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 Sun Zenghui, Yang Huaxiong, Li Daming 

(Xi'an United Ultrafiltration Purification Equipment Co., Ltd., Xi'an, Shaanxi)

Abstract: In the summer under high-temperature conditions, more than half of the air compressors and purification equipment in the air compression station have experienced overload and even triggered alarms and shut down. The main reasons for this are as follows: First, insufficient attention was paid to the circulating cooling water system, manifested as insufficient cooling water volume, high water temperature, and deviation in water quality; second, when selecting unit products, the pressure, temperature coefficients, and extreme working conditions were not considered, resulting in an under-sized selection; the system lacks a backup machine. Third, the air compressor factory paid insufficient attention to the end cooler and even cut corners, causing the exhaust temperature of the air compressor to be generally high, with water-cooled units reaching over 45℃ and air-cooled units even over 55℃. The regulations for the purification industry specify working conditions: in Europe, it is 35℃; in the United States, it is 38℃. In China, the industry standards include both working conditions. Some manufacturing plants, to cope with high-temperature working conditions, set the maximum allowable inlet temperature for adsorption dryers at 40℃, and for refrigeration dryers, under special design, the maximum allowable inlet temperature is 45℃. However, the actual operating conditions are not satisfactory. 

This article analyzes the impact of high temperatures on the purification equipment of the compressed air station, and proposes countermeasures and solutions. 

1. The influence of environmental temperature and humidity on the compressed air system: 

1.1 Atmospheric Conditions 

(Temperature and relative humidity) determine the total amount of water entering the air compressor system. Additionally, high temperatures and high humidity levels can affect the outlet temperature of the circulating water, thereby determining the liquid water discharge volume of the separator and filter, as well as the absolute moisture content (g/m³) of the saturated humid air entering the dryer. It is worth noting that the amount of water vapor entering the dryer is not directly related to the atmospheric conditions; it is merely a single-valued function of the saturated humid air temperature. Because even when the atmospheric relative humidity is as low as a very dry 20%, when compressed to 7 bar and cooled to room temperature, supersaturated humid air has formed, and the condensed water it carries is discharged through various separators and filters. Therefore, the factors determining the moisture content吸入量 of the dryer are only the cooling water temperature (for air-cooled machines, it is the ambient temperature) and the cooling and separation efficiency.

1.2, Purification product standards and specified operating conditions: 

As is well known, no single technology or product can solve all problems. One of the important aspects of product standards is to specify the applicable scenarios, scope and related conditions for such products. Generally, the specified operating conditions for refrigeration-type compressed air dryers (as per JB/T10526-2005) and for adsorption-type compressed air dryers (as per JB/T10532-2005) are exactly the same (see Table 1). The inlet air temperature is specified in two ways: Scheme A/Scheme B are respectively set at 35℃/38℃. The former is suitable for temperate regions, while the latter is suitable for subtropical regions. Although China has a vast territory, most areas still have hot climates in summer with high temperatures. The main difference between the north and south is the humidity level and the duration of high temperatures. Therefore, most production plants have adopted Scheme B, that is, the inlet air temperature is 38℃ ± 1℃. As mentioned earlier, the inlet air temperature mainly depends on the cooling water temperature. Considering a temperature difference of about 5℃ for heat exchange, the cooling water is specified as 30℃ ± 3℃. In actual calculations, 32℃ is mostly used. It is worth noting that the specified cooling air inlet temperature and the ambient temperature in Table 1 for Scheme B are both 38℃ ± 3℃. This value contradicts the inlet air temperature of the dryer, which is 38℃. In reality, the exhaust temperature of most air compressors with air cooling is 8-15℃ higher than the ambient temperature. These types of machines become the hardest-hit areas during summer's high-temperature conditions. Table 1: Operating Conditions Specified in the Product Standards for Dryers

1.3 The Impact of High Temperature on Adsorption Dryers:
Table 2: Comparison Table of Saturation Temperature and Absolute Moisture Content

From the above table, it can be seen that when the temperature rises from 38℃ to 48℃, the moisture content increases by 63%. Additionally, from the "isobaric adsorption graph", it can be observed that at the working point of the dryer (25~45℃), the isobaric adsorption line is highly sensitive to temperature. As the temperature increases, the adsorption capacity of the adsorbent significantly decreases. Combining these two factors and considering statistical laws, it can be roughly concluded that for every 1℃ increase in temperature, the working load of the dryer increases by approximately 10%. 

