardness passing through to service and can increase the pressure drop across the softener. At the design flow rates shown there would be a pressure drop of between 10 and 15 psi across the plant. All automatic water softeners need a minimum supply water pressure of 25 psi. in the case of duplex units, this pressure must be available at the design flow rate so that there is sufficient pressure to allow a regeneration to occur whilst the demand flow is passing through the service column. They are designed to operate up to at least 100 psi.

All these softeners require an electrical supply. 240 volt or 24 volt are standard options.


Note: To prevent damage to the vessels this equipment must be protected from negative pressure from the drain or the supply. A vacuum release valve is recommended on the inlet supply and is supplied by us on all equipment using vessels of 370mm diameter upwards. The maximum water temperature permissible is 45°C.


Automatic water softeners need a supply of appropriate salt to make the required brine for regeneration purposes. Salt is most commonly supplied in 25kg bags of either granular or pellet type. This type of salt is manufactured specifically for water softening purposes and has a very high purity level. Sometimes, on very large systems, PVD salt can be supplied in bulk. Other types of salt should not be used due to the levels or impurity or additives.

The only attention required from the use is to check, on a regular basis, that the level of salt in the brine tank is kept topped up to ensure a saturated brine solution is available for regeneration.

Although softeners are very reliable, as with any other piece of essential plant, routine servicing is strongly recommended. This is generally straightforward and will ensure many years of reliable service from the plant.