Single stage vs two stage with same specs

[6PTsocket]

Recently I ranted against HF selling a single stage as an alternative to the 5hp 2 stage with the same specs. Unless they are flat out lying, why is a single stage three cylinder that is 165 psi max and the same cfm worse that a 2 stage. I admit I did not check duty cycle or pump speed and I doubt they even give it. CH used to make a 4 cyl single stage that put out a lot of air. What is the advantage of a 2 stage if the specs are the same? Thanks.

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[brollona]

First thing that falls on my mind is size. Also , I don't know for small compressors , but for larger ones used in industrial sector cooling between stages is big deal.

 

[u3b3rg33k]

Recently I ranted against HF selling a single stage as an alternative to the 5hp 2 stage with the same specs. Unless they are flat out lying, why is a single stage three cylinder that is 165 psi max and the same cfm worse that a 2 stage. I admit I did not check duty cycle or pump speed and I doubt they even give it. CH used to make a 4 cyl single stage that put out a lot of air. What is the advantage of a 2 stage if the specs are the same? Thanks.

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they're probably being creative. like a car saying 30mpg, 300hp.
it's 30mpg OR 300hp. not getting both at the same time.
Single stage is probably 13cfm @40psi OR 165PSI, while the two stage is 13cfm @ 165PSI. TINSTAAFL.

 

[pcmeiners]

Are the outputs the same using standard cubic foot per minute measurement?
Single stage has a much higher output temperature then two stage. If the CFM is measured at the higher temperature, the single stage is less efficient.
Due to the higher temperature, the single stage will be more stressed in operation, parts and oil

 

[DerekV]

Single stage compressors start to get really inefficient past something like ~110psi. Exponentially inefficient might be a better way to describe it. Inefficiency means heat which translates into shorter component life, namely the reed valves. A single stage compressor set for 165psi is not long for this life...my guess...MAYBE a year of weekend hobby use.

 

[seber]

I'm pretty sure that either you are misreading the spec or they are flat out lying. No single stage compressor is going to get 165 psi for more than a few minutes before it destroys itself.

 

[sberry]

Lot of speculation about the service life. In fact all.

 

[DerekV]

Lot of speculation about the service life. In fact all.

Not sure who this is directed towards, but if it’s me I have first hand experience building single stage air compressors that are set beyond their actual efficiency range (~110 psi) using Harbor Freight pumps. Two different pumps, two different compressor builds. Both lasted about a year pumping to 145psi, which is what the boxes said they were rated for. Tested with several pressure gauges to confirm it wasn’t a gauge issue.Failure mode on both were the reed valves.

At 165psi, there’s no way they’d last more than a dozen hours. If they did, two stage pumps wouldn’t be nearly as popular.

No speculation necessary.

 

[u3b3rg33k]

^^ in addition, 2 stage is more efficient than 1 stage, in in terms of CFM/HP. it's not just pump lifespan that suffers, but your utility bill, too. that might not matter to the tire-filler, but anyone who uses a LOT of air probably cares about TCO (total cost of ownership).

 

[sberry]

Not sure who this is directed towards, but if it’s me I have first hand experience building single stage air compressors that are set beyond their actual efficiency range (~110 psi) using Harbor Freight pumps. Two different pumps, two different compressor builds. Both lasted about a year pumping to 145psi, which is what the boxes said they were rated for. Tested with several pressure gauges to confirm it wasn’t a gauge issue.Failure mode on both were the reed valves.

At 165psi, there’s no way they’d last more than a dozen hours. If they did, two stage pumps wouldn’t be nearly as popular.

No speculation necessary.

I understand the effecienCy but the speculation about service life here is mostly speculative. I agree it's going to be shorter. Would be interesting to see the warranty and find out what the design spec is on some of the higher ones are.
I agree 110 could be optimum and the uper end of single being 135. That is about ideal for in to 50 ft 3/8 hose real, heavy 1/2 impact. Gun works great while tank is real full, drops fast after that. 2 stage is turning on when that is turning off. Doesn't matter where it is in the charge cycle to maintain constant pressure to a limit of course.

 

[Citation]

Are the outputs the same using standard cubic foot per minute measurement?
Single stage has a much higher output temperature then two stage. If the CFM is measured at the higher temperature, the single stage is less efficient.
Due to the higher temperature, the single stage will be more stressed in operation, parts and oil

I agree with the higher outlet temp comment (more below) but I was under the impression CFM was measured at the pump inlet. However, if measured at the outlet again I could see the issue.



OP, in theory a two stage isn't more efficient but to make that theory true would be difficult. To step back a moment, when you compress air you raise the temp and pressure at the same time.

So in one case your outlet air temp is say 200F and another case say 250F, both at 150psi. Which pump is delivering more air into the tank? Well if you let that air cool to room temp you will find the 200F outlet temp air cools to 121 psi while the 250 outlet air cools to 111 psi. (PV=nRT based calculations, BIG assumption that the outlet temp air and tank temp air are the same)

So the pump that discharges the hotter air actually discharges less room temp air even though the pressure and volume of hot outlet air is the same.

So why is two stage better? Well it's better because you can install an inner cooler which helps pull heat out of the air before you compress it a second time. Is it better than an after cooler? Not sure. Both cool the air (and thus drop the pressure) before the air gets to the pressure switch. However, I suspect that a two stage system should generally have better cooling in most cases so your outlet air temps will be lower.

How much this matters for a garage user? Probably not much (not sure about moister in painting) but like many power tool choices we are often motivated by technical differences that might not actually matter in our application.

