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ASK THE EXPERTS 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:
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:
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.
There are other things that could cause these problems:
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: Generally this is not true, but it's often touted by over-zealous marketing/sales personnel who use these soft-start controls on their compressor packages. The usual demand change is based on a high average kW reading taken over a 15- to 30-minute time frame. The power spike we are discussing is 1 to 2 seconds in duration. The soft starts generally experience 50 to 70 percent of the power spike of Full Voltage, for a duration of 3 to 7 seconds.
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:
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:
Conflicting data on inlet and discharge piping We are installing new 1000HP 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:
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:
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Air Power USA Inc. | P.O. Box 292 | Pickerington, OH 43147 | Ph: 740.862.4112 | Fax: 740.862.8464
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