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'Receiver Cut Off' on a compressor


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HI there,

 

I have a cheap Chinese compressor, the ubiquitous As-186. It's working ok, but gets very hot and cuts on and off a lot more than it used to, so I'm keeping an eye on it in case it's time for an upgrade.

 

So I had a look online, and one of the ones the caught my eye was the Sil Air 15D. It seems well liked, and very quiet, and Mike did an excellent review of it on here. However, I was surprised to read/hear about the 'Receiver Cut-Off' hiss, which is louder than the compressor itself. If I understand it correctly, basically when the tank is full some air escapes through the receiver cut off and makes a noise. As far as I know, this only happens when the tank gets full, so basically every time the compressor kicks in and fills the tank.

 

What I don't understand is why is this cut-off needed. With my As-186, when the tank is full the compressor just stops, without any noise or release of pressure. Is this a cheap compressor v decent compressor difference? Or a oil-less v oil compressor difference? It seems to me that the Sil Air 15D is almost a no-brainer (if you have the money) considering how quiet it is, but the receiver cut-off is a major downside to that quietness?

 

Any advice appreciated, thanks!

 

 

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The noise is the "unloader valve" operating & happens at the end of each motor run cycle. The valve unloads pressure in the pump to reduce starting load on the next motor run cycle, these are typically fitted to DIY workshop & commercial compressors for the same reason. Without the unloader valve being present a more powerful & consequently louder pump would be required - catch 22.

 

The valve expels air for somewhere in the region of 1 second as it's only releasing pressure from a relatively small volume & the noise is about as loud or slightly quieter than the normal running of an oil-free compressor in the 25 L/Min class (such as the AS-186).

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Thanks for your reply. So, does the AS-186 not have one of these as it's a cheap compressor and basically its motor has to work a bit harder as the starting load isn't reduced?

 

Do all good compressors have these unloader valves, or just the ones that use Oil?

 

Thanks...

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Oil free airbrush compressors, including the AS-186 are not fitted with these - more to do with the task in hand than price or quality;

The operation of the unloader valve requires a pressure control unit (the "control box" with the on/off switch) that's capable of opening the unloader valve at motor shut-off, the in-line pressure switches typically fitted to automatic oil free compressors are not capable of this. A pressure control unit adds weight, size, complexity & price to the compressor & on a run of the mill oil free model where quiet operation isn't the specific goal its more effective to specify the motor to suit the load. 

 

To rant on a bit further..... Another reason for none of these bits being required on the typical oil-free airbrush compressor is the valve arrangement - oil free compressors lend themselves to leaking easily while the motor isn't running (from the cylinder, not the receiver), easing the motor load at start-up, which is not so much the case with oil lubricated. The average automatic oil-free model charges to somewhere between 40>60psi, oil lubricated typically charge to around 90psi or more, which would be another obstacle in starting if there wasn't an unloader valve.

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Excellent reply, thanks! So, hopefully the last silly question. Considering that the pressure will be reduced by airbrushing, and the tank won't be full when the compressor needs to kick in again, why does the unloader valve need to kick in when it's full? It's going to be half empty (or whatever the level is) before the compressor needs to start again, and therefore shouldn't take too high a load from the compressor?

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Between the motor / pump & tank there is a non return valve - the unloader valve is connected between the pump & non return valve, so is only exhausts air from the pump compression chamber & a few inches of plumbing. The unloder is effectiveley isolated from the receiver & doesn't have a clue what going on pressure wise, it simply closes when the compressor pump is running & opens when it isn't.

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Thanks, re-reading your first answer made me realise you were talking about the pump, not the tank.

 

Thanks for explaining it all, much appreciated!

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