So here is my follow up on this project. Since my last post I upgraded the fan to a 16" AC Infinity fan rated for 3060 cfm. In the install process, I enlarged the intake and exhaust compartment openings in the compartment to 17" sq. and enlarged the intake and exhaust hoods to accomodate the larger openings.
Prior to this the opening on the intake was about 13" sq. and on the exhaust about the same. The original fan was 12" and rated at 830 cfm.
Using a poor quality anemometer with the 12 in fan that was initially installed I measured an average air flow of about 502 cfm and in 85 minutes of generator operation I measured a temperature rise in the compartment from 75F to 125F or a difference of 50F which in my opinion represented a problem that needed to be solved. I operate under the principle that failures are sudden but damage is progressive.
The installed 16" fan is rated for 3060 cfm but my anemometer showed an average air flow of approx. 1,983 cfm which, although is lower than expected, was far better than the 12" fan output. This time over the same 85 minute test the compartment temperature rose from 78.8F to 88.8F or only 10F.
The hoods on either end are lined with MLV and mineral wool fiber to reduce sound levels and there is no noticeable difference in sound between the 12 & 16 inch fan.
I graphed the results and with the 12" fan the curve flattened at approx. 43 min or 122F and rising slightly from that point on.
With the 16" fan temperatures hit a peak at 59 min. at about 89F and dipped slightly and oscillated up and down. I am comfortable in saying that trmps. stabilized at about 88- 89 deg.F
I live in the mountains in Central America and temperature year round fluctuates between 82-68 F. so I am very happy with the ventilation system now. I do not see this presenting any problems, particularly because it is a backup for a supplemental battery powered charger/inverter system and quite possibly the generator many never be used.
Before undertaking this project I watched many many youtube videos and read many articles on generator sheds. Just as you have probably experienced in your careers everybody is an engineer from the plumber to the electrician to the pipe fitter to the A/C technician. The same applies to backyard engineers who build generator sheds. I have not seem one article that talks about compartment temperatures. I know one guy who built a compartment and installed an 8" fan and says "no problem my 9 kw Preditor ran for 6 hours straight and nothing happened". This seems to be the prevailing attitude until you have a failure and then it is WTF happened. Generator repairs can be expensive.
What did I learn from doing this project, and let this be a lesson for all of you who want to tackle a similar project even on a smaller scale with a plastic garden shed as opposed to what I designed and built? You will not save money by building your own compartment as opposed say to purchasing a standby system from Kohler, Champion, Generac Onan/Cummins or another supplier. The compartment and hookup including a transfer switch will be at least double the generator cost. In my case it was 3 times and I did the electrical installation and used a simple Reliance single circuit transfer switch.
When you do it yourself you have to engineer the exhaust piping, the sound insulation, and of course ventilation but don't forget the hydraulic roof supports/struts so you can raise a roof that is in excess of 200 lbs. On top of that you might want a bimetallic switch in the low oil circuit in case the fan fails or something else causes the compartment to overheat. With a packaged standby system the engineering is already done by guys/gals with a lot more experience than you have, and you don't have to get really creative to work out small details like exhaust outlets, mufflers etc. There are no teething problems to solve with an off the shelf standby system. They have long ago gone through the learning process that you are struggling with, and they have survived all the associated brain damage.
Financially the project is full of "I forgot to order this or that" or "this wasn't right so I have to order something different" and before you know it you have blasted through the budget and are committed, so you are committed and have no choice but to move ahead and cannot just stop and leave it unfinished.
Soundproofing is a real headache to get right and has to be done in a meticulous way just to get marginal results and the material you require is not cheap. It can be painstaking and you have, if you choose the materials I did, fight with the mineral fibers in your skin etc. In my case due to lots of considerations the compartment front is about 5' from the side wall of the bedroom (cinder block home). Bedroom sound levels are 42 dba or about the sound level in a library.
Personally as a word of advice think also about a solar powerwall. These offer instant hands free changover without worrying about whether your motor will start and they are silent and will provide protection during any short term outage. In my case it is 2-3 hours and since 2013 it has never let me down.
. Installed in a temperate climate, with an average ambient temperature range of between 68F-85F year round. I installed a small vent fan of approx 850 cfm. Most small generator mfrs. do not publish operating environment temperature limit info, presumably because they do not want generators installed in compartments, and so if they have info, they intentionally withold it. Published info might also become problematic if there are warranty issues. I have installed a propane conversion kit and run it on propane only rather than gas. My concern recently, while running it for over an hour, the compartment interior reached an equilibrium delta T of 50 deg F under load. In other words the outside temp. was 75 F and the interior of the compartment reached 125 F.