The heat exchanger in a deep hole drilling coolant system works silently — until it stops working effectively. Fouled heat exchanger surfaces reduce cooling capacity, coolant temperature rises, tool life drops, and coolant chemistry degrades faster. Regular cleaning restores cooling performance and extends the life of the coolant and the chiller.
Heat Exchanger Types
Type Comparison
| Type | Construction | Cooling Efficiency | Fouling Susceptibility | Cleaning Difficulty | Application |
|---|
| Shell and tube | Tubes inside a cylindrical shell | Moderate | Moderate — tube interior fouls | Moderate — can rod tubes | Large systems, high flow |
| Plate (gasketed) | Stack of corrugated plates with gaskets | High | High — narrow plate gaps foul easily | Easy — plates can be opened and cleaned | Most deep hole drilling applications |
| Brazed plate | Sealed plate stack (no gaskets) | High | High — narrow gaps | Difficult — cannot open — chemical cleaning only | Compact systems, no maintenance access needed |
| Tube-in-tube | Concentric tubes | Low | Low — large passages | Easy — can brush inner tube | Low-capacity applications |
Common Installation
| Heat Exchanger Location | Coolant Circuit | Chiller / Cooling Water Circuit |
|---|
| Between pump and machine | Hot coolant from pump → cooled to machine | Chilled water from chiller → warm return |
| Coolant tank loop | Warm coolant from tank → cooled to tank | Chilled water from chiller → warm return |
| Chiller integrated | Refrigerant circuit cools coolant directly | Not applicable — refrigerant-based |
Fouling Types and Effects
Fouling Identification
| Type | Cause | Appearance | Effect on Performance |
|---|
| Scale (hard water) | Calcium and magnesium deposits from hard water | White or gray hard crust | Insulation — reduces heat transfer by 20–60% |
| Biofilm (biological) | Bacterial growth on surfaces | Slimy, dark layer | Insulation + flow restriction |
| Particulate (chips, fines) | Fine chips and debris settling in low-flow areas | Dark, gritty deposit | Flow restriction, erosion of surfaces |
| Oil fouling | Tramp oil coating on heat transfer surfaces | Sticky, greasy layer | Insulation — reduces heat transfer |
| Corrosion products | Rust from system components | Red-brown deposit | Insulation + surface damage |
Fouling Effects
| Fouling Thickness | Heat Transfer Reduction | Coolant Temperature Rise (compared to clean) | Effect on Drilling |
|---|
| 0.1 mm (light) | 10–20% | + 2–4°C | Minimal — tool life slightly reduced |
| 0.3 mm (moderate) | 25–40% | + 5–8°C | Noticeable — tool life reduced, coolant degrades faster |
| 0.5 mm (heavy) | 45–60% | + 8–15°C | Significant — coolant temperature exceeds 40°C, chip evacuation affected |
| 1.0 mm (severe) | 65–80% | + 15–25°C | Critical — coolant temperature may reach 50°C, system alarms |
Cleaning Methods
Chemical Descaling
| Step | Action | Detail |
|---|
| 1 | Isolate heat exchanger | Close isolation valves on both circuits |
| 2 | Drain both circuits | Coolant side and chiller water side |
| 3 | Prepare cleaning solution | For scale: 5–10% phosphoric or citric acid. For biofilm: 0.5% hydrogen peroxide |
| 4 | Connect cleaning pump | Circulate solution through the coolant side only |
| 5 | Circulate cleaning solution | 30–60 minutes at 40–50°C (if heating available) |
| 6 | Monitor solution pH | When pH stabilizes (stops rising), descaling is complete |
| 7 | Drain cleaning solution | Dispose per local regulations |
| 8 | Flush with clean water | 10–15 minutes circulation |
| 9 | Flush with neutralizing solution (if acid used) | Baking soda solution — circulate 5 minutes |
| 10 | Final flush with clean water | Until discharge is neutral pH |
| 11 | Refill both circuits | Coolant on process side, water/glycol on chiller side |
| 12 | Return to service | Verify pressure drop and temperature differential |
Mechanical Cleaning (Plate Heat Exchanger)
| Step | Action | Detail |
|---|
| 1 | Isolate and drain both circuits | — |
| 2 | Measure and record plate pack thickness | For reassembly reference |
| 3 | Remove heat exchanger from mounting | If necessary for access |
| 4 | Remove tie bolts | Note number and position of tightening sequence |
| 5 | Separate plates carefully | Mark orientation — each plate is identical orientation |
| 6 | Remove gaskets (if replacing) | Or clean in place if reusing |
| 7 | Clean each plate with soft brush | Non-abrasive brush — do not damage plate surface |
| 8 | Rinse plates with clean water | — |
| 9 | Inspect plates for damage | Cracks, pitting, deformation — replace if damaged |
| 10 | Inspect gaskets | Replace if hardened, cracked, or compressed |
| 11 | Reassemble plates in correct order | — |
| 12 | Install new gaskets (if replacing) | Lubricate gasket grooves per manufacturer |
| 13 | Tighten tie bolts | Cross-pattern — tighten to specified dimension |
| 14 | Pressure test | Per manufacturer specification |
| 15 | Reinstall in system | — |
| 16 | Refill and return to service | — |
