When I first built my computer, I installed the power supply with the fan sucking air from the outside, into the power supply, then out the back. Because computer is housed in a SFF (Small Form Factor) case, it got hot rather quickly. Here is a picture of the case I use (Silverstone Sugo SG03B-F):
After turning the power supply or PSU (power supply unit) around, to suck air from inside the computer, and blow it out of the exhaust at the back; there was a big difference!
Further cooling ideas.
I also used a FOX-1 PCI Exhaust fan, which further cooled it. It is relatively quiet, although you can get quieter ones if you are prepared to pay the money.