Knowledge about dew points

What is dew point? What does it relate to?

The temperature of the unsaturated air is called "dew point" when the partial pressure of water vapor is kept unchanged (that is, the absolute water content is kept unchanged) and the temperature is lowered so that it reaches the saturated state. When the temperature drops to the dew point, condensation droplets are released from 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.

What is the "pressure dew point"?

After the wet air is compressed, the water vapor density increases, the temperature does not rise, when the compressed air cools, the relative humidity increases, when the temperature continues to drop to the relative humidity of 100%, there are water droplets precipitated from the compressed air, then the humidity is the "pressure dew point" of the compressed air.

What is the relationship between "pressure dew point" and "atmospheric dew point"?

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

What instruments are 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 phosphorus pentoxide and lithium chloride as an electrolyte. 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.

What should I 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 the connecting pipe must be dry (at least 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-moisture meter" 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.

Where in the dryer should the "pressure dew point" of compressed air be measured?

The "pressure 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.

Can the evaporation temperature be used instead of the "pressure dew point"?

In a cold dryer, the evaporation temperature (evaporation pressure) reading cannot be used to replace the "pressure dew point" of compressed air. 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℃); The temperature to which the compressed air can be cooled is always higher than the refrigerant evaporation temperature. In addition, the separation efficiency of the "gas-water separator" between the evaporator and the precooler can not be 100%, and there is always a part of the small water droplets that are not separated will enter the precooler with the airflow, and there will be "secondary evaporation" to reduce to water vapor, 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 compressed air.

Under what circumstances can the "pressure dew point" be replaced by a measurement of temperature?

The steps of intermittent sampling to measure air "pressure dew point" with SHAW dewpoint meter in industrial field are quite troublesome, 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 by the "gas-water separator" cannot reach 100%, but in the case of good condensate discharge between the precooler and the evaporator, The condensate that enters the "gas-water separator" and needs to be removed through the "gas-water separator" accounts for only a small part of the total condensate water. So using this method to measure the "pressure dew point" error 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 that's where the compressed air is at its lowest temperature. There is also this method to measure the "dew point temperature" of the finished gas in the original imported cold and dry machines abroad.

 Atmospheric dew point - moisture content relationship table