As an important production power, compressed air is used in all aspects of the industrial field. During the production of compressed air, the water in the air will enter the compressed air system together with the compressed air. The moisture in the compressed air will lead to the corrosion of the compressed air pipeline and the propagation of microorganisms; If the water is not removed, the condensate formed will accumulate at the low point of the system, which will pose a potential threat to industrial production, such as the failure of air control elements, increased wear of equipment, or directly lead to the stop of the production process.

Traditional refrigerated dryers and adsorption dryers have long been well-known products. Most of these dryers are installed in the air compression station and dry the compressed air of the whole system behind the compressor. We know that each user has different requirements for the dryness of compressed air at the point of use of compressed air, and different dryness requirements also occur in the compressed air system of the same user. Therefore, the compressed air drying method is to dry only the parts actually needed according to the required dryness. Whether for test gas, workshop gas or field gas, mobile gas or fixed gas, compressed air users put forward higher requirements for the timeliness and reliability of compressed air drying. It is precisely on the basis of the demand for drying the compressed air at the point of use that the film infiltration type compressed air dryer was born. At first, the membrane dryer was a solution for the use point of small amount of gas, and later evolved into various suitable application fields. 2. Molecular membrane characteristics

The polymer permeation membrane materials have the characteristics of water molecule permeation and diffusion. As shown in Figure 1, if there is gas partial pressure (different concentrations) at both ends of the molecular membrane, the gas molecules will diffuse through the osmotic membrane from the side with high partial pressure to the side with low partial pressure.

The diffusion rate of gas molecules through the polymer membrane depends on three aspects:

a. The structure of the permeating material through which diffusion is required;

b. Size of gas molecules

C. Vaporization temperature of gas

Through continuous experiments in the laboratory, scientists found that there is a kind of synthetic polymer membrane. At room temperature, as shown in Figure 2, the diffusion speed of water vapor molecules through the polymer membrane is 20,000 times faster than that of oxygen molecules. This synthetic molecular membrane is an ideal material for separating water molecules from other gas molecules. This characteristic makes this synthetic polymer membrane become the basic material for manufacturing membrane dryers.

3. Structural composition of polymer membrane

At the beginning of the use of polymer membranes, because only the basic materials of permeation membranes were used, the selectivity of molecular membranes to gases was relatively low. As shown in Figure 3, this means that the gas with a lower diffusion rate can also pass through the membrane substrate material, including nitrogen, especially oxygen (up to 5% penetration). That is to say, the low selectivity permeation membrane will cause leakage in a certain amount and change the proportion and structure of various gas components in the air, which is not suitable for use in breathing air.

At the same time, the gas molecules directly pass through the membrane wall, which will cause the dirt in the compressed air to accumulate on the membrane surface and affect the service life of the membrane. The permeation of other gases on the membrane surface is used as the blowback gas, so the blowback gas volume is a constant based on pressure. It is not possible to adjust the blowback gas volume, so it has low flexibility. Therefore, it can not adapt to the application of large flow rate, and the loss of blowback volume is also large.

With the progress of technology, the laboratory is trying to solve the problems of low selectivity osmosis membrane. Several years later, highly selective permeation membranes with different technologies were manufactured. Taking the high selective permeation membrane of Bakerol Company as an example, a layer of coating is attached to the inner side of the high selective membrane, as shown in Figure 4, which basically achieves the ideal effect that water molecules can penetrate the permeation membrane.

Due to the low cost and simple manufacturing of low selectivity osmotic membrane, there are a large number of low selectivity osmotic membrane dryers in the market. The way to distinguish low selectivity osmotic membrane dryers is to close the dryer outlet and measure whether there is still compressed air consumption. If compressed air is still consumed, a low selectivity osmotic membrane is used. If there is no compressed air consumption, a highly selective osmotic membrane is used.

4. Formation of hollow polymer membrane

The macromolecular raw materials of the ingredients are controlled by temperature in the raw material furnace. After adding additives, they flow out of the raw material furnace in liquid form. The raw materials grow rapidly after encountering water. Through precise geometric shape control, they become slender hollow tubes as shown in Figure 4. Under the control of underwater constant temperature and excipients, the membrane continues to grow and gradually forms strength. At this time, the coating starts to be sprayed on the inner wall of the midair pipe. After the constant temperature and constant speed control at different stages of the 100 meter long underwater, the hollow osmosis membrane reaches the strength and then leads out of the water to enter the spool for winding. The finished hollow high selective permeation membrane has a length of several thousand meters for each spool.

