16 Replies Latest reply on Jul 1, 2013 3:38 PM by Jared Conway

# Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

Hi,

I have been simulating different heat exchangers in solidworks flow simulation, where hot gas @510C transfers its heat to the cold water@90.55C. After simulation, I got the temperatures and specific heats of the both fluids by using surface parameters tool at the exit. From these values I tried to balance energy transfer between gas and water using

mCpdt formula.(where m= mass flow rate, Cp= Specific heat, dt = Temperature difference) but the energy is not balanced, for example heat lost by the gas = 9.5kW, Heat gained by the water =7.5kWm, I wonder where does the remaining 2kW heat is?

What's the difference between average temperature and Bulk temperature in the surface parameters results.

I attached a heat exchanger model.

Thank you very much,

Vamsi Mokkapati.

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

can you post the raw data that you used for your calculations? did you measure with a surface parameter or a goal?

also, in surface parameters, you can plot enthalpy directly, have you checked that parameter?

bulk vs average is described in the help and also the solidworks kb. are you seeing very different values between the 2?

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

Water : m= 3.8328kg/s, Cp=((4205.8+4206.3)/2)=4206.05J/kgK, Ti=363.705K, To=364.18K(average temperature)

Q=mCpdt = 3.8328*4206.05*(364.18-363.705) = 7657.25W

Nitrogen : m=0.1724kg/s, Cp=((1118.0+1103.9)/2)=1110.95J/kgK, Ti=783.15K, To=724.68K(average temperature)

Q=mCpdt = 0.1724*1110.95*(783.15-724.68) = 11198.62W

around 3.5kW difference

I measured with a surface parameter.

Now I also checked with enthalpy rate in surface parameters.

Enthalpy Rate Difference

Water : 8575.409W

Nitrogen : 9628.489W , around 1kW difference.

I am seeing very much big difference between bulk and average temperatures of nitrogen (724.68-average, 732.62-Bulk), I think Thats quite high difference.

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

vamsi, i didn't look at your model, was there any other heat being added or removed from your system?

have you tried improving the mesh to see if that makes a difference?

the only other thing i can think of is how you're measuring these. can you post pictures of the faces that you're selecting?

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

I dont think there is any other heat source or heat sink, Since I selected the shell wall as adiabatic wall and I also selected the lids for all the openings as Insulating material.

I am exporting surface parameters to the excel file, where the average temperature of the fluid is given for the respective surfaces, that's how I selected the values of the temperature.

Is that the right way to do it? Or is there any other easier way to do that?

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

should be ok. and your method seems fine. goals are nicer but i don't think you can currently create an enthalpy goal.

let us know whether and improved mesh sorts out the balance problem.

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

Vamsi,

You might try running the heat exchanger tutorial with your conditions to see if you get the same heat differential.  If the tutorial balances, then you have a problem with you input or model.  If the tutorial does not balance, then the explanation resides with your VAR, or Jared as he has been helpful in your case.

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

thats a really good idea david

or just running the problem with air/water just to check it out

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

For tutorial, the difference is reduced, not much higher difference, I changed the model and tried again, for the new design the difference is also reduced i.e around 0.5kw, looks like that is acceptable...

Thank you very much David and Jared.

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

vamsi, if you want a second set of eyes on it, send it over to me at jared@hawkridgesys.com. Thanks!

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

You mean the new model that I designed?

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

yep

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

Vamsi,

I had almost the same problem.

In my case it was a cooler for sugar syrup. Cooling liquid is water. Also my energy balance was always wrong, typically 15-20% (m1*cp1*dt1 m2 *cp2*dt2).

I simplified the model as far as possible until it was only a tube in a tube, but without success. Also refining the mesh didn’t help.

Coincidentally I run a simulation where I defined bulk average fluid temperature as surface goals on the 2 liquid outlet lids. Finally, the energy balance was correct (difference < 1%). Using the average fluid temperature to calculate the energy balance, I still get a difference of 15% between the two energies.

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

@ Sirio

1) As Jared mentioned have you tried comparing the total enthalpy rate between two surface?

2) As per the definition of bulk temperature and average temperature, what I felt is considering bulk temperate as the temperature at the exit is quite reasonable.What do you think?

3) I talked to my professor about <5% difference in the energy balance that I am getting from the simulation, he says that is acceptable.

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

vamsi, did you ever go through a mesh convergence test on your model?

i got the one you sent me on friday, my first note was that you had specified level 3 mesh but hadn't specified a minimum wall or gap. this is highly recommended. also once you've run it at 3, then 3+ min wall or gap, you run at level 5 or with local controls to make sure that you have convergence on your values.

i skipped most and just went to a solution adaptive mesh and got a difference of 26W which is very good considering the delta for one parameter was 5 and the other was 70.

one thing that might be affecting your model is that you have interferences between lids and components and between components. these should be removed, i have seen them affect surface and volume parameters.

the other comment i ahve is the model your components on the origin. it allows you to use the default planes automatically. take a look and let us know how ti goes.

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

Vamsi, Jared

The total enthalpy rate doesn’t help much. I think it should be anyway 0, neither is energy extracted, nor added to the system. In fact, the simulation gives a value of only 7.3 W. I am more interested in the outlet temperatures of the fluids. To check if the results are reasonable, I verify the energy balance.

As an example, I took the heat exchanger from the SolidWorks tutorial, HX.SLDASM (in lesson 5). I run the simulation with the default settings. The results are: water outlet temperature: 303.7 K, air outlet temperature: 433.4 K.
The (internal) energy change of the water is:
ΔUw = 0.1 kg/s * 4184 J/(kg*K) * (303.7 - 293.2)K = 4393.2 W

And of the air:
ΔUA = 0.15 kg/s * 1021 J/(kg*K) * (450 - 433.4)K = 2542.3 W

As both values should be equal, one or both outlet temperatures are wrong!

Choosing the bulk average temperature from the Flow Simulation Results Features (of the same simulation run) the energies are as following:

ΔUw = 0.1 kg/s * 4184 J/(kg*K) * (299.03 - 293.2)K = 2439.3 W

and:

ΔUA = 0.15 kg/s * 1021 J/(kg*K) * (450 - 434)K = 2450.4 W

This result is excellent!

I still don’t understand exactly what the difference between temperature and bulk temperature is. In the help I find “… the Bulk Average is a value averaged over the flow through the surface, i.e. considering the mass flow rate as well.” Does it mean that it gives more weight to the temperature where the flow is higher? E.g. in the centre of the outlet lid?

• ###### Re: Energy balance between two fluids in shell and tube heat exchanger is not Matching, Why?

bulk average is exactly as you have defined it

when yo usay "total enthalpy" i think you mean total enthalpy chang and both of us would agree it should be zero as per the conversation in this thread. that being said, there are meshing and reporting limitations that factor in which is why there can be a bit of a delta.

also to note, there is a surface parameter called enthalpy. which is what makes the method that you used potentially irrelevant. but it is a way to back up that calculation.

so at this point, i think we're all on the same page about this.