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Fin Surface Area Condenser-Evaporator (2P-MA) Simscape Fluids

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Hello everyone,
I am currently modelling a CO2 heat pump cycle in Simscape Fluids based on measured values of a real-world heat pump and have a question regarding the Condenser-Evaporator (2P-MA)-block.
The Condenser-Evaporator (2P-MA)-block is used to model the real-world finned tube heat exchanger of the heat pump's evaporator and condenser. As the heat transfer between the 2P-(CO2) and Moist Air-side was too little at the correct 2P enthalpy difference and mass flow, I checked the block parameters and noticed that each fluid side has a separate fin area.
The real finned tube heat exchanger has 387 rectangular fins with an area of 12 238 mm^2/fin (rectangular area with tube bores subtracted). Therefore I set the fin area of the 2P-side to 9 472 212 mm^2 and on the moist air side to 0 mm^2, but the heat transfer is too little. If I set both sides to 9 472 212 mm^2, the heat transfer is too high.
  1. How are the respective fin areas defined in the case of a finned tube heat exchanger (is the moist air side really 0 mm^2)?
  2. How is the port area of the moist-air side defined? (I assume, in flow direction, the heat exchangers width x height – fin amount x fin height x fin width?)
Thank you for your help!

Accepted Answer

Yifeng Tang
Yifeng Tang on 30 Jun 2023
In the parameter dialog, the fin area for the MA side are for those thin foil/sheet you see on the outside of the tubes. The way you calculate the total area sounds reasonable.
The fin area for the 2P side, on the other hand, refers to any wavy or fin-like features INSIDE the tubes. They are most common in plate heat exchangers where the plates have wavy patterns to increase surface areas. For the type of heat exchanger you show, there MAY be some internal spirals. If so, you can estimate a fin area from that. If it's smooth, you can leave the fin area to be zero and reply only on the surface area of the pipe, from cross section & hydraulic diameter calculation.
Then, you may use the fin efficiency on the MA side as a tuning nob to better match your data.
  2 Comments
Matthias Schmidt
Matthias Schmidt on 3 Jul 2023
Thank you that was very helpful! Are my assumptions regarding the MA-port area correct? (changes in area seem to have only very little impact anyway)
Yifeng Tang
Yifeng Tang on 7 Jul 2023
Hi Matthis,
The way you define the port area sounds very reasonable. I would expect the fin area in the flow direction (not the total heat transfer area) should be relatively small compare to the heat exchanger width * height. Using just width*height may be also OK I think. I'd be surprised if it makes much a difference. The pressure loss is calculated mainly from the pipe geometry & arrangement, not from this port area.

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