John Honeck

If you read the resume you know, I know a thing or two about boilers. I started a new group on google groups, follow the above link to see it. The following is a sampling of the type of conversation I hope to get into, its a reply I wrote to a poster about surface blowdown.

You will need to know the pressure of the steam drum to do any
calculations. The degree of superheat isn’t relevant as that energy is
added to the steam.

When you refer to blowdown I assume you mean surface or continuous
blowdown. Bottom blowdown is usually too dirty and intermittent to
consider heat recovery from it. Usually we use the energy from the
surface blowdown and recover it in two different places, as low
pressure flash steam, and with a heat exchanger to preheat the boiler
make-up water.

I’ll run through some of the calculations, but you will need to know
the operating pressure of the boiler, the low pressure equalizing line,
and temperature of make-up water to complete the Calcs yourself.

First off, lets discuss the process. You say that you have a 1%
blowdown rate. For those who don’t fully understand I’ll
elaborate. Let’s say the boiler is running at 100,000 lbs/hr of
steam production. So we are bringing in boiler feedwater with a
certain amount of dissolved solids in it and heating it to boiling temp
and letting the water vapor leave as pressurized steam. As the steam
evaporates it leaves behind the dissolved solids in the water. If we
let it run like this the concentration of solids in the boiler water
would raise higher and higher. Too high of solids concentration leads
to problems such as priming etc. To control the solids left behind, we
drain off some of the boiler water continuously which is replaced with
feedwater with a lower concentration of solids. So using the example
of 100,000 lbs/hr capacity we’ll actually have a feedwater flow rate
of 101,000 lbs/hr with a blowdown rate of 1000 lbs/hr.

The boiler water is at operating pressure and mostly at saturated steam
temperature, so let’s assume for discussion you are running at 250
psig. Referring to a steam table at 250 psig the temperature is 406 F
and the sensible heat in Btu/lb is 382. At 1000 lbs/hr that’s
382,000 btu/hr going down the drain which is probably worth recovering.

Generally there are two components of heat recovery. The first is
flash steam recovery. In our example we are going to drain the water
at 250 psig and saturated temp. This water will go through valves and
to a lower pressure, where it can’t exist as water at this
temperature and pressure, some will flash off as stem. So we take
advantage of this and design the system around it. Generally we take
the blowdown to a flash separator, which is nothing more than a
pressure vessel that brings the water in the side tangentially to the
walls. The water swirls around the outside of the tank, flash steam
rises out the vent on top, and water goes out the drain. (For a
picture visit http://www.pennseparator.com/pbs12.htm) The first
component of our heat recovery is generally to connect this vent to a
low pressure steam usage, such the preheat steam supplied to the
boilers deareator.

Let’s further assume for this example that your dearator is operating
at 10 psig, and the flash steam from our blowdown will go to this.

>From the steam charts steam at 10 psig has a sensible heat of 207

btu/lb, latent of 953 btu/lb at 239 F. So our water is going to go
from having a sensible heat of 382 btu/lb to 207 btu/lb, the difference
is 175 btu/lb. This times the flow of 1000 lbs/hr means we have
175,000 btu/hr of energy released as flash steam. From the steam chart
above 175,000/953=183 lbs/hr of flash steam at 10 psig is produced.
Another calculation is to take the difference of 175 divided by the
latent heat at the equalizing pressure 175/953=0.183 which means we are
producing 18.3% flash steam.

Now we still have the water left over, 1000-183 = 817 lbs/hr of water
at 239 f is available. We’ll then take this water through a heat
exchanger. On one side of the heat exchanger is the cold fresh make-up
water for the boiler system, let’s assume this is at 60 F, we’ll
also assume further that we selected this heat exchanger with an
approach of 20 F, so our blowdown water will be cooled to 80 F before
going to the drain, the energy recovered is then 817 lbs/hr X 1.0
Btu/lb-F X (239-80)F = 129,903 btu/hr.

So our total energy recovered is 175,000 + 129,903 = 304,903 btu/hr.
Our original blowdown had 382,000 btu/hr of energy, so we are
recovering nearly 80% of the blowdown energy available. Selecting the
heat exchanger with a tighter approach will increase the amount of
energy recovered, but maybe be cost prohibitive depending on the size
of the unit.

There are factory built systems above, I’ve personally used the ones
by Penn Separator the most, and have a lot of confidence in their
abilities. Visit http://www.pennseparator.com/home.htm for more
information.

I hope that helps, if you have any more questions, or need to get a
source to size and price them let me know (send an email to the email
address in the profile). John.