Appearance
A 3 mm steel chip in water-based coolant at 30°C settles at approximately 0.3 m/s — fast enough to fall out of suspension within seconds if the coolant flow velocity in the tank is below 0.15 m/s. The same chip in high-viscosity oil-based coolant settles at one-tenth that speed, requiring 10 times the tank volume to achieve the same separation efficiency. The settling time calculation — balancing chip terminal velocity against coolant flow velocity in the tank — determines whether the coolant returning to the pump is clean or laden with abrasive particles that will destroy seals, erode bushings, and block drill coolant holes.
Chip Settling Fundamentals
Terminal Velocity Calculation by Settling Regime
| Regime | Particle Size Range | Reynolds Number | Velocity Equation | Applicability |
|---|---|---|---|---|
| Stokes | <0.1 mm | Re < 0.1 | Vt = (d² × (ρp − ρf) × g) / (18 × µ) | Fine particles — silt — small chip fragments |
| Intermediate | 0.1–10 mm | 0.1 < Re < 500 | Vt = [4 × (ρp − ρf)² × g² × d³ / (225 × ρf × µ)]^(1/3) | Typical BTA chips — 0.5–8 mm particles |
| Newton | >10 mm | 500 < Re < 2×10⁵ | Vt = √[4 × d × (ρp − ρf) × g / (3 × Cd × ρf)] | Large chips — heavy steel — turbulent settling |
Settling Velocity by Particle Size and Material
| Particle Size (mm) | Steel Chip — Water-Based Coolant (m/s) | Steel Chip — Oil-Based Coolant (10 cSt) (m/s) | Aluminum Chip — Water-Based Coolant (m/s) | Aluminum Chip — Oil-Based Coolant (10 cSt) (m/s) |
|---|---|---|---|---|
| 0.1 | 0.008 | 0.0008 | 0.003 | 0.0003 |
| 0.5 | 0.08 | 0.008 | 0.03 | 0.003 |
| 1.0 | 0.18 | 0.018 | 0.07 | 0.007 |
| 2.0 | 0.28 | 0.028 | 0.11 | 0.011 |
| 3.0 | 0.32 | 0.032 | 0.13 | 0.013 |
| 5.0 | 0.38 | 0.038 | 0.16 | 0.016 |
Tank Design and Sizing
Settling Tank Design Parameters
| Tank Parameter | Recommended Value | Impact on Settling | Calculation Method |
|---|---|---|---|
| Flow velocity (horizontal) | ≤0.15 m/s (fine) — ≤0.30 m/s (coarse) | Higher velocity resuspends settled chips — lower velocity improves capture | Velocity = Flow rate / Tank cross-section area |
| Length-to-width ratio | 3:1 to 5:1 | Longer tanks provide more settling distance — wider tanks reduce velocity | Tank length = Required settling distance |
| Effective volume utilization | 60–80% | Dead zones at inlet and outlet reduce effective settling volume | Effective volume = Total volume × utilization factor |
| Minimum residence time | 2–5 minutes (steel) — 5–15 minutes (aluminum) | Longer residence improves fine particle capture | Residence time = Effective volume / Flow rate |
| Baffle spacing | 2–4× tank width | Proper spacing prevents short-circuit flow | Baffles at 1/3 and 2/3 of tank length |
| Tank depth | 0.5–1.5 m | Deeper tanks allow taller chip accumulation before cleaning | Depth ≥ Chip removal mechanism clearance |
Sizing Examples by Machine Type
| Machine Type | Coolant Flow (L/min) | Min Tank Volume (L) | Recommended Tank Volume (L) | Residence Time (min) | Tank Dimensions (L × W × D in m) |
|---|---|---|---|---|---|
| Small gun drill | 50 | 150 | 250 | 5.0 | 1.5 × 0.5 × 0.6 |
| Standard gun drill | 150 | 450 | 750 | 5.0 | 2.0 × 0.8 × 0.8 |
| Small BTA | 300 | 900 | 1500 | 5.0 | 3.0 × 1.0 × 0.8 |
| Medium BTA | 600 | 1800 | 3000 | 5.0 | 4.0 × 1.2 × 1.0 |
| Large BTA | 1000 | 3000 | 5000 | 5.0 | 5.0 × 1.5 × 1.2 |
| Heavy-duty BTA | 1500 | 4500 | 7500 | 5.0 | 6.0 × 1.8 × 1.2 |
FAQ
How is chip terminal velocity calculated for coolant tank design?
