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>> Bob Miles: What I’d like to do now is turn over the presentation to Shawn Crowe.
Shawn will be looking at opportunities related to the supply side.
>> Shawn Crowe: Thank you, Bob. I am Shawn Crowe and today I will be focusing on the
supply side of the compressed air systems. There are plenty of opportunities on the supply
side of the compressed air systems, but due to their nature, they tend to have higher
investment costs. The demand side is often considered first and often more attractive
due to their payback. As Bob mentioned earlier, 76 percent of your cost of compressed air
is tied up in energy over the first 10 years. So you cannot neglect your supply side of
your system.
The following slides are going to be focusing on control. There are six types of controls:
Stop/Start, Load/Unload, Dual/Auto Dual, Modulating, Variable Displacement and Variable Frequency
Drive. A Start/ Stop Compressor is just as it sounds. When you need the demand, your
compressor will be started up by the controls. Otherwise, it will be shut off.
Load/Unload allows your compressor to continuously run. It loads the compressor when needed and
idles it when you don’t need it. A Dual/Auto Dual functions the same way as your Load/Unload
with an additional feature; it has a timer so your compressor will shut off when it’s
not needed. Modulating – It restricts air inlets to change the output of your compressor.
Variable Displacement – It is also known as the rotor linked adjustment. It allows
for a reduction displacement without changing the pressure on your compressor. It’s usually
done with a spiral, a turn, or a poppet valve. A variable frequency drive actually changes
the speed of the motor on your compressor to adjust for the CFMs needed.
For today, we will be focusing on Load, Dual and Variable Frequency drive controls. But
first on to our next question: “What type of control does your facility have on your
compressor system?” You should be seeing five options available to you: Load/Unload,
Dual/Auto Dual, Modulating, Variable Displacement and Variable Frequency. Most people have answered.
It seems like we have a variety of answers here. It seems like the majority has the Variable
Frequency which hopefully is being used for a trim motor. If employed properly it can
very energy..it can increase the energy of your system. Load/Unload seems also to be
very popular, which is of no surprise. It can be easily put on your compressed air system.
We are going to talk about Load/Unload in detail a little more. The first bullet here
actually surprises most people. Thirty percent of the rated load of your compressor will
be used when your air compressor is unloaded. So this means if you have a 100 horse power
motor, 30 horse power worth of work is going to be done, with not adding anything to your
compressed air system. This can obviously be bad if you have an inefficient setup. Mainly
if you are unloaded for long periods of time because you are spending money on energy that
you are not putting to useful work.
A Load/Unload control on a compressor should be used for typically a one-compressor system
where you are cycling on and off frequently. This, of course, is better than the Start/Stop
because you’re not having to turn on and off the compressor frequently, which could
decrease the longevity of your compressor.
On to Dual/Auto Dual, with the additional feature of the timer, actually eliminates
the periods of time being unloaded that we are worried about. So this means that you
can apply it to a multi-compressor system as a base compressor. When I am talking about
base compressors, I am distinguishing between two types of compressors and a multi-compressor
system. The first being your base. What happens is you have a base demand you know is going
to be needed and these compressors tend to run at 100 percent. The reason why this is
important is because a compressor other than when it’s off is typically most efficient
when it’s at 100 percent capacity.
The other type of compressor is your trim compressor. What this does is it meets the
demand by fluctuating, cycling on and off or its variable frequency ramping up and down
to meet the changes in demand in your system.
Now on to Variable Frequency. Variable Frequency Drive can be very useful on your compressor
system. If you have a sensitive piece of equipment that doesn’t like large pressure fluctuations,
this can be a great control for you. Due to its ability to vary the motor speed, and more
accurately meet the CFMs needed to make up the difference of CFM loss in your system,
it maintains a very narrow pressure band compared to the other motors. It also, due to its nature,
has soft start properties where it ramps up. Which means, it doesn’t have the initial
hard pull most motors have when they are trying to get over the inertial forces. Due to this
nature, a VFD can be started up often without reducing the motor life. The other great thing
about the VFD is that you are extending just the amount of energy you need to produce the
CFMs needed for your system. You are not spending more energy to get the same results. Of course,
your question is “Is a VFD always the best choice and is it always cost effective?”
The answer is simply, no. That’s why we are going to discuss when to use a vfd.
