BBSE3006: Air Conditioning and Refrigeration II http://www.hku.hk/bse/bbse3006/ Air-side Systems: Air Duct Design Dr. Sam C M Hui Department of Mechanical Engineering Department of Mechanical Engineering The University of Hong Kong E-mail: cmhui@hku hk E mail: cmhui@hku.hk Jan 2008
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BBSE3006: Air Conditioning and Refrigeration IIg ghttp://www.hku.hk/bse/bbse3006/
Air-side Systems: Air Duct DesignDr. Sam C M Hui
Department of Mechanical EngineeringDepartment of Mechanical EngineeringThe University of Hong Kong
• Minimum & recommended thicknessMinimum & recommended thickness• ASHRAE standard or local codes
• Temperature rise curves• Temperature rise curves• Depends on air velocity, duct dimensions & insulation
Temperature rise from duct heat gain(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Duct Properties
• Frictional losses: Darcey-Weisbach EquationFrictional losses: Darcey Weisbach Equation• Hf = friction head loss, or Δpf = pressure loss
• f = friction factor (dimensionless)• L = length of duct or pipe (m)• D = diameter of duct or pipe (m)• v = mean air velocity in duct (m/s)• g = gravitational constant (m/s2)• ρ = density of fluid (kg/m3)• g c = dimensional constant, for SI unit, g c = 1
Mode of airflow when air passes over and aroundMode of airflow when air passes over and aroundsurface protuberances of the duct wall
(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Duct Properties
• Duct friction chartDuct friction chart• Colebrook formula
R h & t t ti• Roughness & temperature corrections• Δpf = Ksr KT KelΔpf cpf sr T el pf,c
• Ksr = correction factor for surface roughness• K = correction factor for air temperature• KT = correction factor for air temperature• Kel = correction factor for elevation
Friction chart for round duct(Source: ASHRAE Handbook Fundamentals 2001)
(Source: ASHRAE Handbook Fundamentals 2001)
Duct Properties
• Circular equivalentCircular equivalent• Hydraulic diameter, Dh = 4 A / P
• A = area (mm2); P = perimeter (mm)• Rectangular duct:g
• Flat oval duct:
Duct Properties
• Dynamic lossesDynamic losses• Result from flow disturbances caused by duct-
d i d fi imounted equipment and fittings• Change airflow path’s direction and/or area• Flow separation & eddies/disturbances
• In dynamic similarity (same Reynolds number &• In dynamic similarity (same Reynolds number & geometrically similar duct fittings), dynamic loss i ti l t th i l itis proportional to their velocity pressure
Duct Properties
• Local or dynamic loss coefficientLocal or dynamic loss coefficient• Ratio of total pressure loss to velocity pressure
Duct Properties
• Duct fittingsg• Elbows
C i di i t d• Converging or diverging tees and wyes• Entrances and exits• Enlargements and contractions
• Means to reduce dynamic losses• Means to reduce dynamic losses• Turning angle, splitter vanes
• ASHRAE duct fitting database• Fitting loss coefficients• Fitting loss coefficients
(Source: ASHRAE Handbook Fundamentals 2001)
Region of eddies andturbulences in a round elbow 5-piece 90o round elbow
(Source: ASHRAE Handbook Fundamentals 2001)
R t l lb th di 2 littRectangular elbow, smooth radius, 2 splitter vanes
Mitered elbow and its secondary flow(Source: ASHRAE Handbook Fundamentals 2001)
Airflow through at l irectangular convergingor diverging wye
(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Entrance ExitEntrance Exit
(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Duct Properties
• Flow resistance, RFlow resistance, R• Total pressure loss Δpt at a specific volume flow rate V
2VRpt
• Flow resistance in series:ns RRRR 21
• Flow resistance in parallel:p
RRRR1111
np RRRR 21
Total pressure loss and flow resistance of a round duct section(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Flow resistance in seriesFlow resistance in series
Flow resistance in parallel(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Flow resistance for a Y connection(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Air Duct Design & Sizing
• Optimal air duct designp g• Optimal duct system layout, space available
S ti f t t b l• Satisfactory system balance• Acceptable sound level• Optimum energy loss and initial cost• Install only necessary balancing devices (dampers)Install only necessary balancing devices (dampers)• Fire codes, duct construction & insulation
• Require comprehensive analysis & care for different transport functionsp
Flow characteristics of a supply duct systemFlow characteristics of a supply duct system
(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
• Main ducts: air flow usually ≤ 15 m/s; air flow noiseMain ducts: air flow usually ≤ 15 m/s; air flow noise must be checked
• With more demanding noise criteria (e g hotels) maxWith more demanding noise criteria (e.g. hotels), max. air velocity: main duct ≤ 10-12.5 m/s, return main duct ≤ 8 m/s, branch ducts ≤ 6 m/s≤ 8 m/s, branch ducts ≤ 6 m/s
• Face velocities for air-handling system components
(Source: ASHRAE Handbook Fundamentals 2001)
Air Duct Design & Sizing
• System balancingSystem balancing• Air volume flow rate meeting design conditions• System balancing using dampers only is not
recommended• Critical path
• Design path of airflow (total flow resistance is maximum))
• How to reduce the dynamic losses?
