The NPL defines dew point as:‘The temperature at which dew, or condensation, forms as you cool a gas. Where the condensate is ice, this is known as the frost point.’
It’s a common parameter in meteorology, hence the name ‘dew point’ but in this article we are going to look at why it’s important to a wide variety of industrial processes. (If you want to find out about meteorological applications for dew point measurements, here is a short explanation from the UK Met Office.)
Unlike other process variables such as temperature, pressure, and flow, moisture has two unique characteristics:
Hygrometry is the measurement of the moisture content of gases, and a hygrometer senses, measures and outputs or displays the relative or absolute humidity in the gas. As well as the term ‘hygrometer’ it may also be called a:
Humidity is simply moisture that is dissolved in a gas. There is always a level of humidity in the air around us, but when it comes to industrial processes, the levels of humidity often need to be controlled.
Relative humidity is a measure of the humidity in terms of its saturation point. As with dissolving solids in a liquid, a gas can only hold a certain amount of dissolved moisture before it starts to condense back to liquid phase. The saturation point varies depending on the temperature of the gas and the pressure with higher temperatures and lower pressures allowing more moisture to be held in the gas as humidity. If the temperature is lowered or the pressure increase, the saturation point changes and the %rh will also increase.
The %rh is a measure of how close to saturation point the humidity in the gas is.
Moisture content is an important parameter for many processes too. Unlike relative humidity or dew point, moisture content doesn’t change with pressure or temperature: it’s a measure of the actual molecules of water in the gas.
It’s stated as parts per million by volume (PPMV)
Dew point is another variable measure of humidity. Instead of looking at the capacity of the gas to hold moisture, dew point measures the temperature at which liquid moisture will start to condense. As with relative humidity, the dew-point temperature is dependent on the pressure of the gas being measured.
It’s stated as °C dew point, °C frost point or °Ctd
Each of these parameters provide operators with a slightly different benefit. Because dew point is a measure of the temperature at which liquid moisture will condense, it is of most benefit in applications where operators need to avoid moisture condensing – for example in pipework where the ambient temperatures may drop to below freezing. Ensuring the dew point temperature of the gas is below the lowest possible ambient temperature means no liquid moisture will condense and there will be no risk of blockages due to ice.
Dew point is also a common parameter for monitoring the output of industrial dryers and may also be stipulated in quality regulations.
There are many options available to operators to measure dew point in industrial processes - the final choice depends on the specific application and often budget:
Moisture can penetrate virtually any surface, render test results useless, result in poor product quality, cause corrosion in tubing, lead to ice formation at low temperatures, cause premature wear and tear and equipment failure as well as react with other chemicals and gases.
Monitoring the dew point in a process gas gives the following benefits:
Excess moisture in pipework may cause corrosion and reduce the integrity of the equipment and possible leaks, or even explosions. In cold conditions, ice may form and block pipes. Controlling moisture reduces the need for unscheduled maintenance and protects staff.
Moisture has a detrimental effect on many final products. In metallurgy, the level of moisture in a furnace needs careful control to prevent weak products, whereas in pharmaceutical production powders must be kept dry to avoid clumping. Low moisture in necessary in refineries to avoid unwanted chemical reactions.
The highest cost for many processes is the heat needed for dryers. Monitoring the output of dryers ensures that the cycle can be stopped as soon as the drying cycle in complete.
Many global industrial bodies agree international standards for quality, which often include moisture or dew point levels. An example of this is the EASEE gas standards for natural gas quality.
Need advice on your trace moisture or humidity measurement application? Contact us here with your question.
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