If you have a single compressor DOE recommends that a 40-85 percent load should be where
you are operating. You can find this by using data loggers. We use them when we go onsite
to figure out the load by mounting them to one of the legs or phases in your compressor.
What happens is the data logger pulls amperage data along with the time stamp it was pull.
From here you can export in Excel and figure out how often your motor is being called upon
in your compressor, and how hard it’s being pulled. The reason why the 40-85 percent range
is picked is because in a VFD, once you start getting on the top end of that load, 90-100
percent range, you actually start losing efficiency on an efficiency curve. VFDs are actually
detrimental on the high end. If you are too low, you simply lose the cost effectiveness
of it and the payback is gone. A VFD is great as a trim unit in the multi-compressor system
I was talking about. Because it more accurately meets the fluctuations in your system when,
let’s say, large intermittent users pull a massive amount of CFMs, you need one to
be able to ramp up and pull back down as needed. Of course if you have those pieces of equipment
that are sensitive the pressure band range is great for VFD control.
Coming to the payback portion, if you are in a state or region that typically has higher
energy costs, you are going to get payback quicker. If you are in New York, your payback
is going to be a lot quicker than it is here in Kentucky, typically. The other thing to
consider is that utilities will offer you incentives to save energy on your air compressor
system. This is because they don’t want to invest capital money in making new plants.
There were programs like this in our state at TVA and at LG&E.
Before you here, we have a three-compressor system. Two compressors are operating at base
and the other one is operating at the trim. Most likely this facility is a three-shift
facility that uses a skeleton crew in the off shift. As you can see here, the trim compressor
is enough to make up for the skeleton section and there you can see that the CFM demand
is well into that 48-85 percent range which means you are actually beginning to get payback
on the VFD at this time. When people start coming in, your production starts ramping
up, you actually have – are called upon- for the space compressor. What happens here
is the base compressor goes straight to a 100 percent and your VFD comes back down.
This is how it should be optimally figured because your VFD is the best at matching the
demand as it changes. And when you get to your major shifts, you need these two compressors.
Now here is the difference between Load/Unload and Dual/Autodual, when it makes a difference.
If you had a Load/Unload compressor, on compressor number two, after the shift ended for the
day at 7:00 o’clock, your compressor would be idling for the time and it wouldn’t actually
be pulled back up to loaded for most of the day. Which means you would be expending 30
percent of the energy without really getting anything.
Now let’s move away from controls for a second and on to storage. Proper storage can
reduce the compressor cycling. This is done because you are basically adding volume to
your system since volume and pressure are directly related. The more volume you have,
you have a CFM pull, the less the pressure is affected. This reduces the cycling because
your controls are based on the pressure of your system. And if your pressure isn’t
dropping as much, your compressor isn’t having to pull as often. The fact that your
compressor will cycle less will actually increase the longevity of it. We do know that those
startups are *** the motors.
Another thing is we all typically have a piece of equipment that is a major user in our facility.
And if it hits intermittently, bad things can happen to your system if storage isn’t
properly implemented. Sometimes it could pull so much air that other pieces of equipment
might not be able to function properly. Proper storage can actually prevent this from happening.
Now before I go further, I’d like to explain there are two different types of storage:
wet and dry. Wet storage is storage before the air treatment center, so before your dryers.
Dry storage is anything after. The third bullet, facilitate separation and migration – of
humidity is what I am talking about in this case – actually is helped by wet storage.
What you do is you allow the air to get in there and natural condensation happens, makes
your dryer work. It doesn’t have to work as hard. This means you’ll save energy in
the long run. The other thing wet storage can do is it will dampen the pulse of a reciprocating
compressor.
Now before you, you see some Best Practices as recommended by the Department of Energy.
They recommend essentially 2:1 ratio between wet and dry storage. Thirty-three percent
of your system should be wet and 67 percent should be dry. This is the ideal case. The
reason why is you get the best of both worlds. With the wet, you get the aid of separation
and the stops in pulsation. With the dry, you have protection from those large users,
which can be detrimental to your system. If your equipment is humidity sensitive, and
you have a large user without the proper dry storage, the air can actually be rushed through
the dryer. Meaning that the water couldn’t have been effectively been pulled out and
of course this could eventually hurt your system.
