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DUCT DESIGN

DUCT DESIGN. (material to be checked with local legislation). Daikin Europe, Consulting sales section. Summary. Introduction Ducts classification Pressure losses in ducts Air diffusers and resisters Methods for duct calculation Daikin ductmeter. 1. Introduction Needs to be considered.

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DUCT DESIGN

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  1. DUCT DESIGN (material to be checked with local legislation) Daikin Europe, Consulting sales section

  2. Summary • Introduction • Ducts classification • Pressure losses in ducts • Air diffusers and resisters • Methods for duct calculation • Daikin ductmeter

  3. 1. IntroductionNeeds to be considered • Space availability • Space air diffusion • Noise level • Duct leakage • Duct heat gains and losses • Balancing • Fire and smoke control • Investment costs • Operating cost of the system

  4. 1. IntroductionNeeds to be considered • Poor or excessive air distribution can cause discomfort, loss of productivity and even adverse health problems. • Lack of sound attenuators may permit objectionable noise levels. • …

  5. 2. Duct classificationBy size • Main duct system: • each branch is connected to the main duct • most widely used • Separate ducting system: “-” more ducting material required more ducting space necessary. “+” economical solution if: • mass produced • round ducts are used.

  6. 2. Duct classificationBy purpose • Air supply duct • Cool or warm air coming out of the air-conditioner, is delivered to the room through the air supply duct. • The air supply duct is generally insulated to prevent heat losses and dew formation. • Return air duct • The air is returned from the room to the air-conditioner through the return air duct. • If the resistance inside the duct is big, pressure inside the duct goes down, leading to a lower efficiency.

  7. 2. Duct classificationBy purpose • Fresh air intake duct. • The fresh air intake duct is connected to the suction side of the machine • Location of the fresh air intake port should prevent sucking in bad smells or dust. • Insulation might be necessary • Separate fresh air system id recommended • Air exhaust duct. • Polluted air is released to the outside through the air exhaust duct. • The material and paint of the air exhaust duct should be selected in function of the composition of the polluted air

  8. 3. Pressure losses in ductsIntroduction • Duct system losses are the irreversible transformation of mechanical energy into heat. • These losses can be divided into friction losses and dynamic (local) losses.

  9. Friction loss in the Zn Iron Plate duct 3. Pressure losses in ducts • Fluid resistance caused by friction in round ducts can be determined by use of the friction chart (based on an absolute roughness of 0.09 mm.) Diameter (mm) Charts for friction losses in round ducts Air volume (m³/h) Air velocity (m/s Friction loss (mmH2O/m)

  10. 3. Pressure losses in ducts: dynamic losses • Dynamic losses are the result of flow disturbances caused by duct mounted equipment and fittings. These fittings include entries, exits, elbows, transitions and junctions. • The dynamic losses due to these fittings can be derived from tables, curves and equations.

  11. 4. Air diffusers & resisters-Types Adjustable flush with hidden pattern control for horizontal draftless air distribution to vertical projection plan C2 Deep outer cone reduces smudging. Inner cone is a plaque. ACP Flange outer cone. Adjustable square diffuser for directional control from horizontal to vertical pattern. Extruded aluminium construction E2 Adjustable square diffuser with a plaque. Extruded aluminium construction EP Extruded aluminium linear slot diffuser for directional control from horizontal to vertical position VTL Extruded aluminium linear grilles with fixed bars KL Double direction supplying grilles with front louvers vertical behind louvers horizontal VHS

  12. 4. Air diffusers & resisters-Types Cylindrical air diffuser with a long throw and low noise NOZZLE Diffusers with unrestricted swivel type inner assembly PK SLIT Return air grilles with fixed bars Return air grilles with filters. Cores are hinged for easy removal and replacement of filter RF PG Return air grilles with punching plates Grilles for exhaust air or intake air with slant louvers GL Return air grilles with a sight ploof core DG

  13. 4. Air diffusers & resisters • Location • needs to be determined with care. • to avoid drafts for the people, the diffuser should be positioned high enough. • care to avoid dew generation on the ceiling surface. • position of the lights, structural beams, etc., needs to be considered

  14. 4. Air diffusers & resisters Design procedure for supply/return grilles • Divide the room into squares or rectangles • Each sq./rectangle to be served by one grille • Rules of thumb: H= height of room W and L: dimensions of Sq/rectangle W or L ≤ 3 * H L ≤ 1.5 * W L=15 m L=7,5 m L=7,5 m W=9 m Variant 2 H= 3m Variant 1

  15. 4. Air diffusers & resistersDesign procedure for supply/return grilles • Outlet velocity The permissible sound level, and as a consequence the maximum air velocity, is determined by the application. dB(A) Application Max. velocity (m/s) 25 Studio – recording room 2 35 Cinema – hospital – library 3 40 Office – school – hotel 4 46 Bank – public hall 5 50 Store – post office 6 70 Factory 10

