Dew Point Demand Controls for Twin-Tower Compressed Air Desiccant Dryers
Compressed Air Purge Air Cycle Control: Dew Point vs. Timer
A compressed air drying system that uses dew point demand controls to shut off the dried purge air flow when the media in the off-line tower has been regenerated and delays tower switchover until the media in the in-service tower has been exhausted.
Item ID: 437
Process Loads & Appliances--Industrial Processes
With a regenerative heatless dryer, 15% of the dried compressed air flow is "lost" as dried purge air to regenerate the dryer desiccant. In twin-tower dryer designs, each tower is often on-line for a four-hour timed interval. Constant purge air cycles are governed by timers. A dew point demand control system can be retrofitted into a heatless or regenerative external heater dryer to sense the actual dew point of the outlet air, delaying tower shift-over during periods of low moisture loads until the drying bed is saturated. With this method of dryer control, the purge flow through the off-line tower is also shut off after drying has been achieved with tower switchover delayed until moisture removal capacity is used up.
An externally heated regenerative dryer uses electrical energy, microwaves, or steam to heat the purge air from 300°F to 400°F before directing it through the off-line drying tower. With this dryer design, dew point demand controls can reduce purge air requirements from 5% to 7% of the dried airflow. The external heater is shut off after providing the energy required for off-line tower regeneration. A dew point demand control can reduce air requirements from 2.5% to 3.0% of dried compressed air flow, compared to 15% for a regenerative heatless dryer. Compressed air requires about 18 kW/100 scfm to produce, so savings are significant.
Baseline Description: Regenerative Heatless Dryer with Timed Switchover
Baseline Energy Use: 1140 kWh per year per hp
Dew point demand controls can reduce the number of regeneration cycles for a compressed air heatless regenerative dryer through monitoring dew point of the compressed air at the dryer exit. The controller bases regeneration on dryer performance instead of a timed cycle. These controls can yield 50% to 80% in energy savings, depending upon maximum allowable dew point. Savings achievable must be calculated on a case-by-case basis. Assume a 200 hp compressor operating constantly for 350 days per year. A 1,000 cfm regenerative desiccant dryer would match the output of the compressor and require 65.14 kWh/day per 100 cfm of delivered air requiring a baseline energy use of about 227,980 kWh per year (or 1,140 kWh/year per hp). Assuming a 50% reduction in purge air requirements, energy savings from adopting the dew point demand technology onto this system amount to about 113,995 kWh/year. This means that total dryer purge requirements are 227,990 kWh/year or 1,140 kWh/compressor hp.
Manufacturer's Energy Savings Claims:
Currently no data available.
Best Estimate of Energy Savings:
"Typical" Savings: 40%
Low and High Energy Savings: 30% to 80%
Energy Savings Reliability: 5 - Comprehensive Analysis
A 350 hp rotary screw air compressor operates with a twin tower desiccant dryer rated at 1,639 scfm. The compressor is capable of delivering 1,700 cfm of air at 100 psig. A standard regenerative heatless dryer requires about 15 cfm of dried and compressed purge air per 100 cfm of rated dryer capacity. The purge air requirement for the dryer in this example is thus 246 scfm.
A replacement dryer is equipped with dew point demand controls and an external heater. The purge requirement is reduced to 7 cfm/100 cfm of compressed air production. An 18 kW heating element is provided for purge air heating. A dew point demand control system can be retrofitted onto a regenerative external heater dryer to sense the actual dew point of the outlet compressed air, delaying tower shift-over during periods of low moisture loads until the drying bed is saturated. With this method of dryer control, the purge flow through the off-line tower is also shut off after drying has been achieved with tower switchover delayed until moisture removal capacity is used up. Data logging indicates a heater run time of only 27.8%. Purge air requirements are thus reduced to an average of about 35 cfm. Net energy savings are the energy use reduction from the air compressor as it now meets reduced airflow requirements (about 210 cfm reduction) less the electrical energy required for the external heating element.
Energy savings, after the heater takeback is considered, amount to about 322,880 kWh/year. This is equivalent to a savings of 922 kWh/year per compressor hp. When contrasted with conventional practice, this amounts to a 922/1140 x100% = 80.8% reduction in dryer-related energy consumption. As the dryer in this example is considerably oversized and thus benefits greatly from the installation of dew point demand controls, a "typical" energy savings of 40% will be assumed.
Energy Use of Emerging Technology:
684 kWh per hp 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 compressed air systems that are adequately served by refrigerative dryers and the market penetration of dew point demand systems for those compressors with twin tower desiccant dryers that already have dew point demand. Without market survey data, it will be assumed that 50% of all compressors have refrigerative dryers and that 10% of the existing twin tower dryers already utilize dew point demand. Incorporation of these assumptions yields a total compressor hp of:
0.5 x 522,318 hp x (0.9) = 235,043 compressor hp. (Given an average compressor size of 300 hp, this would amount to 783 dryer retrofits).
Regional Technical Potential:
0.11 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)
Installed first cost per: hp
Emerging Technology Unit Cost (Equipment Only): $74.00
Emerging Technology Installation Cost (Labor, Disposal, Etc.): $0.01
Baseline Technology Unit Cost (Equipment Only): $0.01
An Atlas Copco quotation (dated 10-14-2014) for a field mountable purge control panel to reduce dryer purge to become dependent upon dewpoint rather than use of a time had a cost of $1,486. This control is for a 20 hp compressor with a desiccant air dryer rated at 106 cfm at 100-psig. Cost is thus $74/hp. It is expected that considerable economies of scale will exist and the first cost should be recomputed when cost data for larger installations is available.
Simple payback, new construction (years): 1.8
Simple payback, retrofit (years): 1.8
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.