Replacing Desiccant with Refrigerated Compressed Air Dryers
Compressed Air Dryer: Refrigerated vs. Desiccant
A strategy to cut compressed air dryer energy use in applications where refrigerated dryer dew point is sufficient, providing air with a higher dew point while reducing electricity use.
Item ID: 69
Process Loads & Appliances--Industrial Processes
Compressed air system designers sometimes install a twin-tower desiccant dryer when a refrigerant dryer would meet their requirements. Regenerative heatless dryers can consume up to 15% of the compressed air supplied to meet purge air requirements. Industries that replace such desiccant dryers with a cycling refrigerated dryer can save money from reduced energy costs and reduced use of desiccant fill.
The first step in selecting a compressed air dryer is understanding the compressed air drying requirements. Dew point is the air temperature at which entrained water vapor condenses. The pressure dew point is the lowest temperature that compressed air can be exposed to without causing condensation of entrained water vapor. The lower the dew point, the more energy a dryer consumes. A pressure dew point of 37 to 40 degrees F is sufficient for most industrial plant air requirements and can provide air of Class 4 quality or above (Ruppelt, 2013) (Vaisala, 2011). The goal is to provide an adequate dew point at the lowest operating cost.
Dryers are selected to deliver a dew point 15°F below the lowest ambient temperature that the compressed air will encounter to accommodate the cooling that occurs when compressed air is expanded to atmospheric pressure. Avoid running compressed air distribution system piping outdoors in colder climates. Refrigerant dryers produce pressure dew points between 35°F and 50°F. Regenerative desiccant dryers provide dew points from -40°F to -150°F. A refrigerant dryer uses about 10 kWh per day of electrical energy per 100 scfm of supplied dried compressed air. A regenerative heatless dryer requires purge air in the amount of 15% of the dryer nameplate rating and consumes 65 kWh per day per 100 scfm of dried compressed air supplied (Pneumatech, 2013). Energy savings of up to 85% are achievable when a refrigerative, rather than a desiccant dryer, can be used for plant air drying purposes.
Baseline Description: 1000 scfm regenerative heatless desiccant dryer
Baseline Energy Use: 237 kWh per year per unit
A 200 hp oil-lubricated rotary screw compressor would produce about 963 cfm of plant air at 100 psig. A refrigerant dryer would require about 35,676 kWh/year to dry this air flow (assuming continuous operation and 10.15 kWh per 24 hours per 100 cfm (Pneumatech, 2013), with a pressure dew point of about 40 degrees F). A regenerative heatless dryer would require about 237,761 kWh annually to provide the same quantity of compressed air with a pressure dew point of about (-)40 degrees F. This analysis assumes a purge air requirement of 15%. This means that a refrigerant dryer would produce an 85% energy savings when producing air with a +40 degree pressure dew point. This is satisfactory to meet the needs of most industrial applications---particularly when the compressed air pipelines are routed exclusively within heated spaces.(Ruppelt, 2013)
Savings are reduced to 83.2% when the refrigerant dryer replaces a regenerative desiccant dryer with an external heater (purge air is reduced to 7 cfm, external heater uses 1.5 kW per 100 cfm). The energy savings is about 84.9% when the desiccant dryer is equipped with a regenerative blower with supplemental heat (3 kW for heater per 100 cfm, 0.5 hp for blower). Savings are reduced to only 63.8% when the baseline desiccant dryer is of the regenerative external heater type and is also equipped with dew point demand. The bottom line is to optimize operating costs by considering dew point requirements when a compressed air dryer selection is made.
Manufacturer's Energy Savings Claims:
"Typical" Savings: 85%
Savings Range: From 64% to 85%
This measure is appropriate with a facility is "overdrying" their air, i.e. using a regenerative desiccant dryer when a refrigerative dryer would provide air at an acceptable dewpoint.
Best Estimate of Energy Savings:
"Typical" Savings: 85%
Low and High Energy Savings: 64% to 85%
Energy Savings Reliability: 3 - Limited Assessment
Energy Use of Emerging Technology:
35.6 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.))
A refrigerative dryer would consume about 35,676 kWh per year when continuously operating to dry the output from a 200 hp rotary screw compressor providing about 963 acfm of plant air. A heatless regenerative twin tower desiccant dryer would consume about 237,760 kWh per year while drying the same amount of air. While the desiccant dryer would provide compressed air with a pressure dew point of (-)40 degrees F versus the (+)40 degrees F for the desiccant dryer, the additional energy consumption produces no benefit when the lower dew point provides no benefits i.e. when it is not required by plant processes. A pressure dew point of 3 to 4 degrees C (37 to 40 degrees F) is adequate for most industrial plants. Use of a refrigerative versus a desiccant dryer for a single 200 hp compressor could result in an annual energy savings of 202,080 kWh, valued at $18,188 given an energy cost of $0.09/kWh.
Currently no data available.
Installed first cost per: unit
Emerging Technology Unit Cost (Equipment Only): $23668.00
Emerging Technology Installation Cost (Labor, Disposal, Etc.): $7100.00
Baseline Technology Unit Cost (Equipment Only): $42000.00
In the new facility or plant expansion cases, a refrigerative dryer costs far less than a desiccant dryer. This energy conservation measure (ECM) involves replacement and/or removal of an existing, operating regenerative heatless desiccant dryer with a new refrigerative dryer. Cost of refrigerative dryers (from Grainger) are $3,646 for a 100 cfm dryer suitable for a 25 hp compressor; $8,575 for a 400 cfm dryers that fits with a 75 hp compressor, $11,834 for a 500 cfm dryer (for a 100 hp compressor), and $14,694 for a dryer capable of treating 750 cfm of compressed air supplied by a 150 hp compressor. All dryers are capable of providing compressed air at a 38 degree F pressure dew point. Grainger didn't have prices for larger dryers. The price for the 1,000 cfm dryer is conservatively approximated as double the price of a 500 cfm unit. Installation costs are assumed to be 30% of the equipment costs.
Simple payback, new construction (years): -1,011.1
Simple payback, retrofit (years): 1,697.0
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.