When facilitating the "Study Your Supply Side" section of the "Fundamentals of Compressed Air Systems" workshop of the Compressed Air Challenge (www.compressedairchallenge.org), I always point out that that there's more to a successful and efficient dryer installation than just sizing the unit properly. For example, it's important to understand what can happen in the piping upstream of the dryers and how to reduce their operating costs. Also, if dryers will be installed in parallel, knowing how to maintain a balanced flow across them is essential. Simply paralleling dryer packages is no guarantee the air flows will balance equally. Differences in the pressure drop across filters, piping and even the dryers themselves can cause unbalanced flow that can overload dryers. So, here, we'll look at examples of some problems.
But first, let's consider what happens in the piping upstream of the dryers:
• The moisture content of the air doubles with every 20°F rise in temperature at the discharge of the compressor.
• Oil carryover increases as the operating temperature of a lubricated compressor goes up.
• In most cases the air leaving the compressor's after-cooler is saturated.
• When the ambient temperature is lower than the pressure dew point temperature of the compressed air, the compressed air will cool as it travels along the pipe and condensate will form.
• The larger the temperature differential is between the compressed air pressure dew point temperature and the ambient temperature, the larger the volume of condensate.
• The longer the piping is between the compressor and dryer, the more the compressed air cools, boosting the volume of condensate created.
• Moisture and oil in vapor form will pass through filters.
• Coalescing filters prefer water over oil.
• Slugging a coalescing filter element with large volumes of condensate increases the wet band in the filter element, causing the velocity of the air flowing through the filter element to rise, which results in condensate carryover and, if the slugs are large enough, the drain may not be able to remove the condensate before some of it carries over. Slugging also can damage the filter element.
• Mist eliminators can handle slugs of condensate; however, like any other filter, if overloaded, they will allow condensate to pass through.
• Failing to drain the filter housing often enough or for a sufficient duration results in condensate carryover. It's crucial to maintain condensate drains because they're the weakest link in the cleanup process.
• Installing a pre-cooler and moisture separator ahead of a desiccant dryer can prevent condensate formation or reduce the amount created and may save energy.
These points are important in troubleshooting problems and ensuring that desiccant dryers are installed so they will deliver air at their rated pressure dew point and achieve their expected desiccant life.
DISAPPOINTING DEW POINT
A few years ago a plant in Texas installed a new 2,600-scfm heated blower purge desiccant dryer equipped with energy savings controls downstream of air-cooled lubricated rotary screw compressors. The compressed air equipment was placed outdoors, which is common in that part of the country. The dryer was sized for a peak flow of 1,635 scfm at inlet conditions of 115°F at 95 psi. A mist eliminator filter was installed upstream of the dryer's pre-filter and an air-to-water heat exchanger was installed downstream of the dryer to eliminate the temperature spike that sometimes occurs at switchover. Yet, even with all these precautions, the dryer only produced a -20°F pressure dew point instead of its rated -40°F level. So, we were asked to determine why.