Experts In Compressed Air

Ask The Experts

The experienced staff at Air Power USA is unmatched in our expertise on compressed air systems and equipment, and we're happy to share some of that expertise with you. Here, Don and Scott Van Ormer address some questions we've received from our clients. Do you have a question for the compressed air system specialists at Air Power USA? If so, drop us a line. Our training programs and seminars and textbooks and manuals may also be of help.


Constant speed compressor motors versus VSDs

I had been told that by replacing or changing all air compressor motors to Variable Speed Drive (VSD), I will lower my electrical energy costs. I have three 100-hp rotary screws and run all three. Should I replace all these with VSDs? - Hammond, Ind.

Air Power USA Answer:

This question cannot be answered accurately without more data about your load profile. To simply say that just installing new VSD drives or a VSD compressor will lower your bill is "way off base".

Here's why:

• A VSD is much more efficient as a trim unit than a similar size constant speed version at loads from 20% to about 75%.

• A variable displacement constant speed compressor of similar size is more efficient than a VSD from loads of about 75% to 100%.

• A VSD applied at full load will draw more power for the same flow than a similar size constant speed (for example, 4 to 6% in a lubricated rotary screw, or up to 13% in an "oil-free" rotary screw.

• The key to correct application of an appropriate trim compressor is identifying the "Load Profile" by shift (including weekends and holidays) to estimate how many hours the trim unit spends in the "sweet zone".

We recommend holding off on taking any major action or spending money on new equipment or piping without a review of the plant load and system dynamics by trained personnel. We often have applied VSDs, Variable Displacement Controlled Units, and sometimes both, when optimizing a specific system and identified set of conditions.Well applied the VSD is a great tool poorly applied it can make the system worse.


Problems holding steady pressure

We have three 200-hp oil-free rotary screw compressors installed to run our plant. All three units run during production at 110 psig. The pressure entering the receiver after the dryers is 80-85 psig. We need a firm 82 psig to the process.A company sold us two 3700-gallon air receivers and a pressure flow controller to hold a steady 85 psig to the process at all times. Our original installation also had two of the units loading and unloading and loading every 20 to 30 seconds, which has caused motor and cooler failures. Our piping is sized correctly - we used the same size pipe as the opening size in the compressor discharge (3 inches). The header is 4 inches. We cannot hold 85 psig in the plant - the air is entering the controller at 84 psig and the system is 78 psig. If the pressure gets up to 90 psig then we do have 85 psig in the system. Overall, we are back to where we were - and bypassing the flow controller in order to run. The compressors are still loading and unloading rapidly. We were told this tank and controllers would fix our problems. We spent a great deal of money to go nowhere. What is wrong? What can we do? - Hayward, Calif.

Air Power USA Answer:

Without more data such as what we would acquire in a system review, it is hard to pinpoint everything exactly. However, here are some very accurate statements that may help:

Interconnecting piping from the compressor discharge through the dryers to the system should be at a velocity of 20 fps or less, never to exceed 30 fps to avoid turbulence-driven back pressure.

Each 200-hp compressor probably delivers about 830 acfm -- the 3-inch discharge line has a velocity of 36 fps at 100 psig and 2490 scfm (all 3 units) a velocity of 52.6 fps in the 4-inch pipe.

We will make an assumption that you are using straight "TEE" connections to the header from the compressor. Each unit has to work its way past the adjacent discharge to move through the piping. At these velocities there will be a great deal of turbulence at each "TEE" connection - particularly the last one. All of this is apparently creating a total pressure loss of up to 26 psig at higher loads (you are not at full load due to the short cycling). Assuming 5 psig is lost in the dryer, this leaves 20 psig or more due to high pipeline velocities, turbulence and basic friction loss.

This high-pressure loss is the cause of the short cycling. As soon as the compressor discharge senses a 10-psid differential, it reacts by loading or unloading. Adding the receiver storage - although large - does no good as control storage after the pressure loss of 20 (+) psig. You could eliminate the short cycling by moving the compressor control sensing point to the receiver - but now the discharge pressure will rise to 120 psig or more, which may overload the motor. If the motor can handle it, your electrical power draw will increase 10 percent or more (1/2 percent per psig). This type of action also can create a very bad safety situation. Something of this nature should only be implemented by trained professionals. A better action would be to correct the piping and eliminate the wasted pressure loss. Check the actual pressure loss across the dryer at full load with the same pressure gauge. If it is more than 5 psig, we may want to review the situation in more detail and ask for more data. Increasing the header size from 4 inches to 8 inches will lower the maximum velocity from 52.6 fps to 15.9 fps. This will not only slow the air down, but also act as storage.

• Even though the 3-inch discharge lines have a velocity of 36 fps, if the "TEE" connections are changed to 30º or 45º angle entry (angled towards the flow direction) into the header, only frictional pressure loss will apply.

