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Ask The Experts

Top 10 Rules for Compressed Air Equipment Purchasing – From ANY Supplier

Written by: Ray Krohn

February 2022

Reimund (Ray) Krohn is Central Air Equipment’s Product Specialist for Engineered Projects. He has been working in the compressed air industry for 22 years and is a US Department of Energy certified AirMaster+.

Air compressors and their associated accessories (air treatment and storage) are essential pieces of equipment for your business – providing you with a utility through the transmission and storage of pneumatic energy that is used for everything from running a rotary tool to moving cylinders, from transporting powders and liquids to cleaning and sealing surfaces, from aiding in the separation of constituent gases to being an active ingredient itself in processes such as the making of cellular concrete or giving ice creams their texture and volume. Compressed air is everywhere, and if you’re involved in Industry at the Plant Operator or Management level, you’re keenly aware of how key they are in day-to-day operation. When the time comes to replace that key piece of equipment in your plant, there are no end of opinions on what new piece of equipment you should buy. Consultants can be brought in, Engineers can be invoked, Prices can be contrasted, Bids can be weighed. There are a lot of things that should be considered before you award that Purchase Order to the compressor supplier you choose. This article is a list of what ought to be considered, first and foremost. It is not by any means an exhaustive list. It can’t be. Every plant, every business, is unique. Every territory (city or county or state or province or country) has different players in the market, with different abilities and different experience to offer you. This list is intended to help you, the consumer, identify what you need to consider before you buy.  It’s not another tool to sell you on a brand or a solution. Take it for what it is worth.

  • The Very Best Piece of Equipment You Can Buy is the Equipment Best Supported Locally.

I have worked for a number of compressor companies, and I can tell you that while there are differences in quality of components or quality of manufacture between the different makes on the market, the biggest difference by far in my experience is the difference in after sales support my customers receive. You can buy the best rated brand of compressor with the best warranty on the market, but if the dealer is 8 hours from your door, then there’s nothing you can do to get his personnel there faster when you need someone in under 8 hours. Count on that. Compressors are rotating equipment. They will fail unexpectedly, even the best of them and while thorough preventative maintenance does drastically reduce failure rates, it does not eliminate them.

  • The Most Efficient Machine (Electrically) is Not Necessarily the Most Reliable.

This is an unpopular truth, but truth none-the-less. These days, companies are being incentivized with taxpayer dollars to invest in “energy saving” equipment.  “Energy Saving” air compressors seems to have come to mean “Variable Speed Drive” compressors, but VSD’s will not save you money regardless of your demand profile. If the compressor is oversized for your needs, then it will better match output to demand efficiently at part load, but they will also consume MORE power (via conversion losses) at 100% load than a standard fixed speed air compressor. Conversely, if the VSD compressor is grossly oversized, then it won’t save you power either; as below 40% capacity VSD performance curves tend to degrade, and operating at that low a flow typically leads to issues elsewhere in the machine, issues which shorten it’s life. VSD’s, being electronic, also require relatively clean environments and stable operating temperatures. They aren’t readily serviced for maintenance, and they are integrated in such a way that they are key to the compressors operation.  When they fail, which they will well before the compressor itself normally would, they render the compressor inoperable; and often “beyond economic repair”.  The key to avoiding the “VSD trap”? Measure your demand accurately, even between shifts, and NEVER “put all of your eggs in one basket” – ie use a series of smaller fixed speed compressors to share the uptake of the plant demand, match load hours, and optimize running kW to CFM throughout your plants demand range. This saves energy without jeopardizing air compressor lifespans, and it builds some redundancy into your system so that when you have an air compressor suddenly go down for service (and you will), you can “limp along” with at least a portion of your system still online and responsive to your needs.

  • Everybody has a Lemon.

