| Author | Message | ||
| Mike's 4106 Rating: N/A |
I have 6 blade 26" 4" pitch fan and I have a 8 blade 28" 3" pitch fan. Will the 8 blade pull more air through the radiator than the 6 blade??? If so how much more air could I expect??? Alot more?? A Little more?? | ||
| Henry Bergman
Rating: N/A |
Best guess without doing the easy math (I'm lazy) is that they would be about the same. Would the 28" fan fit inside the shroud? (if you have one) Changing the diameter of the drive pully (also if you have one) for a faster fan speed MIGHT help, but you can reach a fan rpm where the fan will cavitate. If possible, it is USUALLY (not always) best to try to keep such stuff as close as stock as possible, unless a repower/more hp demands cooling improvments. Good luck. | ||
| Marc Bourget Rating: N/A |
Henry points out a very important factor, whether the fan blades will cavitate. Until the tip speed of the fan approaches sonic, the fans will, more or less, behave like a pump and they have pump performance curves. Fans at speed store so much kinetic energy that they resemble small IED's (improved explosive devices) The positive divergent occilations that could develop from cavitation could release all that stored energy with really bad results. More importantly, the replacement fan could depend on the supply of a certain mass of air. All else being equal, too high a pressure drop (from a fan sized too large for the air that will pass through a certain radiator) will promote cavitation and possible fan self-destruction. The fan/radiator combination is a critical exercise in coordination. I wouldn't switch fans unless the Mfg (ex. MCI, etc.) indicates it's a suitable replacement. Otherwise, you need to ID the Fan Mfg, part number of both and see if you can get the performance curves to see if they're compatible. BTW, this is close to rocket science, IYKWIM, with the possibility of serious consequences, to boot! | ||
| mcswain Rating: N/A |
Just been thru this . The fan blades is supose to be 1/2 in the shroud. | ||
| Gary Stadler (Boogiethecat)
Rating: N/A |
When I built my generator trailer (15kw, silent) I put the fan halfway in the shroud. It cavitated like crazy. The cure (and a lot more airflow) was to create a "tube" around the fan about 2" thick, and the "tube" stuck halfway in the shroud. Now the entire blade is surrounded and the cavitation quit immediately... | ||
| Tom Caffrey (Pvcces)
Rating: N/A |
Gary, could you describe the tube setup a little more? I/m unsure what you mean by 2" thick. About how much clearance from the fan did that leave? Thanks. Tom Caffrey PD4106-2576 Suncatcher Ketchikan, Alaska | ||
| Marc Bourget Rating: N/A |
Gary, Are you describing a 2" cylinder slightly larger than the fan which is a continuation of the shroud? The better approach is to have a continuous curve from the shroud past the fan blades. This curve is specifically calculated according to a pressure distribution curve to reduce drag, deliver maximum air to the fan "disk" with a minimum of turbulence. If you grafted a "tube" onto the shroud or used the 2" tube to make a ducted fan setup, you increased the fan efficiency but didn't take full advantage of what's available. But in your case, it's not that significant if this is the bus you reported such excellent cooling on. Great that you're preventing blade cavitation. | ||
| Gary Stadler (Boogiethecat)
Rating: N/A |
Marc, you're close to correct. This isn't my bus, it's my silent generator trailer I built for Burning Man. Kubota 4 banger driving a 20kw genhead. Unfortunately the Kubota craps out at about 15k but it's plenty for my application. Basically I took a old dog automotive electric fan apart and extracted it's "duct" which I then bolted to the fan shroud of the gene's radiator. It is somewhat like a tube the diameter of the fan (plus an inch) and about two inches thick, long, or whatever you want to call it. It does offer some curvature so the airflow thru it is not so abrupt as if it had a square interface edge to the shroud. The front and rear surfaces of the fan are entirely within this "tube/duct". It instantly eliminated any possibility of cavitation which was quite pronounced prior to doing this. | ||
| Bob Belter
Rating: N/A |
Ahoy, Busnuts, The fan and inlet shroud really ought to look like the inlet to a jet engine and nacelle. Blade tip clearance is important, with “zero” clearance being ideal, but clumsy in practice. Any sharp corners induce flow separation. BTW, liquids cavitate, but with gasses -- air – the somewhat similar condition is called flow separation. A stalled wing is an example of flow separation, (and one needs to deal with the condition properly). Marc is correct in that the flow separation can cause damage to blades. That the blades don’t fall off often is a tribute to how robust the things are, not due to the refinement of the fan-shroud installation. Many of the configurations which I have looked at are horrible from an aerodynamic standpoint. And -- the discussions on these web sites often involve cooling faults. Some problems are due to the degraded condition of the shroud and fan, where it is simply a marginal poor design, which just barely works when new. If you can’t alter – improve the design, all you can do is try to get as close to new condition as you can, and live with it. If you are able to make changes, a bigger radiator and fan with a proper shroud will do wonders for your cooling. The power required is also reduced. Some installations are putting 40 or 50 HP into the fan, and you have to buy this fuel, but it does NOT help to make the wheels go ‘round. Enjoy /s/ Bob | ||
| Richard Bowyer (Drivingmisslazy)
Rating: N/A |
Gary, how do you detect cavitation (flow separation)? What are the symptoms? Richard | ||
| Dale Waller (Happycampersrus)
Rating: N/A |
Well, Seeing how folks are too lazy or they are trying to OVER complicate your question with redesigning a fan shroud and assuming your bus isn't powered by a GE J52 jet engine LOL. I placed a call to Flexalite. The fellow I spoke with explained that the 2 more fan blades and larger diameter will pull quite a bit more air. Also not to start any battles, but you should have 3/4" of clearance between the fan and shroud to allow motor movement per the book. I to have been through a repower and found that a 6v71 came stock with a 6 blade and a 6v92 has a 8 blade. I hope this helps answer your question. If not do a search and get the phone # and give them a call. | ||
