Appearance
A deep hole drilling coolant tank with an open surface area of 3 m² in a warm shop can lose 50–150 liters of water per week to evaporation. That water must be replaced — every liter of evaporated water takes with it the coolant additives that keep the coolant stable, and every liter of make-up water added dilutes the remaining coolant chemistry. Over the course of a year, the water lost to evaporation from a single coolant tank can exceed 5,000–10,000 liters — water that was purchased, treated, and mixed with coolant concentrate. A floating cover — a simple polypropylene or HDPE blanket floating on the coolant surface — reduces this loss by 80–95% and pays for itself in water and coolant savings within 6–18 months.
Evaporation Mechanisms
Factors Affecting Evaporation Rate
| Factor | Effect on Evaporation | Typical Range | Control Method |
|---|---|---|---|
| Coolant temperature | Higher temperature = higher evaporation rate | 5–15 L/m²/day at 30°C — 15–40 L/m²/day at 50°C | Temperature control — cooling system |
| Ambient temperature | Higher ambient = higher evaporation | Affects coolant temperature indirectly | Shop climate control |
| Relative humidity | Lower humidity = higher evaporation | 100% humidity = no evaporation — 30% humidity = maximum evaporation | Shop humidity control — covers unaffected by humidity |
| Air movement over tank | Higher air velocity = higher evaporation | Still air: base rate — 5 m/s wind: 3–5× base rate | Tank enclosure — covers eliminate wind effect |
| Tank surface area | Larger area = more evaporation | Proportional to area — 2 m² tank loses half of 4 m² tank | Cover reduces effective evaporation area to near zero |
| Coolant concentration | Higher concentration = slightly lower evaporation | 5–10% reduction at 10% concentration vs water | Minor effect — not a practical control method |
| Tramp oil layer | Oil layer reduces evaporation | 20–50% reduction with continuous oil layer | Oil layer is inconsistent — not reliable for control |
Evaporation Rate Estimation
| Condition | Typical Evaporation Rate (mm/day) | Typical Evaporation Rate (L/m²/day) | Notes |
|---|---|---|---|
| Coolant at 25°C — 60% RH — still air | 2–4 mm/day | 2–4 L/m²/day | Low evaporation — cool coolant — moderate humidity |
| Coolant at 35°C — 50% RH — still air | 5–10 mm/day | 5–10 L/m²/day | Moderate evaporation — typical operating temperature |
| Coolant at 45°C — 40% RH — moving air | 10–20 mm/day | 10–20 L/m²/day | High evaporation — warm coolant — air movement |
| Coolant at 55°C — 30% RH — air movement | 15–30 mm/day | 15–30 L/m²/day | Very high evaporation — hot coolant — dry air |
Annual evaporation calculation: Tank surface area (m²) × average evaporation rate (L/m²/day) × operating days per year.
Example: 4 m² tank — 8 L/m²/day average — 300 operating days: 4 × 8 × 300 = 9,600 L/year evaporated.
Floating Cover Types
Cover Design Comparison
| Cover Type | Material | Coverage | Evaporation Reduction | Cost per m² | Best For | Limitations |
|---|---|---|---|---|---|---|
| Solid rigid cover — custom | HDPE — polypropylene — 3–6 mm | Full surface | 95–98% | $100–200 | Rectangular tanks — consistent level — permanent | Must be custom-fit — cannot adapt to level changes — requires support |
| Flexible membrane cover | EPDM — polyurethane — PVC — 1–3 mm | Full surface | 90–95% | $50–100 | Irregular tank shapes — variable coolant level — retrofits | Less durable than rigid — may wrinkle — requires anchoring |
| Modular floating panels | HDPE panels — interlocking | Full surface | 90–95% | $80–150 | Large tanks — tanks requiring periodic access | Gaps between panels — panels can separate — requires maintenance |
| Floating foam blanket | Closed-cell polyurethane foam — 20–50 mm | Full surface — cut to fit | 80–90% | $30–60 | Low-cost option — temporary — irregular tanks | Absorbs coolant over time — degrades — replaced annually |
| Hollow plastic balls (ball blanket) | Polypropylene — 20–50 mm diameter balls | Partial — 91% coverage (single layer) | 70–80% (single layer) — 85–90% (double layer) | $20–40 | Irregular tanks — tanks with frequent access — simple installation | Less effective than continuous cover — balls can escape — need replenishment |
| Rigid lid with seal | Aluminum — SS — with gasket | Full — sealed | 99% | $200–400 | Tanks requiring complete sealing — vapor containment | Highest cost — requires hinge or lifting mechanism — access is more difficult |
Cover Material Compatibility
| Material | Coolant Compatibility | Temperature Range | UV Resistance | Chemical Resistance | Recommended Life |
|---|---|---|---|---|---|
| HDPE (high-density polyethylene) | Excellent | −40°C to 80°C | Poor — degrades in sunlight | Excellent — most chemicals | 5–10 years |
| Polypropylene (PP) | Excellent | −10°C to 100°C | Poor — degrades in sunlight | Excellent — most chemicals | 5–10 years |
| EPDM rubber | Excellent | −40°C to 120°C | Good — with UV stabilizers | Good — most coolants | 5–8 years |
| Polyurethane | Good | −20°C to 80°C | Moderate | Good — most coolants | 3–5 years |
| PVC (polyvinyl chloride) | Good | −10°C to 60°C | Moderate | Moderate — some chemical sensitivity | 3–5 years |
