Pinion

September 13, 2016Ken Bean

The next component to discuss in the centrifugal compressor is the pinion. Basically a shaft that carries several key components of the compressor. Below you can see a photo of a pinion that is set in a lathe for repair but this gives a great view of the bare pinion only.

As you can see in the photo, there is a gearing cut in the center of the pinion (shaft) which will be used to rotate the assembly and then on each end of the pinion there are some raised areas and some smooth areas.

The pinion carries several other key components of the compressor including the impeller or impellers if the pinion is a double hung design as the way pictured. Below is a photo of a pinion with two impellers mounted.

The photo below demonstrates a layout of the various other components mounted on the pinion.

As the photo shows, in addition to the impeller on the left end of the shaft; moving to the right you can next see the air seal is shown followed by the oil seal and finally the high speed bearing that carries the weight of the impeller. Moving down the pinion you can see where the bullgear meshes with the gearing on the pinion. This gearing will be discussed at a later time as well as the seals and bearings.

Not shown, there would be at a minimum another bearing to the right of the bullgear so that the 2 bearings are completely supporting the entire pinion assembly.

FYI, the term pinion assembly or rotating assembly or cartridge assembly are all terms used which simply mean the entire rotating assembly which would include the pinion, impeller, bearings and seals.

If this happens to be a double pinion design, then the entire arrangement would be duplicated on the right side of the pinion shaft past the bullgear which would include another bearing (already mentioned), another oil seal, air seal and impeller.

Cool The Air

September 6, 2016Ken Bean

We’ve discussed how the air is compressed through each component in a centrifugal compressor. Now that the air/gas has been compressed in the 1st stage, it has an elevated temperature. An example on a 400 HP machine with the air inlet temperature at 75 degrees F. the air temperature as it exits the 1st stage is approximately 242 degrees F.

As previously discussed, the ability of the compressor to meet the required performance is based on inlet pressure, inlet temperature and humidity levels. With cooler air being more dense it is also more easily compressed. Since the air leaving our 1st stage is at 242 degrees, we need to cool the air down before we move it to the 2nd stage of compression.

Enter the cooler, the next component of discussion in our centrifugal compressor. Unless, there is a very specialized application the manufacturer will supply a cooler between each stage of compression and a final cooler (after-cooler) after the final stage of compression to cool the air prior to the next stage of compression or enter the plant distribution system.

Normally on compressors as large as centrifugals the cooling medium will be water although some manufacturers do provide air to air coolers up to certain horsepower machines.

The photo above is a fairly normal centrifugal compressor gearbox and cooler casting. This is the base building block of the compressor. The 3 square holes near the bottom of the casting is where the 2 intercoolers and single aftercooler will be located. the circular holes toward the top are where the impellers and diffusers will reside and the scroll will be mounted to the outside of the circular holes once the impeller and diffuser are mounted.

Remember I stated this is a fairly typical gearbox/cooler casting. Manufacturers will incorporate a single casting design which can be utilized for several HP sizes. This saves the manufacturer money rather then a casting design for each single HP compressors.

Other designs can be found as well. One design is shown below where the cooler or cooling tubes surround the inlet air path. As you can see the air is being drawn down the center toward the impeller. The air exits the impeller/diffuser to each side where it then channels back through the cooler. The main issue with this design is the difficulty in disassembly to make any repairs unlike the casting design shown above where the coolers can be independently & easily removed.

Best in class design can accomplish even more. You can see in the photos below that certain manufacturers create separate gearbox and cooler housing castings. The thought process here is that the gearbox can still be usable for a range of HP sizes but the cooler sizing type can also be customized. By bolting the gearbox casting to the cooler casting you complete the casting assembly. In the event a client needs special large or smaller coolers or perhaps the client needs an API compressor then the gearbox casting and cooler requirements can be totally customized.

Moving on to the actual working of the cooler. The cooler itself contains a series of tubes, the inner portion of the photo below. Best design has the water flowing through the tubes although certain manufacturers reverse this and have air flowing through the tubes. The air enters the shell of the cooler and flows by the tubes containing the water. The air transfers the heat to the cooling water flowing through the tubes and the water then exits the cooler flowing back to a cooling tower, chiller or in rare cases a drain if the plant is using city water with a once through design. (A very expensive alternate) Most often the tubes are also surrounded by fins which aides in the heat transfer process.

