smithart-reints

7/26/2019 smithart-reints

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Welcome

Green HVAC Ductwork Done Right!

John Reints, PE

Eugene Smithart, PE


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High Velocity Duct Design Tips:

Fundamentals of High Velocity,Static Regain, Duct Design

Key Advantages

Dos Donts

Biggest Misconceptions

Best Rooftop Acoustics Idea

Real World Examples

In Summary


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Fundamentals of Static Regain

VP

SP

hf

Tp = Vp + Sp - hf

A B C D


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1000-1300

600-900

600-700

Main ductsb

Branch ductsb

Branch risersb

700-900

600

500

1200-1800

800-1000

800

Designation Residences Schools, theaters,

public buildings

Industrial

buildings

Recommended velocities, fpm

Table 6 ASHRAE Handbook, 1972, page 481

Recommended and Maximum DuctVelocities for Low Velocity Systems(rectangular duct)

Vp = (fpm/4005)2


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6,000

5,000

4,500

4,000

3,5003,000

2,500

CFM carried by the duct Maximum FPM

Recommended maximum duct velocitiesfor high-velocity systems

60,000

40,000

25,000

15,000

10,0006,000

3,000

40,000

25,000

15,000

10,000

6,0003,000

1,000

to

to

to

to

toto

to

Table 7 ASHRAE Handbook, 1972, page 481

Recommended and Maximum DuctVelocities for High Velocity Systems

(round duct)


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Key Advantages:

Lower cost

Lower duct heat pick up

Less duct leakage

Less space


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$0.25-$1.00/sq.ft. Installed CostSavings

Rectangular Duct 0.5#/sq.ft.

Rectangular Duct $3.00 - $3.50/sq.ft. High Velocity Static Regain RoundDuct 0.25#/sq.ft. (50% less) Round Duct $2.00 - $2.50/sq.ft. Saving of at least $0.25 to $1.00/sq.ft.($100 to $400/ton)


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Relative Installed Cost vs. Aspect Ratio

200190180170160150140130120110100

90807060

1:1 2:1 3:1 4:1 5:1 6:1 7:1

rectangular duct

round duct

(%)

aspect ratio


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Duct Heat Gain vs. Aspect Ratio

200

180

160

140

120

100

80

60

40

202:1 3:1 4:1 5:1 6:1 7:1 8:1

rectangular duct

round duct

9:1 10:1

(%)

aspect ratio


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Duct Heat Pickup


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Duct Leakage:

Leakage:

Rectangular Duct

traditionally hard to get below 10 percent

today its hard to get below 3%. Round Duct can get below 1 percent

Fan BHP:

(CFM2/CFM

1)3 = BHP

2/BHP

1

(110/101)3 = 29% increase in fan brake horsepower

(103/101)3 = 6% increase in fan brake horsepower


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drive connection

slip connection

D U C T M A T E


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Coupling

Gasketing


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Less Space


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Biggest Misconception

I cant use round ductbecause it wont fit!


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Relative Installed Cost vs. Aspect Ratio

200190180170160

15014013012011010090

807060

1:1 2:1 3:1 4:1 5:1 6:1 7:1

rectangular duct

round duct

(%)

aspect ratio


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U.S. Market

Rectangular

remains theprimary ductwork


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Bellmouth Fitting


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Go the Distance

4-6 duct diameter


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33% upstream flow


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Reducer Fitting


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Y Fitting


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45 - 90

90


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branchtapsmooth entrance


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Donts


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topbranch

sharp edge


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Divided Flow Fittings

Fitting

Y-2 plus 45 elbows

Capped cross withconical tabs

Capped cross

Capped cross with1 cushion head



Loss Coefficient (2)

0.22

5.4

5.0

5.0

6.0

6.4


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Sound transmission below 250 Hz via duct breakout isoften a major acoustical limitation for many rooftopinstallations. Excessive low-frequency noise associated

with fan noise and air turbulence in the region of thedischarge section of the fan and the first duct elbow results

in duct rumble, which is difficult to attenuate. Thisproblem is often worsened by the presence of a high-aspect-ratio duct at the discharge section of the fan.

Rectangular ducts with duct lagging are often ineffective inreducing duct breakout noise. Using either a single- ordual-wall round duct with a radiused elbow coming off thedischarge section of the fan can reduce duct breakout. If

space does not allow for the use of a single duct, the ductcan be split into several parallel round ducts.

2003 ASHRAE Applications Manual, p. 47.7

Rooftop Sound Attenuation


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Lower air leakage210+%

Shorter installation hours30+%

Lower fan energy use6-30+%

Lower weight25+%

In Summary...

SPIDA


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Experience

Air to every box quietly

Plus $0.25 to $1.00/sq.ft. ($100to $400/ton) savings


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JOHN V REINTS, PE

817 DORKEN LANE, DEKALB, ILLINOIS, 60115630-327-5425 or [email protected]

30 Years of Designing High Velocity, Static Regain, Round Duct Systems


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Thank You

Green HVAC Duct Design

Done Right!


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6 Real World Examples

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