Dissolved air in coolant is invisible — until it comes out of solution and causes cavitation. The bubbles collapse with explosive force, eroding metal surfaces, causing noise and vibration, and disrupting coolant flow. In deep hole drilling, air in the coolant is particularly damaging because the high pressures and flow velocities create ideal conditions for cavitation. Preventing air from entering the coolant system is far more effective than dealing with cavitation damage after it occurs.
How Air Dissolves in Coolant
Air Solubility in Coolant
| Parameter | Effect on Air Solubility | Explanation |
|---|
| Pressure | Higher pressure = more dissolved air (Henry's Law) | Air dissolves proportional to partial pressure |
| Temperature | Higher temperature = less dissolved air | Coolant holds less dissolved air as it warms |
| Coolant type | Water-based holds less air than oil-based | Water-based coolants have lower air solubility |
| Agitation | More agitation = more air entrainment | Turbulence at return lines and tank inlets pulls air in |
How Air Enters the System
| Entry Point | Mechanism | Air Form | Prevention |
|---|
| Tank return line | Coolant splashing into tank creates bubbles | Entrained air (bubbles) | Submerge return line below coolant surface |
| Tank surface | Vortex at pump suction pulls air into pump | Entrained air | Install vortex breaker, maintain coolant level |
| Leaking suction line | Air pulled in through loose fitting or shaft seal | Entrained air | Repair suction-side leaks |
| Coolant mixing | Adding coolant concentrate creates bubbles | Entrained air | Mix slowly, allow to settle |
| Agitation | High flow velocity at tank inlet | Entrained air | Baffle tank, reduce inlet velocity |
| Temperature rise | Air comes out of solution as coolant warms | Dissolved air → entrained air | Control coolant temperature |
Dissolved Air vs Entrained Air
| Property | Dissolved Air | Entrained Air |
|---|
| Visibility | Invisible — cannot see | Visible as bubbles |
| Location | Uniform throughout coolant | Concentrated at high points, tank surface |
| Effect on pump | Causes cavitation when pressure drops | Causes noise, pressure fluctuation |
| Removal method | De-aeration (vacuum, heat) | Baffling, settling, filtration |
| Measurement | Dissolved oxygen meter | Visual — cloudiness |
Cavitation Mechanisms
Cavitation Types in Coolant Systems
| Cavitation Type | Location | Cause | Damage Pattern |
|---|
| Pump cavitation | Pump inlet, impeller eye | Low inlet pressure — air or vapor bubbles form | Pitting on impeller vane surfaces |
| Valve cavitation | Downstream of throttle valve | Pressure drop below vapor pressure | Erosion on valve seat and body |
| Orifice cavitation | At flow restrictions | High velocity — pressure drop | Erosion downstream of restriction |
| Pipe cavitation | At elbows, tees, sudden expansions | Flow separation — local low pressure | Pitting at downstream wall |
| Flow meter cavitation | At flow meter throat | Velocity increase — pressure drop | Erosion of flow meter element |
Cavitation Damage Progression
| Stage | Damage | Detectable By | Action |
|---|
| 1 — Incubation | No measurable material loss | Cannot detect | Prevent conditions |
| 2 — Pitting initiation | Small pits on surface (10–100 µm) | Visual inspection under magnification | Investigate cause |
| 3 — Pitting growth | Pits enlarge and connect | Visual — rough surface | Reduce severity |
| 4 — Material loss | Surface layer removed | Visible erosion — metal loss | Repair or replace component |
| 5 — Perforation | Hole through component wall | Leak | Replace component |
Cavitation Resistance of Common Materials
| Material | Cavitation Resistance | Typical Application |
|---|
| Stainless steel (316) | Good | Pump impellers, housings |
| Duplex stainless steel | Very good | High-pressure pump components |
| Bronze | Moderate | Lower-pressure pump components |
| Cast iron | Poor | Older pump housings |
| Ni-resist (austenitic cast iron) | Good | Pump components |
| Ceramic | Excellent (brittle) | Seal faces, bushings |
| Hard chrome plating | Good | Impeller coating |
Symptoms of Air in Coolant
| Symptom | Cause | Diagnostic Check |
|---|
| Pump noise — crackling, gravel sound | Cavitation from air or vapor bubbles | Listen at pump — check inlet pressure |
| Pressure gauge fluctuation | Air bubbles passing through pump | Observe gauge needle |
