Supply Side Review: Heat of Compression-Type Dryers

The supply side of a compressed air system has many critical parts that factor in to how well the system operates and how easily it can be maintained.   Dryers for the compressed air play a key role within the supply side are available in many form factors and fitments.  Today we will discuss heat of compression-type dryers.

Heat of compression-type dryer- Twin Tower Version

Heat of compression-type dryers are a regenerative desiccant dryer that take the heat from the act of compression to regenerate the desiccant.  By using this cycle they are grouped as a heat reactivated dryer rather than membrane technology, deliquescent type, or refrigerant type dryers.   They are also manufactured into two separate types.

The single vessel-type heat of compression-type dryer offers a no cycling action in order to provide continuous drying of throughput air.  The drying process is performed within a single pressure vessel with a rotating desiccant drum.  The vessel is divided into two air streams, one is a portion of air taken straight off the hot air exhaust from the air compressor which is used to provide the heat to dry the desiccant. The second air stream is the remainder of the air compressor output after it has been processed through the after-cooler. This same air stream passes through the drying section within the rotating desiccant drum where the air is then dried.  The hot air stream that was used for regeneration passes through a cooler just before it gets reintroduced to the main air stream all before entering the desiccant bed.  The air exits from the desiccant bed and is passed on to the next point in the supply side before distribution to the demand side of the system.

The  twin tower heat of compression-type dryer operates on the same theory and has a slightly different process.  This system divides the air process into two separate towers.  There is a saturated tower (vessel) that holds all of the desiccant.  This desiccant is regenerated by all of the hot air leaving the compressor discharge.  The total flow of compressed air then flows through an after-cooler before entering the second tower (vessel) which dries the air and then passes the air flow to the next stage within the supply side to then be distributed to the demand side of the system.

The heat of compression-type dryers do require a large amount of heat and escalated temperatures in order to successfully perform the regeneration of the desiccant.  Due to this they are mainly observed being used on systems which are based on a lubricant-free rotary screw compressor or a centrifugal compressor.

No matter the type of dryer your system has in place, EXAIR still recommends to place a redundant point of use filter on the demand side of the system.  This helps to reduce contamination from piping, collection during dryer down time, and acts as a fail safe to protect your process.  If you would like to discuss supply side or demand side factors of your compressed air system please contact us.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

 

Heat of compression image: Compressed Air Challenge: Drive down your energy costs with heat of compression recovery: https://www.plantservices.com/articles/2013/03-heat-of-compression-recovery/

 

Heat Recovery from an Air Compressor

On the whole most of us are quite aware of the considerable savings that can be accomplished by wise use and recovery of energy.   One way that a plant can save substantially is to capture the energy that an electric motor adds to the compressed air from the air compressor.  As much as 80% to 93% of the electrical energy used by an industrial air compressor is converted to heat.  A properly designed heat recovery system can capture anywhere between 50% to 90% of this energy and convert it to useful energy.

The heat recovered is sufficient in most cases to use in supplemental ways such as heating water and space heating, however generally there is not enough energy to produce steam directly.

IngersollRand_R-series-R110
Ingersoll Rand Rotary Screw Compressor

 

Packaged air cooled rotary screw compressor lend themselves easily to heat recovery, supplemental heating or other hot air uses very well due to their enclosed design.  Since ambient air is directed across the compressors aftercooler and lubricant cooler where the heat can be easily collected from both the compressed air and the lubricant.

Packaged coolers are normally enclosed cabinets that feature integral heat exchangers and fans.  This type of system only needs ducting and an additional fan to minimize back pressure on the air compressors cooling fan.  This arrangement can be controlled with a simple thermostat operated vent on a hinge and when the extra heat is not required it can be ducted outside the facility.

The recovered energy can be used for space heating, industrial drying, preheating aspirated air for oil burners or  other applications requiring warm air.  Typically there is approximately 50,000 Btu/Hr of energy available from each 100 SCFM of capacity (at full load).  The temperature differential is somewhere between 30°F – 40°F above the air inlet temperature and the recovery efficiency is commonly found to be 80% – 90%.

We all know the old saying there is “no free lunch” and that principle applies here.  If the supply air is not from outside the plant a drop in the static pressure could occur in the compressor cabinet thereby reducing the efficiency of the compressor.  If you choose to use outside air for makeup, you might need some return air to keep the air above freezing to avoid compressor damage.

Heat recovery is generally not utilized with water cooled compressors since an extra stage of heat exchange is required and the efficiency of recovering that heat is normally in the 50% – 60% range.

To calculate annual energy savings:

Energy Savings (Btu/Yr) = 0.80 * compressor bhp * 2,545 Btu/bhp-hour * hours of operation.

If we consider a 50 HP compressor:

.080 * 50bhp * 2,545 Btu/bhp-hour * 2080 hrs/year =  211,744,000 Btu/yr

Where 0.80 is the recoverable heat as a percentage of the units output, 2,545 is the conversion factor.

Cost savings in dollars per year = [(energy savings in Btu/yr)/Btu/fuel) x ($/unit fuel)]/primary heater efficiency.

If you would like to discuss saving money by reducing compressed air demand and/or any EXAIR product,  I would enjoy hearing from you…give me a call.

Steve Harrison
Application Engineer
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Photo courtesy of Ingersoll Rand CC BY 3.0, https://en.wikipedia.org/w/index.php?curid=32093890