Refrigeration Cycle Simulator: Master Vapor Compression HVAC Systems

✓ Verified Content: All equations, formulas, and reference data in this simulation have been verified by the Simulations4All engineering team against authoritative sources including MIT OpenCourseWare, ASHRAE Handbooks, and NIST refrigerant property databases. See verification log

Introduction

Here's a counterintuitive efficiency metric: your refrigerator removes 300 watts of heat from the cold compartment while consuming only 100 watts of electricity. That's not a violation of thermodynamics; it's the magic of heat pumping. The second law tells us you can move more heat than you consume in work, as long as you're moving it from cold to hot. The question is how much more.

Energy in must equal energy out: for every watt of compressor work you input, that watt plus whatever heat you absorbed from the cold space must be rejected at the condenser. Thermal engineers find that COP values of 3-5 are typical for air conditioning, meaning you move 3-5 watts of heat for every watt of electricity. No real refrigeration cycle achieves Carnot COP because you lose energy to compressor inefficiency, pressure drops through heat exchangers, and the irreversible expansion through the throttling valve.

In practice, you lose energy to superheat (running warmer than saturation temperature at the compressor inlet to prevent liquid slugging that would destroy the compressor). You lose energy to subcooling (dropping below saturation at the condenser exit to prevent flashing in the liquid line). And you lose energy to the temperature differences needed to actually drive heat transfer through your evaporator and condenser coils. Experienced designers know that every degree of approach temperature costs you COP.

This interactive simulator lets you explore how operating conditions (evaporator and condenser temperatures, superheat, subcooling, and refrigerant choice) affect COP and system capacity. Visualizing the cycle on pressure-enthalpy (P-h) diagrams develops the energy accounting intuition that separates adequate HVAC design from excellent thermal system engineering.

How to Use This Simulation

Energy in must equal energy out. The work you put into the compressor, plus the heat absorbed at the evaporator, equals the heat rejected at the condenser: W + Qevap = Qcond. This simulation tracks every kJ/kg around the cycle.

Main Controls

Input Parameters

Output Display

The results panel shows complete cycle performance:

• COP: Coefficient of Performance = Qevap/W (cooling) or Qcond/W (heating). Compare to Carnot COP to see the efficiency gap
• Cooling Capacity (kJ/kg): Heat absorbed at evaporator = h1 - h4
• Heat Rejected (kJ/kg): Heat released at condenser = h2 - h3. Always equals Qevap + W
• Compressor Work (kJ/kg): Work input = h2 - h1. The entropy generated here is your primary loss
• Evaporator Pressure (kPa): Low-side pressure, determined by Tevap
• Condenser Pressure (kPa): High-side pressure, determined by Tcond
• Pressure Ratio: Pcond/Pevap. Higher ratio means more compressor work
• Mass Flow (g/s per kW): Refrigerant flow rate needed for 1 kW cooling capacity

Understanding the P-h Diagram

The pressure-enthalpy diagram is the refrigeration engineer's workhorse:

Tips for Exploration

1. Start with Home AC preset (Tevap = 5°C, Tcond = 40°C): See the baseline cycle. COP around 3-4 is typical
2. Reduce the temperature lift: Lower Tcond from 40°C to 35°C. Watch COP improve significantly. The second law tells us smaller temperature differences mean less work
3. Compare refrigerants: Switch from R-134a to R-410A. Notice R-410A runs higher pressures but similar COP. Ammonia (R-717) shows excellent properties
4. Add superheat and subcooling: Increase SH from 0 to 10°C, then SC from 0 to 5°C. Superheat costs capacity; subcooling improves it. In practice, you need both for reliable operation
5. Try Refrigerator preset: Tevap = -20°C for frozen food. COP drops because the temperature lift increases dramatically

Carnot Comparison

Always benchmark against Carnot COP:

The gap comes from superheat losses, compressor inefficiency, pressure drops, and heat exchanger approach temperatures.

What Is Vapor Compression Refrigeration?

The vapor compression refrigeration cycle is a thermodynamic process that transfers heat from a low-temperature reservoir to a high-temperature reservoir using the phase-change properties of a working fluid (refrigerant) [1]. Unlike heat transfer by conduction or convection, which naturally flows from hot to cold, refrigeration systems use mechanical work to "pump" heat in the opposite direction.

The cycle consists of four fundamental processes operating in a closed loop:

The refrigerant enters the evaporator as a low-pressure liquid-vapor mixture, absorbs heat from the cold space, and exits as a low-pressure vapor [2]. The compressor then raises the pressure and temperature of this vapor. In the condenser, the high-pressure vapor rejects heat to the surroundings and condenses to a liquid. Finally, the expansion valve reduces the pressure back to evaporator conditions, completing the cycle.