| Reduced coolant flow | Air displaces coolant — reduces pump efficiency | Measure flow at drill |
| Drill running hot | Less coolant actually reaching drill | Check drill temperature after short run |
| Inconsistent chip evacuation | Intermittent coolant delivery | Check chip form — chip packing |
| Bubbles visible in return line | Entrained air not settling in tank | Look at return line sight glass |
| Foaming in tank | Air entrainment + coolant chemistry | Observe tank surface |
Critical Air Levels
| Measurement | Condition | Effect | Action |
|---|
| < 2% entrained air | Normal | Minimal effect on pumping | None |
| 2–5% entrained air | Elevated | Reduced pump efficiency, noise | Investigate source |
| 5–10% entrained air | High | Cavitation risk, flow reduction | Correct within 1 week |
| > 10% entrained air | Critical | Cavitation damage, drilling problems | Immediate correction |
Measurement Methods
Dissolved Air Measurement
| Method | Equipment | Procedure | Interpretation |
|---|
| Dissolved oxygen meter | DO meter | Submerge sensor in coolant sample | Compare to saturation at coolant temperature |
| Vacuum extraction | Vacuum pump + graduated cylinder | Extract dissolved air from sample under vacuum | Volume of air released = dissolved air content |
| Gas chromatography | Laboratory GC | Analyze coolant sample headspace | Precise composition — laboratory only |
Entrained Air Measurement
| Method | Equipment | Procedure | Interpretation |
|---|
| Visual — settling test | Graduated cylinder | Fill cylinder — let settle 5 minutes — measure bubble rise | Bubbles rising = entrained air |
| Density comparison | Densitometer | Measure coolant density vs air-free density | Lower density = entrained air |
| Ultrasonic | Ultrasonic flow meter with air detection | Air bubbles reflect ultrasound differently | Some meters show air content |
| Pressure fluctuation | Pressure transducer + data logger | Record pressure variation at pump outlet | Fluctuation amplitude correlates to air |
Prevention Strategies
System Design for Air Prevention
| Design Feature | Purpose | Implementation |
|---|
| Submerged return line | Prevents air entrainment from splashing | Return line discharge below minimum coolant level |
| Baffled tank | Allows air bubbles to rise and separate | Baffles between return and suction compartments |
| Vortex breaker at suction | Prevents vortex from pulling air into pump | Plate or cone over suction pipe opening |
| Large suction line diameter | Reduces velocity — prevents low pressure at pump inlet | 1.5–2× pump inlet diameter |
| Pump location below tank (flooded suction) | Ensures positive inlet pressure | Mount pump below minimum tank level |
| De-aeration section in tank | Provides residence time for air to separate | Calm zone between return and suction |
Operational Practices
| Practice | Benefit | Implementation |
|---|
| Maintain coolant level | Prevents vortex at suction | Check daily — fill to operating level |
| Control coolant temperature | Reduces air coming out of solution | Keep below 40°C |
| Minimize tank turbulence | Allows air bubbles to rise | Baffle returns, reduce flow velocity |
| Use anti-foam additive | Helps bubbles coalesce and rise | Add per manufacturer dosage |
| Mix coolant slowly | Minimizes air entrainment during mixing | Add concentrate to water, not reverse |
| Avoid suction-side leaks | Prevents air from being pulled into pump | Check fittings and shaft seal |
De-Aeration Equipment
| Equipment | How It Works | Air Removal Efficiency | Best For |
|---|
| Tank baffling | Creates quiet zone — bubbles rise naturally | 50–80% (after settling) | Continuous removal |
| Vacuum de-aerator | Pulls vacuum on coolant — dissolved air comes out | > 95% | Central systems, critical applications |
| Centrifugal de-aerator | Centrifugal force separates air from liquid | 80–95% | High-flow systems |
| Coalescing media | Air bubbles coalesce on media surface and rise | 70–90% | Small to medium systems |
| Heated de-aeration | Heat reduces air solubility — air comes out of solution | 60–80% | Systems with heater already installed |
Corrective Actions
| Symptom | Immediate Action | Follow-Up |
|---|
| Cavitation noise from pump | Stop pump — check suction strainer and level | Install vacuum gauge at pump inlet |
| Bubbles visible in return line | Check coolant level — check return line submergence | Add baffles if needed |
| Pressure fluctuation | Bleed air at high points | Check for suction leaks |
