Trying to correct another writer on a political/science type discussion page, but am at a job site and don't have my usual "fundemental" engineering reference books.

So send me back to the classroom: He (a political writer) is talking about the visible exhaust gasses from a (typical) power plant; and is somehow confusing and combining his descriptions of steam condenstion (from trace steam and steam traps), the visible exhaust gas plumes (which he calls "condensation"), actual condensing steam (described as if from the turbine exhaust !), and the "steam" visible above the cooling towers.

Of the four, he does properly identify the condensing water vapor coming "out" of the cooling tower. In the cooling towers, cool, somewhat dry air is pulled through the hot service water leaving the condensors, the outside air removes the heat (not water!) from the hot service water and then is pulled up the cooling tower exhaust. The hot air is pulled up and away from the heat exchange surface either by natural draft (in a tower) or by fans (forced draft).

No problems explaining that process.

The small amounts of trace steam that are visible are easy to explain. Some is from leaks of course, but very little. Vents, blowdown pipes, steam traps, drains, .... None of the trace steam that might be in any generic photograph (of a properly maintained plant at least) would be from safety reliefs or from the combustion process itself though, and none of this vented steam would exhaust from the stack itself.

Third source the writer mentions is "condensation" of the water vapor: I assume the original writer is talking about the water vapor coming from combustion - but he would be correct only if natural gas or oil were being burned though, right? "Pure" dry coal has almost no hydrocarbons by weight - so it "should" burn the carbon directly into CO2 - and that produces no water, only CO2.
2C + 2O2 -> 2CO2

If you had a oil burner (rare nowdays) with hydrocarbons as fuel you'd get a modest amount of H2O as a combustion product. With a natural gas plant, you'd still get hot water vapor coming out of the exhaust stack, but only a little.

(I don't think the writer has somehow confused the steam condensing in the main condensor with the water vapor coming out of the stack, but that's also possible.)

Fourth source of visible vapors would be water vapor from the surrounding air condensing out in the stack - but I can't remember the circumstances (hot or cold day, humid or dry surrounding air, greater or lessor amount of fuel being burned) that make this plume somedays, but invisible others.

Can somebody verify the cause why sometimes there is a visible stack plume, and sometimes there is no visible plume?

All of the white smoke-looking stuff you see off of a cooling tower is warm, moist air contacting the cooler surrounding air and the excess humidity in the warm air condensing into a fog (think of a glass of iced liquid sweating, it is the same thing).  The reason it is intermittent is that generally the outside air temperature is above the dew point of the air that went through the cooling tower.

The amount of process steam that exhausts directly to atmosphere is tiny and is not visible more than a few feet away from the source.

Any stoichemetric combustion results in CO2 and water as byproducts.  This water is also in the from of humidity and it is very common for the water vapor to condense after exiting the stack as the stack gas temperature falls below its dew point.  Again this can be intermittent because the scrubbers associated with modern stacks cool the exhaust gases considerably and often the ambient air temp is above the dew point of the stack gases.

The nastier possible combustion products (e.g., SOx and NOx) are invisible.  Plants work very hard to ensure that their stack gases are as clean as possible and the plumes that you do see are generally benign.

Our plant has a wet Scrubber before our smoke stack. Caustic (for Ph control) is add, then a venturi pressure drop. This cleans our flue gases, but it give a white billowy plume from the stack. It can be more or less depending on our load. We also have 2 large coal fired boiler that share a common stack(globle valve). when the outside temperture gets below about 30* F you can see a slight vapor. When both boilers are firing hard it is more promanant, then if only one boiler is online at half load we may see very little vapor. This vapor is the kind that is invisible for the first foot or so then condensing rapidly causing a light transparent vapor. 

Arguing with a journalist is like wrestling with a pig.  Both of you will get dirty but the pig likes it.

Most visible plumes that the public is aware of are either (1) the cooling tower plume, air saturated with water as described in a post above, and/or (2) products of combustion or the effects of the clean up of products of combustion.  To the trained eye, the deaerator plume in a fossil plant is visible and detectable, but so minor in comparison to 1 & 2 above so as to be negligible.  If you can see the steam from leaks and steam traps from the road you have SERIOUS problems.

The steam that drives the turbines that condenses in the condenser produces no visible effects.

The public, including that writer who wants to think of those plumes as "pollution" most often won't be dissuaded by the facts, no matter what they are.  It is futile to try to educate untechnical people with techincal facts.

Your handle states that you are Nuclear, and nuclear power is the cleanest and safest out there, but to those who hate it (as well as any other source of power other than wind or solar) will only see the cooling tower plume and think to themselves 'look at all that pollution.'

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