blowdown on boilers vs. cooling towers

[mufan1]

Hi,
since starting my carreer with a water treatment company, I’ve always calculated boiler blowdown using the following formula:

BD = evap rate / (cycles -1)

This is an equation that I learned in cooling tower training but I’ve always used it for boilers too by replacing evap rate with steamrate. However, after a customer looked at me funny when I calculated their boiler blowdown I found the formula below, which gives you a different answer:

(1/cycles) * steamload = BD

Can you help me understand why there is a difference between the way cooling tower blowdown and boiler blowdown are calculated? I can't wrap my mind around why there would be a difference in the way cycles are calculated for the two since they both have makeup, blowdown, cycles, and steam. What am I missing here?


thanks.

 

[rmw]

Cooling tower loses a percentage of the liquid to evaporation and hence concentrates the cycle build up as a function of evaporation while boilers evaporate every unit of water that enters the boiler one pound in, one pound out. When it evaporates, it leaves behind the chemicals it brought in with it and the dissolved and suspended solids as well.

There is a difference, however. In a relatively tight steam system will have less 'stuff' in the water to concentrate in a boiler than a CT will, because a CT acts just like a sponge to pull out of the air whatever the air brings in with it, plus having to deal with what ever the make-up water brings in with it too.

I don't know if your formulae are right or not, so this is not an endorsement of those. You might get hold of a book called "steam" by B&W and see if it has any info about boilers as to what the practical limits are. Sorry, it has just been way too many years since I did boiler chemistry to remember the parameters.

rmw

 

[MJCronin]

The appropriate amount of boiler blowdown is empirical and depends a lot on the quality of the make-up water fed to the system.

It really has no similarity to cooling tower blowdown.

Recall that there are two typcal types of boiler blowdown: Continuous blowdown and muddrum blowndown.

The continuous blowdown draws from just beneath the water level in the steam drum. It is designed to draw all lighter undesireables from near the liquid surface. It's function is a lot like the skimmer on a swimming pool.

Continuous blowdown is routed to a blowdown tank and usually adjusted via a needle valve. Flow is typically 0.3 to 2% of total steam flow. The boiler vendor's recommendations are usually followed.

Intermittent or "muddrum" blowdown is done about once per shift or per day. and it is simply a blast from the bottom drums of the boiler. This is done, of course, to purge dirt and sediment and is performed more often after internal cleaning activities.

 

[racookpe1978]

Well, the two formulas are very similar:

(Flow rate)/(cycles) or (flow rate)/(cycles -1)

For an emperical formula - is there enough difference to matter?

Or is the continuous surface blowdown mentioned above part of the missing "1"