Surface temperature Calculations (HX)

[Farhan Ahmad]

Hi,

Could anyone please help me regarding calculating the surface temperature of HX tube and tubes are covered by aluminium sheet? In this case, the 5.02m^2 area is exposed to sun, 500kW/m^2 peak flux on surface, total no of tubes 20 are arranged in parallel manner in one row (parallel flow) and covered with aluminum sheet and the spacing between tubes is 2mm.
I have already calculated total thermal resistance, heat transfer to fluid , heat absorbed by tube, heat losses include reflection and convection. But the outlet temperature is not according to desired output.So what approach should i follow to solve this problem? The attachment includes the equations which i'm using to calculate the surface temperature.

Thanks

 

[IRstuff]

Is this for school? Student posting is forbidden.

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[Farhan Ahmad]

It's not for school. It's about thermodynamic analysis for designing of Tubular solar thermal receiver(Thermal rating 3.3 MW). Thanks

 

[IRstuff]

Where are you getting the 500 kW/m^2 "flux" from? Maximum solar flux is only 1100 W/m^2

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[Farhan Ahmad]

DNI of any particular area varies with time so i am considering peak average flux 800w/m^2. The allowable peak flux for various HTF is different and in my case,considering water as HTF so 0.2MW/m^2 is valid. If you have solar field of 700 heliostats with surface area of 7.2m^2 and concentration ratio of around 500 then the total allowable peak flux of receiver area of 6m^2 is 500kW/m^2. Thanks

 

[IRstuff]

You're asking people to help troubleshoot calculations that you don't share, for an application you've not provided no information.

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[Farhan Ahmad]

My question is regarding surface temperature, i already have posted the equations. And mostly calculations are about the basic thermodynamic analysis. The crux of this problem is that the solar radiations hit on surface and i need to know best approach to calculate the surface temperature? What sort of the calculations do you need to solve this problem? Thanks

 

[IRstuff]

You've already given a bunch of equations that are potentially applicable. The question is how did you calculate your result and why do you think it's not correct?

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[Farhan Ahmad]

I am uncertain about the energy balance equation because that's according to the geometry of surface. I think i might have not included some aspect of surface or something is missing while taking account all factor in above mentioned equations.That's why the outlet temp isn't correct. Eventually those equations could be used to get the average surface temperature Ts? Thanks

 

[racookpe1978]

No, we cannot. You STILL have not shown a diagram/sketch of the heat exchanger arrangement (plan and elevation views), nor where the aluminum "cover" is nor how it is to be used - maybe as a shade for the heat exchanger?

is the heat exchanger being heated by the sun, or losing its heat by natural convectin to the (hot ?) or "cool" air flowing by the heat exchanger tubes that are themselves shaded under the aluminum sheet?

Is the Al sheet re-radiating its heat energy to the top tubes of the heat exchanger?

No info, no valid answers. Only bad guesses.

 

[Farhan Ahmad]

HI rackoope
I have attached some rough diagrams of the arrangement of the tube & other stuff. I think you can get some good idea from this scenario. Thanks

 

[IRstuff]

OK, that helps. Now, how did you use your equations?

You continue to make it difficult to help you.

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[Farhan Ahmad]

Energy balance eq in case of heat conduction through Fin;

In fin case, the total area is considered carefully as area of conduction isn't simple.

Energy balance eq in case of heat transfer through tube;

The total thermal resistance is combination of conduction and convection.

By solving above those eq simultaneously, Ts for tube and fin can be determined. After it, average surface temperature is considered as overall surface temperature.Thanks

 

[chicopee]

What is the difference between your calculated and your measured temperature? When I took heat transfer years ago, our professor use to say that if your calculated values were within 50% of the measured values, you did pretty good. So where do you stand now?