3 Zone Motor Home A/C

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3 Zone Motor Home A/C

Postby bluescarecrow » Mon Feb 20, 2017 1:31 pm


I am converting an MCI Coach to a Motorhome. I do not want to install A/C units on the roof for several reasons... Rooftop A/C units are noisy... I don't want to increase the height of the Coach... I don't want to compromise the integrity of the roof with possible leaks... I don't want to have to run a generator to run the units as I'm traveling down the highway.

I'm building an assembly with a Sprag (one way) clutch, similar to that in a helicopter, that would allow the compressor to be turned by the engine or a 220 VAC electric motor; no one has ever done this before.

I'd like too install 2 small evaporators with squirrel cage fans, one in the living room and one in the bedroom the 3rd evaporator is in the drivers compartment; it came with the coach and I'd like to use it. I have the mechanical knowledge to install all of the parts; the system will be checked and charged by a local A/C company.

However, I am not an A/C Technician, I'm a retired Motion Picture Special Effects Supervisor no one wants to help me design this system. Every A/C company tells me to just install 2 rooftop A/C units and be done with it. The problem with that is that you have to run a generator as you are driving down the street; so you are using diesel fuel to run the engine and diesel fuel to run the generator; that's wasteful, expensive and makes no sense.

The System...

Three electrically valved zones operated by thermostats. One TM31 Compressor, EZ-Clip hoses and fittings, one Condenser with 12 VDC electrical fan, Dryer etc.

The thing that I am most concerned about is if only one zone is calling, will the compressor pressure be too high, since it was made to handle all 3 zones at the same time? Should there be a relief valve included in the system?

Any help would be greatly appreciated.

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Re: 3 Zone Motor Home A/C

Postby Tom Greenleaf » Tue Feb 21, 2017 4:03 pm

1st off welcome to this now quite slow bulletin board.

This type project wouldn't be my thing but have some clue of loads on an engine and keeping the motor that drives this thing as realistic as possible no doubt works hard enough to just move and might have A/C for just front area now not knowing what it is or looks like much at all.

A real good reason for running generator in larger moving things like a motor home or I do have some experience with large yachts most was two zone, two separate systems entirely ran off "shore" power or on board generators. Advantage is condenser is a small ambient water heat exchanger using water not air.

General idea is you only have one chance for a condenser with a vehicle's grille and TMK one compressor. Even large passenger vehicles just share that if they have "rear" A/C is really just adding another evaporator with lines to it's own box with fan and controlled assorted ways still requires engine running that moves the vehicle.

The RV assortment I would think you don't want it's engine running all the time and doubt you'll get enough BTU out of ONE compressor belt driven for 3 zones on a motor home that is really just splitting up what the one can do.

I'm mostly lost to help you with this. Understand the clearance of roof tops and potential for leaks but the placement seems right for them to work at a glance.

Another here still comes by now and then highly into big rig A/C may shed some light for you or ideas.

I'm at a loss for you and thread will stay alive for others but do suggest you find an RV specific place/site for what folks do that want what you want in a Motor Home and for how you'll use it,

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Re: 3 Zone Motor Home A/C

Postby cornbinder89 » Wed Feb 22, 2017 3:05 am

1st and formost, you need a receiver capable of storing all the refrigerant that the system can hold. So if the system holds 5 lbs you need to size the receiver volume enough to store 5 lbs of liquid refrigerant. This is what prevents pressure problems when only useing one or two zones. The problem I see with the sprag clutch arraignment is it needs to be lubed somehow, sprag clutches can't run dry.
If all the "zones" evaporators are going to run approx the same temp when in use, than no special valveing is needed. A solonoid valve ahead of the Tx on the evaporator to close off the zone when not in use is all that is needed, this is how it is done on sleeper units in trucks.
The big drawback with "zones" is all the "extra" refrigerant that is not needed when running only 1 or 2 evaporator must be stored as high pressure liquid in the receiver. Long and short of it is it takes HP to keep the pressure high enough to keep the extra stuff in liquid form. This means there will be little gained in terms of efficiency by shutting down one evaporator. This is where separate roof mounts can save energy, as each runs as a its own system, so when only one is in use it takes 1/3 the power of 3 units.
Most Coaches I have worked on had a large condenser with multiple fans located above the engine compartment, but I know that some MCI's had a special "bay" in the luggage area for the condenser, what ever you use, Size matters, and any skimping on the condenser or the airflow to it, will really cut into the cooling.
There are some refrigeration units for trailers that have both a diesel and electric motor to drive the compressor, so the diesel can be shut down when 3 phase is available but I haven't looked into how the solve the "clutching" problem.
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Re: 3 Zone Motor Home A/C

Postby cornbinder89 » Wed Feb 22, 2017 3:10 am

Oh, and I forgot to mention most coaches are NOT 12 volt but 24 volt, they may have a 12 volt supply to run common 12 volt accessories (and even the headlights, if they are sealed beam) but the rest of the coach will be 24 volt, which is good and bad. It means the amperage will be less but things like blower motors are less common, you'll just have to do a little more digging to find them, and most likely be a special order, not in stock at the local truck parts supply.
Another problem that I have wrestled with, when you build a custom system, it is very hard to get the amount of refrigerant correct. I have found no guidance on how to determine amount needed, so have been reduced to trial and error, starting low and working up. The hard part is hitting the right amount without going over, Keep good notes as you go so you know how much you put in when you finely get it right.
I am sure a engineer with a specialty in thermodynamics can mathematically determine the amount, but I've never come across anything that told me how.
Oil capacity is another thing that you have to determine, but IIRC that compressor has a sightglass for the oil level, so you can run the system and keep checking and adding oil until the level in the compressor stays within the safe range. Oil in an A/C system gets carried thruout the whole system, and if you change the volume, more oil may be needed to keep from starving the compressor.
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Re: 3 Zone Motor Home A/C

Postby cornbinder89 » Wed Feb 22, 2017 4:01 am

More off the wall, and outside the box thinking:
In Australia and the Pacific rim, 3 phase VFD (variable frequency drive) window units are becoming the "norm". On these the compressor speed is varied to match the heat load. The alternating current mains is rectified to DC and then inverted to 3 phase A/C but not at a constant frequency.
SO,You likely have 275-300 amp 24 volt oil cooled alternator on the coach (Delco DN50). What if you could design 3 small 3 phase VFD units? They wouldn't have to be roof mounts, but you would need 3 smaller condensers. The advantage of 3 smaller independent systems is the efficiency I mentioned above. The VFD means you turn the compressor just fast enough to match the evaporator heat load and no faster, this means no storing excess refrigerant at pressure and no cycling of the compressor, so the load on the alternator or generator would be less.
3 phase compressors are fairly small and widely available in hermetically sealed units, and have the added benefit that many are scroll type compressors that are easyer to start than piston types.
This is getting out of my area of knowledge, but in theory is all possible.
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