The coolant return tank is the bridge between the drilling process and the coolant supply — it receives hot, chip-laden coolant from the machine, allows chips to settle, releases entrained air, and delivers cleaner coolant to the pump tank. A well-designed return tank is the foundation of a reliable coolant system.
Return Tank Functions
| Function | Purpose | Design Feature |
|---|
| Chip settling | Allows heavy chips to fall out of suspension | Low flow velocity — adequate residence time |
| Air release | Allows entrained air bubbles to rise and escape | Calm surface — adequate surface area |
| Coolant storage | Holds reserve coolant volume | Adequate capacity — minimum 5–10× pump flow per minute |
| Temperature equalization | Mixes hot return coolant with tank volume | Adequate volume — baffles for mixing |
| Flow smoothing | Dampens return flow fluctuations | Baffles — weirs — multiple compartments |
Sizing Guidelines
Tank Volume
| Machine Type | Recommended Tank Volume | Minimum Tank Volume |
|---|
| Single gun drilling machine (< 10 mm) | 200–500 L | 150 L |
| Single gun drilling machine (10–25 mm) | 500–1500 L | 300 L |
| Multi-spindle gun drilling | 1000–3000 L | 500 L |
| BTA drilling (small — < 40 mm) | 1500–3000 L | 1000 L |
| BTA drilling (medium — 40–80 mm) | 3000–6000 L | 2000 L |
| BTA drilling (large — > 80 mm) | 5000–10000 L | 3000 L |
| Central coolant system (multiple machines) | 10000–50000 L | 5000 L |
Sizing Rule of Thumb
| Sizing Method | Formula | Example |
|---|
| Based on flow rate | Tank volume (L) = 5–10 × pump flow (L/min) | 200 L/min pump → 1000–2000 L tank |
| Based on chip load | Tank volume (L) = 2–4 × hourly chip volume (cm³) | 5000 cm³/h chips → 10000–20000 L |
| Based on settling time | Tank length (m) = flow velocity (m/s) × settling time (s) | 0.1 m/s × 120 s = 12 m length |
Surface Area Requirements
| Parameter | Recommended Value | Why |
|---|
| Tank surface area | 0.5–1 m² per 1000 L/min flow | Allows air bubbles to rise and break |
| Coolant depth | 1–1.5 m | Deep enough for settling — not so deep that sludge resuspends |
| Freeboard (above coolant level) | 150–300 mm | Prevents overflow from return surges |
Design Features
Compartment Configuration
| Compartment | Function | Design Details |
|---|
| Return compartment (first) | Receives coolant from machine — heaviest chips settle | Baffled inlet — flow spreader — deep section |
| Settling compartment (middle) | Fine chips settle — air rises | Low velocity — long flow path |
| Clean compartment (last) | Cleanest coolant — feeds pump tank | Outlet at opposite end from return — skimmed surface |
| Sludge collection (bottom) | Accumulates settled sludge | Sloped bottom — drain at low point |
Baffle Design
| Baffle Type | Purpose | Design | Location |
|---|
| Inlet baffle | Spreads return flow — reduces velocity | Perforated plate or deflection plate | At return line inlet |
| Underflow baffle | Forces flow downward — prevents short-circuiting | Baffle from near surface to near bottom | Between compartments |
| Overflow weir | Skims surface — removes floating oil | Adjustable weir plate | Before clean compartment |
| Perforated baffle | Distributes flow evenly | Holes 20–40 mm diameter | Across flow path |
Baffle Spacing and Flow Path
| Parameter | Recommended Value | Why |
|---|
| Compartment length | 3–5× compartment width | Long flow path for settling |
| Baffle gap (under or over) | 100–300 mm | Adequate for flow — not too large |
| Number of compartments | 2–4 | Progressive cleaning |
| Flow path length (total) | 10–20× tank width | Ensures adequate settling time |
Flow Path Design
| Design Consideration | Best Practice | Benefit |
|---|
| Inlet location | One end of tank — submerged below surface | Reduces aeration, distributes flow |
| Outlet location | Opposite end from inlet — at surface | Skims cleanest coolant |
| Flow direction | Horizontal — with underflow/overflow baffles | Maximizes settling path |
| Flow velocity (settling zone) | < 0.05 m/s (50 mm/s) | Allows chips to settle |
| Flow velocity (return zone) | < 0.3 m/s | Prevents resuspension of settled chips |
Chip Settling Calculations
Settling Velocity
| Chip Type | Particle Size | Settling Velocity (in still water) | Settling Time (1 m depth) |
|---|
| Steel chips — large | 5–10 mm | 0.3–0.5 m/s | 2–3 seconds |
| Steel chips — medium | 1–5 mm | 0.1–0.3 m/s | 3–10 seconds |
| Steel chips — fine | 0.1–1 mm | 0.01–0.1 m/s | 10–100 seconds |
| Steel fines (swarf) | < 0.1 mm | 0.001–0.01 m/s | 100–1000 seconds |
| Cast iron fines | < 0.1 mm | 0.0005–0.005 m/s | 200–2000 seconds |
Required Settling Length
| Flow Velocity | Chip to Settle | Required Tank Length |
|---|
| 0.05 m/s | Medium chips (1 mm) | 1.5–2 m |
| 0.05 m/s | Fine chips (0.1 mm) | 5–10 m |
| 0.03 m/s | Fine chips (0.1 mm) | 3–6 m |
| 0.01 m/s | Very fine chips | 1–3 m |
Return Tank Materials
| Material | Corrosion Resistance | Cost | Typical Use |
