Compressed Air
Here at the Titus Company we think of compressed air as the
fourth utility and so strive to make your compressed air
system as reliable as your electrical, water and steam
supplies. What can make this challenging is knowing what the
true needs of the system are now and in the future. A very
simple mistake in sizing, type, placement of the equipment or
future requirements can be very costly in the short term and
into the future. The following is a very simple explanation of
what a compressed air system does.
There are 5 main things the system needs to be doing at all
times:
- Compressing The Air
- Cooling The Air
- Cleaning The Air
- Drying The Air
- Delivering The Air
Compressing the Air
Oil flooded rotary screw compressors are the primary
compressor of choice these days and fully 95% of them are air
cooled. While these machines are relatively simple to
understand and operate, how the machines are controlled(i.e.
variable speed, on/off, load/unload, modulated or other) is
very important to understand. Will there be multiple
compressors, and, if so, how will the machines interact to
provide compressed air energy as efficiently and reliably as
possible. Will your production stop if the primary compressor
goes down, and, if so, should a backup compressor also be
considered? Also important to consider is how much air is
required by the plant now and in the future.
Where to install the compressed air equipment is another
important consideration. By their nature, compressors create
a lot of heat and need a considerable amount of clean, cool air
to operate well. They are also better installed out of the
weather and protected from the elements. Rotary screw
compressors will have oil and air outlet temperatures in the
200 °F range. A typical 50 HP compressor will require
approximately 4,000 CFM of cooling air supplied from an
internal fan, and with a fresh air room requirement of 8,000
CFM. The fresh-air room requirement is generally the
customer’s responsibility and in most cases requires an
exhaust fan. The exhaust fan should be temperature
controlled. As with all rotating equipment, the cooler the
running temperature, the longer the operating life will
generally be . This cooling will have an impact on the quality of
the air the compressor delivers to the compressed air
treatment equipment. Most compressors have a filtration
system for the cooling air before it enters the air cooled heat
exchangers. The cooling air should be as dust free as possible.
The heat generated by the compressor can be recaptured and
ducted into a space that requires heat or can be recaptured by
using a compressor that is water cooled. The heat generated
is considerable and should not be wasted if at all possible.
The quality of the compressed air supplied to the plant starts at
the compressor intake filter. So whatever is in the air where
the compressor is installed will get into your compressed air.
If it is not possible to install the compressor in an area that is
free from chemicals, dust or anything else you don’t want in
your compressed air, then moving the compressor intake filter
location should be a serious consideration.
Basically, what happens in a compressor is that it sucks in a
cubic foot, 1728 cubic inches (12 x 12 x 12 inches) of clean
cool air and turns it into a hot, oily, water-saturated cube that
is now approximately 222 cubic inches in volume and is 6 x 6 x
6 inches in size. You now have a lot of potential energy
because those 222 cubic inches would really like to be 1728
cubic inches in size again. Thus ends the compression phase of
the process. Now this hot, oily, water-saturated air has to be
cooled, cleaned, dried and delivered to where you want to use
it. The compressor still has some work to do though. The first
thing the compressor needs to do is get back as much oil as it
can from the compressed air. This is generally done with a
air/oil separator. This separator is specifically designed to
remove the oil from the compressed air. Some of the
compressor oil will make it out with the air. Typical ratings for
rotary screw compressor oil discharge are in the ppm range,
but if the separator is not properly maintained, oil discharge
levels can be measured in gallons. Gallons of compressor oil
in the downstream air treatment equipment or your process
equipment is generally not a good thing.
Cooling The Compressed Air
The 4,000 CFM of cooling air now has to do its job. The 4,000
CFM of air is forced through the compressor after-cooler by a
fan. Generally, these after-coolers arepart of the compressor
package but can be installed as a stand-alone unit. These
after-coolers look very similar to the radiator on your car.
They have relatively small openings and these openings can
get clogged. When they do, the effectiveness of the radiator
(after-cooler) is reduced. The compressed air is passing
through the tubes and the cooling air is flowing past the tubes
and between the cooling fins and removing the heat from the
compressed air. Generally, these coolers have two sections.
One section is cooling the air and the other section is used to
cool the oil.
As the air is compressed it gets hotter. We call this the "heat
of compression” and that temperature rise gives the air a
higher capacity to hold water. Now, we want to remove that
heat, and once that process begins, the water will condense
and go back to its liquid form.
Let’s talk about water for a moment. The air the compressor
sucked in had some water in it and generally we measure that
by using relative humidity. From here on out we will talk
about dew point as a measurement of the water content of the
compressed air. But let’s first figure out how much water we
are talking about. We will assume that the air coming out of
the compressor is saturated and this will be the case unless the
relative humidity of the air the compressor is sucking in is very
low.
If you have one 50 HP compressor delivering 225 SCFM of
compressed air at 100 PSIG and 100°F and do nothing with that
air after cooling it to 100°F you will put approximately 5
gallons of water into your compressed air system every 8
hours. Let’s say your after-cooler is not performing and the
outlet temperature goes from 100°F to 120°. Then the amount
of water going into the system will be approximately 9 gallons
of water in an 8 hour period. Now, you may never see all of
this water but if the compressed air is cooling as it travels in
the piping network or whenever the pressure is reduced, then
you will see the water. Bottom line you should take care to
make sure you have adequate cooling for the compressor and
related equipment.
The first step in removing this water is a air/moisture
separator designed to remove the liquid water from the
compressed air. This separator is generally installed at the
outlet of the compressor and should have an automatic
condensate trap to dispel the water from the separator. We
will talk about condensate drains in a little while.