| Marc Bourget Rating: N/A |
You're assuming (3/4" clearance) that the fan is mounted on the engine, usually (except front engines like Blue Birds) it is not. Mr. Belter, an aeronautical engineer, spoke of better-best, not "only". The larger fan had 25% less pitch (3" vs 4") but 33% greater number of blades. (Nice facts, logical assumption, but they mean little with out the performance data!) On reason the 1st fan may have tolerated more pitch is that it had less capacity. You start trying to move more air you'll drop the pressure inside the shroud cavity. This drops air density, increasing the potential for a blade stall, i.e. cavitation. I'm guessing that blade stall (short of disintegration) is accompanied by a change in sound and extra vibration. | ||
| Dale Waller (Happycampersrus)
Rating: N/A |
NO Marc, NOT assuming. The RTS V drive manual states this as probably a 4106 manual. Most GM V drive engines have the fan mounted to the engine. I am assuming that you didn't notice what bus Mike has. And Also as I stated I have hands on experience with this for example a RTS 6v71 has a 6 blade with a 4 core radiator, a RTS 6v92 has a 8 blade with a 4 core radiator. I would believe that DD used a 6 blade on a 6v71 for a couple reasons. #1 That was all it required to properly cool it #2 would be it would require more HP for the 8 blade. So your idea about the fans cavitating is out the window because DD used both fans on the same radiator. | ||
| Marc Bourget Rating: N/A |
Au contraire mon camper-person! Dale said: "The RTS V drive manual states this as probably a 4106 manual." Did you mean "was" instead of "as"? Do you mean 4 row radiator rather than 4 core radiator? I'll assume you meant a 4 row radiator. I don't follow the reference as you jump from RTS to 4106. IIRC, the 4106 is an intercity rather than a transit bus. I admit (obviously), I'm not familiar with the various RTS systems. Were the fans both driven at the same speed? I have a 6V71 engine/trans assembly from a fishbowl transit. I believe the fan was driven by a shaft isolated from engine vibration by u-joints. I can check this a few minutes from now because I'll be visiting my shop. Was the RTS 6V71 version's fan mounted to the engine or was it too driven by an articulated drive shaft? Assuming both are 4 row, are you sure it's the same radiator? There's a lot more to cool with a 6V92 as compared to a 6V71. Is it possible that tube size, fin spacing/count were different? Finally, my comment referencing front drive busses was intended to limit my comments to busses where the fan was NOT mounted on the engine. It wasn't a universal statement that all busses besides BBs had remote mounted fans. | ||
| Dale Waller (Happycampersrus)
Rating: N/A |
Marc, AS probably a 4106 manual would state would have been a better choice of words. I will confirm that the 4106 manual states this also tomorrow for my own piece of mind. And if one wishes to get picky about 4 core or 4 row (same thing)that's ok. Just explain what an "improved explosive device" is. your words see third post. Improvised explosive device is the term you are seeking. Everyone makes a miss lick every once in a while. Yes both radiators have the same part# as I researched this on my Fishbowl repower a 6v71 to a 6v92. Once more and the last time, most GM V Drives use a Torus fan drive mounted on the front of the engine. the manuals of those coaches will tell you check for the proper clearance between the fan and shroud. Marc said, "Finally, my comment referencing front drive busses was intended to limit my comments to busses where the fan was NOT mounted on the engine. It wasn't a universal statement that all busses besides BBs had remote mounted fans." I can't understand your comment. I am not talking about anything other than V Drives. They have both types of fan drives. Mike email me off board and we will solve you fan problems. Dale ASE Certified Heavy Equipment Mechanic since 1994 | ||
| Gary Stadler (Boogiethecat)
Rating: N/A |
Richard, in the case of my generator, when it's working properly (no cavitation) the airflow exiting the radiator (and the sound of the fan if you open the service door) sounds smooth and sweet. When cavitation happens, it suddenly switches to a very loud buffeting sound with lots of low-frequency rumble that is just about what it sounds like if you roll your car window down halfway when you're driving at 75 mph on a freeway. It's VERY obvious, and it switches back an forth as fast as if someone was flipping a switch. MY particular system was 'on the edge' and could choose either mode, or flip flop back and forth between them. Once I put on the little duct, it was stable from then on with no tendency whatsoever to cavitate. and... Once again, my practical experience finds itself at odds with some of Marc'c comments: "this is close to rocket science, IYKWIM, with the possibility of serious consequences" and "I wouldn't switch fans unless the Mfg (ex. MCI, etc.) indicates it's a suitable replacement" On my Crown I went from the stock metal bladed fan (6 blades, medium pitch) to an 8 bladed high pitch fan. Yes the consequences were serious... the system switched from marginal performance to some serious cooling !! It's not rocket science by any means. You switch the fan, run a test on a known grade/similar conditions to that same grade without the new fan, and watch the temp. gauge. If you know your bus, you'll know instantly if it's working better or worse 'cause the gauge will climb faster/higher than it ever did before, or not depending on if the switch worked or not. And unlike a rocket, if it didn't work you can slow down and go home, change the fan back and try again. The most it could cost you is a hundred bucks for a fan that didn't work. Rockets don't have that luxury, thus the science that needs be done prior to launch. Land-barges are much more forgiving...  (and oh ouch, if it really gets too hot you can use your mister on the way home!!!) Cheers Gary (Message edited by boogiethecat on June 08, 2006) |