Mechanical Cleaning (Shell and Tube)
| Step | Action | Detail |
|---|
| 1 | Isolate and drain both circuits | — |
| 2 | Remove tube side covers | Access to tube interior |
| 3 | Insert cleaning brush or rod into each tube | Nylon or brass brush — not steel (avoids scoring) |
| 4 | Push brush through each tube | Multiple passes if necessary |
| 5 | Flush tube side with water | After brushing all tubes |
| 6 | Clean shell side (if accessible) | Remove baffles if possible |
| 7 | Reinstall covers with new gaskets | — |
| 8 | Pressure test | — |
| 9 | Refill and return to service | — |
Backflushing (In-Situ Cleaning)
| Step | Action | Detail |
|---|
| 1 | Reverse the flow direction through the heat exchanger | Requires valving to reverse inlet and outlet |
| 2 | Flush at maximum flow rate for 5–10 minutes | Dislodges loose debris |
| 3 | Return to normal flow direction | — |
| 4 | Repeat if necessary | — |
| 5 | Check pressure drop across heat exchanger | Should be lower than before backflushing |
Cleaning Frequency
| System Condition | Cleaning Frequency | Method |
|---|
| Treated water (softened or DI) + good coolant maintenance | Annually | Chemical descaling or mechanical |
| City water (hard, > 100 ppm) | Every 6 months | Chemical descaling |
| Open loop cooling tower (worst case) | Every 3 months | Chemical + mechanical |
| Biofilm detected (slimy return water) | Immediate + every 3 months | Chemical (biocide) + mechanical |
| Temperature rise of 5°C above clean baseline | Clean immediately | Per type |
Preventative Measures
| Measure | Effect | Implementation |
|---|
| Water treatment | Prevents scale formation | Softener or RO on make-up water |
| Coolant maintenance | Prevents biofilm and oil fouling | Regular concentration, pH, biocide checks |
| Filtration | Prevents particulate fouling | Proper micron rating, regular filter changes |
| Strainer on chiller water inlet | Prevents debris from entering heat exchanger | 500 µm strainer |
| Temperature monitoring | Detects fouling early | Log coolant temperature daily |
| Pressure drop monitoring | Detects flow restriction | Install pressure gauges across heat exchanger |
FAQ
How often should a coolant heat exchanger be cleaned?
Clean annually for systems using treated water and well-maintained coolant. Clean every 6 months for systems using hard city water. Clean immediately when coolant temperature rises 5°C above the clean baseline, or when the pressure drop across the heat exchanger increases by 25%. Regular cleaning prevents the progressive fouling that leads to chiller overload and system alarms.
How do I know when a heat exchanger needs cleaning?
Monitor the coolant temperature at the heat exchanger outlet. If the outlet temperature is 5°C or more above the clean baseline (the temperature when the heat exchanger was new or freshly cleaned), cleaning is needed. Also monitor the pressure drop across the heat exchanger — a 25% increase above baseline indicates fouling. Additional signs: the chiller runs continuously without reaching set point, or the coolant temperature alarm activates more frequently.
Can I clean a heat exchanger without removing it from the system?
Yes — chemical cleaning and backflushing can be performed in-situ. Chemical cleaning: circulate a descaling solution (phosphoric or citric acid for scale, hydrogen peroxide for biofilm) through the coolant side of the heat exchanger, then flush thoroughly. Backflushing: reverse the flow direction to dislodge loose debris. For severe fouling or plate heat exchangers with gaskets, mechanical cleaning requires disassembly.
What chemicals are safe for cleaning coolant heat exchangers?
For scale removal: phosphoric acid (5–10%) or citric acid (5–10%) — both are effective and compatible with stainless steel plates. For biofilm removal: hydrogen peroxide (0.5–1%) or peracetic acid (0.02–0.05%). Never use hydrochloric (muriatic) acid — it attacks stainless steel and causes pitting. Never mix cleaning chemicals. Always flush thoroughly after chemical cleaning and neutralize acid with a baking soda solution before the final rinse.
What causes a heat exchanger to foul quickly?
Rapid fouling is caused by: hard water (high calcium/magnesium — scale forms quickly), inadequate coolant maintenance (low pH allows bacterial growth that forms biofilm), high tramp oil content (oil coats heat exchanger surfaces), poor filtration (fines enter the heat exchanger and settle in low-flow areas), or oversized heat exchanger (low flow velocity allows debris to settle).
A clean heat exchanger maintains coolant temperature within the optimal range — typically 25–35°C. When temperature rises above 40°C, tool life decreases, coolant chemistry degrades faster, and chip evacuation becomes less efficient. Regular cleaning is a simple maintenance task that prevents these problems. Monitor temperature, clean on schedule, and use treated water to minimize fouling. This article reflects industry practice as of 2026.
