What would be the best option if the air demand required are 100%, 85% and 70%?

Option (1):
Loading Power = 0.1643kW/cfm * 3066cfm * 8000 hours = 4,029,950 kWh
Unloading Power = 0 kWh
Total Energy Cost / year = 4,029,950 kWh * RM 0.21/kWh = RM 846,289

Option (2)
Loading Power = 0.1672kW/cfm * 3066cfm * 8000 hours = 4,101,081kWh
Unloading Power = (3242 – 3066)/3242) * 50kW *2 * 8000 hours =43,430kWh
Total Energy Cost / year = (4,101,081kWh + 43,430kWh) * RM 0.21/kWh = RM 870,347

Option (3)
Loading Power = (0.1672kW/cfm*1621cfm) * 8000 + 246kW * 8000 = 4,136,250 kWh
Unloading Power = 0 kWh
Total Energy Cost / year = (4,136,250kWh + 0 kWh) * RM 0.21/kWh = RM 868,612

Comparison for Maximum Load Demand:
For a Maximum (100%) Constant Plant Air demand Option (1) is the most economical. A large and higher efficiency compressor offers energy savings during peak load periods.

Option (1)
Loading Power = 0.1643kW/cfm * 2,606cfm * 8000 hours = 3,425,326 kWh
Unloading Power = (3066-2606) / 3066) *150 kW * 8000 hours = 180,039 kWh
Total Energy Cost / year = (3,425,326 + 180, 039) kWh * RM 0.21/kWh = RM 757,126

Option (2)
Loading Power = 0.1672kW/cfm * 2,606cfm * 8000 hours = 3,485,785 kWh
Unloading Power = ((3242 – 2606)/3242) * 50kW * 2 * 8000 hours = 156,940kWh
Total Energy Cost / year = (3,485,785kWh + 156,940kWh) * RM 0.21/kWh = RM 764,972

Option (3)
Loading Power = (0.1672kW/cfm*1621cfm) * 8000 + 169kW * 8000 = 3,520,250 kWh
Unloading Power = 0 kWh
Total Energy Cost / year = (3,520,250 + 0) kWh * RM 0.21/kWh = RM 739,252

Comparisons for 85% Part Load Demand:
For a Part Air Load (85%) or fluctuating demand, Option (3) is the most economical because it utilized a medium size high energy efficiency air compressor as base load and an additional Air compressor with VSD as a top load compressor to meet the requirements of air demand of the plant. VSD (Variable Speed Drive) eliminates unloading energy wastage and maximum energy savings can be achieved. Cost of running one large compressor in Option (1) is higher because of lower energy efficiency at fluctuation and part load.

Option (1)
Loading Power = 0.1643kW/cfm * 2,146.2cfm * 8000 hours = 2,820,965 kWh
Unloading Power = ((3066 – 2146) / 3066) * 150 kW * 8000 hours = 360,078 kWh
Total Energy Cost / year = (2,820,965 + 360,078) kWh * RM 0.21/kWh = RM 668,019

Option (2)
Loading Power = 0.1672kW/cfm * 2,146.2cfm * 8000 hours = 2,870,757kWh
Unloading Power = (3242 – 2146)/3242 * 50kW * 2 * 8000 hours = 270,450kWh
Total Energy Cost / year = (2,870,757+ 270,450) kWh * RM 0.21/kWh = RM 659,653

Option (3)
Loading Power = (0.1672kW/cfm*1621cfm)*8000 + 90.7kW*8000 = 2,893,849 kWh
Unloading Power = 0 kWh
Total Energy Cost / year = (2,893,849 + 0) kWh * RM 0.21/kWh = RM 607,708

Comparisons for 70% Part Load Demand:

For a Part Air Load (70%) or fluctuating demand, Option (3) is still the most economical because it utilized a medium size high energy efficiency air compressor as base load and an additional Air compressor with VSD as a top load compressor to meet the requirements of air demand of the plant . VSD(Variable Speed Drive) eliminates unloading energy wastage and maximum energy savings can be achieved. Cost of running one large compressor in Option (1) and Option (2) is higher because of lower energy efficiency at part load.

Conclusion:

From the example above, we can conclude that the selection of the compressor combination depends on the type of demand whether it is constant demand or fluctuating demand. For a constant demand, one large size compressor just sufficient to meet the demand of the air requirement and it is more economical. However this is not usually the case because user need margin to cater for losses Most of the typical plant consumption is highly fluctuating due to changes in production capacity. This air consumption varies depend on the production shifts , day, night, weekends public holidays and the increase of production capacity On part load or fluctuating demand it is more economical to run with one medium size compressor on base load and top up with an additional compressor fitted with Variable Speed Drive because VSD eliminates unnecessary unloading energy wastage.