Sizing Water Softeners for Unfired Steam Generators

This paper discusses recommending a water softening treatment for use with Cemline Unfired Steam Generators. We recommend a duplex softening system with automatic alternation. A water softening system takes approximately three hours to regenerate and within a single system, the possibility exists of a three hour period during which raw water is introduced into the unfired steam generator. With a duplex softener with alternation, when one of the softeners needs to be regenerated, the other automatically switches over and provides softened water to the unfired steam generator.

Our recommendation for sizing water softener is as follows:
Take the maximum gallons/day able to be generated by an unfired steam generator. Calculate this number by multiplying the total number of pounds/hour that the generator is rated for by 24 and dividing this number by 8.33 (# of water per gallon). For example, take 1000#/hour x 24 hours in a day and then divide by 8.33 to determine the maximum usage of 2,881 gallons/day. Multiply the maximum usage with the hardness of the grains of water in question. If, for instance, the water hardness is 8 grains, multiply 2,881 x 8 to arrive at 23,049 maximum grains of hardness to be removed in one 24 hour period.

Water softeners are sized in grains of hardness which are removed between regeneration periods. Allow for approximately six hours to recharge the brine in the brine tank which makes four regenerations possible in a 24-hour period. Divide the total of grains arrived by four (in our example, 23,049/4 = 5,762 grains {round to 6,000 grains}) to reach the required size per tank in the duplex system.

Water softeners do not go to zero grain hardness. Typically, 0.5 to 1.5 grains of hardness is the range of softening achieved in a water softener and this level of hardness is acceptable in a Cemline Unfired Steam Generator.

While this method will somewhat oversize the softener, it will provide complete protection for the unfired steam generator in the event that it is steaming at full capacity for a 24-hour period.

We also recommend the total dissolved solid blow down system to keep the interior of the boiler clean in case of hardness (TDS) carry over from the water softener and to remove any dissolved solids (other than calcium) which might precipate in the steaming process. We understand modern water softeners have very little if any chloride carry over, but the engineer should verify with the water softener manufacturer that there is little or no chloride retention from the softener.

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