Ventilation for Blower enclosure

[curtis2004]

Hi Everyone,

I'm working on a project of supplying of enclosure for air blower, and need to calculate ball park of cooling air flow requirements. We have a blower with 400 HP motor (4,200 VAC/3/60) which will take outside air at 5 C/40 F and supply 7,000 cubic meter an hour/4,100 CFM at 15 psi pressure. Air discharge temperature 96 C/205 F and total efficiency of the blower system is 67%.
Motor is TEFC with 300 BHP at operational conditions and efficiency of 95%.
I need to calculate amount of outside air (5 C/40 F) in order to keep inside temperature in a range of +25C+30C/+77F+86F range.
Where do I start?

Thank you all,
Curtis

 

[Drazen]

you need to calculate motor heat dissipation, than you calculate air flow quantity via hear rejection by ventilation equation.

this can give you clue: www.cat.com/cda/files/3375314/7/Engine

 

[curtis2004]

Hi Drazen,
I look through your link. It seems it is about genset engines. I have electric motor driven air blower. Do you think it is similar?
Thank you,
Curtis

 

[ChasBean1]

15 psi means psia, or psig? You are discharging 205°F. But you need to keep the inside temperature 77-86°F. Good luck with that.

 

[Drazen]

curtis,

basic equation for heat rejection by ventilation is generic, but more proficient use of it is developed by generator and compressor manufacturers. so i believe you can apply it after carefully studying differences of air patterns described and finally deciding on it.

do not forger that these are tips, i can give you some idea on how to work with that, but i cannot actually solve it and make final decision instead of you.

this is one of steps described. the other is to properly calculate heat rejection, which, as chas mentioned can be tricky. i assume there is some good insulation between air path and outside casing, which i understood as motor casing outside of air stream. if not, you will have to specify adequate insulation. there are ashrae and neca guides on how to calculate heat rejection, but it should be expected that blower manufacturer has some valid declaration.

if all the mentioned is not sufficient for you to get grip on what you need to do and be reliable that you understand each step, the most professional thing you can do is - to ask help from senior engineer.

 

[curtis2004]

ChasBean1,
Discharge is 15 psig. Discharge is piped out from enclosure to process piping. So affect of 205F discharge air temperature is negligable (heat gains by radiation of uninsulated piping).
I probably didn't discribe process properly since I can feel some confusion.
Air Blower is taking outside air at 40 F and discharging into process piping at 15 psig pressure and 205 F temperature. Blower with electric motor is enclosed
by noise enclosure. Total efficiency of motor+blower is 67%, which means that 33% of motor's BHP is turned into heat (Am I right?). I assume that portion of this total heat is taken by air discharged at 15 psig and 205 F into process line. The remaining heat is dissipated inside enclosure.
My question was how to calculate this heat rejection rate given temperatures and 300 BHP motor and 67% efficiency.
I'm pretty sure this is common engineering task for HVAC engineers. I have HVAC sub-contractor doing calculations, however, would like to do rough calculations myself just to double check him.
Thank you,
Curtis

 

[MTES93]

I believe you are correct, heat gain in the enclosure would be the sum of the motor and blower inefficiencies plus any external gains. Not sure I understand your layout... you have 4,100 CFM of OA at 40 degrees F coming into the enclosure or 4,100 cfm of air at 15 psig leaving your enclosure. Q = 300 x . 33 x 2,547 = 254,700 Btu/h in either case. Not sure of your blower inlet conditions... if you are kicking out 4,100 cfm at 15 psig on the discharge or is it 4,100 CFM at your inlet condition. Either way you can figure out the actual mass flow..

I maybe too confused to help!

 

[MintJulep]

Step 1 in solving problems is writing a clear definition of the problem. You don't seem to have that yet.

What is inside and outside of the enclosure?

 

[curtis2004]

OK. I`ll try one more time.
400 HP motor with blower is inside enclosure. Blower is taking air from outdoors (4100 CFM at atm. pressure and 40F) and discharging to process piping (which goes outside of enclosure) at 15 psig and 205F.
Enclosure will be furnished with cooling air fan which will take air from outside (atm. pressure and 40F) and supply into enclosure to keep inside temperature below 80F. Enclosure will also have garvity louvers for cooling air vent outside.

I need to find cooling fan capacity for these conditiond. I made some calculations:
1. Qem=2545 x (P/Em) x Fum x Flm = 1,122,794 BTU/hr = 18,713 BTU/min

2. This is total heat generated by motor + blower. Some of this total heat is taken out by process air which is discharged into piping at 15 psig and 205F. I calculated this one by formulae: Qprocess = Ma x Cp (Tdischarge - Tambient)= 321.7 x 0.24 x (205-40)=12,383 BTU/min,
where Ma=4,100 (CFM) x 0.076 lbs/cu.ft air density = 312.7 lbs/min.

3. Heat rejected inside enclosure will be a difference between the total heat generated and the heat taken out by hot stream of process air:
Qrej = Qem - Qprocess = 6,330 BTU/min.

4. Mass flow of air required to keep temperature inside enclosure at 80 is calculated W = Qrej/Cp/(80-40) = 659.4 lbs/min

5. Fan air flow required F = W/density = 659.4/0.076 = 8,676 ACFM at atm pressure and 40F temperature.

I know that I also need to take care of other electrical equipment. I also know that temperature outside will vary from -40F to +40F (it is used only in winter). In order to overcome these I will supply VFD for my fan.
How about overall approach solving this issue?

Thank you,

Curtis