How to decide between compressor types?

[fogell21]

Hi all,

This may not be the right place to be posting this but hopefully I am close.

Long story short, I am currently in a more "Project Coordinator" type job than actual engineering and am currently looking at upgrading our compressed air system. We have 2 compressors in our facility at this time, one is a newer 750cfm (150hp) model and the other is an old beat up item and I have no specs on it. We have money in our budget to purchase new compressed air equipment in order to get our cfm level up to 1500cfm (our requirement with the new production equipment we are getting) and I have gotten quotes from several suppliers.

I have basically been offered both variable displacement as well as variable speed and am quite sure I understand the benefits of both and how they fit into our process. Our process isn't going to necesarily have a constant cfm usage, as it depends on how casting is going and what product we are casting that day. Could anybody briefly explain to me the difference between variable speed and variable displacement? And please give your opinion as to which is better.

Thanks guys.

 

[BigInch]

Variable speed changes the speed of the equipment to change the flow and pressure. Different cycle speeds of a cylinder will give you different flowrates for each speed. 5 RPM * 1 CF = 1 CFM, 10 RPM * 1 CF = 10 CFM. Variable displacement runs at the same speed, but the cylinder volume is varied, so 5 RPM * 1 CF = 5 CFM, 5 RPM * 10 CF = 50 CFM.

You haven't said enough about your intentions or your process to say anything else, but generally I avoid variable speed anythings, unless its a diesel engine.




We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[zdas04]

I took a class at Ariel in beautiful Mt Vernon a few years ago and the instructor explained various capacity control schemes as "If you have never compressed the gas then the effeciency on the part you haven't compressed is 100%". His point was that variable speed was the most effecient means of capacity control by a long margin.

On the other hand the effeciency numbers quoted for a recip compressor are "volumetric effeciency". One of the factors in that calculation is "unswept volume" or "clearance". Since, a control scheme that changes displacement is really just changing the clearance (since the cylinder, and piston are fairly rigid), you are decreasing the effeciency (i.e., increasing the energy cost per unit volume) when you increase clearance.

1500 CFM (I think of it as 2.16 MMCF/day) is not a small energy load if you are running with significant compression ratios. All other things being equal, I'd go with the variable speed.

David

 

[BigInch]

My point is that it would probably be better to get the process to run constantly at 1000 cfm demand and optimize your compressor for that volume rather than accomodating your production line at whatever they happen to want to run that day.... as far as designing the compressor goes. Whether that is possible or not, is a question you should settle with production. It is ALWAYS more efficient to optimize your setup for running at one constant speed, be that pumping, compressing or building widgets.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[zdas04]

BigInch,
You have to define what your "process" is. The OP's process is casting, not compressing air. Most anything he can do to optimize the casting process has a chance of positive economics. Optimizing an air compressor for optimum efficiency probably has marginal to negative economics.

The air compressor is a tool of production like a chain fall or a fork lift. Tools of production need to be optimized to never be an impediment to production. So you oversize them. You make them tougher than they need to be. You optimize them toward the high end of process needs. The OP is putting in an air compressor that has significant over capacity in normal operations. His air-demand is too high for storing a significant reservoir of compressed air, so his compressor needs to be flexible. If he can provide the same process need for flexibility with either variable speed or variable capacity then he can save a few bucks by going with variable speed.

Even when a compressor (or pump or widget) is the process (like in a natural gas compressor station for example), it will rarely have a constant throughput requirement. Suction pressure will fluctuate. Discharge pressure will fluctuate. Required volume moved will fluctuate. You can design for constant speed as these things change, but (for compressors, I don't know much about widgets), that comes at a cost of significant throttling of the suction or discharge flow to maintain a constant speed requirement--I contend that taking a 50 psid drop across a suction controller and a 200 psid drop across a discharge controller as I saw in a compressor station recently is not more effecient than letting the speed match currently prevailing needs. I'm not buying your always statement.

David

 

[fogell21]

Without divulging too much information, the main use of compressed air will be our "burnout" units, which fluctuate on time and BTU's with each different pattern. The units are capable of requiring up to 150cfm each (there may be up to 14 total in the future) but our process right now runs them between 80-100cfm (we currently have 3). The short term we are upgrading to a total of 9 burnouts so our estimated maximum cfm is 100x9 = 900cfm.

The other users of compressed air are 3 coating robots, hand chiseling tools, dust collector (air valves) and our machining area (several Mazaks and Mori Seiki machines, as well as some festo cylinders on our leak test equipment).

We really don't have a good time study to say what is being used when and for how long,etc as we don't have all this equipment in our facility yet. The way I look at it, is the casting process is always shuffling, and what I mean by that is we will likely never have 9 burnout stations running at the exact same pace, they will be starting and stopping at different points meaning our usage will fluctuate.

