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Sizing circular ductwork

You need to size circular ductwork according to the flow rate (L/s) through eachsection of pipework. You can use a friction loss chart or ductulator to help you sizethe ductwork for a heating or cooling system.

Commercial heating and cooling systems are more complex than domesticsystems. When sizing circular ductwork for a commercial system you will need to!

analyse the layout of the ductwork materials and fittings used throughoutthe system

perform calculations "ased on material and fitting specifications

size ductwork to ensure the system operates efficiently and effecti#ely("alanced system).

$he following diagram outlines the features of a sizing chart that has "eendesigned for sizing circular ductwork and fittings in a commercial heating and

cooling scenario.

$he following demonstration steps through the process of sizing circular ductworkfor a commercial heating and cooling system.

%se the worked example&s Backand Nextna#igation to #iew the demonstration.

Worked example ' of

$his worked example steps through the process of sizing circular ductwork for acommercial heating and cooling system.

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Worked example of

Size straight lengths of ductwork

Looking at the ductwork sizing chart you can see that ductwork section * has anairflow rate of '+, L/s. $o help us identify the correct ductwork size for section *draw a line across the friction loss chart from the '+, L/s airflow rate.

-tarting at a friction loss of .+ a/m draw a line up from the .+ a/m friction lossrating.

$he intersection of these two lines will determine the ductwork size. We can seethat +, mm is a suita"le size for ductwork section *.

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Worked example 0 of

Determine air velocity for ductwork

We can use the friction loss chart to determine the #elocity of air through the duct.

1or example for ductwork section * identify the point where the airflow rate of'+, L/s and friction loss of .+ a intersect along the #elocity line of the frictionloss chart.

$he intersection falls "etween a #elocity of 2 m/s and ', m/s. We can select thenext a#aila"le air #elocity cur#e of 3.+ m/s for ductwork section *.

Worked example of

Correct friction loss through ductwork

4ach straight section of ductwork in this system was sized at an initial friction lossof .+ a/m. $his pressure loss falls within the suggested pressure loss range. We

will need to correct the friction loss to help us size a "alanced system.

$he correction is determined "y identifying the duct size for each straight sectionand then selecting the nearest pressure loss on the friction loss chart.

1or example ductwork section * would ha#e a corrected friction loss of 0. a.

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Worked example of

Calculate pressure drops for ductwork

$he pressure drop for each straight section of ductwork can "e calculated "ymultiplying the length of each section "y the corrected pressure loss. 1or examplethe pressure drop of section * would "e ''.3 a.

You can enter the result of this calculation in the pressure drop column in theductwork section of your sizing chart.

Worked example 6 of

Calculating the velocity pressure of the air passing through the

ductwork

7efore you can calculate the #elocity pressure of the air passing through theductwork you will need to know the air temperature density and #elocity.

*ll air conditioning e5uipment is rated for -tandard air. -tandard air has atemperature of '. 8C density of '.,' kg/m0and pressure of ','0+ a.9enerally the #alues for standard air are used in calculations for air conditioningprocesses and e5uipment selection unless the temperature of the air #aries "ymore then a"out ', 8C.

:nce you know the air temperature and density you can use this e5uation tocalculate the #elocity pressure.

$he #elocity pressure of the air passing through a section of ductwork carrying-tandard air at a #elocity of 3.+ m/s can "e calculated with this e5uation.

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$he #elocity pressure of the air passing through this section of ductwork is +.'+a.

Worked example 2 of

Determine fitting type

$here are a large #ariety of fittings that can "e used in a commercial heating orcooling system. 1ittings are often specified "y the mechanical ser#ices engineer or"uilding architect.

*lways refer to your layout and specifications to determine the type of fittings touse in your system. $his system is using $44 '' and 74;< ,'' type fittings.

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Worked example 3 of

Fitting 1 B!ND "11

$he 74;< ,'' fitting has a cur#ed straight=through air path.

Worked example ', of

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B!ND "11 #dentify radius$to$diameter ratio %&'D(

Looking at the 74;< ,'' fitting specifications we can see that it has a default radius ratio

of '.,.

Worked example '' of

B!ND "11 Determine the fitting)s *t factor

$he 74;< ,'' fittings that will "e used in this system ha#e a radius ratio (>/

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Worked example ' of

B!ND "11 Determine velocity pressure

$he #elocity pressure of each 74;< ,'' fitting is determined "y the #elocitypressure of each section of connected ductwork.

Looking at the sizing chart you can see that the A section ductwork has a #elocity

pressure of '. a. $he AB@ 74;< ,'' fitting will also ha#e a #elocity pressure'. a.

4nter this a amount into the #elocity pressure column in the AB@ 74;< ,'' fittingsection of your sizing chart.

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Worked example '0 of

B!ND "11 Calculate pressure drop

$he pressure drop of each 74;< ,'' fitting can "e calculated "y multiplying the

fitting&s #elocity pressure "y its @t #alue.

$he following e5uation descri"es how to calculate the pressure drop of the AB@74;< ,'' fitting.

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Worked example ' of

Fitting + ,!! -11

$he $44 '' fitting has a straight$throughand a .ranched air path. You willneed to determine the pressure drop for each of these air paths.

Worked example '+ of

,!! -11 Determine velocity ratio of straight$through air path

$he #elocity ratio of the $44 '' fitting&s straight=through air path can "edetermined "y identifying the fitting&s entry #elocity and exit #elocity.

1or example the CB* $44 '' fitting has an air path with an exit #elocity of 2.3m/s and an entry #elocity of 3.+ m/s.

$he #elocity ratio for this fitting would "e expressed as 2.3!3.+.

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Worked example ' of

,!! -11 Calculate velocity ratio %/D'/0( of straight$through air

path

$he #elocity ratio (

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Worked example '6 of

,!! -11 Determine *t of straight$through air path

$o help us determine the @t factor of each $44 '' fitting we can draw a lineacross the straight=through air path ta"le from the fitting&s #elocity ratio.

$he CB* $44 '' fitting has a @t factor of , "ased on a straight=through air path#elocity of 3.0 m/s.

Worked example '2 of

,!! -11 Calculate pressure drop of straight$through air path

$he pressure drop can "e calculated "y multiplying the #elocity pressure of asection of straight=length ductwork "y the @t factor of a $44 '' fitting&s straight=through air path.

$he following e5uation descri"es how to calculate the pressure drop of CB* $44'' fittings straight=through air path.

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Worked example '3 of

,!! -11 Determine velocity ratio %/B'/0( of .ranch air path

$he #elocity ratio (7/%) of each $44 '' fitting&s "ranch air path can "ecalculated "y di#iding the fitting&s exit #elocity "y the fitting&s entry #elocity.

$he following e5uation descri"es how to calculate the 7/%of the 7B* $44 ''fitting.

Worked example , of

Calculate entry and exit area of ductwork

Calculate the entry and exit area of each section of ductwork "y con#erting theductwork&s diameter to an area (m). You can use Drto calculate the area of theductwork.

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1or example ductwork section 7 has a diameter of +, mm. $his diametercon#erts to an area of ,.,3 m.

Worked example ' of

Calculate area ratio %B2D(

$he area ratio (*7!*

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$he following e5uation descri"es how to calculate the pressure of the 7B* $44 ''fitting&s "ranch air path.

Worked example 0 of

Determine run loss

*dd all of the straight ductwork sectionslosses and fitting lossestogether todetermine each run&s pressure loss.

Worked example of

Determine the #ndex

:nce each run loss has "een calculated identify the run with the highest loss andcall it the Endex.

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Worked example + of

Calculate pressure re3uired to .alance the system

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