Theermal conductivity formula for sch 40

[salman]

Is there any table or formula from which we can determine the thermal conductivity for schedule 40 3/4 or 1 or 2 inch piping i.e how many btu per ft of 1 inch of schedule 40 pipe . for construction of condensor.

[ChillGuy]

Hi Salman,

I think I have an idea what you are trying to do and the answer may be a little more complex than what you are asking for. It is not necessarily a simple formula, but a set of heat transfer calculations that need to look at the physical construction of the condenser and the fluids involved.

Since we are on a refrigeration site and under the NH3 section I'll assume you want to condense ammonia.

From experience I know for carbon steel construction with the ammonia (R-717) on the shell side and water on the tube side it is common to have an overall heat transfer co-efficient of up to 380 Btu/hr ft2 F.

This fits the basic formula

Q = UAT where

Q = Amount of heat transfered, Btu/hr
U = overall heat transfer co-efficient as above.
A = heat transfer surface area of tubes (pipe) sqft
T = LMTD, F, log mean temperature difference. You can get details on this from Wikipedia if you're not already familiar with it.

You should know your Q, your T from planned operating conditions, I've given you U, so now you can calculate the amount of surface area, then break that down into number of tubes/pipes and length.

I recommend that you select the number of tubes so that you maintain a water velocity of 3 - 5 fps to avoid fouling. Then adjust the length of pipe necessary to get the heat transfer surface area.

If you are not making a shell and tube style condenser, or you want to condense the ammonia on the tubeside, or there is some fluid other than ammonia & water, then the U value I've provided is not appropriate and should not be used.

I am also expecting the amonia is condensing around 90F to 120F which is where I normally see this for.

There are a number of other things to be considered, like design pressure to contain the amonia, ensuring adequate flow distribution of the water (requires some pressure drop) so I trust you will address those issues in proper time.

Hope this information helps and isn't just enough to get you into technical trouble!

[US Iceman]

I'll take a different approach.

The heat transfer through a pipe is different than a flat plate.

Second part of this is: Are you transferring heat into the pipe or from the inside to the exterior surface?

From a purely static heat transfer point, the heat transfer is based on one or two mechanisms. If you consider conductivity of the material then you need the material thickness and the temperature difference. If you are trying to find forced convection then you need to determine the inside or outside film coefficients and the the wall conductivity to find the heat transferred.