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Heat transfer through foam.
3

Heat transfer through foam.

Heat transfer through foam.

(OP)
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?

RE: Heat transfer through foam.

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

RE: Heat transfer through foam.

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

RE: Heat transfer through foam.

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"

RE: Heat transfer through foam.

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

RE: Heat transfer through foam.

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



RE: Heat transfer through foam.

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.

RE: Heat transfer through foam.

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

RE: Heat transfer through foam.

i think willard3 may be correct - homework problem.

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

-pmover

RE: Heat transfer through foam.

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


RE: Heat transfer through foam.

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

RE: Heat transfer through foam.

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"

RE: Heat transfer through foam.

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

RE: Heat transfer through foam.

(OP)
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.

RE: Heat transfer through foam.

2
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

RE: Heat transfer through foam.

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

 

RE: Heat transfer through foam.

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

I2I

RE: Heat transfer through foam.

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"

RE: Heat transfer through foam.

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



RE: Heat transfer through foam.

sailoday

A typical car heater/defroster blows hot air into the passenger compartment.  The heating (and cooling in the summer) is done in the engine compartment. The two compartments are separate (usually by a metal wall). The piping for heating system does not directly interact with the passenger.

In so far as "providing constructive input on why an idea may not work," I obviously mistakingly thought that providing input as to the dangers and liabilities associated with having someone sit on a seat with a pipe under him/her and behind his/her legs holding 180°F - 196°F (82°C - 91°C) was sufficient.  Perhaps my wording was a mite too terse to properly get the concept across that even a minor pinhole leak inside the passenger compartment could release that hot fluid directly onto the driver/passenger or that even a minute quanity of fluid at those temperatures might cause damage to human skin, much less distract the driver leading to an accident.

Furthermore, because it appears that the hot fluid piping would be encased in foam, there is a danger that the fluid might be adsorbed by the foam, depriving the engine of coolant while also causing the above-mentioned damage.

Additionally, because of the possibility of an automobile being involved in an accident, there is a danger of a coolant pipe being severed (especially due to sudden seat movement.) This could lead to a large quantity of extremely hot fluid being suddenly released unto the human occupants of the car.  This could result in 2nd or 3rd degree burns.

I should also point out that, at least, the American Justice system has taken a dim view of manufacturer's whose products end up hurting people, especially when such hurt could be avoided through a reasonable design.

Finally, and to interject a somewhat humorous note:  It's not my bottom that gets cold, it's my feet! Perhaps kmasine would do better to turn his inventive gaze onto a way of running all that engine coolant underneath the automobile to heat the underside and provide a warm floorboard.

Patricia Lougheed

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

RE: Heat transfer through foam.

Car seats have to move, so flexible hose is required.  Break in hose means passengers get sprayed with scalding coolant, real nice.

TTFN



RE: Heat transfer through foam.

vpl

i agree with 99.9% of what you say.

but my wife really really likes her car seat warmers.

magicme

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

RE: Heat transfer through foam.

Patricia,

Au contraire.  At least not in a Ford.  The heater core is easily accessible from the inside of the passenger compartment, and luckily so.  Fords are notorious for leaking heater cores. (in my opinion.)  If it leaks, it leaks right onto the carpet in the passenger foot well.  I've replaced several.  They are not hard to get to.

rmw

PS: the air conditioning evaporator is also inside the 'airspace' of the passenger compartment meaning that if it leaks, freon enters the passenger air.  Those systems are not isolated from the passenger area.

RE: Heat transfer through foam.

I believe car seat heaters are electrical resistance wires embedded within silicone rubber: Flexible and instant-on (don't have to wait for thermal mass of cold coolant & piping to warm), plus have a temperature sensor to prevent overheating.

RE: Heat transfer through foam.

kmasine:
I believe that you already understood how potentialy dangerous your idea is.
This should give you an indication which way your idea should take...

RE: Heat transfer through foam.

The AIAA paper covers seat cooling, but on page 5 a figure shows a thin electric 'Heater mat construction' just below the top fabric.

See some basic electric heater details in 'Thermal Simulation of Textile Heating, e.g. Car Seat' here:
http://www.ltnt.ethz.ch/teaching/Master-Sc-LTNT.pdf

You can buy a 'GM & Mopar Licensed Car Seat Heater Kit - Model Specific' here: http://www.sportsimportsltd.com/heatelcarsea.html
Some details:
"Power requirements vary by kit and range between 51W (3.7A @ 13.8V) & 84W (6.1A @ 13.8V).
Temperature: High - 113 to 119 degree F
Temperature: Medium - 110-116 degree F
Temperature: Low - 97 to 103 degree F
Heating Element: .25" thick Polyester Foam w/Nylon Scrim"

Why anyone would want to sit on a rigid seat containing a cold metal-coolant heat sink that doesn't heat up until the engine warms is beyong the understanding of the OEM's & myself.

 

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