9 replies to this topic

[P Thakur]

Dear Everyone,

 

I am trying to model Ammonia Heat Pump in HYSYS with condensing temperature at 56 C & evaporation temperature as 2 C. This was the method I followed in HYSYS.

1. Defined a Ammonia stream at 2 C & vapor fraction 1 and sent it to compressor.
2. Compressor outlet temperature was provided at 56 C.
3. Pressure at Compressor outlet was calculated by 8.2 bar whereas based on literature it should have been 24 bara
4. If I remove the efficiency from default values and try to input 24 bara & 56 C, then it shows negative efficiency. 
5. I am connecting compressor outlet to Condensor inlet. I understand that in Condensor of heat pump, there is no temperature change & pressure change. Only phase changes. However, when I try to make DP = 0 & DT = 0 across Condensor, it gives me 0 heat duty. If I try to manually input heat duty and DT=0, it gives me unrealistic pressure drop. 

Please help on how to properly model compressor, condensor, JT valve in HYSYS.

[Nikolay_]

Hello,

I've attached HYSYS case of refrigeration unit. It was created for learning.

Attached Files

•  Refrigeration unit.zip   79.32KB   33 downloads

Edited by Nikolai T, 15 June 2017 - 02:04 AM.

[P Thakur]

Dear Nikolai,

 

Thanks for sending the example. However, I find it intriguing that there is temperature change in condensor & evaporator whereas my understanding is that there should not be any temperature change in condensor and evaporator. Correct me if my understanding is wrong.

[Bobby Strain]

Seems to me you are lacking a fundamental background in thermodynamics. So, If you don't understand processes you will never build a model. So, learn a bit about compression and vaporization and condensation. Your model is nowhere close to the actual process.

 

Bobby

[P Thakur]

Dear Bobby,

 

Could you guide me?

[Bobby Strain]

You seem to be a student. That's where you should post student questions as members will be more responsive. You should look for information on refrigeration systems. You might find a good example from York or Frick. If you have a HYSYS license, you can probably find examples on the AspenTech support site. To start with basics, get a Mollier diagram for ammonia and trace the cycle. Also, you can probably find good information on centrifugal compressors at Elliott or Dresser.

 

Bobby

Edited by Bobby Strain, 15 June 2017 - 03:41 PM.

[Saml]

You are making the mistake that most (almost all) engineering students make today. You jump into a simulator without understanding what you are simulating.  Hysys and Aspen are basically a thermodynamic properties calculator, but you shuld know what properties to calculate, the degrees of freedom of the system and the variables to fix.

 

In your particular case, it seem that you are fixed to the idea that "in a pure substance condensation at constant pressure, the temperature is constant". That is true. But that does not mean "In every condenser, the fluid enter at its saturation point". In many cases the fluid enter the condenser with a temperature higher than the condensation, that is, superheated.

 

Second, please take the advice of Bobby, get an ammonia Mollier or a TS diagram (I prefer the later for this case, as isentropic evolutions are vertical lines) and understand the process on the diagram. It is not difficult, jut google "ammonia ts diagram refrigeration" and go to images. You will probably find the TS diagram with the cycle superimposed.

 

Also, take into account that a compression efficiency less than 100% (every real world case) means that on the TS diagram you end at a point that is to the right of  where you started.

 

After you understand what the cycle is and what it does then go to Hysys.

Edited by Saml, 15 June 2017 - 09:11 PM.

[P Thakur]

Dear Bobby & Saml, 

 

I am a Process Engineer with 5 years experience but my experience with refrigeration is limited.

 

I was trying to utilize ammonia for cooling my process side (process inlet temperature = ranges from 50 - 100 C; process outlet temperature 40 C) in evaporators & use heat of ammonia to meet district heating requirement.)

 

I had already gone through Mollier Diagram of Ammonia. But yes, I overlooked the fact that condensation will happen from superheated region to sub-cooled region. Therefore, I agree there will be temperature change in Condensor. I think this answers regarding condensors

 

Kindly refer page 47 of attached presentation. But still the compressor pressure calculated by HYSYS is lower than literature values. Please advise.

Attached Files

•  Heat_pump_presentation.pdf   36.4MB   50 downloads

[serra]

in general, before to start a simulation, you may wish to verify the accuracy of thermodynamic models against available data,

solving mass and heat balance make sure to test accuracy for vapor pressure (fugacities), H, S ...

for NH3 usually I select extended versions of Peng Robinson (PRX, in Prode Properties, not sure about the name in Aspen) or BWR,

errors (in vapor pressure , H, S, V) are acceptable for this kind of applications,

in your example

 

2 C -> 4.62 Bar.a

56 C -> 23.69 Bar.a

89 C -> 50.14 Bar.a

 

take care that std. EOSs (as Peng Robinson) can return large errors for liquid phase, meaning latent heat, enthalpy , entropy ...

[P Thakur]

Dear Everyone,

 

It was my mistake. I was reading the Mollier Diagram incorrectly. Hysys is giving good results.