Question Dive Shop Compressor Power Consumption

[happy-diver]

I put an industrial fan above the door for cooling outside and slick 50 inside for cooling inside

 

[tbone1004]

I haven't decided how much information I am going to share on this project yet, but there has been a couple significant technological developments in the last 15 years that could make a nearly maintenance free compressor stage possible. No oil changes and no valve replacements even after years of continuous use. Still have to change the breathing air filters though. I could probably just describe the whole thing and nobody would believe that it would work anyway, but I want to make a prototype or two and see what happens. 95% chance that it won't work the way I think it might anyway and I will post what I tried and why it didn't work.

It seems that the consensus is that electricity consumption is not a big factor, which is what I assumed based on the initial numbers I looked at, but I wasn't sure if there was something I was missing.

Most shop owners aren't knowledgeable enough to even tell you what their actual consumable cost is for the compressor and they peanut butter math it out across all of the bills and don't actually track their production. If you made a compressor 30% more efficient I would say less than 1% of dive shops in the world would notice, and if you made it 50% less efficient maybe 2% would notice.

US National Average is call it $0.17/kwh. Rule of thumb for compressors is 1hp/cfm so the easy rule of thumb is $0.01/cf for electricity. Obviously that is based on where in the country you are and goes up and down with power factors for single vs. three phase, etc. but call it $0.01/cf for direct electricity costs *and yes it is lower than that if you actually math it but if you swap that 1hp=1kw etc. it all comes out close enough*. You can't be making that big of a change in efficiency on the pump to where it would really swing the cost.

 

[joebob24]

$0.17/cf for electricity

I think your math is off a little. I like where you are going with this, but I think you need to divide by 60 to convert from cubic feet per minute to kilowatt hours. That would make it $0.00283 per cubic feet, or $0.22 to fill an aluminum 80. Seems pretty insignificant, even though that could add up to $500 for the shop filling 600 tanks a week.

 

[tbone1004]

I think your math is off a little. I like where you are going with this, but I think you need to divide by 60 to convert from cubic feet per minute to kilowatt hours. That would make it $0.00283 per cubic feet, or $0.22 to fill an aluminum 80. Seems pretty insignificant, even though that could add up to $500 for the shop filling 600 tanks a week.

correct, sorry, coffee hadn't kicked in.

 

[TrimixToo]

A wise dive shop owner (yes, there are a few) once told me he *never* complained about the electric bill, which was largely driven by his compressors (both single-phase 240V). When he was using them a lot he was making a *lot* more money than when he used them less often.

By contrast, when I spend a lot of time in the basement with the machine tools, my CFO definitely sees it in the electric bill. But as many of those hours result in significant savings, she does not complain...

 

[joebob24]

Less efficient will also mean more heat. Watts are energy. If you use them they have to do sometime. Either be efficient and compress air, or not efficient and make heat. In the winter extra heat may be fine, although electric heating (which is what you would be doing) is usually the most expensive option. In the summer, it can add to air conditioning loads if run indoors. Around here, they are all indoors.

This one took me a bit to wrap my head around and calculate, but I think I finally came up with some heat output numbers that are interesting. Compressing air releases a lot of heat even if the compressor is 100% efficient. A 100% efficient compressor taking 1 cfm from atmospheric to 4000 psi releases requires 268 watts of input and releases 268 watts of heat. Any loss of efficiency does just turn into heat, so a 50% efficient compressor requires 536 watts of input per cfm and releases 536 watts of heat. Essentially compressors are also just 100% efficient space heaters.

This means that a shop running a 50% efficient 15 cfm compressor is essentially running a 8kw heater in the shop. Doesn't sound ideal, but if the 1hp per cfm is accurate from the previous post, then compressors are running less than 50% efficient already.

 

[tbone1004]

This one took me a bit to wrap my head around and calculate, but I think I finally came up with some heat output numbers that are interesting. Compressing air releases a lot of heat even if the compressor is 100% efficient. A 100% efficient compressor taking 1 cfm from atmospheric to 4000 psi releases requires 268 watts of input and releases 268 watts of heat. Any loss of efficiency does just turn into heat, so a 50% efficient compressor requires 536 watts of input per cfm and releases 536 watts of heat. Essentially compressors are also just 100% efficient space heaters.