• We would expect the air to reach the receivers now at or near 100 psig, and consequently, air also would reach the flow controller. This should give you a constant 85 psig to the system (properly adjusted), and also allow you to perhaps lower the discharge pressure and reduce the energy input.

There are other things that could cause these problems: Plugged or fouled dryer.

• Dead head piping.

• Improper sizing of the regulator part of the Flow Controller:

• Regulators are sized to handle "flow rate", not "rate of flow".

• If the Regulator is sized for 2500 scfm and the average flow is 2000 scfm, it will regulate well during normal flow. If during the day you had a single event that used 500 scfm in 30 seconds. This is an additional 1000 cfm to the rate of flow. Now the regulator is trying to handle a 3000 scfm rate of flow during that event. The regulator now becomes a choke and restricts the airflow.

• Your regulators appear to start "tracking" at somewhere between 90 psig and 85 psig. This can be corrected by a more effectively-sized regulator, depending on the type and brand of Flow Controller. Regulation correctly applied to a system can operate at a ½ psig differential or less if required.

For more information on piping and turbulence-driven back pressure consult our book "Energy Savings in Compressed Air".


Full-voltage starts

Is it true that a full-voltage start on my compressor motors will significantly increase my electrical bill compared to such soft starts or Star Delta, Wye Delta, etc. because of the “power spike”? – Waxahachie, Texas

Air Power USA Answer:

Certainly not! At loads from 90 percent to 100 percent, the modulating controls are almost always more power efficient than any two-step in a lubricant-cooled rotary screw. Regardless of brand, two-step control performance on these type units is absolutely dependent on establishing enough idle time to allow the unit to reach “full blow down” to the lowest kW in order to achieve “real power savings”. Many of these rotary types have blow-down times from 40 seconds to several minutes depending on size and brand. Some of the newer, more modern and engineered packages utilizing proper synthetic lubricant/coolant have reduced these times to less than 10 seconds.

Ninety percent of the two-step rotaries we review in plant audits do not reach full blow-down for any appreciable time due to poor piping and/or not enough effective storage.

Here’s some additional information on the subject:
• Contrary to popular myths, 1 gallon of effective storage per cfm of air is not enough for proper part-load operation of a two-step Capacity Control.
• The most power-efficient unloading control for lubricant-cooled rotary compressors at loads from 75 to 100 percent is the variable displacement.
• The most power-efficient unloading control for lubricant-cooled rotary compressors from 20- to 75-percent loads is the variable speed drive.

All major changes to the system, including controls, should be preceded by a full-system review by trained professionals.


Modulation control in lubricated rotary screw compressors

Is it true that modulation control in lubricated rotary screw compressors always is less efficient than two-step control (“on line-offline” or “full on – full off”)? – Muncie, Ind.

Air Power USA Answer:

Certainly not! At loads from 90 percent to 100 percent, the modulating controls are almost always more power efficient than any two-step in a lubricant-cooled rotary screw. Regardless of brand, two-step control performance on these type units is absolutely dependent on establishing enough idle time to allow the unit to reach “full blow down” to the lowest kW in order to achieve “real power savings”. Many of these rotary types have blow-down times from 40 seconds to several minutes depending on size and brand. Some of the newer, more modern and engineered packages utilizing proper synthetic lubricant/coolant have reduced these times to less than 10 seconds.

Ninety percent of the two-step rotaries we review in plant audits do not reach full blow-down for any appreciable time due to poor piping and/or not enough effective storage.

Here’s some additional information on the subject:

• Contrary to popular myths, 1 gallon of effective storage per cfm of air is not enough for proper part-load operation of a two-step Capacity Control.

• The most power-efficient unloading control for lubricant-cooled rotary compressors at loads from 75 to 100 percent is the variable displacement.

• The most power-efficient unloading control for lubricant-cooled rotary compressors from 20- to 75-percent loads is the variable speed drive.

• All major changes to the system, including controls, should be preceded by a full-system review by trained professionals.


Lower discharge pressure for energy savings

Is it true that anytime I can lower discharge pressure of a compressor I will save about ½% per psig lowered? – Appleton, Wis.

Air Power USA Answer:

This is true only on positive displacement compressors (vane, screw, reciprocating, etc.) but not on dynamic (centrifugal, regenerative blower, etc.). For example, when the discharge pressure rises on a centrifugal compressor, the flow goes down but the power draw remains about the same. Conversely, when the pressure is lower the flow increases with the power at basically the same rate. This is while operating outside the control band.


Using amperage to calculate airflow

Can I calculate the airflow from my compressor by measuring the amperage and the discharge pressure? – Dubuque, Iowa

Air Power USA Answer:

About the only time this works is when tracking full-load and no-load amps on a two-step controlled unit. With these, we can calculate the percentage at full load and percentage at no load to come up with an average load.

