This article first appeared in Practical Engineering 1940 Vol1 No5. The information that is contained herein is accurate as of 1940. This article describes modern methods employed in Industrial Engineering at the time.
Apart from the simple direct addition of chemicals, such as soda ash, or tribasic phosphate of soda, it will be remembered that there are two standard methods of softening water for steam and hot-water heating boilers. These are the use of lime and soda ash, and of base exchange plant. Both of these have inherent advantages and disadvantages, according to the conditions, but another principle, the importance of which is now being more and more realised, is a combination of both systems.
The water is first softened with lime and soda ash, which removes both the temporary and permanent hardness down to about 5-6 degs. total (grains per gallon), as well as any acid and finely suspended mud that may be present. Most modern types of lime and soda ash plant also now have separate closed pressure sand filters for the final clear water, equal to a town’s supply, and claimed to be much superior to the wood wool filter.
Advantages of the Dual System
The almost completely softened water is then passed through a base exchange plant, which automatically reduces the hardness to zero (according to the soap test), under conditions that only a very small amount of soluble salts are added to the water. If this latter method is used direct, there is apt to be a large amount of such salts left in solution, which causes trouble in steam generation with frothing and priming, while the lime and soda ash principle may give partially softened water with as much as 5-10 degs. total hardness unless scientific methods of control are adopted, especially when the raw water varies in quality.
This dual system has obvious advantages, both for large engineering works and other industrial steam generation plant and for power stations, while a new and interesting field is in the modern liner, for softening the boiler make-up water. Experience in the latter connection has shown also that in many cases the base exchange plant can be operated in conjunction with lime treatment plant instead of lime and soda ash, when the hard; water contains little permanent hardness (calcium sulphate).
New Softening Plant
A good example is the softening plant recently installed by the Paterson Engineering Co., Ltd., of London (Windsor House, Kingsway, W.C.2), on board the Cunard-White Star liner Mauritania. The plant is on the principle of lime cream treatment, filtration through closed pressure filters, and base exchange treatment, having a normal capacity of 100 tons (22,400 gallons) per 24 hours, that is 9,3001bs. per hour, with a settling time in the reaction tanks of three hours.
When the permanent hardness is relatively small in amount it can be taken care of by the base exchange plant direct; that is, the addition of soda ash can be eliminated. Alternatively, if the permanent hardness is high and the temporary hardness low, then the lime treatment is eliminated and soda ash used, and whilst the best methods to adopt depend upon the individual conditions not only the quality of the water but also the type and operating rates of the boiler, there are many industrial establishments where such dual combinations of water softening for boiler feed purposes can be employed to great advantage.