Hot air ducting with in-line axial fans 5

[vij36]

Dear All,

I have a hot air ducting requirement for a Diesel Generator radiator side.
It was a 2003 year model with open room working condition.
The DG set is acoustically enclosed recently.

The site has space constraints at the radiator side. The space available in front of radiator is only 1 meter.
So planning to have a plenum with this 1 meter depth in front of radiator and have cut-outs at the top of the plenum so that in-line axial fans can be fitted which sucks radiator hot air and in turn exhausts the same hot air above the roof.

The radiator hot air as specified by DG manufacturer is 24,000 CFM. So planning to have two axial fans with about 15,000 CFM each. The height of plenum is about 2 meter. So static pressure will be negligible per my assessment. radiator fan has 0.5 inch WC static pressure.

Please advice.

 

[EnergyProfessional]

Is there a specific question?

Make sure the fan ahs the temperature rating required.

 

[vij36]

Sorry. I was not specific.
My question is :
How to calculate the static pressure offered by the 2 meter plenum duct for deciding the fan?

Duct length is 2 mtr. Width is 2 mtr and height is 1 mtr.
One online calculator for calculating SP which has inputs of CFM, duct length and dia shows friction loss as zero.

Static Pressure Calculator

CFM 24,000
Duct dia 64 inch (equivalent of 2 m x 1 m rectangular duct)
Duct length 20 ft (taken higher side)

I would like to know if this calculation is correct since friction loss with this calculator resulting in zero.

 

[EnergyProfessional]

A wide plenum will have near-zero pressure drop. It is basically a very oversized duct. But you have transitions going in and out that create pressure drop. Also add louvers etc.

You can do the normal pressure drop sans the plenum, then add a tiny amount. Also consider if the plenum has obstacles going through (conduits, or structural elements. This is a common problem with plenum ceilings and floor - tons of cables and other things in there while the design relies on near-zero pressure drop. No way to calculate that.

 

[itsmoked]

You say "inline"? That means a fan in front of a fan. Is that really what you're doing?

Is there any way you can use a single larger fan? Multiple fans bring lots of other problems and issues when trying to move air thru large square radiators. If you look at big CAT units I don't think you'll ever see multiple fans - always a big single.

I just decommissioned a 1.7MW Cat generator that was massively engineered to be sound attenuated. It used a single large fan that drew air the length of the the system around the engine/generator and auxiliary equipment.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BronYrAur]

I am guessing that "inline' is just referring to the axial fans. Two fans, individually connected to the plenum. I would put backdraft dampers on them in case one dies. Otherwise you may re-circulate through the non-operating fan. Did you select the fans based on the anticipated air temperature? Not sure how "hot" the air will be.

 

[EnergyProfessional]

For safety reasons, a single fan is better since failure of either fan will prevent generator from cooling. it also reduces control complexity. If this is a life-safety generator, this is even more important.

inquire with generator supplier if their fan can provide the added pressure, or if they have options for a larger fan. I'm sure you are not the first one to require such duct added.

in general generators should be outside... there are a 1000 reasons why installation inside is problematic. This is just one.

 

[vij36]

Dear Sirs,
Thank you all for the kind advices.

Have Attached a rough schematic of generator layout. Apologies for such a sub-par schematic.
You could see a sea green color box beside the pink vertical columns which is the generator with green doors. Two pink colored circles are the planned axial fans which are to be fitted in top cut-outs of plenum duct shown in yellow color. The roof is about 2 ft above of the proposed axial fans.

I have tried to calculate the dynamic pressure caused by the air flow changing direction from the radiator ( horizontal) towards the roof( vertical).

Do not know if this is the right calculation.
Dynamic pressure

Dynamic pressure = (Density) * (Velocity)2 / 2

As per the above formula:
CFM of 24,000
duct cross sectional area 21 sqft(2m x 1m)
Velocity comes to 5.67 m/s.
Hor air density took 1 kg/m3.
Dynamic pressure results in 16 Pa which is only 0.064 in WC. I need to calculate the SP of roof exhaust hood above plenum duct.

As per the generator manual the radiator fan can handle 0.5 inch wc. I feel this is well above sum of dynamic pressure (results in air flow direction change) and SP of exhaust hood above roof(yet to be calculated).

Thank you.

 

[willard3]

There may also be a "system effect" that can be calculated from SMACNA publications.

 

[Gholam Baghery]

HI
The pressure drop in your design is very low. So, you can use the duct but I am not sure about the two other fans that you were talking about. It seems not necessary at all. However, you can use the exhaust of the Generator for helping the outlet air. It would be more effective. if you put the exhaust outlet perpendicular to the duct the exiting hot air help to vacuum in the duct to make more speed for the air which passes through the radiator.
please look at the below image.

