How expansion valve auto AC works
Different types of auto AC
Auto AC can use either an expansion valve or orifice tube design. This post explains how an expansion valve auto AC system works.
What is an expansion valve?
An expansion valve meters the flow of liquid refrigerant into the evaporator where it changes from a liquid to a gas by absorbing heat from the cabin air. The expansion valve can have a remote sensing bulb or an integral sensing device (called an H block). This diagram shows a remote sensing bulb. The remote sensing bulb is connected to the expansion valve by copper tubing filled with an expanding gas. This tubing is referred to as a capillary tube.
As the sensing bulb detects more heat, the gas in the bulb and capillary tube expands, causing the diaphragm in the expansion valve to open and allow more liquid refrigerant into the evaporator. The additional refrigerant lowers the temperature at the sensing bulb, causing the gas to contract and shut off or slow the flow of refrigerant inside the expansion valve.
What happens inside the evaporator?
Liquid refrigerant sprays into the evaporator. Hot cabin air flows across the evaporator. The liquid refrigerant absorbs this cabin heat, which causes it to boil and change state from a liquid to a gas, much like water boils on a stove and turns into steam. The heat required to make the refrigerant change from a liquid to a gas, is coming from the cabin air that passes across the evaporator.
Once the refrigerant changes state to a gas, it continues to pick up more heat from the cabin air. This additional heat is referred to as superheat. A certain amount of superheat is common in all expansion valve systems, which is different than an orifice tube style system. The superheated gas then leaves the evaporator and flows into the compressor.
What the compressor does
The superheated gaseous refrigerant enters the compressor where it’s compressed into a high-pressure gas and pushed into the condenser.
The job of the condenser
The condenser has three jobs:
1) De-superheat the refrigerant. Remove the extra heat created during both the superheat phase in the evaporator and the extra heat created during the compression cycle.
2) Remove more heat from the gaseous refrigerant to the point where it condenses back into a liquid.
3) Sub-cool the refrigerant. This sub-cooling is an important process because it reduces the temperature of the high-pressure liquid refrigerant closer to its evaporating temperature so it can more easily changes state inside the evaporator. Sub-cooling increases the net refrigeration effect of the system.
Temperature drop across the condenser
Carmakers use serpentine/parallel flow and tube and fin style condensers. You can measure the efficiency of the condenser by measuring the temperature drop between the inlet and outlet of the condenser.
In an expansion valve system, you should see a 60°F — 80°F drop for serpentine or parallel flow condenser and a 20°F — 50°F drop for a tube and fin condenser. If your readings don’t show that, check for clogged fins, incorrect fan speed or internal blockage.
©, 2020 Rick MuscoplatPosted on by Rick Muscoplat