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.

Single Tower & Membrane Air Dryers

May 17, 2016Ken Bean

We’re in the home stretch now discussing the final types of compressed air dryers. Previously we’ve discussed why compressed air dryers are needed in most industrial systems. We’ve reviewed refrigerated dryers and various types of regenerative dryers. That brings us to the final few types of compressed air dryers, the deliquescent and membrane type compressed air dryers. A note of importance: The dryers discussed in todays post are not normally associated with large industrial manufacturing plants.

Single Tower Dryers

Single tower dryers are used in more specialized applications and are available in 2 different types. Typically these are point-of-use applications such as moisture-sensitive pneumatic tools, valves or other equipment. Pressure dew points from single tower desiccant dryers are as low as -40°F. Since these types of dryers are not self-regenerative, the desiccant must be replaced, or regenerated outside of the dryer, according to the amount of usage.

Adsorbing Dryer

The single tower dryers that adsorbs water vapor with traditional desiccants (activated alumina) are capable of -40 degree F dew points. This type of dryer will stay on line until the desiccant reaches saturation at which point the desiccant is changed or the desiccant is removed and regenerated outside of the drying vessel. As you can imagine, the air flow is typically very low as the the single tower adsorbing dryer requires a great deal of maintenance.

Absorbing or Deliquescent

Single Tower

Deliquescent Type Dryer

The deliquescent dryer is most often the thought when single tower dryers are discussed. The deliquescent desiccant type dryer uses a hygroscopic desiccant material having a high affinity for water. The desiccant absorbs the water vapor and is dissolved in the liquid formed. These hygroscopic materials are blended with ingredients to control the pH of the effluent and to prevent corrosion, caking and channeling. The desiccant is consumed only when moist air is passing through the dryer.

Deliquescent dryers normally are designed to give a dew point depression from 20 degree F to 50 degree F at an inlet temperature or 100 degree F. This means that with air entering at 100 degree F and 100 PSIG, a leaving pressure dew point of 80 degree F to 50 degree F may be obtained (a reduction of 20 degree F to 50 degree F from the inlet pressure dew point). This type of dryer actually dries the air to a specific relative humidity rather than to a specific dew point. The deliquescent dryer can actually handle large flow rates up to 6000 SCFM @ 100 PSIG but require large amounts of desiccant material to maintain the desired dew point suppression making maintenance an even larger burden.

Advantages of Single Tower Deliquescent Desiccant Type Dryers include:

Low initial capital and installation cost.
Low pressure drop.
No moving parts.
Requires no electrical power.
Can be installed outdoors.
Can be used in hazardous, dirty or corrosive environments.

Disadvantages of Single Tower Deliquescent Type Dryers include:

Limited suppression of dew point.
Desiccant bed must be refilled, replaced or externally regenerated periodically.
Drainage of dissolved solution (Absorbing).
Regular periodic maintenance.
Desiccant material can carry over into down-stream piping if there is no after-filter and if the dryer is not drained regularly. Certain desiccant materials may have a damaging effect on down- stream piping and equipment.
Some desiccant materials may melt or fuse together at temperatures above 80°F.

Membrane Type Dryers

Membrane technology has advanced considerably in recent years. The structure of the membrane is such that molecules of certain gases (such as oxygen) are able to pass through (permeate) a semi-permeable membrane faster than others (such as nitrogen) leaving a concentration of the desired gas (nitrogen) at the outlet of the generator.

When used as a dryer in a compressed air system, specially designed membranes allow water vapor (a gas) to pass through the membrane pores faster than the other gases (air) reducing the amount of water vapor in the air stream at the outlet of the membrane dryer, suppressing the dew point. The dew point achieved normally is 40 degree F but lower dew points to -40 degree F can be achieved at the expense of additional purge air loss.

Advantages of Membrane Type Dryers include:

Low installation cost.
Low operating cost.
Can be installed outdoors.
Can be used in hazardous atmospheres.
No moving parts.

