Simulators · Refrigerant Loop

Refrigerant Loop Simulator HVAC

A directional model, not a thermodynamic one — the pressures are real P-T-table lookups, but the magnitudes each knob moves are illustrative, tuned to teach which way the state travels. Turn a knob and watch the gauges, the loop, and the alarms react — then flip the mode to Heat and the reversing valve swaps the coils: the outdoor coil evaporates, frost becomes the story, and Defrost shows why a heat pump blows cold air twice a day. New to the cycle? Start with the basics →

Mode
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The loop

The refrigerant loop, live A closed rectangular vapor-compression loop with a heat-pump reversing valve. The top bar is always the outdoor coil and the bottom bar the indoor coil; which one condenses follows the selected mode. In cooling mode the loop runs clockwise: a compressor on the left drives hot high-pressure vapor up through the reversing valve into the outdoor coil spanning the top, which rejects heat to outdoor air and drops the refrigerant to a warm high-pressure liquid; the liquid line runs down the right side to a metering device, which flashes it to a cold low-pressure mix; that mix crosses the indoor coil along the bottom, absorbing heat from indoor air and boiling back to a cool low-pressure vapor that returns through the reversing valve to the compressor. In heating mode the valve flips and the loop runs counterclockwise: discharge gas goes DOWN to the indoor coil, which condenses and heats the indoor air, while the outdoor coil evaporates — running below the outdoor temperature and, on a cold damp day, below freezing, where a frost overlay builds on the top bar. The Defrost scenario flips the valve back to the cooling direction with the outdoor air lanes stopped: a temporary cooling-cycle run that melts the frost while the indoor duct blows cool air. Four pipe inks track four states wherever they flow: hot-gas orange on the discharge leg, warm-liquid amber on the liquid leg, cold-mix muted blue on the expansion leg, cool-vapor blue on the suction leg — the warm-to-cool jump landing at the metering device, where it physically belongs. Inside each coil bar the refrigerant rides a serpentine tube circuit painted with a live color gradient: the condensing run blends hot-gas orange into liquid amber at a point that tracks subcooling — flash gas stretches the blend past the exit — and the evaporating run blends cold-mix blue into vapor blue at a point that tracks superheat. A four-way reversing-valve symbol sits between the compressor and the coil legs, and both compressor lines terminate at it in every mode: discharge always leaves the compressor's fixed top port into the valve, and suction always returns from the valve into the compressor's fixed bottom port. The valve's internal passages — drawn by the pipes themselves and flipping with the mode — are what swap which coil each line serves; the compressor's own ports never trade places. When a coil frosts or freezes, a frost overlay appears on that coil and its suction leg. The compressor symbol is a top-down scroll set — two interleaved spirals, the moving one orbiting the fixed one in a small circle, running against the spirals' outward winding — the compression direction — at a rate that follows the compressor stage. Thin dashed air lanes cross both coil faces vertically, on the crossflow axis — outdoor air rises through the top coil, indoor air drops through the bottom coil — entering in dim ink and leaving tinted by the coil's work: warmed orange off the condensing coil, cooled blue off the evaporating coil, frost-tinted once a coil ices, and stopped on the outdoor face during defrost. Each face's lane density follows its own airflow knob. Live pressure, superheat, subcooling and alarm values are reported in the gauges and readouts below. REVERSING VALVE HIGH SIDE · HOT LOW SIDE · COOL COMPRESSOR CONDENSER warm air → outdoors METERING EVAPORATOR cool air → indoors hot vapor warm liquid cold mix cool vapor

Gauges & readouts

Manifold R-410A
Low-side suction pressure gauge LOW PRESSURE SIDE —psig SUCTION High-side discharge pressure gauge HIGH PRESSURE SIDE —psig HEAD
Superheat
—
Subcooling
—
Evap sat temp
—
Cond sat temp
—
Suction
—
Head
—
Indoor air in → out
—
Outdoor air in → out
—
Run, on Coil freeze, off Floodback, off High head, off
Turn a knob to run the loop.
Indoor coil airflow
— CFM/ton · floor 400

The two gauges read like a manifold set: the low-side needle follows suction pressure, the high-side needle follows head. Each dial's inner scale is that refrigerant's own saturation-temperature ring — the P-T curve, on the gauge face. Superheat is referenced to the dew curve, subcooling to the bubble curve. An airside-starved coil can freeze with normal superheat — the refrigerant side looks fine while the coil ices, which is exactly why the freeze alarm fires on its own. In heating mode the same trap moves outdoors: a frost-choked outdoor coil starves at normal superheat too, and only the diving suction pressure gives it away.

