help needed for calculating heat loss through boiler wall

Shah · January 2021

Hi. I was looking at the boilers installed in my refinery and i found that there is a lot of heat loss through the walls of the boiler. I understand that it may be a refractory damage but its easier for me in my current scenario to proceed with thermal insulating the walls. in order to sell this idea to my management, i needed to calculate the current heat loss and compare it to what the heat loss will be after thermally insulating the walls.
So i measured the wall temperatures and assumed an ambient temperature of 70F. I assumed a thermal conductivity coefficient for CS (assuming that the plate is CS and assumed a thickness of 25mm for the plate). i then used Q=(k.A.Temp difference)/thickness to calculate the heat loss. but that calcualtion doesnt feel right.

reason i say that is the temp i measured on wall is on the outside and not the inside of boiler. so does that mean that the thermal coeff i used should be of air rather than CS? and if that is the case, what is the amount of thickness i assume for the air?

Can anyone please help me in properly calculating the current heat loss from my boiler walls?

Following are the actual values i measured.
Wall Temp = T1 = 250F
Wall Surface Area = A = 32sqft
Wall thickness (assumed) = d = 25mm
k for CS = 26.017 BTU/hr.ft.F
Ambient Temp = T2 = 70F (assumed)
Formula used: Q = k.A.(T1-T2)/d

acwagner · January 2021

You need the inside boiler temperature, not the exterior surface temp. Inside temp will be whatever temperature the boiler maintains for whatever process is ongoing in the boiler.

Then run the calculation just for the steel wall. Then run it for a steel wall with the insulation in series. Make sure your units of measure all match (you have mixed imperial and metric listed)

Note that your equation is for a flat surface. I assume the boiler is a cylinder, so it won't be entirely accurate.

Also, that equation is just for conduction, and does not take into account the loss through convection and radiation that will apply for the bare steel surface. There are separate equations for those. I believe those are generally ignored when insulation is added.

SteamingatMohawk · January 2021

If you haven't, try talking to the engineering/tech support people at the boiler manufacturer. In many, but not all cases, I have gotten excellent help.

Jamie Hall · January 2021

A couple of things to add to @acwagner 's commentary -- the basic thrust of which is spot on: what you are actually interested in is the heat loss through the wall and jacket assembly.

The part which is most variable is determining the exterior surface temperature of the assembly. This is what ultimately determines the heat loss, as the system reaches an equilibrium between heat input on the inside and heat output -- through conduction and radiation -- on the outside. You'll not be too far off, though, in assuming that the free air temperature is 70.

Shah · January 2021

Okay, that makes total sense. thank you @SteamingatMohawk @acwagner & @Jamie Hall. thanks for the insight.

i realise i need the inside temperature. and i got that too. But now i have one more question.

I calculated the heat loss through the steel plate based on inside temp of boiler and surface temp measured by temp gun. And now i want to see what the heat loss will be after insulation is installed. do i run the same calculations with making the following changes to the temperatures?

Initial Condition (without insulation)
Wall Temp = T1 = 600F (inside boiler temp)
Outside Surface Temp = T2 = 250F (measured via gun)

Final Condition (with insulation)
Wall Temp = T1 = Outside Surface Temp 250F (measured via gun)
Outside Insulation Surface Temp = T2 = 125F (assuming 50% surface temp reduction as stated by the manufacturer of the insulation)

Running the same formula based on these two temperature conditions and subtracting the results, should give me the amount of heat i am ensuring remains inside the boiler and not escape it. is that correct?

Jamie Hall · January 2021

That works.

SteamingatMohawk · January 2021

If you understand heat transfer through multiple "walls", this might help.

https://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html

I've been away from my college class on heat transfer since 1968, so I'm rusty to say the least. And I couldn't find my heat transfer text book to give a better explanation.

The calculation includes conduction through the walls and convection on the inside and outside surfaces. The inside and outside temperatures are the bulk temperature of the fluids, not the surface temperatures on the inside and outside of the walls.

Radiation is not included.

In what you wrote above about thickness of air, you don't calculate that. The heat transfer between the metal or insulation and the corresponding bulk fluid is convection, which is the hci for the inside coefficient and hco for the outside coefficient.

Anyone more recently familiar with heat transfer calculations is welcome to add to this discussion and correct any mistakes I have made.

Good luck.

help needed for calculating heat loss through boiler wall

Comments

Categories