I don’t care how wonderful “Brand X” has been in your experience, even “Brand X” has a lemon, and they know it. People have asked me why CAE is involved with so many different compressor brands. The fact of the matter is, we recognized that there is no “one brand solution” that fits every need. We have also learned that where “Brand X” might have a truly reliable and well priced 50hp (a difficult combination to find, to be sure), their 75hp model can’t make it outside the warranty period. Why? Well, perhaps the 50hp model and the 75hp model both share the same airend (which helps offset cost of manufacture), but at 75hp the torque and RPM of the rotors is such that the machine itself can’t take the strain. Conversely, “Brand Y” might have a fantastic track record with their 75hp models, but their 50hp is priced well above the average market value and has a poor kW:100 cfm ratio. Why? It too shares the same airend design as their 75hp, but that means it’s airend is oversized and so the cost of manufacturer it shares with it’s 75hp cousin is also higher (and back pressure losses due to low RPM reduce efficiency). How do you avoid this trap? Even if you have a good record with the Brand you’re considering, ask for references on the model you are looking at.  Don’t accept old model references either. Things change. Sometimes Gen 1 of a unit is well built (or Gen 2, or Gen 3), but the next Gen could have been built with cost savings in mind, or with components sourced from new sub-suppliers. If you can, get references for the model and generation of machine you are considering. No seller should ever be unwilling to share references with you. 

  • Seeing is Believing.

If you are looking at a brand you are not familiar with, and at this stage you should be doing just that – you’re only investigating options – finding out what’s on the market locally…then ask to see a machine. In this day and age, electronic brochures cost little to produce and nothing to email. A company willing to invest even a modest amount of money in building a website can make their product look fantastic, but seeing is believing. The most customer-focused distributors have warehouse inventory and even a show floor where we can demonstrate our equipment in operation, but even if you’re dealing with a small dealer, they should have some local sales under their belt that they can take you in to see. Go in to see that equipment with personnel you will be trusting to work with it, assuming you buy it. It’s important to let those who interact with these machines regularly and who know what to look for have a voice in the decision making. Not the sole voice; but A voice. Those people will think of things you and I might not; such as “how are we going to get this compressor in that room?”, or “how is that cooler orientation going to fit existing ductwork”? If you don’t consider those things up front, then you better pad your purchasing budget substantially.

  • The Cost of Ownership Exceeds the Cost of Purchase in No Time Flat

Remember Rule #1? Well, there’s a second part to it. Every compressor dealer operates on a different business model, so it’s important to not only consider how geographically close they are to you, but also who they have on staff and what they have in stock. It makes no difference if the dealer you’ve chosen is right next door but only employs technicians who were once shown how to spin a filter, but don’t have the wherewithal to troubleshoot a problem. You need to know that when your machine is in trouble and you don’t know why, your dealer has someone on board who can figure it out quick. Time is money, after all. You also need to know that they have the parts and equipment ready at hand to fix it; OR that they have an alternative that they can offer you to “get you out of trouble” while they work to get the things you need and make things right. That might be a rental machine, it might be a new machine, it might be a refurbished machine. Whatever they have, they better have it handy, because down time is costly, and no warranty covers it.

  • After Sales Support Has a Price, But is it a Fair One?

As in Rule #5 above, Rule #6 asks the question: “What does this level of support (staff, parts stock, machine stock), cost my dealer, and are they asking a fair price for that?” It should go without saying that after sales support requires the dealer invest more in their business. Both in wages and in inventory. The question though is whether that dealer is asking a reasonable amount for that investment. Small dealers with little stock and fewer staff needn’t charge much for their services, because their services are poor, and they know it. Big dealers can charge more, but that doesn’t give them license to take advantage of their clients. These days, competition in many markets is such that dealers need to target a reasonable profit to remain competitive, but in smaller markets, and in older businesses which developed when markets were new, it is not uncommon to see higher dealer mark ups on OEM parts. Why? Too often, only because that is the company culture.  How do you avoid this trap? Get a Parts and Service Quote for the Compressor you are considering up front. Demand complete transparency. You need to know what to expect when it comes to annual maintenance costs on this machine, should you purchase it.  

  •   Don’t Buy From “A Buddy”.

Whole company policies have been written around this, and in some companies may even be grounds for discipline or worse.  Whether you have met an engaging salesman who always seems to have tickets to a game, or you have a high school friend you’ve been having drinks with who also happens to sell equipment, DON’T make a company decision based purely on a relationship. Having a relationship shouldn’t preclude the two of you from doing business, but in no way should a relationship (past or present) determine whether such business should ever be done. The money you are investing in equipment is company money, and company money is intended to benefit the company, not you and not your buddy.  

