I recently received a call from the operator of a ready mix batch plant here in Denver. Let’s call him Jon. Jon reported using a 15 horsepower screw compressor to drive control valves, pneumatic pumps, and various other air operated devices within the plant. He runs the compressed air at a discharge pressure of 105 to 120 PSIG, flowing up to 47 SCFM. He was having problems with water in his compressed air system and asked that I come in for a visit.
Like many ready mix plants in the Rocky Mountain region, this plant was largely enclosed to protect equipment from harsh weather. In the south by contrast, it’s quite common to see plants with minimal enclosures. Similarly mobile batch plants used at construction sites have few enclosures. Yet even enclosed plants – like the one Jon operates – have outdoor compressed air lines that serve bulk tanks. Water is especially prone to condensing in these lines. Indeed Jon reported that his most troublesome pneumatic devices were tampers at the top of outdoor cement tanks.
Batch plants are also inherently dusty. Dust from aggregates and cement inevitably wafts into the air. This happens regardless of whether the plant has a building enclosure or not. A dusty atmosphere poses a challenge to any air cooled piece of equipment. Refrigerated compressed air dryers are almost always air cooled in the size range appropriate for ready mix batch plants. As a refrigerated dryer’s condenser becomes caked with cement and aggregate dust, the dryer loses performance and eventually fails.
Indeed Jon had a small stack – three units – of failed refrigerated compressed air dryers. Each had died from dust.
Jon had seen some of our online videos explaining the features and benefits of single tower deliquescent dryers. With no moving parts or power requirement, these types of dryers are well suited for dusty operating conditions. Jon wanted my help sizing and locating this class of dryer for his plant.
After a survey of the plant, we decided that a model D8 could be installed immediately upstream of a receiver tank positioned about 75 linear pipe feet away from the compressor. This distance allows the compressed air to cool close to the ambient air temperature, reducing the moisture load to the dryer. In locations where a long pipe run upstream of the dryer is not practical, we recommend installing a trim cooler. Remember cooling compressed air prior to a deliquescent dryer is critical for optimizing performance.
Jon made several comments during the visit about how his water problem is attributable to temperature flux (heating, cooling, and re-heating) of the compressed air as it passes through different portions of the plant. I explained to Jon that this is half right. Water has been forming in his air lines because warm humid air discharged from the compressor condenses when it cools. To be more exact, water condenses when the air cools below its dew point. Heating the air back up does not produce condensation.
Another important point I made to Jon is that the dryer is only responsible for removing humidity from the air line. Other devices – separators and filters – are responsible for removing liquid water.
Here is a typical system layout showing the various components in a well-designed air system using a single tower deliquescent dryer.