 

[Citation]

I'm pretty sure that either you are misreading the spec or they are flat out lying. No single stage compressor is going to get 165 psi for more than a few minutes before it destroys itself.

I would agree with the concern though many of the portable compressors are single stage and well over 165 psi
https://www.ereplacementparts.com/d...carry-compressor-parts-c-1009_3079_14914.html

 

[Citation]

^^ in addition, 2 stage is more efficient than 1 stage, in in terms of CFM/HP. it's not just pump lifespan that suffers, but your utility bill, too. that might not matter to the tire-filler, but anyone who uses a LOT of air probably cares about TCO (total cost of ownership).

I've always read this but based only on looking at 120V compressor specs I'm not sure it's true. Consider CH made several models of belt drive, oil lubed pump compressors. I'm making an assumption that they are all about the same power in (15A @ 115V). The single stage model I have is rated at 5.7 CFM A 90 psi, 125psi peak (per the label). There is also a CH/Kobalt 2 stage model at Lowes with similar motor specs. It's peak pressure is 175 psi and 5.1 CFM @ 90 psi. I know that isn't true proof but it seems to also align with what I've seen of low end 80 gallon "5 hp" models with similar motor current draw. The single stage ~135-145 psi models have better flow numbers vs the 2 stage 155-175psi models. Of course I'm talking about lower end stuff here. This is my observation rather than something based on physics.

 

[u3b3rg33k]

observed efficiency is generally much lower than theoretical efficiency. for example: diesel engines. higher CR = more efficiency, but in practice, cars ended up at like 16-18:1 instead of 21:1 or higher, because the gains just weren't there in the real world. with 120V compressors, my guess is the motor efficiency matters more. not sure how many 2 stage 120V pumps are out there.

as for 2 vs 1 stage on coolers, intercooler = good for keeping 2nd stage head temps down/VE up. does very little for stage 1 I would think. aftercooler is good for water removal. if it were me, I'd measure inlet CFM and not even wonder about output "CFM".

 

[finn]

Your Diesel engine analogy isn’t very good, though.

Old IDI Diesels typically have compression ratios in the neighborhood of 21:1 because of the heat losses inherent withe pushing all of the air into the prechamber, where the fuel is injected and combustion initiates. Heat energy is lost again when the burning mixture expands into the main chamber.

Direct injection combustion is usually best in the neighborhood of 16-17:1, depending on fuel system pressure capabilities and after treatment technologies.

Compression ratio is usually boxed in by cold starting and smoke generation limits.

Within a narrow range, efficiency trade offs are minimal, compared to other performance parameters. The biggest factor in efficiency in a diesel is the fact that there’s no throttle to induce pumping losses. Cr is boxed in by smoke, starting, and cylinder pressure limits.

Direct injection gasoline engines do see higher efficiency with higher cr, , and the cooling effect of fuel expansion in the chamber reduce knock and make the higher cr feasible.

 

[sberry]

Injection also better due to no fuel in the cyl till its needed.

 

[pcmeiners]

"I agree with the higher outlet temp comment (more below) but I was under the impression CFM was measured at the pump inlet."

Yes CFM is supposed to be measured at the inlet, but there are many manufacturers which take liberties in not adhering to the standards , just like horsepower ratings Sears gets away with.

The amount of power saved using efficient intercoolers is substantial..

http://cascousa.com/compressed-air-101/types-of-compressors/single-versus-multi-stage-compressors/

One of my compressors with a high efficiency Intercooler, using a DB1290 cooler (over sized)....,.

https://www.garagejournal.com/forum/showthread.php?p=556948&highlight=db1290

 

[u3b3rg33k]

Your Diesel engine analogy isn’t very good, though.

Old IDI Diesels typically have compression ratios in the neighborhood of 21:1 because of the heat losses inherent withe pushing all of the air into the prechamber, where the fuel is injected and combustion initiates. Heat energy is lost again when the burning mixture expands into the main chamber.

Direct injection combustion is usually best in the neighborhood of 16-17:1, depending on fuel system pressure capabilities and after treatment technologies.

Compression ratio is usually boxed in by cold starting and smoke generation limits.

Within a narrow range, efficiency trade offs are minimal, compared to other performance parameters. The biggest factor in efficiency in a diesel is the fact that there’s no throttle to induce pumping losses. Cr is boxed in by smoke, starting, and cylinder pressure limits.

Direct injection gasoline engines do see higher efficiency with higher cr, , and the cooling effect of fuel expansion in the chamber reduce knock and make the higher cr feasible.

this is what I'm talking about:

pumping losses have been massively reduced on newer engines thanks to variable valve timing and lift systems. my car doesn't use the throttle at all - it's basically there as a failsafe in case the fancy valve adjustment system fails.

Injection also better due to no fuel in the cyl till its needed.

I think it's mostly this. diesels are allowed lean burn, gassers are not, due to emissions regulations. you get a little bit of the effect with DI, but not to the level you see in a diesel engine.

 

[6PTsocket]

I'm pretty sure that either you are misreading the spec or they are flat out lying. No single stage compressor is going to get 165 psi for more than a few minutes before it destroys itself.

It was printed right on the tank in a big graphic. It caught my eye because it used to be 145 for an earlier version of that compressor. High max pressure single stages are not unknown. I think there are 20 and 30 gal DeWalts that are 155 psi. I am not saying it is a good idea, just commenting on it.

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[6PTsocket]

Thanks, everyone for the info.

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[crguy]

Would you expect anything but ******** from HF? Think about it.