| Author | Message | ||
| David Ljung Madison (Daveola)
Registered Member Username: Daveola Post Number: 77 Registered: 11-2005 Posted From: 63.204.157.4 Rating: N/A |
I had posted before about adding an auxillary air compressor and it was suggested that I tee off at the bus compressor. Things have changed a bit since then - I now have a tee off the wet tank up by the front of the bus for taking air out (for running a pump and tools and the like), I'm assuming this would be a reasonable place to also push air in with an electric compressor? I figure it can still go through the bus dryer before it gets to the brakes and the like. Any thoughts on this idea? Am I right that it will still use the bus air dryer? Speaking of the air dryer - has anyone here done the maintenance and taken it apart to check if the dessicant needed replacing? How often does this need to be done? | ||
| David Ljung Madison (Daveola)
Registered Member Username: Daveola Post Number: 78 Registered: 11-2005 Posted From: 63.204.157.4 Rating: N/A |
And speaking of the air dryer again, has anyone tried these "dessicant inline dryers" such as: http://www.jaggerswholesale.com/servlet/the-448174/Dessicant-Snake-Promotion/Detail If you've tried them - do they work well, and how long do they generally last? | ||
| RJ Long (Rjlong)
Registered Member Username: Rjlong Post Number: 1027 Registered: 12-2000 Posted From: 71.195.113.23 Rating: N/A |
David - Do I understand you correctly, in that you put a Tee just downstream from the wet tank (first tank after the compressor)? Or did you locate the Tee between the compressor and the wet tank? Please clarify, thanks. . . | ||
| David Ljung Madison (Daveola)
Registered Member Username: Daveola Post Number: 79 Registered: 11-2005 Posted From: 63.204.157.4 Rating: N/A |
Actually I just put a tee directly on the tank (where there was a fitting for something else). So I can take air right out of the wet tank, and possibly pump right back in. | ||
| Kyle Brandt (Kyle4501)
Registered Member Username: Kyle4501 Post Number: 192 Registered: 9-2004 Posted From: 65.23.106.193 Rating: N/A |
Where is the dessicant tank/ dryer on your bus? before the wet tank or after? On my 4501, the air compressor feeds the dryer then the air goes to the 'wet' tank then to the rest of the system. My preference is to add the necessary air lines to add the aux air to the system between the engine compressor & dryer. As for the 'inline dryer' link you posted, I did not see specifications on it concerning the amount of water it would remove, I'm guessing not much. The standard dryer has a water purge on it everytime the air compressor govenor cycles. That combined with the quanity of dessicant it holds implies there is a LOT of water to be removed from the compressed air. Good luck! kyle4501 | ||
| David Ljung Madison (Daveola)
Registered Member Username: Daveola Post Number: 80 Registered: 11-2005 Posted From: 63.204.157.4 Rating: N/A |
Why would they call the tank that holds the dry air the "wet tank"?? I'll have to look at my bus air schematics and see if I can figure it out. | ||
| Buswarrior (Buswarrior)
Registered Member Username: Buswarrior Post Number: 899 Registered: 12-2000 Posted From: 65.92.191.187 Rating: |
Hello David. The first tank after the compressor is by tradition called the "wet tank". A more formal name is Supply Tank, with the "dry tank" (or tanks) being formally called Service Tanks (primary and secondary for a post 1975 dual air brake system.) Air dryer technology is rather recent, with Bendix introducing it in 1970. To this day, an air dryer is optional equipment, though you'd be foolish not to spec' one in a new vehicle, and it is a good project to retrofit one from the scrap yard into a vehicle without one. The air dryer should be plumbed in between the compressor and the first tank (wet tank) in order to get most of the moisture out of the incoming charge. There are minimum distances the air dryer must be away from the compressor in order that the incoming charge is not too hot to render the desicant useless. A little known fact is that when the air dryer's purge valve opens at the end of the compressing cycle, (loud sneeze) part of its functioning closes off the connection between the compressor and the wet tank. If the purge valve in the air dryer fails to return to the closed position when air pressure drops to cut-in,(freezes in winter with the heater failed, gummed up from years of neglect) you will not be able to build air pressure. Grizzled veterans from the frozen north have been known, under winter conditions, to drop the air pressure below cut-in just prior to shut down to ensure that the purge valve is in the proper position to allow air to build. That way, if it freezes, air can build, it just won't purge. I would be inclined to leave your install the way it is, auxiliary air compressor directly to the wet tank, bypassing the air dryer. You will only get a small amount of moisture in the wet tank due to building air with your electric compressor in a park before motoring off, perhaps no more noticeable than the small mist you would get normally, from what gets by the air dryer. And, an air dryer is only capable of drying somewhere around 10 cubic feet of air and then it is saturated. So, you won't hurt anything