The coolant pump is the largest energy consumer on a deep hole drilling machine, typically using 60 to 70 percent of the total machine power. I have spent considerable effort optimizing pump energy consumption because the savings add up quickly on machines that run multiple shifts. The key is reducing energy use without affecting the coolant pressure and flow that the drilling process requires.
Energy Consumption Baseline
Before optimizing, I measure the baseline energy consumption of the coolant pump under different operating conditions. The baseline measurement identifies the largest opportunities for savings.
| Machine Size | Pump Motor Power | Operating Hours per Year | Annual Energy at Full Speed | Annual Energy Cost ($0.12/kWh) |
|---|---|---|---|---|
| Small gun drill (10mm capacity) | 5 kW | 4,000 | 20,000 kWh | $2,400 |
| Medium gun drill (25mm capacity) | 15 kW | 6,000 | 90,000 kWh | $10,800 |
| Large BTA machine (50mm capacity) | 30 kW | 8,000 | 240,000 kWh | $28,800 |
| Multiple machine cell (4 machines) | 60 kW total | 6,000 | 360,000 kWh | $43,200 |
The first observation from this data is that a medium machine running two shifts consumes $10,800 per year in pump energy alone. A 20 percent reduction saves over $2,000 per year per machine. For a shop with multiple machines, the savings become significant.
Matching Pump Pressure to Job Requirements
The first step in energy optimization is matching the pump pressure to the job requirements. I have found that many operators run the pump at maximum pressure for every job, even when lower pressure would work perfectly well.
The required coolant pressure depends on the drill diameter, the hole depth, and the material being drilled. A 12mm gun drill drilling to 200mm depth in mild steel needs much less pressure than a 6mm gun drill drilling to 500mm depth in stainless steel.
I have developed a simple guideline for setting coolant pressure based on the drill diameter and the length-to-diameter ratio. For L/D under 20:1, I start at 800 to 1000 psi and adjust upward if chip evacuation is poor. For L/D over 40:1, I run the pump at maximum pressure because chip evacuation is the limiting factor.
Reducing the pressure from maximum to the minimum needed for good chip evacuation saves energy and reduces heat generation in the coolant. The pump energy consumption is proportional to the flow rate times the pressure. Reducing pressure by 30 percent reduces pump energy consumption by approximately 20 percent.
Variable Frequency Drive Implementation
A variable frequency drive on the pump motor can reduce energy consumption during idle times. When the machine is between cycles — during part loading and unloading — the pump can run at reduced speed. A VFD reduces pump speed to 50 percent during idle, which cuts energy consumption by approximately 80 percent during those periods.
| Operating State | Pump Speed | Power Consumption | Time Percentage | Energy per Hour |
|---|---|---|---|---|
| Drilling (full power) | 100% | 15 kW | 60% | 9.0 kWh |
| Idle (no VFD) | 100% | 15 kW | 40% | 6.0 kWh |
| Idle (with VFD) | 50% | 3 kW | 40% | 1.2 kWh |
| Total per hour (no VFD) | - | - | 100% | 15.0 kWh |
| Total per hour (with VFD) | - | - | 100% | 10.2 kWh |
The table shows a typical cycle where the machine is drilling 60 percent of the time and idle 40 percent of the time. The VFD reduces energy consumption from 15.0 kWh per hour to 10.2 kWh per hour, a 32 percent reduction. Over a 6,000-hour year, that saves about 28,800 kWh or $3,456 at $0.12 per kWh.
I have seen VFD installations pay for themselves in 12 to 18 months on machines that run multiple shifts. The VFD also provides soft starting, which reduces electrical stress on the motor and mechanical stress on the pump.
Pump Type Selection and Efficiency
The pump type also affects energy efficiency. I have compared the efficiency of different pump types at the pressures required for deep hole drilling.
Positive displacement pumps, such as piston pumps or progressive cavity pumps, maintain their efficiency at high pressures. Centrifugal pumps, which are common on general-purpose machines, lose efficiency rapidly as the pressure increases above their design point.
For gun drilling operations requiring 1500 psi or higher, a positive displacement pump is 15 to 20 percent more efficient than a centrifugal pump at the same pressure and flow. The higher initial cost of the positive displacement pump is recovered through energy savings within two to three years.
I have replaced centrifugal pumps with positive displacement pumps on several machines and measured the energy savings. In one case, a 15 kW centrifugal pump was replaced with a 12 kW positive displacement pump that delivered the same pressure and flow. The energy savings of 3 kW running 6,000 hours per year saved $2,160 annually.
Coolant Temperature Effects on Pump Energy
Coolant temperature affects pump energy because cold coolant has higher viscosity and requires more pump power. I set the coolant temperature to 25 to 30 degrees Celsius, which balances viscosity with cooling performance.
A coolant chiller that maintains temperature in this range provides consistent pump energy consumption throughout the day. Without a chiller, the coolant temperature can vary from 15 degrees Celsius in the morning to 40 degrees Celsius in the afternoon. The pump draws more power in the morning when the coolant is cold and less in the afternoon when the coolant is warm.
The energy savings from operating at the optimal temperature are small compared to the other optimization measures, but the consistency in pump performance is valuable for process control.
Key Takeaways
- The coolant pump uses 60 to 70 percent of total machine power, with a medium gun drill consuming $10,800 per year in pump energy at $0.12/kWh.
- Matching pump pressure to the specific job requirements — 800-1000 psi for L/D under 20:1, maximum pressure for L/D over 40:1 — reduces energy consumption by approximately 20 percent.
- A VFD on the pump motor reduces idle energy consumption by 80 percent, paying for itself in 12 to 18 months on multi-shift machines with typical 40 percent idle time.
- Positive displacement pumps are 15 to 20 percent more efficient than centrifugal pumps at the high pressures required for gun drilling above 1500 psi.
- Maintaining coolant temperature at 25 to 30 degrees Celsius provides consistent pump power consumption and extends motor life.
- I measure baseline energy consumption before optimization to identify the largest savings opportunities and verify the savings after implementing changes.