Another negative impact of increased intake air temperature is that the adsorption heat significantly increases and leads to a substantial rise in the exhaust temperature of the dryer (8-20℃, the higher the intake air temperature, the greater the amount of adsorption heat generated). Since the cooling process mostly uses the product gas from the dryer outlet, this undoubtedly raises the initial temperature of the cooling air. At this time, the temperature can even reach 50-60℃, which inevitably results in incomplete cooling. Before the cooling process is completed, the temperature of the tower remains as high as 80-100℃. After the switch, for a period of time, there is a serious drift in the outlet temperature and dew point of the adsorption tower, with the temperature and dew point generally increasing by more than 20℃, and the duration exceeds half an hour. 

1.4 Impact of High Temperature on the Cold Dryer: 

The increase in the inlet temperature of the dehumidifier not only directly increases the heat load of the refrigeration system, but also increases the moisture content in the inlet gas. When water vapor condenses, the system needs to provide additional latent heat for the phase change, and both of these require more cooling capacity or energy consumption. High-temperature gas will also increase the load on the evaporator, which may cause the dew point of the product gas to rise, resulting in a decrease in the water removal capacity of the dehumidifier. In addition, an increase in the ambient temperature or the temperature of the cooling water will directly affect the heat exchange efficiency of the condenser, leading to a significant decrease in the system's cooling capacity. An increase in the temperature of the condenser will also inevitably cause an increase in condensation pressure. When this exceeds the limit that the equipment can withstand, it may even cause equipment and personal safety accidents. 

1.5 The impact of high temperature on the oil removal filter: 

The compressed air filter is currently formulating product standards. However, in the measurement method (GB/T13277.2-2015) and the experimental method (GB/T30475.1~2-2013), both the inlet air temperature and the ambient temperature are specified as 20℃. It is worth noting that the residual oil content indicators provided by the filter manufacturer for the majority of cases are based on the performance indicators provided by the filter element supplier at 20℃. However, the actual operating temperature of the filter is much higher than 20℃, and its residual oil content indicator is several times, even up to 10 times or more, higher than the indicator provided by the filter manufacturer. (See Table 3) Table 3: Oil/Gas Ratio of Injected Screw Oil at Different Temperatures

Although this table does not indicate the absolute oil content, it can be seen that for every 5°C increase in temperature, the volume of oil vapor doubles. This conclusion is consistent with the statistical data that "for every 5°C increase in temperature, the residual oil content doubles". This is because modern precision/super-precision technologies have an oil-soluble aerosol removal rate of up to 99% - 99.99%. Any filtration method has no effect on gaseous oil vapor.

As the temperature decreases, the oil vapor will condense into liquid oil aerosol, and the oil vapor can also partially deposit on the surface of the adsorbent. Lubricating oil can cause the adsorbent to lose some of its activity, and in severe cases, it can lead to oil poisoning, causing the adsorbent to fail.

2. Countermeasures:

As mentioned above: An increase in intake air temperature will cause the load of the dryer and filter to increase, performance to decline, and even endanger equipment and personal safety. From the statistical data: The exhaust temperature of the air compressor in summer is much higher than the ideal working temperature and specified conditions of the dryer. Therefore, the following three methods can be adopted to remedy or compensate for this situation.

2.1 When the cooling water temperature can be guaranteed to be ≤ 32°C, a low-temperature difference high-efficiency water cooler (the temperature difference between the inlet and outlet water of the cooler is designed to be 3°C) can be added between the air compressor and the adsorption dryer. The temperature difference △t between the inlet and outlet air of the cooler is 10°C. 

2.2 When there is no cooling water available on site or the temperature of the cooling water itself is higher than 35℃, an additional low-temperature difference water cooler should be installed. It can use chilled water or a self-provided water chiller. The full flow of chilled water or the mixture with the circulating water should enter the cooler. The temperature difference between the inlet and outlet of the cooler, △t, is 10 to 15℃.

2.3 Under the same conditions as above, pre-cooling combined with adsorption can be adopted. The pre-cooling machine is different from the traditional dry air machine. The former is mainly used for cooling and reducing temperature rather than freezing and removing water. That is, it uses the refrigerant cooling method to limit the reduction of the compressed air temperature, making it suitable for the adsorption dryer to work effectively within a reasonable temperature range. Currently, this application product belongs to non-standard customization and urgently needs to be standardized to form a series of products.

2.4 Correct selection: When the actual working pressure is low and the working temperature is high, the following correction table can be referred to: Table 3: Correction Coefficients for Pressure and Temperature (DH Sample)