Disadvantages of the Membrane Type Dryers include:

Limited to low capacity systems.
High purge air loss (15 to 20%) to achieve required pressure dew points.
Membrane may be fouled by oil or other contaminants (pre-filtration required).

Heated Regenerative Air Dryer

May 8, 2016Ken Bean

Last week we began our review of regenerative dryers and covered the heatless type. This week we discuss the various types of heated regenerative dryers.

Heated Regenerative Dryer

Heat reactivated regenerative desiccant dryers may have internal or external heat applied by heaters.

There are several different types of heat reactivated regenerative dryers and below we discuss each type along with their advantages and disadvantages. In all heated type regenerative dryers the heat source can be provided by either electric or steam with the most common being electric.

Internally Heated Regenerative Dryer

In the internal type, heaters are embedded in the desiccant bed. This reduces the amount of purge air required for regeneration to less than 10%. The purge air plus radiant heat is used to regenerate the off line desiccant bed. It is also important to consider a cooling cycle to decrease the temperature of the bed prior to bringing it back on line to prevent elevated air temperatures going downstream where the excess heat could damage components. It should be known that in all heater regenerative dryers, when the off line tower is brought back on line there will normally be hotter than average air moving into the process. Even when the air temperature would not cause a problem, the elevated temperatures cause a dew point spike because of the higher temperatures. These can be eliminated with engineering foresight to allow adequate cool down time or by utilizing a period of time that allows dry air to continue to flow through the bed, known as a cooling sweep. Another factor with internal heated regenerative dryers is where earlier stated that activated alumina was typically used in regenerative dryers, this is not the case with an internally heated unit. Due to the heaters being in such close proximity (although normally contained within wells) to the desiccant the temperatures right on the desiccant reaches extremes that do no make activated alumina ideal for this application. Normally a silica gel desiccant is used in these units which can better withstand the increased temperatures. However, silica gel is very sensitive to liquid water and will rupture if subjected to this condition where the pre-filter might not remove all water in a liquid form. To mitigate this potential a layer of activated alumina would be used at the inlet side of the desiccant bed. While performing well, the layered bed arrangement is more difficult when desiccant changes are required and the silica gel desiccant is more expensive than it’s activated alumina counterpart.

Advantages of Internally Heated Regenerative Desiccant Type Dryers include:

Low dew points can be achieved without potential freeze-up.
Reduced level of purge air required.

Disadvantages of Internally Heated Regenerative Desiccant Type Dryers include:

Periodic replacement of the desiccant bed (typically 3-5 years)
Layered bed is more difficult to change and more expensive
Oil aerosols can coat the desiccant material, rendering it useless if adequate pre-filtering is not maintained.
Although reduced, Purge air is required.

Externally Heater Regenerative Dryer

In externally heated regenerative desiccant dryers, the purge air is heated to an elevated temperature with the heater being located outside of the desiccant bed. The heated air then passes through the desiccant bed. to achieve the desired regeneration the amount of purge air is approximately 10% of the dryers rated flow.

Advantages of Externally Heated Regenerative Desiccant Type Dryers include:

Low dew points can be achieved without potential freeze-up.
Activated Alumina desiccant

Disadvantages of Externally Heated Regenerative Desiccant Type Dryers include:

Periodic replacement of the desiccant bed (typically 3-5 years)
Oil aerosols can coat the desiccant material, rendering it useless if adequate pre-filtering is not maintained.
Although reduced (less than heatless), Purge air is required.

Blower Purge Regenerative Dryer

A heated blower purge type regenerative air dryer incorporates the same (usually larger) external heater as the externally heated type dryer. In this type unit the purge air from the compressed air system can be eliminated by incorporating a blower which uses atmospheric air in place of the compressed air that has been discussed on the previous type of units. The trade off here is the blower consumes additional electricity but is decidedly more efficient than the air compressor itself. This also allows all of the compressed air to be utilized by the plant as none is required for dryer regeneration.

