Water softeners are ion exchange systems designed to remove scale-forming calcium and magnesium ions prior to boiler, cooling, and reverse osmosis systems. Many different sizes and configurations of softeners are available, with selection based upon factors such as water quality, system demand, and the overall application. In industrial applications, it is imperative that a properly sized softener is selected to ensure that hardness is not allowed to pass to critical system components. Both over and under-sized softeners can result in serious problems due to hard water in these systems.
Sizing a Water Softener
Softeners Must Be Properly Sized To Prevent Hard Water Problems
To size a water softener, four factors must be taken into account:
Determining the hardness of the raw water and the resin exchange capacity is usually straightforward. You first need to know the total hardness of the water to be softened and understand how much it varies. The resin exchange capacity is then determined by the salt dosage during regeneration (How to Calculate Softener Capacity). However, it’s not always so simple to determine how much water needs to be softened and the maximum flow rate, especially in industrial applications.
Determining Water Usage
In general, softeners should be sized to regenerate once every 1-3 days, and in some cases, once every 12 hours. More frequent regenerations allow for smaller softener units, reducing capital costs; however, if regeneration is too frequent, the resin may degrade prematurely, valves wear more quickly, the brine solution may not be properly saturated, and there is a risk that the demand may exceed the capacity. If regeneration is too infrequent, flow path short circuits (or bypassing) can form in the resin bed (also known as “channeling”), resulting in hard water passing through the bed.
In order to regenerate the resin no more than once per day, the daily amount of soft water used must be determined. How this is done can vary greatly, depending upon the application. If softening water upstream of an RO unit, for example, the demand for water may be fairly constant. In this case, the water requirement can be determined simply by averaging the amount of water used over a period of time. However, the soft water demand for a steam boiler system may fluctuate significantly due to seasonality and/or variations in condensate return. In this case, soft water requirements should be calculated by factoring in the maximum system load and minimum amount of condensate return expected.
Problems with over and under-sizing the softener may occur if careful attention is not paid to the setup of the entire system. The maximum system load may not be the same as total system capacity. Factoring in redundancies, such as boilers that are operated lead-lag, can result in drastic oversizing based upon excess capacity. In addition, consideration must be given to whether the softened water is blended for use in tower systems, and to the amount of reject water that also has to be included when sizing softeners upstream of a reverse osmosis (RO) system.
Flow Rate Requirements
The flow rate though the softener is just as important as the overall size of the resin bed. If the rate of flow is too high, compaction of the resin bed may occur. Bed compaction decreases the amount of time that water is in contact with the resin, resulting in reduced softening capacity. Pressure drops increase which can decrease flow, outlet pressure, and potentially damage resin beads. If the flow rate is too low, poor distribution of water over the resin can cause channeling. In either case, the end result is hard water passing through the softening system.
There are two types of flow rate that are used when sizing a softener, continuous flow rate and peak flow rate. Continuous flow is the average maximum flow rate though the system, and is generally the flow rate that should be used for sizing the softener. Peak flow rate is the maximum allowed flow though the softener during times of extraordinary demand. Peak flow should only be reached intermittently, and only for short periods of time or the integrity of the resin bed may be compromised. Generally, softeners should not be sized based upon peak flow rates that last longer than 30 minutes to 1 hour.
If the flow rate is highly variable, a demand flow softening configuration may also be recommended. A demand flow configuration operates softener tanks in parallel, and allows for higher flow thorough the softening system at times of peak demand. As demand for soft water increases, available resin tanks are brought online to accommodate the load. As demand decreases, idle tanks are taken offline to prevent low-flow channeling through the unneeded resin tanks. A soft water storage tank with subsequent repressurization pumps could be also considered as an alternative strategy to handle peak demands rather than sizing a larger softener system.
Careful consideration of all factors related to sizing a water softener is required so that vital system components are protected from hardness scaling issues. Chem-Aqua has a full line of softeners designed to meet any demands and a team of engineers to help guide you through the selection process.
Written by: Kevin Blake
If you like this blog article, you may also be interested in Softener Regeneration in Four Easy Steps.