Cleaning Cooling Towers
BACKGROUND
This procedure is generally written for the cleaning open, recirculating cooling water systems requiring acid cleaning due to scale formation on heat exchanger/condenser tubes and/or generally throughout the system.
PREPARATION FOR CLEANING
Prior to any cleaning operation, if at all possible, a sample of the deposit causing the loss of heat transfer should be obtained for analysis. The deposit could be other than typical scale and other methods of cleaning might be more appropriate. In more severe cases, both alkaline and acid base cleaning could be required. In general, for scales comprised primarily of iron oxides, 1104, an ammonium citrate-based product, is used. For scales comprised primarily of carbonate based materials, HSS-CID, highly inhibited hydrochloric acid, is used.
ON-STREAM CLEANING PROCEDURE
An on-stream acid cleanup is one that is basically done “on-the-fly” while the system continues to operate. This type of cleaning will generally work to restore a portion of the heat transfer lost due to deposit and will allow continued operation until a more thorough cleaning can be done. This procedure will normally take from six hours to a couple of days depending on how well the deposit responds to the lower pH required for scale removal. Monitoring heat transfer (or in HVAC, head pressures) will indicate the scale removal progress made during cleaning.
To accomplish the cleaning, the pH of the tower recirculating water is reduced to a pH of 4 to 5 while the system is operating. The inhibitor levels in the system should be increased to 3 to 4 times normal to minimize corrosion in the system. When it is determined the amount of heat transfer required is satisfactorily restored, then the system should be blown down heavily to raise the pH back to a more normal alkaline level and the inhibitor level should be maintained at 3 to 4 times normal for 24 to 48 hours to rapidly passivate the system.
OUT-OF-SERVICE CLEANING
In more severely fouled systems, cleaning when the system is not in service will be required. For this, there are a number of options available. Probably the best option is to isolate the effected heat exchanger(s) and piping and then recirculate the appropriate cleaning acid through that portion of the system. Typically, is used at a 5% to 10% concentration. This mixture will rapidly remove calcium carbonate scale as well as iron and other calcium based deposits. The acid is recirculated for 3 to 6 hours or until the pH stabilizes and it is apparent that the cleaning is either completed or has progressed as far as possible with the solution in the system.
In some systems, it may be impossible to isolate the equipment to be cleaned and the acid will need to be recirculated throughout the entire system including the tower and the tower basin. Under these circumstances, extreme care must be taken as most newer tower systems are fabricated with galvanized metal. There is no currently known acid inhibitor that can protect galvanizing from attack by strong acids such as hydrochloric or sulfuric. Currently, S-CID or SL-CID (inhibited sulfamic acid) are the products of choice when it is necessary to do this type of cleaning. The usually recommended solution strength is 3% to 6% based on the entire water content of the system. This acid solution is then recirculated for 4 to 8 hours or until the pH stabilizes and it is apparent that the cleaning is either completed or has progressed as far as possible with the solution in the system.
ACID NEUTRALIZATION
After cleaning, the systems should be drained and flushed. Then the system should be refilled and recirculated and an alkaline material should be added to raise the pH to over 8. Soda ash is probably the best and least expensive of the materials which may be used. When the pH is stabilized above 8, the normal treatment may be added at 3-4 times the normal recommended dosage or even better, a PT1133, specially formulated pretreatment chemical may be used.
The spent acid in the system may need to be neutralized prior to discharge. However, do not neutralize it in the system as this will cause the contaminants removed to be precipitated out of solution which could cause severe problems in the cleaned system.
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For more information, please contact us at 800-658-7716 or email our sales consultant at [email protected].
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