Thread: Sound Insulation

Originally Posted by John Samuel

Would love to measure airspeed with the exhaust duct on and off, but don't want to spend that type of loot on an instrument that would get used for a few days and then get parked up.

Air flow meters are very useful for setting up, testing and modifying ducting and inlets.

The problem is finding one that gives a direct read out of the air speeds that such a high speed unit will generate. The cheaper meters (eg $15 on ebay) are all propellor driven units and have a typical max readings of 30 m/s but unfortunately this not sufficient to measure the peak air speed of a 2000 cfm flow in an 8" duct and higher speeds in narrower ducts.

There are some propellor driven units that go to 50 m/s but no propellor driven units are really designed to use inside ducting as;
- the opening required to insert these into a duct would so large it completely disrupts the vacuum.
- in narrow pipes the prop significantly blocks the flow giving an incorrect reading.
- to accurately measure the flow the air speed near the duct wall should be measured. This is impossible to do with a prop type sensor and difficult enough with a hot wire anemometer.

A less disruptive sensor suitable for insertion into a length of test duct is the hot wire or themistor anemometer which has a similar cross section to a car aerial but these are generally expensive and even these only measure a maximum of 30 m/s.

The second type of sensor suited to use inside a test dust is the Pitot tube connected to a pressure gauge. These can measure any speed by measuring pressures which then have to be converted to air flows so again some (relatively simple) math is needed. A basic Pitot tube is about $60 and a pressure meter is about the same so if you are prepared to undertake the maths this is the cheapest full

Just placing an anemometer at the entrance of a duct will not give a true reading of the volume flow, especially if it is anywhere near a junction. They are not even useful for simple comparative purposes if the inlets have a different shape. As described by BP to measure flow a length of straight piductpe is needed with the small hole for the sensor that can be easily sealed. Air speed as a function of distance across the width of the duct should be measured and some maths is required to measure an accurate flow. Just measuring the max air flow at the same point can be used for simple comparisons of different inlets etc.

It is still possible to use even a 20 m/s max anemometer to measure working air speeds if in ducts if the test ducting that has a greater cross sectional area than the ducting being tested. So to measure flow in VC hose and 2" duct I use a 4" test duct, and for 4" ducting I use a piece of 6" duct. I have a length of (Skip rescued) 9" duct that I plan to setup to measure the flow in 6" ducting.
Thus a 20 m/s max hot wire anemometer can measure a max air speed of 80 m/s in a 2" duct using a 4" test duct, and a max of 45 m/s in a 4" duct using a 6" test duct, and 45 m/s in a 6" duct using an 9" test duct.

Clear as mud?

All perfectly clear, Bob.

I never considered the propeller models.

I use a bit of stats in my work, so a little maths is not a problem.

The Pitot tube sounds like the way to go. The cheapest hot wire unit I could find was $300.00. Pass.

Any idea about suppliers? My initial searches turned up only tubes for model aircraft.

Cheerio!

Originally Posted by John Samuel

All perfectly clear, Bob.

I never considered the propeller models.

I use a bit of stats in my work, so a little maths is not a problem.

The Pitot tube sounds like the way to go. The cheapest hot wire unit I could find was $300.00. Pass.

Any idea about suppliers? My initial searches turned up only tubes for model aircraft.

Cheerio!

Dwyer instruments are one supplier. Bill Pentz website has all the details for doing this - let me know if you can't find it.

Bob,
How about using a water manometer to measure air pressure at the fan outlet? I kow it is very difficult to calculate accurate flow from the pressure, but you should be able to get a comparison value to evaluate how changes in ducting impact the pressure losses due to ducting changes.
A water manometer is easy and cheap to make.

Originally Posted by HardingPens

Bob,
How about using a water manometer to measure air pressure at the fan outlet? I kow it is very difficult to calculate accurate flow from the pressure, but you should be able to get a comparison value to evaluate how changes in ducting impact the pressure losses due to ducting changes.
A water manometer is easy and cheap to make.

A manometer can be easily used to measure pressure within a flow but a single pressure reading point relative to atmosphere will not give a flow. Both ends of the manometer need to be in the flow - one with the flow impacting directly into the end of a manometer tube and the other at right angles to the flow. The problem is that cut PVC or PE tubing will not give accurate pressure readings as the shape of the ends interfere with the flow and hence the pressure and which is why Pitot tubes are used. A well made and calibrated Pitot tube minimally interferes with the flow that is being monitored. Especially for 4" or smaller ducting these tubes need to be as narrow as possible and as the right angle flow pressure needs to be measured by at least 5 points around the tube this is not easy to make. A small pitot tube can be connected to a manometer thereby saving a few $.

The decision not to build a cupboard for my cyclone has been made.

Ductwork went up yesterday. Noise levels are at 63dB (3M away at 1.5M high - at 1M away the noise rises to about 71dB) with air being drawn through the table saw. At that level, it is difficult to justify the time and cost of an insulated enclosure. Also, having a naked cyclone keeps the footprint minimised.

