Engine room ventilation system

[kkkasturi]

How to calculate the air needed to cool the engine room and maintanin the temperature not to exceed 120F( for marine duty)with 2 engines of 1401bhp and 2-generator sets. What factors should i consider? i thought of taking the engineer room volume, heat dissipation from the engine and air intake for combustion into consideration.I have to size the blowers to supply that particular amount of air. Any help would be appreciated.

 

[Clausius2]

Are the engines taking the air from the room, or do they have an external air intake?

i) calculate the heat dissipated by each engine. How? That's pretty difficult. I don't know. Try to estimate it. Surely they radiates and convect heat.

ii) the air flow which enters will be approximately

$$\dot m=\frac{\dot Q_{engines}}{c_p (T_{room}-T_{external})}$$

by the way if you don't take into account the latent load of the air flow (changes on room humidity).

 

[kkkasturi]

Thanx Clausius2,
Yes the engines are sucking air from the atmosphere and i do have the heat dissippation from the engines.Can you help me in calculating the pressure drop in the engine room. I think it has a n effect in sizing the ventilation.

 

[Clausius2]

Thanx Clausius2,
Yes the engines are sucking air from the atmosphere and i do have the heat dissippation from the engines.Can you help me in calculating the pressure drop in the engine room. I think it has a n effect in sizing the ventilation.

What do you mean with "pressure drop"? What drop?

 

[russ_watters]

Pressure drop is not relevant at this stage of the game, kkkasturi - you start by calculating required airflow, then design the ductwork/louvers/fans to achieve it.

 

[kkkasturi]

Thanx Russ_watters,Hope i get the calculations rite

 

[Artman]

Since there is combustion happening in the room you will need to keep a postive condition in the space. Just bring in more air than you take out plus the air used in combustion.

If you are not cooling the incoming air, you will just be limiting the temperature rise. This creates a temperature difference that "moves" with the incoming air temperature. So you need to consider the highest temperature your equipment can be subjected to. This is important if the room contains electical equipment, or other equipment that is temperature sensitive.

Once you know your highest possible temperature, (Say a piece of equipment in the room has a maximum temperature rating of 105 deg F), your temperature difference becomes a point less than this, (Troom,from Clausius2's calculation), and the incoming air temperature Texternal. You may not always be able to keep the room cooler than this requirement using only external air (on a 105 deg F day you will be hotter than 105 deg F in the room because of heat added from the equipment). So you may need to decide on a compromise using a higher than average incoming air temperature to give you a higher CFM of air. This will give you a greater safety factor.

 

[kkkasturi]

Thanx artman, Yes that is what exactly i was looking for. i need to maintain the room temperature under 120F.
Clausius iam having difficulty with the units, can you help me with the units please. i found the heat rejected by the engine which is 4885 BTU/min.

 

[Clausius2]

Look at my location.

Here we are familiar with SI units. I don't know how much is a BTU (I have never used it, and I don't feel like to look for it in a table).

When there will be a Spanish Thermal Unit (STU)?

 

[russ_watters]

One thing I need to clarify: you said

Yes the engines are sucking air from the atmosphere...

First, that's called combustion air. Second, that implies to me that the air is coming directly from outside the building, through a dedicated duct, and not being sucked from inside the building itself. That makes a huge difference here (though mostly, practically). Could you clarify?

If we can get all these assumptions straight, this really is a simple problem. Do you know how much heat is being rejected into the space? (it should say in the documentation for the equipment) Also, is there a radiator and is it ducted separately?

 

[Artman]

The BTU calculation is:

CFM=(BTUH/1000)/(.00108 * (Troom-Texternal))

CFM = Cubic Feet Per Minute of air at Texternal temperature
BTUH = The sensible heat of rejection of the equipment in Btuh
Troom = maximum design temperature of the room at design external temp.
Texternal = maximum design outside air temperature

 

[kkkasturi]

Thanx Artman.
russ-watters, Yes the combustion air is coming from atmosphere(not from the engine room) through a duct. the heat released by the engines is 2*4885 BTU/min(9770 BTU/min for 2 engines) and we dont have a radiator. The engine room volume is 3220 cu.ft and the engine room temperature shuold not exceed 120F. Is there a standard number of air changes in the engine room?

 

[Artman]

Thanx Artman.
russ-watters, Yes the combustion air is coming from atmosphere(not from the engine room) through a duct. the heat released by the engines is 2*4885 BTU/min(9770 BTU/min for 2 engines) and we dont have a radiator. The engine room volume is 3220 cu.ft and the engine room temperature shuold not exceed 120F. Is there a standard number of air changes in the engine room?

