- Hi, John here. In this video, we're going
to be looking at deaerators. I'm gonna show you three
different deaerator designs. I'll show you all of the connections and the systems associated
with deaerators. I'll explain to you how they work, and then I'll tell you exactly why we have deaerators in the first place. If you're working in the
power engineering industry or most industrial settings
that use steam systems, then you're highly likely
to encounter a deaerator. A deaerator has three main purposes. Its primary purpose is to
release non-condensable gases from the boiler feedwater. Its secondary purpose is to
heat the incoming make-up water before it's sent to the boiler and to heat the condensate
that's returned from the steam system before it's
sent back to the boiler again. Its third and final purpose is to allow us to store a certain amount
of water that can be sent to the boiler as the steam
demand on the boiler fluctuates. So it gives us a storage
capacity of boiler feedwater. When we're talking about
non-condensable gases the two gases that
we're most interested in are oxygen and carbon dioxide. Also referred to as CO2. These non-condensable gases are present in the boiler feedwater
and they're referred to as dissolved gases because they are dissolved into the water. The problem with having
oxygen in the boiler feedwater is that it can lead to corrosion. The problem with having carbon dioxide in the boiler feedwater is that it makes the water more acidic. A typical boiler will use feedwater that has a pH value of
between eight to 11. So if we have excess CO2 in the feedwater we're going to have
feedwater with a lower pH, and this is not desired because
it may lead to corrosion. So in order to protect our boiler and a lot of the components
associated with the boiler including piping and
valves, we need to remove as much oxygen and CO2 from the boiler feedwater as possible. A typical deaerator will be
designed to mechanically remove oxygen down to a level of
about seven parts per billion. Any oxygen that remains after being stripped by the deaerator
will be removed chemically. These oxygen scavenging chemicals are typically sodium
sulfite and hydrazine. Let's now have a look at
our first deaerator design. I'll explain to you exactly how it works and how we can get those
non-condensable gases out of the boiler feedwater
and how at the same time we can increase the water's temperature in order that we don't
thermal shock the boiler when we send the feedwater to the boiler. So here's our first 3D
model of a deaerator. I'll do a little spin so
you can have a look at it. You can see it's horizontally orientated. It's actually a pressure vessel, although the pressure associated with the deaerator is quite low. You're looking at about .5
bar which is about seven psi, and the temperature of
the deaerator should be approximately 105 degrees celsius
or 217 degrees fahrenheit. Pressures and temperatures do vary based upon the deaerator design. Let's have a look at the systems that the deaerator is associated with. We can see here we've
got a condensate inlet. This is a return from a steam system. When the steam has done its
work and given up its heat it's gonna condense and
we're gonna need to return that condensate, that
water, back to the deaerator where we can increase
its temperature again and remove some of
those condensable gases, if they're present, before we
send it back to the boiler. So that is a steam system
return, or condensate inlet. We have a steam and a gas vent. That is this connection here. Although the deaerator is
under a very low pressure, as I mentioned before,
about .5 bar or seven psi, sometimes a little lower than that, we actually have a vent that vents to atmosphere continuously, and this vent allows us to constantly vent the non-condensable gases that are being liberated from the water. It's not possible to separate
the non-condensable gases from the steam in an efficient manner, so what we do is we actually
allow a bit of the steam escape with the non-condensable gases. For this reason you'll often
sea a little bit of steam always escaping the deaerator vent. And this is normal. You have to be careful that
you don't vent too much steam 'cause this would be a waste of energy, but at the same time,
you also need to ensure that you're removing all of
the non-condensable gases. If you don't remove all
the non-condensable gases what'll actually happen is you
will have an area inside here that is full of non-condensable gas, or we'll say air, and this
is known as air blanketing. If you have air blanketing
then it is very difficult to reduce the oxygen and
CO2 levels of the water to an acceptable level. Let's go over here and look
at the next connection. We've got a steam inlet over here. We use the steam to heat up the water within the deaerator. This is low pressure steam, and sometimes in a power station it will be exhaust steam from the turbine. It is a heat source. As we increase the temperature
of the water inside the deaerator the solubility
of the condensable gases, the solubility of the
dissolved gases reduces. That means as we increase the temperature, more oxygen is liberated from
the water and so is more CO2. So this is actually what we want. So we need to increase the