Advantages of Externally Heated Blower Purge Regenerative Desiccant Type Dryers include:

Low dew points can be achieved without potential freeze-up.
Activated Alumina desiccant
No purge air required
All compressed air is available for plant use

Disadvantages of Externally Heated Blower Purge Regenerative Desiccant Type Dryers include:

Periodic replacement of the desiccant bed (typically 3-5 years)
Oil aerosols can coat the desiccant material, rendering it useless if adequate pre-filtering is not maintained.
Blower consumes additional electric

Heat of Compression Type Regenerative Dryers

The heat of compression (HOC) type regenerative dryer is a twin tower type dryer as previously discussed with the primary distinction that the heat required for the regeneration process is taken from the compressor. This process utilizes the heat of compression, thus the naming convention. Some caution must be incorporated into the thought process of utilizing any of these design types. Obviously, the compressor must have the capacity to continuously offer the heat needed for regeneration. In situations where the compressor is running at part load or unloaded, inadequate heat levels could be experienced. If this event occurs there is not enough heat from the compression process to regenerate the offline tower resulting in elevated dew points downstream. While these units can be incorporated into both 2 stage oil free rotary screw compressors as well as centrifugal compressors, the more reliable of these applications are with the 2 stage rotary screw type as the heat levels are typically higher than with a 3 stage centrifugal compressor.

The primary design of the HOC dryer directs all the discharged air from the compressor to the HOC dryer. When using an HOC dryer, the compressor after-cooler is not used. Remember, we need the high temperature air for regeneration. This hot air is first directed to the offline tower or tower that needs regeneration. The hot air at this point is not saturated with water because of the elevated temperature (350 degree F). The air migrates through the offline tower picking up the moisture that is in the desiccant bed from it’s previous work cycle. The air is then directed to an after-cooler where the air is cooled to 100 degrees F and then to a separator to remove the condensed moisture. All of the air is then directed to the online or working bed where it is dried and then exits the dryer to the plant for use.

A second type of design termed “Inter-stage Heat of Compression Dryer” is used on centrifugal compressors by some manufacturers, continues to utilize the compressor after-cooler, cooling the air to 100 degrees F and directing this air to the online or working desiccant bed. A portion of air is taken from the the discharge of the 2nd stage of the compressor where the air is still at elevated temperatures. This air is directed to the offline tower to be used for regeneration where it sweeps the moisture from the bed and the air is then directed to the intercooler downstream of the 2nd stage of compression from where it was taken and prior to the inlet of the 3rd stage. The air passes through the intercooler and into the 3rd stage of compression where it is then compressed to its final pressure and continues the normal flow path through the after-cooler and into the working (online) dryer tower.

Typically 350 degree F air required to regenerate the desiccant bed although 400 degrees F is ideal.

Advantages of Heat of Compression Regenerative Desiccant Type Dryers include:

Low electrical installation cost
Low power cost
No purge air required (Standard Type)
All compressed air is available for plant use (Standard Type)

Disadvantages of Heat of Compression Regenerative Desiccant Type Dryers include:

Applicable to only oil free compressors
One unit required for each compressor (typically)
Applicable only to compressors having a continuously high discharge temperature
Inconsistent Dew point
Susceptible to changing ambient and inlet air temperatures
Booster heater required for low load conditions
Slight amount of purge air is required for sweep Cycle (Inter-stage Heat of Compression Dryer)

Filtration

To protect the desiccant bed a coalescing filter is required upstream of the dryer to remove any liquid water and/or vapors that have been ingested by the compressor. To protect downstream equipment from desiccant dust or “fines”, a particulate filter downstream of the dryer also is also recommended.

While on the topic of filtration I would like to interject my personal opinion that a heated type regenerative dryer should NOT be used with lubricated type air compressors. I have witnessed instances where oil contamination from the compressor reached and collected on the desiccant. This not only renders the desiccant useless but is also a fire hazard as bed temperatures can reach over 400 degrees F.

