So, after years of procrastination, I finally got around to this little project: Using a refrigerator compressor as a pump for pneumatic spearguns.
The idea is not new at all. In fact, it was mentioned on this very forum years ago and there are a lot of build-threads on an Italian forum as well. So, the inspiration was there, the research was easy and Youtube helped with the rest.
This wont be a very detailed build-log as it's all pretty straight forward. Where I did do things slightly different, I'll make sure to point it out.
You can use a new or old pump. I happened to buy a new one, which is likely a Danfoss clone, from an online shop in China (where I live) and had them ship it without any gas in it.
In regards to the size of these things, I think they can all put out enough pressure. But the bigger ones move more air, so it will be faster to take a gun up to pressure. I still opted for one of the smaller ones. They take up a little less space, weigh a lot less and are cheaper, too.
This is what I bought though mine is a 75w model (pic below shows a 65w version). Apologies for the vendor's pic as I forgot to snap one myself when I unboxed it:
I have a box of misc. pipe fittings lying around from building my vacuum pump and the Scuba Tank "Pump" project, so for this one, I just needed a bit of copper pipe and elbow fittings, which I soldered to the stubs on the compressor:
I actually ended up using a quite soft solder as I didn't have any luck with the stiffer stuff with more copper in it.
As you can tell from the pic, I used a U-bolt to hold the T-fitting in place on the alu frame to add some rigidity to the piping. A side bonus of that setup is that if any of my soldering joints were to give, nothing can really come flying at me. But I test drove this compressor already and the soldering job is holding up fine. The trick is to make sure your joints are well abraded, solvent-cleaned and fluxed.
On my Scuba Fill Adapter I have a safety valve, but I don't on this compressor build. I opted to leave it out for two reasons; first, I don't have another safety valve around and secondly, the compressor pump is quite slow so should leave me enough time to shut it off safely. If I change my mind, I can easily swap out the T-fitting with a four-way one and add the valve.
You can go about this a 1000 different ways when you choose your fittings, especially because you can modify and solder fittings on easily. E.g. the fitting that goes from the outlet elbow to the T-fitting is a barbed brass hose fitting that I drilled out for the copper tube. Here's a pic showing how it looks un-modified:
As for the pressure gauge, I knew I wanted one that went up to 40 bar as my Mirage guns will normally be pumped to 28-33bar. These gauges come in different sizes named after the size of their dials. Here are three sizes: 40, 50 and 60mm (all to 40bar):
In the end, I went for the bigger 60mm one. It makes it easy to read:
One thing I did do differently than any of the build-threads I have come across was to change out the oil in the compressor. The compressor motor sits in an oil bath but if I understand correctly, a bit of that oil gets pushed around with the gas in the whole system (in the case of an AC/fridge/freezer). Since I don't know if the compressor oil is any good in guns, I changed to the same oil I use in my guns. Might be a bit risky, but so be it.
Compressor oil on the left, gun oil to the right:
To change the oil, I poured out the original oil, flushed with a solvent, let it evaporate and then poured in an equal amount of new gun oil.
I've seen other guys add an air filter on the intake line which seems like a good idea to protect both the gun and the compressor from debris getting into them:
I attached the filter with a bit of PVC tube onto a barb fitting that I soldered onto an elbow which in turn is soldered onto the air intake. If I ever wanted to use this compressor as a vacuum pump, it is as easy as unplugging the air filter and plugging the vac hose on instead:
One thing I knew I wanted on this build was a handle. The compressor weighs 6.5kg and there's no obvious place to grab it for moving it around, so I added a T-handle. It also serves to lock in the alu mounting frame on the top:
Here it is in its almost final beauty - I will add an on-off switch later on:
Forgot to say that the high pressure hose is from a vendor that sells parts for PCP guns (pre-charged pneumatic air rifles and such). Those guns run at around 200 bar, so this is more than safe for my use. It has quick-couplings on both ends. But you can easily leave the hose out, a lot of builders have done that.
The pump valve adapter on the gun side of the hose is a bit of a Frankenstein rig. Its heart is the Salvimar fitting at the front (this one is for Mares, but they make and sell them for Cressi and Salvi, too) and then some adapters to get the male quick-coupling fitting mounted. This is one thing that could be made as a single part on a lathe, which could be a latter upgrade;-)
The idea is not new at all. In fact, it was mentioned on this very forum years ago and there are a lot of build-threads on an Italian forum as well. So, the inspiration was there, the research was easy and Youtube helped with the rest.
