Factory a/c on bus

[Jim Blackwood]

Seb, the thinking at this point is that a pair of sealed rotary compressors with about a 5 ton seer 22 or better rating would handle the job and MIGHT be able to run on a combination of 50 amp shore line and genset, using both for drawdown and then just the shoreline to maintain. More analysis needed at this point, state of the art was seer 24 last time I looked.

As for the condenser and evap fans,  switching those to VFD drive would enable variable speed by demand and conditions and also allow the use of less expensive motors if desired. Personally there is absolutely no way I'd ever even think of considering forking out 2 grand for one of those motors. Too many other good options.

Jim

[luvrbus]

I really like this idea, if the system is in good shape.

Hope to hear more along these lines.

Seb

Not going to happen using the O5G Carrier taking 15 to 18 hp with a 18 hp motor on 3 phase 230 volts drawing 60 amps you would need a 15k generator just for AC,Jims idea may work but I wonder what will happen with all the valves and evaporators that used R22 like his DL   

[petarm1]

Thanks for all the input. Every little bit helps. And it was good that buswarrior thought not to be rude. And i did try to look at other threads

[chessie4905]

I would think that the horsepower required to run a compressor even at half the capacity of the factory unit, would require 50 amp shore power 30 amp at many campgrounds wont cut it. Add in also the current draw of the condenser fan, blower fans, and you are drawing some serious current.

[Jim Blackwood]

Um, well first off my '96 DL uses r134, says so right there on the dryer.

Then secondly the Carrier compressor will be out of the circuit if the engine isn't running, otherwise why would I be looking at canister type sealed compressors?

Then thirdly, the efficiency of the Carrier pump has no bearing on amp draw. The only requirement is that there be enough amperage available to run the sealed compressors and that is entirely dependent on the Seer rating. So let's compare apples to apples. Most of the RV aircon units out there are probably seer 12 at best. So a 5 ton pump with a seer 24 rating is going to draw the same current as a 2-1/2 ton pump with a seer 12 rating. This is extremely important in determining current loads and is crucial in a high powered RV system.

Finally, the MCI OTR AC system uses TXV's. (Thermal expansion valves) this is an on-demand valve that regulates the introduction of liquid freon into the evaporator based on evaporator temperature. This means quite a lot of things actually, but one of those things is that it is possible to gain quite a lot of control over how much freon is dumped how quickly into the evaporator, and if your compressor(s) output is not quite up to full system capacity the possibility exists to still release freon at a slow enough rate to keep the liquid line full, which is required in order for the condenser to operate correctly.

It means basically de-rating the system while operating on shore power, but that only means that the condenser and evaporator become more efficient at doing their job.

The challenges are to minimize the plumbing changes, maximize cooling capacity, and remain within limits on power consumption. Done right, this system has the potential to be as good or better than any mini-split system, and still operate normally while underway.

Jim

[chessie4905]

Go for it!

[Jim Blackwood]

Yeah, it isn't going to happen overnight, and before I evacuate my system I'll need to know exactly what changes need to be made and have all the hardware on hand, though it should be possible to start upgrading the blower and fan motors ahead of time. I'd estimate at least a year or two and realistically it might be wise to double that.

Jim

[luvrbus]

Um, well first off my '96 DL uses r134, says so right there on the dryer.

Then secondly the Carrier compressor will be out of the circuit if the engine isn't running, otherwise why would I be looking at canister type sealed compressors?

Then thirdly, the efficiency of the Carrier pump has no bearing on amp draw. The only requirement is that there be enough amperage available to run the sealed compressors and that is entirely dependent on the Seer rating. So let's compare apples to apples. Most of the RV aircon units out there are probably seer 12 at best. So a 5 ton pump with a seer 24 rating is going to draw the same current as a 2-1/2 ton pump with a seer 12 rating. This is extremely important in determining current loads and is crucial in a high powered RV system.

Finally, the MCI OTR AC system uses TXV's. (Thermal expansion valves) this is an on-demand valve that regulates the introduction of liquid freon into the evaporator based on evaporator temperature. This means quite a lot of things actually, but one of those things is that it is possible to gain quite a lot of control over how much freon is dumped how quickly into the evaporator, and if your compressor(s) output is not quite up to full system capacity the possibility exists to still release freon at a slow enough rate to keep the liquid line full, which is required in order for the condenser to operate correctly.