The pic shows how I have tried to insulate the blower. Polyester fibre is held in place with a mastic-like product called Sureform (designed as a roof flashing). It seems to be worth about 3-4 dB in sound reduction, but the majority of the noise came from the exhaust. Many years ago I saw a big diesel generator treated a similar way. The exhaust and air intakes were heavily insulated, and you could barely hear it from 5 metres away.

Start work on the machine ports today so I can hook up 150mm flexy. Tablesaw is done ... next comes the horizontal/vertical sander ... and so on. The combo jointer/thicknesser will be the most difficult to modify. The machine structure limits what can be done, and I need to get a lot more air flowing through the machine to keep the volume up. Getting that air in so that it flows either over the cutter or the exit path of the shavings/dust is a bit of a trick, but I have a few ideas.

Originally Posted by BobL

A less disruptive sensor suitable for insertion into a length of test duct is the hot wire or themistor anemometer which has a similar cross section to a car aerial but these are generally expensive and even these only measure a maximum of 30 m/s.

I wonder if the automotive type would read sufficient velocity.

Originally Posted by Chris Parks

I wonder if the automotive type would read sufficient velocity.

Most automotive air flow meters would have too small a throat to use to measure DC flows ie they would stymie the flow they are trying to measure. They might be OK for VCs and VC tools.

Was talking to Chris Parks' son Drew a couple of days ago. He mentioned in passing that the VFD for my cyclone should not be set below 40Hz.

Just for kicks, tested the noise levels (air drawn through the TS) at three different Hz levels:
* At 60 Hz ... 63 dB
* At 50 Hz ... 55 dB
* At 40 Hz ... 45 dB

Of course, a lot of air flow is lost as the frequency is dropped, and it is my intention to run it at 60Hz. However, a cyclone without a VFD would run at 50Hz, and 55 dB is pretty quiet for a cyclone.

The reason for limiting the low speed end of things is motor cooling. yes, you can run it slower but the cooling function of the motor suffers due to the fan speed being lower. With something like this unless you are monitoring the motor temp then safe is better than a damaged motor. It could possibly be run slower but then all the smoke might escape.

I cant for the life of me understand why you would run a variable frequency drive on a blower in a dust extraction system.

Its not like there is any good reason to reduce the fan speed and thus reduce the air flow....... we constantly strugle to keep air flow up.

besides if the motor is designed for 50Hz australian supplies it will be less efficient at other frequencies.

If the syeten has been designed properly and arround 50Hz supplies.....increasing or decreasing the fan speed and thus the suction pressure and the desgned airflow, will change all the engineering done

What do you possibly hope to achieve.

cheers

Originally Posted by soundman

What do you possibly hope to achieve.

It's not about reducing the impeller speed but increasing it. Impeller efficiency is strongly dependent on rate of rotation so almost irrespective of design, increasing the AC frequency from 50 to 60 Hz will give a nominal increase of 20% in air movement. The chinese DCs that are sold into the US and Canada all use the same impellers as the ones sold into Oz, it's just that ours a 20% less efficient. If a DC uses a 3Phase motor it is well worth the couple of hundred bucks needed to add a VSD. Most 3Phase motors (even those designed for 50 Hz Operation) are over engineered and will easily cope with 60Hz and the increase in current required to move more air, so engineering is not usually an issue. I really kick myself for not getting a 3Phase version of my DC so that I could add a VSD to it. Fortunately used 3Phase motors are incredibly cheap, I picked up a 5HP for $40 a few months back - if the opportunity arises I will be changing my DC 1 Phase motor to 3Phase.

I have a good reason to slow mine down and it works. I tune two stroke motors as part of racing in superkarts so I slow my DE down enough that it captures all the smoke and gets it outside the shed. I just run a length of flexible line to the exhaust and it captures all the smoke.

The original reason we started using VFD's for the Cleaview Cyclones was because the Leeson motor was multivoltage and it allowed us to run 230V three phase at 60hz and as Bob pointed out it ensures a gain of at least 20% but I think it is slightly higher than that IIRC. Now we have begun using Ozzie supplied motors we still use a VFD to run it at 60hz. No amount of design will lift the airflow to what can be had with 60hz, if you can do it let me know how and I will save a lot of people a fair bit of money.

Originally Posted by Chris Parks

I have a good reason to slow mine down and it works. I tune two stroke motors as part of racing in superkarts so I slow my DE down enough that it captures all the smoke and gets it outside the shed. I just run a length of flexible line to the exhaust and it captures all the smoke.

I assume you don't have a filter on your cyclone otherwise the unburnt fuel and oil plus all the two stroke lube additives etc could gum the filter up. For this reason I don't use my dust extractor when welding but have a separate 500 cfm extractor fan.