Electrical Room Ventilation

[tankanghan]

i have an application where i need 13,000 cfm for an electrical room with a 42KW heat dissipation. I assumed about 10 degree temperature rise in space for this...is this true? or is it more like 12degF rise. ..so for a 95 Ambient outside, the space inside willbe arounf 105F...??

Also, im thinking of doing 2 speed and control based on space temperature, such that at colder weather the fan wont cycle as much...is this a typical strategy??

 

[MintJulep]

Is there a question in the first paragraph?

105F seems correct for the given conditions.

I don't see how a control based on space temperature will respond to the outside ambient to reduce fan cycling. It might change the time that the fan runs at high or low speed, but it won't have any effect at all on starts and stops.

 

[tankanghan]

Thanks for the response....I also will need a direct drive on one corner of this area where maintenance is a big issue, but there are no 2 speed direct drive fan...so im thinking now of getting that 13,000 cfm direct drive fan single speed on the said corner, and then a 8,000 hi/4,000 low, two speed fan BELT drive fan for the center area, such that at most time, when the ambient outside temperature is cold the low speed of the belt drive-2-speed will run. Then at certain tempertaure say above 89F, the hig speed 8,000 cfm will run and at temp above 93 both fan run....Is this normal control strategy for this type of ventilation??? Thanks

 

[MintJulep]

What is the desired goal of the control?

The governing equations for the system are relatively simple.

Figure out what you want to accomplish with the control and model it.

 

[tankanghan]

Thanks again, i appreciate the comment.
the desired is so as not to use as much airflow during the colder ambient. If i just have one speed 13,000 cfm fan, at low outdoor air, the space will be colder and the fan might just cycle on space temperature. Maybe ths is not the proper way??
So for outside air around 20F, the low speed 4,000 cfm will give indoor space is 53F. At lower ambient, the fan may not run at all or just cycle based on space temperature. I dont know how long the space could heat up to 80F so the fan may not run at all.
Then at rising space temperature, the two speed fan can go on, first with low speed, and then as the space temperature goes higher with the high speed, and the further space temperature, the second direct drive fan will come on....
Does this make sense??

 

[jasno999]

Aren't you missing a whole bunch of data? In reality don't you need to determine the UA values for the walls and ceiling and then figure out what you heat transfer out of the room is naturally. Then you can determine what you need to do to maintain a 10F temp difference on a 105 outdoor air day......????

Seems pretty basic. Wit ha one speed fan you are goign to see it cycle to maintain the desired indoor air temperature. YOu can reduce cycling if you schedule the fan to continue running until room temperature is 10F below your setpoint or even 20F below the setpoint if you can handle a room temp that is 20F below the design point....

Otherwise you could spend some more money and get a fan that has a VFD )Variable frequency drive) on it and then you can have it modulate speeds to meet the room temperature. Higher up front cost but would save some cash in terms of wated energy later on.

 

[tankanghan]

Thanks again, i appreciate the comment.
The space is sandwich between two airconditioned space....i assumed that the room load is very small compared to the 42 kw equipment heat

 

[Jabba007]

I second the VFD approach. It's how I would come at it.

Write your control sequence so that the fan is at high speed at... 90 or 95F in the room, and at minumum speed at say... 65F, and OFF at 55F. Something like that. You fill in the temp points on the curve that you want.

Remember that there will be a minimum speed at which the fan will become unstable.

Jabba

 

[tankanghan]

Thanks to all who comment...it appear the vfd is better approach and maybe less expensive...so the fan will basicallyt ramp down to what the cfmm is needed to maintain 90 or 95F in the room and the above the 95F then it will run full speed....Thanks again

 

[mike7941]

Just two more thoughts on this

Isn't most electrical equipment rated for 104F (40C)? Not the most profound difference, but if you are documenting your calculations. . .

Second, is there any reason to ventilate the space below 80F room temp? From an energy stand point 'off' saves the most. Cooling down to 70F would be comfortable for the people, but thats not why the fan is there

 

[KiwiMace]

Hmmm.. I think some value engineering might be in order. Not worth the engineering cost.

Also, Mike7941 has a good point. If your electric room is a transformer, just stick a fan operated by a thermostat and walk away. If this is 42kW of VFD's, then I strongly recommend air-conditioning the space. Insulate well, and the operating cost will likely be little more than the much larger fan.

 

[lowedogg]

kiwimace hit the nail on the head. what kind of electrical equipment are you trying to cool? if it is just transformers, i agree with using a simple on/off fan w/ line-voltage t-stat. if the electrical equipment is anything more sensitive than a transformer, it really should be fully climate controlled.

 

[tankanghan]

Thanks to all. It is basically transformers.

CFM = (42KW x 3412)btuH / (1.08 x 10 delta T F) = ~13,300 cfm at peak

so It will cycle at say 10F ambient, right?

so instead of that, with VFD at 10F outside i just need 1,560 cfm air not the full 13,300 with 95F inside. ....

Does this argument make sense? am i missing something??

will the 42KW heat load ever heat the space above 95F when the ambient is low?? Im afraid with just the t-stat and fan, it will cycle a lot during low ambient days??