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Flexible Duct Size

Flexible Duct Size

Flexible Duct Size

Hello Everybody,
In our recent project, we have proposed acoustic flexible duct sizes in a tabular form. The first size is 100 mm for 40 l/s which will provide friction rate of 4.35 p/m (0.56 In.Wg) and 5 m/s (1024 fpm) velocity. The proposed flexible ducts are pre-fabricated type with acoustic lining to maintain low noise level. The flexible duct shall connect to the slot diffusers which has plenum with internal acoustic lining. The plenum shall be sized to maintain air velocity not more than 2.5 m/s.

I do understand that due to small flexible duct size, the velocity and pressure drop is on a higher side and changing the duct sizes from 100 mm to 150 mm will have significant cost impact on the project which I cannot allow. Based on above duct sizes, will there be any noise related issues in the office spaces.

RE: Flexible Duct Size

It depends.
How long are the flexible duct runs?

Regarding sound, the answer is yes and no. Unless the flexible duct length is very short (less than 0.5m) you will hear it as the air through the duct starts and stops and the flexible duct expands/contracts. Once pressurized they are very quiet.

I am glad you are concerned with your project's cost. However, have you considered the long term costs for using smaller duct? The increased friction requires a larger fan (at least with more ESP) with a larger motor which will cost more up front and use more energy in the long run. Larger motor may also mean larger electrical panels and switchgear also adding cost to your project.

RE: Flexible Duct Size

Thanks dbill74. The flexible ducts are 0.5 to 0.6 m long. I do accept your recommendations to minimize operational cost but the main problem is contract is awarded to the contractor and any increase in duct sizes may cost variations to the contract which I am trying to avoid.If there is enough static in the fan to achieve design air flow with noise levels within acceptable limits, we will retain 100 mm duct size for 40 l/s flow rate.

RE: Flexible Duct Size

Does the flexible hose manufacturer have any data on sound level in their brochures?

RE: Flexible Duct Size

They don't have any sound level data in their brochure but we have requested to provide it to assess the impact on sound level.

RE: Flexible Duct Size

Recommended noise rate for office area is RC 25-35 according to ASHRAE Sound and Vibration Controls. It had better to look at octave band sound pressure levels. For office (assume NR30), noise levels are 57db, 48db, 41db, 35db, 31db, 29db and 28db for 63Hz, 125Hz, 250Hz, 500Hz, 1000Hz, 2000Hz and 4000Hz respectively. I think flexible duct with your size and flow rate is not a problem for acoustical concern since i calculated roughly that it produces 15db, 13db, 10db, 6db,0db, 0db, 0db in that Hz order. Moreover, i do not think that you will get proper data about flexible duct's acoustic performance from the manufacturer since you are expected to calculate it. Furthermore, if you have 5m/s velocity in volume damper (if any), that will be problem. Please check this out also.
The more length flex duct has, the more sound attenuation it has. (limited to max. 1.5m length for flex duct not to break it) I assume your fan has enough capacity to meet the pressure drop. For such small ducts, try to avoid to have 5m/s velocity.

RE: Flexible Duct Size

@Emrelug84, Thanks for your comprehensive response. Much appreciated. As per attachment of flexible duct connection, you can see each flexible duct has got one volume damper which is subjected to 5 m/s velocity. Moreover, the lentghs of flexible ducts are shorter, so we cannot have more sound attenuation.

RE: Flexible Duct Size

The greater the pressure drop in the flex duct, the more noise you will have

Reduce pressure drop, reduce noise.

RE: Flexible Duct Size

Any increase in flexible duct size will cost variations to the contract which I am trying to void. The pressure drop as per 100 mm size and 40 l/s flow rate is 4.35 pa/m (0.56 In/Wg). I am just trying to figure out if this duct size and flow rate will have any noise related issues in the offices.

RE: Flexible Duct Size

As mentioned by Emrelug84, duct size is not a concern for acoustical concern but volume damper with 5 m/s velocity may be a problem. We are currently trying to figure out if there is any noise related issue with the damper.

RE: Flexible Duct Size

Although flexible duct sizes mentioned in a table on "General Notes and Legend Drawing" but it is also mentioned in the same drawing that contractor to read "General Notes and Legend Drawing" in conjunction with project specifications and contract drawings. As per project specifications about flexible ductwork, transmission and dynamic insertion loss is mentioned as per octave band frequencies which need to be complied. I believe to achieve required transmission and dynamic insertion losses, contractor may have to increase the flexible duct sizes without any variation. Please kindly advise.

The dynamic insertion loss of 1m of the acoustic flexible ductwork shall be no less than the values stated within HVAC Flexible Ductwork, Table D within Appendix A for dynamic insertion loss.

A. TABLE D - Table of HVAC Flexible Ductwork Dynamic Insertion Loss

Plant Reference Octave Band Centre Frequency [Hz]
63 125 250 500 1k 2k 4k 8k
Dynamic Insertion Loss for 1 m of acoustic flexible ductwork dB 3 4 10 11 11 11 6 5

The transmission loss of the acoustic flexible ductwork shall be no less than the values stated within HVAC Flexible Ductwork, Table E within Appendix A for transmission loss.

A. TABLE E - Table of HVAC Flexible Ductwork Transmission Loss

Plant Reference Octave Band Centre Frequency [Hz]
63 125 250 500 1k 2k 4k 8k
Transmission Loss for acoustic flexible ductwork dB 4 7 8 10 13 13 13 14

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