Video summary

Lecture -12 Vapour Compression Refrigeration Systems(contd.)

Main summary

Key takeaways

Educational

Main ideas and lessons

1) Why single-stage vapor compression systems can fail

  • Single-stage systems are adequate only when “temperature lift” is small (i.e., the condenser temperature minus the evaporator temperature).
  • When the temperature lift is large, performance worsens for multiple reasons (qualitatively explained using TS/PS and PH diagrams):

As evaporator temperature decreases (with condenser temperature fixed):

  • Throttling losses increase
  • Superheat losses increase
  • Compressor discharge temperature increases
  • Refrigeration effect decreases
  • Compression work increases
  • Quality of vapor at evaporator inlet increases and specific volume at compressor inlet increases

Consequences:

  • COP decreases
  • Refrigeration capacity decreases
  • Compressor life decreases rapidly (mainly due to high discharge temperature → lubrication inefficiency → wear and tear)

As condenser temperature increases (with evaporator temperature fixed), performance again degrades because the lift increases.

2) When multi-stage (multi-pressure) systems are needed

  • Single-stage becomes economically unviable when temperature lift exceeds practical limits.
  • Practical guidance:
    • Single-stage: evaporator temperature typically above −30°C
    • Two-stage: −30°C to −60°C
    • Three-stage: below −60°C
  • Multi-stage systems are used not only for large lifts, but also when different refrigeration temperature levels are required.

3) Example of multi-evaporator / multi-stage use for different loads

Dairy plant example:

  • Ice cream: about −30°C
  • Milk chilling: about +2°C

Options discussed:

  • Two independent refrigeration systems
    • One for −30°C, one for +2°C
  • Single system operating at −30°C even for milk chilling (inefficient; can cause issues like frosting)
  • Multi-operator (multi-evaporator / multi-pressure) system
    • One evaporator / low-side pressure operates at −30°C
    • Another operates at +2°C

Domestic refrigerator note:

  • Thermodynamic inefficiency occurs if the same evaporator serves both:
    • freezer (~−8 to −25°C, achieved by operating around −30°C)
    • fresh food (0 to 5°C)
  • Multi-evaporator systems could be thermodynamically better, but often not economical due to extra components—benefits typically justify economics only at large scale.

Multi-stage concepts: Flash gas removal and intercooling

4) Flash gas removal (flash gas is “bad” if it reaches the evaporator)

  • Flash gas definition:
    • Vapor formed during throttling/expansion of refrigerant liquid in the expansion device.
  • Key issue:
    • Flash gas must be recompressed back to condenser pressure.
    • It does not contribute further to refrigeration effect because its cooling potential was already “spent” by flashing.
    • It also increases evaporator pressure drop.
  • Ideal conceptual goal:
    • Remove flash gas immediately as soon as it forms and recompress it, avoiding delivery to the evaporator.

Practical approach: Flash tank

  • Use a flash tank at an intermediate pressure between evaporator and condenser.
    • Refrigerant from condenser is expanded to intermediate pressure using a float valve
    • In the flash tank, liquid and vapor separate
    • Saturated liquid → sent to the evaporator (then throttled to evaporator pressure)
    • Vapor → goes to the compressor (or is further pressure-managed depending on configuration)

Design requirement:

  • Vapor leaving the flash tank should be pure vapor (no entrained droplets).
  • Achieved by maintaining vapor upward velocity typically < 1 m/s (as cited).

5) Intercooling (reduce compressor work and discharge temperature)

  • Principle:
    • In multi-stage compression, compressor work can be reduced by cooling vapor between stages, lowering the specific volume at the inlet to the next compression.

Qualitative thermodynamic basis:

  • Work reduces because cooling decreases vapor specific volume and alters enthalpy/pressure-region relationships (supported by PV/PH/PS diagram reasoning).

Benefits:

  • Reduces specific work input
  • Reduces compressor discharge temperature
  • Better discharge temperatures → improved lubrication → longer compressor life

Ways to achieve intercooling:

  1. Water-cooled heat exchanger between compressor stages
  2. Flash-tank-based intercooling
    • Superheated vapor from the low-stage compressor is bubbled through liquid in the flash tank → desuperheats toward saturation
  3. Combination of both

Notes on effectiveness (depends on refrigerant)

  • Liquid flash-tank intercooling drawbacks:
    • Heat rejected to refrigerant can generate extra vapor, increasing mass flow handled by the high-stage compressor.
    • Total power impact depends on whether saved specific work outweighs increased mass-flow power.

