Heat transmitted in convection from a pipe

[Karol]

Homework Statement

A vertical steam pipe of outside diameter 7.5 cm and height 4 m has its outer surface at the constant temperature of 95⁰C. The surrounding air is at atmospheric pressure and at 20⁰C.
How much heat is delivered to the air by natural convection in 1 hr?

Homework Equations

$$\mbox{The heat convection current H: }H=hA\,\Delta t$$
$$\mbox{Coefficient of naural convection in air at atmospheric pressure for a vertical pipe (diameter D): }h=1.00\times10^{-4\left(\frac{\Delta t}{D}\right)^{1/4}}$$

The Attempt at a Solution

$$h=1.00\times10^{-4\left(\frac{75}{7.5}\right)^{1/4}=0.00018$$
$$H=0.00018\cdot\frac{\pi\cdot 7.5^2}{4}400\cdot 75=235.7\left[Cal/Sec\right]=848,469\left[Cal/hr\right]$$
The answer, according to the book, should be 454,000.
If this isn't the place for this subject, please guide me to another sight that deals with those kind of problems.

 

[rock.freak667]

I see you have '400' in your final line. I don't think that belongs there, else you are essentially multiplying by volume and not cross-sectional area 'A'.

 

[cmpersson]

I have a similar question. I'm trying to determine the heat loss in our steam pipes both outside exposed to the wind(forced convection) and inside our plant(free convection). I'm having some trouble in determining the heat transfer coefficient, h for both situations. When I calculate the Reynold's number for the wind blowing on the pipes, its so large that I can't use any of the normal equations to find the Nusselt number. The equations I'm using are
Nu = hl/k where h is heat transfer coefficient, l is a characteristic length (I'm using the diameter of the pipe and insulation), and k is the thermal conductivity of the insulation.
and
Nu = .023Re^.8 * Pr^.3 this range is valid for approx 100< Re < 50,000
My Re is 300,000+

Suggestions?
Thanks

 

[rock.freak667]

I have a similar question. I'm trying to determine the heat loss in our steam pipes both outside exposed to the wind(forced convection) and inside our plant(free convection). I'm having some trouble in determining the heat transfer coefficient, h for both situations. When I calculate the Reynold's number for the wind blowing on the pipes, its so large that I can't use any of the normal equations to find the Nusselt number. The equations I'm using are
Nu = hl/k where h is heat transfer coefficient, l is a characteristic length (I'm using the diameter of the pipe and insulation), and k is the thermal conductivity of the insulation.
and
Nu = .023Re^.8 * Pr^.3 this range is valid for approx 100< Re < 50,000
My Re is 300,000+

Suggestions?
Thanks

you should start your own thread, but your Re is outside the range for that equation, thus it cannot be used for your problem.