|---|
| Mild steel (painted) | Moderate | Low | General — oil-based coolants |
| Stainless steel (304) | Excellent | High | Water-based coolants |
| Stainless steel (316) | Excellent — highest | Very high | Corrosive coolants |
| Polyethylene / polypropylene | Excellent | Moderate | Small tanks — retrofits |
| Concrete (lined) | Good — if lined | High | Large central systems |
| Fiberglass (FRP) | Excellent | High | Large — corrosive environments |
Common Design Problems
| Problem | Cause | Solution |
|---|
| Coolant aeration at pump inlet | Return flow too turbulent — air not released | Add baffles — increase surface area — submerge return line |
| Chips carry over to clean compartment | Flow velocity too high — settling time insufficient | Increase tank volume — improve baffling — reduce flow |
| Short-circuiting — coolant goes directly from inlet to outlet | No baffles or poorly placed baffles | Add underflow/overflow baffles |
| Sludge resuspension | Flow velocity near tank bottom too high | Keep bottom flow < 0.03 m/s |
| Foaming in return tank | Coolant degradation, aeration, contamination | Check coolant quality — improve air release |
| Temperature rise through tank | Tank volume too small for heat load | Increase volume — add chiller |
| Vortex at pump suction | Return line too close to pump suction | Relocate suction — add vortex breaker |
Troubleshooting
| Symptom | Likely Cause | Corrective Action |
|---|
| Chips in clean compartment | Tank too small or flow too fast | Increase tank volume or add baffles |
| Air bubbles in pump supply | Return turbulence — insufficient air release | Submerge return — add de-aeration baffle |
| Coolant overflowing tank | Return flow exceeds drain capacity | Increase tank size or return line diameter |
| Sludge layer too thick | Insufficient tank cleaning schedule | Clean tank more frequently |
| Temperature too high | Tank volume insufficient for heat load | Increase volume or add chiller |
| Foaming | Coolant degradation or mechanical aeration | Check coolant — reduce turbulence |
FAQ
How big should a coolant return tank be for deep hole drilling?
The return tank should hold 5–10 times the pump flow rate per minute. For example, a pump delivering 200 L/min needs a return tank of 1000–2000 L. The tank must provide adequate residence time for chips to settle and air to release. Larger is always better — an oversized tank provides more settling time, better temperature stability, and greater tolerance for flow fluctuations.
What is the best baffle design for a return tank?
Use a series of underflow and overflow baffles that force the coolant to travel a long path from inlet to outlet. Three compartments is typical: return compartment (with inlet baffle to spread flow), settling compartment (long zone with low velocity), and clean compartment (with outlet at the surface). Baffle gaps should be 100–300 mm — large enough for flow but not so large that coolant short-circuits.
How do I prevent chips from reaching the pump?
Design the return tank with adequate volume and baffling so that chips have time to settle before coolant reaches the pump suction. Use multiple compartments with underflow/overflow baffles that force a long flow path. Keep flow velocity below 0.05 m/s in the settling zone. Install a suction strainer at the pump inlet as a final defense. For fine chips that do not settle, install a filtration system (centrifuge or paper band filter).
Why is my return tank overflowing?
Return tank overflow is caused by: return flow rate exceeding the tank's drain capacity (tank too small for the pump flow), clogged return line strainer or filter (restricts flow out of the tank), incorrect tank level setpoint (tank fills too high before pump starts), or return line not submerged (aerated coolant occupies more volume). Measure actual return flow and compare to tank drain capacity — the tank must be able to pass the full pump flow plus any bypass flow.
What is the correct flow velocity in a coolant return tank?
In the settling zone, flow velocity should be below 0.05 m/s (50 mm/s) to allow chip settling. At this velocity, medium chips (1–5 mm) settle within seconds. In the return zone near the inlet, velocity can be higher (up to 0.3 m/s). Near the tank bottom, keep velocity below 0.03 m/s to prevent sludge resuspension. Design the tank cross-section area to achieve these velocities at the maximum expected return flow rate.
The coolant return tank is a critical component that directly affects coolant quality and pump performance. Size it for 5–10 minutes of pump flow, design with multiple baffled compartments for progressive settling, keep flow velocity below 0.05 m/s in the settling zone, and provide adequate surface area for air release. A well-designed return tank delivers clean, de-aerated coolant to the pump — the foundation of reliable deep hole drilling. This article reflects industry practice as of 2026.