  16. 5. Duct calculationMain steps • Draw schematically the duct network • Establish the airflows for each section of the duct • Calculate dimensions of ducts, based on airflow velocities (see following 4 methods) • Draw on scale the duct network, mentioning the special fittings (elbows, junctions, regulating flaps, etc.) • Calculate the total pressure loss, to choose the corect ventilator (the correct ESP) The velocities are decreasing from the ventilator to the far end of duct

  17. 5. Methods for duct calculation • 4 main methodsrelated to the selection of air velocities along the main duct sections: • The method of equal air velocities • The method of reducing the air velocities • The method of equal frictions • The method of static pressure regain

  18. Friction loss in the Zn Iron Plate duct 5. Methods for duct calculation • The method of equal friction: • Establish a maximum air velocity next to the fan • Determine the friction loss by using the chart • Use the same friction loss along the whole duct system • Determin the duct dimensions and the speed by inputing the friction loss and the airflow into the chart. Diameter (mm) Air volume (m³/h) Air velocity (m/s Friction loss (mmH2O/m)

  19. 5. Methods for duct calculation Advantages/difficulties of the equal friction method: “+” The velocities resulting from charts are already decreasing along the duct- no arbitrary selection in this case “-” Calculation may become difficult in case of many dynamic losses (elbows, etc.): • Dynamic losses are determined from charts • Add dynamic losses and divide the sum to the total duct length • Add the result to the constant value of the friction loss • Use the new value to determin the duct dimensions and air velocity from charts • Difficulty: the dynamic losses have to be recalculated by using several attempts (velocities) Conclusion: precise method, easy to use for straight ducts

  20. 6. Duct calculation by equal friction methodThe Daikin ductmeter

  21. 6. Duct calculation with the Daikin ductmeter • Assume that one packaged air conditioner is used to cool 4 rooms of a public building • Given data: • Total air volume: 84 m³/min • Discharged air flow volume from each universal resister: 21 m³/min • Passing air velocity through supply and return grille: aprox. 4 m/s

  22. 6. Duct calculation with the Daikin ductmeter • STEP 1 • Draw schematic view of the duct system • Make notes for air volume, air velocity, etc • Mark clearly the elbow, the branch parts, the air discharge outlet • Select one main ducting route (where the maximum static pressure loss occures) K 21 m³/min F 1 m 2,5 m C D E 2 m 3,5 m 3,5 m 1,5 m G • STEP 2 • Find the static pressure loss of the main duct route • Select the corresponding air velocity and duct size 1 m 1 m B 1 m A 21 m³/min 21 m³/min 21 m³/min

  23. 6. Duct calculation with the Daikin ductmeter • Select the air velocity for the main duct in accordance with the desirable air velocity, mentioned in the table:

  24. 6. Duct calculation with the Daikin ductmeter • INPUT: • the total air flow is 84 m³/min • select the velocity 6 m/s for the source section (B-C) of the main duct • OUTPUT from the ductmeter: • friction loss: 0,07 mm H2O/m • size of round duct: ǿ 53 cm or, • size of rectangular duct: Lxl=75 cm x 35 cm Friction loss Air volume Air velocity Round duct Long side length Rectangular duct Short side length Long side length < 7 x short side length

  25. 6. Duct calculation with the Daikin ductmeter STEP 3: Find the local friction loss of the main duct • Find the total friction loss at the elbow or branch parts: • Input the velocity into the local friction loss calculation table • In our case, to v=6 m/s corresponds 0,4 mm H2O friction loss in the elbow “W“shows the diameter of the round duct or the long side of the rectangular duct w

  26. Equivalent length of main duct (A K) 6. Duct calculation with the Daikin ductmeter STEP 4: Calculate the required external static pressure • ESP > total friction loss main duct + local friction loss + 1 mm H2O • In our example: ESP > (0,07 x 13,5) + 4,4 + 1= 6,4 mm Necessary ESP = 7 mm H2O STEP 5: Dimensioning of the branch ducts • Obtain the duct size of each branch duct based on the same standard friction loss as for the main duct:0,07 mm H2O/m • Input standard friction loss and airflow and use the ductmeter to find velocity and duct size Complete the calculation tables for main ducts and branch ducts

  27. Table for main duct calculation Reducer Main ducting route

  28. Table for branch duct calculation

  29. Remember the main steps of duct calculation ! • Draw schematically the duct network • Establish the airflows for each section of the duct • Calculate dimensions of ducts, based on airflow velocities • Draw on scale the duct network, mentioning the special fittings (elbows, junctions, regulating flaps, etc.) • Calculate the total pressure loss, to choose the corect ventilator (the correct ESP) The velocities are decreasing from the ventilator to the far end of duct

  30. THANK YOU FOR YOUR ATTENTION !

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