Question on air compressors:::

[Old Beaver]

Hi guys, over the next few months I will be having a rethink of the layout of my workshop. As I do blacksmithing/welding etc I am going to build a separate forge to the rear of my property. This will let me get all my metal working tools and those that cause sparks out into a safer work space, leaving me much needed space in my wee joinery shop. One change that is going to take place is I am going to fit my air compressor outside under it's own we shed; more like dog shed size. Currently I have a 100ltr tank compressor and I have had this for 40 years and still going strong. I am thinking in putting in a larger compressor, something along the size of 150-200 ltrs, but more importantly, 3HP, 14CFM and single phase. Size available will determine tank size.
I have 2 questions:
1. I have looked at compressors that are 14 cfm air displacement, but then they say 11 cfm free air delivery / 300L/min. I am not sure what the bolt writing mean completely. E.g. Does free air mean the air that comes to the tool????????? If so what does 14cfm mean???? I would appreciate it if someone could explain the bolt writing above to me.

2. What compressors are you guys using? What's your thoughts on them? If it packed in would you buy the same make and size and why?

I have some limitation here in N. Ireland to what I can purchase, but since it's 40 yrs since my last purchase, I have no clue what's a good buy.
Thanks

 

[Ozi]

I think you will find very few modern compressors working in 40 years time, they are practically disposable items now. Have you thought about buying a smaller unit and keeping your current machine and manifolding them together (obviously running at the same pressure limits), then you can run just one if not using a lot of air and save a bit on the power and if one fails you are not left flat as it were. Just one question when was the last time your safety valve was checked. I've inspected a lot of older machines that were very good but had safety valves gummed up to the point where only the high safety factors in construction of the rest of the machine prevented explosion. the 14 / 11 shows you the volumetric efficiency you are only ever going to get 11 and that will drop as load increases.

 

[Old Beaver]

I think you will find very few modern compressors working in 40 years time, they are practically disposable items now. Have you thought about buying a smaller unit and keeping your current machine and manifolding them together (obviously running at the same pressure limits), then you can run just one if not using a lot of air and save a bit on the power and if one fails you are not left flat as it were. Just one question when was the last time your safety valve was checked. I've inspected a lot of older machines that were very good but had safety valves gummed up to the point where only the high safety factors in construction of the rest of the machine prevented explosion. the 14 / 11 shows you the volumetric efficiency you are only ever going to get 11 and that will drop as load increases.

Safety Valve!!! That needs checking. Any tips on how to do that??

 

[Ozi]

The way it's done is hydraulically, we used a thing called a bucket pump and a calibrated gauge. I don't imagine you have those to hand. I would ask a local garage who does their insurance inspections, your looking for a tame inspector who might test your valve when he's doing theirs rather than contacting the insurance company direct who will get all unnecessarily financial about it. My old boss said he would sack anyone who took more than the price of a cup of coffee for a side job but we might have done a few free on the side in the name of general safety

 

[sometimewoodworker]

Currently I have a 100ltr tank compressor and I have had this for 40 years and still going strong. I am thinking in putting in a larger compressor, something along the size of 150-200 ltrs, but more importantly, 3HP, 14CFM and single phase. Size available will determine tank size.

What compressors are you guys using? What's your thoughts on them? If it packed in would you buy the same make and size and why?

an important point that is missing is what are you using it for as that makes a big difference to a decision on size and kind of compressor that works for you. Also If you DO have some big tools that require a lot of air (but you use them only once in a while) it’s better to install a bigger compressed air receiver than to buy a bigger reciprocating compressor or even a rotary screw compressor.

So do you need a bigger compressor or just a much bigger tank that you could fit near the compressor or in the workshop?

in the last few years there is a completely different compressor motor that has become available on the domestic market like these.

for me I have a 3 cylinder Reciprocating compressor yes I would buy it again but I would get a big receiving cylinder as well.

