toprudder wrote:
In fact, if done correctly, the cooling effect will be more efficient. The Onan in my MH will actually overheat if the door is left removed, since it depends on the ducted airflow being forced through the interior to cool everything.
Let's expand on Bob's observation about the open door on the Onan compartment:
By analogy; on most Personal Computers, air is drawn in the lower front, up over the disk drives and exhausted out the upper fan. We all know it is inadvisable to leave a PC case open because overheating will occur. Why? Air flow is disrupted and no longer follows the prescribed path for cooling.
Last week the temps in our area were "comfortable", thus I did NOT turn on the three large outside A/C units. Rather, I elected to turn on the large attic/ceiling exhaust fan at the top of the stairs. Nancy and I were sitting in the family room watching TV. The door to the screen porch behind us was open. We were both uncomfortably warm. So, I crossed the room - opened a window on the other side and closed the door to the porch. The result? Well, air began to circulate in a new path which did a much better job of cooling us. Of course, with temps at 100 or above this week, we are back to the A/C units.
There are several factors in designing an enclosure that must be considered if success is to be achieved. Obviously, one is the path of that air flow. On the Chinese gennys there are two major air flow streams. The first is into the engine shroud where the fan moves air up and across the cylinder head. There are two plates fastened to the cylinder fins to further direct this air. If these plates were not there, the engine would overheat. Removing or altering the plates will have negative impact on cooling.
On the generator end, there louvers in the cap at the very end. Inside is a fan (end close to the engine - not the cap) that pulls air into the generator and exhausts it at the bottom of the engine end. This air flow is critical to cooling the AVR and wiring inside the generator.
There are several things you DO NOT want to do in using an enclosure. One is, as Bob noted, recirculating heated air. Heated air needs to exit quickly and be immediately displaced with cooler air in the original flow pattern. The enclosure I described using ยฝโ thick Hardi-Baker Concrete panels with a 2" opening around the lower perimeter will allow this. The natural air flow pattern of the engine moves heated air DOWN toward the crankshaft end.
If the bottom is not left open, further sound reduction may be achieved. But, air flow is disrupted. This is where a booster fan becomes valuable. The fan should be placed either on the bottom of the enclosure, directly under the out-take from the generator head or directly behind the out-take on the back of the generator.
The end of the generator (not the engine) where fresh air enters must be OUTSIDE the enclosure. The big, round yellow object in the photo is the generator end cap.
The air intake for the engine shroud must not be compromised - meaning NO space should be left between the grill in the shroud and the outside grill. In the photo below a 6" to 8" ducting adapter was used to funnel outside air into the intake grill of the engine shroud. If I had left this space open, a considerable amount of inside heated air would have been recirculated.
The opening to bring air into the box must be at a minimum at least the same area as the grill on the engine shroud. Don't forget that adding a mesh grill to the opening will reduce air flow.
An aux cooling fan should NOT be placed in front of the engine shroud so as to blow air in. Rather, the fan should be an exhaust fan to PULL air out of the compartment. This (exhausting air)is critical to establishing an efficient air flow pattern.
Selection of the fan is also critical. Conventional bladed fans are designed for free air movement. The pitch on their blades is low and the blades are extremely wide. When these fans are introduced ino an environment where some static pressure is present, they cavitate and air flow is drastically reduced. Stay away from this type of fan โ it will not work!
In selecting a fan, think about the design for a boat propeller or the prop on an airplane. They are designed to grab and move as much air (water) as possible for the power of the motor. Some slip will always be there, but these blades are designed to minimize slip and really grab the air. Therefore, you want to look for something that has multiple high pitch, narrow blades. The fan only needs to be capable of something close to 250 CFM (with a negative static pressure in the exhaust mode). Some fans that work very well for this purpose are โmuffinโ fans (110 VAC) that were removed from surplus electronic equipment. These fans can be combined for needed air flow. For example, two surplus 6โ muffin fans found at Northern Tool placed side by side easily provided 250 CFM together with a low noise level and less than 200 watts of power taken away from the genny. I used an 8โ 110VAC 300 CFM fan from surplus that met these specs on my enclosure. Fans that move more than 250 CFM of air can actually be counter productive and disrupt the design cooling paths. Larger fans will also waste power you want to use from the generator.
Alternately, squirell cage fans as used in "blower" applications might also be considered. They can move a significant amount of air with little slippage.
Additional air flow into the generator end will not hurt. This added air flow benefits the overall cooling and evacuation of hot air. A muffin fan (from an old computer) will fit inside an extension tube if you cut or sand the corners off the fan. In this location a duct booster fan also will work well. Be sure this fan blows INTO the opening and DOES NOT suck air out.
It is not rocket science โ but there is a science to doing it so that air flow is not compromised. Donโt just buy a big fan and hope for the best. Consider the negative pressure that can occur in the box, possible cavitation of the fan blades and respective air flow.
Of course, you realize that you MUST remove or totally isolate the muffler from the enclosure for it to be effective. It is also a good idea to have the fuel tank outside the enclosure so that the possibility of spillage on hot components is minimized.
I also advise using a small, inexpensive dial thermometer to check on inside temps. Anything over 180 degrees in the exhaust air stream should be cause for reducing load or shutting down the genny.
For those planning on building their own box this summer, I hope this information helps.
