Natural ventilation (outlet) from Marine Machinery Room

[Achiro]

Dear all,

would appreciate if anyone is willing to help me with following:

I manage to figured out the ventilation calculation for duct/grilles sizing for E.R. inlet (but need to double check, air-in is not the issue).

Calculated following:

1. Airflow for combustion from engines (Value X - m[sup]3[/sup]/s)
2. Airflow for evacuation of heat emission (Engines, pipes, electro cabinets) (Value Y - m[sup]3[/sup]/s)
3. Total airflow required (Value Q - m[sup]3[/sup]/s) / devided by 2 (two ducts)
4. Maximum airflow velocity shall not exceed (v): 6 m/s
5. Duct sizing (minimum): A = Q/v (m[sup]2[/sup])
6. Air Inlet grilles have a pressure drop of following: ∆p = 1,4 x v[sup]2[/sup]
7. Calculation of new flow velocity (∆p, ω, where ξ=2,5 (minor loss coef.); ρ=1,13kg/m3 (air density))- v/ω =1,15
Velocity needs to be increased by 15%; Q[sub](1)[/sub] = Q x 1,15 (m[sup]3[/sup]/s)
8. v[sub](1)[/sub] = Q[sub](1)[/sub]/A = 8 m/s
9. Second iteration: ∆p = 1,4 x v[sub](1)[/sub] ; ω[sub](1)[/sub]
10. Fan in air out requires minimum of free flow rate of Q[sub](1)[/sub] - condition satisfied.

(Air density: 1,13 ; Base temperature: 30 degree celsius; Peak temp: 40 degree celsius;
temp. difference: ∆T =10; Minor loss coef: ξ=2,5; Specific heat capacity of air: c=1,01)

My question is following:

How to calculate minimum Air-OUT ducts and Grilles size, in a way to avoid use of fan pushing air out? (natural outlet of air)?
The ducts/grilles must be sufficient to exhaust this amount of air - without creating a large back pressure issue.
Which formula do I need to use?

In attachment preliminary plan and side views.

Thank you very much all in advance.

 

[MintJulep]

Why is combustion air in an exhaust calculation?

 

[Achiro]

This is airflow for combustion, needed air supply for engines; in air INLET calculation for engine room, not exhaust.
The calculation shown in OP is air IN, just to show where do I get so far.

I get stuck on calculating the air OUT, to be precise, optimal duct dimension without fan usage.

 

[chicopee]

The exhaust air will be pushed out by the inlet fan, regardless, so I would use an exhaust fan from which you can size the outlet duct, then keep the inlet duct the same size.

 

[Achiro]

Hi Chicopee,

Appreciate your reply.

But I have a problem, the actual duct is already smaller than inlet duct (drawn on sketch I have uploaded). I need to prove that the actual duct can exhaust air without the exhaust fan (I would like to avoid exhaust fan because several reasons). How can I check that the duct size for air out is enough?

Thank you in advance.

 

[chicopee]

Once thing is for certain, discounting all air infiltration, the mass rate(lbm/unit of time) of air coming in will be equal to the mass rate(lbm/unit of time) of the exhausted air. By contrast the CFM at inlet will, or will not be nearly the same as the CFM at outlet; It will all depend on the temperature of the engine room. Assume air temperature in is less than room temperature( under steady state condition) then, temperature of exhausted air will be the same as room temperature. With an increase in temperature of the exhausted air, its CFM will be less than CFM of air in.

 

[MintJulep]

The exhaust air will be the air input - the engine combustion air.

The engines are effectively exhaust fans.

You need to determine the pressure of the space relative to the outside.

Compare that to the system curve of the exhaust duct at the exhaust flow rate.

 

[chicopee]

MintJulep "The engines are effectively exhaust fans" I would have thought that the engines would have their separate air intakes but if not yes you are right.

 

[Achiro]

Thank you both of you.

Please I will appreciate if anyone can help me calculate room pressure.

I have room volume input, inside temperature, outside temperature (10C degree difference).
How do I calculate the pressure?

The engines get the air from the engine room, do not have separate intake.

 

[willard3]

A manometer and time will give you the answers.

 

[Achiro]

Ok, suppose I won't get answers easily.
Nevermind.
Thank you anyway.

 

[LittleInch]

But are you missing a trick here?

Surely if volume X is bigger than volume Y, then you don't need any exhaust air as the engines will simply suck it all in and blow it out through the exhaust? you might need to duct the air around the room a bit and have a small fan for when the engines aren't running, but won't the engines running lower the pressure in the room if you allow the in and out grills to just flow air in?

And I don't really understand the initial quote " How to calculate minimum Air-OUT ducts and Grilles size, in a way to avoid use of fan pushing air out? (natural outlet of air)?"

If you don't have positive pressure in the from compared to outside air then there will be no flow.

Once you know how much, if any air needs to be exhausted then you seem to have the formula to work it out.

The thing is that for any flow out, you need fan flow, it's just that this is fan flow on the way in, not on the way out.

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

 

[trashcanman]

Without a pressure differential, there is NO fluid flow. The only sure fire way I know of to ensure and adequate exhaust rate is mechanical ventilation with a fan and duct and a way to measure the air flow.