Duct Design In Central Air Conditioning System Engineering Essay

Earlier the usage of air-conditioning for comfort intent was considered a epicurean but now-a-day, it has been a necessity in utmost climatic conditions, such as utmost cold and hot in western states. Window air conditioners are preferred for office suites while big centralized units are installed for conditioning the auditorium, infirmaries etc. The right appraisal of chilling burden of an auditorium is really complicated due to many factors such as out-of-door temperature, humidness, air escape into the learned infinite, residents, measure of fresh air taken in and solar burden etc. The writers have calculated chilling burden, measure of supply air ( in cmm ) of an auditorium utilizing CLTD method which is more accurate. Equal clash method is adopted to plan the canal which is simple as compared with the other canal design methods. Using CFD package, analysis of air flow sing speed and force per unit area is done in chief canal and in cubitus. The writers concluded that the combine consequence of proper canal design method & A ; CFD could be used to accomplish proper air distribution inside the auditorium with minimal force per unit area loss, running cost & A ; energy ingestion

Introduction: –

Without A/C system, individual working in industry is experiencing unhealthy, uncomfortable and inefficient. In order to accomplish required chilling burden, proper method is required. Proper air distribution is achieved with proper canal design which leads minimal losingss in the system, suited choice of fan with high efficiency, optimal air speed in canal, recess and mercantile establishment of fan. Today some package ‘s are available to gauge chilling burden, to plan the canal, to choose the fan etc. Life has become easier due to AUTOCAD package to pull works layout. CFD as a analysis tool has the ability to set up steadfast quantitative informations sing air gesture and can foretell unstable features and force per unit area derived functions to a really low degree that are by experimentation impossible during experimentation. Analysis of air flow in canal with inactive force per unit area and speed force per unit area is made easier

and faster in Fluent package. The demand of air conditioner is that it must supply equal chilling to the residents in the learned infinite under a broad assortment of ambient conditions. A normal healthy individual feels comfortable at 25A°C DBT, 50 % RH with 9 to 12m/min air speed. Human comfort is influenced with the physiological conditions determined by the internal heat coevals.

Large figure of research workers have worked on canal design with assorted methods. The part of assorted research workers on the subjects is briefed as follows.

1. design for high speed canal system is done by finding the force per unit area losingss, ciphering the noise degree, finding the out of balance force per unit areas & A ; optimising this against the entire cost of the system.

2. VAV optimisation process was applied to the three VAV canal systems to look into the impact of changing airflow rates on the size of canal systems. For comparing intents, other canal design methods, such as, equal clash, inactive regain, and the T-Method, were besides applied to the canal systems

3. Design optimisation of industrial canals is achieved with CFD. The CFD analysis has offered a comprehensive scope of end product including speed distribution, force per unit area profiles and turbulency degrees

The chief intent of this paper is make proper computation of chilling burden, supply air measure utilizing proper method, make up one’s mind size of chief canal & A ; elbow utilizing proper canal design method so individual seating in the auditorium can experience comfy.


Cooling Load Temperature Differential ( CLTD ) / Cooling Load Factors ( CLF ) / Solar Cooling Load ( SCL ) is used to cipher the chilling burden of an auditorium. It was an effort to simplify two stairss TFM & A ; TETD /TA methods into a individual measure technique that allowed continuing straight from natural informations to chilling burden. A series of factors were taken from chilling load computation consequences as ‘Equivalent Temperature Difference ‘ for usage in traditional status ( q = UA dT ) equation. The learned air ( cooled or heated ) from the air conditioning equipment must be decently distributed to suites or infinites to be conditioned in order to supply comfort conditions. When the conditioned air can non be supplied straight from the air conditioning equipment to the infinites to be conditioned, so the canals are installed. The canal systems convey the learned air from air conditioning equipment to proper air distribution points or air supply mercantile establishments in the room and carry the return air from the room back to the air conditioning equipment for reconditioning and recirculation. The learned air ( cooled or heated ) from the air conditioning equipment must be decently distributed to suites or infinites to be conditioned in order to supply comfort conditions. It may be noted that canal system for proper distribution of conditioned air cost about 20 to 30 % of entire cost of equipments required

Duct stuff is normally made from galvanized Fe sheet metal, Al sheet metal or black steel. But now a twenty-four hours, the usage of non-metal canals has increased. The rosin bonded glass fibre canals are used because they are rather strong and easy to fabricate harmonizing to desired form and size. They are used in low speed applications less than 600m/min and for inactive force per unit areas below 5mm of H2O gage.

