Presentation on theme: "(material to be checked with local legislation)"— Presentation transcript:
1 (material to be checked with local legislation) 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 resistersMethods for duct calculationDaikin ductmeter
3 1. Introduction Needs to be considered Space availabilitySpace air diffusionNoise levelDuct leakageDuct heat gains and lossesBalancingFire and smoke controlInvestment costsOperating cost of the systemWhen designing a duct system, the following items need to be considered to meet the requirements of the customer:
4 1. Introduction Needs 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.…Poor duct design can lead to many problems
5 2. Duct classification By size Main duct system:each branch is connected to the main ductmost widely usedSeparate ducting system:“-” more ducting material requiredmore ducting space necessary.“+” economical solution if:mass producedround ducts are used.In a main duct system is each branch duct connected to the main duct. The main duct system is the most widely used system from an economical point of view.The separate ducting system needs more ducting material and requires more ducting space. It could prove to be an economical solution if only mass produced round ducts are used.
6 2. Duct classification By purpose Air supply ductCool 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 ductThe 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.Air supply ductCool 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 and more fresh air gets into the air-conditioner leading to a lower efficiency.
7 2. Duct classification By purpose Fresh air intake duct.The fresh air intake duct is connected to the suction side of the machineLocation of the fresh air intake port should prevent sucking in bad smells or dust.Insulation might be necessarySeparate fresh air system id recommendedAir 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 airFresh air intake duct.The fresh air intake duct is connected to the suction side of the machine and provides ventilation to the room.Special attention needs to be paid to the location of the fresh air intake port in order to prevent sucking in bad smells or dust.Insulation might be necessary to prevent heat losses and dew formation.The fresh air intake duct needs to be designed taking into consideration the ventilation needs during times that the air-conditioner is stopped.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 ducts Introduction Duct system losses are the irreversible transformation of mechanical energy into heat.These losses can be divided into friction losses and dynamic (local) losses.Duct system losses are the irreversible transformation of mechanical energy into heat. These losses can be divided into friction losses and dynamic losses.
9 3. Pressure losses in ducts Friction loss in the Zn Iron Plate duct3. Pressure losses in ductsDiameter (mm)Charts for friction losses inround ductsAir volume (m³/h)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.)Air velocity (m/sFluid resistance caused by friction in round ducts can be determined by use of the friction chart (Fig …). This chart is based on standard air flowing through round galvanized ducts with beaded slip couplings on 1200 mm centers, equivalent to an absolute roughness of 0.09 mm.Changes in barometric pressure, temperature and humidity have an impact on the air density, air viscosity and Reynolds number. No corrections to Fig. … are needed for:duct materials with a medium smooth roughness factortemperature variations in the order of +/- 15 K from 20°Celevations to 500 mduct pressures +/- 5 kPa relative to the ambient pressureFor duct materials other than those categorized as medium smooth in table … , and for variations in temperature, barometric pressure (elevation), and duct pressures (outside the range listed), calculate the friction loss in a duct by the Altshul-Tsal and Darcy equations.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 planC2Deep outer cone reduces smudging.Inner cone is a plaque.ACPFlange outer cone. Adjustable square diffuser for directional control from horizontal to vertical pattern. Extruded aluminium constructionE2Adjustable square diffuser with a plaque. Extruded aluminium constructionEPExtruded aluminium linear slot diffuser for directional control from horizontal to vertical positionVTLExtruded aluminium linear grilles with fixed barsKLDouble direction supplying grilles with front louvers vertical behind louvers horizontalVHS
12 4. Air diffusers & resisters- Types Cylindrical air diffuser with a long throw and low noiseNOZZLEDiffusers with unrestricted swivel type inner assemblyPKSLITReturn air grilles with fixed barsReturn air grilles with filters. Cores are hinged for easy removal and replacement of filterRFPGReturn air grilles with punching platesGrilles for exhaust air or intake air with slant louversGLReturn air grilles with a sight ploof coreDG
13 4. Air diffusers & resisters Locationneeds 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 consideredLocationThe resister needs to be positioned with care.In order to avoid drafts for the people, the resister should be positioned high enough.Special care needs to be taken to avoid dew generation on the ceiling surface.In addition, the position of the resister needs to be considered in function of the lights, structural beams, … .
