use of demineralized ater to natural draft cooling tower

[vinay0003]

hi i am a chemical engineer at zinc smelter plant and i plan to add demineralized water to the natural cooling towers that we are using now i want to know what are the implications in this matter the inlet water temp into cooling tower is 72c and the demineralized water is around 45c my point is that addition of dm water reduces coc...

 

[rmw]

You must own a bank if you can afford to add DM water as make up to a CT.

What is the point? Unless of course your source of make up water is already full of chemicals to the point of the COC concentrations.

Is this DM water free surplus from some other process?

If not, this seems a colossal waste of money to me; unless you have plenty of money or extra DM water.

You didn't give us much information on what the option is for MU water.

rmw

 

[vinay0003]

the dm water that i plan to use is excess water that has been circulated to the boiler my main concerned is that dm water is known to be agressive on metals so will it affect the process

 

[vinay0003]

the plant is located in area where water is very scarce we are actually buying water at 800rs for 1 m3....and make up water is highly contaminated ground water...we are planning on adding 300m3/day of dm water to the make up water of CT the CT circulation rate is 3300m3/hr and make up water is 40m3/hr the hold up tank is 900m3 and evaporation is 40m3/hr.
but the pipes are made of mild steel so will the use of dm water cause corrosion problems. presently we are using 8 coc

 

[bimr]

If your cooling tower is already operating at 8 COC, addiing demineralized water to the existing cooling water will not have any effect.

The corrosion problem with demineralized water would only occur if you were operating on straight demineralized water. Once you mix the demineralized water into the cooling water, there will be no effect.

In water short areas, it is common to dispose of wastewater (that still is of relatively good quality) into the cooling tower system. RO reject streams, boiler blowdown, etc. are typically put into the cooling water system.

 

[vinay0003]

If your cooling tower is already operating at 8 COC, addiing demineralized water to the existing cooling water will not have any effect.

Can u xplain y ? how come it doesnt have any effect
and also delta T is now 9c(inlet 45 outlet 36) and wet bulb temp is 30c around here, the CT is now only 60% effective suggest anyways on improving the effectiveness of the CT

 

[bimr]

Should have said no effect on the corrosiveness of the water. I asssume that is what you are concerned with.

If you add demineralized water to another water, the demineralized water addition will simply decrease the water's chemical parameters in proportion to the amount of demineralized water that is added.

For example, add 50% demineralized water, then the hardness, alkalinity, sodium, etc. will decrease 50%.

The addition of demineralized water will in effect increase the cooling tower's COC because the blended incoming water will now be less mineralized.

Demineralized water by itself is corrosive because the water lacks hardness and alkalinity. You typically want a water to be slightly scaling as determined by the Langelier index. However, since you are adding the demineralized water into the cooling tower, the demineralized water will be diluting the chemical content.

It looks like you are planning to add 300 m3/day demineralized water to CT with existing 960 m3/day. That will reduce your incoming water mineral content approximately 1/3 or (300/960 x TDS mg/l).

Cooling tower COC's are typically limited by the hardness, alkalinity, and TSS. You will still have the same limitations.

The water temperature does not appear to be problem. What is your concern with it? You should consider the average water temperature of the blended water across the CT.

 

[vinay0003]

thank u for the information and one last query we are adding dm water that is at 70c to make up water of 36c will it be a problem

 

[bimr]

The effect of the temperature on the water properties will not be significant at that temperature if that is what you concerned with.