Thread: Dust extractors noise ratings

Originally Posted by Ambrosia

Actually, this is incorrect, but a common misunderstanding. For pumps and fans, maximum power is absorbed at maximum flow. Throttling the flow reduces required power with the minimum at no flow.

See the curve below which is typical.
Max flow = max power
Min flow = min power

Leigh

I agree. Air at STP has a density of 1.2kg/m^2 so shifting 2000 m^3/hr means about 2.4 TONS of air is moved per hour which requires considerable amount of energy to be expended. If only half of that is moved only half the energy is needed.

I first noticed this went I was playing around with a lab fan back in the 1970's. The motor specs showed it as a 1A/240V motor (~1/3 HP) and was originally set up to draw air through a rectangular opening that I thought I would double the area of the opening to get more air through it. Th net result was the motor got very hot and the thermal cut out switch stopped the motor. When I put an ammeter on it I found it was drawing ~3A which at 240V mean the motor was developing much more than 1/2HP. I got around this by putting several layers of coarse filter media across the opening and this provided enough resistance to reduce the flow so that only ~1.5A was being drawn.

This is one reason why modifying DC impellers to utilise bigger ducting should be accompanied by measuring the current drawn by the motor.

Runningman - for what it is worth (given the above caveats), I get a 90dB reading at the air intake of my DC setup. I usually have the air intake located at my lathe and less than half a metre from my ears. That is where the noise is loudest and the most important to me. My fan unit is in a cement cellar below the workshop, so is not the main source of noise inside the workshop.

My DC setup has a 3HP standard straight impeller unit running through a full (1:3 cone ratio) cyclone with the smallest duct size being 6". Having given those details, I don't expect my setup and numbers are readily applicable to other workshops.

I always use good quality ear plugs whenever the system is running, which is most of the time I am in my workshop.

Originally Posted by NeilS

Runningman - for what it is worth (given the above caveats), I get a 90dB reading at the air intake of my DC setup. I usually have the air intake located at my lathe and less than half a metre from my ears. That is where the noise is loudest and the most important to me. My fan unit is in a cement cellar below the workshop, so is not the main source of noise inside the workshop.

Neil, how far away from the air intake was the SPL meter for your 90 dB reading?

When I measure the SPL 1m from the air intake on my lathe with the VFD on 50Hz, I measure 81.5 dB with no Bell mouth hood, and 80.5 with the hood.
At around 0.5m away, the SPL rises dramatically, now 88.5 with the hood and 86.5 without.

Originally Posted by NeilS

Runningman - for what it is worth (given the above caveats), I get a 90dB reading at the air intake of my DC setup. I usually have the air intake located at my lathe and less than half a metre from my ears. That is where the noise is loudest and the most important to me. My fan unit is in a cement cellar below the workshop, so is not the main source of noise inside the workshop.

My DC setup has a 3HP standard straight impeller unit running through a full (1:3 cone ratio) cyclone with the smallest duct size being 6". Having given those details, I don't expect my setup and numbers are readily applicable to other workshops.

I always use good quality ear plugs whenever the system is running, which is most of the time I am in my workshop.

Thanks Neil for the info.

90dB, That’s quite noisy at the hose entry at the lathe, hate to think what the noise level is at the DC itself.
I’ve noticed plenty of 3/4hp DC units for sale, they’re probably useless at suction I imagine


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Originally Posted by Runningman

Thanks Neil for the info.
90dB, That’s quite noisy at the hose entry at the lathe, hate to think what the noise level is at the DC itself.

Most of that is not likely to be noise reflecting back down the ducting from the DC itself but the noise generated by turbulent air being sucked into the pipe inlet. The noise emitted by one 100 mm duct opening/inlet when connected to a large DC can be what I call, a "screaming banshee".

I’ve noticed plenty of 3/4hp DC units for sale, they’re probably useless at suction I imagine

They are OK for picking up chips but not much chop at fine dust collection.

Originally Posted by BobL

Neil, how far away from the air intake was the SPL meter for your 90 dB reading?

When I measure the SPL 1m from the air intake on my lathe with the VFD on 50Hz, I measure 81.5 dB with no Bell mouth hood, and 80.5 with the hood.
At around 0.5m away, the SPL rises dramatically, now 88.5 with the hood and 86.5 without.

Bob, that was at 0.5m from from the inlet with MB hood, as that is where my ears are in relation to it, which is my primary interest.

I did once take some readings outside my workshop on the neighbours boundary (about 10m away) to see what that was in case I got any complaints, but I couldn't pick up any level above background.