Heat Loss from an Uninsulated Pipe
Heat Loss from an Uninsulated Pipe
(OP)
Dear All,
Need your help..
I've a got situation where I need to condense and cool a vapour stream from 260 deg C to about 200 deg C using natural convection or by leaving it bare. I need to know the amount of uninsulated length needed to do that. The process line is a 1/2" tubing.
Thanks in advance,
Timewrap
Need your help..
I've a got situation where I need to condense and cool a vapour stream from 260 deg C to about 200 deg C using natural convection or by leaving it bare. I need to know the amount of uninsulated length needed to do that. The process line is a 1/2" tubing.
Thanks in advance,
Timewrap





RE: Heat Loss from an Uninsulated Pipe
thread391-257339: Finned tube natural convection
I think it related to finned tube largely, buit armed with a suitable "U"-value you should have what you need.
Regards,
SNORGY.
RE: Heat Loss from an Uninsulated Pipe
Can you tell me how this equation works??
Nusset number to be calculated based on the calculated Prantl and Rayliegh number (Both based on air properties. After I get Nu, I can calculate H-T coefficient.
The unit of Q is Btu/hr, I need to calculate heat loss Btu/linear foot. Since I need to know the length of the pipe to be left uninsulated to get to the desired temperature.
Timewrap..
RE: Heat Loss from an Uninsulated Pipe
I assume heat is lost not just by natural convection but also by radiation. A combined heat transfer coefficient for the given data on horizontal pipes taken from very old (Petroleum Refiner-1959) nomographs would be 3.5 BTU/(hr.ft2.oF).
For ΔT I'd take the difference between the wall temperature, Twall, and the surrounding air T∞.
The results would be within [±] 30% of the true value.
RE: Heat Loss from an Uninsulated Pipe
Q=(kA(T2-T1))/l
Where:
Q: heat transferred
k: thermal coductivity
A: heat transfer surface area
T2: hot side temp.
T1: cold side temp.
l: thk. of barrier
RE: Heat Loss from an Uninsulated Pipe
Thanks for the e-mail.
I was using a Heat transfer coefficient of 5 btu/hr-deg-ft2, but 3.5 is a more conservative value.
The heat area is the cross section of the pipe, it does not take into account the length of the piping.
Once I solve the heat transfer equation it will give me total heat loss in btu/hr. I need to calculate the heat loss per foot of piping. Can I assume that the total heat loss I get from the equation is the heat loss per foot of piping.
Any comments would be appreciated
Timewrap..
RE: Heat Loss from an Uninsulated Pipe
As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"
RE: Heat Loss from an Uninsulated Pipe
Q = M*cp*deltaT (1)
Q = thermal power to reduce the stream temperature from 260 °C to 200 °C [W]
M = vapour mass flow rate [kg/s]
Cp = vapour specific heat [J/(kg °C)]
deltaT = temperature decrease [°C]
Cooling process doesn't take into account change of phase
Q = h*S*deltaT (2)
h = heat transfer coefficient [W/m2 °C]
S = pi*D*L lateral pipe area [m2]. Where D = pipe outer diameter and L = pipe length (your unknown value)
Equal the expression (1) and (2) and solve for L.
Note: As noticed by 25362 you should take into account both natural convection and radiative heat transfer.
RE: Heat Loss from an Uninsulated Pipe
It is IMO very important to notice that as cooling proceeds the driving force (deltaT) of the process diminishes. Moreover, as the vapour temperature drops, so does the wall pipe temperature and the properties of air needed to calculate the heat transfer coefficient vary as temperature varies.
RE: Heat Loss from an Uninsulated Pipe
The heat loss is related to the external pipe surface not its cross-section. Therefore, knowing the external surface per unit length will give you the loss per unit length.
RE: Heat Loss from an Uninsulated Pipe
Hope your post was addressed to the OP as I have obviously considered the external surface area (so involving the length of the pipe which is the unknown value)