| Closed-cell PE foam | Good | −40°C to 80°C | Poor — degrades | Good | 1–3 years |
| Aluminum | Good (with coating) | Unlimited | Excellent | Moderate — may corrode with some coolants | 10+ years |
| Stainless steel (304/316) | Excellent | Unlimited | Excellent | Excellent | 15+ years |
Selection Criteria
| Criterion | Consideration | Recommendation |
|---|---|---|
| Tank shape | Rectangular — round — irregular | Rectangular: rigid panels or flexible membrane. Round: flexible membrane or ball blanket. Irregular: flexible membrane or foam blanket |
| Coolant level variation | Does level change significantly during operation? | Yes (variable): flexible membrane or ball blanket. No (consistent): rigid panels or solid cover |
| Access frequency | How often is the tank opened for cleaning or pump service? | Frequent (weekly): ball blanket or modular panels. Infrequent (quarterly): rigid cover or membrane |
| Coolant temperature | Normal operating temperature range | < 60°C: most materials OK. > 60°C: polypropylene — EPDM — stainless steel |
| Tank size | Surface area | Small (< 2 m²): any type. Medium (2–10 m²): rigid panels or membrane. Large (> 10 m²): modular panels or membrane |
| Chemical environment | Coolant type — tramp oil — chemicals present | Most coolants: HDPE or PP. Aggressive chemicals: check compatibility |
| Budget | Initial investment vs ongoing savings | Low: ball blanket or foam. Moderate: membrane or modular panels. High: custom rigid cover |
Installation
| Step | Action | Cover Type | Detail |
|---|---|---|---|
| 1 | Measure tank surface accurately | All | Length — width — diameter — corners — obstacles (pump columns — return lines — level sensors) |
| 2 | Clean tank surface | All | Remove floating debris — tramp oil — chips from surface before installing cover |
| 3 | Install edge guides or anchors | Rigid — membrane | Attach guide strips or anchor points to tank walls — ensure cover stays in position as level changes |
| 4 | Place cover on coolant surface | All | Lower carefully — avoid trapping air under cover — allow cover to float freely |
| 5 | Cut openings for obstructions | All | Cut cover material to fit around pump columns — return pipes — level sensors — use grommets or seals at openings |
| 6 | Install access panels or hatches | Rigid — membrane | Install small hinged covers or flaps at access points — allows inspection without removing main cover |
| 7 | Seal edges | Rigid — membrane | Seal around edges — use flexible seal or wiper — prevents vapor escape at edges while allowing level movement |
| 8 | Verify free movement | Rails — guides must allow vertical movement | Cover must rise and fall with coolant level without binding — check at high and low operating levels |
| 9 | Test operation | All | Run coolant pump — operate machine — observe cover movement — check for binding — verify seals at obstructions |
Maintenance
| Task | Frequency | Procedure | Notes |
|---|---|---|---|
| Inspect cover surface | Weekly | Visual check for damage — wear — debris accumulation | Remove any chips or debris from cover surface — debris adds weight and can sink cover |
| Check edge seals | Weekly | Verify seals are intact — no gaps — cover moves freely with level | Worn seals allow vapor escape — reduce evaporation reduction |
| Clean cover surface | Monthly | Wipe or rinse off coolant residue — dried coolant — chip debris | Accumulated debris adds weight — reduces buoyancy — may sink cover sections |
| Inspect for sinking | Monthly | Check that cover is floating — not partially submerged | Holes — cuts — or accumulated debris can cause cover to sink — repair or clean immediately |
| Check at obstructions | Monthly | Verify openings at pump columns and return lines are still sealed | Seals at openings wear — coolant vapor escapes through gaps |
| Replace damaged sections | As needed | Cut out damaged section — patch with compatible material | Small damage grows — repair promptly to prevent cover failure |
| Full cover inspection | Annually | Remove cover (if possible) — inspect both sides — clean — repair or replace | Bottom side may have biological growth — coolant residue buildup |
Cost-Benefit Analysis
| Parameter | Without Cover | With Floating Cover | Savings |
|---|---|---|---|
| Annual evaporation loss | 9,600 L | 960 L (90% reduction) | 8,640 L |
| Water cost ($0.005/L) | $48 | $5 | $43 |
| Coolant concentrate cost (5% mix — $5/L concentrate) | $2,400 | $240 | $2,160 |
| Total annual water + coolant cost | $2,448 | $245 | $2,203 |
| Cover cost (4 m² — flexible membrane — $75/m² + installation) | — | $400 installed | — |
| Payback period | — | — | 2–3 months |
| 5-year total cost | $12,240 | $1,625 (cover replacement at year 5) | $10,615 |
Additional savings: reduced labor for coolant concentration adjustment (less frequent adjustment needed — evaporation only removes water — concentrate stays — concentration drifts less). Reduced coolant disposal cost (less make-up water added — less coolant volume to dispose at end of life). Reduced water treatment cost (less make-up water needs softening or deionization).
