HVAC pressure confusion

[cunconbkhp]

Hello everyone!

For a simple validation case test, i want to apply HVAC system to pressurize a compartment to 40 Pa and the expected volume flow of the fan (blower) is 3500 m3/hour.  

My fds file is attached .  To estimate the loss coefficient in the extraction duct : i've applied the momentum equation with steady state assumption. The diameter of the duct is D=40 cm.

The leakage is not accounted yet to simplify the calcul. So the volume flow of the extraction duct must be equal to 3500 m3/hour. So the air velocity in the extraction duct is :

3500/3600/(pi*(D**2)/4)=7.7367 m/s

From hand calcul with  delta_P=40 ,u =7.7367 m/s,rho=1.2 kg/m3  we have K (Loss coefficient) of the duct = (2*deltaP)/rho/(u**2)= 1.113775

With the fan, from the quadratic fan equation with VF_fan=3500 m3/h and delta_P = 40 we have :

3500/3600 = VF_MAX*((delta_P_MAX-40)/delta_P_MAX) **0.5

with VF_MAX=1 m3/s we have delta_P_MAX = 730.1408450704215 Pa

But when i apply VF_MAX , delta_P_Max and K found by the calcul above to the simulation, delta_P predicted is about 60 Pa, not 40 Pa. I'm very confused and can't find the error in my hand calcul.

I really need your advices. Thanks in advance!   

  

[dr_jfloyd]

You need to look at the local pressure at the outlet for the fan not the local pressure in the center of the compartment.

[cunconbkhp]

Dear dr_jfloyd, 

Thanks for your answer. I have added more Pressure node devices and found that :

+  Relative pressure at the outlet of the fan is 41 Pa like expected 

+  Relative pressure at the inlet of the extraction duct is about 50 Pa 

+  The compartment pressure is about 60 Pa

I'm still confused at this point, there is not leakage in the compartment, so what makes the differene of pressure from the fan outlet to the compartment and to  the duct inlet. Maybe the height of the compartment (bernouli equation)?

 Do you think my assumption of the volume flow conservation is valid?

Best regards ,

[dr_jfloyd]

The pressure at a point is the background pressure plus any hydrostatic pressure (rho g h) plus any perturbation pressure (due to the flow field). 

[cunconbkhp]

Everything is clear now. 

So is there anyway with FDS  to obtain a  stable pressure of 40 Pa in the compartment (zone pressure)?

Thank you!

[dr_jfloyd]

You could adjust the duct loss until you get 3500 m3/hr and 40 Pa zone pressure