 

[EnergyProfessional]

Gholam Baghery: do you have actual experience with such layout? I mean an actually existing application that was approved by JHA?

I doubt mixing radiator air and engine exhaust is legal or recommended. If the radiator fan fails, exhaust could seep into the generator room. At least JHA could see it that way.
Also exhaust is corrosive, so the duct material and location where you can discharge it has different requirements. I also would run that idea by generator manufacturer what they think about the engine exhaust going through addl. duct paths.

Not saying it is impossible, but a lot to look into.

 

[Gholam Baghery]

This type of design can be found in many military vehicles such as BMP1 and BMP2. Also, I used this type of cooling system in a 1.2 MW generator in Iraq during the Gulf War. The diesel engine of BMP armored personnel carriers does not have a rotating fan or turbine and has radiators and ejectors that work with this system. Look at the attached file.
I'm sure you are well aware that the diagram above is just an outline and should be designed to create a vacuum in the duct, and if the fans do not work, the exhaust path should create the necessary suction in the duct.
Regarding the corrosion of the exhaust smoke, you are right and it should be calculated to obtain the life span of it.

 

[EnergyProfessional]

Gholam_Baghery: I think the OP inquired about application in a building, with the associated requirements. I don't doubt the physics of your suggestion. But it has to be approvable by JHA.

A design without fan is simpler and could potentially be more reliable. But it also would require approval by the generator manufacturer (if you want any warranty, or use it for emergency loads). The vehicles you talk about I assume came that way from the factory.

 

[Gholam Baghery]

Hi
Thanks for your response. Yes, you are right, it works in the physics approach, however, it must be approved by JHA as well. I did not consider getting approved. Regarding the vehicle, it has made in 1970 and it is still being produced, but with many new weapons, but the engine and mechanical system are the same and it uses the exhaust smoke for cooling the radiator.

 

[itsmoked]

Gholam Baghery; I learned something here! Thanks.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[FacEngrPE]

Does this generator have an on board engine driven fan? This is the usual case when an engine skid has a fan installed. If it does the engine supplied cooling fan often can meet it''s required capacity through a short duct. The installation manual may provide some guidance.

The lack of depth available for a plenum can be replaced by width. If using the engine fan, the rule of thumb is to keep the cross sectional area (including louver free area) of the duct equal to the area of the radiator outlet.

Occasionally generator sets are provided with radiators having electric fans. If that turns out to be the case, the fan and radiator assembly could be relocated to the roof.

Ambient temperature in the cooling duct can approach engine cooling water temperature. You should look at fan assemblies that keep the motor out of the hot air stream, to avoid the need for selecting a motor for operating in 100C ambient.

 

[vij36]

Dear Sirs,

Unfortunately i am not having suficient expertise to implement Gholam Baghery's advice. I apologise.

The radiator fan is engine mounted and belt driven. Radiator fan (4 ft x 4 ft) cross sectional area is 1.5 sqm. . The Kohler customer care on DG sets is not responding to my calls to get info on radiator fan SP.
Since now I am not able to come to a conclusion whether to have extra in-line duct axial fans in addition to this engine driven fan because Some say engine mounted fan can easily throw hot air out and some say additional axial fans are required. So I am planning below.

Install the plenum duct with exhaust vent. The vents are above roof. Plenum dimensions are 1 mtr depth , 2 mtr width and 1.5 mtr height. Cross sectional area is 3 sqm. Calculate static pressure of plenum duct , the exhaust vents and the velocity pressure due to hot air changing direction. I feel this can be calculated with a meter which I have to look out. May be a U kind of tube or any pressure meter?. In this area i have to workout now. And see if the in-built radiator fan can overcome this pressure. (Radiator fan SP is 12.5 mm WC). Whatever additional pressure beyond 12 mm SP of radiator fan, install axial fans to suit that. I am guessing the additional static and dynamic pressure due to duct and of air change may not cross beyond 10 mm. So, two fans each with 10 mm SP and with an air flow of 10,000 CFM can give sufficient suction to hot air and disperse rhe same above roof.

 

[nuuvox000]

I assume the generator already has a fan currently? Are you planning on removing this? I wouldn't run it inline with new fans.

You're not going to be able to calculate an exact pressure drop unless you do some intense calculations and probably CFD analysis. It sounds like the manufacturer specified 0.5 in w.c. with their fan? I would do 1 inch w.c. if it were me. If you get ever more air exhausted, it's not going to matter much and won't really increase the electrical much. But if you don't exhaust enough air you'll be in trouble.

Prop fans aren't the best at pushing air far distances; the closer you can get them to the roof the better.