What the Orifice Tube Does
The orifice tube is a small, simple part with a big job. Like the expansion valve, it separates the high-pressure and low-pressure sides of the A/C system and creates the pressure drop that makes refrigerant cold. The difference is that an orifice tube has a fixed opening. It has no moving parts and can't adjust its flow.
Vehicles use either an orifice tube or an expansion valve, not both. Orifice tube systems are common on many domestic trucks, SUVs, and cars.
Where It's Located
The orifice tube sits inside the liquid line, the smaller metal line running from the condenser to the evaporator. It's usually near the evaporator inlet at the firewall, or at a connection near the condenser outlet. Because it's inside the line, you can't see it without opening the system. On some vehicles, it's built into the line and the whole line section is replaced.
What It's Made Of
- A calibrated brass tube in the center with a very small, precisely sized hole that meters refrigerant.
- Mesh filter screens on the inlet and outlet ends that catch debris before it can block the tiny opening.
- O-rings that seal the tube inside the line so all the refrigerant has to pass through the opening.
- A plastic body, often color-coded, to indicate the size of the opening for a specific vehicle.
How the Orifice Tube Operates
1. High-pressure liquid arrives
Warm, high-pressure liquid refrigerant flows from the condenser through the liquid line and reaches the orifice tube. In these systems there is no receiver dryer, so the refrigerant goes straight from the condenser to the orifice tube.
2. Filtering
The inlet screen catches any debris in the refrigerant before it reaches the small metering opening.
3. The pressure drop
The refrigerant is forced through the narrow brass tube. Coming out the other side, the pressure drops sharply, and the refrigerant becomes a cold, low-pressure mix of liquid and vapor that flows into the evaporator to absorb heat from the cabin air.
4. Controlling cooling without adjusting flow
Since the orifice tube can't change its opening, the system controls cooling in other ways. Most orifice tube systems cycle the compressor clutch on and off using a pressure or temperature switch, which keeps the evaporator from getting cold enough to freeze. Some newer systems use a variable displacement compressor instead.
5. The accumulator catches extra liquid
Because flow is fixed, the evaporator can't always boil off all of the refrigerant. Any leftover liquid collects in the accumulator after the evaporator, so only vapor reaches the compressor.
Orifice Tube vs. Expansion Valve
| Orifice Tube | Expansion Valve (TXV) | |
|---|---|---|
| Opening | Fixed | Adjusts with temperature |
| Moving parts | None | Diaphragm, spring, valve |
| Paired with | Accumulator | Receiver dryer |
| Cooling control | Compressor cycling | Valve adjusts flow |
| Cost | Inexpensive part | More expensive part |
Common Causes of Orifice Tube Problems
Clogging from compressor debris
This is the most common problem. When a compressor wears or fails, metal particles travel through the system and collect on the orifice tube's inlet screen. A heavily clogged orifice tube is often one of the first clues that the compressor is failing.
Desiccant and sludge
A broken-down desiccant bag can release beads that block the screen. Moisture contamination can also create sludge that builds up on it.
Freezing
Moisture in the system can freeze at the orifice tube, where the refrigerant gets coldest. This blocks flow and causes cooling to stop, then return after the ice melts.
Wrong size or damaged O-rings
An incorrectly sized orifice tube causes the evaporator to be under- or over-fed. Damaged O-rings let refrigerant bypass the opening, which reduces the pressure drop and cooling.
Signs of a Clogged Orifice Tube
- Warm or weak airflow from the vents
- Cooling that stops and starts
- Frost on the liquid line near the orifice tube location
- Low pressure on the low side and high pressure on the high side
- Compressor cycling on and off very rapidly
Why It's Replaced During Other Repairs
The orifice tube is inexpensive, and a used one may hold debris that can damage new parts. It's typically replaced whenever the system is opened for a major repair, especially along with the compressor and accumulator. Whatever is found on the old screen also helps diagnose what's happening inside the system.
Related Topics
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