Elec. Transformer Heat Load

[tlhoo]

What do you typically use for the heat rejection for transformers in elec. rooms? I've heard 5% of the kVA rating, but this seems to get my airflows a little high. I have an 10'x12' elec room with (2) 45 kVA and (1) 30 kVA transformers, and using the 5% rejection, I get a cooling load of about 650 CFM (even with 85 degree design temp), which is about 37 ACH. Seems high to me.

Thanks in advance.

 

[ChasBean1]

I didn't look into calcs, but three transformers in a room that size - sounds like you're in the right ballpark. Answer is VAV without reheat. VAV max = 650 cfm, VAV min = 0-100 cfm range; maybe pick 50 cfm to maintain some airflow.

 

[pablo02]

I use the 5% value for transformers 150 kVA and smaller... (50 watts per kVA)

 

[imok2]

Rule of thumb for an electrical rooms ~ 2cfm/sq ft or 10-12 air changes/hr, however, if you determine that you have a 6000 watt load then you have a 20,000+ btu load which under normal office conditions would need about 650 CFM, but there your dealing with more latent heat, whereas in this case it's mostly sensible, therefor, the heat removable efficiency( sensible heat factor) will be greater.

 

[kico]

you have received some good rule of thumbs. however, to get a feel for how good they are, you should look at transformer's literature and get actual heat rejected to space. most mfr provide this info in their catalogue.