5. Working principle of membrane dryer

As shown in Figure 5, wet compressed air enters the hollow membrane tube through the upper inlet, and then flows through the membrane tube to the bottom. Because the partial pressure of water vapor inside and outside the membrane is different, the water molecules will diffuse from the inside of the membrane with higher partial pressure to the outside of the membrane with lower partial pressure, and the dry compressed air will be obtained at the bottom. A small part of this dry compressed air is led out for expansion and decompression to form extremely dry compressed air. The extremely dry compressed air after decompression is introduced outside the osmotic membrane to purge the diffused water molecules. In this way, the distribution gradient of water molecules inside and outside the hollow molecular membrane is increased, and the diffusion speed of water molecules is accelerated. As a result, the humidity of compressed air at the bottom of the osmotic membrane drops sharply, so as to dry the compressed air.

6. Composition of membrane dryer

The structure of the membrane dryer is shown in Figure 6, which is composed of an upper end cover, a shell and a tube core. The tube core is composed of a plurality of hollow membrane permeation tubes.

The flow direction of compressed air is shown in Figure 7. The moist compressed air enters from the inlet on the end cover, and then flows through the central sleeve of the tube core to the bottom of the dryer.

The compressed air changes direction, flows from the inside of the membrane permeable fiber tube from bottom to top, and then flows out from the top of the tube core.

After the dry compressed air flows out of the pipe core, it is delivered to the rear end use point through the outlet of the end cover. 7. Impact of environmental parameters

Because the drying effect of the membrane dryer depends on the diffusion speed of water molecules, and the diffusion speed of water molecules is related to the partial pressure gradient of water molecules. Because the greater the working pressure of compressed air, the greater the gradient of water molecules formed inside and outside the membrane. Therefore, the greater the working pressure, the better the drying effect of the membrane dryer.

Secondly, molecular membrane is used to carry out osmotic diffusion separation of water molecules by water pressure gradient. Therefore, at the outlet of the membrane dryer, more dry compressed air can always be obtained than at the inlet. This is different from the traditional freezing dryer and adsorption dryer. After membrane separation and drying, the compressed air does not reach a constant pressure dew point value, but a constant relative humidity value RH. This is more realistic for users of compressed air.

Based on the above analysis, the amount of residual water behind the membrane dryer is obviously related to the water content of the compressed air at the inlet. The higher the water content at the inlet, the greater the residual water, and vice versa. That is to say, the drying effect at the outlet of the membrane dryer is related to the dew point of the compressed air pressure at the inlet. Therefore, the membrane dryer provides a constant "dew point drop". No matter what the dew point of the upstream compressed air pressure is, the compressed air can always be further dried after being dried by the membrane dryer. This outstanding advantage is beyond the reach of the cold dryer and the suction dryer. When the dew point of compressed air pressure at the inlet of the dryer is lower than the processing capacity of the dryer (such as 3 ℃), or when the dew point of compressed air pressure at the inlet of the dryer is lower than the processing capacity of the dryer (such as - 40 ℃), the dryer or the dryer will no longer have any drying effect. In this case, the osmotic film dryer can further improve the dryness of compressed air. 8. Precautions in the use of membrane dryer

Because the polymer membrane separates water molecules by molecular gap, it has the precision of. Therefore, to ensure the reliable operation of the drying film group, the compressed air shall meet the following conditions:

a. Compressed air shall not contain liquid condensate;

b. The oil content of compressed air is less than 0.01mg/m3;

c. Cannot have more than 1 μ M of particulate impurities.

In order to ensure the normal operation of the osmotic membrane dryer, a reliable filter of high grade is necessary.

As shown in Figure 8, the important installation principles of the membrane dryer are:

a. Meet the filtering accuracy requirements;

b. There shall be no extended connecting pipe between the filter and the membrane dryer (to prevent condensate generated after the compressed air cools);

c. Only anti-corrosion connecting pipe fittings can be used between the filter and the membrane dryer;

d. Clean the inside of the pipe before installing the drying pipe;

e. Do not use liquid sealant (Loctite, Delo...);

f. Ensure that there is no installation stress during installation.

The membrane dryer is mainly used for direct drying before the compressed air consumption point. The recommended installation diagram is shown in Figure 9. 9. Practical case analysis

Since the birth of the membrane dryer, it has occupied the market with rapid development. So far, as shown in Figure 10, membrane dryers have outstanding advantages and strong competitiveness in many application fields:

9.1 Case I: Application on railway locomotives

On railway locomotives, compressed air is mainly used for braking systems. Based on the fault principle, the brake shoe opens under pressure, and locks when the pressure is lost. The brake system is a system on the locomotive.

