Using high conductivity water / un demineralized water as cooling water & effect on heat exchang

[RASitepu]

Dear all,
in my plant, we used raw water or undemineralized water as cooling water. Is that giving a bad effect on the heat exchanger tube, for example higher rate of corrosion or fouling? The treatment that is used for the cooling water is only NaClO injection.
Because I know most of cooling water especially in coal fired power plant use demineralized water as cooling water.
Thank you.

 

[don1980]

All raw untreated water isn't the same, so it's not possible to answer that question with a yes or no. The mineral content of water varies widely, depending on the specific source of that water and the local geological conditions. Whether treatment is needed depends on whether this is once-through water or a closed loop cooling system, and the chemical analysis of the water source. The pH and the mineral content are the main details to know, and to manage.

If it's a closed loop system like a cooling tower, then it's always necessary to actively monitor the water chemistry and to treat it with additives. If it's once-through water, then the need for treatment entirely depends on the chemical analysis of the source water.

Regardless of whether it's once-through water or a closed loop system, you need to prevent the temperature rise from getting too high (above ~120F). Otherwise calcium carbonate will precipitate out of solution and build up on the internal surfaces. Exchangers need to be designed to ensure that the cooling side outlet temperature doesn't get too hot and necessitate frequent outages for cleaning.

 

[zdas04]

RASitepu, in addition to Don1980's excellent comments, you should look to the sea for answers. A steam ship uses a LOT of cooling water that is not great quality. Condensers on even relatively small loads are huge and pass a very large quantity of water, with a very small dT, and it is once-through. When a condenser starts losing effectiveness, you usually find an octopus or stingray draped across the tubes instead of scaling. It doesn't take much of a strainer to keep stingray's out, but marine engineers don't put strainers on those huge pipes because the cost/benefit isn't there. In other words, as long as the dT is low and the outlet temp is below about 120°F, precipitation and phase-change scale are manageable without chemicals. It is when we start doing closed-loop and high dT that things get ugly.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[bimr]

Coal fired power plants generally DO NOT use demineralized water as cooling water. It is too expensive.

The quality of the raw water determines the cycles of concentration for the cooling tower. If the raw water was of very poor quality (Sea Water for example), you would use the raw water once through.

Generally, you would want to have a water quality that will allow 6-9 cycles of concentration for economical operation of the cooling tower. So the contaminants of concern such as hardness would concentrate 6-9 times, but still be below the concentrations that will not have a detrimental effect on the cooling tower operations and condenser operations.

 

[ashtree]

The demineralised water in power stations is generally only used for boiler feed. Gas fired power stations sometimes use demineralised water for power enhancement by injecting it into the intake of the gas turbine to reduce intake temperatures.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"