From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: 120 Volt Air Conditioner for Truck or Car
Message-ID: <ph127vofs43ooo9053rg8n3j4u71ice11n@4ax.com>
Date: Thu, 13 Mar 2003 17:55:09 -0500

On Thu, 13 Mar 2003 12:43:06 -0600, clyde9ZZ@ZZpipeline.com wrote:

>I don't have a camper or RV but I'd like to be able to have air
>conditioning for situations where I'd pull over and sleep a few hours
>or overnight on the road in my car or truck. What I'm thinking of is
>either mounting the equipment in the truck bed or on a receiver hitch
>carrier behind the car. I would like to get one of those 5,200 BTU
>window air conditioners and hook it up to a Honda EU1000i generators.
>I don't need a generator for a refrigerator, lights, or TV, only the
>A/C.

Let me make sure I understand what you want to do.  You want to supply AC to
your car or truck while you're stopped.  Nothing to do with an RV.

Assuming that is your objective, my comments follow.

>
>My questions:
>
>1. Does the air conditioner have to PHYSICALLY be mounted either
>inside or partially inside the vehicle in order to get the thermostat
>to respond properly and enable the A/C to cycle on and off repeatedly.
>Reason for this is that if I mount it in the truck bed, I'd run a
>flexible 6" round duct to an opening in the truck or car. If it's
>mounted remote and the thermostat isn't in the car or truck, it seems
>that the compressor and fan would run continuously.

Most window units have the thermo bulb in front of the evaporator so that it
sees the returning air.  Assuming you plumb in a return line (a must if you
want this to work) there will be no problem.

>
>2. Presuming this all DOES work, will the small 5,200 BTU unit get the
>small space in the vehicle so cold as to be unbearable?  (these small
>units are built for a maximum room size of 10' X 15') Will I have to
>crack the window a bit to allow humidity to escape?

That isn't nearly enough capacity.  Car ACs start at about 2 tons (24,000 btu)
for small cars and go up.  The problem is all the greenhouse (glass) and the
lack of insulation.  If you want this to work in the daytime, I'd not look at
anything LESS than a 24kbtu unit.

>
>3. Most importantly, presuming the A/C unit cycles on and off, "how
>long will the unit run"? The unit is rated on their website at running
>on full load for about 4 hours and about 7 hours on partial load. Can
>anyone with any experience with this type of unit in either a boat or
>RV give me any info?

The answer is "awhile" :-)  That's like asking "how hot does it get".  It
depends.  You're not going to be able to do this with a 5kbtu unit so it's
time to look upward.

I don't think this is practical in a car unless you are willing to do some
major butchery and lose the use of your trunk so let's talk about pickups.

Let me suggest some alternatives.  The first alternative is an industrial spot
cooler.  This is a self-contained high capacity unit that is designed to
provide spot cooling in a hot factory environment.  It consists of a box about
the size of a console TV that contains all the AC stuff.  IT has one or two
outlets that receive flex hoses to conduct the air to the spot to be cooled.
The unit has enough capacity to take in hot ambient air and cool it so that no
return line is necessary.  These things are fairly expensive new but I see
them for sale used/surplus fairly often.  A search of the net and/or ebay will
probably turn up one.  You're probably looking at at least a 3kw genny to run
one of these and possibly more.

The second alternative is to parallel the existing car AC compressor with an
electric compressor and then use your truck's existing AC while stopped.
Install an RV converter to supply the necessary 12 volts for the fan and
controls.  There are several approaches that I can think of.

One, mount a 2-2.5 ton R12 (r134a) hermetic compressor somewhere convenient
and plumb it in parallel to the engine driven compressor.  This is exactly the
approach used by Grumman for a "roach coach" they built for the Air Force.  I
bought one surplus.  When underway the engine compressor drove the AC and
refrigeration.  When stopped and plugged in (or on generator) a hermetic
compressor took over.

Two, obtain one of the electric AC compressors designed for electric vehicles.
These generally pack more capacity into a smaller package and are more
expensive.  Googling around the web should turn up some suppliers.

