How to temporary chill a car with non-working AC.

The A/C system in an `89 old Honda is completely dead. They
quoted a price of about $1500 to fix but the car's worth as much as
that. I like to put in a 700 watt 117V portable or standard home air
conditioner in the trunk but worry it may drain the battery. Is it
possible to make a difference by continuously pumping chilled
waters (filled with icicles) into the entire liquid line (tube) that runs
thru the evaporator?

Thanks

 

Find and get an automotive swamp cooler. They were popular before car A/C
became common.

 

Just carry a 450 oz. Slurpee!




____________________________________
Do not write below this line. Reserved for me.

 

Um.
That would be so much trouble, so messy, and not very effective. When
you say the system is "completely dead" - what is bad? if you can do the
labor, the parts won't cost too much. Changing a compressor (if thats
whats bad) is a matter of unbolting the a/c lines, taking the belt off,
and unbolting the housing.

 

I like to put in a 700 watt 117V portable or standard home air

You've GOT to be trolling. How long an entension cord do you have?

A car a/c unit is equivalent to cooling an entire house. A 700 watt
window unit isn't up to the job anyway.

JM

 

Well, first, I'd suggest shopping around on the repair of your car's
a/c. Depending on exactly what is wrong, you may get some quotes
below $1500. But on to the other ideas...

The kind of inverter you need to do justice to this application is
pretty big: in ballpark figures, I'd go with a 1500 W continuous/3000
W peak inverter of at least "modified sine wave" output quality ("pure
sine wave", i.e., what the power company gives you, would be even
better, at higher cost). The ante is probably six or seven hundred
dollars.

The issues are that (a) motors draw more at startup than in continuous
operation and (b) an inadequate inverter slowly murders them with
undervoltage, like operating at home in a brownout.

On the input side, we can do another finite-napkin model and figure
that after accounting for inefficiencies, you're using 1400 W, at 14
V, which comes out to 100 A. That's a lot of current; you'd want to
run a husky wire, fused at the battery end, like the kids with the
enormous stereos have to do. It also represents a very large
fraction of a typical car's alternator output. These arguments still
apply even if you look at things more closely and decide that my
safety margins are too large.

Finally, you have to expose the home air conditioner's coils to the
outside world or else you're defeating your own purpose by dumping
that excess heat into the cabin. And don't forget to support it very
solidly.

Without dragging out some rather dusty textbooks and a calculator, I
think that as a working fluid, this would be far, far less effective
than compressed freon. It would also eliminate any chance that any
part of your a/c system is salvageable. And you'd have to arrange a
pump.

You might be better off getting an ice chest and some of those quilted
blankets of "blue ice" and making cold packs for your body. This
would at least be tidier than what I had to do on a trip across the
Mojave when my air conditioner went on the blink -- an ice chest full
of ice water, into which I could dip a towel for my head and neck and
shoulders. It must have been a sight to behold, sort of Lawrence of
Arabia meets the Beverly Hillbillies.


You might be better off with a portable evaporative or "swamp" cooler
meant for use in the car -- see for instance
http://www.evaporative-cooling.com/evaporative-cooling_faq.html#anchor93613
These only work well if you are in a dry climate. For high humidity
environments, there are devices that blow air across an ice bed:
http://www.evaporative-cooling.com/istr1.html
Neither of these are anywhere near as effective as a compressor-type
air conditioner in good working order, but they do take the edge off
the heat, and their 12V power draw is reasonable.

Best of luck,
--Joe

 

=========================

I have a good air conditioner sitting in the middle of my garage . . . I
sometimes us it to HEAT the room on a chilly morning. An air conditioner
generates more HEAT than COLD, if the backside is not vented out a window.

Are you planning to cut the hole thru the top of the trunk, or the side
of the fender?

catnip.... is that what you've been smoking? Buy a bag of party ice and
sit it on the back seat, or your lap.

 

An air conditioner generates neither heat nor cold. All it does is *move*
heat.

 

An air conditioner generates neither heat nor cold. All it does is *move*
heat.>

Partially true. It also converts a lot of electricity into heat in the process
of "moving" the heat.

Quent

 

Actually, it doesn't move heat, either. It moves energy. Heat is just
how energy is transfered between the condenser (or evaporator) and the
rest of the world.

 

But it moves the energy at less than 100% efficiency. That means there is
some extra work that shows up as heat.