If you know the input power on any positive displacement compressor and knowing the type of compressor (type of unloading control, motor data, etc.), you can come pretty close to estimating flow by measuring input power.

Amperage is not a true reflection of power – kW is a power measurement. With amperage, voltage and power factor, you can calculate kW. If you measure these three (which are required) with a hand-held motor analyzer, you will get kW and can calculate probable motor efficiency (ME).

Power measurement in any form is not a good estimating tool for centrifugal compressors except to indicate whether the unit is in turndown or not. To accurately estimate the flow without a flowmeter, you need the original performance curve if it is available, along with the inlet and discharge pressures and inlet temperatures. Even with a flowmeter, we usually estimate as close as possible in order to double-check.


Inlet guide vanes on a centrifugal compressor

Is it true that inlet guide vanes on a centrifugal compressor will increase the amount of “turn down”? – Milford, Mass.

Air Power USA Answer:

No. Inlet guide vanes (IGV) used instead of an inlet butterfly valve (IBV) will not increase the amount of turndown. They will improve specific power (efficiency or scfm/kW) of the compressor throughout the turndown range.

Many people, including us at Air Power, believe the IGV allows much smoother turndown, enabling and encouraging the operator to set the controls to full-effective turndown. For best performance, controls should be reset seasonably unless you are running a control system that calculates inlet conditions and changes in its program and automatically readjust itself.


Loose foundations

My 350-hp XLE has come loose on the foundation. We have installed new hold-down bolts and clamps, but they don’t hold. What should I do? -Tulsa, Okla.

Air Power USA Answer:

Without seeing the installation, it is hard to give an exact recommendation, But here are some key thoughts:

The only way to reset the unit on the foundation is to move the compressor, clean the old grout off the foundation, and reinstall new grout and the unit.

• This can’t be done if the foundation is crumbling, cracked and/or soaked with oil. If any of these conditions exist, the foundation will have to be removed down to solid, clean concrete and then rebuilt back up before regrouting.

• Another option is to cut the foundation down to floor level and install a proper “inertia base” instead of a new foundation. Professionals should be called in to review this option.

• Be sure you are not in Inlet or Discharge piping “critical length” (to avoid). This data is available in the instruction manual. If you can’t find this data, we have most of the engineering sheets and can help you if you send us the compressor model (ie., Bore, Stroke) and RPM. These “critical lengths” can also be calculated with some additional information.


Conflicting data on inlet and discharge piping

We are installing new 1000-hp centrifugal compressors in our plant. We have received conflicting data on the proper material for inlet and discharge piping! The OEM recommends stainless steel, but the engineering firm wants to use galvanized schedule 40 pipe. What do you say? – Birmingham, Ala.

Air Power USA Answer:

The question of galvanized piping comes up often in compressed air system piping, as does the question of using stainless steel instead of schedule 40 black iron for the nominal 100 psig air systems. Let’s look at inlet and discharge piping separately.

Inlet Piping:
The proper inlet pipe brings the air from the filter to the compressor with no pressure loss and should not create operational problems with any type of self-contamination on the inside. It is important to realize that the ambient inlet air condition may well dictate the selection of one type of pipe over another.

Galvanized inlet piping has the advantage of resisting corrosion better than standard iron pipe. However, over time when the corrosion does set in, the galvanizing material then peels off. The inlet pipe is now a producer of potentially very damaging, solid contaminants between the filter and the compressor. This would be particularly dangerous to the mechanical integrity of a centrifugal compressor.

During high-humidity weather it is quite conceivable that condensation will form in the inlet pipe (therefore, the OEM installation manual recommends a drain valve be installed on the pipe before the inlet). Condensation in the pipe will obviously accelerate the time frame before the coating breaks down. This time frame is dependent upon where the thinnest portion of the coating is applied.

Stainless steel inlet pipe is the best possible material for such large-diameter, low-pressure inlet air, as long as it is installed properly and the inside is properly cleaned.

There are also many grades of plastic material suitable for inlet air piping.

Summary: We recommend either stainless steel or proper plastic-type material for inlet piping and do not recommend galvanized piping.

Discharge Piping:

Here we have more complex considerations:

• The discharge air from the compressors can be at 250 to 350ºF (for centrifugal, oil-free rotary screw and reciprocating types), or from 200 to 220ºF (for lubricant-cooled rotary screw compressors), so the pipe must be able to withstand those temperatures.

• Even if there is an aftercooler that drops the temperature to 100ºF, consideration must be given as to the consequences if the aftercooler were to fail.

• Compressed-air-generated condensate tends to be acidic. In oil-free compressors (such as centrifugals and oil-free rotary screws), it is usually very aggressive.

