1. May 10, 2010

febbie22

Hey ive got this question and the answer that is giving is 219.12 but i keep getting 319 could someone please help me out heres the question.

Consider a person standing in a room. The room air temperature is
maintained at 22OC at all times. The inner surface of the walls, floors and
the ceiling of the room are observed to be at an average temperature of
10OC.
Determine the total rate of heat transfer from this person if the exposed
surface area and the average outer surface temperature of the person
are 1.4 m2 and 30OC. Take the convective heat transfer coefficient to be
6 W/m2 OC and the emissivity of the human skin to be 0.95

any help would be great

2. May 10, 2010

Mapes

Can you please show your calculations? I get 219.13 W without any problems. Make sure to check your units.

3. May 10, 2010

febbie22

cheers for replying here what i done:

Q radiation - 0.95 * 5.67*10-8 * 1.4 * ((273+30)^4 -(273+10)^4) = 151.9

Q convection - 6 * 1.4 * (30-10) = 168

Qtotal = 319.9

god knows what im doing wrong probably something really stupid, how did you do it

cheers

4. May 10, 2010

Mapes

Is convective heat transferred from the person directly to the walls?

5. May 10, 2010

febbie22

i'm not really sure to be honest, how did you get 219

6. May 10, 2010

Mapes

Convection is defined as heat transfer via the movement of a fluid (like air). So I'm not sure why you'd have the temperature difference that drives convection be (30-10)°C when 10°C is the temperature of the walls. Know what I mean?

7. May 10, 2010

febbie22

yeah i know what you mean i actually used the 220C in the equation but the final answer was coming smaller than the actual answer so i tried the 100C