temperature of the water until we come close to the saturation temperature of the water. The saturation temperature
is the boiling point. We don't want our water to boil, because we want to keep it in liquid form. But we do want to get it to
within two degrees celsius or two degrees fahrenheit
of the boiling point. If we can do this then
we are massively reducing the amount of oxygen and CO2 that will be present in the water. And at the same time, reducing
the likelihood of corrosion because we're removing
the non-condensable gases. Over here we have a safety relief valve. We may have a safety relief
valve on the deaerator, but you may also have a water column. It really does depend upon the design. Remember, deaerators are not operating at very high pressures. So sometimes the water
column will be sufficient. Although many, many deaerators do have a safety relief valve, this is definitely not unusual. So how are we going to
liberate those gases and also heat up the water? Well this is our first
design of deaerator. We feed the condensate and also
the make-up water sometimes in through this connection here. And if we zoom in we can see
it comes out of a spray nozzle. The spray nozzle will be a
spring-loaded spray nozzle normally manufactured
from stainless steel. As the water enters a
water box around here or sometimes slightly higher up, perhaps this section here, the pressure inside the
water box will increase. It will overcome the spring
pressure of the valve and the water will spray
out of this spray nozzle. The reason we spray the water out is because we want to have
a good contact surface area between the water and the steam, which is actually filling
up this whole section here. When we have a good contact
surface area between two phases, such as water and steam, we have a very good heat transfer rate. This allows us to
quickly heat up the water and liberate a lot of those non-condensable gases very quickly, and they'll accumulate
within this dome here. Also this section here is
referred to as a spray shroud. And those non-condensable gases will then travel through the vent. I'll zoom in over here. They'll travel up and out
and through the vents. The vent itself here looks very large. You can see up in the corner. Although it looks large,
don't be deceived. What you'll actually have is a round plate with a hole bored in the middle. And you'll use that to restrict
the flow through the vent. This round plate has a hole in the middle, and it's actually
referred to as an orifice. That's just a snazzy
engineering word for hole. And by increasing the size
of the hole in the plate you can vent more air and steam, and if you reduce the size of the plate then you will vent less air and steam. So very simple but also very effective. Once we've sprayed the water and it's been heated up by the steam, the water will then drop
down through this tray here. You can see that the yellow arrows actually indicate the steam, and the steam is traveling up. The water's coming down. And it's going to land in our deaerator, particularly in this section here. This is a preheating section. We're heating up the water
which will normally come up to, say a level of around here. And we're heating up that water to liberate more of the
non-condensable gases. But also that is a secondary
function of the deaerator, we need to preheat it and get it ready to be fed to the boiler. The steam connection, as
you can see over here, it comes down. We have a sparger steam pipe. Sometimes people refer to
it simply as a sparge pipe. And you can see the steam
will come down here, come along here, and we
will spread the steam out, and it's going to come
out below the water level within the feedwater tank. And it's gonna bubble
up through the water, again, heating the water and releasing some of those non-condensable gases. So that's essentially how
this type of deaerator works. This type of deaerator is known
as a spray type deaerator. The reason we have the baffle plate here is to restrict the flow slightly so that we can have the hotter water coming over to this side on the right. And we call this side on the
right the deaeration section, whereas the side on the left
is the preheating section. Once the water's been heated and dearated, it's gonna come over to this pipe here and this connection here
is for boiler feedwater, and the water will drop down and go to our boiler feedwater pumps. Important to realize here that condensate from the steam system is
sent to the deaerator, but once it's been
deaerated and heated again it becomes feedwater. Make-up water is water
we add to the system to replace any losses we might have had is totally untreated. It will have high levels
of oxygen and CO2, and we need to strip those out. Once the make-up water
enters the deaerator and is treated it becomes feedwater. Let's go and have a look at a
slightly different design now, which is a spray scrubber design. So here we have a
slightly different design. Let me go in a little bit here and show you some of the parts. You can see we have an air and steam vent, same as before. We've got some make-up water
coming into the system now. That wasn't shown before. We've got a safety relief valve. We've got a condensate return. And then we have quite a different setup inside the deaerator, which we will discuss in a moment, and some steam coming in as well. So some of the design is quite similar to the deaerator we saw before, but there are some differences. You can see here that we've
