Duct Design 2007

11ME 425 - Air Distribution & ASHRAE Outlet Selection

ME 425Keith E. Elder, P.E.

Duct Design

Duct DesignThe purpose of air conditioning ductwork is to deliver air fromthe fan to the diffusers which distribute the air to the room.Air Moves Through the Ductwork in Response to a PressureDifference Created by the FanThe necessary pressure difference will be a function of the waythe ductwork is laid out and sized.The objective of duct design is to size the duct so as tominimize the pressure drop through the duct, while keeping thesize (and cost) of the ductwork to a minimum.Proper duct design requires a knowledge of the factors thateffect pressure drop and velocity in the duct.

22ME 425 - Air Distribution & ASHRAE Outlet Selection

Duct System Pressure

Total Pressure, TP, is Related to the Energy in the AirStream, and is equal to:TP = Static Pressure + Velocity Pressure

Static pressure and velocity pressure increase anddecrease as the air proceeds through the ductwork,depending on the cross-sectional area of the flow.The total pressure of the airstream decreases as theair proceeds through the the ductwork due to theconversion of mechanical energy to heat caused byfriction

Duct System Pressure

33ME 425 - Air Distribution & ASHRAE Outlet Selection

Velocity Pressure

Where:VP = Velocity Pressure, inches H2OV = Velocity, feet per minute

VP V

4005

2

Static Pressure Losses

Frictional Losses Due to fluid viscosity and turbulence in the flow through the

ductwork, and occur along the entire length of the ductwork

Dynamic Losses Result from flow disturbances caused by fittings that change

the airflow direction or area.

44ME 425 - Air Distribution & ASHRAE Outlet Selection

Frictional Loss CalculationVelocity Reduction Method Velocity at the fan discharge is pre-selected. The duct

system is designed to provide progressively lower ductvelocities as the air proceeds from the main duct to thebranches.

Static Regain Method The ducts are sized so the increase in static pressure at

each take-off offsets the pressure loss of the succeedingsection of ductwork.

Equal Friction Method The duct is sized to result in a constant pressure loss per

unit length of duct.

Equal Friction Method

Frictional Losses, p, can be approximated:

Rectangular Ductwork is converted to round:

p f LdV

0 03

10001 221 82

..

.

D ab

a be

130

0 625

0 25..

.

55ME 425 - Air Distribution & ASHRAE Outlet Selection

Equal Friction Chart

66ME 425 - Air Distribution & ASHRAE Outlet Selection

Maximum Duct VelocityControlling Factor Controlling Factor - Duct Friction

Application Noise Generation Main Ducts Branch DuctsMain Ducts Supply Return Supply Return

Residences 600 1000 800 600 600ApartmentsHotel Bedrooms 1000 1500 1300 1200 1000Hospital BedroomsPrivate OfficesDirectors Rooms 1200 2000 1500 1600 1200LibrariesTheatres 800 1300 1100 1000 800AuditoriumsGeneral OfficesHigh Class Restaurants 1500 2000 1500 1600 1200High Class StoresBanksAverage Stores 1800 2000 1500 1600 1200CafeteriasIndustrial 2500 3000 1800 2200 1500

* From Carrier Air System Design Manual

Approximate Ductwork Cost

Dimension Gauge* lb/SFUp to 12 26 0.90613 to 30 24 1.15631 TO 54 22 1.40655 TO 84 20 1.65684 and Over 18 2.156

Galvanized Steel = $8.25 per lb**

* Source: Uniform Mechanical Code**Source: Recent bids includes fittings & everything

77ME 425 - Air Distribution & ASHRAE Outlet Selection

Economical Duct DesignMinimize aspect ratio (a/b)Minimize total pounds of sheet metal andMinimize number of reductions (2-inch rule)

Duct Sizing ExerciseAn office space has a sensible peak cooling load of29,400 Btu/hr. The inside design temperature is 78 Fand the supply air temperature is 60F. The duct mustbe lined and yet fit under the beam shown below andnot exceed a friction loss of 0.10 inches per 100 feet.

Determine themost economicalduct size.

88ME 425 - Air Distribution & ASHRAE Outlet Selection

Dynamic Loss Calculation

pf= C

op

v,o

Fitting Loss Exercise

1000 CFM is flowing througha 16 x 12 rectangular duct.

400 CFM flows through an8 x 12 branch to another room, and the remaining600 CFM continues along the 12 x 12 main duct.

What is the pressure drop, in inches of water,through the branch?

99ME 425 - Air Distribution & ASHRAE Outlet Selection

Homework

Read 05F35.1-25The ductwork shown below will serve several openoffice spaces. Size the ductwork and calculate themaximum pressure drop. The maximum verticalspace is 12 inches and the branch fittings are of thesame type as the example in the handout.

1200 CFM

900 CFM750 CFM

FROMFAN 1200 FPM

70 ft

30 ft

40 ft

80 ft

40 ft

30 ft

30 ft