6 replies to this topic

[suhair]

hello everyone,

actually, i'm working toward the dynamic simulation for refrigeration unit. the system is closed loop and contain three operation unit which is condensor, chiller and compressor. the condensor using cooling water in the tube side while refrigerant at the shell side. chiller using ethylene glycol in the tube side and refrigerant at the shell side. the type of refrigerant used is freon. there is a lot of question i would to ask.

1. what is the suitable fluid package we can use? is it correct to use Activity model?

2. what type of compressor need to use? since, the 186kW being used as a heat supply to the compressor.

3. i would like to make a level transmitter at condensor and also the chiller unit, but i face the problem because the " d_Holdup Level(d_Holdup Level_1d_Holdup Level_1)" for both unit display <empty>. what parameter we need to adjust?

4. another problem, during integrator. i get an error msg about numerical error is occured. what does it mean? is there any case sensitive in condensor and compressor system.

5. i have set the phase for outlet chiller is vapor (which is 1.0000) but why during running simulation the value display for liquid or aques phase? is it we need to specify the flowrate because during dynamic simulation, the specify value for cooling water suddenly dissapear and change to 0.


i hope, there is someone to help me out from this challenge. thank you.

[suhair]

hello..

erm..just want to add a little bit..
mostly..we called it as a chiller unit in plant..

and why my system does not want to flow through the compressor unit? is there anyone know mostly in dynamic simulation, what factor are affect the flow of the stream?

[daryon]

Hi,

Are you using HYSYS dynamics?

I have a small amount of experience using HYSYS in dynamics mode and found it can be hard work getting the simulation set-up right and working robustly.

The first thing you should be doing is making sure the simualtion is built correctly in steady state mode before switching to dynamic mode. You should be specifying pressure and flow only for streams, other specs will be needed for unit operations like compressors and heat exchangers. The built-in dynamic assistant can be used to ensure you have a correct set of pressure -flow specs and should tell you if you have overspecified the simulation. I don't have a massive amount of expercience with dynamic mode so can't really be much help without sitting down in front of the simulation and fumbling my way through till it works. But I've tried to help with your questions below:

1. Freon's are normally HCFCs these days, and i think these molecules are quite polar, therefore you are probably right to use activity model. Not sure which one is best for this application. Activity model should be used for refrigerant, but you might be better off using PR EOS for the glycol. You can use different fluid packages for different streams that are idenpendent in HYSYS.

2. You would probably use a screw compressor but i don't think you can model this in HYSYS. So i'd probably go for a recip.

3. How are you modelling the condenser and chiller? From my limited experience the easiest way to model these is using 2-phase separators. A 2-phase separator can be used to model a heat exchanger. Look on the dynamics tab of the separator and you will see a option for heat exchanger. This allow you to set up heat transfer coefficients for each phase on tube and shell side and to maintain the level on the shell side. You need to specify the geometry of the exchanger to allow the dynamic solver to run.

4. Could be anything, it doesn't sound like you have the simualtion set-up right.

5. Do you mean the condenser? You should sepcifying the cooling water flowrate not the vapour fraction. HYSYS in dynamic mode works on pressures and flows. You should be using a control loop to vary the cooling water flowrate if required. Often condensers are set up with fixed cooling water flowrate.

I'm sorry i can't be more help, i am certainly no expert and I think you have a challenging simualtion on your hands. By the way what are you trying to achieve from the simulating this system?

[suhair]

Hello daryon..

thanks for your information..

ya.i'm using HYSYS dynamic simulation..for the chiller and condensor, i'm try to using heat exchanger because in the real plant..both of this unit is shell and tube heat exchanger. may be i will try to use phase separator. for this dynamic simulation, i want to make it pseudo to the carnot cycle termodynamic concept.

just starting with chiller. i want to simulate it with freon (both liquid and gas phase) flowing into shell of chiller and all of it will be vaporize at the outlet of chiller to become a freon gaseous. at the tube side contain the chilled water which will be used for plant operation. then, freon gaseous will flowing into compressor as a saturated gas. the compressor will compress the gas and become superheated gas and flowing through to condensor. in condensor, the superheated gas will condense and become saturated liquid. also freon flowing at the shell side of condensor. the cooling water is flowing at the tube side. then, the saturated liquid of freon will flowing to expansion valve to flash the liquid to be a mixture of gas and liquid phase. then, the process will cycle back to the chiller unit.

i think, i want to simulate this process and the temperature of chilled water(inlet and outlet), temperature and pressure for compressor suction, pressure of compressor discharge, temperature of cooling water (inlet and outlet), and the flowrate is known.

[suhair]

i'm still confuse..how to make the freon flowing in compressor?
because i think, in HYSYS dynamic simulation... freon can be flow by the factor of pressure drop.
so, how to make it flow in compressor?since compressor have a pressure increase inside it.

other than that, i got the small amount of flow rate go through to another unit operation..
how to solve it? to ensure that the flow rate displayed as my specified value. why we cannot change the flow rate value when change it into dynamic?

[vicini]

i don't know much about hysys and you know less. First off, freon 134a or whatever should be in the componet package, if not, sell the hysys and get a real simulation program.

If you can't get a steadystae model of a refrigeration loop going, then, go back to school and learn how they work.

Then for dynamic simulation, learn how the controls on a refrigeration loop works. There is 1 level valve in a noneconomized system and a suction control valve and a hot gas bypass valve if you are useing a level controlled chiller.

I'll leave it to you to fiquire out a flooded chiller, it has a superheater valve that acts like a hot gas bypass valve.

[asproegypt]

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