Heat pumps: Questions about compressor pump refrigerant gas and evaporator

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Discussion Overview

The discussion revolves around the operation of heat pumps, specifically focusing on the role of the compressor, the behavior of refrigerant gas, and the thermodynamic processes involved in heat transfer. Participants explore concepts related to thermodynamics, including pressure-volume and temperature-entropy diagrams, as well as the mechanics of piston compressors.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants question how the compressor increases pressure within a sealed unit and whether it pumps refrigerant gas from the evaporator into the high-pressure space.
  • One participant compares the compression process to a bicycle pump, noting the fixed volume of refrigerant and questioning how compression occurs without adding more gas.
  • Another participant suggests that the compressor operates by trapping a portion of gas and compressing it, while also seeking clarification on the concept of "compression space."
  • There are inquiries about the nature of evaporation and condensation occurring at constant pressure, with analogies drawn to cooking pasta under a lid.
  • Participants express uncertainty about the time scales involved in the evaporation, compression, condensation, and expansion processes within the heat pump cycle.
  • Questions arise regarding typical temperatures and pressures for a propane heat pump and how these relate to the pressure-volume diagram.

Areas of Agreement / Disagreement

Participants do not reach a consensus on several key points, including the mechanics of compression in a fixed volume and the implications of constant pressure during evaporation and condensation. Multiple competing views and uncertainties remain throughout the discussion.

Contextual Notes

Participants express varying levels of familiarity with thermodynamics, with some referencing prior education and others seeking foundational understanding. The discussion includes references to external resources, such as Wikipedia, but does not resolve the complexities of the thermodynamic processes involved.

  • #31
GreenWombat said:
Hi Russ. In your contribution (Saturday, 24 August), you wrote that the pressure drop across the evaporator was “too small to be visible on the P-V diagram and can be ignored.” For me, that was brilliant because it resolved the contradiction of (1) the pressure-volume diagram showing constant volume and (2) refrigerant moving across the evaporator without any pressure drop.
My error: I meant "the pressure-volume diagram showing constant pressure"
 

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