It is this fluctuation in our usage on the burnout stations that leads me to belive that a variable speed may be better as we may end up running at 70% or lower capacity most of the time.

Does this help answer a few of the unknowns?

 

[BigInch]

Its true.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[zdas04]

Oh, I guess that settles it.

 

[fogell21]

So you both agree that a variable speed would be the best way to go? I am new at this and I didn't want to just make a decision based on what the guy trying to sell his product was saying, I guess I am a bit skeptical of sales people.

Thanks.

 

[zdas04]

I'm not sure that BigInch and I will ever agree on this subject. We're coming from a different starting point. Neither one of us is wrong about this particular subject.

When I have a varying capacity requirement that is greater than I can handle with some sort of unloader (turn valve or slide valve on a screw, valve unloaders on a recip), then I'll always go toward variable speed rather than variable capacity. BigInch can speak for himself.

David

 

[ccfowler]

fogell21,

What type of compressors are being considered, reciprocating, screw, centrifugal, ...? What is the required system pressure?

Most likely, you would be wise to pay more attention to receiver sizing to cover reasonable demand variations. Remember that the amount of work required varies with the type of compressor chosen. The minimum work is achieved by best approximating an isothermal (constant temperature) process. For a typical compressed air system operating at 100 psig, a 2-stage reciprocating compressor with decent intercooling and aftercooling will cost significantly more than a single stage compressor, but it will use much less energy, and it will last much longer and be more reliable because the maximum temperatures are much lower. Operating a single stage compressor at variable speed will still require more energy than a 2-stage machine. Also, the compressor may not tolerate variable speed operation very well if the speeds go too low or too high.

Based on my experience and observations, it is very difficult to beat a big, slow running multi-stage reciprocating compressor for energy efficiency and reliability. Since you seem to be needing to supply a nominally constant pressure system, varing the speed of a centrifugal compressor would make no sense because its discharge pressure is highly speed dependent.

Rather than worrying about having one compressor cover the performance range needed, I would look very seriously at the probable actual duty cycles required. It may make much more sense to consider three 50% compressors or even four 35% compressors. Reliability of the supply of compressed air to your process(es) is what really matters.

Variable speed drives add additional potential reliability risks with no (yes NO!) efficiency advantage for powering reciprocating compressors. To accommodate varying flow requirements, recips can be operated at idle for reasonable periods of time with astoundingly low power consumption requirements. If your process has significant periods of time with relatively small flow requirements, you may want to consider one quite small compressor to cover these periods and then an assortment of larger compressors to cover other periods. The process needs will control a rational compressor selection process.

Don't forget that a variable speed drive imposes parasitic losses of its own on the order of 5% or more in addition to significantly increasing motor and control costs. As I have often stated, variable speed drives, when properly applied to suitable applications, can provide dramatic energy savings (I have seen as much as 90% savings at low loads), but they are anything but a universal answer to any or every problem. An industiral air compression system is almost certainly not a suitable application for a variable speed drive.

Compressors and compressed air systems are expensive, but false savings on these can impose staggering costs on your processes if they are not sufficiently reliable and durable.

Be careful when considering so-called non-lubricated air compressors. Any compressor will concentrate ambient oil vapors or fumes, so if any portion of your process has any specific sensitivity to oil, filtration and separation equipment will still be required. Non-lubricated compressors can perform quite well, but they are not likely to compare favorably with lubricated compressors for very long-term durability, reliability, and maintenance requirements. I've seen and heard many claims favoring non-lubricated reciprocating compressors, but I have never seen a case where a non-lubricated compressor performed as well as claimed in terms of durability and maintenance requirements. I have seen low speed lubricated multi-stage reciprocating air compressors provide durability, reliability, and performance greatly exceeding all claims, guarantees, and expectations.

For really serious energy savings, you will be wise to pay very close attention to compressed air system leakage, and more importantly, to air usages that can be better served by non-compressed air based items. Compressed air should only be used where it is the only reasonable choice. For example, an air motor may be the prudent choice where a safety consideration controls, but otherwise, an electric motor would require dramatically less total energy. If significant amounts of compressed air are required at different pressure levels, serious consideration should be given to multiple compressed air systems. It may be reasonable to have a lower pressure system serve as the main one with booster compressors drawing air from that system to serve higher pressure systems.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[CaracasEC]

Hi Fogell21,
The problem of positive displacement compressors such as reciprocating compressor is the high maintenance cost. Wear bands, piston rings, piston packings, valve plates, and so on are just some of the spares that should always be at hand. Capacity control could be suction unloaders and VFD's.
I recommend that for an air system (Plant Air and Instrument Air)centrifugal type is better especially in terms of maintainability. It could be integrally geared compressor package and downstream of this equipment will be the plant air receiver, instrument air receiver and air dryer unit. Maintenance cost for this type of compressor is more economical than a reciprocating compressor. Capacity control for a centrifugal compressor could be suction throttling, inlet guide vane, VFds etc.

