Questions regarding System Effect 3

[David_HVAC_Learning]

So, I've been reading this paper from Carrier in regards to the application of fans in commercial HVAC Equipment (Link to White Paper below) and I have a few questions regarding system effect. First, here is a following excerpt from the paper (Page 6), saying,

"Since the velocity profile at the outlet of a housed fan is not uniform, adequate straight duct distance must be allowed for the velocity profile to fully develop. (See Figure 7.) If a turn or bend in the duct takes place before this distance, the fan’s external static pressure (ESP) capability will diminish."

So, the thing I don't get is that, why is it so important for there not to be a bend coming out of a scroll housed centrifugal fan in comparison to bends and turns that happen all along a duct system? Why is it important that this one area (aside from the inlet) not have any bends or turns? I assume the velocity profile will be distorted in other bends in the ductwork, so why is it important for external static pressure for this one area? I don't know if there is a physics explanation to this or if it's just an observed phenomenon. Any input on this would be appreciated. Thank you. |

Link to Paper:

https://www.shareddocs.com/hvac/docs/1001/Public/0F/04-581070-01.pdf

 

[MintJulep]

Fundamentally, friction and momentum.

Higher velocity means higher friction losses.

Higher velocity means more momentum means harder to change direction.

These things use the fan's capacity without providing useful work (such as moving the air further along the system).

There's no fundamental difference at a fan discharge compared to any other change in direction, but at the discharge the velocity is high, therefore the losses are high.

 

[MEP.Learner]

hello,
not sure if your question is as per the figure or just a general MV question.

But for the figure 7 above, for blow thru "fan and coil arrangement", u can see the turbulence of air or the "velocity profile" of the air within the duct. If the min. distance is not complied, the cooling / heating thru the coil will be affected. (uneven heating / cooling).

For general MV case, i believe the intend is always to achieve the airflow.
if due to site constraint that this "after fan" distance cannot be maintained, there might be air noise issue, etc.

 

[David_HVAC_Learning]

Thank you for your replies, in regards to my question, it was more of trying to understand why system effect is so important at inlet and outlet from a physics perspective rather than the repercussions for not designing for it.

I'd like to try to give a response to MintJulep's comment so I can try to encapsulate it in my head. So, when the flow is coming out of the fan (particularly, the scroll centrifugal fan), the velocity distribution is uneven, so one side of it as higher than the other. From what I gathered from MintJulep's comment, the reason why we care so much about having enough duct length for the inlet and outlet (outlet in this case), is because since the velocity is so high on one side of the distribution, its associated losses are going to be much greater. Here I refer to the minor head loss equation, H_f = K*(v^2/2g) which according to said equation, the higher velocity the fluid has, the greater the minor head loss will be. So if we don't allow the velocity distribution to even out, then its peak velocity will be higher thus resulting in a higher minor head loss. And just to chime in from a physics perspective, I would assume that if air has a lot of kinetic energy imparted to it (so high velocity), and something gets in its way in order to change direction such as an elbow, the impact of the air onto the duct will cause a loss of kinetic energy as the impact is not elastic (kinetic energy is transformed another type of energy). So I guess in other words, the higher velocity that something is moving, the harder it'll be to make it change direction, which would then generally result in a greater associated loss of kinetic energy.

If someone thinks I've got this completely wrong, please let me know. Thank you.

 

[HVAC-Novice]

Same reason you also are not supposed to have fittings closes to each other.

Some fan manufacturer selection software can account for some fan-effect. Not sure how realistic those are accounted for, though. but the energy loss is a real thing if you install a fan against manufacturer recommendation.

I suspect this also creates more noise. Sometimes you don't have a choice and need a more powerful motor.

 

[LittleInch]

Also any bend will induce swirl which also adds to frictional losses.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.