It means basically de-rating the system while operating on shore power, but that only means that the condenser and evaporator become more efficient at doing their job.

The challenges are to minimize the plumbing changes, maximize cooling capacity, and remain within limits on power consumption. Done right, this system has the potential to be as good or better than any mini-split system, and still operate normally while underway.

Jim

It has been changed the 1996 DL3 left MCI with R-22 pull the front  evaporator cover and you can read R-22 only they don't work as good on R134 as they do R-22

[luvrbus]

Yeah, it isn't going to happen overnight, and before I evacuate my system I'll need to know exactly what changes need to be made and have all the hardware on hand, though it should be possible to start upgrading the blower and fan motors ahead of time. I'd estimate at least a year or two and realistically it might be wise to double that.

Jim

The condenser fans are huge they 53 amps @ 24 volt and the condenser is huge what and how would one replace those and keep the OTR air ?,I have seen it tried with 6 electric fans and it didn't work

[Jim Blackwood]

I've had that experience with r134 conversions from r11, but then again I've also seen systems that seemed to work better. The AC on this bus works but just how well I won't get to find out until summer is in full swing. So far I'm pretty satisfied with it, though from the sight glasses it appears it could use a little more freon. Some of that may depend on whether or not they changed the compressor when they crossed over. I suspect that the very large capacity of the system would tend to cover up any shortfall. How that might affect cooling capacity vs amperage input I'm not sure at this point.

On the condenser I'm thinking that a basic 1/2 hp 3 phase motor is going to move enough air. Maybe I'm wrong but do you know what rpm those fans run? That'd give me what I need to find out I think. I can measure the pitch of the blades and with blade count maybe find an online calculator to help sort it out. With a VFD (variable frequency drive) those can be throttled back or overdriven based on demand and the current draw should be less. I can probably find a VFD that takes straight DC if it makes sense to do that, considering that the way they work is to input AC, convert it to DC internally and then output it as 3 phase AC. Might even be possible to run one on either/or, which would be good for OTR and shore. But finding those surplus could be a challenge. Might just be easier to access the internal buss.

As a rule 3 phase motors are pretty efficient, very available as surplus, and economical to buy. The availability of VFDs as surplus has declined as people began to learn they could use them to run 3 phase from split phase 220 but you still can find them occasionally, especially in the under 2hp range. I wish I'd bought a few more of them back when they were dirt cheap though as I'm using all of the ones I have in the shop. Anyway, I agree that slapping pancake fans on the condenser isn't going to do much. That thing is about 3 inches thick which means a lot of air resistance. You need a healthy pressure differential, something that a pancake isn't very good for. I'm not sure why they didn't use a squirrel cage but no doubt there was a reason. Maybe space constraints.

Jim

[Jcparmley]

Richard do you know what kind of compressor you bus has?  I would assume it's much cheaper than the 05g in the MCI.

The GM system using the rotary compressors seems simple compared to what I've seen installed in some MCI buses. Even the dual rotary compressor setup. Wonder if it would be possible to replace the Carrier units with the GM style compressors? Our 4106 had the dual setup and it worked great. Our current bus, the 4108, has just a single rotary compressor. Takes about 20 minutes to get the system fully running, but once it's going it keeps up quite well. I don't know the specs, but I'd suspect that the rotary GM compressors don't rob nearly as much power from the engine as the larger Carrier units.

[chessie4905]

It is a GM A-6. Been used in millions of cars and trucks from 60s to the 90s. It has a capacity of about 42,000 btu's per single unit. Can be operated either rh or lh. Some 4905 used two in tandem in place of the big 3 cylinder. I wonder if you could use one, and kick in the other if needed. Are very reliable. Only repairs I've ever seen with them is the front seal leak and easily repaired with the few special tools required, and front clutch slip and burn from low oil, freon or improper cooling or overfill with refrigerant.

[richard5933]

What he said.

Newer models are a bit more specific with right-hand and left-hand models, as well as a few differences depending on which oil they come pre-loaded with, but basically the same compressor that's been used for decades. Our bus could have had 2 of them mounted side-by-side using a manifold so they both pump the same refrigerant.

Seems like a much more simple and reliable system than the MCI setup.