Alternative Evaporation Control Methods
| Method | Evaporation Reduction | Capital Cost | Operating Cost | Advantages | Disadvantages |
|---|---|---|---|---|---|
| Floating cover | 70–98% | Low–Moderate | None | Most effective — passive — no energy | Requires maintenance — must be compatible with tank |
| Tank enclosure / sealing | 90–99% | Moderate–High | None | Complete vapor containment — also contains mist | Limits access — may require ventilation — higher cost |
| Coolant temperature reduction | 20–50% per 10°C reduction | Moderate (cooling system) | Energy for cooling | Also benefits tool life — coolant stability | Limited reduction vs covers — energy cost |
| Shop humidity control | 30–60% (humidifying) | High | High (energy for humidification) | Improves operator comfort | Very high cost for significant evaporation reduction |
| Tramp oil layer | 20–50% | Low (oil skimmer — then return oil) | None if oil is from system | Uses existing tramp oil | Inconsistent layer — oil can cause other problems — limited reduction |
FAQ
How much coolant is lost to evaporation from open coolant tanks?
The amount of coolant lost to evaporation from open coolant tanks in deep hole drilling depends on the temperature of the coolant, the ambient humidity, air movement over the tank surface, and the tank surface area. At typical operating conditions (coolant at 35°C, 50% relative humidity, still air): evaporation rate is approximately 5–10 mm of water depth per day — this translates to 5–10 liters per day per square meter of tank surface area. For a typical single-machine coolant tank (2–4 m² surface area): 10–40 L/day — 50–200 L/week — 2,500–10,000 L/year. For a central coolant system with a larger tank (10–20 m²): 50–200 L/day — 250–1,000 L/week — 12,500–50,000 L/year. These are significant volumes — and the cost is not just the water. The coolant concentrate and additives remain in the tank when water evaporates — so the coolant concentration increases — requiring dilution with make-up water. But the concentration is never perfectly adjusted — it cycles between over-concentrated (before water addition) and under-concentrated (after water addition) — the instability affects coolant performance. A floating cover reduces evaporation by 80–95%, dramatically reducing water consumption, stabilizing coolant concentration, and reducing the labor required for concentration management. The payback period for a floating cover on a single-machine tank is typically 2–6 months — on a central system tank, it is typically 1–3 months.
What type of floating cover is best for a deep hole drilling coolant tank?
The best type of floating cover for a deep hole drilling coolant tank depends on the tank geometry and access requirements: for rectangular tanks with consistent coolant level (most common in deep hole drilling), a flexible membrane cover (EPDM or polyurethane, 1–3 mm thick) is the best choice — it provides 90–95% evaporation reduction, conforms to the tank shape (no custom fabrication needed — cut to size on site), adapts to coolant level changes (rises and falls with the level without binding), and is moderate in cost ($50–100 per m²). For larger tanks (over 10 m²) or tanks that require frequent access (weekly or more), modular floating panels (HDPE panels — interlocking) are the best choice — individual panels can be removed for tank access without removing the entire cover. For tanks with irregular shapes or many obstructions (pump columns, return pipes, level sensors), hollow plastic balls (ball blanket — 20–50 mm polypropylene balls) are the most practical choice — they conform to any shape, are easy to install around obstructions, and individual balls can be removed for access. The trade-off: ball blankets provide only 70–80% evaporation reduction (single layer) or 85–90% (double layer) — less than a continuous membrane. For tanks where complete vapor containment is required (regulatory requirements — aggressive coolant chemistry), a rigid lid with gasket seal provides 99% evaporation reduction but is the most expensive option. The most common recommendation for deep hole drilling coolant tanks: flexible membrane cover for single-machine tanks — modular floating panels for central system tanks.
How do I install a floating cover on an existing coolant tank?