The receivers should be based on your trim size. People don’t think that way. They
think “capacity” or they need to add up all three. The reason why, is because your
storage is making up for the fluctuation in your system. Which as I explained earlier,
is maintained by your trim compressor. It is different for a lubricated vs. non-lubricated
rotary screw. A lubricated rotary screw, DOE recommends three to five gallons per CFM.
A non-lubricated – about two gallons per CFM.
Below we see a block diagram of the system. Here would be our wet and here would be our
dry. I can’t stress enough how important a block diagram is. In fact, DOE has the block
diagram as the first step on their guidelines to maintain the compressed air system. We
have a thing around here: You can’t manage what you don’t know. A block diagram is
a step you need to take to really understand your system so you can better manage it.
Now we are going to move on to dryers. There are four types of dryers: refrigerated, regenerative,
deliquescent and membrane. A refrigerated dryer uses refrigerant to cool and condensed
water. Regenerative uses desiccant that adsorbs H20 into small pores of the material. A deliquescent
uses desiccant that absorbs H20 and dissolves into the water with it. Because the desiccant
in these deliquescent dissolve into liquid you actually have to replace them more regularly.
A membrane allows for gas like 02 and N2 to exit through the membrane, while the water
is stopped and pulled out of another pore.
For the purposes of this presentation, we are going to discuss refrigerative and regenerative.
We typically see these more often at facilities. The refrigerated dryer. There are two types
of a refrigerated dryer: cycling vs. non-cycling. A cycling is at it sounds, it actually chills
the thermal mass, will cycle your compressor on and off as needed. How it does this it
actually has an internal thermostat that tells when to kick on the compressor In the dryer
versus when it needs to be running. By doing this it allows you to save energy through
your dryers. It also, since it doesn’t always circulate the refrigerant, will increase the
life of your refrigerant. DOE predicts about 60 percent increased life of your refrigerant.
The problem with this is it typically is more expensive, scaring users away. Non-cycling
provides a more rapid response. It is completely cycling refrigerant the whole time. So that
means your dew point doesn’t fluctuate as much with this. A refrigerated dryer has a
low initial capital cost, relatively low operating cost and it’s pretty easy to maintain. The
disadvantage is that it doesn’t have the dew point capabilities as some other dryers.
So if humidity is a problem in your system that you have to address, you may not be able
to use this type of dryer.
The regenerative dryer typically works in a twin tower system. One tower is where your
compressed air is going to be running through and going to have the water pulled out. The
other is going to be regenerated, often by purged air. There are two types: heatless
and heated regenerative. A heatless regenerative uses just purged air. There are no external
heaters. So you tend to use about 10 to 18 percent of the rated CFMs on your dryer to
just regenerate the material. The heated will use some type of external heat, steam, or
electric and can reduce your purge by five to eight percent. If you use blowers instead
of your compressed air, you can eliminate the purge required, which could save you money
considering that, as Bob explained, air compressors tend to be an inefficient process. They are
a moderate cost to operate. You have to replace the desiccant every three to five years and
it tends to be a high capital cost initially.
Now, we are going to give you an opportunity known as heat recovery. Your compressor if
puts off a lot of heat. The DOE says about 93 percent of the electrical energy that you
will expend will be converted to heat. With the proper heat recovery system, you could
actually recover about 50 to 90 percent of that heat. The rule of thumb is one Horse
Power of compressed air equates to 2,545 BTUs per hour of available heat for use. This means
for a 100 HP compressor, if you could recover 80 percent of this heat, you will have about
200,000 BTUs per hour that you could use. If you have a heated room next to you and
it’s the cold season, why not replace some of your HVAC with this to take the load off
the HVAC. All you would need is some ventilation. Something you also have to consider, though,
is that in the cooling season, you need a way to remove that. You don’t want to make
your cooling equipment work harder.
Now on to the compressor intake. The location of your compressor is very, very key and the
increase of heat actually hurts your compressor. If you put your compressor in a confined space,
you are actually hurting the compressor because it’s producing the heat that’s actually
making it work harder. DOE says that a 20 degree decrease in temperature will actually
decrease the cost of the compressor by about four percent. Basically you don’t want to
choke the intake either by putting it too close to the wall. This means that you should
probably have it in line with outdoor air, preferably from the shaded side of the building.
Dampers should also be considered as the humidity is a big factor since water won’t compress.
You need to take that into consideration when it rains outside.
There are some of the best practices I have for you on the supply side. Now I am going
to kick it over to Cheryl Eakle who will be talking about the demand side.