What a Metering Device Does in the Cycle
The metering device is the intentional restriction between the high-pressure liquid line (leaving the condenser) and the low-pressure side (entering the evaporator). Its job is to: (1) create a pressure drop, (2) meter (control) how much refrigerant flows into the evaporator, and (3) deliver refrigerant to the evaporator as a low-pressure liquid–vapor mixture that can absorb heat effectively.
Common metering devices you will see are:
- Variable metering: TXV/TEV (thermostatic expansion valve)
- Fixed metering: fixed orifice (piston), capillary tube
Flow Path: Where the Pressure Drop Happens
The metering device sits right before the evaporator inlet. The key change is a sharp drop from high-side pressure to low-side pressure.
High side (high pressure, mostly liquid) Low side (low pressure, mixture) ┌───────────┐ ┌───────────────┐ ┌──────────────┐ ┌──────────────┐ ┌───────────┐ │ Compressor│-->│ Condenser │-->│ Liquid line │-->│ Metering dev.│-->│ Evaporator│ └───────────┘ └───────────────┘ └──────────────┘ └──────────────┘ └───────────┘ (TXV/TEV, fixed orifice, cap tube) (boils, absorbs heat)After the metering device, the refrigerant is at a lower pressure, so its saturation temperature is lower. That is what enables the evaporator to run cold enough to absorb heat from the space or product being cooled.
Fixed vs Variable Metering: Conceptual Comparison
Fixed metering (fixed orifice / capillary tube)
A fixed device has a restriction that does not actively adjust itself. Flow through it mainly depends on the pressure difference across it and the refrigerant conditions feeding it.
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- Behavior: “Same restriction all the time.”
- What it tends to do: If load increases, evaporator pressure may rise and the pressure difference across the device may change; the device does not actively correct superheat, so evaporator feeding can become less ideal.
- Where used: Many small systems and some applications where simplicity and cost matter.
Variable metering (TXV/TEV)
A TXV/TEV changes its opening to maintain a target evaporator outlet superheat (it is trying to ensure the evaporator is properly fed while protecting the compressor from liquid floodback).
- Behavior: “Restriction changes as conditions change.”
- What it tends to do: If load increases and the evaporator starts boiling more refrigerant, the valve can open to feed more, keeping superheat near its setting.
- Where used: Many comfort cooling and refrigeration systems where load varies and stable control is important.
| Feature | Fixed Orifice / Cap Tube | TXV/TEV |
|---|---|---|
| Restriction size | Fixed | Adjustable |
| Primary “control target” | None directly (flow follows pressures) | Superheat at evaporator outlet |
| Response to load change | Indirect, can drift | Direct, actively compensates |
| Typical symptoms when mismatched | Too much/too little feeding depending on conditions | Hunting if misadjusted or unstable conditions |
The Expansion (Throttling) Process: What Changes and What Mostly Doesn’t
Across a metering device, the refrigerant experiences a throttling (expansion through a restriction). In practical HVAC/R terms:
- Pressure drops sharply from high side to low side.
- Temperature drops to match the new lower saturation temperature on the low side.
- A portion of the liquid “flashes” to vapor immediately after the restriction, creating a liquid–vapor mixture entering the evaporator.
- Enthalpy is approximately constant across the device (often described as “no useful work, no significant heat transfer in the valve”). You do not need heavy math here: think of it as the refrigerant trading some of its liquid form into vapor form to satisfy the new low-pressure condition, rather than “losing energy” to the surroundings.
A helpful mental picture: the metering device does not “create cold” by removing heat; it creates the conditions (low pressure → low saturation temperature) that allow the evaporator to absorb heat.
Why a pressure drop creates a colder evaporator
When pressure is reduced, the saturation temperature corresponding to that pressure is lower. So if the metering device drops the refrigerant from condensing pressure down to evaporating pressure, the refrigerant entering the evaporator is now able to boil at a much lower temperature.
Practical takeaway: evaporator pressure is a temperature control lever. Lower evaporator pressure corresponds to a lower boiling (saturation) temperature, which generally means colder coil temperatures (within design limits).
Step-by-Step: What Happens From Liquid Line to Evaporator Inlet
- High-pressure liquid arrives at the metering device inlet (typically subcooled or near-saturated liquid depending on system conditions).
- Refrigerant is forced through a restriction (valve port, piston/orifice, or long capillary tube).
- Pressure drops rapidly to the evaporator (low-side) pressure.
- Some liquid flashes to vapor immediately after the restriction because at the new lower pressure, not all of the refrigerant can remain liquid.
- A low-pressure mixture enters the evaporator. The remaining liquid portion can now boil along the evaporator length, absorbing heat from the load.
Short Scenario: Load Change Response (TXV vs Fixed Orifice)
Scenario setup
Assume a system is running steadily, then the cooling load suddenly increases (for example, a door opens on a walk-in box, or indoor heat load rises). The evaporator now has more heat available to boil refrigerant.
What a TXV/TEV tends to do
- More heat in the evaporator boils refrigerant faster, which tends to increase superheat at the evaporator outlet.
- The TXV sensing bulb “sees” the outlet temperature rise (relative to saturation at that pressure) and the valve opens more.
- More refrigerant flow enters the evaporator, increasing the amount of liquid available to boil.
- Result: superheat is pulled back toward the valve’s setting, and evaporator feeding stays closer to ideal under changing load.
What a fixed orifice/capillary tends to do
- The restriction does not actively respond to superheat.
- As load increases, evaporator conditions shift; depending on how pressures change, the system may become underfed (superheat rises, less effective coil usage) or sometimes swing the other way under different conditions.
- Result: performance can vary more with load and operating conditions because the device does not “aim” for a specific superheat.
Practical Notes for Beginners (What to Watch For Conceptually)
- Metering device = pressure-drop + flow control. If you remember only one thing, remember that.
- Expansion is not a heat-removal step; it is a pressure-reduction step that causes flashing and sets up low-temperature boiling in the evaporator.
- TXV/TEV is a feedback device: it adjusts flow to maintain a target evaporator outlet superheat.
- Fixed devices are condition-dependent: they rely on system pressure differences and are less adaptive to varying load.
