Can it be done? (A/C Dilema)

[Newbob]

I would like to know if it is possible (reasonably) to run my 2 A/C units which normally run of the Cummins without running it. That is, what would have to be done to run those same units off of a generator or some custom transmission/ gear assembly to run the compressors.

An odd question I know. Give me some reasonable discussion on the matter ( not just - " why would you want to- you're crazy" and I'll share my plan with all the curious minds out there :-)

Thanks in advance to all of those giving it some thought.

[bevans6]

How are they driven by the Cummins?  Two separate compressors?

Brian

[robertglines1]

Don't know how many hp is needed.  To cool bus they say high idle or about 1000rpm. That said the electric side of condenser and evap fans are around 50 amps each.I am referring to the factory compressors engine driven.  As far as roof tops they are energy hogs requiring (15000 btu) around 18amps to start and 14 amps at 120 volt to run. Minni-split (mine ) require 6 amps at 120 volt to run. (inverter type) amps x volts = Watts  if that will help you rough figure.  the number of batteries in a bus are not mostly for starting but for the onset of AC  load. My coach had 4ea 31's but really only requires the amp load of 2 for starting circuit.   FWIW   Bob  

[chessie4905]

   An auxiliary power unit generator of sufficient capacity to start both units or a smaller one which would have wiring and relays to prevent both compressors to attempt to start at the same time. What brand a/c's and btu ratings? This is assuming that you are referring to rooftop units or splits.

[pvcces]

You said "can it be done?" The answer is yes; it has been done. As for the generator, if I put our engine on fast idle, we get a full 220 amps. That's about 2,700 watts.

I think that if you are serious about pursuing this, there are a bunch of side issues to deal with. Spend some time reading up on how it is done. You're going to want the right generator, the right inverter, the right batteries and the right wiring. Efficient A/C equipment will help a lot.

Good luck in your studies.

Tom Caffrey

[TomC]

If you have a 24v system, then a 270amp alternator running at 28.2 volts will give you a peak of 7614 watts. This will easily keep up with either a single 4,000watt inverter, or two 2,800watt inverters stacked to give you 5,600watts of 120vac power. Which ever you do, there still will be a drag on the main engine. But not as much fuel used as running the generator. I personally just prefer to run the Diesel generator. Good Luck, TomC

[Oonrahnjay]

        On a completely different tack, you could leave your system as it is and add an electrically driven A/C pump (such as Red Dot $$$$) and a separately mounted evaporator pancaked in front of or behind your stock evaporator (and separate condenser).  When you're on the road, you use the engine-driven compressor and evaporator; when you're parked you use whatever electrical system (shore power or generator) to provide cooling via the other system.
       This works but it seems pretty wasteful overly complicated to me.  But you do get cool air out of the same vents pushed by the same fans ... but that's about all.  But maybe considering the principle will give you ideas.

[bevans6]

I think the original poster has to post again before I will post a thought on this question.

Brian

[Newbob]

Op here. - Most of these posts have to do with the electrical load on the system - that's the easy part I think. Its the rest of the system I am concerned with. Like the compressors  and such - I don't know too much about the intimate details of how Bluebird has the A/C running off the cummins engine.

Yes, I could just put in a rooftop unit and call it a day but was very curious if the former could be done without too much more trouble and expense. Like one poster said - using the same units , fans, condensers etc that are already installed in the bus. - Simply put, I don't know if the components that the A/C units depend on the engine for can be replaced by another mechanical component (feasibly).

Regards

[Oonrahnjay]

Op here. - Most of these posts have to do with the electrical load on the system - that's the easy part I think. Its the rest of the system I am concerned with. Like the compressors  and such - I don't know too much about the intimate details of how Bluebird has the A/C running off the cummins engine.

Yes, I could just put in a rooftop unit and call it a day but was very curious if the former could be done without too much more trouble and expense. Like one poster said - using the same units , fans, condensers etc that are already installed in the bus. - Simply put, I don't know if the components that the A/C units depend on the engine for can be replaced by another mechanical component (feasibly).

Regards

      I asked a similar question of Lamar Welch in Georgia -- and if anybody knows, Lamar does.  I asked him if there was some way to do "Y" connectors with one-way valves so that "Freon" (let's just call it that now, OK???) could be compressed by an engine-driven compressor or by an electrical compressor like the Red Dot when parked.  Easy, pressure from the engine driven while moving and engine making power and from "the other" when not.  He said it's not feasible and described what I wrote about above -- if what you want is for the cold air to come out of the same vents and be blown by the same fans.  But that's not really the same thing.

[bevans6]

Do you want the compressors to run off an electric motor or a gas/diesel motor?  Do you know the BTU rating of each system?  If electric, do you want to be able to run it from a generator, or just when you are plugged in to an external power source?

