Dew Point Monitoring Prevents Production Downtime and Product Rejection
Compressed air users often assume the air flowing from the pipe system is clean and dry. However, this is rarely the case and besides dust and dirt particles, moisture in particular is the most underestimated threat. This not only applicable for the correct functioning and service life of the compressed air installation itself, but also for the processes and components controlled by compressed air. Correct control and accurate monitoring of the moisture content with dew point sensors is therefore more important than many people think!
Moisture can freeze in compressed air systems and cause rust and pitting in pipes and components. It can also flush out the lubricant resulting in accelerated tool wear and damage to valves and cylinders. Moist air is also a rewarding breeding ground for bacteria, which especially in the food and pharmaceutical industries can lead to product rejection and costly production downtime. It is therefore strange that many companies limit themselves to measuring only basic quantities such as pressure, flow and (absorbed) power. Because it is precisely dew point measurements that can prevent a lot of problems and (unnecessary) costs.
VPVision processes all results of flow, pressure, dew point, temperature, and energy use with which the system behavior of compressed air systems can be continuously monitored and analyzed.
Moisture Content and Dew Point
The dew point is expressed in degrees Fahrenheit and is a measure of the amount of water vapor in (compressed) air or in a gas. We explicitly refer to pressure dew point for compressed air because the dew point temperature is measured at a pressure that is usually a factor 6 to 8 higher than the atmospheric pressure. This is important because changing the pressure of a gas also changes the dew point temperature. The lower the pressure, the lower the dew point. For example, if atmospheric air with a relative humidity of 30 to 50% is compressed to a pressure of 100 psig, that air becomes 100% saturated.
The current compressed air temperature (which is higher than the ambient temperature) is the actual pressure dew point. As soon as the temperature drops, the moisture in the compressed air will condense, allowing many liters of water per week to enter the system.
Causes of Moisture Problems
Moisture problems can have various causes. A common occurrence is the flooding of water separators or combined oil/water separators behind the compressor due to mechanical problems such as a stuck float. If this stays unnoticed, water flows unobstructed into the compressed air system and can collect in a buffer tank. Investing in a float drain, a timer drain, or an electronic condensate drain system is therefore not a superfluous luxury. Clogged cooling elements of aftercoolers and oil coolers, but also ignorance about the operation of refrigerated dryers in relation to the ambient temperature are other causes of moisture problems. For example, if a refrigerated dryer after the (wet) buffer tank cools the air to a pressure dew point of (actually) 50°F and in winter and on cool evenings the ambient temperature of the pipe network drops to 40°F, this is often ignored. But that 10-degree temperature drop alone, creates about 1.45 gallons of condensation water in the pipe system during a 40-hour working week and with 24/7 production even 6 gallons! With dew point measurement behind the cooler, this can be discovered very quickly, and measures can be taken on time. So, when choosing a dryer and the pressure dew point that is achieved, the average ambient temperature must be taken into account in addition to the requirements that the process places on the compressed air!
Installing Dew Point Meters
Mirror, capacitive metal oxide and polymer sensors are the three best-known instruments for measuring the dew point. Capacitive polymer sensors are, however, the best protected against dust and dirt, insensitive to condensation, have good long-term stability and have an attractive price/performance ratio. With these sensors, a change in capacitance is translated into the dew point temperature, displayed in degrees Celsius or Fahrenheit. VP Dew Point Sensors in particular have a unique internal heating system which allows them to recover very quickly after exposure to a lot of moisture. This can be easily the case, for example, if dryers, water separators and/or drains do not function properly.
The big question is of course, how many dew point sensors should be installed in a compressed air system and where they should be placed in order to realize reliable measurements. The simplest (starting) solution is to install a dew point sensor just after the dryer and in front of the dry tank. In this way, the dryer is monitored for correct operation. It is also possible to place the sensor after the dry tank, but then take into account a delay in the measurement signal. If there are two or more dryers in parallel, it is recommended to install a dew point sensor after each dryer. After all, if only one sensor is used in the central pipe to the buffer tank, it will not immediately be possible to determine which dryer is causing problems in the event of a deviating measurement. It is also wise to install an extra dew point sensor in the supply line of critical processes. If something goes wrong with the dew point, timely action can be taken and costly production downtime can be avoided.
The Power of Combined Measurements
To determine the causes of a deviating dew point quickly and in a targeted manner, it is necessary to perform additional measurements. By installing a 3-in-1 VPFlowScope sensor after the dryer, mass flow, pressure and temperature are also measured. As soon as the dew point rises, it will then be possible to quickly see what is causing this, for example, a rising refrigeration dryer inlet temperature and/or a high airflow. A decrease in flow and/or pressure can also be an indication that the dryer is contaminated internally, while flow measurements can also be used to monitor the leakage level. This is important because, apart from the energy loss, condensation can flow back into the pipe network via leaks, which risk increases with lower dew point applications.
Additional measurements can also be used to visualize the pressure loss across the dryer and filter installation, so that the moment of filter replacement can be predicted precisely. By also measuring the power, in combination with the other measurements, the efficiency of the dryer can be calculated and compared with that of other dryers. This can then be used to optimize maintenance, for example, and also to see afterwards whether the right choices have been made when purchasing the dryer(s).
In the context of energy savings, it is important to take a critical look at the pressure dew point actually required and/or the need to cool all air centrally. Decentralized drying, only for the processes that require it, is also an option. Often people choose ‘out of safety’ for air that is too dry, with a pressure dew point that is too low. However, this will cost an unnecessary amount of energy. A good guideline for determining the correct dew point is the ISO 8573-1:2010 standard. Here the dew point values are divided into seven classes: Class 0 to Class 6. Class 0 (Defined as anything better than Class 1) is the highest category that only applies in rare cases, for example when compressed air is needed in the highest category cleanrooms. Class 1 has a pressure dew point of -94 °F, Class 6 of +50 °F.
Properly analyzing what is really needed can save a lot of money both when investing in the dryer installation and during subsequent operation. To illustrate, the energy consumption of a refrigerated dryer is roughly 0.8 kW/100 CFM, while an adsorption dryer requires about five times as much energy, i.e. about 3 to 4 kW/100 CFM!
Wet and Dry Storage Tanks
It is recommended to install a wet tank downstream of the air compressor and upstream of the compressed air dryer. It is also recommended to install a separate ‘dry air storage tank’ downstream of the dryer. This protects the dryer against overload and also makes it possible to size the dryer based on the average flow instead of on a (short-term) peak demand. As a result, a smaller dryer can generally be chosen. In addition, the extra tank helps to achieve a more stable system pressure and can even have a beneficial effect on compressor sizing and control.
The base for a healthy and optimally profitable compressed air installation is permanent monitoring, in which dew point measurements are combined with flow, pressure, temperature and power measurements. By displaying everything clearly in a monitoring system specifically developed for this purpose, such as VPVision, the system behavior can be monitored and analyzed 24/7, 365 days a year.
Fluctuations in demand, in dew point, a compressor temperature that is too high; everything is visualized in good time and an alarm is issued if things get out of line. It also provides extremely valuable information for maintenance optimization, to make the right investment decisions for future expansion and for optimization of the total compressed air system. Permanent monitoring extends equipment life, reduces maintenance and energy costs, and prevents product loss and production downtime.
This article was created in collaboration with Compressed Air Challenge Instructor Frank Moskowitz and Pascal van Putten, CEO of VPInstruments from Delft, Netherlands. It was published in the Compressed Air Best Practices magazine.