This means that a shop running a 50% efficient 15 cfm compressor is essentially running a 8kw heater in the shop. Doesn't sound ideal, but if the 1hp per cfm is accurate from the previous post, then compressors are running less than 50% efficient already.

Part of that also has to do with the number of stages, the more stages the more efficient in theory but in practice that washes out with more rotating mass and friction. Best to assume the shop compressors are running to 4500psi vs 4000psi for reference and typically the current "low pressure" pumps are designed for 5000psi. In theory yes it is 0.5hp/cfm at 100% efficiency, and we do use the 1hp/cfm as a swag number for two reasons, first is that it makes for easy math, but second because it also pretty comfortably correlates to standard motor sizes. I.e. 5hp motor can run 5cfm. If you math everything back out it's actually around 1:1.2 vs 1:1 so 5hp motor typically is good for a 6cfm pump, 15hp for 18cfm, etc. Will be interested to see what you're doing to make the efficiency drop that much though, it's either in production of more heat or you're blowing off a lot more....

Don't discount the benefit of multi stages though, they are very important.... you have a LOT of stuff that needs to get knocked out of atmospheric air before it hits the filters and if you don't have enough interstage cooling you are not going to be able to knock it out.

 

[broncobowsher]

I can't say I have been around enough dive compressors to see how different a 3-stage is from a 4-stage.
But I have been around plenty of shop compressors. There is a huge jump in performance going from a single stage to a 2-stage. That's accounting for all the different sizes of storage tanks, HP, CFM ratings, etc.

I've watched the amp draw of a single stage climb as the pressure rose. To a point and the amp draw leveled off and then started decreasing. As it reached its peak, the output dropped off. Got pretty slow to reach the final pressure. Clearly was exceeding its optimal output pressure. Never seen that in a multi stage compressor.

 

[iain/hsm]

Does anyone know how big of a factor the cost of electricity and the efficiency of the compressor is for a dive shop? Is this something that is compared between units when purchasing?

This question is IMHO never ever asked or considered. Even with professional dive shops the two questions they always ask are how long to fill a standard 80 cylinder and how much. That's it, nothing else is ever asked.

Now to be fair my experience with the American scuba market is based purely on a four day DEMA show each year repeated some 20 odd times. But non the less the running cost or maintenance cost or even the cost of spare parts is never ever mentioned or one could assume never even considered.

The conclusion is that price and how long to fill are the only questions relevant to the average scuba dive shop. Hence why so much crap engineered and junk built scuba compressors are on the market all with fancy coloured stripes and the obligatory wow bling factor of a Harley to entice these natives.

Now to answer your specific question more fully the size of the motor fitted does not always align to the amount of power required. Clearly the cheap end of the market use as little as they can get away with while the better designed compressors in general use a larger motor than is required, Reason being that the cost of a motor by the pallet load is not that much different for say a 7Kw than for a 9 Kw motor.

The question of power required most compressor manufacturers and the various retail distributors and suppliers are reluctant to divulge for obvious reasons.

However for a non comparing comparison by way of example the RIX SA-6 compressing ambient air at 14.7 PSIA (ATA) atmospheric pressure to 5000 PSIG (Gauge) pressure at 6 SCFM flow will require 4.87 BHP or 3.63 Kw (Calculated at an atmospheric air temperature of 104 DegF)

Now once the above information is fully understood I would have hoped that fitting at the factory a 5.5 BHP 4 Kw motor would be better understood as an advantage. Sadly however for the most part it is manipulated by others as a disadvantage who for reasons best know to themselves hawk and bleat that the Chinese/ Italian piece of crap motor they fit is better and more efficient as its only 5 BHP. Go figure. Iain

 

[broncobowsher]

Going off memory, but don't electric motors start dropping efficiency as they reach there rated power? Maybe it is just the cheap crap motors? I'm going off a memory of looking at an electric motor dyno sheet from many years ago. There were several factors, torque, RPM, volts, amps... Enough you could map electrical HP (volts x amps to get watts) and mechanical HP (torque x RPM / 5252 if using ft/lb torque). It may have just been that one motor I was looking at, but things got ugly at the top of the scale.