There is a third option.  I've seen this done once.  machine a second pulley
to attach to the compressor side of the clutch on the existing compressor.
Belt this into an electric motor mounted near the compressor.  Install an
overrun clutch on the motor.  When the engine is running the compressor is
driven through the electric clutch.  The belt and motor pulley spin but the
motor does not thanks to the overrun clutch.  When the engine is off and the
electric clutch is disengaged, the electric motor drives the compressor
directly.

The person who did this was a machinist and had a machine shop in his
basement.  I'm sure you could hire it done but the cost would probably be
fairly high.

yet another option dispenses with the generator completely.  Simply mount a
small gas engine, a small alternator (for the fan) and a second automotive
compressor on a baseplate, put it in the bed of the truck and parallel it in
with the existing compressor.  This would be the most straightforward and
cheapest method.  Say, $300 for a compressor, maybe $60 for a small alternator
and about $350 for a good OHV Honda engine plus maybe $100 in misc materials.
You're still well under a kilobuck.  If you put quick connects on the
refrigerant and fuel lines, you can even remove the cooling unit from the
truck when you don't need it.

If I had a client ask me to supply cooling to a truck cab today, I'd go with
the engine driven compressor.  Very easy and straightforward to install.  You
don't even need a thermostat - just tie into the existing clutch control
circuit.  If it had to be electric, I'd go with the hermetic unit.

John

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: 120 Volt Air Conditioner for Truck or Car
Message-ID: <p6r57vsi7r4s2f6c5oaeiqbtp60arjtsdr@4ax.com>
Date: Sat, 15 Mar 2003 04:17:49 -0500

On Thu, 13 Mar 2003 18:26:41 -0500, "Captain Dondo"
<captain@thebigsandbox.com> wrote:

>On Thu, 13 Mar 2003 17:55:09 +0000, Neon John wrote:
>
>> One, mount a 2-2.5 ton R12 (r134a) hermetic compressor somewhere
>> convenient and plumb it in parallel to the engine driven compressor.
>> This is exactly the approach used by Grumman for a "roach coach" they
>> built for the Air Force.  I bought one surplus.  When underway the
>> engine compressor drove the AC and refrigeration.  When stopped and
>> plugged in (or on generator) a hermetic compressor took over.
>>
>>
>
>You don't still happen to have that setup do you?

Wish I did.  Someone wanted it a LOT worse than I did at the time.

>
>That's exactly what I've been trying to figure out.  Saves space, it's
>fairly $$$ but it uses almost all of the existing stuff.  Wiring would be
>a bitch - you'd have to separate the fans that run off the starting
>battery and run them from the house battery.  Mostly I can't find a
>hermetic compressor that would work in that application.
>
>I'd love to see how it was done - even a parts list.

This one was trivially simple.  All fans were 12 volts with a cab-over
external condenser mounted on the roof. No condenser in front of the radiator.
As I understand it, these coaches were used to serve food to airmen on the
flight lines, thus the engine was always running.  Didn't want the extra heat
load on the engine cooling system while idling.

RV type converter to run the fans while on shore power. Liquid and suction
lines simply split to go to the various refrigeration loads.  The hermetic
compressor was simply teed into the suction and discharge lines of the vehicle
compressor.  I believe there were check valves in the lines but not sure.  The
only real requirement of the compressor is that it be able to start against
full head pressure.  Any compressor designed to work in an expansion valve
cycle will do.  Add a kickstarter to make sure.

The compressors (both) were controlled by suction pressure.  There was a small
suction accumulator to help prevent short cycling at light load.  Each
individual cooling load had its own thermostat and solenoid valve in the
liquid line.

A DPDT relay with the coil hooked to the shore power supply did the automatic
transfer from engine to hermetic compressor transfer.

this was all refrigeration/AC with no freezer loads.  If there was a freezer
load, then a back pressure regulator valve would have been necessary in the
suction lines from the refrigeration/AC loads to prevent frosting.

Since this unit was not designed to be autonomous on 12 volts like an RV,
there was no need to try and separate the wiring.  Nothing worked unless the
engine was running or shore power was available.

If I was building such a rig now, I'd use 120 VAC high efficiency AC ball
bearing fans and run them on inverter(s).  Better efficiency and longer
service life than the brushed motors used on this thing.