Extending the Lifespan of Dryers: An Analysis of Hande Oil-Water Separators

With the development and progress of the domestic automotive industry, vehicle safety has received increasing attention from society. At the national level, following the release of standards for passenger car braking systems, on October 10, 2014, the National Standardization Administration issued GB12676-2014 'Technical Requirements and Test Methods for Commercial Vehicle and Trailer Brake Systems,' which, from the perspective of laws and regulations, provides a general framework for automotive industry professionals in designing vehicle braking systems.

As is well known, in the current domestic commercial vehicle sector, the vast majority of braking systems are air brake systems. Whether it is a standalone air compressor system used in pure electric vehicles or the conventional vehicle air compressor system, during operation, it inevitably introduces more or less oil-water mixtures and impurities into the downstream braking system. Especially for vehicles using conventional internal combustion engines, part of the engine oil residue is carried into the downstream pipeline during operation.

Excessive oil-water mixtures and impurities can cause leaks or sticking in brake system components, rust and blockage in pipelines, leading to abnormal component function, affecting braking system performance, and potentially causing vehicle safety issues. The reliable performance of a braking system depends on every component of the system. The failure or abnormal operation of any single component can significantly reduce the system’s performance, posing potential braking safety risks for the vehicle.

Providing clean compressed air for the braking system can effectively ensure the normal operation of brake system components. Currently, the dryers equipped in braking systems on the market can filter water vapor from compressed air, but they have almost no effect on separating oil mist. Moreover, domestic commercial vehicles are often used in non-standard environments, drying maintenance is infrequent, and the drying tanks are rarely replaced for long periods, which easily leads to oil-water mixtures and impurities entering the downstream system, causing abnormalities in braking components. Based on this situation, oil-water separators, as efficient devices for separating and filtering oil-water mixtures and impurities, are gradually being widely applied in domestic commercial vehicles.

● Function of the Oil-Water Separator

An oil-water separator, also known as a condenser, is sometimes referred to by some OEMs as a "compressed air oil-water separator." The oil-water separator serves as the first line of defense in ensuring the compressed air in the braking system is clean. Under extreme operating conditions, oil-water mixtures and impurities in the system can severely affect the performance and lifespan of braking system components. The oil-water separator is generally installed between the air compressor and the air dryer and requires minimal maintenance during long-term use.

When properly matched, the oil-water separator can filter (separate) up to 90% of oil-water mixtures and impurities, and the maintenance cycle (or lifespan) of the dryer cartridge can be extended by at least three times. A well-matched oil-water separator and dryer can provide clean compressed air for the braking system, ensuring stable and reliable braking performance.

● Working Principle of the Oil-Water Separator

In 1989, HALDEX launched its first electronically controlled oil-water separator (Figure 1, left). In 2010, HALDEX introduced a pneumatically controlled oil-water separator (Figure 1, right). The former is mature and stable, while the latter features a simpler structure and higher cost-performance ratio. Currently, both types of oil-water separators are widely used domestically, with the former mainly used in buses and the latter primarily in trucks.

Figure 1 Electric and Pneumatic Oil-Water Separator

The oil-water separator consists of an upper and a lower part. The upper part is mainly responsible for separating the oil-water mixture and impurities; the lower part is mainly responsible for discharging the oil-water mixture and impurities from the separator. The lower part is equipped with either an electric or pneumatic automatic drain valve.

The working principle of the oil-water separator is shown in Figure 2. Compressed air enters the spiral pipe through the side air inlet. Inside the oil-water separator, the compressed air flows through a downward spiral space, and under the action of centrifugal force, the oil-water mixture and impurities are left on the baffle. When the compressed air reaches the bottom of the spiral track, it flows upward through a vertical passage and exits through the top outlet. Meanwhile, the condensed oil-water mixture and impurities on the baffle flow down the spiral pipe under gravity into the bottom collection chamber.

Figure 2 Oil-Water Separator

For electrically controlled oil-water separators, the lower part is an electrically controlled automatic drain valve. When the drain valve is powered and activated, the oil-water mixture and impurities in the bottom collector will be discharged. The electrically controlled automatic drain valve is generally controlled by a timer relay or by taking power from the brake light. For pneumatically controlled oil-water separators, the lower part is a pneumatically controlled automatic drain valve, as shown in Figure 3.

Figure 3 Pneumatically Controlled Automatic Drain Valve

When the oil-water separator is supplying pressurized air to the brake system, as shown in Figure (a), the air pressure at the oil-water separator (p1) increases, causing the intake valve diaphragm to open. Compressed air flows through the intake valve and is stored in the oil-water separator collection chamber (p2) until the pressure on both sides of the diaphragm returns to balance, at which point the intake valve diaphragm closes.

When the unload valve of the air dryer opens, the air pressure between the air compressor and the dryer decreases, opening the exhaust valve diaphragm, as shown in Figure (b). Oil-water mixtures and impurities are discharged through the exhaust valve into the atmosphere or flow into an external collection container. When the pressure in the oil-water separator collector (p2) drops to a certain level, the diaphragm closes.

● Oil-Water Separator Matching Requirements

The mounting bracket of the oil-water separator must be strong enough to prevent damage to the piping connections and valve body caused by vehicle vibrations. The oil-water separator should be installed in a location with sufficient space for installation, maintenance, periodic inspection, and ventilation. It should also be located away from the wheels to avoid valve body contamination from tire splashes and to prevent damage to the oil-water separator (including the bracket, piping, and fittings) from minor external impacts to the vehicle.

