Utilising on board aircon with Genset option.

[Sitec]

Hi all. Firstly, mods if there is a better location for this, please feel free to relocate it. Sooo, I have a thread going in the Unimog and MB section, and am about to start my truck build. I have a Mercedes 1222A 4x4 truck, and I have also purchased an ex mine truck/bus body. The body needs modification to suit my needs, but it's the right length and cost a lot less than it would to build one from scratch. The body (front pictured) came with a huge air conditioning system. I would like to be able to utilise it when we are stationary and the truck engine is not running. I have a large Honda EU70is generator (complete overkill I know, but it is also used to run the house when power goes out and in the event of bush fires etc, so is always on hand). I have space on/under the truck to carry it, and would like to devise a system where I can run the inbuilt aircon by means of a motor driven by the Genset.... Has anyone any ideas, input or suggestions? You all probably think I'm mad, but it does have the tendency to get a little hot here in Oz in summer. We don't plan to be in remote bush in the height of summer (48 deg +), but we know that even on a 35 deg day it gets pretty hot in an aluminium clad metal box! Thoughts and suggestions please. Cheers, Simon.

 

[luthj]

The built in AC has an engine driven compressor from what I see.

So your options are thus.

1: move compressor and drive it with an AC electric motor via belt drive.
2: Add a second electrical compressor in parallel with engine unit. You would possibly need one way valves depending on compressor designs.
3: Remove engine compressor and replace with 220v electrical compressor.
4: Rip the whole thing out, and install a bone stock home split unit.

The engine driven compressor is not going to be very efficient compared to a home/stationary hermetically sealed unit (electric driven).

Appropriately sized electric compressors are not terribly expensive, and should be available form china direct, or from a local HVAC equipment supplier. You will need to incorporate the high/low pressure cut-out switches into the compressors control circuit (they normally control the clutch for the engine driven unit). An appropriate oil will need to be selected that is compatible with the new compressor and refrigerant used. The system likely currently has 134A, and that should be fine for most compressors. If the system can handle a higher pressure, many home/stationary compressors can create the much higher pressures used with high efficiency refrigerants. Though this is not necessary, especially for a system that is not reverse cycle capable (heat pump).

With a compressor replacement, you will need someone to fabricate a few lines, and possibly weld/braze some custom adapters. Nothing difficult for a skilled HVAC tech (commercial techs are often more experienced that home install guys). The AC lines can have the rubber hoses removed and new length units crimped on. With a few tools you can do this yourself.

 

[Hillbilly Heaven]

What about removing what is there and replacing it with a 220 volt split unit. You could mount the outside unit where the old system is and the inside unit wherever you want.

 

[dwh]

Um...230v in Oz , methinks.

I imagine that's an off-the-shelf unit. Probably from some company that also makes commercial reefer units for trucks and trailers. In the U.S. that'd be either Thermo-King or Carrier, in Oz...dunno.

But I bet you can get hold of the manufacturer and ask them. They may have exactly the 230v compressor you'd need.

 

[Ozrockrat]

Actually it’s 240 volt.

My suggestion is also to go with a mini split unit. Just make sure to vibration proof the tubes on the condenser and evaporator and make sure the compressor has adequate mounting rubbers.

We used these on mining equipment and they worked a treat. Also a lot cheaper than dedicated “equipment” units.

 

[Sitec]

Thanks all. the engine driven compressor is huge, and good enough to keep the body reasonably cool even on 40 deg days. Once I have modified the floor to suit my needs it will all be well insulated to minimize thermal bridging etc so the AC should be more effective. We prob wont have a huge need for AC at night but it would be a 'nice to have'. With what I already have, setting up a motor to run the existing compressor would be the most cost effective and simplest approach. There are a lot of Mine 'dongers' (living quarters) which have household units as suggested.. Just noot sure how they'll cope with being hammered on corrugated washboard roads...

 

[luthj]

There are two main concerns with fitting an electric compressor.

First, the compressor you have may have a refrigerant control valve. There are two type of this valve. An internal mechanical kind, and an external electrical kind. Compressors with these RCVs are variable displacement, and adjust the stroke based on either low side pressure, or a control signal from the engine. Variable displacement compressors work in conjunction with the temperature expansion valve (TXV) to reduce power consumption and providing consistent outlet vent temperatures. If you are fitting a constant displacement electrical compressor (as opposed to a belt drive for your current unit), you will need to evaluate if it will perform properly. It probably will, but its something to consider.

The other type of compressor is called a fixed displacement unit, not very common on OEM designed systems after 2000, these units have constant volume, and rely on the clutch to cut in/out at low cooling load. If you have a fixed displacement unit, you don't need to worry about what compressor you use (other than it needs to be sized appropriately).

If the compressor has an electrical external RCV, then you need to evaluate how you will control evap temps, as the system may not properly prevent freeze up without the electronic feedback.

Mostly likely the system you have uses the TCV and RCV to control compressor load and evap temps mechanically, as this is the simplest and most reliable method.


If you go the belt drive from electric motor route, you will need to choose an operating RPM, assuming an engine RPM a bit off idle (1000-1500RPM) you can calculate the compressor speed from the pulley ratios. You will want the lowest speed that meets your cooling needs, as that will keep energy usage down, and extend compressor life. You may need to fit a small flywheel to the motor depending on the compressors pull-down when the clutch engages.

It may be easier to disable the clutch (assuming the compressor has one, some don't), and instead use the clutch control signal to switch the motor on/off as needed. A properly sized motor with starter capacitor should be able to start against the compressors load, even with a decent pressure differential.