A few other items that are important to note:

The tubes in the cooler can be either straight or U-Tubes. A straight tube design has the water entering one end of the tube, flow straight through to the other end where it exits the cooler assembly. A U-tube design (cheaper) has the water enter one end of the tube, flows to the opposite end of the cooler where the tubes bends 90 degrees and flows the water back to the originating end of the cooler where it exits. A U-tube cooler can only be chemically cleaned! A straight tube design offers the ability to remove both end caps and be mechanically rodded for cleaning. Please allocate the extra money (if required) and specify straight tube coolers!

The tubes and fins can be made of special materials depending on the service of the machine. Normal material of construction is copper tubes and aluminum fins.

You will normally hear of coolers being rated in approach temperature such as a 10 degree approach or a 15 degree approach. This simply means that the air leaving the cooler will be X degrees higher than the cooling medium – in our case normally water. Where X is the approach rating. So if a cooler is rated at a 10 degree approach and we have 80 degree water, the air leaving the cooler will be 90 degrees.

So now we’ve cooled the air down to an acceptable temperature for use in the next phase – either the next stage of compression or plant use.

If you read my dryer series you might remember when you have hot air and then cool it down you also condense water. The same thing just happened in the inter or after cooler. We had hot air (which is capable of holding large amounts of water vapor) and cooled it down so the water vapor condenses changing it to a liquid state. Now we certainly do not want liquid water going downstream to our plant and we REALLY do not want liquid water going into our next impeller! Therefore the design of the cooler casting must be such that the liquid water drops out of the air stream to a low collection point where an automatic drain valve will disperse the water to a drain outside the cooler casting.

A final note on condensate. We’re discussing water & metals which equals corrosion over a period of time. Air passages can be sprayed with an anti-corrosion coating for additional protection. Some manufacturers charge extra for this item and some provide it as standard. Yes, coating also wear away but any additional protection from corrosion is a good thing! Remember, your impellers are spinning at high speeds (up to 100, 000 RPM) with very close tolerances. A piece of rust slag hitting an impeller is not a good idea!

Rise & Collect

August 30, 2016August 30, 2016Ken Bean

We’ve previously discussed that high velocity air leaving the impeller impacts the blades on the diffuser which slows the air which causes the air pressure to rise. The next component in a centrifugal compressor is the scroll. Sometimes referred to as the volute.

Once the air passes through the diffuser blades it enters the scroll. As you can see in the above picture, the air passes around the scroll and exits at the top right discharge port. The air passing through the scroll is further reduced in velocity in which again causes a rise in pressure.

At this point the first stage of compression is finalized resulting in the final pressure from the 1st stage of compression which typically results in a discharge pressure of approximately 14 PSIG on a 100 pound compressor design. Remember, the aerodynamic engineer can alter the various aspects of the impeller, diffuser and scroll to achieve various outcomes.

It’s also important to note that we likely started with an inlet pressure to the compressor at approximately 14.3 PSIA. The absolute atmospheric pressure at the location of the compressor. This location and subsequent absolute pressure are critical considerations for the compressor. A machine designed for installation at the beach (sea level) with an absolute pressure of 14.7 will not have the same performance if it is moved to a mountain in Denver with an atmospheric pressure of 12 PSIA.

Always assure you’re using the correct readings, whether PSIG or PSIA. Gauge pressure vs. absolute pressure makes a huge difference. It’s also interesting to note that most of the work related to pressure increase on a centrifugal compressor is done in subsequent stages.

The secondary function of the scroll is to provide a smooth collection of the air where it will be passed to the next section of the compressor. The next section could be discharge to the plant, discharge to the next stage of compression or most commonly to a cooler.

Slow The Air Down–Raise The Pressure

August 23, 2016Ken Bean

We previously discussed how to get the air moving and the role of the impeller in a centrifugal air compressor. Now that we have air moving at a high rate of speed we need to slow it down. Slow it down? Speed up, Slow down – sounds like old people driving on Sunday morning!

