Heat transfer through foam. 3

[kmasine]

I currently have a design to heat an automobile seat by placing a copper coil under the foam of the seat and piping hot engine coolant through the coil. How can I model the amount of heat transfer to the top of the seat if the bottom and sides of the coil are insulated so the heat can only go up. I realize I'll need to know the conduction coefficient of the seat foam and the flowrate, but how do I go about calculating the heat transfered upward?

 

[willard3]

Sounds like homework to me...........

 

[reidh]

Measure the flowrate of the coolant, and the coolant temperature into and out of the seat. You can then calculate heat transfer Q:
Q=mdot*Cp*delta T

The heat transferred from the coolant must then go to the seat.


It can then be approximated by a simple heat transfer equation:

Qdot=-(kA*delta T)/delta X

-Reidh

 

[magicme]

adding a footnote to what reidh stated.....

the "deltaT" in the second equation would be the temperature difference between the hot fluid and the top of the seat.
which means you will have to regulate and measure these during the test, to back-calculate k for the seat.

regards

magicme

------------------------------------
"not all that glitters is gold"

 

[finder1]

What if a test is not possible or you want to determine K by analysis. How would you do that?

 

[IRstuff]

A good book on heat transfer for electronics applications will include a section on spreading resistance.

I would, however, question your assertion that the coil is insulated on the sides and bottom, since the insulation is probably not that different than the urethane foam between the coil and the seat cover.

TTFN

 

[vpl]

Are you serious about piping hot engine coolant through the seat? This sounds dangerous. What are you doing to prevent a leak developing in the piping? You could end up scalding your driver and causing a serious accident.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[sailoday28]

Don't forget that someone will be sitting on the seat, compressing some of the foam. You will have to include the contact resistance of the person and the cushion.
With good contact, I would assume the surface temp to be that of the person's rear end.

Regards

 

[pmover]

i think willard3 may be correct - homework problem.

then again, i have heard of the cooled/heated seats in the newer luxurymobiles.

-pmover

 

[Darken99]

Kmasine,

Not that this design won't work, but you might want to investigate how long it takes engine coolant to get hot enough to be effective. Engine coolant doesn't start flowing until the temperature hits 82-91 C. If it is cold out, it would take much longer for your coolant to get to a sufficient temperature. Also there is a large deviation between the coldest and hottest coolant temperatures. Since you would have to design based on the hottest coolant temperature. Does the coolant get warm enough at -30 Degrees when you are removing the coolant from storage to warm your seat.

Regards

 

[reidh]

VPL,

I agree it is dangerous to plumb coolant through the passenger compartment. However, compared with the guy who invented/suggested the front windshield defroster that piped exhaust gas up the front of your windshield, the seat heater is incredibly safe and well thought out (if only the internet could capture my sarcasm).

-Reidh

 

[magicme]

on second thought.... my wife's car has electric seat heaters.... best thing since....well, since seat heaters.
these must be a simpler design than piping coolant around the passenger compartment.

<?> .... exhaust gases to heat the windshield .... bet the test dummies are dead and gone.

magicme

------------------------------------
"not all that glitters is gold"

 

[Darken99]

http://www.coolorheat.com/newsreleases.html

They use a stand alone fluid system in foam to heat & cool their seat covers.

 

[reidh]

Darken99,

Your statement "Engine coolant doesn't start flowing until the temperature hits 82-91 C" is a bit misleading. Assuming a mechanical water pump, the coolant is flowing as long as the engine is turning (also assuming a properly primed system). The heater core on your car is always getting coolant flow, which is why it can heat up before the thermostat opens.

-Reidh

 

[kmasine]

I have to say most of the concerns and comments that everyone has come up with are valid and things I have already thought of. The coolant would be routed like stated earlier, directly from the block or heads (depending on vehicle) and back to the pump just like a heater core. The thermostat would be bypassed until it opens.

I also must agree with the fact that the seat foam will be similar in k value to the surrounding insulation. However, with compressed foam and good insulation the heat should find the smallest resistance through the foam.

 

[MikeHalloran]

After you figured out for yourself that someone is going to get burned, as has been pointed out here too politely, why did you continue to pursue this dumb idea?

It's a _guaranteed_ lawsuit, and a guaranteed slam- dunk for the plaintiff.

Not to mention the criminal penalties.











Mike Halloran
Pembroke Pines, FL, USA

 

[sailoday28]

vpl (Nuclear)
reidh (Automotive)
loran (Mechanical)
I believe that a typical car heater/defroster is in the passenger compartment. Coolant is the source of heat.

Why then, is this (dumb???)idea more dangerous?

Let's give constructive input on why an idea may not work.

Regards

 

[insult2injury]

If the engine coolant leaks in heater/defroster, does it come into to contact with the passengers?

I2I

 

[magicme]

okay, i'll list my anxiety issues with this concept.

1. the problem already has an elegant solution .... electric heaters .... why re-solve this problem?
2. troublesome details piping hot fluid from engine to circulate around movable passenger seats. spring a leak in a movable joint and you loose engine coolant PLUS you get hot fluid squitring in the passenger compartment.
3. how to regulate the temperature? valves to each seat ?

i'll stop there

magicme

------------------------------------
"not all that glitters is gold"

 

[IRstuff]

The seats in my car have about a 4 inch adjustment range. This means that the coolant requires a flexible connection inside the passenger compartment. If that breaks, you get your passengers sprayed with scalding coolant, which should be quite exciting.

The liquid connection means that the seats cannot be removed without a hazardous material cleanup.

Any puncture damage to the seat can disable your car as well as dump hazardous liquid into the passenger comparment.

In order to get coolant that far, you need hose which either runs inside the compartment or outside. If inside, then it'll need to be insulated to prevent contact burns, and will chew up volume in the leg area. If outside, then you run the risk of it getting damaged from road debris and killing your car.

TTFN