| Foaming in tank | Add anti-foam — check concentration | Review coolant quality |
| Vortex at pump suction | Fill tank — install vortex breaker | Check suction line design |
Long-Term Corrective Actions
| Problem | Long-Term Fix | Timeline |
|---|
| Air entrainment from return | Extend return line below coolant surface | Next maintenance |
| Tank turbulence | Add baffling between return and suction | Next tank cleaning |
| Vortex at suction | Install vortex breaker | Next shutdown |
| Suction line air leaks | Replace fittings, seals, gaskets | Immediate |
| Coolant overheating | Add chiller or improve cooling | As budget allows |
| High dissolved air content | Install de-aeration equipment | As needed |
Preventive Maintenance
| Task | Frequency | Benefit |
|---|
| Check coolant level | Daily | Prevents vortex at suction |
| Observe tank surface for bubbles | Daily | Detects aeration early |
| Listen for pump cavitation | Weekly | Catches developing problem |
| Check return line submergence | Monthly | Ensures proper design |
| Clean suction strainer | Monthly | Maintains pump inlet flow |
| Measure entrained air (settling test) | Monthly | Trend monitoring |
| Check suction line fittings for leaks | Monthly | Prevents air ingress |
| Maintain coolant temperature | Continuous | Prevents dissolved air release |
FAQ
What is dissolved air in coolant and why is it a problem?
Dissolved air is air that is invisibly mixed into the coolant at the molecular level — it is not visible as bubbles. Under normal conditions, dissolved air causes no problems. But when coolant pressure drops (at pump inlets, through valves, or at flow restrictions), dissolved air can come out of solution, forming bubbles that collapse violently — this is cavitation. Cavitation erodes pump impellers, valve seats, and piping, and causes noise, vibration, and reduced flow.
How do I know if I have air in my coolant system?
Check for these signs: pump noise that sounds like gravel or crackling (cavitation), pressure gauge needle that fluctuates rapidly, bubbles visible in the return line sight glass, foaming on the tank surface, and reduced drilling performance (drill running hot, inconsistent chip evacuation). For a simple test: fill a clear graduated cylinder with coolant — if bubbles rise to the surface over several minutes, you have entrained air.
What causes air to get into a coolant system?
Air enters the coolant system through: splashing at the return line (most common — return line not submerged below coolant surface), vortex at the pump suction (low coolant level, no vortex breaker), suction side leaks (air pulled in through loose fittings, worn pump shaft seal, porous hose), coolant mixing (agitation during concentrate addition introduces air), and temperature rise (air comes out of solution as coolant warms — coolant holds less dissolved air at higher temperature).
How do I prevent cavitation from dissolved air?
Design the system with a submerged return line (below minimum coolant level), adequate tank baffling (to allow air to separate), flooded pump suction (pump below tank level), and proper suction line sizing (1.5–2× pump inlet diameter). Operate the system with correct coolant level (check daily), controlled temperature (below 40°C), and no suction-side air leaks. For existing cavitation problems, install a vacuum gauge at the pump inlet and correct any restriction.
How do I remove dissolved air from coolant?
Dissolved air cannot be removed by settling or filtration — it is dissolved at the molecular level. It must be removed by de-aeration: vacuum de-aeration (pulling vacuum on the coolant causes dissolved air to come out of solution — most effective), centrifugal de-aeration (centrifugal force separates air from coolant), or heating (heating coolant reduces air solubility — air comes out of solution). For most machine coolant systems, preventing air entry is more practical than removing dissolved air after it is in the system.
Dissolved air is an invisible problem in deep hole drilling coolant systems — it causes cavitation that erodes pumps and disrupts drilling performance. Prevention is the best strategy: submerged return lines, proper tank baffling, correct coolant level, and suction-side leak prevention. Monitor for signs of aeration (pump noise, pressure fluctuation, bubbles) and correct the root cause before cavitation damage occurs. This article reflects industry practice as of 2026.
