Room pressure after stopping the pressurization

[aungkyawtun]

Hi Guys,

Our project has a compartment for temporary refugee. The purpose is to muster all the persons on the FPSO (a vessel) when there is emergency shut down.

Design concept is to keep the room pressure to keep positive pressure 50 Pa after 60 mins from pressurization fans are stopping, the room leakage area is 0.05m2.

Would appreciate if anyone can explain how to calculate the initial pressure of the room before the fans are stopping when the hydrocarbon gas is detected at the air inlet of these pressurization fans.

 

[3DDave]

Is this room the volume of a football stadium?

 

[aungkyawtun]

@3DDave, 50 m x 20 m x 3.6 m.

 

[Snickster]

ASHRAE 53 provides a method for determining infiltration air flowrate through cracks and overall room based on differential pressure and door/window/room construction. It should also apply to air exiting a room due to higher pressure in the room than outside.

Possibly you can do a calculation of the flowrate and overall flow exiting the room on a stepwise time basis such as every minute for example (or use greater or lesser time steps). In each step you would calculate the flow out of room based on differential pressure and total flow and then calculate the resulting pressure in room after that one step based on the total volume out. Then reapeat next step calculation and so on using the resulting pressure of the previous time step until you reach atmospheric pressure.

I have done this type of stepwise calculations using an Excel spreadsheet where you can set up each time step as one line of the spreadsheet following follwed by the next time step on the next line down to the end. Keep adding time steps until the pressure reaches atmospheric.

The attached gives a typical calculation using ASHRAE 53.

(PS if you are really good in differential calculus you may be able to set up an differential equation to solve for time}

Now I am not sure how breathing of the people inside of the room would change the pressure so you should research this.

 

[aungkyawtun]

@Snickster,
Thanks for your sharing. Agree with you that calculus is required to find out.

 

[Snickster]

I did not say calculus is required but just ideal if you could set up a differential equation.

I said that you can set up an excel spreadsheet with calculations based on a stepwise analysis.

Using ASHRAE 53 calculation methods you can calculate flow out of room using the equations for a given constant pressure calculated during the previous step. Each time step would have equal time for step. In first time step you assume constant pressure in room and calculate flowrate out. Then with flowrate out you can use ideal gas equations to calculate resulting pressure. Then in next time step you assume constant lower pressure that was calculated from previous step and calculate flowrate then resulting pressure to plug into the next time step. Then keep on in similar manner in following time steps until pressure is ATM. With excel you can cut and paste each calculation step to make the next calculation step so even if you need a thousand time steps to get an accurate enough answer it would not be that difficult cutting and pasting with Excel.