1. Calculation and comparison of water content in compressed air:

1. Under the condition that the atmospheric temperature is 30℃ and the relative humidity is 70%, the volume flow rate of the air compressor is 14.3m3/min. What is the amount of water Q1 inhaled after 24 hours of work?

According to the atmospheric pressure dew point/moisture content table, the moisture content g1 at 30℃ is 30.38g/ m3

Q1 = g1 bits of Vt

=30.38× 70% ×14.3×60×24

=437910g

=437.91kg.

2. The pressure dew point after passing the freeze dryer is 15℃. Under the pressure 0.7MPa, what is the water content Q2 after 24 hours through the cold dryer?

At this temperature, the atmospheric dew point is -13℃, and the atmospheric dew point/moisture content table shows that g2 is 1.8764g/ m3.

Q2= g2Vt

= 1.876 * 14.3 * 60 * 24

=38630g

=38.63kg.

3. After the adsorption dryer, the pressure dew point is -35℃, and the pressure is 0.7MPa.

What is the water content Q3 24 hours after passing through the drying machine?

Under this pressure dew point, the atmospheric dew point is -53℃, and the atmospheric dew point/moisture content table shows that g3 is 0.04g/m3.

Q3=g3Vt

= 0.04 * 14.3 * 60 * 24

=824g

=0.824kg.

The above calculation is the saturated water content in the compressed air, in addition to the above 38.63Kg of water through the refrigerated dryer into the compressed air pipeline, the rest of the water in addition to a part of the filter, cold dryer, storage tank drainage valve excluded, there is a considerable part of the compressed air pipeline, after the temperature difference is constantly changing, In addition to the humid compressed air, there is a large amount of liquid water after the freeze dryer, which brings great harm to the equipment and production. Therefore, only through the adsorption dryer can the water adsorption in the compressed air be fundamentally removed, so as to fundamentally solve the harm of the water in the compressed air to the equipment and production.

2. Knowledge about dew point

1. What is dew point? What does it relate to?

The unsaturated air reduces the temperature while keeping the partial pressure of water vapor unchanged, so that the temperature when it reaches the saturated state is called the dew point, and when the temperature falls to the dew point, there are condensation droplets in the wet air.

The dew point of wet air is not only related to temperature, but also related to the amount of moisture in the wet air, the dew point of high water content is high, and the dew point of low water content is low.

2. What is the pressure dew point?

After the wet air is compressed, the density of water vapor increases, and when the compressed air cools, the relative humidity increases. When the temperature continues to drop to the full relative humidity, water droplets precipitate from the compressed air, and the temperature is the pressure dew point of the compressed air.

3. What is the relationship between pressure dew point and atmospheric dew point?

The corresponding relationship between pressure dew point and normal pressure dew point is related to compression ratio, and is generally expressed in charts. In the case of the same pressure dew point, the higher the compression ratio, the lower the corresponding normal pressure dew point. For example, when the pressure dew point of 0.7MPa compressed air is 2 ° C, it is equivalent to the normal pressure dew point of -23 ° C. When the pressure is increased to 1.0MPa, the same pressure dew point is 2 ° C, the corresponding atmospheric pressure dew point is -28 ° C.

4. What instrument is used to measure the dew point of compressed air?

Although the unit of pressure dew point is ° C, its connotation is the water content of compressed air. So measuring the dew point is actually measuring the water content of the air. There are many instruments for measuring the dew point of compressed air, such as "mirror dew point meter" with nitrogen and ether as a cold source, and "electrolytic hygrometer" with electrolytes such as phosphorus pentoxide and lithium chloride. At present, the industry generally uses a dedicated gas dew point meter to measure the dew point of compressed air, such as the SHAW dew point meter in the United Kingdom, which can measure the range of -80 ° C. In addition to the German ESTO dew point instrument.

5. What should we pay attention to when measuring the dew point of compressed air with a dew point meter?

When measuring the dew point of the air with a dew point meter, especially when the moisture content of the air being measured is very low, the operation should be very careful and patient. The gas sampling equipment and connecting pipe core must be dry (at least a little drier than the measured gas), the pipe connection should be completely sealed, the gas flow rate should be selected according to the regulations, and there is a long enough pretreatment time, a little careless, will bring a lot of error. It has been proved that there is a great error when using phosphorus pentoxide as electrolyte "micro-water measuring instrument" to measure the pressure dew point of the compressed air treated by the cold and dry machine. According to the manufacturer, this is because the compressed air will produce secondary electrolysis during the test, so that the reading value is higher than the actual value. And the water content of the compressed air treated by the cold and dry machine is about 1000ppm, which has exceeded the measuring range of the instrument. Therefore, such instruments should not be used when measuring the dew point of the compressed air treated by the cold and dry machine.

6. In which part of the dryer should the pressure dew point of compressed air be measured?

The dew-point of compressed air should be measured with a dew-point meter. The sampling point should be placed in the exhaust pipe of the dryer, and the sample gas should not contain liquid water droplets. The dew points at the other sampling sites are all inaccurate.

7. Can evaporation temperature be used instead of pressure dew point?

The evaporation temperature (evaporation pressure) reading is not a substitute for the pressure dew point of compressed air in a cold dryer. This is because in the evaporator with limited heat exchange area, there is a non-negligible temperature difference between the compressed air and the refrigerant evaporation temperature in the heat exchange process (sometimes up to 4~6 ° C), and the compressed air can be cooled to a temperature that is always higher than the refrigerant evaporation temperature. In addition, the separation efficiency of the gas-water separator between the evaporator and the precooler is impossible to completely remove water, and there is always a part of the small water droplets that are not separated will enter the precooler with the air flow, and there will be secondary evaporation and reduction into water steam, so that the water content of the compressed air increases and the dew point rises. Therefore, in this case, the measured refrigerant evaporation temperature is always lower than the actual pressure dew point of the air.

8. Under what circumstances can temperature measurement be used instead of pressure dew point?

It is very difficult to measure the dew point of air pressure by intermittent sampling with SHAW dew point meter in industrial field, and the test results are often affected by incomplete test conditions. Therefore, in cases where the requirements are not very strict, a thermometer is often used to approximate the pressure dew point of compressed air.

The theoretical basis for measuring the pressure dew point of compressed air with a thermometer is: If the compressed air entered the precooler through the gas-water separator after being forced to cool by the evaporator, the separation efficiency of the condensate water carried in the gas-water separator cannot achieve complete removal of water, but in the case of good discharge of condensate water between the precooler and the evaporator, the condensate water entering the gas-water separator and needs to be removed by gas-water separation only accounts for a small part of the total condensate water. Therefore, the error of measuring the pressure dew point by this method is not very large.

When using this method to measure the pressure dew point of compressed air, the temperature measurement point should be selected at the end of the evaporator of the cold and dry machine or in the gas-water separator. Because at this point the compressed air is cooler. In foreign original imported dry machine also has this method to measure the product dew point temperature.