Heat of Compression Desiccant Dryers
Air Compressor Desiccant Dryers: Heat-of-Compression vs. Purge Air
Use waste heat from an air compressor to regenerate the desiccant in the off-line tower, thereby saving energy through reduced purge air requirements and/or reduced use of supplemental electric heating.
Item ID: 2
Process Loads & Appliances--Industrial Processes
Regenerative, heatless, twin-tower desiccant dryers use compressed and dried "purge air" for off-line tower regeneration. Up to 15% of the dryer's nameplate rating is used for moisture removal (i.e. for a dryer rated for 1,000 acfm, up to 150 acfm of purge air is required). Moisture-laden purge air is vented to the atmosphere. Some dryers use steam or electric heaters to remove moisture with reduced purge requirements.
Most compressed air systems do not employ heat recovery. A regenerative heatless dryer uses approximately 65 kWh of electrical energy per 100 scfm of compressed and dried air per day. About 80% to 93% of the electrical energy input to an air compressor is lost as waste heat. A heat of compression (HOC) dryer uses this waste heat to reduce purge air and steam or electrical heating requirements. HOC dryers are used with centrifugal and lubricant-free rotary screw compressors.
With a HOC dryer, hot compressed air goes directly into the regenerating tower, then to the aftercooler. Purge air and external heater energy requirements are eliminated. HOC dryers are only compatible with oil-free compressors. Disadvantages include increased piping and installation costs, the necessity for the compressor and the dryer to be in close proximity, dryer effectiveness (dew point may increase to 0ºF to (-)20ºF), and pressure drops when the split stream of regeneration air is reintroduced into the compressed air flow. This is not a retrofit technology, but should be considered for new construction and when existing dryers reach end of life. HOC dryers are commercially available.
Baseline Description: Regenerative Heatless Dryer
Baseline Energy Use: 281661 kWh per year per unit
Assumes a regenerative heatless dryer serving a 250 hp oil-free compressor that runs fully loaded (supplying 1300 cfm) for 8,000 hours per year. The regenerative heatless dryer would require only 0.8 kW/100 cfm so the dryer power requirement would be 10.4 kW (from Compressed Air Challenge Level 1 Training, H1-16). A conventional regenerative heatless dryer would require 65 kWh/day per 100 cfm of treated air given a 15% purge air requirement (Pneumatech). Given 8,000 hours per year of operation, the post HOC dryer annual energy use would be reduced from 281,661 kWh/year to 83,200 kWh/year (by 70.5%).
Manufacturer's Energy Savings Claims:
Currently no data available.
Best Estimate of Energy Savings:
"Typical" Savings: 70%
Energy Savings Reliability: 4 - Extensive Assessment
Note that other desiccant dryer efficiency measures are available for dryers serving oil-lubricated compressors (ET #437). Dew point demand controls delay tower switchover until the in-service tower has exhausted its moisture capture capability. Then the purge air flow to the tower being regenerated switches off when all moisture has been removed. This approach greatly reduces purge air losses (versus operating the towers with a timed switchover such as every two minutes---which requires a constant purge airflow).
Energy Use of Emerging Technology:
84,498.3 kWh per unit per year
Energy Use of an Emerging Technology is based upon the following algorithm.
Baseline Energy Use - (Baseline Energy Use * Best Estimate of Energy Savings (either Typical savings OR the high range of savings.))
Potential number of units replaced by this technology:
Compressed air systems greater than 50 hp account for about 79,682 GWh of electrical energy consumption in the U.S. (From the DOE's "United States Industrial Electric Motor Systems Market Opportunities Assessment"). The number of motors by air compressor size range are 51--100 hp, 36,450 motors; 101--200 hp, 27,288 motors; 201--500 hp, 16,275 motors, 501--1,000 hp, 2,642 motors, and 1000+ hp, 3,848 motors. A conservative estimate of the total compressor drive motor horsepower is made by multiplying the number of motors times the lower value of the motor horsepower range. This yields: 1,858,950 + 2,756,088 + 3,271,275 + 1,323,642 + 3,848,000 or 13,057,955 drive motor hp in the U.S. The compressor motor hp will be pro-rated by population to estimate the total compressor hp in the Northwest region. 0.04 x 13,057,955 hp = 522,318 total motor hp. This number must be reduced to reflect the load served by oil-free compressors that are served by refrigerative dryers and the market penetration of heat-of-compression and dew point demand systems for those compressors with twin tower desiccant dryers. Without market survey data, it will be assumed that 50% of all compressors have refrigerative dryers and that 50% of the remaining compressors are oil free. It is further assumed that 50% of the twin tower dryers already utilize heat-of-compression drying or dew point demand. Incorporation of these assumptions yields a total compressor hp of:
0.5 x 0.5 x 0.5 x 522,318 hp = 65,290 compressor hp. Divide by 250 hp per compressor to obtain an estimate of installations:
65,290 / 250 = 261
Regional Technical Potential:
0.05 TWh per year
Regional Technical Potential of an Emerging Technology is calculated as follows:
Baseline Energy Use * Estimate of Energy Savings (either Typical savings OR the high range of savings) * Technical Potential (potential number of units replaced by the Emerging Technology)
Currently no data available.
Simple payback, new construction (years): N/A
Simple payback, retrofit (years): N/A
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.
Reference and Citations:
Air Best Practices,
Compressed Air Best Practices program
A summary of the technology from the Compressed Air Best Practices program.
Compressed Air Dryers
Describes various types of compressed air dryers and their applications.