I've been looking at this for 4-5 years now.. ever since the first time I drove the deuce from Dallas to Austin in July. These are my opinions on ths 'problem'.
First, I'd like to comment about the ice chest and fan combo. The amount of cooling needed for a space is expressed in BTU per hour. 12,000 BTU/Hr is called a "ton". This means it is equivalent to the amount of heat absorbed by melting a ton of ice to water. So, we can see that 50 LBs of ice in a cooler with a fan is going to make 300 BTU. Not per hour, that is total, then the ice is gone and you are just humidifying with water.
Next, the kind you have to add water to. This is a "swamp cooler" or water-evaporator and won't work well when the humidity is high because it adds humidity to the cooled space.
The unmodified deuce cab, by my calcs, takes about 3 tons to get it to 65 degrees in 100 degree weather at 55MPH (engine heat blast) assuming every hole is sealed. Adding 3/4" rubber foam insulation (Nomaco K-flex) on the floor and firewall, hardtop ceiling, and rear of the cab, as well as on the doors (inside them is possible?) could reduce this enough to hit 70 with 1.5-2 tons. Memphis equipment uses this material for insulation inside the cab. Major air leaks difficult to seal would be at the roill-down windows where they 'seal' to the top frame and the doors. Memphis could be considered somewhat of an expert at M35 cab cooling, because they do it for money. (they farm it out to a local custom A/C shop actually)
One must also be very diligent when looking at BTU claims not just for a self contained A/C unit, but for the pieces one might install to make a system. Thisis because there are standards and not all makers adhere to them. Sort of like the old hi-fi stereo ratings of "RMS power" and "music power".
I took this from Kysor's pages, but niether Kysor nor Red Dot seem to offer an all-electric A/C unit, meaning a compressor has to be mounted on the engne. DCAirCo does, but it is only 9000 BTU.
"Kysor’s published BTU ratings are based upon IMACA Standard 200. This standard was established to provide uniform methods, instrumentation, facilities, test procedures, and methods of calculations for conducting tests on vehicle air conditioners to determine their capacity and related performance. Kysor adheres to the purpose of IMACA 200. However, some competitors choose to list their ratings based upon other criteria. One such measure is 80°F wet bulb and a 36°F refrigerant. Based upon that method we have restated our performance ratings by unit in the right hand column below. Always attempt to determine how a competitive system is being rated prior to making any comparison. The IMACA rating should be used for condenser and compressor sizing."
I was considering using two self contained roof mounted units side by side to stay cool in Texas. Most take a full 25A. That leaves me with 10A reserve on a 60A alternator. This is why I have asked about more amps from an alternator in other threads. The units are pretty wide. How wide is the M35 hardtop? How much weight can it safely support without bending/cratering?
I found one model of totally self contained unit of the 24VDC 25A type. It is 9000BTU, so two would be needed in hot climates. They are 30" wide and have side vents, so side by side mounting could cause issues. This is the above mentioned DCAirCo DC9000 dc powered air conditioner.
I found a larger capacity one in the past, 13500BTU. - link is lost or gone..
There are separate DC-operated components available like a condensing unit which has to be mounted somewhere. One is the "Renegade Auto Cool".
From work of Devilman we also know that an A/C condenser cannot be put in front of the M35's radiator because it will dump too much heat there and overheat the truck. Devilman built up a custom truck with A/C - that would be a good thread to see.
Some rooftop units that include a condensing unit dump the heat out the back. If you have an M109 or an M35 with a shelter, there is alot of wind pressure behind the cab that could oppose the fan-forced airflow.
Some roof top units are designed to pressurize the cab to avoid sucking in dust, but any time you do that you are also blowing hard-earned cold air out any crack or hole.
How much do those 9000 BTU 24V A/C's cost? I did not see prices, which could mean they are high. It could be cheaper to use a big RV one and a sinewave inverter over-rated for motor starting. A 120V 10 Amp coleman 13500BTU model is about $650 1200W - that is right at 50 amps at 24V assuming a perfect inverter. That is alot of continuous amps from a 60A alternator.
In the end analysis, the most practical way to get lots on BTUs out of the cab is probably to:
1.) Mount the biggest compressor (displacement) you can find on the engine. This is important because the M35 engine turns slow unlike a 3000RPM cruising speed car engine, so a small compressor won't give as much cooling, especially while running 2000RPM and the like. Having the smallest available compressor pulley could help this.
2.) Mount a condenser under the truck or bed or somewhere like the passenger fender so that it will get lots of airflow.
3.) Use a roof top evaporator/cooling unit only if necessary. If you remove the passenger seat and install another driver seat, a large evap unit could fit between the seats leaving the roof clear (for RADARs, gun mount rings, etc..)
4.) Buy large, like 25,000 to 36,000 BTU and use an evap. with 400+ CFM. You can always turn down the A/C if it gets too cold, but you can't increase its mechanical capacity if it is not cold enough, especially when sitting idling at 800RPM in traffic in August.
Sorry to ramble on, this is just my opinion having had alot of hands-on with air conditioners in general including building two 120V units from piece-parts and having worked at the theory and engineering end of this M35 cab cooling problem for a few years. The fact that I have not yet installed an A/C in the cab means I'm still not satisfied with available DC powered solutions. But I'm pretty tired of waiting. It's a shame the military always seems to focus on putting heaters everywhere, but seldom considers air conditioning! Ask any soldier who's been to Iraq about that one.