 

[ccfowler]

fogell21,

CaracasEC's cautions about recip compressors is valid to a point, and they should be kept in mind. The relatively high speed compressors that are typically chosen for their lower initial cost can have substantial maintenance requirements, and due to their higher speed, they consume more energy than larger, low speed recips.

With just reasonable, normal attention in the form of keeping filters clean, cooling jacket temperatures proper, inter-stage cooling temperatures proper, and maintaining proper lubrication, it is not unusual for low speed, multiple stage reciprocating compressors to operate continuously for decades between scheduled major maintenance outages. For the normal, contant pressure system that you indicate, these are almost always the most energy efficient choice and the most durable and reliable if flow variations sre significant.

If your normal minimum flow requirement is sufficiently high, the choice of a multi-stage centrifugal compressor may be an excellent choice for minimizing initial system cost along with achieving good efficiency. The use of low speed, multi-stage recips may be a very good way to handle variations in flow requirements while keeping the centrifugal compressor running at full load.

I disagree with use of VSD's for any of these compressors. Because the discharge pressure of a centrifugal compressor is dependent upon speed, very little energy can be saved by reducing speed while attempting to maintain the discharge pressure while the VSD imposes its 5% or more parasitic losses.

You will need to work out ehat makes the most sense for your system's needs. You would do well to give serious consideration to multi-stage oil-flooded screw compressors for part of your flow needs, too.



Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[CaracasEC]

Hi Fogell21,
For an air compressor, i recommend that you use an inlet guide vane in the suction plus a blow-off valve at the discharge considering that you are maintaining a constant discharge pressure. There is always misconception between inlet guide vanes and throttling valves such as butterfly valves on compressor systems. Inlet guide vanes varies the inlet angle to the impeller blades with minimal friction loss varying compressor capacity while the butterfly valve as a throttling valve throttles the flow and have more friction loss which is equal to wasted energy. For air compressor discharge, a blow-off valve is more efficient than having a minimum bypass flow which will need an anti-surge protection. This system is usually used if the process gas is toxic/flammable but for air this will not be a problem.

 

[Smokerr]

Keep in mind for recip use, VFDs can go down to 25% of motor speed, but the compressor has a minimum speed needed for lubrication. You always have to ensure that the VFD maintain at least the minimum compressor speed.

Also, add on VFDs are bad things (and possibly package units).

There is only recently attention being given to the VFDs and shaft currents eating up bearings on the motors. Unless its packaged and you ensure they have done their homework in shaft current protection/elimination you can have bearing failures often (I had it manifest in 3 months on a fan drive).

While we have had good success with the use of ceramic hybrid bearings, its also recommended that a shaft grounding system (like Aegis) be used as part of the complete protection package (bleeds off the potential rather than letting it build up to potentially dangerous levels).

If for any selected compressor type that has a VFD they say, what problem? Run.

 

[MechEngNCPE]

Why not go rotary screw or centrifugal? 150hp seems to be enough to breach into these machines. Also consider a larger receiver to bring down your hp requirements.

 

[Smokerr]

I have had no experience with centrifugal so no opinion on those.

Screws in my opinion are a pain. Oil carry over is an issue and takes very good setup at the point of service. They also wear out (preferred operation mode is running in load unload if you have to rather than on off which is harder still on them). 30k is the limit on our Trane refrigerant screws. Replace the air end at a very high cost. You of course need at least on backup or some other type of backups for them regardless .

I had a system with Coalescing filters, thermal mass dryer with the same setup and got oil hundreds of feet away in a small sub service coalescing. No idea why it finally separated out there, good thing as it was sensitive stuff (controls ergo the final filter). Also with the Sullair, we had fitting start to leak on the pipes, one time fittings, replace, leak, replace leak.

Personally I like the Quincy QR25s, I think 25 hp is the biggest (100 cfm roughly). Three of those and you are covered (one for backup). It has an oil pump, you can got to continuous run and load unload for the lead unit. Loaded they are rated for 100% duty cycle (air cooled recips are only 50%)
For service ease, the vertical models are easier to work on. 15 hp I think is the biggest there.

Champion also makes a pressure lubricated model. Have not had one, but it looks pretty good. It is limited to 70% duty cycle (lighter build, iron sleeve in an aluminum jug I believe).