 

[Old Beaver]

OK. This amount of detail is all new to me so I hope I am reading you correctly, e.g. receiver is the holding tank, correct???? Use> general air blowing around the shop, tyre pressure, spraying paint, air die grinders. Hopefully drills and files. Currently when using my die grinders the air drops quickly and the compressor runs all the time, but never catches up unless I shop. That means the grinder runs slow. Some of the air files run at a higher CFM. I really want to have my set up outside the shop as I have little space.

 

[Spectric]

One change that is going to take place is I am going to fit my air compressor outside under it's own we shed; more like dog shed size. Currently I have a 100ltr tank compressor and I have had this for 40 years and still going strong. I am thinking in putting in a larger compressor, something along the size of 150-200 ltrs, but more importantly, 3HP, 14CFM and single phase.

What size is your current compressor, it may be that you just need to add an extra receiver. How long does it take to reach pressure, and what is that pressure?

 

[sometimewoodworker]

OK. This amount of detail is all new to me so I hope I am reading you correctly, e.g. receiver is the holding tank, correct????

Yes, in the pictures above it’s with wheels on it

general air blowing around the shop, tyre pressure, spraying paint, air die grinders. Hopefully drills and files. Currently when using my die grinders the air drops quickly and the compressor runs all the time, but never catches up unless I shop (stop?).

that definitely means you need a larger compressor, or quite possibly a second one that kicks in when spraying or using your die grinders. Personally I would not sell the existing one since it’s still giving good service and the used value is probably minimal. That would also give you a much larger receiving tank volume for less than if you bought completely new.

Regrettably just adding in another receiver will probably not be good enough if you are needing to die grind a lot as it will certainly extend the time before the compressor kicks in but that’s only going to help a little since the compressor can’t keep up.

 

[Davey44]

I was much in your situation a couple of years ago. I started off with a cheap'ish 2hp 50 litre job from Aldi pumping out a stated 7cfm. Realistically that 7 should probab;y have been 5. However, it sufficed for several years until I needed to start using air driven nibblers and the odd spray job. I searched around for a while, mainly for the used/like new advertised versions. I ended up buying one with a 3hp motor, belt-driven, with a 120 litre tank and a claimed 17cfm. It had been used in one of the many RAC recovery/repair vans and I paid only £150 instead of the £315 new price. It had apparently been fitted in one of the vans that had been 'retired early' because of a 'little bump'. It's wonderfully quiet compared with the direct drive Aldi version and has worked hard for me ever since. I've used it for all kinds of jobs about which I'd had no idea about when I bought it. Examples include two air chisels thumping away cleaning up bricks, spraying our garden fences and cleaning out filters on the extraction units and vacuum cleaners.

 

[BillK]

Displacement is something like car engine capacity, where free air/FAD is what it delivers - like your car's bhp. The FAD is what really matters. Rough rule of thumb is 3cfm delivered per hp of pump power. 10-ish delivered is typical of 3hp motors. Tank size relates more to how often the motor will run to top it up.
Match your tools air demand roughly to the comp's capability. And whether it needs more like bursts or constant running. Like I do a fair lot of spraying using 3hp comps so look for guns wanting around 10cfm or less. And something like an air sander wanting 15cfm or more at 90psi continuous is going to struggle.

3hp might need 16A for start-up, one of mine is on 16A and the other needed to go on 32A to cope.

 

[Old Beaver]

What size is your current compressor, it may be that you just need to add an extra receiver. How long does it take to reach pressure, and what is that pressure?

100 ltr and is 40 yrs old. From memory about 90psi but drop very fast. If is starts on its own, due to a little leakage, 5-7 mins to fill. If I'm spraying or using a die grinder it runs a lot of the time. The CFM is low, can't remember offhand what it is but really not enough for running air tools. I won't be running lots or heavy stuff but I would like options. I'm retired so it does not get as much use as it did.