This is a thought problem we sometimes give to mechanical engineering job
applicants.

You have a closed,well insulated room with a refrigerator in it. It is
running and you hold the door open.
What happens to the air temp in the room?

Erich

 

Sure, but the fact remains, an aircon does not "generate" heat or cold.

Of course the ambient temp increases.

Now here's a fun one: It is common knowledge that there is no such thing
as a 100 percent efficient machine. Given that, consider an electric space
heater with a 20-foot electric cord and a high-speed motor-driven blower.
Is the efficiency of such a device 100 percent, or some lesser figure?
Support your answer.

 

You turkey! Heat *is* energy.

It is measured in BTUs, Calories, Joules, etc.
Specifically (no pun intended), heat is kinetic energy.
It can be converted to potential energy (storage).
It can be transmitted (conduction, convection, radiation)
or it can be converted (storage again).

A heat pump moves *heat*. It exchanges kinetic energy across a temperature
gradient using a reverse carnot process.

Coefficient of heating is always coefficient of cooling plus one.
Therefore, more heat is added than is removed during the process.

Homework questions:

1) Why is the coefficient of heating always equal to the coefficient of
cooling plus one? (hint: all losses occur during transitional phase
(less-than-ideal adiabatic compression, fluid transport, friction).

2) What is the maximum theoretical Carnot COP, and how does this relate to
temperature difference at the diabatic ends (input and output)?




NOTE:
Whatever you do, don't ask our resident troll Paul Milligan to do these
homework questions for you!

Why, he'll even flame you worse than I just did!

 

No, because some of the energy will be converted into light assuming
the elements glow, and the blower will convert some energy into heat,
but also some into sound waves. I assume the 20ft cord is inside the
heated area, as the resistance of the cord will contribute to the
heating.

So, at least because of the element glow, it'll be damn close to 100,
but now quite.

 

Efficiency in the thermodynamic sense relates work output to energy
input. So it's obvious that the efficiency must be less than zero.

Can you have a situation where the energy you input to the system be
equal to the energy output? Sure. You have the work output from the
motor, plus the heat generated by the motor. You also have the heat
generated by heating element. Add that up and one should expect to get
very close to the electricity input to the device. But that doesn't
make it 100% efficient. It just means that you were able to close the
energy balance.

 

A system does not contain "heat". Heat is simply a method of
transfering energy between the system and it's surroundings.

No it isn't.

You can't store "heat". You can store energy. You seem to have your
definitions all screwed up. "storage" is not potential energy.
Potential energy is mgh (mass*gravity*height above a reference plane).
Kinetic energy is energy due to motion of an object with respect to a
reference state.

In a refrigeration system, heat absorbed by the working fluid, which
results in an increase in the enthalpy of the fluid (not potential energy).

No, it moves energy. It does not exchange kinetic energy. There is no
kinetic energy in a "reverse carnot process".

You showing off your lack of thermodynamics knowledge is a flame? You
don't even know what potential energy is!

 

Closing the energy balance doesn't mean the efficiency is 100%.

 

Sorry, it's not a yes/no question.

Try again?

 

Assuming a substantial room without windows, the sound will vibrate
the walls, ceiling and floor, and be absorbed as heat. The motion of
the air will also be converted to heat by drag against the surfaces.
So, pretty much 100% is correct.

However, the power company lost some 30% of the electrical energy in
transmitting it to that room. Efficiency = 70%.

JM

 

| Find and get an automotive swamp cooler. They were popular before car A/C
| became common.
|
| On Wed, 22 Sep 2004, Burt Squareman wrote:
|
| > The A/C system in an `89 old Honda is completely dead. They
| > quoted a price of about $1500 to fix but the car's worth as much as
| > that. I like to put in a 700 watt 117V portable or standard home air
| > conditioner in the trunk but worry it may drain the battery. Is it
| > possible to make a difference by continuously pumping chilled
| > waters (filled with icicles) into the entire liquid line (tube) that runs
| > thru the evaporator?
| >
| > Thanks
| >
| >

Swamp coolers are relatively ineffective when the humidity is very high since
they use water evaporation process to cool the air down to the "web-bulb"
temperature. At 100% humidity...the wet-bulb temperature is essentially the
dame as ambient. But if you live in low-humidity areas (say the southwestern
US) it would work nicely...