got make-up water coming in. And it is going into the water box, and as the pressure in
the water box increases, the valves are going to open, and the water's gonna be
sprayed into the deaerator. So that's much the same
as what we saw before, but we can also see that the condensate is not going into the water box. It's actually being fed
straight into the deaerator. Now, why would we do this? Well, the reason we do this
is because make-up water has a high concentration
of oxygen and CO2, depending on where you get the water from. It may be a river, it may be a lake, it may be groundwater,
it may be city water, and that means we're going
to need to treat that water a lot more than perhaps a condensate. Let's imagine for a moment
that our steam system is fully enclosed, it's
totally sealed off, there's no chance of any oxygen or CO2 getting into the system. Then we can assume that the condensate that we sent back to the
deaerator is actually quite clean. That means it has low levels of O2, low levels of CO2 because it did not encounter any oxygen or CO2 in the system. So it cannot have absorbed
any oxygen or CO2. So there's no need to
treat that condensate to the same level as for the
make-up water, for example. So that's the reason why the make-up water will pass through the spray nozzles and the condensate will not. It will just be returned to
the inside of the deaerator. Now that's not to say that we don't need to do anything with the condensate. We still do. And what we're gonna do with
it is still try and liberate any non-condensable gases
if they are present, but we're also going to
eat up that condensate before we send it back to the boiler. So we have our water
that sprayed out here. This is our make-up water. It's gonna fall into this tray here. The condensate comes down as well. And the water, this mixture of
condensate and make-up water will then go into this pipe here. And let's follow the pipe down. You can see it's coming
down here, down here, and it's going into this section here. And what we have is a scrubber. This is a spray scrubber type deaerator, because of the scrubber. Inside the scrubber we have a steam inlet. You see our steam
connection comes down here, and the steam itself will come down here, and it will come out
of these sort of vents or these little long holes, the grills, maybe, we could call them. Steam will come out and it
will bubble through the water. Remember, the water's draining down here. It's probably gonna fill
up the entire scrubber up to about here. The entire thing is full of water. The steam comes down. The fresh steam heats up the water, liberates some of those
non-condensable gases, scrubs it essentially, that's what we refer to as scrubbing, and slowly as the water
passes up through the scrubber the non-condensable
gases will be released, the water will be heated,
and the steam will accumulate at the top of the deaerator in
the steam space around here, and the water level will
be somewhere around here. So that's a spray scrubber type deaerator. Once the water's been heated and the non-condensable
gases have been removed, we can then send the feedwater out through the connection here, and it will go usually to a single or multi-staged centrifugal pump or series of pumps before
it's fed to the boiler. Although it's not shown on this
model or any of the others, keep in mind that chemical
dosing often occurs in the deaerator, on
the make-up water line, or after the deaerator before the boiler. The position of chemical
dosing really does depend upon the steam system design. For example, if you're
drawing make-up water from a reverse osmosis plant, the water tends to be quite acidic. So you'll need to treat that water before it comes to the deaerator. So you'll add chemical treatment
on the make-up water line. Other times, it won't be acceptable to chemically dose the deaerator, so you'll dose the boiler feedwater as it come out of the deaerator. But as I mentioned it really
does depend upon the design, but do keep a lookout for
your chemical dosing position because you're highly likely to have one. The deaerator's only gonna strip down the oxygen to seven parts per billion, and you'll need to use
oxygen scavenging chemicals to remove the rest of that oxygen. CO2 is not such a big issue because when you heat water close to its saturation temperature
or boiling point almost all of that CO2 is going to be removed from the water. Let's go and have a look at
that final design of deaerator, which is slightly different
from the other two that we've seen thus far. So here's our final design of deaerator. As you can see the deaerator shown looks different to the
other two that we have seen. This type of deaerator is known
as a spray tray deaerator. Many of the connections
are the same as before. We can see that we've got condensate not going into the deaerator. It's actually going
into the feedwater tank. It's important to make a distinction here between the feedwater
tank and the deaerator. When we have a design like this, the deaerator is actually the top section. The section above the feedwater tank. We deaerator in this section. Once the water drops down
