Dimensioning of Air Ducts in Buildings with Comfort Ventilation

Transcription

1 South-EasternUniversity of Applied Sciences Overview on Dimensioning of Air Ducts in Buildings with Comfort Ventilation Heikki Salomaa Senior Lecturer, M.Sc.(Tech.) South-Eastern University of Applied Sciences

2 Comfort ventilation Comfort ventilation is used in - office buildings - commercial buildings - school buildings - hospitals - dwellings etc. where is enough space for the ducts In industrial ventilation (high air velocities may be necessary) and e.g. in ships (lack of space) is used different dimensioning methods as in comfort ventilated buildings 2

3 AIR DUCTS Connection duct Supply air device Cleanout cover Branch duct Adjusting valve and inspection cover Main duct

4 Air flow rates Air flow rates are basing on the - use purpose of the spaces (D2, SIL = ICC: S1, S2, S3) - cooling demand - heating demand (air heating) - impurity loads - quality demand of the indoor air The use, loads and quality demands of the building can be changed many times during the life cycle

5 Dimension of Air Ducts The dimension of ducts has influence on the - quality level of indoor air - energy use of the fan - regulation of air flow rates - noise level caused by air flow Correct dimensioning creates good operation conditions to regulating valves and air terminal devices Furthermore the reasonable sizing has an important influence on the costs (constructions, air ducts, re-regulation ) during possible repairs, where the use purpose of the spaces will be changed

6 Different dimension methods of air ducts The total pressure losses of air ducts must be known in the selection of a proper fan Air flow rates in a completed duct network will be automatically adjusted so, that the pressure drop in of every branch will be the same Traditional dimensioning methods of air ducts are the use of - empirical air flow velocities - a constant frictional pressure loss (Pa/m) - a method basing on the recovery of static pressure Nowadays the trend has been the use of a method developed by VTT basing on the average duct pressure level and the control of fan power demand (if there is enough space for the ducts)

7 Well-sized air ducts basing on VTT-guidelines The basis of dimensioning is the - right choice of the average duct pressure level - control of fan power demand Air flow rates are controlled by adjusting devices (air terminal units, control valves etc.) Pressure losses of air ducts are very low, because applied flow velocities are really low No need to take into account of frictional pressure losses Low velocities help to keep sound levels within the limits Improper dimensioning of air ducts Proper dimensioning Selection of air duct sizes 7

8 When the temperature outside is lower than indoors, thermal forces have influence on ventilation air flow rates (chimney effect) These forces are harmful, because they change the air flow rates of the building and so also the ventilation The changes of air flow rates are depending on - thermal pressure difference - the pressure level in the ducts - the tightness of the air ducts - the tightness of the building envelope Thermal forces in air ducts p T T Δp = pressure difference in the duct, Pa (caused by temperature differences) T s = temperature of indoor air, K T u = outdoor air temperature in K ρ s = indoor air density, kg/m 3 g = gravity = 9,81 m/s 2 H = height of the flue, m Thermal pressure difference is 5 6 Pa, when = s T ρsgh - floor-to-floor height is 2,8 m - outdoor temperature is -20 o C u u 8

9 The pressure level of an air duct The pressure level inside the air ducts should be roughly triple so high as the pressure difference caused by thermal forces In that case the deviation of air flow rates will be acceptable 5 10% The lowest operation pressure of a duct and a regulating device should be ca. 100 Pa in a building with 5 6 flats Pressure losses should be altogether Pa The highest pressure drops of the adjusting valves and air terminal units should be Pa (in high buildings even more) to be able to control air flow rates and sound level of the space p = T s T T Thermal pressure difference is Pa in a building With 5 6 flats, when u ΔT > 40 o C u sgh ρ 9

10 The average pressure level in the air duct The upper pressure level limit of a flow control device is set by - the noise production in the adjusting device - the noise caused by leakage air of air ducts - the allowed sound level in the space Noise production < Pa Adjusting of air flow rates > Pa Thermal forces (-20 o C) > Pa Tightness class of air duct must be C 10

