How to approx. Total pressures and Flow (CFM)

[aero_ub]

Here is and Example.

There is a system with 2-20ft dia. fans, in parallel, pulling air through a bunch of cooling coils with a cross-sectional area of about 800 ft^2.

One fan is running and the other is isolated by backdraft dampers. In this situation the flow through the unit is 842,770 CFM and a total pressure (static + velocity) of 0.853" wg.

If the backdraft dampers on the second fan failed and air was allowed to enter through the second 20' Dia. by 10' high fan stack into the unit...how can I approx. the new total pressure inside the unit and the resulting airflow. Also what % of that air is coming through the coils and what % is coming through the open 'hole' in the unit?

Thanks for any input
aero

 

[Q_Goest]

Very briefly, fans have a characteristic "curve" which you can obtain from the manufacturer. You need to determine the flow resistance which you can do two ways, either by summing the resistances as given per Crane #410 or by using the Darcy Weisbach equation and determining the resistance from your known flow and pressure drop.

the flow through the unit is 842,770 CFM and a total pressure (static + velocity) of 0.853" wg.

I'd suggest doing it both ways and see if they come out close, then trying to pin down a usable number for resistance.

Once you have the resistance, you simply put that back into the equation for flow and find the point where that flow will cross your fan curve for a single fan. I realize that's a very brief, so if you don't understand any part of that let me know and I'll clarify.

 

[ravenprp]

engr

To approximate the total pressure and flow in this scenario, you can use the fan laws, specifically the continuity equation and the pressure rise equation. The continuity equation states that the mass flow rate through a system is constant, so we can use it to calculate the new flow rate through the unit. The pressure rise equation states that the total pressure rise across a fan is proportional to the fan speed squared, so we can use it to calculate the new total pressure inside the unit.

First, we need to determine the new flow rate through the unit. Since the mass flow rate is constant, we can set the initial flow rate (842,770 CFM) equal to the new flow rate (Q) and solve for Q. This will give us the new flow rate through the unit, which will be a combination of the air coming through the coils and the air coming through the open hole in the unit.

Next, we can use the pressure rise equation to calculate the new total pressure inside the unit. We know that the initial total pressure is 0.853" wg, and we can assume that the fan speed remains constant. Therefore, we can set the initial total pressure equal to the new total pressure (P) and solve for P. This will give us the new total pressure inside the unit.

To determine the percentage of air coming through the coils and the open hole, we can use the continuity equation again. We know the new flow rate (Q) and we can calculate the flow rate through the coils by multiplying the cross-sectional area of the coils (800 ft^2) by the velocity of the air. The remaining flow rate will be the air coming through the open hole. We can then calculate the percentage of air coming through each by dividing the flow rate through the coils and the open hole by the total flow rate (Q).

Overall, using the fan laws and the continuity equation, we can approximate the new total pressure and flow in this scenario and determine the percentage of air coming through the coils and the open hole.