into this holding tank it becomes feedwater. And that's why we call
this our feedwater tank. I know that might seem fairly obvious, but a lot of the time when
people talk about deaerators, they're referring to the entire package, the feedwater tank and the deaerator. But it's important to make a
distinction between the two. And the deaerator is usually mounted above the feedwater tank. Although, as we've seen, it's also sometimes
inside the tank itself. Let's have a look at our connections. We've got make-up water. This is the water that really
does need to be treated as it enters the system. We've seen that the condensate goes straight to the feedwater tank so it's most likely a closed steam system. Got a relief valve connection. We have a vent. The deaerator, which is the entire piece we're looking at here. You can see that the
water's being sprayed out into this section and it's going to land on these trays. Here's one tray, here's another tray, here's another tray. And these top trays are
referred to as heating trays. Sometimes you'll actually
refer to these trays as first-stage trays. The trays lower down, that
would be these ones here, these are referred to
as air separating trays, also sometimes referred
to as deaeration trays or secondary stage trays. We've got an access door on the side which allows us to get to the trays, although looks slightly small here. And we've got a steam inlet. The steam comes in. You can see that it actually comes in, comes down, it'll go around the trays, and then it will come up
and pass through the trays, and it's flowing in a counter
flow direction to the water. Sometimes people call
counter flow, contraflow. So don't be thrown off by that term. So we have a counter of contraflow flow through the deaerator,
the steam is coming up, the water is coming down. The trays give us a very
large contact surface area between the water and the steam. That gives us a high heat transfer rate and allows us to liberate a lot of those non-condensable gases very quickly whilst also heating up the water. Believe it or not, about 95%
of the non-condensable gases are actually stripped
out in this section here, with the remainder being
removed by the trays. The trays, though, are required. If we don't have the trays
then we won't be able to strip the oxygen down
to seven parts per billion. The non-condensable gases
go out through the vent, once again, with a little bit of steam. The deaerator water falls
into the lowest section here, and it will drop down into this hole and it will be discharged
to the feedwater tank, and the feedwater level
will be approximately here, which allows us to
maintain a buffer of steam in the top of the tank. We also have a steam
inlet on this side here. You see we've got some low pressure steam, or exhaust steam, coming
in through the pipe, goes to our steam sparge
pipe or sparger pipe, comes along here and it will bubble up through the feedwater,
heating the water up, liberating some of those
non-condensable gases again, and then the water is ready
to be sent to the boiler, or specifically the
boiler feedwater pumps. If you do have chemical dosing inside the feedwater tank with this design it is ideal because we've got steam bubbling up through the water. It agitates the water and this means that if we add chemicals
inside the feedwater tank then those chemicals are gonna mix very effectively with the water, which means that the chemicals are going to reliably do their job. There's no point adding
chemicals to a tank that's just completely
stationary and not agitated, because chemicals are
not gonna mix properly and they won't work effectively. Once the deaeratored and
heated up water is ready it's gonna go out through
the exit discharge here. And it will go to the pumps. We also have a drain on
the feedwater tank here, and we may need to drain
the deaerator periodically, perhaps once a year when we
do our boiler inspections. And we will be able to inspect
the inside of the deaerator, looking for things like corrosion,
cracks, scale formation, sedimentation, and anything like that. So in summary, deaerators
are designed to liberate non-condensable gases from feedwater, which can cause corrosion in the system. Deaerators also increase the
temperature of the feedwater so that we don't thermal shock the boiler when the water enters the boiler. And finally, deaerators, or specifically the feedwater tank itself, gives us a reserve
storage capacity of water that we can send to the
boiler as steam demands on the boiler fluctuate. The typical storage capacity
for a feedwater tank is 10 minutes when the
boiler is under full load. There are two main designs of deaerator. These are the spray
type and the tray type. Deaerators allow us to mechanically remove non-condensable gases from the feedwater. After we've done this we're
going to chemically remove the remaining oxygen
using oxygen scavengers. These are typically sodium
sulfite and hydrazine. If you like this video then please do like it or share it on social media. We really do appreciate it. Please do subscribe to
the YouTube channel. And if you wanna learn
more about deaerators then check out our introduction
to steam video course or our introduction to
deaerators video course. Thank you very much for your time. And I hope to see you
on another video soon.
