ESP For Fan Located at roof

[Russell vp]

Dear All,
Am a Mechanical Engineer dealing with Installation of HVAC system in Buildings. Have came across below issue while dealing with one of my project.

Have few Fans Located on roof of a 10 floor building which is Extracting Air from Below floor (first floor). I given the ESP calculation to a third party and the ESP calculated came very high compared to the designed ESP.

The reason is that he added a factor Pxgxh (P=Density of Air, 1.225 kg/m3., g=Gravity, 9.8 m/s2., h=Riser length of duct in meter (m) - 60 Meters). His statement was that since the flow is agaist the Gravity , this factor needs to be considered. ( Here in this case, the factor itself came = 721 pa ie: 1.225x9.81x60)

Please advise the correct method of doing the static pressure calculation for a Fan located in roof extracting air from below floor.

Note: Am capable of calculating the static pressure loss in the ducting and fittings due to friction.

 

[GT-EGR]

I would say it sounds technically correct, but maybe something in the calculation is flawed which is why you are getting such a high number. I would say you don't use the height method - but compare the difference in absolute air pressure from the bottom of the building to the top, and that would be the additional stack static you need to account for. I think you actually end up with the same answer though.

You definitely have to account for is the stack effect of a tall building - which usually creates a problem of having your lower and upper floors operating at different interior space pressures. So if you calculate your static pressure, and then don't account for the fact that in the winter your lower floor may be 0.25"wg negative, and your whole system is only 0.5"wg, then you will fall short of getting the airflow you want.

I don't design high rises, so I don't have any experience with building height causing this issue.

 

[Emrelug84]

Hi Russell,

The calculation is wrong. The phenomena is called as "thermal gravity effect". You have to compare the air inside the duct and ambient air. when air in the duct is hot, then it rises even roof fan is not in operation. Air is cold then it goes downward. How it is calculated is as follows: you have to consider the worst case. when outside air is hot and the inside air cold, then it is necessary to put extra power to make air go upward.
density of air= 1.225kg/m3
density of ambient air (hottest time) = ? (you enter the value according to your project data)
thermal gravity effect = (density of air in duct - density of ambient air) x gravity x height
For example you have 45oC and 0m altitude, ambient air density is equal to 1.11kg/m3. then thermal gravity effect is calculated as:
TGE (thermal gravity effect) = (1.225-1.11)kg/m3 x 9.81m/s2 x 60m = 68Pa.
The third party guys forgot to take the ambient air density in the calculation.
For more details, please look at ASHRAE Fundamentals 2015 Chapter 21 Duct Design Thermal Gravity Effect (Stack Effect)
Regards,
Emre

 

[Russell vp]

Thanks lot guys.. Your response were really helpful to understand better the scenario...

 

[ChasBean1]

You're all messed up. Everything cancels. It's all about friction losses. See me after class.. Lol