3 replies to this topic

[Dudesons123]

A 10 m long copper pipe with an inner diameter of 2 cm and a wall thickness of 2mm is covered with a 5cm layer of fiberglass insulation. An electrical heating blanket, which generates heat at a flux 1kWm⁻², is wrapped outside this layer of insulation. The thickness of the heating blanket can be neglected. An outer layer of fiberglass of thickness 2 cm surrounds the heating blanket.

 

Water enters the pipe at 15^◦C with a mass flow rate of 0.2kgs⁻¹. The heat transfer coefficient due to forced convection of the water is h = 3000 W m⁻¹ K⁻¹. The air surrounding the pipe is at 30^◦C. The heat transfer coefficient due to natural convection of the air is h_o = 5 W m⁻¹ K⁻¹. 

 

 

What is the exit temperature of the water? Neglect the resistance to heat transfer due to the heating blanket, although you should account for the heat generated by it.

 

My exit temperature is 15.3°C

 

My workings are shown in the files.

Attached Files

•  Piping and insulations.JPG   106.87KB   2 downloads
•  Exit temperature 1.JPG   99.08KB   0 downloads
•  Exit temperature 2.JPG   73.78KB   0 downloads

Edited by Dudesons123, 23 August 2017 - 12:50 PM.

[breizh]

Hi ,

To support your work , consider the document attached , you may revise it for your specific assignment .

As stated in a previous post , it is time for you to " work as an engineer " and gather all input you need :using your favorite engine :

 

No info about the tube ,conductivity of pipe ,

No info about the insulation material, conductivity of insulation mat

etc

 

 

Others will be agree with me , you need also to use Excel  for iterative or repetitive calculations .

 

Hope this is going to help you and others a bit ,

 

Good luck .

Breizh

Edited by breizh, 23 August 2017 - 05:38 AM.

[Dudesons123]

Hi Breizh,

 

Thanks for replying. I was only given a datasheet for liquid water and fiberglass for this particular question and also the    information given in the question:

 

liquid water:

• heat capacity: 4.2 kJ kg^-1K^-1
• viscosity: 10^-3 Pa s
• thermal conductivity: 0.6 W m^-1 K^-1
• heat of vaporization: 2400 kJ kg^-1

fiberglass:

• thermal conductivity: 0.04 W m^-1 K^-1

No information was given about the thermal conductivity of copper pipe. We weren't even given information about Gnielinski correlation so I am unfamiliar with the concept. I carried out an energy balance without taking into consideration the Nusselt number, Reynold's number or Prandtl's number. 

 

Thanks.

Edited by Dudesons123, 23 August 2017 - 01:19 PM.

[breizh]

hi ,

Yes indeed you need to perform an heat balance and don't forget the resistance due to fiberglass .

 

my document is just there to show how you should present your work .

 

notes : - wrapping a electrical element above  the 1rst layer of insulation does not make any sense to me, purely academic.

            - important for you to understand the meaning of the 'magic' numbers Re,Pr, Nu and others very much used in Chem Eng

 

 

Good luck

 

Breizh