Pressure Loss Through Symmetrical Wye

[greenen]

Hello all, first time poster here. I have a HVAC pressure loss problem and I am stuck. Here is the background info. I am working on an HVAC renovation project for a commercial office and we are redoing a portion of the return duct work. I am calculating the pressure drop in the return line to confirm that our new air handling unit will not be starved for air and to see if any of the portions of the duct work need to be redone.

The contractor fortunately used lots of rigid duct and has several symmetrical wye fittings, ASHRAE fitting ED5-9. The fitting has two 8" lines coming together with a separation of 60 degrees and they converge into another 8" line. Each inlet line has 180 cfm giving a combined cfm of 360.

Here is my issue, I am trying to find the total pressure loss through this fitting. In the ASHRAE tables for the this fitting with this airflow and geometry my local loss coefficient or "C" value is a negative. This means that the air stream actually gains energy after passing through the fitting. In the ASHRAE Fundamentals it explains that this can happen when one air stream is moving faster than the other. The faster air stream imparts energy to the slower air stream making the slower stream gain energy. My problem is that my two incoming air streams are the same velocity, same cfm, same duct diameter. Yet the tables say that each flow stream has the same negative C value thus each flow stream gains energy. This does not seem right.

What I think is happening is I have 2 8" lines reducing to 1 8" line, the fitting is acting as a nozzle speeding up the airflow. Thus my velocity pressure through the fitting goes up for both air streams, hence the negative C value. But, as a result my static pressure must go down and total pressure must go down. How do I find that drop in total pressure?

Thank you for any help.

 

[berkshire]

Without getting a velocity increase you would need an 11inch diameter duct on the main duct. You are right the 8" main branch is speeding up the airflow. It has to in order to carry the flow you are asking it to . Your 180 cfm is going to have a velocity of 516 fpm. doubling the flow to 360 cfm is going to require a velocity increase to 1031 Fpm . Which brings the next question , what is the design flow rate of the existing ducting?

You are judged not by what you know, but by what you can do.

 

[willard3]

Total pressure = Static Pressure + Velocity pressure.

Don't overthink this, it's straightforward.

 

[Waramanga]

The OP doesn't think this is straight forward clearly. I am not familiar with ASHRAE notation, what is the C value and what formula do you use it in?

 

[greenen]

Thanks all for the replies. The design flow rate of the duct work is around 1000 fpm. Because it is mostly existing duct work, I simply need to check that the total return pressure loss is not too high. With this system the design pressure drop is around 0.15 in. w.g.

As for what the ASHRAE C value is, it is a dimensionless constant that reflects the losses air experiences as it goes through a fitting. The higher the C value the higher the losses. The total pressure drop across a fitting is found by multiplying the C value times the velocity pressure. At least that is my understanding.

 

[tys90]

Total pressure drop through a fitting (referred to as Dynamic Loss in ASHRAE) is your coefficient times velocity pressure. Just because the number is negative doesn't change the equation. See example 7 in your the Duct Design chapter.

 

[Drazen]

it is enough that velocity increases and your dynamic energy will increase, and possibly total energy.

that's ok for wanting to understand everything, but some engineering simplification exist for generally complex processes - so follow you ashrae table.

i would be more concerned about what you call rigid duct. what is material of duct, and what is condition of the inner surface? that would be much more influential to your pressure loss estimate that subtleties of a one single fitting.