As compressors draw air from atmosphere, there is a need to perform a drying operation if the application demands. ISO 8573-1 sets out the purity classes of compressed air, numbered 0-9. Each class indicates the levels of specific contaminant – in this case water – allowable in a cubic metre of compressed air.
“Typically, general-purpose air is ISO quality class 4 for water contamination, which is equivalent to a +3°C pressure dew point,” explains Keith Atkinson, sales director – compressed air treatment and gas generation at Ingersoll Rand, of which CompAir is a division. Below dew point, water will condense out of the air on to interior surfaces.
In refrigeration dryers, an air-to-refrigerant heat exchanger reduces the temperature of the warm, wet compressed air, lowering its moisture retention capability until it effectively starts to ‘rain’ inside the chamber. The system expels water into a collection line, while the dried compressed air exits into the system.
“If you want even drier air, perhaps down to -40°C or -70°C pressure dew point for applications such as electronics, pharmaceuticals or food, then a desiccant dryer is normally the way forward,” says Atkinson. These pass compressed air across a tower/vessel filled with desiccant. Moisture molecules attach to the desiccant beads, allowing dry air to pass out. This is the chemical process of adsorption. However, gradual saturation of the desiccant occurs, demanding regular regeneration to regain drying capacity. Desiccant dryers therefore have two towers/vessels, the first to dry the incoming compressed air and the second to provide regeneration. The towers switch tasks when requiring regeneration.
ENERGY EFFICIENCY
Atkinson suggests that most plants today choose energy-efficient dryers, as today’s high energy prices make for swift return on investment.
“A standard non-cycling refrigeration dryer is either on or off, while an energy-efficient model will switch on and off automatically in line with your compressed air demand profile,” says Atkinson. “Furthermore, CompAir refrigeration dryers not only cool the air, but also an integral thermal mass. Once the thermal mass reaches a certain level, the dryer recognises it has sufficient stored cooling capacity and switches off the refrigerant compressor to save energy.”
Achieving energy efficiency with desiccant dryers is more challenging due to the need for regeneration. Aside from conventional methods, such as using compressed air or supplementary heating, another option for desiccant regeneration is to take heat from the process compression phase, known as heat-of-compression (HOC) technology. However, HOC dryers are only compatible with oil-free compressors, and usually incur increased piping and installation costs.
In any case, once installed, both refrigeration and desiccant dryers require consistent monitoring during use.
“Cleanliness is a key factor,” states Atkinson. “Filters against the fan inlet can get blocked, particularly in a dusty environment such as a cement works. For desiccant dryers, an additional check is to calibrate the hygrometer on a regular basis, every one or two years.”
FOOD FOR THOUGHT
Andy Lill, quality air business development manager at Atlas Copco in the UK, says that for 80-85% of applications, class 4 moisture content is acceptable, lending itself to refrigeration dryer technology. Although this dryer type is typically energy-efficient, it does of course use refrigerant gases, which are ozone-depleting materials. Today, many companies monitor these type of emissions, especially as any potential leaks could be penalised by the Environment Agency.
He adds that the company has just switched to a new refrigerant gas, as some very low global warming potential refrigerants are semi-flammable, which makes them unsuitable for industrial use, because if refrigerant leaks into the compressed air circuit, it would send air that could contain a flammable gas into the customer’s application. “We’re at the mercy of the refrigerant gas industry to provide a solution as regulations tighten further in the years ahead.”
Lill points out that many companies that would previously only consider a refrigerant dryer are now scrutinising more expensive desiccant technology, even if they do not require higher levels of moisture removal. The trade-off with dryers of this type is that they are slightly less energy-efficient than refrigerant models, although the latest technology innovations help.
“When pushing compressed air through a dryer there is back pressure, which in turn means the compressor has to work harder, demanding more energy consumption. Instead of using an off-the-shelf heat exchanger, we engineer our own to ensure very low pressure differential,” explains Lill.
For plant engineers, the heat exchanger should be an area of focus. These devices need a throughput of ambient air to remove heat generated by the refrigeration process. “You need a well-ventilated area with cool air supplied across the heat exchanger – through a grill on side of unit – to ensure efficient operation and good moisture removal,” says Lill.
IN THE PIPELINE
David De Pril, director of product management – Europe, at compressor brand ELGi, points out yet further key issues of not drying compressed air. “Any dust that passes through the system will cling to the sides of the pipework and, over time, cause a blockage. These problems relate to general industrial air. The issues are arguably worse in what we call process air applications, where compressed air comes into contact with the product. If this air contacts medical or food ingredients, perhaps in powder form, any moisture could change its characteristics, including its density. You could end up dosing the wrong quantity of ingredient.”
He adds that refrigerant dryers are cost-effective and suitable for most general industrial applications. “They are also compact and, in many cases, can integrate within the dimensional envelope of the compressor,” says De Pril. “In contrast, desiccant dryers are more bulky and expensive, but offer higher levels of dryness.”
While De Pril points out that ELGi dryers are compatible with any compressor brand, he says that understanding capacity is key: “If the compressor’s capacity is higher than that of an under-specified dryer, you will no longer reach the dew point. They dryer will not have time to cool the air and remove the moisture at a sufficient speed. Right-sizing the application is very important.”
Energy efficiency is another highly important factor in the selection of a compressed air dryer, says De Pril. “Sure enough, there is a price differential between an energy-saving dryer and a standard dryer, but the life of a compressed air system is typically 10 years. Over 10 years, the energy-saving dryer will recoup the difference in purchase price several times over because it consumes less energy.”
Another huge contributor to total cost of ownership is of course maintenance requirements. Here, De Pril suggests a little housekeeping can go a long way. “Take care of system filters [before the dryer] as they will protect your dryer from exposure to excessive dust loads,” he explains. “Another tip is to monitor your dew point. If there is a change in dew point, it can lead to nasty surprises over time. Most dryers feature a dew point monitor, although it is often advisable to site further dew-point meters downstream, close to more sensitive applications.”