The smaller tube is the high-pressure line. The larger-diameter tube is the low-pressure line, which is the one you're looking for. It runs from the compressor back towards the passenger compartment.
Low (suction)side is the larger of the two lines, high side is smaller. Suction line should be cool or cold, high side is hot.
The low side line of the system will have a larger diameter metal pipe than the high-side. The low side service port is usually located between the AC compressor and evaporator.
Type 3 refrigerants are called low-pressure refrigerants as they are used in low-pressure appliances. Type 3 refrigerants include: R-11 (CFC) R-123 (HCFC)
The low pressure side of the refrigeration cycle occurs primarily in the indoor unit. Cold, low pressure refrigerant leaves the expansion valve and enters the evaporator coil. The blower fan moves warm indoor air over the coil, and the refrigerant absorbs heat from the air. As it does so, it turns into gas form.
Note that the four-part cycle is divided at the center into a high side and a low side This refers to the pressures of the refrigerant in each side of the system.
Anyway, getting back to this article, if the air conditioning system had R-410A, we know the pressure on the low side of the system will be between 102 and 145 PSIG regardless of the heat load conditions (except for extreme circumstances).
Liquid Line: This smaller, uninsulated line carries high-pressure, high-temperature liquid refrigerant from the condenser coil to the expansion valve and evaporator coil inside the home.
The suction line connects the evaporator to the compressor, the discharge line connects the compressor to the condenser, and the liquid line connects the condenser to the expansion device. The expansion device is typically located at the end of the liquid line, at the inlet to the evaporator.
The larger line typically carries a cool gas and is insulated. This is commonly referred to as the suction line, but it is also called the return line or vapor line. The smaller uninsulated line typically carries a warm liquid. It is most often called the liquid line.
Generally speaking, the larger hose is the low pressure side (suction) and the smaller hose is high pressure (discharge). The ports on your compressor should be labeled as such.
It's attached to the front of the engine and powered by a belt and pulley. It can be at the top or down along the side of the engine, as shown in these two examples: One way to tell the compressor apart from other belt-driven parts – such as the alternator – is to turn the engine on but turn the A/C off.
Connect R22 manifold gauge middle hose onto R22 refrigerant container. Flip R22 container upright and open the valve. Watch left and right gauges to see when pressure equalizes. Left-hand gauge should read below 60psi.
R11 is a colorless and odorless CFC refrigerant that was completely banned from production under the Montreal Protocol for depleting the ozone. It is a low-pressure refrigerant that was commonly used in centrifugal chillers, and before the 1970's it was used as a propellant for aerosols.
Tube-side materials in R-410A coils need to be thicker due to the higher operating pressures associated with R-410A relative to R-22. We're not arguing that there's not a difference in operating pressure between the two. There is – roughly 130 PSI for R-410A evaporators compared to about 70 for R-22.
For R22 refrigerant: High pressure is approximately 1.75 MPa. Low pressure is approximately 0.6 MPa.
The A/C compressor can also be identified because it has two metal tubes coming out of it. The two tubes are different sizes. The smaller tube is the high-pressure line. The larger-diameter tube is the low-pressure line, which is the one you're looking for.
1 The compressor which pumps the refrigerant around the system, is the heart of the air conditioner. Before the compressor, the refrigerant is a gas at low pressure. Because of the compressor, the gas becomes high pressure, gets heated and flows towards the condenser.
The low side port, or the low-pressure service port, is important because it's the point where you connect to recharge the AC refrigerant. Its location varies by vehicle, but you can find it by starting at the compressor. There should be two tubes coming out of the compressor.