Throttling in Refrigeration Cycles

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Discussion Overview

The discussion centers on the throttling process in refrigeration cycles, specifically addressing the relationship between enthalpy and temperature during this process. Participants explore the implications of isenthalpic processes and the observed temperature drop in the TS diagram.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant states that throttling is considered isenthalpic, leading to the expectation that temperature should remain constant as enthalpy does.
  • Another participant questions the specific process being referred to, distinguishing between isenthalpic and isentropic processes.
  • A participant asserts that while enthalpy is generally proportional to temperature, this relationship holds true only for ideal gases, suggesting a limitation in the initial claim.
  • Another participant explains that the temperature drop observed is due to a partial phase change from liquid to vapor during the expansion process, where energy is drawn from within the system rather than from external heat transfer.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between enthalpy and temperature, particularly in the context of non-ideal gases and phase changes. The discussion remains unresolved regarding the implications of these factors on the throttling process.

Contextual Notes

There is a lack of consensus on the applicability of the ideal gas assumption to the discussion, and the role of phase changes in the throttling process is not fully explored. Additionally, the definitions of isenthalpic and isentropic processes may require clarification.

multiviya
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Hi all,

The process of throttling in refrigeration is taken to be isenthalpic. And as enthalpy remains constant then so should the temperature. But in the TS diag of the process it shows a drop of temperature from Thigher to Tlower. Please help me regarding this.

Thanks!
 
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for isenthalpic process, for which temperature decreases despite enthalpy remaining constant. But from what i know enthalpy is directly proportional to temperature, so temp should remain constant when enthalpy does. But clearly that's not the case in RAC cycle. Plz explain
 
Hi multiviya,
multiviya said:
But from what i know enthalpy is directly proportional to temperature, so temp should remain constant when enthalpy does.
This is only true for an ideal gas.
 
RefrigerationTS.png

The reason is that there is a (partial) phase change from liquid to liquid & vapor during the expansion process.

Normally, to go from liquid to gas, you need to add energy to break the molecular bonds, i.e. boil the liquid. But since we have a process which have no heat transfer to or from the surroundings by definition, the energy needed has to come from within, thus the temperature drop.
 

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