Chip terminal velocity is calculated based on Stokes' law for small particles and modified for larger particles that deviate from Stokes assumptions. For typical BTA drilling chips in the 0.5–5 mm range, the terminal velocity falls in the intermediate settling regime between Stokes and Newton. The simplified calculation procedure is: determine the chip equivalent spherical diameter (for irregular shapes, use 0.7–0.9 × the longest dimension), calculate the Reynolds number using an assumed velocity, select the appropriate settling regime equation, solve for terminal velocity, and iterate if necessary. For steel chips in water-based coolant, a practical approximation is 0.28–0.38 m/s terminal velocity for 2–5 mm particles. For oil-based coolants with viscosity 10× higher than water, the terminal velocity is approximately 10× lower — making adequate tank sizing even more critical.
What is the minimum residence time for effective chip settling in BTA systems?
The minimum residence time for effective chip settling in BTA coolant systems is 2–5 minutes for typical steel and cast iron drilling chips in water-based coolants, assuming the tank flow velocity is maintained below 0.15–0.30 m/s. Aluminum chips require 5–15 minutes due to their lower density and correspondingly lower terminal velocity. Oil-based coolants with higher viscosity require residence times 3–10× longer than water-based coolants for the same particle size. These residence times assume proper tank design with baffles to prevent short-circuit flow and effective chip removal systems to prevent re-suspension of settled chips. Tanks with poor baffle design or inadequate chip removal may require 2–3× the calculated residence time to achieve acceptable settling performance.
How do baffle design and placement affect settling efficiency?
Baffle design and placement are critical factors in settling efficiency because they prevent short-circuit flow (where coolant flows directly from inlet to outlet without passing through the full settling volume), reduce flow velocity and turbulence in the settling zone, distribute the flow evenly across the tank width, and create dedicated settling stages (coarse chips settle in the first chamber, fine particles in subsequent chambers). The recommended baffle configuration is a three-chamber tank with underflow baffles — the first baffle submerged 50–70% of the tank depth, located 15–25% from the inlet, and the second baffle with similar submergence located 60–75% from the inlet. This configuration forces the coolant to flow under the first baffle and over the second, doubling the effective flow path and preventing floating solids from reaching the outlet.
What chip removal system is best for a deep hole drilling coolant tank?
The best chip removal system depends on the chip type and volume. Drag chain (hinged steel belt) conveyors are the most common and robust choice for BTA steel and cast iron chips — they handle heavy chip loads, tolerate large and stringy chips, and can be installed at the bottom of the settling tank to continuously remove settled chips. Flight conveyors (scraper-type) are suitable for lighter chip loads and smaller installations — they are lower cost but less robust for heavy chip loads. Screw conveyors work well for smaller chips and fine particles but tend to jam on long or stringy chips. For chip volumes above 500 kg per shift, a drag chain conveyor with programmed intermittent operation (2–5 minutes of operation per hour) provides the best balance of chip removal efficiency and maintenance requirements. The conveyor drive should be located outside the tank to prevent coolant exposure.
How does coolant viscosity affect settling time?
Coolant viscosity directly and significantly affects chip settling time — the terminal settling velocity is inversely proportional to coolant viscosity in the Stokes regime and approximately inversely proportional to viscosity^(1/3) in the intermediate regime. For water-based emulsions with viscosity close to water (≈1 cP at 30°C), steel chips settle rapidly. For oil-based coolants with viscosity of 10–30 cP at operating temperature, the settling velocity of the same chip is reduced by a factor of approximately 3–10. The practical implication is that a machine using oil-based coolant requires 3–10× the tank volume of an equivalent machine using water-based coolant, or must use auxiliary filtration equipment (centrifugal separators, paper bed filters, or hydrocyclones) to achieve acceptable coolant cleanliness. Temperature also affects viscosity — coolant at 20°C has approximately 30–50% higher viscosity than at 40°C, meaning settling efficiency improves as the coolant warms up during machine operation.
Disclaimer: The settling time calculations, tank sizing data, and design parameters provided in this article are general guidelines based on fluid dynamics principles and industry-standard practices for deep hole drilling coolant systems. Actual settling performance varies with chip morphology, coolant properties, tank geometry, and operating conditions. Tank design should be validated through computational fluid dynamics (CFD) analysis or physical flow modeling for critical applications. The authors and publisher assume no liability for any damages or losses arising from the use of this information — always consult qualified engineering personnel for coolant system design. Content is for informational purposes only and does not constitute professional engineering advice. Verify all parameters with qualified personnel before implementation as of 2026.