Desirability of compressed air drying: compressed air with high humidity will lead to corrosion of brake shoe cylinder, which will cause brake failure or failure to operate.

Advantages of membrane dryer: increase the reliability of the whole system, reduce maintenance, suitable for frequent startup and shutdown, suitable for locomotive lightweight, almost no floor space, suitable for mobile use, no moving parts, no wearing parts, unlimited installation location, no power consumption, etc.

The installation diagram is shown in Figure 11.

9.2 Case II: Outdoor application of oilfield in winter

In winter, the outdoor temperature of northern oil fields sometimes reaches more than - 40 ℃. However, oil production facilities are all in outdoor environment, and pipeline control valves need to be driven by compressed air. It is a big challenge to prevent the compressed air pipeline from freezing in winter and ensure that all valves can move flexibly.

Compressed air drying: even if the compressed air treated by the suction dryer reaches - 40 ℃ when it is used outdoors, it will still produce condensate because the ambient temperature is lower than the dew point of the compressed air pressure, and then cause freezing to block the pipes or valves, thus forming a fault. It is required that the dew point of the pressure at the use point is lower than the ambient temperature, which is the condition that the compressed air outdoor needs to meet.

Advantages of membrane dryer: it can provide a fixed dew point drop, which enables the membrane dryer to solve this problem. The compressed air is cooled to the same temperature as the outdoor air in the outdoor air storage tank, and the dew point is further reduced after passing through the membrane dryer to ensure that the pressure dew point of the air consumption point is lower than the ambient temperature, thus avoiding the problem of condensation and freezing.

9.3 Case 3: Application on laser cutting machine

In industry, laser cutting machines and laser welding machines use high-power CO2 laser beams to operate. Starting from the laser generator, the beam is guided to the laser head through the mirror optical system channel.

Compressed air drying: laser loss will greatly reduce the working efficiency of the laser machine. Keeping the optical path clean is accomplished by compressed air. If the dryness of compressed air is not enough, the laser beam will be lost due to condensation on the mirror surface.

Advantages of membrane dryer: increase the efficiency of the whole system, improve the reliability of the system, and reduce maintenance.

9.4 Case 4: Application of devices

The hospital uses an integrated respiratory system, each of which is about 30 liters/minute (on average, each healthy person breathes 7 liters/minute).

Compressed air drying: the compressed air reaches the breathing standard. Besides the condensate, it also removes the smell and oil mist, which can only be achieved by using activated carbon. However, the activated carbon will soon saturate when the compressed air humidity is high, so it can not continue to adsorb other substances. Therefore, the compressed air shall be dried first to ensure the normal operation of the activated carbon filter.

Advantages of the membrane dryer: long service life, stable and reliable, less maintenance, meeting the air consumption rules of the respiratory system, convenient for mobile use, and suitable for intermittent air use.

The process flow diagram is shown in Figure 13.

9.5 Case 5: Application on CMM

The CMM adopts a ruby measuring head and makes the measuring head contact the measured object precisely and move continuously through the air bearing. At the same time, three X/Y/Z axes of the CMM have three grating rulers to record the coordinate value of the current position of the ruby measuring head (that is, the actual size of the measured value workpiece), and output it to the computer software for processing to draw a three-dimensional graph of the measured workpiece.

Compressed air drying: Both the air bearing and the grating ruler need high cleanliness compressed air for driving and protection. The compressed air with insufficient dryness will greatly affect the movement accuracy of the air bearing and the measurement accuracy of the grating ruler.

Advantages of the membrane dryer: The high reliability of the membrane dryer makes the high-value measuring machine equipment well protected, increases the life of the whole system, improves the measurement accuracy of the system, and reduces maintenance.

The application photo is shown in Figure 14. 10. Conclusion

The membrane dryer has its characteristic advantages, and its high-quality characteristics are summarized as follows:

·Dry air is available immediately without pre operation time;

·The principle of steam concentration difference ensures the drying process;

·Permeation area guaranteed by high selective hollow membrane;

·Low gas consumption and compact structure (TWIST 60 technology);

·Maintenance free;

·The temperature of the compressed air flowing through does not change;

·Ensure constant relative humidity;

·After drying, the air composition remains unchanged, and the breathing air certification is obtained;

·Small size, no power consumption