When to use: Visualize the vapor-compression refrigeration cycle on a pressure-enthalpy (P-h) diagram. Adjust evaporating and condensing temperatures, superheat, and subcooling to see how each parameter shifts the cycle, affects compressor work, and changes system COP in real time.
| State | Location | Pressure (psia) | Enthalpy (BTU/lb) | Description |
|---|---|---|---|---|
| ① | Compressor Inlet | 316 | 116.2 | Sat. vapor + 10°F superheat |
| ② | Compressor Outlet | 1548 | 199.6 | Superheated vapor (discharge) |
| ③ | TXV Inlet | 1548 | 50.8 | Sat. liquid − 10°F subcooling |
| ④ | Evaporator Inlet | 316 | 50.8 | Two-phase mixture (flash) |
This simulator visualizes the vapor-compression refrigeration cycle on an animated pressure-enthalpy (P-h) diagram, calculating COP, compressor work, evaporator cooling effect, and condenser heat rejection for R-410A, R-32, and R-22. Engineers use it to analyze how operating conditions affect system efficiency, select superheat and subcooling targets, and understand the thermodynamic trade-offs in refrigeration system design.
The vapor-compression cycle consists of four processes on the P-h diagram. At State 1 (compressor inlet), refrigerant is superheated vapor at low pressure. The compressor raises it to high pressure and temperature (State 2, discharge). The condenser rejects heat at high pressure, condensing vapor to liquid and subcooling it to State 3. The expansion device (TXV or EEV) drops pressure isenthalpically to State 4, creating a two-phase mixture. The evaporator absorbs heat at low pressure, boiling the refrigerant back to superheated vapor at State 1.
COP (Coefficient of Performance) = Q_evap / W_comp = (h1 − h4) / (h2 − h1). A higher evaporating temperature or lower condensing temperature increases COP by reducing the pressure ratio the compressor must overcome. Superheat (typically 10°F) ensures only vapor enters the compressor. Subcooling (typically 10°F) prevents flash gas before the expansion valve and increases net refrigerating effect.
Compressor isentropic efficiency is approximately 75–85% for scroll and reciprocating compressors and 80–90% for centrifugal compressors at full load. Actual work is: W_actual = W_ideal / η_isentropic. The ratio of high-side to low-side pressure (compression ratio) drives compressor work — higher ratios reduce efficiency significantly.
ASHRAE Standard 15 (Safety Standard for Refrigeration Systems) governs refrigerant system design, installation, and operation including machinery room requirements, leak detection, and maximum charge sizes. ASHRAE Standard 34 classifies refrigerant safety groups and sets toxicity and flammability designations.
ARI Standard 540 governs hermetic refrigerant motor-compressor ratings and efficiency. ARI Standard 210/240 defines performance test conditions for unitary air conditioning equipment (95°F outdoor, 80°F DB/67°F WB indoor for cooling). ASHRAE Refrigeration Handbook provides P-h diagram data and design guidance for all common refrigerants.
Each 1°F increase in evaporating temperature improves COP by approximately 2–3%; each 1°F decrease in condensing temperature improves COP by 1–2%. This is why condenser water temperature reset and high-SEER air-cooled condensers are economically justified despite higher first cost. Superheat must be maintained above 0°F to prevent liquid refrigerant from entering the compressor (slugging), which can cause catastrophic valve damage.
Subcooling increases the net refrigerating effect without additional compressor work, improving overall system efficiency. Subcooling is typically achieved by running the condenser at capacity (condensing at lower temperature), by a liquid suction heat exchanger (LLSL-HX), or by a dedicated subcooler. A LLSL-HX transfers heat from the liquid line to the suction line, simultaneously subcooling the liquid and superheating the suction gas.
Select a refrigerant type and adjust the evaporating temperature (typical range 35–45°F for comfort cooling) and condensing temperature (typical range 100–120°F for air-cooled, 90–110°F for water-cooled). Adjust superheat and subcooling to match your equipment targets — 10°F is standard for both.
Watch the cycle path shift on the P-h diagram as you adjust parameters. Observe how raising condensing temperature stretches the cycle upward (more compressor work) while raising evaporating temperature compresses it (less work). The COP and pressure ratio results quantify the efficiency trade-offs. The state point table shows pressure and enthalpy at each cycle point for equipment verification.
COP (Coefficient of Performance) = cooling output / electrical input. A residential air conditioner with SEER 16 has an approximate COP of 4.7 at ARI rating conditions. Water-cooled centrifugal chillers can achieve COP of 6.0–7.0 at full load, 8.0–9.0 at part load. Air-cooled scroll compressor systems typically achieve COP of 2.5–3.5 depending on condensing temperature.
Superheat is the temperature of refrigerant vapor above its saturation temperature at the same pressure. At the compressor inlet, 10°F of superheat means the vapor is 10°F warmer than the boiling point at that pressure. This ensures only dry vapor enters the compressor — liquid refrigerant is incompressible and would damage valves and pistons if it reached the compressor. TXVs and EEVs control superheat by modulating refrigerant flow.
Subcooling is the temperature of liquid refrigerant below its condensing (saturation) temperature at the same pressure. 10°F of subcooling means the liquid is 10°F cooler than the condensing point. This increases the net refrigerating effect (h1 − h4) without changing compressor work, directly improving COP. It also prevents flash gas in the liquid line, which can cause erratic expansion valve operation.
R-410A has a Global Warming Potential (GWP) of 2,088 — over 2,000 times more potent than CO₂ as a greenhouse gas over 100 years. The EPA AIM Act and Montreal Protocol Kigali Amendment require significant GWP reductions in HVAC refrigerants. R-410A is being replaced by lower-GWP alternatives: R-32 (GWP 675), R-454B (GWP 466), and R-32 blends. New residential systems must switch by 2025 per EPA regulations.
Compressor work increases with pressure ratio (P_high / P_low). As condensing temperature rises or evaporating temperature falls, pressure ratio increases, requiring more compressor work per unit of cooling. Scroll compressors are most efficient at pressure ratios of 2.5–4.0; above 5.0, efficiency drops sharply. Two-stage compression improves efficiency at high ratios by dividing the compression into two steps with intercooling.
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