blowdown on boilers vs. cooling towers
blowdown on boilers vs. cooling towers
(OP)
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.
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.





RE: blowdown on boilers vs. cooling towers
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
RE: blowdown on boilers vs. cooling towers
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.
RE: blowdown on boilers vs. cooling towers
(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"