To install a floating cover on an existing coolant tank: measure the tank surface accurately — include all obstructions (pump columns, return pipes, level sensors, baffles, agitators). Clean the coolant surface — remove any floating debris, tramp oil, or chip accumulation from the surface where the cover will float. For a flexible membrane cover: cut the cover material to the tank dimensions — allow 50–100 mm extra on each side for edge sealing. Cut openings for obstructions — mark the position of each obstruction — cut a slit from the edge of the cover to the obstruction location — cut a circular opening around the obstruction (slightly smaller than the obstruction diameter — the material will stretch). Place the cover on the coolant surface — start at one end and gradually lower the cover onto the surface — avoid trapping air under the cover (air bubbles under the cover create raised areas that reduce contact and evaporation protection). Adjust the cover around obstructions — fit the slits and openings around each obstruction — use weights (clean stainless steel washers or weights sealed in a plastic pouch) at the edges of the slit to keep the cover in position around obstructions. Install edge guides if needed — for tanks where the coolant level varies significantly, install guide strips or a float guide system on the tank walls to keep the cover centered as it rises and falls. Verify free movement — fill the tank to the normal operating level — check that the cover floats freely — check that it can rise and fall with level changes without binding on edges or obstructions. Test with the pump running — operate the coolant pump — observe the cover during pump operation — check that the cover does not get sucked into the pump suction or block return flow. The installation should take 1–2 hours for a flexible membrane cover on a typical single-machine tank.
Do floating covers cause problems with coolant quality?
Floating covers, when properly selected and maintained, do not cause coolant quality problems — and can actually improve coolant quality by stabilizing coolant concentration (reducing the cycling between over-concentrated and under-concentrated that occurs when evaporation is replaced with intermittent water addition). Potential concerns and mitigation: bacterial growth under the cover — the cover creates a dark, warm environment that can promote bacterial growth if the coolant is already contaminated. Prevention: maintain proper coolant biocide levels — test coolant weekly for bacterial count — if bacteria are present, treat with biocide (the cover does not prevent biocide access — biocide circulates with the coolant). Debris accumulation on the cover — chips and debris can accumulate on the cover surface — the added weight can sink the cover partially. Prevention: clean the cover surface weekly — remove accumulated chips and debris — most debris can be rinsed back into the coolant or removed manually. Cover material leaching — low-quality cover materials can leach plasticizers or other chemicals into the coolant. Prevention: use materials compatible with coolant (HDPE, polypropylene, EPDM — all are chemically stable in water-based coolants — do not leach). Reduced access for inspection — a floating cover hides the coolant surface — you cannot see floating tramp oil or surface debris. Mitigation: install a small inspection port or clear window in the cover — use it to check the coolant surface condition periodically. Overall: the benefits of evaporation reduction (water savings, coolant stability, reduced labor) far outweigh the minor operational adjustments needed — and most potential problems are prevented with basic maintenance (weekly cleaning, regular coolant testing).
How do I maintain a floating cover on my coolant tank?
Floating cover maintenance: weekly — inspect the cover surface for damage, debris accumulation, and proper floating position. Remove any chips, swarf, or debris from the cover surface (accumulated weight can sink the cover — debris can also abrade the cover material). Check the cover edge position — ensure it is not caught on tank edges or obstructions. Monthly — clean the cover surface thoroughly (wipe with a clean cloth or rinse with a hose — remove all dried coolant residue and debris). Check the cover material for cuts, tears, or wear (small damage can be repaired with compatible patch material — larger damage requires section replacement). Check seals around obstructions (pump columns, return pipes — worn seals allow vapor escape). Verify the cover is floating freely (should rise and fall with coolant level without binding). Annually — remove the cover (if practical) — inspect both sides — clean both sides. Check the bottom side for biological growth (clean with appropriate biocide if present — this indicates coolant bacteria issue). Check for material degradation (UV damage — chemical attack — embrittlement). Replace the cover if it shows significant wear or degradation (typical life: 3–8 years depending on material and conditions). Ball blankets: inspect ball condition monthly — replace balls that have collapsed, cracked, or become waterlogged — add new balls to maintain coverage (balls are lost through normal operation — replace approximately 10–20% per year). The most important maintenance task is keeping the cover clean — a cover that sinks from accumulated debris is not reducing evaporation and can interfere with pump operation.
Coolant evaporation from open tanks is a significant source of water and coolant loss — 5,000–10,000 L/year for a typical single-machine system. A floating cover reduces this loss by 80–95% — pays for itself in 2–6 months — and continues saving water and coolant for the life of the cover. Select a flexible membrane cover (EPDM or polyurethane) for most tanks — modular panels for large or frequently accessed tanks — ball blankets for irregular tanks or tanks with many obstructions. Install with proper edge sealing — cut openings for obstructions — allow free movement with level changes. Clean the cover surface weekly — inspect for damage monthly — replace every 3–8 years. Floating covers are the single most cost-effective method for reducing coolant consumption and stabilizing coolant chemistry in deep hole drilling operations. This article reflects industry practice as of 2026.