by going direct. Air dryer maintenance: In a fleet setting, I would replace the desicant in an air dryer this time of year, leading into winter, and that would that until next fall. I would also carefully check the heater in the air dryer for functioning and the condition of the wiring/power source. Maintenance Manual will detail how to check the heater. For a busnut, annually check for the heater and wiring, and desicant every so many years, depending on the mileage you do. Mine has been in there for over 7 years, and there is no water at all showing when I drain the wet tank. I think I might open it up this fall, just so it doesn't rust completely solid.... If the desicant age is unknown to you, change it. Then you know when, and how many cycles/hours/miles have been run through it, moving forward. happy coaching! buswarrior | ||
| David Ljung Madison (Daveola)
Registered Member Username: Daveola Post Number: 81 Registered: 11-2005 Posted From: 63.204.157.4 Rating: N/A |
Buswarrior - awesome amount of information, thanks for that. If my air dryer is before the "wet" tank, and presuming I would like to have a compressor that keeps my bus air up all the time, is it really such a good idea to bypass the dryer with my electric compressor? I ran a test and filled the wet tank with the compressor and then emptied it with a hose, and there was more mist than I would have expected coming out - is this just because of the cooling effect of the decompressed air?? | ||
| Buswarrior (Buswarrior)
Registered Member Username: Buswarrior Post Number: 900 Registered: 12-2000 Posted From: 65.92.191.187 Rating: N/A |
Hello David. As long as you drain the wet tank periodically, you'll be fine. Prior to air dryers, the wet tank caught all the moisture, and drivers just drained the tanks regularly. If you get a lot of water, drain it a little more frequently, if you get little to none, you can let it go longer. I'd leave the aux compressor bypassing the dryer because it is already done, and in the unlikely event that the dryer purge valve sticks, you could at least air up with the aux compressor. Curious, why would you want the coach always aired up? If the coach doesn't hold air fairly well by itself, that aux compressor is going to be cycling a lot... happy coaching! buswarrior | ||
| David Ljung Madison (Daveola)
Registered Member Username: Daveola Post Number: 82 Registered: 11-2005 Posted From: 63.204.157.4 Rating: N/A |
The only problem is I saw a lot of moisture coming out of the hose that I hooked up to the tee after filling the tank with the electric compressor. It's odd, because when I open that hose after filling with the bus compressor I don't see that, but I also don't see that when I empty the tank on the electric compressor. It's only on the bus tank when it's filled by the electric compressor. Anyways, I want to keep air pressure up in an attempt to keep the suspension bags full. After about a day the bus sinks, and it seems like it would be easier to level (using, say, planks of wood) if the bags stay full. It tilts considerably more once the bus starts to sink. It would be nice to be able to open the baggage locks whenever I lock - someday I'll replace them with electric, but that's on my endless todo list.  | ||
| Tim Jones (Torquester)
Registered Member Username: Torquester Post Number: 90 Registered: 12-2000 Posted From: 12.73.174.148 Rating: N/A |
Hi All, Bendix says, all air introduced into the system should be filtered. This means, introduced into the system in front of the dryer. I'll give my vote for following their instructions. Simple and safe. Tq | ||
| Tim Strommen (Tim_strommen)
Registered Member Username: Tim_strommen Post Number: 236 Registered: 11-2004 Posted From: 24.6.107.46 Rating: N/A |
There's a few things that could be recommended based on what you are seeing: 1) "...after about a day the bus sinks..." I think this indicates that there is either a leak in your air system and/or there is enough movement or weight shift in the chassis to cause your air-leveling valves to alter the pressure of the airbags. The way I have my bus parked in the storage yard, it will teeter-totter the suspension all the way to no air pressure in about 18 hours. You might try dragging some air lines to the cab from the outputs of the auto-leveling vales (between the leveling valve and the air bags) - and placing line locks on them. This would allow you to get the bus level while the engine is running, and lock-out the airbags while you're stationary. Thus if the system pressure dropped, the air bags would be individually isolated. I believe that Bendix offeres an accesory isolation valve with as electrical switch in it (NO/NC) - you would be wise to configure isolation valves in such a fashion as to turn on a sounder AND warning light if the ignition is turned On and any one of the valvles are "locked out" (like a 3+ input "OR" circuit). 