  • Check Their Credentials.

When you are making a purchase, you’re not just buying equipment – you’re buying experience. Just because a company boasts of having “100 years of experience” on staff, doesn’t mean the person who puts together your recommendation has been working for them long enough to satisfy their probationary period. Make sure that whoever is building you a proposal understands what it is they are proposing. If they don’t, then ask to speak to someone who does. Nobody should be excluded because of lack of experience, but the decision you are making is an important one. You can’t afford to help someone build job experience when their work is going to be prone to error.

  • Always Look at Your Total (Turnkey) Investment Costs.

Maybe “Brand J” has a fantastic price, good references, and great performance… but “Brand J” is also 1,000 lbs heavier and 2’ wider than your existing compressor. Installing it in your plant means you have to either relocate your compressor room, or take out a wall and expand it. Installation costs can easily be 80%, 100%, or even 150% of the equipment costs.  You need to work out exactly what those costs will be before you sign that purchase order. Maybe “Brand J” is the right fit for you, but you need to put it up on a mezzanine, or on the loading dock, or in an independent dealer supplied building to get it to work. A business case can be made for the right solution, but we need to identify the challenges. Talk to your salesman about where and how you want to install their machine…. assuming that they have the credentials to speak to that (see Rule #8, above).

  • Ask for An Introduction to the Team.

You have vetted your dealer, ensured they can support you locally, seen their quoted machines, gotten their references, checked their prices and stock, made sure that the person who provided the quote knows what you need and how their proposal answers those needs and where it will fit in your system. Now you need to understand who to go to when things go wrong. The staff at your dealer are there to help you once you’ve made your purchase. They aren’t your employees, but they are on your team. They need to understand not just what you’ve bought but how it fits in your system. If you’re buying from a dealer you already know, maybe they know you already – but don’t assume so. If you have never dealt with them before, bring them in to the plant.  Get a quote on servicing your other compressors (assuming you have more than one). Show them your process. Maybe they can point things out that haven’t been done, or that could be done better. Either way, when it comes to trouble shooting at your shop, it’s a rule in our industry to not start in the compressor, but on the floor. Work your way to the problem, back to front. Issues that first occurred on line #3 might be related to line #3 exclusively. The technician needs to be a detective, and to ferret that out. Some familiarity with your system will save you time and money. If you can do this before you install the new system, you will be better off for it.

What is the ‘Hold-Up’ In Considering Pneumatic Hold-Up Time in Your Facility?

Up to 43% Savings in Compressed Air System Capital Costs

Written by Ray Krohn

April 2021

Reimund (Ray) Krohn is Central Air Equipment’s Product Specialist for Engineered Projects.  He has been working in the compressed air industry for 22 years and is a US Department of Energy certified AirMaster+.

What is “Hold-Up Time”?   In Pneumatic power systems, “hold up time” is the length of a time a compressed air supply can continue to meet downstream pressure demands after the failure of the upstream compressors – ie:  With zero system input.   Compressed air is used to store Energy.   An air compressor consumes direct energy (typically in our systems via AC power), and converts it to potential energy by means of compression.   As the compressed air is released to atmosphere through devices and regulated end uses downstream, that potential energy performs “work” – opening and closing or holding shut valves, conveying media, turning turbines, etc.  When downstream demands are critical – when a complete system shutdown can cost not just lost revenue but lost lives, “Plant Hold-Up Time” becomes a paramount issue.  A sudden loss of pressure due to system failure must be avoided – or at least delayed until other processes can be brought online or losses can be mitigated.    This article addresses another way to do just that.

If your employed in the Oil and Gas Sector and are involved in compressed air systems, then you have undoubtedly seen the gigantic compressed air storage tanks at sites where Plant pneumatics are integral components of safety systems.   You may have wondered how those towering vessels were sized and may even have wondered what they might have cost.  What you likely haven’t considered is whether there was a better way to store that volume of air.  A less costly way. 

Average Instrument Air System designs demand 20 minutes worth of compressed air storage at base load plant demands, be available in the event of a complete system failure.   In the case of system sized for 100 cfm average outlet flow at -40C PDP at average 120 psig skid edge pressure, this would mean we would need to have stored 20 minutes x 100 cfm = 2000 cubic feet (14,960 US Gallons) of compressed air at pressure.   But at what pressure?