It may be made in round, rectangular or square forms. From an economical point of position, the round canals are preferred because the round form can transport more air in less infinite. This means that, less canal stuff, less canal surface clash and less insularity is needed. For rectangular canal, form is determined sing minimal facet ratio. Calculation of inactive force per unit area, dynamic force per unit area & A ; entire force per unit area in canal is of import. The force per unit area in canal is normally expressed in millimeter of H2O. Rise in force per unit area in fan is known as fan entire force per unit area ( FTP ) . FTP & A ; supply air flow rate ( in cmm ) are used to choose the fan. Here the force per unit area is lost due to clash between the traveling atoms of fluid ( i.e. air ) and interior surface of canal. When the force per unit area loss occurs in a consecutive canal, it is normally termed as clash loss. The force per unit area is besides lost dynamically at the alterations of way such as in decompression sicknesss, elbows etc. and the alterations of cross subdivision of canal ; this type of force per unit area loss is normally termed as dynamic loss.

General regulations used to plan air-duct:

I ) Air should be conveyed every bit straight as possible to conserve on power, stuff and infinite.

two ) Sudden alterations in way should be avoided. When decompression sicknesss are indispensable, turning vanes should be used to minimise force per unit area loss.

three ) Air speeds in canals should be within allowable bounds to minimise noise.

four ) Diverging subdivisions should be made gradual. The angle of divergency should non transcend 200.

V ) Rectangular canals should be made as about square as possible. This will see minimal canal surface, and therefore cost, for the same air transporting capacity. Dampers should be provided in each subdivision mercantile establishment for equilibrating the system.

six ) Avoid canal obstructors

Methods of Duct Design:

I ) Equal Friction Method

two ) Velocity Reduction Method

three ) Static Regain Method

In Equal Friction Method the frictional force per unit area bead per unit length of the canal is maintained changeless throughout the canal system. Due to its simpleness, this method is used in the thesis to plan the canal.

Cooling Load Calculation for an Auditorium:

Data collected for an bing auditorium, Pune

No. of doors: 9 Entry and issue.

1 Back phase.

2. No. of Windowss: 8

3. No. of Chairs: Dress circle 704

Balcony 202

Entire 906

4. Equipments: Entire point holding 11.78 KW

Design Conditionss

1. Latitude of Pune: 18A°6A? North.

2. Design Day: 25th May.

( Max. temperature during last hebdomad of may, from last 5 old ages data. )

3. Outside conditions: DBT = 38A° C

( Outdoor ) RH= 50 %

4. Indoor conditions: DBT = 22A°

RH= 55 %

5. Daily scope ( DR ) : 10A° 6. Time: 4:00 autopsy

7. Average outside temperature on design twenty-four hours

Ta= To – ( DR/2 ) = 38 – ( 10/2 ) = 91.6A° F

Entire Cooling Load

Entire = QS + QL = 2.55 + 4.54 = 7.09 kilowatt

Entire reasonable burden on chilling spiral = 248132.64 W

Entire latent burden on chilling spiral = 276989.83 W

Therefore, Entire chilling spiral burden = 525122.47 W

Therefore, Refrigeration burden =525.122/3.516=150TR

Safety factor of 5 % due to duct heat addition


Cooling burden = 157.5 TR a‰? 160 TR

Fig.1 Air distribution system & A ; air flow rate

Assuming by-pass factor ( BPF ) 0.15, mass flow rate is calculated. Mass flow rate of air, MS = 19.28 kg/s

( cmm ) s = 1288.88 a‰? 1290

Calculation of Friction Loss in Duct & A ; Duct Diameter

Rearranging fanning equation, Colebrook-White relation & A ; Fritzsche relation

a?†Pf = ( 0.01422 C 1.852 L ) / D 1.269

a?†Pf = ( 0.002268 Qv1.852 L ) / D4.973 millimeter of H20

a?†Pf = ( 0.012199 C 2.4865 L ) / Qv0.6343 N/m2

Recommended speed in canal for auditorium, C = 6.5 m/ s or 390 mpm

.So, a?†Pf / L = 0.36629 N/m2 /m

= 0.03669 mmH2O /m a‰? 0.04 mmH2O/m

a‰? 0.04 in w. /100 foot

( In industry criterion value for force per unit area loss = 0.1 in.w./100 ft. = 0.846 N/m2 /m )

Round Duct Diameter = 1.187 m

For rectangular canal dimension, minimal facet ratio 1.3 is taken. a/b = 1.3 ( a= breadth, b= tallness )

( it should be kept every bit low as possible to maintain clash losingss moderately low and thereby avoid extra energy ingestion. )

D = 1.265 ( a * B ) 0.6 / ( a + B ) 0.2

Large dimension ( a ) =1.24 m & A ; little dimension ( B ) = 0.9543 m

Fig.2: Pressure fluctuation in a Duct System:

Fan Total Pressure = 45.47 millimeter H2O = 446 N / M2

Fan Static Pressure= 359.49 N / m2 = 1.42 ” Water

Duct Design utilizing Equal Friction Method

First measuring of all dimensions are done for auditorium and with AUTOCAD package, drawings are made ( front position and top position ) . It is shown below

In the equal clash method, the frictional force per unit area bead per unit length of the canal is maintained changeless throughout the canal system. The process is to choose a suited speed in the chief canal from sound degree considerations. Knowing the air flow rate and the speed in the chief canal, the size and clash loss are determined from the clash chart. The staying canals are so sized keeping the clash loss per unit length at this value for their several air-flow rates.

