The expansion device is a deliberate restriction between the high-pressure liquid and the low-pressure evaporator, and it is what lets the two sides sit at different pressures. As liquid squeezes through, part of it flashes to vapour and the energy that takes chills the remainder to the low-pressure boiling temperature, ready to absorb heat in the cold space.
Part of Refrigeration
Step three of the cycle: the restriction that drops the refrigerant’s pressure and temperature, and the three ways of building one.
The Restriction Makes The Pressure Difference Possible, And The Cold Follows From It
The compressor raises pressure, but it is the restriction that keeps the two sides apart. Without a real narrowing, pressure equalises through the open path and nothing cools. The refrigeration carrier calls this the critical step and says a restriction is as important as the compressor.
The cooling is real physics, not an effect of sudden squeezing. When the liquid drops to the low pressure it is hotter than the new boiling temperature, so some of it boils at once. The boiling takes heat from the rest of the liquid, which cools to the low-pressure boiling temperature. That is the same energy bookkeeping the corpus describes for latent heat in Sensible vs Latent Heat. The flashed vapour does no cooling in the box, because it has already been vaporised: the more of the liquid that flashes at the restriction, the less is left to do the useful work in the evaporator.
Three Ways To Build A Restriction
| Device | How it works | Good for | Weak point |
|---|---|---|---|
| Capillary tube | A long, narrow tube; its length and bore set the flow | Small, steady, sealed machines | Fixed flow; blocks easily; sized by trial |
| Needle valve | A tapered pin in a seat, adjusted by hand | A first build, where you tune by observation | An operator can close it fully |
| Float valve | A float opens a valve as liquid level rises | Larger plants with a liquid reservoir | Mechanical complexity |
The carrier’s absorption example uses a capillary of 1 to 2 mm bore and 1 to 2 m length, or a partly closed needle valve. These are the carrier’s figures and the right size for any particular machine has to be found by observation: too restrictive starves the evaporator, too open floods it and warms it.
What You Need
- Capillary tubing in the right material for the fluid (steel for ammonia), or a needle valve with a finely adjustable stem.
- A filter or strainer just before the restriction, because dirt, scale and frozen moisture block narrow bores.
- Gauges on both sides to see what the restriction is doing.
Method: Setting A Needle Valve
Step 1: Start the machine with the valve slightly open. Watch the high-side and low-side gauges.
Step 2: Close it a little at a time and wait. When it is right, the evaporator coil is cold along most of its length and the vapour leaving it is not wet or cold enough to frost the line far past the cold box.
Step 3: Back off if the evaporator warms or the low side pressure climbs. That means too much liquid is passing and the evaporator is flooding.
Step 4: Lock the setting where it cannot be changed casually. If the valve can be closed fully while heat is still being applied, the high side can keep rising. The restriction is not a safety device and must not be treated as one.
Worked Example: Why Some Of The Liquid Must Flash
Qualitatively, using the corpus latent-heat idea: if the liquid arrives hot and leaves at a colder temperature, the heat it gives up has to go somewhere. It goes into boiling off a fraction of the liquid. A bigger drop in temperature needs a larger flashed fraction. No figures are given here, because the fraction depends on the fluid and the two temperatures and has to come from that fluid’s property tables.
Common Mistakes
- No filter before the restriction. A tiny scale particle or a drop of frozen moisture is enough to stop a capillary.
- Water in the system. Moisture freezes at the cold end of a restriction and blocks it; it also corrodes.
- Re-sizing a capillary by cutting it short without checking. A shorter tube passes more flow.
- Closing a needle valve against a running heat source. The pressure behind it keeps climbing.
Safety
Cold liquid refrigerant and a possible blockage
Liquid leaving a restriction is far below freezing and can freeze skin on contact; for ammonia the liquid is also corrosive. A blocked restriction on a heat-driven machine lets the high side keep rising while the fire is lit, so the relief device must be on the high side and the heat must be one you can cut quickly. See Pressure Vessels.
What’s Next
- Learn next: Evaporation — where the chilled liquid does its work
- Combine with: Troubleshooting — symptoms of a blocked or mis-set restriction