The drainage outlets of the oil-water separator and the automatic drain valve must be installed vertically downward, with an inclination angle not exceeding 15°. The surrounding heat source should be relatively small, and the ambient temperature is recommended to be between -40℃ and 80℃. The installation location should be as close as possible to the air dryer to achieve the optimal separation and drainage effect.

To prevent water accumulation, the pipeline from the air compressor to the oil-water separator must have a certain continuous slope, and the inner diameter of the connection interface between different pipes should not be less than 1/2 of the inner diameter of the connecting pipe. Otherwise, a vertical extension of the connecting pipe from the air compressor outlet is required to form a slope toward the oil-water separator.

A similar sloped gradient is also needed from the oil-water separator to the dryer. The material, length, and size of the connecting pipe from the air compressor to the oil-water separator should be designed and selected to ensure that the compressed air temperature reaching the oil-water separator is within the range of 5℃ to 65℃. Too low an inlet air temperature should be avoided, as it may cause condensation water to freeze in the pipeline or at the oil-water separator inlet in cold environments; likewise, too high an inlet air temperature should also be avoided, as it may damage the sealing components of the oil-water separator or the automatic drain valve, reducing the separation and drainage performance of the oil-water separator.

The air compressor pipeline should not significantly obstruct the internal airflow, as this increases the risk of pipe freezing. At the same time, the use of 90° elbows should be avoided as much as possible.

Main technical parameters

● Maintenance of Oil-Water Separator

The regular maintenance interval of the oil-water separator mainly depends on the working environment of the separator. If the product is found to be malfunctioning during vehicle testing or operation, necessary inspection and maintenance should be carried out immediately. After disassembly and maintenance, check whether the product functions normally and whether there is any air leakage. If it is found during actual use that the recommended maintenance cycle is too long or too short, users can adjust it according to their own circumstances.

The generally recommended maintenance periods are as follows: city buses: 1 year or 50,000 km; urban buses and trucks: 1–2 years.

For air leaks caused by carbon deposits or oil contamination, the inside of the aluminum housing of the oil-water separator needs to be cleaned regularly, while other components can be cleaned or replaced as needed. The filter screens, valve cores, O-rings, and gaskets of the automatic water drain valve should also be cleaned or replaced as appropriate.

● Common Faults of Oil-Water Separator

In the aftermarket, the main fault mode of oil-water separators is air leakage at the exhaust port. This is mainly due to long-term lack of maintenance, leading to excessive accumulation of oil-water mixture and impurities, causing wear of the automatic drain valve core (electrically controlled), exhaust diaphragm (pneumatically controlled), and sealing components, resulting in air leakage; or in winter, ice formation in the accumulated oil-water mixture and impurities can cause the valve core or exhaust diaphragm to jam, leading to air leakage.

For situations lacking maintenance, users must perform timely maintenance according to installation requirements. Proper matching and use, along with reasonable maintenance, are fundamental to ensuring the normal operation of the oil-water separator.

For the fault caused by winter freezing, it is mainly concentrated in the application market of electronically controlled oil-water separators in the central and northern regions of China. The author suggests that the vehicle oil-water separator be maintained before winter to remove the oil-water mixture and impurities that may accumulate. At the same time, the following methods can be used to help avoid air leakage caused by freezing in winter.

The automatic discharge valve of the oil-water separator is heated by a heater. Generally, there are two designs, using the automatic water release valve body coil and the extra coil for heating. The author does not recommend additional coil heating, its structure is complex and there are safety hazards caused by overheating. After testing, by continuously supplying power to the main body coil, the coil is heated as a heating element, and it can withstand continuous heating for 72 hours at room temperature (20°C); The coil power is 12~15 watts, simulating the driving conditions of the vehicle air compressor at -40°C ambient temperature, and the automatic water release valve can be heated to more than 0°C within 6 minutes, which can meet the requirements of end customers.

The time relay is used to control the interval sewage discharge of the automatic water discharge valve. The electronically controlled oil-water separator is generally set to use a brake signal and discharge once every time it brakes. For long-distance trucks and buses with fewer brakings, not braking for a long time may lead to excessive accumulation of oil-water mixtures and impurities, resulting in ice and air leakage. The scheme controlled by time relay can effectively increase the number of exhausts, achieve the purpose of draining the oil-water mixture and impurities, and avoid freezing and air leakage.

● Summary

The quality of air compressor products on the market is uneven, there is a problem of large oil channeling, before matching the oil-water separator, the problem of high claims for dryers is common in major domestic OEMs. This situation also confirms that the brake system components are very likely to work in a poor environment containing oil-water mixtures and impurities, and there are certain hidden dangers in vehicle braking safety. In the European market, oil-water separators began to be used in commercial vehicles in 1989, and so far they have been widely used in first-tier brands such as Volvo, Mercedes-Benz, MAN and Iveco.

Major domestic truck and bus OEMs are also paying more and more attention to and configuring oil-water separators, and for China IV models, many OEMs have used oil-water separators as standard. At present, the configuration rate of oil-water separators for heavy and medium-sized commercial vehicles in China is only about 5%, as an efficient and cost-effective product, the author believes that in the next few years, the configuration rate of oil-water separators will be greatly improved.