Being honest, adding another receiver, I have no clue what that would look like or how to do it. My thinking is, right or wrong, the bigger motor and tank with better CFM will keep the air flow up and my e.g. die grinder running at full capacity for longer. Hope that make sense. When I say outside I mean outside the shop but under cover with air feeding into my main shop and the new smithy shop I will build later this year.

 

[Old Beaver]

Yes, in the pictures above it’s with wheels on it

that definitely means you need a larger compressor, or quite possibly a second one that kicks in when spraying or using your die grinders. Personally I would not sell the existing one since it’s still giving good service and the used value is probably minimal. That would also give you a much larger receiving tank volume for less than if you bought completely new.

Regrettably just adding in another receiver will probably not be good enough if you are needing to die grind a lot as it will certainly extend the time before the compressor kicks in but that’s only going to help a little since the compressor can’t keep up.

Yes I am aware I would get peanut for the old one, although when I bought 40 years ago It was about £100, I think?? A lot back then and I was just starting out. I said in my last reply to another post that while I understand the concept of an additional receiver, I have no clue what it would look like or how to do it. As you say, my CFM would change and once I use the air down to the refill trigger I'm back with a die grinder that is half power. Once I build my smithy shop/forge I will use air a lot more. The other variable is my youngest son had just finished serving his time as a joiner, so he has all the modern ideas and tools. I am old school. I use contact adhesive out of a tin and spreader, he sprays it on as he demonstrated to me last night as we were gluing on 1.0 mm steel unto the new security door. Wow that was an education for an old school boy .

 

[Old Beaver]

I was much in your situation a couple of years ago. I started off with a cheap'ish 2hp 50 litre job from Aldi pumping out a stated 7cfm. Realistically that 7 should probab;y have been 5. However, it sufficed for several years until I needed to start using air driven nibblers and the odd spray job. I searched around for a while, mainly for the used/like new advertised versions. I ended up buying one with a 3hp motor, belt-driven, with a 120 litre tank and a claimed 17cfm. It had been used in one of the many RAC recovery/repair vans and I paid only £150 instead of the £315 new price. It had apparently been fitted in one of the vans that had been 'retired early' because of a 'little bump'. It's wonderfully quietT

Yes I think I want a 3hp belt drive one. My CFM isn't very much but I can't remember what it is. I don't even think it is 7 CFM but that is based on very little evidence. There is no doubt, having a better machine will open up the options for me that I want; a nibbler being one of them.

 

[Old Beaver]

Displacement is something like car engine capacity, where free air/FAD is what it delivers - like your car's bhp. The FAD is what really matters. Rough rule of thumb is 3cfm delivered per hp of pump power. 10-ish delivered is typical of 3hp motors. Tank size relates more to how often the motor will run to top it up.
Match your tools air demand roughly to the comp's capability. And whether it needs more like bursts or constant running. Like I do a fair lot of spraying using 3hp comps so look for guns wanting around 10cfm or less. And something like an air sander wanting 15cfm or more at 90psi continuous is going to struggle.

3hp might need 16A for start-up, one of mine is on 16A and the other needed to go on 32A to cope.

Yes 16Amp sockets is not problem. I have a machine running of one and my new welder runs of 16 amp so I put in a new consumer unit and so I could add more 16amp sockets.

 

[Old Beaver]

Guys thanks you all for your help. If I have not replied to you I apologise. I do feel I need a stronger quicker pump system so 3hp is what I will aim for. I can get one with 14CFM with 11CFM free air. I believe this is what will power my tools. I'm not in a rush, retired, you know what I mean. So I will research more and as I said to someone, my youngest son has just complete his time as a joiner and he will most likely use it more than me.
Thanks again for your help and stay safe in your shop/sites and do good work.

 

[Ollie78]

I have a Nu air 150 litre, belt drive 3hp. Its great and not expensive. Has a handy built in regulator as well. (do shop around some people are asking £200 quid more than others for the identical machine).
Had mine 5 years or so no problems at all, runs of 13amp plug too.

Ollie