On the P-T chart

Where the cycle sits on the refrigerant's own saturation curve. The low-side point rides the dew curve — the gap to its right is superheat; the high-side point rides the bubble curve — the gap to its left is subcooling. Starve the coil and the low-side point slides left past the 32 °F freeze line. In heating mode the same line reads as the frost line instead: the outdoor coil normally runs left of it on a cold day, so the line only turns red when the ambient is in the frost-accumulation band and ice is actually building.

The P-T strip reads best on a wider screen — at phone width the temperature axis compresses. Every reading updates regardless; a laptop just gives the curve room.
Saturation curve Superheat Subcooling 32 °F freeze

Controls

Compressor stage
100%
100%
75 °F
10 °F
90 °F
100%
Fullscreen view

About this model

This is a directional model, not a thermodynamic one. The pressures are real lookups on the selected refrigerant's published P-T table — the same tables the P-T calculator reads — and superheat and subcooling are honest subtractions against the dew and bubble curves. But the magnitudes each knob moves are illustrative, tuned so the state travels the way it does in the field, not to reproduce a specific machine. There is no enthalpy or P-h data behind it, so read it for direction and relationships in plain terms — a real property model is a later upgrade. One framing note: every refrigerant runs the same design cycle per mode — in cooling a 40 °F evaporator against a 105 °F condenser, in heating its own shared anchor set (roughly 27 °F / 105 °F at the 47 °F rating point) — so the pressures compare apples-to-apples across refrigerants. R-404A in real life usually runs colder boxes.

Heat mode is the same machine with the reversing valve flipped. The bars keep their hardware identity — top is always the outdoor coil, bottom always the indoor coil — and the CONDENSER / EVAPORATOR labels swap instead, because that is what the valve actually changes. In heating the outdoor coil runs about 20 °F below the outdoor air, so on a cold day it sits below freezing by design — a below-32 coil is normal heat-pump operation, not an alarm. What turns it into a fault is accumulation: below roughly 40 °F outdoors the coil frosts faster than it sheds, the frost chokes the coil's own airflow, the coil pulls colder, and the spiral runs until a defrost breaks it. The Defrost scenario shows that honestly: defrost is a temporary cooling-mode run — the valve flips back, the outdoor fan stops so the hot coil melts its ice, and the indoor duct blows cool air until the cycle ends. That is the "my heat pump is blowing cold air and steaming" service call, working as designed. The cold-weather capacity fade is modeled too: as the suction falls, less heat reaches the indoor coil, so below the 47 °F rating point the condensing temperature droops with the ambient — watch the head gauge first, then the supply air slide from 95 °F toward roughly 88 °F at the 17 °F rating point, and cooler still in deep cold. That fade is exactly why auxiliary heat exists.

The headline is the airside-versus-refrigerant-side split. Starve the coil and the suction pressure dives, the evaporator saturation temperature drops below freezing, and the coil ices — all while superheat sits at a perfectly normal 10 °F. Nothing on the refrigerant side is wrong; the coil simply cannot breathe. That is why the freeze alarm fires on its own, independently of superheat, and it is the exact trap behind the coil-flow minimum in VAV systems — confirm the airflow itself with the Equipment Airflow Check. A superheat that looks fine never means the system is.

One glide caveat: freeze detection uses the dew (superheat-side) saturation temperature as a single stand-in for the coil temperature. On a large-glide blend like R-407C the coil inlet runs colder than that by roughly the glide, so a real coil would start frosting a touch earlier than the single number here suggests. On R-410A and the other low-glide refrigerants that distinction all but vanishes.

The two air readouts are sensible-only. The indoor pair is a dry-bulb split — at design, the classic 75 °F return dropping to a 55 °F supply — scaled by capacity over airflow, and the outdoor pair shows the matching discharge-air rise across the condenser. There is no humidity model behind either number: a real supply-air reading rides latent load and coil dehumidification, so read these as the dry-bulb direction, not a psychrometric result.

A theory check, a learning aid, and a second opinion — not a charging tool. The magnitudes here are illustrative, and no toy loop should stand in for the machine's own protections. The low-pressure cutout, the freeze-stat, the high-pressure switch, the defrost control, and the manufacturer's charging chart are what actually keep the equipment alive: a coil starved of air ices solid and hands the compressor a slug of liquid — indoors in cooling, outdoors in heating when frost runs past the defrost's reach — and a mischarge cooks or floods it in either mode. Diagnose a charge, a freeze, or a defrost complaint off the gauges, the superheat and subcooling on the real machine, and the data plate — never off a simulator.

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