I went in search of a great description for today’s topic, the diffuser. Below is an excerpt from Wikipedia:

As the flow continues into and through the centrifugal impeller, the impeller forces the flow to spin faster and faster. According to a form of Euler‘s fluid dynamics equation, known as pump and turbine equation, the energy input to the fluid is proportional to the flow’s local spinning velocity multiplied by the local impeller tangential velocity.

In many cases the flow leaving centrifugal impeller is near the speed of sound (340 metres/second). The flow then typically flows through a stationary compressor causing it to decelerate. These stationary compressors are actually static guide vanes where energy transformation takes place. As described in Bernoulli’s principle, this reduction in velocity causes the pressure to rise leading to a compressed fluid.

What?

I’m glad you’ve made it to this point. I’m sure you’re thinking, “Principles & Equations, I just want to know how a centrifugal compressor works?”

Here goes: The next piece of the centrifugal compressor is the diffuser.

The air leaving the tips of the spinning impeller at high speed now impacts on the stationary diffuser to slow the air down. By slowing the velocity of the air, a rise in pressure is created.

Imagine a car hitting a wall. Until reaching the wall the car moving with only slight resistance from atmospheric air. Note that there is some slight pressure against the car at this point which will be important for later discussion’s. But once it hits the wall the pressure is increased and the increase in pressure collapses the metal of the car.

Same thing when the high speed air hits the diffuser. An increase in pressure, which is what we’re really looking for – compressed air.

Notice the diffuser below is not just a plain wall. It has blades on the surface as well. We don’t really want the air to completely stop (like a car hitting a wall). We just want to slow it down a bit so the pressure will increase.

If you recall discussing the impeller, aerodynamic engineers determine the speed of the impeller along with the length and depth of the impeller blades so the air is moving at the desired velocity. The same is true with the diffuser. The engineer determines the number, length and depth of the blades on the diffuser to slow the high speed air to the pre-determined amount to get just the right amount of pressure rise while maintaining the desired flow of air to the next component of the compressor.

Centrifugal Compressor: Moving The Air

August 16, 2016Ken Bean

A recent blog post discussed a presentation to a valued client on the topic of centrifugal compressor components, operation, maintenance and troubleshooting. This client owns and operates 20+ centrifugal compressors ranging from 1000 to 5000 horsepower. We were surprised to see the number of attendees to the class. Our thought process was with the number of compressors at this facility, everyone knew everything about centrifugal compressors. We were wrong. So you don’t misunderstand, there are lots of capable personnel at the facility but with the constant turnover of position changes, promotions and general churn, there were plenty that were looking for education.

I surmised that if this plant could use some refresher, then a lot of other people would likely be looking for information as well. For the next few weeks I’ll be covering some of the items from that presentation.

The photo below is the basic component of a centrifugal compressor: The impeller or often called the wheel.

The wheel is the primary rotating component that moves the air.

The wheel is mounted on the pinion which lies horizontally in the compressor. You’ll note that the pinion below has two impellers, one on each end. This is a common configuration although it is equally common to have only one impeller on a pinion.

The air enters the impeller at the small end and the vanes grab the air and accelerate it through the vanes to the larger end where it is basically thrown from the fins. The increasing velocity of the air is the beginning point of how the pressure is increased in a centrifugal compressor.

Notice the variation of the blades of the impeller from the two photos above. The bottom picture shows a much shallower blade as well as the blade being shorter. The depth, length and angle of the impeller blades is one of the keys to how the aerodynamic engineers achieve various performance from the unit. The material strength and amount of material is also critical as these impellers can turn up to 70,000 RPM and higher. Obviously, at these speeds you certainly do not want a blade breaking off which would complete wreck the compressor.

In todays modern engineering and machining world, these impellers are typically cut from a blank stock piece of material using a 5 axis milling machine to achieve the precise characteristics the engineer has determined is required to meet the performance requested by the end user. ie: a given flow (ICFM) at a given pressure. Some manufacturers will create impellers by a casting or forging process although these will result in a less sophisticated component. The design of the impeller is two-fold, in that it achieves the flow & pressure characteristics required while offer the optimum efficiency for the compressor’s energy requirements. Again, this is where the best of engineering design creates the most reliable and efficient compressor.

