Automotive air conditioning condensers all adopt the air-cooled type and are components that are difficult to arrange. Experiments have proven that the cooling method where the engine water tank and the condenser are installed in the same vertical plane (arranged vertically, one above the other) is relatively good. The water tank is installed above the condenser, and they share a single fan, which can better solve the heat dissipation problem.

It is generally installed in front of the water tank (at the front end of the vehicle head), sharing a single cooling fan. A fan-forced cooling and heat dissipation method is adopted.

Structures of Tube-Fin Type and Tube-and-Belt Type Condensers

Parallel Flow Type

It is modified on the basis of the tube-and-belt type, but the flat tubes are not bent into a coiled belt shape but cut off individually. There is a header pipe at each end. The header pipes are segmented and separated by separators in the middle, which play the role of refrigerant distribution and collection, and the number of tubes in each segment is unequal. When the refrigerant enters the condenser, it is in a gaseous state with the maximum mass volume, and the number of tubes is also the largest. As the refrigerant gradually condenses into a liquid, its mass volume gradually decreases, the occupied volume gradually reduces, and the number of tubes decreases accordingly. This structural design with variable channel cross-sectional area makes the most rational use of the effective volume of the condenser, optimizes the flow and heat exchange of the refrigerant. Under the same windward area, the heat exchange capacity of the parallel flow structure is increased by more than 30% compared with the serpentine tube-and-belt type, the air-side resistance remains basically unchanged or even smaller (due to reduced thickness), and the refrigerant-side flow resistance is reduced to only 20%-30% of that of the serpentine tube-and-belt type. It is the most suitable replacement model of condenser for converting from R12 to R134a.

Evaporator:

In cars, it is installed near the driver's seat, and the air supply blows from front to back.

For medium and large-sized vehicles, it is generally installed at the rear of the carriage. Multiple air outlets are arranged on the four walls inside the vehicle, and cold air is blown into the vehicle through air ducts by fans.

The evaporator and the fan are often integrated into one unit, known as the evaporation assembly. It includes the evaporator, expansion valve, supply fan, thermostat, fan speed regulator, louvered air outlet, thermal insulation layer, water baffle, drain pipe and box shell. Some also include heaters, defrosting devices, etc.

The evaporators of automotive air conditioners are specially made, including tube-fin type, tube-and-belt type and laminated type. The tube-fin type is made by sleeving aluminum heat dissipation fins on copper tubes and then expanding the copper tubes, which is the same as ordinary local air conditioners. The tube-and-belt type is formed by overlapping and welding perforated flat tubes with serpentine heat dissipation plates. Although its manufacturing is complex, its heat exchange efficiency is about 10% higher than that of the tube-fin type.

Tube-Fin Type Evaporator

Tube-and-Belt Type Evaporator

The laminated type is composed of two aluminum plates stamped into complex shapes and stacked together to form refrigerant channels, with serpentine heat dissipation fin belts sandwiched between every two channels. This structure is complex to manufacture and has high welding requirements, but its heat exchange efficiency is about 20% higher than that of the tube-fin type.

Receiver Drier

It is composed of a liquid storage tank, filter screen, desiccant, absorption tube, etc.

Functions

Receives liquid refrigerant from the condenser and delivers it to the expansion valve at a certain flow rate.

Filters out dirt such as dust, grease, and metal particles in the system.

Absorbs moisture in the refrigerant.

A fusible plug is usually installed on the top, and various pressure switches and service valves are installed on the cylinder body or head.

The condition of the refrigerant can be observed through the sight glass (peep glass) on the liquid storage type filter drier.

The desiccant used in the R134a receiver drier is XH-7 type molecular sieve.

The sight glass has two functions: first, to indicate the flow state of the refrigerant. When there is a lack of liquid or moisture content, the observation glass can show different colors and bubbles. It appears light blue when the moisture content is extremely low, and light red when the moisture content increases. When the system is operating normally with an appropriate refrigerant charge (the liquid level in the receiver maintains a certain height), a small amount of bubbles can be seen occasionally; when the speed changes, there are slightly more bubbles, but they all disappear quickly. If the charge is too large, the receiver acts as a section of thick pipe, and no bubbles can be seen; if the charge is too small, a large number of bubbles or foam will be observed. Second, it is used to observe the flow of refrigeration oil in the pipeline.

Expansion Valve

The throttling device of automotive air conditioners mainly adopts the thermal expansion valve (TXV for short), whose working principle is the same as that of ordinary thermal expansion valves, and its structure is also very similar. According to the characteristic that cars require rapid cooling, ball valves are widely used in expansion valves for cars in recent years. The selected expansion valve is generally 20%-30% larger than the refrigerating capacity of the evaporator. The cooling capacity of car air conditioners is 2.56-2.8kW, and an expansion valve with a capacity of 3.83kW (or 1 ton of refrigeration, TR) can be selected.

H-type Expansion Valve

It is an integral type expansion valve, also known as a block valve. Its working principle is the same as that of the externally balanced type.

It is installed between the inlet and outlet pipes of the evaporator, and the sensing end of the valve is directly connected to the suction pipe, so it is not affected by the environment and has high sensitivity.

The H-type expansion valve eliminates the external balance pipe, temperature sensing bulb and its capillary tube of the externally balanced expansion valve, making it directly connected to the inlet and outlet of the evaporator. Later, the connecting joints were simplified, evolving into the currently common block valve structure.

The H-type expansion valve improves the sensitivity of the thermal expansion valve, can quickly control the temperature, enhances the vibration resistance and reliability of the valve, is easy to install, and has no restrictions on the installation position. Therefore, it is widely used in automotive air conditioners at present.

Bypass Valve

The compressor is driven by the automobile engine. When the vehicle speed increases, the compressor speed also increases, leading to an increase in suction volume and a decrease in suction pressure until frost forms on the surface of the evaporator. At this time, the refrigerant flow should be controlled for defrosting, and the commonly used method is to install a solenoid bypass valve.

The functions of the bypass valve are: to prevent an abnormal drop in evaporation pressure, control the temperature inside the vehicle within the specified range, prevent frost formation on the evaporator, or play a defrosting role when frost forms on the evaporator.

The opening and closing of the bypass valve are controlled by a low-pressure switch or a temperature switch.

There are two types of working circuits for the bypass valve:

Connect the bypass valve between the receiver and the suction valve of the compressor. When the outlet air temperature of the evaporator is lower than the specified value, the control circuit opens the bypass valve. A part of the high-temperature and high-pressure gaseous refrigerant is directly sucked into the compressor, thereby reducing the refrigerant circulation rate, and the temperature of the cold air blown from the evaporator rises accordingly.

Install the bypass valve between the compressor outlet and the evaporator inlet. When Po ↓, part of the high-pressure vapor is directly shunted to the evaporator, thereby increasing Po and To and playing a defrosting role, so it is also called a defrosting bypass valve or defroster.