At this point in the process we now have cool (100°F), wet,
compressed air with a dew point of 100°F. The air remains
saturated. Let’s talk a little bit about dew point (DP). Dew
point, like relative humidity (RH), is a measurement of the
amount of water in air, but, unlike RH, it is not relative to the
temperature of the air. In all of our discussions the DP we are
talking about is DP at pressure. You should be very careful
when selecting compressed air treatment products because
some manufactures will quote based on atmospheric DP. For
example if the dew point of the air you were sucking into the
compressor had a DP of -40°F after you compressed it to 100
PSIG, the DP would be -4°F.
Dew point is basically the temperature point where the air will
not hold any more water. So, if you had air at a dew point of
100°F and you raised the temperature of the air to 120°F, the
dew point would still be 100°F. Now let’s say you lowered the
temperature of the air to 90°F. What you have done is lowered
the air’s ability to hold water, so the water condenses out of
the air in the form of liquid. If you could remove all of the
liquid water that has condensed out the dew point of the air
would now be 90°F.
Cleaning The Air
So, now we have compressed the air to 100 PSIG, cooled it to
100°F and removed the bulk liquid water. The air is now
usable, but for 99% of the applications we work on, this air
still needs some work. This is where we would apply
compressed air treatment products. The first step in this part
of the process is filtration. Filtration costs money beyond just
the replacement cost of the elements. The additional costs
come into play because most filtration products need velocity
across a filter media to work properly, and this causes
pressure drop. How this will increase your costs is simple.
You paid the power company for the electricity to compress the
air to 100 PSIG and now after the filtration process you may
only have – let’s say – 95 PSIG. The compressor may need to
come on more often or stay on longer to maintain system
pressure at the set-point required for your system. For this
reason it is important to reduce system pressure drop
whenever and wherever possible.
The primary goal of the filtration process is to remove solid
particulate contaminants, liquid water, and oil. The solid
particulate removal is relatively straight-forward. With the
filter media you create a path for the air to travel through that
blocks everything above a given micron size. Typically today
when we are talking about filtration we are talking about
particle sizes in the 1 to 0.01 micron range for pre-filters
(particulate or coalescing) and 5 to 10 microns for after-filters
(particulate). So how big is a micron?
- In one inch there are 25,400 microns
- One micron is 0.00004 inches
- Most people can see something that is 40 microns
- A white blood cell is about 25 microns
So, the filters in today’s systems are capturing very small
particles. The reason for this in a pre-filter is to keep liquid
water and/or oil out of the treatment equipment and/or your
processes. The filters we use to do this are called coalescing-
type filters and the 1 micron filter is typically in place to
protect the 0.01 micron filter. The liquid water you will see
draining out of the filter bowls has been coalesced from water
droplets too small to even see in some cases. There are some
very complicated processes taking place in the filter media to
make them work. You will hear people talk about Brownian
Movement, Inertial Impaction or Direct Interception. Bottom
line, coalescing filters work on the basic principle that if you
force two, 0.01 micron sized drops of liquid into an opening
(filter media), at the same time, that is only 0.01 microns in
size, the two water droplets will be too large to make it
through and they will join (coalesce) with other droplets of
liquid and eventually drain down the filter element into the
bottom of the filter bowl. Once this water and oil mixture has
collected in the bottom of the filter bowl, it will be discharged
by the condensate drain traps. You can imagine that any
significant amount of dirt that may make its way into the filter
media will cause a higher pressure drop. So while most filters
of this type will start out with a pressure drop in the 2 to 3
PSID (Pounds Per Square Inch Drop) range that pressure drop
number can go up significantly. The selection of these filters is
a very important part of the compressed air system. If the
filter is too small, you will have increased your pressure drop
and perhaps reduced its performance. A filter bowl that is too
small may cause the velocity in the filter bowl to be so high
that it picks the water off of the drain layer of the filter
element and carries it down stream (generally referred to as
re-entrainment). Pick a filter that is too big and you have
reduced your pressure drop, but perhaps also reduced the
coalescing action, because the velocity across the filter media
is so low it will not force the small droplets of water or oil
together.
You will hear claims that this filter or that filter will remove
99.9% of the water or oil in a compressed air stream and while
this may be true, remember it is 99.9% of the water and oil
that is in a liquid form.
After-filters are used to collect solid particulates from the air
stream, these filters range in micron size from 1 to 10
microns. Some can be custom designed for various
temperatures and micron rating.
Drying The Air
The general category for all this equipment is called
compressed air treatment products, which may also include
filtration, but for our discussion and this section we will just
consider dryers. There are basically three ways to dry
compressed air; refrigeration, membrane and desiccant dryers.
But first let’s talk about dew point. The following is the
definition from Wikipedia, the free encyclopedia:
The dew point is the temperature to which a given parcel of air
must be cooled, at constant barometric pressure, for water
vapor to condense into water. The condensed water is called
dew. The dew point is a saturation point. When the dew point
temperature falls below freezing it is often called the frost
point, as the water vapor no longer creates dew but instead
creates frost or hoarfrost by deposition.
The dew point is associated with relative humidity. A high
relative humidity indicates that the dew point is closer to the
current air temperature. Relative humidity of 100% indicates
the dew point is equal to the current temperature and the air is
maximally saturated with water. When the dew point remains
constant and temperature increases, relative humidity will
decrease.
When you purchase a dryer, the first question you need to
answer is, "at what dew point does the compressed air in my
system need to be?” This may be dependant on various
processes or just dry enough to keep the water out of your
compressed air.
