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

"Daniel J. Stern" > wrote in message
n.umich.edu...
> On Thu, 23 Sep 2004, motsco_ _ wrote:
>
> > An air conditioner generates more HEAT than COLD
>
> An air conditioner generates neither heat nor cold. All it does is *move*
> heat.
>
>

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

On Fri, 24 Sep 2004, Kathy and Erich Coiner wrote:

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

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

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

> 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?

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.

On Thu, 23 Sep 2004 18:05:32 -0400, Threeducks > wrote:

>Daniel J. Stern wrote:
>> On Thu, 23 Sep 2004, motsco_ _ wrote:
>>
>>
>>>An air conditioner generates more HEAT than COLD
>>
>>
>> An air conditioner generates neither heat nor cold. All it does is *move*
>> heat.
>>
>
>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.

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! :-)








--
-john
wide-open at throttle dot info

"Daniel J. Stern" > wrote in message in.umich.edu>...
> On Fri, 24 Sep 2004, Kathy and Erich Coiner wrote:
>
> > > An air conditioner generates neither heat nor cold. All it does is *move*
> > > heat.
>
> > But it moves the energy at less than 100% efficiency. That means there is
> > some extra work that shows up as heat.
>
> Sure, but the fact remains, an aircon does not "generate" heat or cold.
>
> > 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?
>
> 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.

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.

Daniel J. Stern wrote:
> On Fri, 24 Sep 2004, Kathy and Erich Coiner wrote:
>
>
>>>An air conditioner generates neither heat nor cold. All it does is *move*
>>>heat.
>
>
>>But it moves the energy at less than 100% efficiency. That means there is
>>some extra work that shows up as heat.
>
>
> Sure, but the fact remains, an aircon does not "generate" heat or cold.
>
>
>>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?
>
>
> 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.

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.

~^Johnny^~ wrote:
> On Thu, 23 Sep 2004 18:05:32 -0400, Threeducks > wrote:
>
>
>>Daniel J. Stern wrote:
>>
>>>On Thu, 23 Sep 2004, motsco_ _ wrote:
>>>
>>>
>>>
>>>>An air conditioner generates more HEAT than COLD
>>>
>>>
>>>An air conditioner generates neither heat nor cold. All it does is *move*
>>>heat.
>>>
>>
>>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.
>
>
> You turkey! Heat *is* energy.

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

>
> It is measured in BTUs, Calories, Joules, etc.

So?

> Specifically (no pun intended), heat is kinetic energy.

No it isn't.

> It can be converted to potential energy (storage).
> It can be transmitted (conduction, convection, radiation)
> or it can be converted (storage again).

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).

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

No, it moves energy. It does not exchange kinetic energy. There is no
kinetic energy in 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! :-)

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

Im anonymous wrote:
> "Daniel J. Stern" > wrote in message in.umich.edu>...
>
>>On Fri, 24 Sep 2004, Kathy and Erich Coiner wrote:
>>
>>
>>>>An air conditioner generates neither heat nor cold. All it does is *move*
>>>>heat.
>>
>>
>>
>>>But it moves the energy at less than 100% efficiency. That means there is
>>>some extra work that shows up as heat.
>>
>>Sure, but the fact remains, an aircon does not "generate" heat or cold.
>>
>>
>>>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?
>>
>>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.
>
>
> 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.

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

On Fri, 24 Sep 2004, Im anonymous wrote:

> > 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.

> No


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

Try again?

(Im anonymous) wrote in message . com>...
> "Daniel J. Stern" > wrote in message in.umich.edu>...
> > On Fri, 24 Sep 2004, Kathy and Erich Coiner wrote:
> >
> > > > An air conditioner generates neither heat nor cold. All it does is *move*
> > > > heat.
>
> > > But it moves the energy at less than 100% efficiency. That means there is
> > > some extra work that shows up as heat.
> >
> > Sure, but the fact remains, an aircon does not "generate" heat or cold.
> >
> > > 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?
> >
> > 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.
>
> 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.

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

"Daniel J. Stern" > wrote in message
n.umich.edu...
| 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...