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. New to the cycle? Start with the basics →

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The loop

The refrigerant loop, live A closed rectangular vapor-compression loop drawn clockwise. A compressor on the left drives hot high-pressure vapor up the discharge line into a condenser 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 evaporator along the bottom, absorbing heat from indoor air and boiling back to a cool low-pressure vapor that returns up the suction line to the compressor. The discharge and liquid-line pipes — up the left side and across the top — are the hot high side, drawn in orange; the expansion and suction pipes — down the right side and along the bottom — are the cool low side, drawn in muted blue. When the evaporator coil is starved of airflow and ices over, a frost overlay appears on the evaporator and the 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 (a circular translation, not a spin) at a rate that follows the compressor stage. Thin dashed air lanes cross both coil faces left to right — outdoor air over the condenser, indoor air over the evaporator — entering in dim ink and leaving tinted by the coil's work: warmed orange past the condenser's midline, cooled blue past the evaporator's, frost-tinted once the coil ices. Each face's lane density follows its own airflow knob, so a starved coil visibly thins its air stream. Live pressure, superheat, subcooling and alarm values are reported in the gauges and readouts below. HIGH SIDE · HOT LOW SIDE · COOL COMPRESSOR CONDENSER heat → outdoor air METERING EVAPORATOR heat ← indoor air 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.

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.

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.

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, 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, and a mischarge cooks or floods it. Diagnose a charge or a freeze off the gauges, the superheat and subcooling on the real machine, and the data plate — never off a simulator.

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