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Effect of the contained refrigerant mass on the efficiency and effectiveness of two-phase heat exchangers.

Number: 2287

Author(s) : ZAINER C., WIMMER K., LANG M., ALMBAUER R.

Summary

The development of high-quality, energy-efficient cooling devices requires extensive experience and special knowledge of the individual components and their interaction as an overall system. In refrigeration processes, the heat exchangers (condenser, evaporator) are components that represent a large uncertainty in the modeling and thus probably also a large potential for increasing efficiency.
Up to now, only the total refrigerant charge of the refrigeration cycle is known, but not how it is distributed in the system during operation. Using an internal 1D algorithm for simulating two-phase heat exchangers (1D-HXM), a fin-and-tube condenser operated with R600a is discretized into 100 elements. In the individual elements, the heat flow, thermodynamic state variables and the contained refrigerant mass can be determined.
The performed simulations show that the condenser contains a lot of liquid refrigerant at many operating conditions – i.e. considered over the tube length, they already reach a subcooling state very early. That means that the condenser contains a big amount of the overall refrigerant mass. By now varying the inlet pressure in minimal steps (0.1 bar steps), significant changes of the refrigerant charge in the condenser occur. Therefore, by holding other boundary conditions constant (mass flow R600a and air, …), the contained refrigerant mass in the condenser can be increased or decreased. It can be seen that in a wide range of this pressure variation a subcooling distance of different lengths occurs. The pressure variation thus changes the location of the phase transition in the condenser. However, with large subcooling proportions air outlet temperature and heat output remain nearly unchanged, only the refrigerant mass in the condenser changes. The heat flow and the air outlet temperature remain constant, because the air temperature approaches the subcooled refrigerant temperature and cannot rise any further, while the refrigerant cannot subcool below the air temperature. Therefore, nearly the same amount of heat can be transferred for e.g. 20 elements subcooled and 10 elements subcooled. This means that the entire heat exchanger (HX) is not optimally utilized and in optimal use it can be dimensioned smaller.
The results show that the condenser can contain a large amount of the overall refrigerant mass. By lowering the pressure an optimum condition can be achieved where the condenser is optimally utilized and only a small refrigerant mass is stored (lower condensing pressure means better COP). Further this condition can be used to evaluate and compare different HX designs with each other. Another result is, that for the validation of a HX simulation model, the geometrical position of the phase transition is needed. Moreover, the refrigerant distribution can lead to considerable problems in the behavior of overall cycles with speed-controlled compressors – which could also make an adjustable throttle indispensable for mass removal from the condenser.

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  • Original title: Effect of the contained refrigerant mass on the efficiency and effectiveness of two-phase heat exchangers.
  • Record ID : 30030647
  • Languages: English
  • Subject: Technology
  • Source: 2022 Purdue Conferences. 19th International Refrigeration and Air-Conditioning Conference at Purdue.
  • Publication date: 2022

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