Off the cuff, back of the envelope thoughts on the above.

A ton of AC is equivalent to 12,000 BTU's

A bus AC system usually has around 10 tons of capacity, or 120,000 BTU's.  Your system might have a dash air system of around 2 tons, and a main air system of around 8 tons.

Each ton of compressor capacity takes around 1 hp of compressor power, so you might have a 2 hp load and an 8 hp load.

The evaporator fans also require power, as do the condenser fans.  The dash air evaporator fan might need 1/3 hp, the main evaporator fan might need 1 hp.  The condenser fan probably is common to both systems and probably needs 2 hp.

You can assume, rule of thumb including motor, drive losses, etc., that each HP will need 1500 watts of electrical power.  If you plan to run the whole thing electrically you can look at 19.5 KW of electrical power.  That's about 165 amps at 120 volts ac, or 80 amps at 240 volt.  That's more than the capacity of a typical campground connection, which is at most 50 amps at 240 VAC.  

The condenser and evaporator fans on the bus are DC, either 12 volt or 24 volt depending on the bus electrical system.  If you want to run them without the bus engine running, you need to provide around 240 amps of 12 volt DC or 120 amps of 24 volt DC.  You can do that with an alternator driven by an electric motor, by a gasoline or diesel motor, or with a honking big switching power supply.

So, there is a reason that these powerful air-conditioning units are run from big engines.  A 10 hp load for the air conditioner compressors is nothing to a 250 hp Cummins.  Likewise the bus alternator is designed to run the fans from DC power at the current levels they require (the condenser fan on my bus was 2 hp and drew up to 80 amps).

So - your options come down to how to drive the compressors, and how to power the fans.  I could, with some time, probably design a jackshaft system to decouple the compressor drive belts from the Cummins and transfer that drive to some other engine. using electric clutches and such, but it would cost a lot of money.  More money than any other AC option that you might have, I fear.  Setting up the compressor system as fully stand-alone, so that it is never powered by the Cummins, would be a lot cheaper - but you would have to run whatever that system was on the road as well.

Most people either keep the factory bus system for on the road use and have a separate roof-top or mini-split based system for while parked, or they just toss the factory system and use the aftermarket system all the time.  The factory OTR AC system is just too big and needs too much power to handily run it from anything but the bus engine, as designed.

Hope this helps you a bit.  BTW, a typical home AC system for a 2,000 square foot house would be around 3 tons capacity, depending on where you live. 

Brian

[Newbob]

Wow, Thanks so much for the detailed response - Good to know those a/c btu horsepower conversion numbers. I would agree - far too much involved. - And that , for me - is saying something! I am almost always willing to do the work and figure things out - simply for engineering and educations sake - I love a challenge!

But in this case, I think the right thing to do is to just add a rooftop unit. I have heard the mini splits mentioned but am not sure whats involved with those in a bus. Are they quieter? I think a regular rooftop unit might be a bit too noisy for my application but I do have a "utlities box" on the back of the bus that houses a furnace, generator, gas bottles and such - maybe I should mount the A/C on that and duct it into the heater ducts into the bus?

[bevans6]

Without knowing what your utilities box looks like, impossible to say what could be done.  AC is best ducted from ceiling height, if you have sufficient air flow floor ducts work as well, of course.  If you are going to go with a remote unit, think about large home window units - people have "deconstructed" them and used them to provide ducted AC.  The biggest challenge with AC in a bus is getting rid of the heat and condensate water produced by the process, and a "utilities box" has some potential for how to deal with that.

Mini-splits can be much quieter, and perhaps twice as expensive, as roof top units.  Main reason is the noise and heat of the compressor and condenser fan can be remoted to a bay with only the evaporator and it's air handler inside.

Brian

[Newbob]

Well, I just did some research on the mini-split systems. Turns out these are just household type systems - not something engineered specifically for RV/coach use. So, I get it. That might work, I'm just getting the furnace fired up here probably today so Ill see how that goes and if I end up using the existing furnace. If so, I may use one of the rooftop units and duct the cold air into the but through the Furnace vents. I do like the mini-split setup and it may work for me as an option if the rooftop unit wont work. To those who are curious, the box looks like a big toy box 52wx24dx32h - made of wood but I am wrapping it in galvanized steel soon. If you are looking at it "sitting" on the rear bumper - the furnace is mounted in the upper left 8"hx about 20" wide while the gas bottle (room for two) sits in a separate compartment below the furnace. The box is divided about in half and I plan on housing the generator and a battery (on an upper shelf) on the right half of the box. The cover lifts as a toy box would and I was considering mounting a rooftop unit on the cover. - This would make the install pretty straightforward.

[bevans6]

Remember that a roof-top AC weighs about 100 - 120 lbs.  You can get them configured for remote control and ducts.

Brian