Determining Heat Loss From an Insulated Tank 5

[jarooney]

I am trying to determine the heat loss from an insulated tank. The insulation is calcium silicate with a 2.5" thickness. The tank construction is cyllindrical 14' ID, 24' ht, carbon steel construction with 0.5" thickness. The product is 50% NaOH, so the idea is to use an electrical heater external to the tank to maintain product temp 80-100F to prevent it from freezing in the winter months. Any help would be greatly appreciated, I have looked everywhere and can't seem to find the help I need

 

[MintJulep]

Everywhere?

 

[Unotec]

I think you can easily find the answer in a book. But if you look more closely, some of these forums will shed some light. Q=mCp DT, Q= UA DT

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[jarooney]

I know that I need to use the Q=UA dT equation because I am assuming that the fluid is static. I am currently an intern in an engineering firm. I have not taken Heat Transfer yet. The engineer I would normally direct these questions to is on vacation until next monday. I was also told to assume a 20 mph wind speed. I am having problems calculating the "U" of the equation. Any help would be greatly appreciated. I have found all of the k values for each material, I am confused as to what the "h" (film coefficient)value even means, if it applies and how to calculate/where to find it if it does

 

[IRstuff]

Well, it's not quite that straightforward. Your heat transfer path consists of conduction through the steel and calcium silicate, followed by convection from the outer surface of the insulation into the air.

So, you have 3 temperatures to be concerned about, the interior temperature, assumed to be the same as the interior surface temperature of the steel, the external surface temperature of the steel, and tha ambient air temperature. You can model the steel/insulation as a single layer of insulation, by replacing the steel with a thinner layer of insulation that matches the thermal conductivity of the steel layer.

The heat transfer coefficient, you'll just have to dig around on the web. You can calculate it from first principles, using the Reynolds, Nusselt, and Prandtl numbers.

You'll then have two simultaneous equations, with same heat flow in both. The objective is to get a value of external insulation surface temperature that results in the same heat flow in both the conduction and convection equations.

TTFN

FAQ731-376

 

[IRstuff]

There's a free heat transfer text that you can download:

http://web.mit.edu/lienhard/

http://www/ahtt.html

TTFN

FAQ731-376

 

[simplemath]

IRstuff:
I found all heat transfer coefficient equations are emphirical or at least semi. could you clarify me on that "You can calculate it(heat transfer coefficient) from first principles"? Thanks.

jarooney:
You can find emphirical equations on cylinder external flow, in the form of Nu=CRe^a*Pr*b, in Fundamentals of Heat and Mass Transfer by Frank P. Incropera.



It boils down to simple math.

 

[quark]

There is an excellent article Predict storage-tank heat transfer precisely by Jimmy Kumana and Samir Kothari that appeared in March'82 Chemical Engineering magazine. I recommend this beyond Holman or Incropera and Dewitt

 

[JoeTank]

jarooney,
While you continue to sort through all the good responses, you may wish to take a look at NFPA-22. This is a National Fire Protection Association standard for fire sprinkler tanks. One of the appendices gives excellent info about the heat loss and design of water heaters to prevent freezing of the water. I know you are dealing with caustic soda, but the reference may help you pull this stuff together.

Joe Tank

 

[rmw]

The tank jacket manufacturers have software that will calculate it for you. Try Tranter, Mueller, and others.

rmw