Training Seminar

May 6, 2016May 6, 2016Ken Bean

Over my 30 year career in the compressed air industry I have led countless seminars. They have varied in topics from general compressors to drying compressed air to control systems and have been presented to engineering firms and a myriad of industrial client types.

I was recently working with an engineer at Eastman Chemical on a water problem he was experiencing in a particular part of the plant. A few weeks after the conclusion of the evaluation I received an email from him asking if there might be an opportunity for them (he and his team) to attend a factory training seminar with the centrifugal compressor company we represent. I asked what in particular they had an interest in learning.

His response was, they would like a presentation on how a centrifugal compressor works, the various components, the control technology and setup along with maintenance recommendations. I told my client that myself along with Scott Mitchell (our service manager) would be glad to do a seminar for them which would save them any travel cost or charges for having factory personnel come in for training. He thought this was an excellent idea and we moved forward to add them to the schedule.

To be honest I was somewhat surprised by the request. I have worked with Eastman for the past 20 years and this is a company that maintains over a dozen centrifugal compressors just for the instrument air system. The units range from 1000 horsepower up to 5000 horsepower each so they are very well versed in centrifugal compressors. I surmised that there must be some new folks at the plant that could benefit from the training seminar.

When we arrived for set up I was again surprised to see 15 attendees ranging from hands on maintenance personnel up to Sr. level engineering that I had worked with for years. I have to admit, knowing the knowledge level in the room made the situation a little intimidating!

A few days after the training seminar I received the email below from my contact that arranged the presentation.

——@eastman.com

On Thu, Apr 28, 2016 at 7:50 AM, ——, Brian –. —— <——-@eastman.com> wrote:

Ken/Scott,

Thanks for your time Tuesday in presenting the material on centrifugal compressors. You obviously put a lot of time into the slides. It was a big help to us all, I got a lot of compliments on your presentation. Maybe we’ll try again one day with a different audience. Anyway, just wanted to say thank you again from all the folks at Eastman.

Regards,

—– ——

Utilities Division

Distribution Services Dept.

_{(Printed with permission, names excluded)}

If we can make an impact at Eastman Chemical, I hope you will trust us to make an impact at your facility. Contact me today to discuss your training requirements.

Regenerative Air Dryer

May 3, 2016Ken Bean

Last weeks post discussed the use and types of refrigerated air dryers. This week we begin a review of the 2nd most popular type of compressed air dryer, the regenerative type. As several different types of regenerative (desiccant) dryers are available, this weeks post will cover the general aspects of the desiccant dryer and the heatless type of regenerative dryer.

These dryers use a desiccant, (typically activated alumina (which adsorbs the water vapor in the air stream. A distinction needs to be made between adsorb and absorb. Adsorb means that the moisture adheres to the desiccant, collecting in the thousands of small pores within each desiccant bead. The composition of the desiccant is not changed, and the moisture can be driven off in a regeneration process by applying dry purge air, by the application of heat, or a combination of both. Absorb means the material that attracts the moisture is dissolved in and used up by the moisture. Absorption takes place in a deliquescent desiccant type dryer.

Regenerative desiccant dryers normally are of twin tower construction. One tower dries the air from the compressor while the desiccant in the other tower is being regenerated after the pressure in the tower has been reduced to atmospheric pressure. Regeneration can be accomplished using a time cycle or on demand by measuring the temperature or humidity in the desiccant towers or by measuring the dew point of the air leaving the on-line tower.

You will find most all regenerative dryers with a stated flow rating at 100 PSIG and 100 degree F incoming air rating for pressure and temperature. The regenerative dryer can remain a viable option with higher or lower pressure and temperature requirements but caution must be used to size the dryer at the worst possible conditions. This being the lowest possible pressure and the highest possible temperature. A regenerative dryer is sized based on how many pounds of desiccant material must be incorporated into each tower such that it has the capacity to adsorb the water vapor load throughout the entire cycle. If the capacity of the desiccant is not adequate to adsorb all water vapor then the moisture that is not absorbed will be passed downstream. The reason for the caution is the moisture contained within the compressed air will vary greatly with higher or lower pressures and temperatures. Most especially temperature.