The stock material can be of carbon steel, stainless steel or in certain cases, exotic materials depending on the particular gas the unit will be compressing. Top rated centrifugal compressors for service in compressing air will use stainless steel material rather than carbon steel to achieve longer component life .

Even More–Atlas Copco

August 2, 2016August 3, 2016Ken Bean

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More From Fluid Energy

July 26, 2016Ken Bean

Last week I announced the new best centrifugal compressor available from Fluid Energy. This week I want to bring even more from our product line additions.

Fluid Energy has always been the premier source for centrifugal compressor applications but we want to offer even more. Being the foremost source for centrifugal compressors allows us a position to help a great number of clients. Our only apprehension was there remained a great number of clients with smaller oil free compressed requirements that we simply could not assist. These smaller applications called for a different technology utilizing oil free screw or oil-less scroll compressors. Over the years Fluid Energy has been approached by a number of manufacturers looking for premier representation of their product line. Unfortunately, none of these manufacturers met the exacting standards that the Fluid Energy team requires from the products it represents.

That Changes Now!

Our distribution agreement with Hitachi positions Fluid Energy to cover ALL of your oil-free compressed air requirements. Hitachi, being the world leader in oil-free rotary screw and oil-less scroll compressor technology, lets us assist facilities previously left to less qualified vendors. Now even the smallest facilities (or small applications within large accounts) can receive the Best-In-Class products and service from Fluid Energy!

The most concerning aspect of oil free rotary screw compressors is the built in failure point – rotor coatings! With no oil in the compression chamber (air end) to act as a sealing mechanism, the rotors of an oil free rotary screw compressor require a coating to act as a seal thus allowing the intermeshing rotors to compress the air. As the coating degrades, the efficiency of the air end diminishes and ultimately will no longer compress the air to the design point which requires a costly air end replacement. A large competitive supplier of oil free rotary screw compressors even states in their operation manual the life expectancy of the air end at 5 years!

Features & Benefits

With Hitachi’s patented HX18, Teflon-free coating over their stainless steel rotors our clients can be assured that air end failures from coating degradation are a thing of the past.
Hitachi’s two stage air cooling system offers a patented stainless steel high pre-cooler which eliminates the opportunity for thermal fatigue of the after cooler thus allowing increase efficiency and reliability.
The standard equipment “motorized isolation valve” assures moisture causing corrosion is eliminated.
The included mist eliminator assures no oil deposits are in the ambient air.
ISO class 0 certification
With Hitachi US headquarters located in Charlotte, NC along with Fluid Energy’s various Southeast maintenance facilities, replacement parts & service are a quick phone call away.
CSG: Customer Satisfaction Guarantee offers a 3 year bumper to bumper warranty including parts & labor with an optional extended air end warranty assure end users their plant will be running at peak efficiency for years to come.
Factory Trained and Certified service technicians.

See Inside The Future

See all the features in the product Video here

Oil-Less Scroll Compressor

For even smaller applications the Hitachi oil-less scroll compressor is the technology to fit your requirements. From 2.5 to 44 horsepower utilizing single & multiple head designs we can fit a compressor to your requirements up to 145 psig. Hitachi actually manufactures the scrolls used on their compressors while the majority of competitors purchase cheaper scroll heads and simply package them on their base. With only two principal manufacturers of scroll type air compressor heads, wouldn’t you feel more secure knowing the manufacturer of your compressor actually understands the primary component?

Learn more about Hitachi’s SRL Scroll series in the product Video

Fluid Energy Announcement

July 19, 2016August 3, 2016Ken Bean

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Keeping Cool

May 31, 2016May 31, 2016Ken Bean

Winters icy grip is beginning to fade from memory while we enjoy the pleasant weather of spring. We’ve not yet begin to think about the dog days of summer but now is the time to begin evaluations on your cooling towers. Remember last summer when your air compressors could barely be kept online due to the cooling water temps. Well, now is the time to prevent the problem from occurring again this year.

Cooling towers and fin fans are the main heat sinks at plants and any bottlenecks there show up in your process equipment.

See this great article by Riyaz Papar to get an insight on what to do before the temps are in the 90s this year.

http://bit.ly/1YU5TfG

Summer Heat Affects Air Compressors

May 24, 2016Ken Bean

SUMMER! It’s almost here. Can you feel the heat?

