Air compressor silencing box query

[Little Jerry]

Gents

Almost completed a box to silence my air compressor and just had one query:

I built a large box with plenty of air flow room on the sides and above. I used a high flow bathroom exhaust fan I had sitting around to move air through the box (the compressor also has a fan as part of the belt drive to cool the motor).

I installed a digital thermometer inside the box.

If the temperature inside the box doesn't change (and is equivalent to outside temp) when the compressor is running does that mean it's most likely running as well as it would if it was in the open air?

I'll post some pics once I have finished the build....

Appreciate any help.

Adam

 

[waddycall]

Depends where in the box you measure it. Needs to be on the exhaust out of the box not in the incoming air stream.

 

[Gareth J]

Is it too late to duct/cowel air to or from the cooling fan of the pump/motor?

You can do it your way by blowing air through the box with a separate fan but eliminating recirculation and keeping hot and cool areas separate is a more elegant an efficient way of doing it. You could also extend the air intake to outside the box so you know that it's drawing fresh air.

 

[AVI_8]

A lot of the noise from a compressor comes from the inlet itself ducting that outside if you can in a non restrictive way will also help to keep the noise down.
Adding something which will absorb sound to the inside of the box will also help. I saw on you tube once someone using carpet underlay stuck to the walls of the box.
I see you’re in Australia so yes keeping an eye on temperature will be important

 

[Little Jerry]

If the air getting exhausted out of the box is the same as the outside air temp (or very close) and the exhausted air flow is strong would that be a good indicator that all is well inside the box? The box itself is 1.7m w x 1.2m h x .7m w so it's not stuffed in tight or anything (the tank is probably 1.5m long and 40cm wide) and I left plenty of room above the cylinder head to whisp the heat away.

My goal was to take the noise down from 90db to 70db and I achieved that already to my surprise while testing with an incomplete box, so I think I will get 65db or less once I finish complete it.

 

[slim_boy_fat]

^^ Would egg crates glued to the inside help with sound absorption?

 

[Little Jerry]

I used 2mm mass loaded vinyl sandwiched between two sheets of plasterboard as a starting point - I always wanted to try MLV so thought this would be a good situation to use it.

I also made the frame out of an old warped 50mm x 100mm hardwood picnic table I had lying around - the thick wood has its own sound blocking properties to.

Given each reduction of approx 10db is perceived by our ears as being half as loud getting a 90db compressor into the 60's is a huge difference....

 

[Gareth J]

If the air getting exhausted out of the box is the same as the outside air temp (or very close) and the exhausted air flow is strong would that be a good indicator that all is well inside the box?

Not necessarily. If air flow paths happen to be very separate, and the extra cooling fan is more or less bypassing the pump altogether, it won't pick up a lot of heat. It probably won't happen like that and probably will be fine but if you want to monitor it, the temperature at the head of the compressor would be more important to focus on. See what it runs at, flat out, maintaining a steady output of compressed air flow at almost it's maximum pressure. Compare boxed and in free air. The difference will tell you if you've got it right, wrong. Or, if the boxed temperature keeps climbing - very wrong!

 

[Memmeddu]

I'd rather make an air intake for the compressor.
Noise is mostly caused by the incoming air entering in the pump , and coming out , vibrations too.
So I would make a support to hold pump and motor together and then attached this entire component to the tank using silent blocks .
A stock automotive air filter and box as intake manifold.
Good oil , and anti vibration hose between the pump and the tank.

 

[not done it yet]

The fan on the pump drive is there to either cool the pump or the the compressed air (usually between primary and compound cylinders on decent pumps). Air flows can cancel each other out, if not properly designed.

The motor is supplied with its own fan (and on cheap direct-drive units it has to cool the pump as well). On cheaper compressors, feeding the motor with warm air may well reduce the permissible continuous run time - which could result in a trip or motor failure.

Memmeddu is right re the intake noise being the largest problem - at least for the slower revving pumps.