Due to 3 fans in the system, there will be 3 chief supply canals. The air supply from three chief canals goes to three different countries

I ) First chief canal – balcony and staying country ( excepting phase ) of one side

two ) Second chief canal – balcony and staying country ( excepting phase ) of other side

three ) Third chief canal – phase

Layout of First Main Duct ( Supply Duct )

Flow rate = 15524 CFM = 7.2 M3 / s,

a?†Pf = 0.04 millimeter H2O /m = 0.4 N /m2/m

Fig. 3: Layout of First Main Duct ( Supply Duct )

Fig 4: Layout of Branch A of First Main Duct ( Supply Duct )

Branch A

From chief canal to first supply mercantile establishment, Flow rate = 2.5 M3 / s = 5390 CFM

a?†Pf = 0.04 millimeter H2O /m = 0.4 N /m2/m

Recommended subdivision canal Velocity = 4.5 m/s

So from friction chart of canal

Diameter of canal = D = 32 ” = 0.8128 m

For rectangular canal, a / B = 1.3

D=1.3 ( a*b ) 0.625/ ( a+b ) 0.25

From this equation, we get a = 0.85 m b= 0.65 m

CFD Analysis:

Duct design is done utilizing equal clash method. For CFD analysis, Fluent 6.1 package is used. Gambit 2.0 is used for patterning the chief canal and cubitus. Geometry and engagement is carefully done in Gambit. Pressure and speed fluctuations are observed in this analysis. Eddies observed in cubitus are due to improper form and speed. All figures from geometry, boundary status to purl are given here. This CFD tool can be used for a whole edifice to analyse air force per unit area and speed fluctuation in all canals

Fig.5: Analysis of Duct sing Inlet Velocity

Fig.6: Analysis of Elbow sing recess Velocity


The consequence analysis is based on canal design of auditorium and bing works. Comparison among these workss can be made with some points.

I ) Duct size: To plan the canal in an auditorium computation of chilling burden and air flow rate is done. By presuming suited air speed ( from 1967 systems and equipment ASHRAE Handbook ) sing noise factor, computation of chief canal is done. Then size of rectangular canal is calculated sing minimal facet ratio.

So maximal attention is taken to use standard regulations to plan the system. Variation occurred in some points are given below




Plant 1

Plant 2




of works

100 TR

100 TR

160 TR



Three chief canals

One chief canals

Three chief canals


Main Duct size

1.2 m* 0.9 m

1.62 m * 1.21m

1.24m * 0.95m

II ) Branch ducting:

Branch connexion with low force per unit area are used.

III ) Equal clash method is used in industry or in bing design or in an auditorium. But equal frictional force per unit area bead used in industry is 0.1 in.w./100ft. or 0.846 N/m2/m. while planing of an auditorium, 0.04 in. tungsten / 100 ft. or 0.4 N/m2/m is used.

Practical cognition is really of import to analyze and do some decision about this point.

IV ) Aspect Ratio: It is unbroken minimal in each design

V ) CFD Analysis: It is a new tool which is applied to detect air gesture through consecutive canal and cubitus. Besides it can be applied for all ducting in a edifice. Due to improper form and speed, Eddies are observed in cubitus with CFD analysis. so by proper form, Eddies are minimized. Analysis of all ducting and fan will cut down force per unit area loss, noise and evidently It saves clip and cost. So CFD is best tool for HVAC system


The undermentioned decisions summarize the experimental consequences presented in this paper

1 ) The chilling load computation of an auditorium is done, using CLTD method and canal design is carried out by equal clash method. All consequences are comparable with bing works.

2 ) The deliberate value of frictional force per unit area bead is less every bit compared to bing works or value used in industry. Due to less value, canal diameter is increased but loss in inactive force per unit area, speed force per unit area can be avoided. Smaller diameter of canal would increase noise degree. So requirement of sound rarefying devices may necessitate. Besides chance of dampers is decreased with increasing diameter. But first cost is increased with increasing canal diameter.

3Due to proper ramification ( with cubitus ) of canals, loss is minimized in this design. But in bing works, there is consecutive ramifying in so many locations, which may increase the force per unit area loss.

4 ) Aspect ratio is kept minimal in this design. So friction loss and extra energy ingestion is decreased

5 ) Pressure loss in canal adjustment is kept minimal by utilizing cubitus with proper form sing really less force per unit area loss coefficient.

6 ) CFD package is used to analyse the air flow in consecutive canal and in cubitus. Eddies are observed due to wrong form of cubitus. So proper form of cubitus and right speed are estimated to minimise the Eddies every bit good as force per unit area loss.

7 ) CFD can be used to analyze force per unit area and speed fluctuation for a whole edifice. So it is a better tool which can be used in HVAC system to salvage clip and cost


Valuable information & A ; layout of works 1 & A ; 2 is taken from bing cardinal air-conditioning works in Pune.