While it might not seem to be that important, a good rule of thumb is that for every 20 degree rise in compressed air temperature, the water load doubles! This means if the incoming air temperature is 120 degree F rather than the standard 100 degree F, the dryer would need to double in size. This can make a tremendous difference in your compressed air budget if a dryer has to double in size.

Advantages of Regenerative Desiccant Type Dryers include:

Very low dew points can be achieved without potential freeze-up.
Moderate cost of operation for the dew points achieved.
Heatless type can be designed to operate pneumatically for remote, mobile or hazardous locations.

Disadvantages of Regenerative Desiccant Type Dryers include:

Relatively high initial capital cost
Periodic replacement of the desiccant bed (typically 3-5 years)
Oil aerosols can coat the desiccant material, rendering it useless if adequate pre-filtering is not maintained.
Purge air usually is required.

Heatless Regenerative Dryer

In the most common type of regenerative dryer, the heatless regenerative desiccant type, no internal or external heaters are used. Purge air requirement can range up to 18% of the total air-flow. The typical regenerative desiccant dryer has a pressure dew point rating of -40°F but dew points to -100°F can be obtained. Often when a regenerative type dryer is required, the heatless type is chosen for the the lower initial investment and the perceived lower operating cost as there is no external heater to consume electricity or steam.

This is however incorrect as the cost of operation is the power consumed by the compressor to generate the purge air requirements. Not only will the compressor have to produce the purge air but often, a larger compressor is required to meet both the plant air demand and the demand for regeneration air.

Let’s assume your plant requirements are 1000 SCFM at 100 PSIG. A typical oil flooded rotary screw compressor requirement will be 250 HP and will deliver 1218 ACFM @ 100 PSIG. We can’t use a 200 HP compressor as the delivery is only 980 ACFM which will not meet our demands. So we now need a heatless regenerative dryer that will handle 1218 ACFM. Looking at our book we find we can use a PPC model 1600DHA. While there is a 1200DHA that will accommodate 1200 SCFM it is slightly undersized and we don’t want to risk the plant dew point so we need to move to the larger size. It’s critical to remember that when determining the regeneration flow that this number must be based from the dryer capacity, not the actual flow rate to the dryer. In this case 18% of 1600 SCFM is 288 SCFM. Now we add the 1000 SCFM that our plant requires plus the 288 SCFM that we will need for regeneration air and our total air from the compressor has to be 1288 SCFM. At this point we see that our 250HP compressor will not deliver enough flow to maintain both the plant requirement and the regeneration flow requirement so we will have to increase our purchased compressor to a 300HP model that will deliver 1480 SCFM.

The majority of clients will feel like they could stay with the 250 HP compressor and use the PPC model 1200DHA dryer as we are only over the max flow by 18 SCFM. Honestly, most compressed air sales companies would also tell the customer this would work just fine and it very well may. A point to consider is in the event the aftercooler gets slightly fouled on the compressor and the discharge temperature increase 5 degrees then our water load increases (by our previous rule of thumb) to 25% greater. Since we are already slightly over our max flow rating on the dryer and now increase the water load by 25% the chances for a catastrophic failure from water contamination is extremely likely.

Also, to think of the cost of operation of the dryer – you are running a 50-60 HP air compressor just to produce the regeneration flow for the dryer. I would state this is not an economical situation!

Advantages of Heatless Regenerative Desiccant Type Dryers include:

Very low dew points can be achieved without potential freeze-up.
Can be designed to operate pneumatically for remote, mobile or hazardous locations.
Low initial investment cost compared to other regenerative types

Disadvantages of Heatless Regenerative Desiccant Type Dryers include:

Periodic replacement of the desiccant bed (typically 3-5 years)
Oil aerosols can coat the desiccant material, rendering it useless if adequate pre-filtering is not maintained.
Purge air is required and can be quite expensive.

Next week we’ll look into the various types of heated regenerative dryers.