This elegant process was first demonstrated by Jacob Perkins in 1834 and refined by Carl von Linde in the 1870s, revolutionizing food preservation and eventually enabling modern air conditioning [3].

Types of Refrigeration Systems

Direct Expansion (DX) Systems

• Refrigerant evaporates directly in the cooling coil
• Used in residential AC, refrigerators, PTACs
• Simple, efficient for smaller applications

Chilled Water Systems

• Refrigeration cycle cools water, which is pumped to cooling coils
• Used in large commercial buildings
• Better zoning control, centralized maintenance

Heat Pump Systems

• Reversible cycle for heating or cooling
• Ground-source (geothermal) or air-source
• COP 3-5 for heating vs. ~1 for electric resistance

Absorption Systems

• Heat-driven rather than compressor-driven
• Uses ammonia-water or lithium bromide-water
• Good for waste heat recovery applications

Key Parameters

Governing Equations

Coefficient of Performance

Cooling Mode:

COP_cooling = Q_evap / W_comp = (h₁ - h₄) / (h₂ - h₁)

Heating Mode (Heat Pump):

COP_heating = Q_cond / W_comp = (h₂ - h₃) / (h₂ - h₁)
COP_heating = COP_cooling + 1

Energy Balances

Evaporator: Q_evap = ṁ(h₁ - h₄) Compressor: W_comp = ṁ(h₂ - h₁) Condenser: Q_cond = ṁ(h₂ - h₃) Expansion Valve: h₃ = h₄ (isenthalpic)

Carnot Comparison

COP_Carnot,cool = T_evap / (T_cond - T_evap)
COP_Carnot,heat = T_cond / (T_cond - T_evap)

Temperatures must be in Kelvin!

Learning Objectives

After completing this simulation, you should be able to:

1. Trace the refrigeration cycle on P-h and T-s diagrams
2. Calculate COP for both cooling and heating modes
3. Explain the function of each component (evaporator, compressor, condenser, expansion valve)
4. Analyze the effect of operating temperatures on system performance
5. Compare refrigerants based on efficiency, GWP, and application suitability
6. Optimize superheat and subcooling for maximum capacity and efficiency

Exploration Activities

Activity 1: Temperature Lift Effect

Objective: Understand why COP decreases with larger temperature difference

Steps:

1. Set T_evap = 10°C, T_cond = 35°C (small lift = 25°C)
2. Record COP
3. Decrease T_evap to -10°C (lift = 45°C)
4. Compare COP values
5. Try T_evap = -25°C (lift = 60°C)

Expected Result: COP drops significantly with increasing temperature lift. A refrigerator (low evap temp) has lower COP than an AC unit.

Activity 2: Refrigerant Comparison

Objective: Compare performance of different refrigerants

Steps:

1. Set fixed conditions: T_evap = 5°C, T_cond = 40°C
2. Record COP, pressures for R-134a
3. Switch to R-410A, R-290, R-717
4. Compare operating pressures and COP
5. Note GWP differences

Expected Result: R-410A has higher pressures, R-717 (ammonia) has highest COP but safety concerns, R-290 (propane) is natural refrigerant with low GWP.

Activity 3: Superheat Optimization

Objective: See how superheat protects compressor but affects performance

Steps:

1. Set superheat = 0°C (dangerous!)
2. Increase to 5, 10, 15, 20°C
3. Track State 1 position on P-h diagram
4. How does COP change?

Expected Result: Some superheat is required to prevent liquid from entering compressor (slugging). Too much reduces COP slightly.

Activity 4: Heat Pump vs AC Mode

Objective: Compare heating and cooling performance

Steps:

1. Set to cooling mode, T_evap = 5°C, T_cond = 40°C
2. Record COP_cooling
3. Switch to heating mode
4. Verify COP_heating = COP_cooling + 1

Expected Result: Heat pumps are very efficient for heating because they "move" heat rather than creating it.

Real-World Applications

Residential Air Conditioning

• Split systems, package units
• R-410A replacing R-22
• Typical COP: 3-4 (SEER 10-16)

Commercial Refrigeration

• Supermarket display cases
• Walk-in coolers/freezers
• R-404A, R-448A, CO₂ systems

Industrial Chillers

• Process cooling
• Data center cooling
• Centrifugal or screw compressors

Automotive HVAC

• R-134a being replaced by R-1234yf
• Electric vehicle heat pumps
• CO₂ systems in some markets

Heat Pump Water Heaters

• COP 2-3 for water heating
• Significant energy savings
• R-134a or CO₂ refrigerant

Reference Data

Refrigerant Comparison

Typical System Performance

Challenge Questions

1. Conceptual: Why does lowering the evaporator temperature decrease COP? Use the Carnot formula to explain the theoretical limit.