Air Duct Design & Sizing
• Reduce dynamic losses of the critical pathy p• Maintain optimum air velocity through duct fittings
E h i d ti f d i l t• Emphasize reduction of dynamic losses nearer to the fan outlet or inlet (high air velocity)
• Proper use of splitter vanes• Set 2 duct fittings as far apart as possibleg p p
• Air duct leakageD l k l ifi i• Duct leakage classification
• ANSI, SMACNA, ASHRAE standards
Air Duct Design & Sizing
• Fire protectionFire protection• Duct material selection• Vertical ducts (using masonry, concrete or clay)• When ducts pass through floors & wallsWhen ducts pass through floors & walls• Use of fire dampers• Filling the gaps between ducts & bldg structure• Duct systems for industrial applications• Duct systems for industrial applications
• Any other fire precautions?
Air Duct Design & Sizing
• Design procedure (computer-aided or manual)g p ( p )• Verify local codes & material availability• Preliminary duct layoutPreliminary duct layout• Divide into consecutive duct sections• Minimise local loss coefficients of duct fittingsMinimise local loss coefficients of duct fittings• Select duct sizing methods• Critical total pressure loss of tentative critical path• Critical total pressure loss of tentative critical path• Size branch ducts & balance total pressure at junctions• Adjust supply flow rates according to duct heat gain• Adjust supply flow rates according to duct heat gain• Resize duct sections, recalculate & balance parallel paths
Ch k d l l & dd tt ti• Check sound level & add necessary attenuation
Air Duct Design & Sizing
• Duct layouty• Symmetric layout is easier to balance
• Smaller main duct & shorter design path• Smaller main duct & shorter design path• For VAV systems, duct looping allows feed from
it di tiopposite direction• Optimise transporting capacity (balance points often
f ll h ’ i i )follow the sun’s position)• Result in smaller main duct
• Compare alternative layouts & reduce fittings• For exposed ducts, appearance & integration with p , pp g
the structure is important
Typical supply duct system with symmetric layout & loopingTypical supply duct system with symmetric layout & looping
(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Air Duct Design & Sizing
• Duct sizing methodsDuct sizing methods• Equal-friction method with maximum velocity
• Duct friction loss per unit length remains constant• Most widely used in normal HVAC applications
• Constant-velocity method• Often for exhaust ventilation system• Often for exhaust ventilation system• Minimum velocity to carry dust is important• Limit velocity to reduce noise
Air Duct Design & Sizing
• Duct sizing methodsg• Static regain method
• Normally used with a computer package for high• Normally used with a computer package for high velocity systems (e.g. in main duct)
• Size air duct so that ↑static pressure nearly offset the• Size air duct so that ↑static pressure nearly offset the pressure loss of succeeding duct section along main duct
• T method• T method• Optimising procedure by minimising life-cycle cost
S t d i (i t i l i i d t)• System condensing (into a single imaginary duct)• Fan selection (optimum system pressure loss)• System expansion (back to original duct system)System expansion (back to original duct system)
Concept of static regain method(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Air Duct Design & Sizing
• Design information requiredDesign information required• Client requirements• Required supply air condition• Type of system suppliedType of system supplied• Ambient conditions• Duct material• Duct insulation• Duct insulation• Duct system layout
• Design outputs• Schematic of ductwork layout & associated plant• Schedule of duct sizes and lengths and fittings• Schedule of duct sizes and lengths, and fittings
Air Duct Design & Sizing
• Duct system characteristicsDuct system characteristics• Supply duct, return duct, or exhaust duct systems
i h i l i b h k ffwith certain pressure loss in branch takeoffs• Duct sizing based on LCC & space optimisation• System balancing through pressure balance of duct paths• Sound level will be checked & analysedSound level will be checked & analysed• Minimise local loss coefficients of duct fittings
S l l fl t dj t d di t d t• Supply volume flow rates adjusted according to duct heat gain
Cost analysis for a duct system(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Air Duct Design & Sizing
• Duct system characteristicsDuct system characteristics• Supply duct, return duct, or exhaust duct systems in
hi h l l ill i hwhich supply outlets or return grilles either mounted directly on duct or have very short connecting duct
• Very small or negligible pressure loss at branch ductsVery small or negligible pressure loss at branch ducts• System balancing depends mainly on sizes of the
successive main duct sectionssuccessive main duct sections
Rectangularsupply duct withpp ytransversal slots
(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Air Duct Design & Sizing
• Duct system characteristicsDuct system characteristics• Industrial exhaust duct systems to transport dust or
h i lother particulates• Require a minimum velocity in all duct sections, such as
12.2 to 20.3 m/s• Select proper configuration of duct fittings to provide a p p g g p
better system balance• Round ducts produce smaller losses & more rigid• Air velocity must not exceed too much, to avoid energy waste• Well-sealed joints & seams to reduce air leakage
Other Factors
• Duct liner• Lined internally on inner surface of duct wall
M i l d f i tt ti & i l ti• Mainly used for noise attenuation & insulation• Fiberglass blanket or boards
• Duct cleaning• Prevent accumulation of dirt & debris• Prevent accumulation of dirt & debris• Agitation device to loosen the dirt & debris• Duct vacuum to extract loosened debris• Sealing of access openingsSealing of access openings
Duct breakout noise(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Other Factors
• Pressure and airflow measurementsPressure and airflow measurements• Pitot tube
• Two concentric tubes• Manometers
• U tube or inclined one
D i f i i• Demonstration of measuring instrument
Pitot tube(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Manometer: U-value
Inclined manometer
(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
Pressure measurements in air ductsPressure measurements in air ducts
(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)
M i i i l & d dMeasuring points in rectangular & round duct transverse(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)