This section describes some real life examples of smart energy efficient compressed air system decisions involving:
- Dramatically reducing desiccant dryer purge air
- Using a very small compressor to supply instrument air on weekends
- Specifying on/off control for a rotary screw compressor
"Before" refers to the existing equipment, operations method, or normal procurement decision.
"After" refers to the energy efficient equipment, operations method or preferred procurement decision.
a. Case 1: Install Purge Controller, Repair Leaks and Lower Pressure
A medium sized furniture manufacturer has two 100 HP screw compressors running in modulating mode feeding a 1,000 cfm desiccant air dryer operating with uncontrolled purge flow. The system runs 6,000 hours a year and is completely turned off on weekends. Problems with end use pressure have been experienced which resulted in a required increase in compressor discharge pressure. Plant personnel attempted to turn off one air compressor to save power but this resulted in low pressure during peak plant flows in the busy daytime production shift. An audit of the facility was done and found the following data:
- Total compressed air production: 450 cfm
- Total compressor energy consumption: 880,000 kWh
- System specific power: 32.9 kW/100 cfm
- Total electrical cost at 10 cents/kWh: $88,000
- Air dryer purge: 150 cfm
- Leaks: 100 cfm
- Drains: 5 cfm
- Average compressor discharge pressure: 120 psi
- Dryer and filter pressure differential: 10 psi
- End use piping differential: 20 psi.
- Required end use pressure: 80 psi
The audit report recommended the following which was implemented by the customer:
- Leak reduction: 50 cfm
- Drainage reduction: 5 cfm
- Plant pressure reduction by 15 psi for a 10% flow reduction: 25 cfm
- Install purge control on dryer with purge reduction: 117 cfm
- Total flow reduction: 197 cfm
- This resulted in a final average compressor loading of 253 cfm
- One compressor could now be turned off and the remaining unit would consume 76 kW x 6000 = 456,000 kWh for a savings of 48%
- Savings with existing compressor modulating control: $42,400 per year.
Further measures are recommended:
- Reduce end use piping differential by 13 psi
- Reduce dryer and filter differential by 7 psi
- Compressor discharge pressure reduction: 20 psi
- Install VSD compressor. VSD compressor average power at 253 cfm@ 100 psi: 50 kW.
This resulted in the following savings:
- New system electrical consumption: 300,000 kWh
- New system specific power: 19.8 kW/100 cfm
- New system electrical cost: $30,000 per year
- Savings: 66%
Additional possible savings:
- Heat recovery
- Convert to refrigerated dryer
- End use reduction
b. Case 2: Use Smaller Compressor During Off-Hours
The company in the previous example installed a dry fire system requiring 5 cfm of compressed air at all times including weekends. No production occurs during these hours. This air demand requires one 100 HP modulating compressor to run for 2,760 hours extra per year.
Plant load on weekend:
- Dryer purge 150 cfm
- Leakage 100 cfm
- Drains 5 cfm
- Fire system 5 cfm
- Power required for 100 HP modulating compressor at 260 cfm = 76 kW
- Total power per year 76 kWx 2,760 hrs = 209,760 kWh
- Operating cost = $20,976 per year (at $0.10 per kWh)
- Use a small 5 HP (4.25 kW) 15 cfm rated reciprocating compressor for only the fire system (isolated with check valve).
- 5 cfm load/15 cfm rated x 4.25 kW = 1.42 kW x 2760 hrs = 3,910 kWh
- Annual cost = $391
- Annual savings = $20,585 (98%)
c. Case 3: On/Off vs. Load/No Load Control
A small automobile dealership is considering upgrading their compressed air system. The current system has a 15 HP reciprocating compressor rated at 50 cfm @ 175 psi. The unit is measured at 13 kW average while running. The unit is installed on an 80 gallon receiver.
The average load at the facility was data logged and it was determined that the average flow is 10 cfm with a peak flow of 50 cfm. The air produced by the compressor is filtered with one particulate filter and one coalescing filter arranged in series, and dried in a 75 cfm direct expansion air dryer with hot gas bypass control consuming 0.88 kW. The pressure drop across the air dryer is 4 psi and the filters pressure differential is 4 psi total. The dryer has a timer drain installed that consumes 2 cfm average. The system operates at an average pressure of 145 psi for 3745 hours per year. Blended power rate is 10 cents per kWh.
The dealership is considering the purchase of a 15 HP screw compressor to replace the ailing reciprocating unit. The compressor salesman indicates a new screw compressor can produce 58 cfm at 150 psi @ 13.5 kW so is therefore more efficient. The plan is to install the compressor on the same 80 gallon receiver, use the same filter and air dryer and operate at 145 psi average pressure.
- Existing compressor consumption = 13 kW x10 cfm /50 cfm rated x 3745 hrs = 9,737 kWh per year x $0.10 per kWh = $974 per year
- Dryer consumption = 0.88 kW x 3745 hrs = 3,296 kWh = $330 per year (Note: It is very common to see these dryers left on all the time 8760 hours per year.)
- Total system cost per year = $1,304.
The new screw compressor will be running in load/unload mode with 1.37 US gallons per cfm storage capacity and a 10 psi pressure band. Due to the pressure drop across the filters and air dryer, the compressor will rapid cycle and remain running for the full operating hours. From the load/unload performance curve (see Figure 7) it can be seen the unit will consume an average of 60% of its full load nameplate power.
Proposed Load/unload with standard dryer
- Proposed compressor consumption = 13.5 kW x.6 x 3745 hrs= 30,335 kWh per year = $3,034 per year.
- Dryer consumption = 0.88 kW x 3745 hrs = 3296 kWh x $0.10 per kWh = $330 per year
- Total system costs = $3,364.
The new screw compressor costs about 2.5 times as much in electricity costs to operate.
Alternative High Efficiency System - On/off operation with measures applied
The following additional measures are applied:
- Pressure differential reduced to 0.5 psi across oversized filters and 3 psi across a new cycling style dryer rated at 0.51 kW.
- Average compressor discharge pressure reduced to 115 psi average. A compressor is selected that shuts off immediately as long as the motor starts are less than 6 per hour.
- Drain reduced to zero by use of an airless drain.
- Storage receiver added to reduce compressor starts to less than 6 per hour for all flows.
- Pressure regulating valve added to reduce shop pressure to a constant 100 psi.
The energy efficiency measures reduce the average air flow and power:
- 2 cfm for the drain reduction
- 2 cfm due to the pressure reduction
- Compressor power consumption reduced by 15% due to 30 psi reduction.
- Dryer power consumption is now proportional to flow.
- Proposed compressor consumption (discharge at lower pressure) = 11.5 kW x 6 cfm/58 rated x3745 hrs = 4,455 kWh per year x $0.10 per kWh = $446 per year.
- Dryer consumption = 0.5 kWx 6 cfm/80 rated cfm x 3745 hrs = 140 kWh x $0.10 per kWh = $14 per year
- Total system costs = $460 per year.
- The efficient system consumes about 35% of the original system and 14% of the load/unload system
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