14 4. Air diffusers & resisters Design procedure for supply/return grillesDivide the room into squares or rectanglesEach sq./rectangle to be served by one grilleRules of thumb:H= height of roomW and L: dimensions of Sq/rectangleW or L ≤ 3 * HL ≤ 1.5 * WL=15 mL=7,5 mL=7,5 mDivide the room into squares or rectangles to be served by one grille respecting the following rules of thumb:W or L ≤ 3 * HW ≤ 1.5 * LW=9 mVariant 2H= 3mVariant 1
15 4. Air diffusers & resisters Design procedure for supply/return grilles Outlet velocityThe 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 235 Cinema – hospital – library 340 Office – school – hotel 446 Bank – public hall 550 Store – post office 670 FactorySelect the diffusers by the maximum allowable air velocity for the room (Table …):Make sure that the maximum spreading radius covers the complete square / rectangle.Make sure that the nearest minimum spreading radiuses do not overlap each other to prevent drafts (Fig. …)
16 5. Duct calculation Main steps Draw schematically the duct networkEstablish the airflows for each section of the ductCalculate 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 methods related to the selection of air velocities along the main duct sections:The method of equal air velocitiesThe method of reducing the air velocitiesThe method of equal frictionsThe method of static pressure regain
18 5. Methods for duct calculation The method of equal velocities:Uses the same air velocity along the main duct“+” simpleeasy equilibration of duct branches“-” big and expensive installationnoisy in case of big air velocitiesConclusion: recommendable for industrial applications
19 5. Methods for duct calculation The method of reducing the air velocities:Establish a maximum air velocity next to the fanChoose an air velocity for each section of the main duct. The velocities are decreasing gradually to the end of the main duct.“+” Provides simple calculation of each sectionGives the possibility to choose velocities according to the required level of comfort“-” Arbitrary selection of velocitiesConclusion: requires personal experience
20 5. Methods for duct calculation Friction loss in the Zn Iron Plate duct5. Methods for duct calculationThe method of equal friction:Establish a maximum air velocity next to the fanDetermine the friction loss by using the chartUse the same friction loss along the whole duct systemDetermin 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/sFriction loss (mmH2O/m)
21 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 chartsAdd dynamic losses and divide the sum to the total duct lengthAdd the result to the constant value of the friction lossUse the new value to determin the duct dimensions and air velocity from chartsDifficulty: the dynamic losses have to be recalculated by using several attempts (velocities)Conclusion: precise method, easy to use for straight ducts
22 6. Duct calculation by equal friction method The Daikin ductmeter
23 6. Duct calculation with the Daikin ductmeter Assume that one packaged air conditioner is used to cool 4 rooms of a public buildingGiven data:Total air volume: 84 m³/minDischarged air flow volume from each universal resister: 21 m³/minPassing air velocity through supply and return grille: aprox. 4 m/s
24 6. Duct calculation with the Daikin ductmeter STEP 1Draw schematic view of the duct systemMake notes for air volume, air velocity, etcMark clearly the elbow, the branch parts, the air discharge outletSelect one main ducting route (where the maximum static pressure loss occures)KF1 m21 m³/min2,5 mCDE3,5 m3,5 m1,5 m2 mGSTEP 2Find the static pressure loss of the main duct routeSelect the corresponding air velocity and duct size1 m1 mB1 mA21 m³/min21 m³/min21 m³/min
25 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:
26 6. Duct calculation with the Daikin ductmeter INPUT:the total air flow is 84 m³/minselect the velocity 6 m/s for the source section (B-C) of the main ductOUTPUT from the ductmeter:friction loss: 0,07 mm H2O/msize of round duct: ǿ 53 cm or,size of rectangular duct: Lxl=75 cm x 35 cmFriction lossAir volumeAir velocityRound ductLong side lengthRectangular ductShort side lengthLong side length < 7 x short side length
27 6. Duct calculation with the Daikin ductmeter STEP 3: Find the local friction loss of the main ductFind the total friction loss at the elbow or branch parts:Input the velocity into the local friction loss calculation tableIn our case, to v=6 m/s corresponds ,4 mm H2O friction loss in the elbow“W“shows the diameter of the round ductor the long side of the rectangular ductw
28 Equivalent length of main duct (A K) 6. Duct calculation with the Daikin ductmeterSTEP 4: Calculate the required external static pressureESP > total friction loss main duct + local friction loss + 1 mm H2OIn our example: ESP > (0,07 x 13,5) + 4,4 + 1= 6,4 mmNecessary ESP = 7 mm H2OSTEP 5: Dimensioning of the branch ductsObtain the duct size of each branch duct based on the same standard friction loss as for the main duct:0,07 mm H2O/mInput standard friction loss and airflow and use the ductmeter to find velocity and duct sizeComplete the calculation tables for main ducts and branch ductsEquivalent length of main duct (A K)
29 Table for main duct calculation ReducerMain ducting route
31 Remember the main steps of duct calculation ! Draw schematically the duct networkEstablish the airflows for each section of the ductCalculate dimensions of ducts, based on airflow velocitiesDraw 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