The incremental capital investment for these controls was estimated at $68,200 ($41,700 for equipment, $15,000 for labor and $11,500 for engineering). We also used this project to introduce fieldbus technology to the plant, which boosted the total capital expenditure for the controls to $102,400. However, the plant was able to take advantage of an incentive program offered by our electrical utility company. It provided one half of the initial estimated investment, or $34,100. Thus, our actual capital cost for the project was $68,300.
|Significant savings||Old System||New System||Change|
3,500 rpm (constant)
|1,750 rpm (maximum)|
||450 gal/min total||660 gal/min average||+47%|
||12.4 kW (estimated)||6.9 kW (average)||-44%|
||1,750 rpm (constant)||1,750 rpm (maximum)|
||6.1 kW (estimated)||1.3 kW (average)||-79%|
||18.5 kW (estimated)||8.2 kW (average)||-56%|
Table 1. Speed controls on pump and fans enable steep cuts in operating costs.
The new tower system has been operating for more than one year. During that time, we have monitored speed and power consumption for the circulation pump and tower fans. These data show that, while the system provides about 47% greater flow, annual operating costs have been cut by more than half, based on current electricity charges of $0.13/kWhr and 8,760 hr/yr. Pump power draw has dropped by almost 45%, while fan power draw is down by nearly 80% (Table 1), thanks to use of VFDs.
We estimate the overall energy savings for this project at $11,700. So, based on the capital cost of $68,300 and an assumed project life of 20 years, we achieved a 13.8% discounted cash flow rate of return. Had we not received the electric company incentive, the total cost for controls of $102,400 still would offer an 8.4% rate of return. We also should realize additional non-quantified benefits:
- Slower motor speeds should increase equipment reliability and decrease maintenance costs
- Fewer pieces of equipment should reduce future costs for replacement parts
- The potential for significant increases in plant capacity and productivity because average speeds are far below maximum values
Overall this project met its three objectives of providing a reliable, capable and economical source for cooling water. The simplicity of the control scheme has worked well and should be adaptable to other systems within the company and throughout the process industries.
Peter Montagna is engineering manager for King Industries, Inc., Norwalk, Ct. E-mail him at PMontagna@KingIndustries.com.