2. Calculation: A refrigeration system operates with T_evap = 0°C and T_cond = 45°C using R-134a. If compressor efficiency is 75%, estimate the actual COP for cooling.

3. Analysis: Why is the expansion valve shown as a vertical line (constant enthalpy) on the P-h diagram? What happens to the refrigerant quality during expansion?

4. Application: A heat pump must provide 10 kW of heating with COP = 4. How much electrical power does the compressor require? How much heat is absorbed from the outdoor air?

5. Design: You need to select a refrigerant for a supermarket freezer case. Compare R-404A (GWP=3922) with R-290 (GWP=3). What factors beyond GWP should influence your choice?

Common Misconceptions

Frequently Asked Questions

Why does COP decrease when evaporator temperature drops?

Lower evaporator temperature increases the temperature lift (T_cond - T_evap), requiring more compressor work to move the same amount of heat. The Carnot COP formula COP = T_evap/(T_cond - T_evap) shows this relationship directly: as T_evap decreases, COP decreases [1].

What's the difference between a refrigerator and a heat pump?

They use identical vapor compression cycles. The difference is which heat transfer is "useful": refrigerators use Q_evap (cooling the inside), while heat pumps use Q_cond (heating the building). Heat pump COP is always higher by exactly 1 because it delivers both the absorbed heat and compressor work [4].

Why is R-22 being phased out?

R-22 (HCFC-22) has an ozone depletion potential (ODP) of 0.05, meaning it damages the stratospheric ozone layer. Under the Montreal Protocol, R-22 production ended in 2020. Modern replacements like R-410A have zero ODP but higher GWP, leading to newer low-GWP alternatives like R-32 and R-454B [6].

What does superheat protect against?

Superheat (typically 5-15°C above saturation) ensures the refrigerant is fully vaporized before entering the compressor. Liquid refrigerant is incompressible and causes "liquid slugging," which is violent mechanical stress that can destroy compressor valves and bearings within seconds [5].

Can COP be greater than 1? Isn't that impossible?

COP > 1 is not only possible but typical for refrigeration systems (COP 2-6 is common). Unlike thermal efficiency, COP is a ratio of desired heat transfer to work input, not a thermodynamic efficiency. Heat pumps don't "create" heat; they move it, which requires less energy than generating heat directly [4].

References

1. MIT OpenCourseWare — 2.43 Advanced Thermodynamics, Lecture Notes on Refrigeration Cycles. Available at: https://ocw.mit.edu/courses/2-43-advanced-thermodynamics-spring-2024/ — Creative Commons BY-NC-SA

2. LibreTexts Engineering — Refrigerator and Heat Pump. Available at: https://eng.libretexts.org/Bookshelves/Mechanical_Engineering/Introduction_to_Engineering_Thermodynamics_(Yan)/06:_Entropy_and_the_Second_Law_of_Thermodynamics/6.03:_Refrigerator_and_heat_pump — Free educational resource

3. Wikipedia — History of Refrigeration. Available at: https://en.wikipedia.org/wiki/History_of_refrigeration — CC BY-SA 3.0

4. U.S. Department of Energy — Energy Saver: Heat Pump Systems. Available at: https://www.energy.gov/energysaver/heat-pump-systems — Public domain

5. Carrier Corporation — Understanding Superheat and Subcooling. Available at: https://www.carrier.com/residential/en/us/products/air-conditioners/ — Technical documentation

6. U.S. EPA — Phaseout of HCFC Refrigerants. Available at: https://www.epa.gov/ods-phaseout — Public domain

7. NIST Chemistry WebBook — Thermophysical Properties of Fluid Systems. Available at: https://webbook.nist.gov/chemistry/fluid/ — Public domain

8. HyperPhysics — Heat Pump. Available at: http://hyperphysics.gsu.edu/hbase/thermo/heatpump.html — Educational use

About the Data

Refrigerant thermophysical properties in this simulation are based on simplified correlations fitted to NIST REFPROP data [7]. The simulator uses Antoine-type equations for saturation pressure and linear approximations for enthalpy and entropy. While accurate enough for educational purposes and preliminary design, professional applications should use NIST REFPROP or manufacturer-specific data for precise calculations.

GWP (Global Warming Potential) values are 100-year values from the IPCC Fifth Assessment Report. Refrigerant critical properties are from ASHRAE Handbook—Fundamentals.

How to Cite

Simulations4All. (2025). Refrigeration Cycle Simulator: Interactive Vapor Compression Analysis Tool. Retrieved from https://simulations4all.com/simulations/refrigeration-cycle-simulator

For academic use:

@misc{simulations4all_refrigeration_2025,
  title={Refrigeration Cycle Simulator},
  author={Simulations4All},
  year={2025},
  url={https://simulations4all.com/simulations/refrigeration-cycle-simulator}
}

Verification Log {#verification-log}