You’re not the only thing affected by Hot Weather –

Your Compressed Air System Suffers As Well!

The pain of hot weather on your compressed air system.

Hot weather brings a myriad of problems for your compressed air system. A complete maintenance plan can minimize the impact. Contact us today for a complete system evaluation!

Pain Points

1: Lower Flow Rates

Increased temperatures reduces the density of the ambient air. This means a lower volume of air is being drawn through the intake. While the decreased horsepower requirement might be helpful for the electric bill, the plant is receiving less air. Hopefully when you initially sized your compressor this was taken into account.

2: Reduced Turndown

The effective operating range of the compressor where efficient regulation through the use of a throttle valve or inlet guide vanes is possible is now reduced due to the elevated intake air temperature.

3: All Temperatures go up

The increase in ambient temperatures also effect the temperature of your cooling water (or air for an air cooled compressor). This means the inter-stage temperature increase further reduces your compressor efficiency.

4: More Water

The elevated ambient temperatures means the air can hold more water vapor. Added to the normal increase in humidity during summer months your filtration and drying system now have a much higher work load to provide the clean dry air your plant needs.

5: Automatic Drains

With the increased water loads, your automatic drains have to cycle more frequently to discharge the moisture removed from the system. This can quickly lead to failures.

What to do?

1: Change the oil.

If using petroleum based lubricants, Summer heat and humidity exact a heavy toll on the compressors oil. High heat and humidity can reduce the life of your oil by as much as half in some cases. Give your compressor a fighting chance by changing the oil and filter before the summer bake starts. Changing oil/fluid on schedule maintains proper viscosity for better lubrication and removes moisture, acids, wear metals and other contaminants. If your compressor is using a synthetic based lubricant perform an oil analysis just before the hot weather begins to assure it’s in the best possible condition.

2: Check the fluid system.

To ensure proper cooling and lubrication, and to prevent unscheduled downtime, ensure there are no restrictions in the compressor’s fluid circulation. Regardless of oil type now is a great time to change the oil filter.

3: Change the inlet filter.

Changing the inlet filter on schedule will keep compression efficiency up and maintain proper operating temperature. Remember there is less air drawn through the inlet so keep restrictions as low as possible.

4: Check your drive couplings.

Direct drive couplings are designed for long life but should be checked for signs of wear to avoid unexpected downtime.

5: Ventilate the compressor room.

Poor ventilation can increase operating temperature, reduce oil life and decrease compressor efficiency. Make sure that you are giving your units enough fresh, cool air to the compressor. The compressor room should have slightly positive pressure. Properly sized louvers and fans may do the job. Consider adding duct work to remove exhaust heat from the room. If you have duct-work with thermostatic controls, make sure it is working properly. Also check other equipment in the compressor room to make sure it is not adding excess heat.

6: Clean the coolers.

Keep the fluid and coolers free of dirt/debris to maintain lowest possible operating and compressed air discharge temperatures. This will make dryers more effective and extend fluid life. Change or clean cooler filter mats if you have them. Keeping the coolers clean is one of the most important things that you can do during the summer months.

7: Check your electrical cabinet.

Dirt and dust can form an insulating layer and build up heat on electrical components. Be sure the cabinet fan works and to clean or replace the filters on the electrical cabinet if present. Use appropriate precautions when cleaning the electrical cabinet!

8: Compressed air treatment equipment.

A majority of air treatment equipment is rated at 100 psig inlet, 100 °F inlet temperatures, and 100°F ambient temperatures. During hot summer months, an increase in any of these conditions can often act to decrease the capacity of the equipment. Keeping the aftercooler clean is the first step.

9: Maintain your dryer.

Refrigerated dryers work best when they have a steady supply of clean and cool air or water. Make sure that your dryer is well ventilated and getting the coolest air or water possible. Clean the condenser. If it is stopped up with dirt and debris it can’t do its job and may cause the dryer to overheat. Also check the refrigerant level.

10: Check all drains on tanks, dryer and filters.

Your dryers and filters work hard to remove the extra water that occurs during the hot, humid summer months. Make sure that your drains are functioning properly so that they get that water out of your compressed air. Many drains have test buttons. Adjust timer settings on timed drains if you have them.