11 The pressure level in air ducts must be high enough because of controlling thermal forces and air flow rates The high pressure will cause noise in the leakage points of the air ducts air ducts must be tight enough The highest allowed leakage air flows per duct area, when test pressure is 250 Pa Noise caused by leaking air Noise caused by leaking air Pressure level of the air duct, Pa Air tightness class 11

12 Recommendations for air duct sizing Air duct sizes are chosen according to maximum air flow rate in a duct Maximum air flow rates of connection ducts equal to the designed max air flow rates of terminal units Main duct sizes are chosen according to the sum air flow rate of individual rooms (if possible) or max one duct dimension smaller Air duct system works the better the lower air velocities and the tighter ducts are used In borderline cases there is better to choose the next bigger duct size of the standard SFS

13 Estimation of duct pressure losses Air velocities in ducts are chosen according to the figure beside, basing on the highest accepted sound pressure levels in rooms and on the adjustability of ducts Quick estimation of pressure losses is done by counting the number of bends and T- pieces Pressure loss coefficients of bends and T-pieces are in average supposed to be: ζ = 1 Pressure losses caused by local flow resistances can be determined from the figure beside These pressure losses are supposed to be a rough value for total pressure losses of ducts Pressure losses caused by friction are not taken into account in this rough estimation Incorrect dimensioning Correct dimensioning 13

14 Magnitude of maximum pressure losses Maximum pressure losses of air ductwork should be maximum 50 % compared with the average pressure level of air ductwork The pressure losses of AHU are not included above Supply and exhaust air fans must be regulated respectively Primary silencers of fans are in machine rooms Secondary silencers may be necessary after air flow controllers 14

15 Summary of demand controlled air ductwork dimensioning In the dimensioning of air ductwork is noticed beside technical and economical view points - sound engineering quality of indoor air (satisfaction, work efficiency) - adjustability of air flow rates (simple initial adjustment) - suitability to demand controlled variable air flow rates - appearance and flexibility to use changes Air ductwork must be airtight (C-class) to avoid - noise problems - dirtiness - growth of fan power and energy use The same duct sizes are used as much as possible - better-looking ductwork, less reducers and lower installation costs 15

16 Target points of air ductwork Energy and control efficient Functional with variable air flow rates allowing load changes Air flows stabile Sound levels low Air tight Easy to design Self regulating and easy to adjust during building process Easy to use (operating mode easy to check) Good-looking for architectures and the public 16

17 Pressure level and authority Aim to design air ductwork, which is symmetric also with demand controlled air flow rates Regulating- and/or terminal devices must have enough authority to adjust air flow rates pressure level (= pressure reduction of devices) must be high enough compared with pressure losses of ducts In design must concentrate on control of duct pressure and noise levels instead of air flow rates Choosing bigger duct dimensions can be built well-working air ductwork in pressure-, flow- and sound properties Limits on pressure levels of regulating and terminal devices are set mostly by their own sound output 17

18 Optimal air ductwork One size bigger duct size compared with ductworks designed by earlier dimension methods Also in borderline cases near highest air velocities, better to choose next bigger duct size The same air duct sizes are used as much as possible The amount of reducers is kept in minimum 18

19 Benefits of choosing next bigger duct size Quantity Abbreviation Choosing bigger duct size Air velocity in a duct v v/2 Pressure losses Δp Δp/3 Sound power level L w L w 7 db Energy demand of a fan P P/2 19

20 Dimension task of air ductwork Dimension an air ductwork shown beside (next slide) and give in table form: duct size, total air flow rate and velocity according to a) traditional empirical dimension methods basing on air velocities b) VTT- dimension method developed by Technical Research Centre of Finland Return your results in table form and be ready to introduce your results during next lessons (number your branch duct starting from above: branch 1, branch 2 and branch 3) 20

21 Example of air ductwork +50 l/s +50 l/s +50 l/s +50 l/s +200 l/s +200l/s +300l/s +150 l/s +300 l/s +400 l/s 21

22 Basics of dimensioning 22