Reported tendencies:

  • Ammonia (NH₃):
    • Power input often decreases with liquid-refrigerant intercooling
    • Discharge temperatures can be higher, and ammonia’s high latent heat reduces vapor generation relative benefit
  • Halocarbons (e.g., R12, R22):
    • Power input often marginally increases, or intercooling is less effective

Typical two-stage system discussed (flash gas removal + intercooling)

6) System description (components and flow logic)

  • High-pressure refrigerant liquid from the condenser:
    • passes through a float valve reducing to intermediate flash tank pressure pᵢ
  • Flash tank separation:
    • Liquid → expansion valve → evaporator at pₑ
    • Vapor → goes toward compression stages
  • Low-stage compression:
    • Evaporator outlet vapor compressed to intermediate pressure
  • Intercooling:
    • Vapor cooled in a water intercooler
    • Then additional cooling in a flash chamber
      • vapor bubbles through liquid; assumed to exit saturated
  • High-stage compression:
    • saturated vapor (plus any generated vapor) compressed to condenser pressure
  • Condenser rejects heat; the cycle repeats.

7) Study-state analysis methodology (mass/energy balance)

Assumptions:

  • Steady-state
  • Neglect kinetic and potential energy changes (ΔK, ΔP ≈ 0)

Method:

  • Apply for each component:
    • Mass balance
    • Energy balance
  • Use thermodynamic behavior for each device:
    • Flash tank: mass + energy balances with vapor/liquid split
    • Expansion valve / float valve: isenthalpic throttling (enthalpy constant)
    • Compressor: power input = mass flow × specific work (from thermodynamic states)
    • Evaporator: refrigeration capacity = mass flow × refrigeration effect
    • Condenser: heat rejection = mass flow × enthalpy change

Compute:

  • COP = cooling (refrigeration capacity) / total compressor work

8) Key advantages of this flash-tank multi-stage system

  • Lower vapor quality / improved quality at evaporator inlet → reduced pressure drop and improved heat transfer
  • Flash gas separated and recompressed from intermediate pressure → reduced throttling losses and improved power/volumetric efficiency
  • Reduced compressor pressure ratios per stage → smaller compressors and lower discharge temperatures

9) Limitation and mitigation: vapor in the expansion valve

  • Problem:

    • Flash tank sends saturated liquid to the expansion valve.
    • Vapor can form in the pipeline due to pressure drops and heat gain (“flashing before throttling”), causing vapor to enter the throttling device.
    • Throttling valves are typically designed to handle liquid only.

  • Mitigation:

    • Use a liquid subcooler
      • additional heat exchange subcools liquid before the expansion device
  • Penalty:
    • Extra vapor may be generated in the flash tank, which must be recompressed.

Next lecture (multi-operator systems and cascade systems)

10) Need for multi-operator systems (multi-evaporator / multi-pressure)

  • Introduced as an extension where multiple evaporators operate at different temperatures.
  • Conceptually:
    • A single-stage system would require both loads to be met by the same evaporator pressure (same evaporator temperature), which is inefficient.
    • A multi-operator approach enables different evaporator temperatures by creating multiple low-side pressures.

11) Cascade refrigeration system (starting point of explanation)

  • Cascade system uses two coupled refrigeration cycles for very large temperature spans.
  • Key coupling element:
    • a cascade condenser, which acts as:
      • the condenser of the low-temperature cycle
      • the evaporator of the high-temperature cycle

Example described in words:

  • Low-temperature side: carbon dioxide (CO₂)
  • High-temperature side: ammonia (NH₃)

Mechanism:

  • CO₂ condenses in the cascade condenser, rejecting heat to the NH₃ evaporator side.
  • NH₃ then compresses and condenses in its own condenser, rejecting heat to the environment.
  • Cascade repeats, and additional stages can be added by stacking additional cycles/cascade levels.

Speakers / sources featured

  • Instructor / Lecturer (unnamed): the video appears to be delivered as a course lecture (e.g., “Lecture -12 … contd.” and “welcome back in the last lecture …”).
  • Other sources/speakers: no other named organizations or individuals are identified in the subtitles.

Original video