• The basic objective of the interconnecting piping is to deliver the air to the filter and dryers and then to the production air system with little or no pressure loss, and certainly with little or no self contamination.

• Galvanized piping will have the same problems once it begins to peel as we described on the inlet application. In all probability, due to the aggressive acid characteristics of the condensate, the galvanized coating life may be much shorter.

Regardless of the plastic type manufactures claim, we never recommend any plastic type material for interconnecting and distribution header piping. Most of these materials carry cautions not to be exposed to temperatures over 200ºF and to avoid any types of oil or lubricants.

Here again stainless steel is our number one recommendation for the interconnecting piping from the compressor to the filter/dryers when the compressed air is oil free. It will obviously resist corrosion much better than standard schedule 40 black iron. Some other considerations:

Most areas will allow schedule 10 stainless steel in lieu of schedule 40 black iron.

• For the same diameter pipe, stainless steel will be much lighter and easier to handle usually lowering the labor cost.

• For welded connections, stainless steel usually just requires one bead, while black iron pipe usually requires three beads (Weld-fill-cover). This should also lower the labor cost.

• Stainless steel does not usually seal well when threaded. It will do much better with Victaulic type connections when welding is not practical.


Pulse-Type Dust Collector

I am trying to calculate the proper receiver storage tank size for our system. I have been given (by Goyen and Donaldson) the following information: Goyen reverse pulse jet needs 2 ft3 of free air or 0.8-1.0 ft3 of air at 100 psi to function properly. Donaldson pulse jet header has a free-air volume of 0.6 ft3 (4” square tubular that is 60 inches long). Which volume should I use (0.6 ft3, 1 ft3, or 2 ft3) to calculate the “rate of flow”? Also, is the calculated required storage size inclusive or exclusive of the dust collector header?

Air Power USA Answer:

The flow used in the calculator is the actual rated flow of the pulse valve itself. What happens after it leaves the pulse valve has no impact on the compressor controls or feed pressure. Usually this volume rating is on the units or at least the specifications. The key data is:

• Pulse jet size in cuft3

• Duration of pulse in seconds

• Number of valves opening simultaneously

• Duration in seconds between each pulse

A pulse jet may be 4 ft3 for ½ second. The rate of flow in cubic feet per minute will be:
4 ft3 x 2 = 8 ft3 per second x 60 = 480 cfm rate of flow

This means that all piping, regulators, valves, etc. must be sized to this flow rate at whatever the operating pressure is. In other words, the effect of moving 4 ft3 in ½ second is very similar to moving 480 cfm and not considering this in sizing will raise the possibility of restrictions in flow and time negatively impacting on performance.

The calculated storage requirement from the formula only calculates the proper tank size to supply the minimum pressure (based on the available pressure drop from system to actuating pressure) at the flow rate and not allowing it to pull down too low. This also isolates the rest of the upstream air system from any negative impact from the short-term, high-magnitude demand.

The same calculation for the duration between pulses (which should be much longer) shows the net flow rate when the receiver refills.


Float Drain

Can a condensate float drain push water upwards in the discharge tubing and if so how many feet up? What is the recommended plumbing from a bottom of a float drain?

Air Power USA Answer:

This float drain question is somewhat complex because it does not specify the type of float drain in question. For example, the float drain could be a small plastic type commonly found in filter housings, ball check, or inverted bucket-type float drains. It may also describe some of the more popular “zero-loss” drains such as level-activated, electric- or pneumatic- actuated drains.

Application of any condensate drain should be sized to handle the maximum expected condensate volume, and most importantly, that the line sizes into and out of the drain should not restrict the expected flow rate.

Basic level-activated condensate drains are designed to restrict high pressure air from going into the condensate discharge line. A small enough line with a short lifting distance to push the condensate may work sometimes, but plumbing an open-ended, or vented, discharge drain line to push the condensate upwards typically is not recommended. Best practice is to plumb a large vented discharge drain line to a collector drum or basin and then pump the condensate to a separate designated location.

Proper compliant condensate disposal should also be considered from approximate calculated condensate levels to lines and vessels. The following projected condensate volume is from a refrigerated air dryer with 70°F atmospheric dew point intake air, delivering a +40°F pressure dewpoint (If a desiccant dryer, there should be almost negligible volume of condensate at the pre-filter):

• Normal oil carryover from lubricated a compressor in condensate is about 140 ppm.
Every 1,000 scfm will deliver a probable maximum of 11 gph of condensate -- normally 2/3 at the after cooler and 1/3 at the refrigerated dryer separator with about a +40°F pressure dew point at 100 psig.

• This term might also describe some of the more popular “zero-loss” drains, i.e., level-activated electric- or pneumatic-actuated drains.

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