Duct Sizing methods 2

[luciferresting]

I know of these 3 methods that are used in the industry.

1.Equal friction method
2. Static regain method
3. T method

But have also heard many saying "Constant velocity" method.
Is this another name for the Equal friction method.

 

[quark]

I didn't hear of constant velocity method and it should not be, definitely, equal friction method. I may be wrong but I can't think of any advantage with equal velocity method.

 

[lilliput1]

For low pressure ductwork, we use equal friction method up to a maximum velocity of 2000 FPM then hoild 2000 FPM for larger duct sizes to avoid noise problems.

 

[cabalen]

Some use a specific range of velocity for duct sizing of main and branches. That is almost the same as equal friction method.

 

[AbbyNormal]

the satanactive method is pretty hot

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[berkshire]

AbbyNormal (Mechanical)
That must be hell to calculate.
B.E.

 

[walkes]

Constant velocity would probably be used for dust collector distribution ductwork.

 

[christyoommen]

Whether u have constant velocity or equal friction, the outcome is the same. because in both cases, the other is constant. For example, in the equal friction method, if you have a friction loss of 0.1 " for every 100 ft of duct, then the velocity is fixed. Likewise in the constant velocity method the friction drop per ft of duct is constant.

 

[luciferresting]

Thank you for your replies but i still am not sure if the method used in sizing ducts using Equal velocity is exactly the same as in Equal friction.

When using the equal friction method, does the velocity remain more or less the same across the entire length of the duct (atleast the main branch)

 

[KiwiMace]

In similar material ductwork these will work out the same, but the way that these two are used are not. Basically, the only difference is which window of the ductulator you are looking at when you set the wheel.

1. Constant Velocity - used in dust extraction etc. Need, say 15m/s duct velocity to maintain particle removal. You set the wheel to 15m/s at the velocity window and read duct size from the V flow.

2. Equal Friction - Very common method of rough sizing supply air etc ductwork. Eg. Use, say 0.8 Pa/m for all duct runs. You set the wheel to 0.8 Pa/m at the friction window and read duct size from the V flow.

 

[luciferresting]

Thanks CinciMace for your reply.

Which means all these three methods are entirely different and are based on applications. But i guess the ductulator has no provision for the Static regain method. Can you suggest the application and methods used for Static regain. Do u suggest a software that i can use for Static regain.

 

[imok2]

Elite software has a good CD on Static regain, however I am only familier with these four:

Static regain method—a method of duct sizing in
which duct velocities are systematically reduced,
allowing a portion of the velocity pressure to
convert to static pressure and offsetting duct
friction losses

Velocity reduction method—a method of duct
sizing in which arbitrary reductions are made in
velocity after each branch or outlet

Total pressure method—a method of duct sizing
that allows the designer to determine all friction
and dynamic losses in each section of a duct
system (including the total system)

Equal friction method—a method of duct sizing in
which the duct friction loss (pressure drop) value
is used constantly throughout the design of a low pressure
system.

Of the four methods of duct sizing listed
above, the equal friction method is the one most commonly used in light
commercial and residential low-pressure systems.
For residential and light commercial duct systems,
the design criteria normally used are pressure drops
of 0.10 in. w.g. (inches of water gauge) for the supply
side of the system and 0.05 in. w.g. for the return

 

[luciferresting]

Thank you imok,
The additional method, u mentioned about Total pressure was very new to me. Thanks once again for a god description.

 

[walkes]

Note to Christyoommen, the two systems you describe are not correct the equal friction method will have varying velocities in the system depending on the size of the ductwork. A 1000 CFM branch will have a velocity of ~940 FPM at 0.1" ( 14" duct), a 2000 CFM branch will have a velocity of 1100 FPM at the same pressure drop (18" duct).
If you were using constant velocity the 2000 CFM branch would be 20" diameter.

 

[willard3]

The total pressure method is a very good one because it lets you see the velocity and static pressures at each take-off.

If you know both static and velocity losses, it will keep you from using high-loss fittings and high velocity flow (noise).

Total pressure is also the only way to analyze an existing duct system to identify problems. It also will allow you to construct pressure gradient diagrams for the whole system.

Specifying fans by total pressure will also allow you to get the fan you want from suppliers and not a cobbed-up substitution.