This wont be a very detailed build-log as it's all pretty straight forward. Where I did do things slightly different, I'll make sure to point it out.
You can use a new or old pump. I happened to buy a new one, which is likely a Danfoss clone, from an online shop in China (where I live) and had them ship it without any gas in it.
In regards to the size of these things, I think they can all put out enough pressure. But the bigger ones move more air, so it will be faster to take a gun up to pressure. I still opted for one of the smaller ones. They take up a little less space, weigh a lot less and are cheaper, too.
This is what I bought though mine is a 75w model (pic below shows a 65w version). Apologies for the vendor's pic as I forgot to snap one myself when I unboxed it:
I have a box of misc. pipe fittings lying around from building my vacuum pump and the Scuba Tank "Pump" project, so for this one, I just needed a bit of copper pipe and elbow fittings, which I soldered to the stubs on the compressor:
I actually ended up using a quite soft solder as I didn't have any luck with the stiffer stuff with more copper in it.
As you can tell from the pic, I used a U-bolt to hold the T-fitting in place on the alu frame to add some rigidity to the piping. A side bonus of that setup is that if any of my soldering joints were to give, nothing can really come flying at me. But I test drove this compressor already and the soldering job is holding up fine. The trick is to make sure your joints are well abraded, solvent-cleaned and fluxed.
On my Scuba Fill Adapter I have a safety valve, but I don't on this compressor build. I opted to leave it out for two reasons; first, I don't have another safety valve around and secondly, the compressor pump is quite slow so should leave me enough time to shut it off safely. If I change my mind, I can easily swap out the T-fitting with a four-way one and add the valve.
You can go about this a 1000 different ways when you choose your fittings, especially because you can modify and solder fittings on easily. E.g. the fitting that goes from the outlet elbow to the T-fitting is a barbed brass hose fitting that I drilled out for the copper tube. Here's a pic showing how it looks un-modified:
As for the pressure gauge, I knew I wanted one that went up to 40 bar as my Mirage guns will normally be pumped to 28-33bar. These gauges come in different sizes named after the size of their dials. Here are three sizes: 40, 50 and 60mm (all to 40bar):
In the end, I went for the bigger 60mm one. It makes it easy to read:
One thing I did do differently than any of the build-threads I have come across was to change out the oil in the compressor. The compressor motor sits in an oil bath but if I understand correctly, a bit of that oil gets pushed around with the gas in the whole system (in the case of an AC/fridge/freezer). Since I don't know if the compressor oil is any good in guns, I changed to the same oil I use in my guns. Might be a bit risky, but so be it.
Compressor oil on the left, gun oil to the right:
To change the oil, I poured out the original oil, flushed with a solvent, let it evaporate and then poured in an equal amount of new gun oil.
I've seen other guys add an air filter on the intake line which seems like a good idea to protect both the gun and the compressor from debris getting into them:
I attached the filter with a bit of PVC tube onto a barb fitting that I soldered onto an elbow which in turn is soldered onto the air intake. If I ever wanted to use this compressor as a vacuum pump, it is as easy as unplugging the air filter and plugging the vac hose on instead:
One thing I knew I wanted on this build was a handle. The compressor weighs 6.5kg and there's no obvious place to grab it for moving it around, so I added a T-handle. It also serves to lock in the alu mounting frame on the top:
Here it is in its almost final beauty - I will add an on-off switch later on:
Forgot to say that the high pressure hose is from a vendor that sells parts for PCP guns (pre-charged pneumatic air rifles and such). Those guns run at around 200 bar, so this is more than safe for my use. It has quick-couplings on both ends. But you can easily leave the hose out, a lot of builders have done that.
The pump valve adapter on the gun side of the hose is a bit of a Frankenstein rig. Its heart is the Salvimar fitting at the front (this one is for Mares, but they make and sell them for Cressi and Salvi, too) and then some adapters to get the male quick-coupling fitting mounted. This is one thing that could be made as a single part on a lathe, which could be a latter upgrade;-)
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