9770 x 60 gives 586,200 btuh. The 3220 cu ft engine room sounds kind of small. Your air to get a 20 deg F delta T (95 to 115) would be 30000 CFM that would be approximately 560 air changes per hour. That is awfully high, but it's a lot of heat in a small space.

 

[kkkasturi]

Thanx a lot Artman

 

[Gokul43201]

Thanx a lot Artman

More appropriate would have been : "That's a lot, artman."

I hope you understand what it takes to put out 30,000 cfms in such a small space. This is truly a large number. Can you imagine air flowing into your outlet/return at thousands of fpm ?

 

[Artman]

More appropriate would have been : "That's a lot, artman."

I hope you understand what it takes to put out 30,000 cfms in such a small space. This is truly a large number. Can you imagine air flowing into your outlet/return at thousands of fpm ?

I agree Gokul. It's going to be a wind tunnel. A better solution might be remote radiators for the engines. Getting the air in and out of the space would not be that difficult. A couple of 30" square high capacity box prop fans could probably do it, one in, one out, but it is an awfully high amount of air movement.

Nope, checked it, 2 in and 2 out, (30" prop fans 15000 CFM each).

 

[russ_watters]

Another question: where is this building located (geographically) and what is the required reliability (what is the usage?). 92F is the HVAC design outside air temperature for Philadelphia, PA, but that's for the purpose of air conditioning. If you are building a power plant (for example) and require extreme reliability, you have to plan for it to be capable of operating with outside air of 100F. Give yourself enough cushion, but don't overdo it.

 

[Gokul43201]

Another question: where is this building located (geographically) and what is the required reliability (what is the usage?).

The OP seems to suggest this is the engine room of some small marine vessel. Don't they typically use water cooling in such cases, or would that be too risky (sea water) ?

 

[russ_watters]

The OP seems to suggest this is the engine room of some small marine vessel. Don't they typically use water cooling in such cases, or would that be too risky (sea water) ?

Yes, they do use seawater and no, it isn't all that risky.

 

[kkkasturi]

russ-watters, our design is for a security craft.
Since we are allowing the deltaT to go higher( once the engines are turned off it won't take long to blow the air out), we came up with reltively small numbers 12800cfm.

 

[Artman]

russ-watters, our design is for a security craft.
Since we are allowing the deltaT to go higher( once the engines are turned off it won't take long to blow the air out), we came up with reltively small numbers 12800cfm.

If one or both engines are off for an extended period during the course of an hour, the heat gain isn't as high so the delta T won't necessarily be any higher. The air will change in the room in approximately 15 seconds. In my opinion, that should be plenty of air.

 

[kkkasturi]

How to calculate the time required to get the temperature of a room from 140F to 120F?
The case here is (marine), the delta T is 40F while the engines are running once the engines are shutdown, i need to calculate the time needed to bring down the temperature of the room to 120F

 

[Artman]

How to calculate the time required to get the temperature of a room from 140F to 120F?
The case here is (marine), the delta T is 40F while the engines are running once the engines are shutdown, i need to calculate the time needed to bring down the temperature of the room to 120F

You can cool the air in the room quickly enough, about 7 or 8 seconds with 100 deg F incoming air, but this does not necessarily cool the contents of the room to 120 deg that fast. That amount of air will cool approximately 65,692 btuh in that time with a 40 deg delta T. How long it will take to cool the room depends on the room's contents, construction weight, color, windows, doors and other heat contributing factors.

For example, your engines will be hot (they will probably rise well above the 140 degrees F of the air) and giving off heat (until their casing cools). How long will it take to actually cool them? When they first shut off they will give off almost as much heat as when they were firing. This number will decrease until the case is cool, how fast this happens depends largely on the mass of the object, how well it conducts heat and surrounding air temperature. (Did I forget anything physicists?)

But the air in the room will cool to 120 deg F in about 1/8 of a minute with 100 deg F incoming air (although the engines, walls, furniture, etc. will still be hot).

Sorry it took so long to reply.

 

[Gokul43201]

This number will decrease until the case is cool, how fast this happens depends largely on the mass of the object, how well it conducts heat and surrounding air temperature.

...and the heat capacity of the object. The time constant for thermal response goes like ~C/K (heat capcity over conductivity).

 

[kkkasturi]

Thanks Artman

 

[kleinjahr]

kkkasturi. These are marine diesels? With no water jacket? Air cooled? Generally marine engines are water cooled, in my experience. The ocean makes a really big heat sink and it's convenient too.

 

[suheri]

if we give large air opening area (for ventilation), how it effect with noise?
thx everybody!

 

[brewnog]

Wow, a 2 year old post revived; well done.

Obviously a large opening in the walls of an engine room will allow engine noise to escape. That's why you put attenuators on ventilation louvres.