Simply because the system was designed to deliver 120 psig at skid edge during normal operation doesn’t mean that the critical end uses need 120 psig to operate.   The engineers who designed that system left safety factors in the skid edge pressure requirement so that end devices wouldn’t perform poorly due to pressure drops in the compressed air distribution network, or due to poor system maintenance.   While minimum operating pressures will vary both by device and by site, the average minimum pressure calculated for will be in the 80 psig range.   This means, if the system when working as intended can deliver 120 psig at skid edge in the flow and purity required, our plant hold up time vessel will be storing a differential pressure of 120 psig – 80 psig =  40 psid.

This differential pressure – the difference between the storage pressure at the start of the failure event and the minimum pressure required to meet the critical needs downstream – is used to reduce the vessel size from the 2000 cubic feet noted above down to a fraction of that size; in this case 369 cubic feet of storage – or 2,760 US Gallons.  Now that’s a much smaller tank; but still a bigger vessel than can be easily fit into a site building.   As a result, we commonly see large vessels installed outdoors on sites – custom engineered vessels designed to handle wind loads and cold temperatures; features that drive up their costs.  They require special foundation work and paint protection against the elements.   Given these tanks are sized to be used only in the unlikely event of a full system failure, there be a more cost effective way to store this volume of air?

The answer is “Yes”, and it’s in the differential.

Look:  If we were consistently delivering only the minimum required plant pressure at skid edge (80 psig in this example), then to have 20 minutes of hold up time for a 100 scfm downstream demand, we would need a 14,960 USG vessel.  80 psig – 80 psig = 0 psid.

By increasing normal skid edge pressure to 120 psig, we have reduced the required tank size for that same 20 minutes of 100 scfm demand at 80 psig to a mere 2,760 US Gallons.    120 psig – 80 psig = 40 psid.

Now imagine instead of storing the hold up time air at 120 psig, we stored it at 500 psig.   500 psig – 80 psig = 420 psid.  Now the required hold up time vessel is only 35.14 cubic feet in size; or 262.8 US Gallons.  That’s a reasonably small tank; easily fit on site in any building, or added directly to the Instrument or Plant air skid.  But can we economically achieve this higher pressure?

While sizing the system to consistently deliver a higher skid edge pressure merely to allow for a smaller hold-up time tank would be both costly and grossly inefficient, the infrequency of the need for “hold up  time” storage means we don’t need to fill such a vessel in seconds or minutes.   That storage is used only in the case of plant wide emergency.  It would be wise to have it ready at commissioning, but if it takes a number of hours to fill that vessel, that is inconsequential to the process.   Since the tank itself is not technically intended for “dry storage” (although it is often used as that), it can be a supplemental tank on the system – fed by the standard air compressors but through a much smaller BOOSTER compressor. 

Booster compressors take the system inlet air at pressure (in this case, 120 psig), and re-pressurize it to allow for higher pressure storage (in this case, 500 psig).   That stored high pressure air can then be isolated from the downstream supply lines via a simple normally open solenoid valve (which opens on system failure) and a precision regulator – regulating it’s outlet flow at a stable (minimum) 80 psig in the event the system crashes.   By adding an additional safety relief valve past this regulator in the system, we ensure high pressure air can’t bypass the regulator and cause problems downstream.   With the hold up time vessel pressure contained, the booster shuts down and will not start again until and unless an event occurs that demands the use of the stored pressure.  

CAE hasn’t just theorized this system design, we have put it into practice.  By installing a small high pressure receiver with a booster compressor and the safeties noted above, we were able to save our client $72,340.00 in capital costs – not including the cost avoidance realized from not having to have a custom vessel shipped separately to site (estimated at $1,280.00), erected on a newly poured engineered concrete pad (est $9,100.00), and tied into the system in the field (est an additional $3,460.00 in labor and materials).  A total savings of $86,180.00 on a package that cost just under $200,000.00 – a whopping 43% savings. 

When it comes to pneumatic power – differential pressure makes all the difference in the world.  Once you understand that, you understand exactly what you need to do to build a Compressed Air System.

Questions on this article, or anything compressed air or nitrogen related? Please email us at