Why is the expansion valve modeled as h4 = h3?

A throttling valve is approximately adiabatic with no shaft work, so the steady-flow energy equation gives an isenthalpic process. Pressure drops sharply while part of the liquid flashes to vapor.

Why does higher condensing temperature lower COP?

A higher condenser temperature raises the high-side pressure and compressor pressure ratio, so more work is needed for roughly the same evaporator heat pickup.

Vapor-Compression Refrigeration Cycle

A vapor-compression refrigerator circulates refrigerant through four idealized components: the evaporator absorbs heat at low pressure, the compressor raises pressure and temperature, the condenser rejects heat at high pressure, and the expansion valve throttles liquid back to the evaporator pressure. This simulator uses a simple refrigerant-like property model to expose the energy bookkeeping: COP_R = (h1 − h4)/(h2 − h1), compressor efficiency raises h2 above the isentropic endpoint, and the expansion valve is treated as isenthalpic with h4 = h3. The p-h sketch marks states 1-2-3-4 and the component schematic shows heat and work flows. It is not a property-table solver: real design needs refrigerant data, two-phase quality, pressure drops, heat-exchanger approach temperatures, compressor maps, superheat control, oil return, and safety limits.

Who it's for: HVAC, refrigeration, thermodynamics, heat-pump, and mechanical engineering introductions.

Key terms

Vapor-compression cycle
COP
Evaporator
Condenser
Expansion valve
p-h diagram

The cycle shows the four ideal components and the energy balances. Real systems require refrigerant property tables, pressure drops, heat-exchanger approach temperatures, compressor maps, oil return, and two-phase details.

Live graphs

How it works

Vapor-compression refrigeration cycle with compressor, condenser, expansion valve, evaporator, COP, cooling capacity, pressure ratio, and p-h sketch.

Key equations

COP_R = q_evap / w_comp = (h1 − h4)/(h2 − h1)

Compressor: h2 = h1 + (h2s − h1)/ηis; valve: h4 = h3

Frequently asked questions

Why is the expansion valve modeled as h4 = h3?: A throttling valve is approximately adiabatic with no shaft work, so the steady-flow energy equation gives an isenthalpic process. Pressure drops sharply while part of the liquid flashes to vapor.
Why does higher condensing temperature lower COP?: A higher condenser temperature raises the high-side pressure and compressor pressure ratio, so more work is needed for roughly the same evaporator heat pickup.

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Frequently asked questions