Tank Fluid Heating with Recirculation and Heat Exchanger ( Continued )

[Pavan Kumar]

Hi All,

In the thread copied, I developed the spreadsheet to calculate the heating required to heat a tank with water with an external HX in recirculation(Updated and corrected spreadsheet attached). I made correction as per the suggestions of Mr. Pierre and Mr. georgeverghese and the spreadsheet makes lot more sense now. I now to account for the tank heat loss to atmosphere assuming no insulation for the worst case scenario. Request your guidance on this.

https://www.eng-tips.com/viewthread.cfm?qid=517615

Thanks and Regards,
Pavan Kumar

 

[pierreick]

Hi,
You may find pointers in the document attached.
Pierre

 

[Pavan Kumar]

Hi Pierre,

The document you sent is in Spanish. Is there any an English version of this paper can be sent to me?.

Thanks and Regards,
Pavan Kumar

 

[pierreick]

Hi,
Nope but not difficult to understand. A Google translation will certainly help.
I've attached another document, more detailed in English.
Pierre

 

[Pavan Kumar]

Hi Pierre,

I already had the paper by Kumana and Kothari. My question was not how to calculate the heat loss through tanks ( which I have to any way). My question was how to incorporate heat loss into the Tank heating with recirculation through an external heat exchanger. I am trying to work out the derivation from DQ Kern book page 629. I wanted to confirm what I am doing is correct. Attached with this an excel spreadsheet where I put the heat balance equation that is the basis for the derivation.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

Sorry I was not able to add the file in my last post. Here it is attached.

Thanks and Regards
Pavan Kumar

 

[georgeverghese]

@Pavan,

"I wanted to confirm what I am doing is correct."

Have briefly seen your new spreadsheet - I dont see how or where you have incorporated the insulation heat loss ?

 

[Pavan Kumar]

Hi georgeverghese,

I am having issues on how to include the tank heat loss to calculate the heating time with recirculation through an external heat exchanger. The problem is with writing the heat balance equation. If I can figure out how to include the heat loss then the problem will be solved. This is what Perry says in page 11-18.

Effect of External Heat Loss or Gain : If heat loss or gain through the vessel walls cannot be neglected, equations that include this heat transfer can be developed by using energy balances similar to those used for the derivations of equations given previously Basically, these equations must be modified by adding a heat-loss or heat-gain term.

A simpler procedure, which is probably acceptable for most practical cases, is to adjust the ratio of UA or Theta either up or down in accordance with the required modification in total heat load over time.

I am particularly interested using what the second paragraph says. Need help in how to do it.

Thanks and Regards,
Pavan Kumar.

 

[georgeverghese]

First, is the heat loss during heating time significant compared to the total heat input? Would guess that if it is < 5-10% of heat input, it can be neglected. If you have adequate insulation, it would most likely be less than 10%.

 

[pierreick]

Hi Pavan,
I can't read your mind.
To support your calculation and evaluate the heat loss consider this chart. This should be good enough to take action.
Pierre

 

[Pavan Kumar]

Hi georgeverghese,

First, is the heat loss during heating time significant compared to the total heat input? Would guess that if it is < 5-10% of heat input, it can be neglected. If you have adequate insulation, it would most likely be less than 10%.

What you say makes sense. In the case where the Heat Loss is greater than 10% of the Heat Input how do you adjust the Heat Transfer area. Perry 9th ed page 11-18 says that the heat UA or Time (Theta) can adjusted in accordance with the heat loss. See below.

[highlight #FCE94F]"A simpler procedure, which is probably acceptable for most practical cases, is to adjust the ratio of UA or Theta either up or down in accordance with the required modification in total heat load over time."[/highlight]

All I want to know is how exactly. I will first calculate the heat loss using Kumana and Kothari's paper and get back today.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

Hi Pierre,

Thank you very much as always for sending useful documents. I will make use of the chart for sure. Unfortunately my tank dimensions are too bog to be able to use this chart.
My tank dia and height are 66 ft each. I will use Kumana's paper for now.

Thanks and Regards,
Pavan Kumar

 

[pierreick]

Hi,
Up to you to decide the methodology but the chart can cope with the dimension of the tank (60 feet or 66 feet length will not make a big difference).
Definitely I need new glasses or a magnifier.
Note: You can easily calculate the surface and then use the chart to get the losses for 2 cases:
with and without insulation, then multiply these heat losses by the wind factor.
This may help you to crosscheck your calculation.
Edit: another link

https://www.chesolver.com/tank-heat-loss/

I do believe you need some insulation.
Good luck
Pierre

 

[Pavan Kumar]

Hi Pierre,

Thank you sharing the online tank heat loss calculator. It is very useful tool. I have completed the development of my spreadsheet for tank heat loss based on Kumana and Kothari's paper and finished my calculations. I tried to verify my results with the Online calculator. Unfortunately it kept saying that Solver failed and Check Input. My spreadsheet is attached for reference.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

Hi georgeverghese,

I have done calculation for one of the tanks in Anaerobic digestion plant, where in the tank is to kept at 37 Deg C. The tank is heated through re-circulation through an external heat exchanger. The tank fluid is already at 37 Deg C and the exchanger only has to compensate for the heat losses to atmosphere. So I calculated the tank heat losses using Kumana and Kothari's paper as a reference and then calculated the required heat transfer area for the heat exchanger. Attached is my calculation spreadsheet. I know the process side( tube side) inlet temperature(t1) to the HX which is 37 Deg C and the shell side( glycol side) inlet temperature(T1) which is at 60 Deg C. I know I cannot cool the glycol below 55 Deg C. So knowing these values I calculated the process side outlet temperature(t2) and the glycol side outlet temperature(T2). After this I calculated the LMTD and with an assumed "U" value of 100 Btu/ft2-hr-Deg F for Glycol-water system I calculated the Heat transfer surface area (A) ft2 required. I then filled the datasheet with this information. With this information I want the vendor to come back to me with a quotation for an exchanger. I want to make sure if my calculations are correct so that my datasheet would also be correct.

Thanks and Regards,
Pavan Kumar

 

[pierreick]

Hi.
Missing data:
Heat loss without insulation vs with insulation to justify the lagging.
Time to heat up, is it acceptable?
I think your calculation is correct, you may need to compare with other methods given to you.

Notes:
The example from the PDF I shared with you (numerical application) in the document you follow, is wrong, starting with the calculation of the Grashof number for liquid, should be 0.98 instead of 98. This means the conclusion of the paper may be reviewed.
Don't forget to perform other iteration(s) as suggested in the paper to recalculate the wall temperature and so on until convergence on T wall.
I believe the roof of the tank should be insulated. Check the impact on losses.
In the document PDF, it's mentioned the insulation material, not the metal sheet to protect the insulation material or cladding material i.e. another resistance to add to the set of equations.

Good luck
Pierre