2) "...I saw a lot of moisture coming out of the hose that I hooked up to the tee after filling the tank with the electric compressor..." I dunno about your compressor - but on my small shop compressor (1/2HP Craftsman circa 1983), with the air coming out of the hose uncapped (no accesory attached) I get about a half cup of water out in about 3 minutes. The risk with air systems and moisture is that the surface tension of water typically puts water right where you don't want it - in stationary control components (like open/closed valves). You say you are running an electric compressor? Why not plumb it into the exsisting air compression setup? From my limited recollection of air systems (forgive any minor mistakes here guys/gals), an air dryer is typically hooked up to the govenor of the air compression system. When the air pressure in a system is at govenor cut-off both the air dryer and the compressor get the same air "control signal". While the air compressor is pumping, the air leaving the compressor begins to cool and sheds its moisture on the outlet line. By having an air dryer in line after a certain length (to ensure enough cooling), the dessicant "grabs" the moisture as the air works its way throught the cartridge on its way to the wet tank. Depending on the type of dryer, when the Govenor sends the cut-out signal to the compressor, the air dryer opens a valve which is actually before the air dryer in the circuit (the purge valve). This causes the air pressure in the compressor outlet line to drop - closing a check valve (one way valve) at the air dryer outlet. The low pressure on the inlet side of the dryer, pushes air from the small tank near the dryer backwards through the dessicant cartridge and blows the moisture out the purge valve - regenerating the moisture capacity of the dessicant cartridge. Of course this process is what I believe Buswarrior was referring to - if the purge valve is stuck open (from moisture freezing as the valve is dumping air) the air coming from the compressor when the govenor kicks in will be immediately dumped overboard, and the check valve would keep what pressure remained in the tanks, there. With this in mind I might suggest this alternative (a bit more work and a little complicated but a little safer?): Have two ways of plumbing the auxilliary air compressor into the system - selectable from a 1-in/2-out selector at the dash. One way will feed the typical compressor line by the engine (in fact, I'd reccomend tee-ing the line at the back of the compressor, with the aux inlet of the system pointing up to allow for mosture drainage from the aux compressor). The other outlet of the selector would go directly to your "wet tank". Then I'd recommend adding an electrical control switch to the govenor controll line (the same one that tells the engine compressor to free-wheel or "run unloaded" and the dryer to dump - as it will still work exactly the same way even when the engine is off - kind of a blessing of these dumb, single task valves). As I recall when the compressor is supposed to be generating air pressure the control line between the compressor, air dryer, and govenor is unpressurized - meaning a simple "low pressure" alarm switch could be used to turn on/off the compressor or a control relay to boost the current (guys a little help on the accuracy of the air line pressure here?). With some simple wiring and a selector valve for the air circuit which includes an electric switch for one position - you can make the whole system automatic and still use the dryer under normal circumstances - or directly feed the air tanks to quickly get pressure if the bozo in the stick-an-staples next to your campsite demostrates how fire retardant his seat covers are with a roman candle . The one draw-back to having the automatic system in a bus at a public campground is that it will run like an automatic system - meaning it won't care if it's 3:00AM if the system needs to be charged, it will charge it (followed by a cute little purge cycle "PSsssssssss!!!!"). A combination of these aproaches is probably in order - the lock-out valves for the airbags, and the auto-compressor setup with the selector valve for toppong off the tanks or quick getaways in the event of an emergency (without idling for 10 minutes to build air). Cheers! -Tim P.S. - If your compressor is 110VAC, I'd recommend a 12/24VDC power supply on a diode (and fused) to power the purge valve heater (if equiped) whenever the electric compressor is available (running or not). This will reduce the likelyhood of the valve sticking. -T | ||
| Tim Strommen (Tim_strommen)
Registered Member Username: Tim_strommen Post Number: 237 Registered: 11-2004 Posted From: 64.186.173.26 Rating: N/A |
With a touch more thought - If you used a check valve and and inversion valve, you could control Shop air entering the compresor line as well. The check valve would only permit the air to enter the invesion valve if the pressure from the shop air is higher than the side towards the inversion valve. The invesion valve controls the flow of air into the compressor line like a "NOT" gate in digital logic (High pressure on the gate port=no throughput from the supply to the outlet, low pressure on the gate port=throughput from the supply to the outlet). Thus the compressor loaded and unloaded cycles would behave as they do now and the purge cycle would continue to ensure the air dryer is doing its job. Having shop air available and using the exisiting system to run the air circuits - is also a neat way to trouble shoot the entire air system (minus the compressor of course) in the event of a problem without the need of running the engine. Cheers! -Tim |
