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In this part of the full HVAC course, we will see about the different duct designing methods that are utilized in the industry.
Distinguishing Factor of ducts with respect to Methods of Duct Designing :
* Equal Friction Method / Constant Pressure Method : Used in Residential or Commercial.
* Velocity Reduction Method.
* Static Regain Method ( For Industries ).
* Constant Velocity Method.
* Equal Friction Method :
In this method, same value of friction loss rate per unit length of duct is used to size each section of the duct in this system. The friction pressure drop or loss per unit length of duct is maintained constant throughout the duct system and sizing of duct is maintained accordingly. Friction loss rate is chosen to result an economical balance between duct cost and energy cost, a higher frictional loss results in a smaller duct but high fan operating cost. Most universally used friction loss rate for supply, return and exhaust air ducts for low and medium pressure ducts is up to 4" of water gauge.
If an equal friction design has a mixture of long and short run of ducts, short duct needs considerable amount of dampering (balance pressure). This is the drawback of the Equal Friction method of design. To balance the pressure of air as we need equal pressure of air of the duct run.
Advantages :
* Automatically reduces air velocities in the direction of air flow for supply air system in duct run.
* Balanced designing with respect to initial and running cost.
* if duct is symmetrical in nature or in design or there is a same length of duct run, then no dampers are required to balance system of air flow, as this method gives equal pressure losses in each branches.
* Quick and easy method of designing which is much more preferable and acceptable in industrial practice.
Disadvantages :
* If equal friction design has a mixture of long and short run of ducts, shortest duct will need considerable amount of dampering, it is the drawback of equal friction method design to balance pressure.
* There is no natural provision of equalizing pressure drop in branches when layout is not symmetrical.
* Not suitable for long-run of ducts. (Not more than 30' from one end to another end ).
Velocity Reduction Method ( Rough Method ) :
Size of main duct is established by selecting of velocity from recommended range as done in equal friction method. In this method, for every duct branch velocities are reduced arbitrarily or reduced velocities are selected based on previous experience, with selected reduced velocities gradually and cfm duct sizes are recommended or designed. This method is though simple but not used in actual practice because it does not take into account of relative pressure losses of various branches. So for balancing system dampers are to be arranged provided at the start of each duct.
Constant Velocity Method :
In this method. velocity to entire duct section is kept constant. Initially, selected velocities are very low as compared to other duct design, which are kept constant. With usage of Constant Velocity there is a reduction in cfm, and as cfm reduces friction loss increases. Whereas velocity remains constant (i,e; Constant velocity cfm decreases, friction loss increases ). Application of this method finds in, high glass palaces, deluxe apartments, villas etc. This method is also used for last / terminal branch design.
Static Regain Method :
Principle of static regain method is to maintain constant static pressure before each terminal and on each branch to provide same flow of air / pressure of air. This is achieved by sizing duct in such a manner after each branch / outlet. Static pressure gain due to reduction in velocity exactly matches branch pressure drop. Static regain method of sizing duct is most often used for high velocity system with long duct run, especially for large installation. With this method, initial velocity and main duct leaving fan is selected in range of 2500 to 4000 fpm.
After initial velocity is selected/ chosen, the velocity in each successive section of duct in main duct is reduced. So that resulting static pressure gain is enough to overcome the friction losses for next section. Result is that, static pressure is same in each junction in main duct run and because of this they are generally same amount of branch outlets and balancing is achieved.
Advantages :
* Possible to design long run and short run of duct for complete regain / pressure.
* Accurate method of duct designing.
* Low balance of equillibrium pressure is required.
Disadvantages :
* Time consuming.
* Experienced person is needed for designing duct by using this method.
Static pressure = Pressure acting on surface of ducts
Static Regain :
Conversion of velocity pressure (Pv) to static pressure (Ps) is known as Static regain. or gaining static pressure is known as static regain.
where, R = Recovery factor depends on shape of fitting that changes velocity . value = 0.7 to 0.9 , can be taken as per fitting.
Aspect Ratio :
It is the ratio of longer side of duct to shorter side of duct. As aspect ratio increases, more metal surface is required for duct, for same cross-sectional area.
Aspect Ratio (AR) = longest side / shortest side
AR = Width of duct / Height of duct
Example 1,
Example 2 ,
As per SMACNA, Maximum permissible aspect ratio should be less than 4 or equal to 4. As that aspect ratio decreases less sheet metal surface in contact with air, that also results in decrease in friction and running cost of design project. Best perfect aspect ratio is equal to 1. i.e; Only for square ducts. As shape changes, the perimeter changes, and friction loss increases with increase in perimeter for same area. Aspect ratio for low velocity and medium velocity is 4:1 . Aspect ratio for high velocity duct is 7 : 1 .
Well, thats it for this part. In this we tried to cover everything related to duct design methods and aspect ratio. Until the next part, Keep learning.. Keep Growing..
Access Previous parts of this course, if you have missed it, by clicking Below..
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