For most data center liquid cooling applications, a brazed plate heat exchanger (BPHE) is the best choice because it offers high heat transfer efficiency, compact size, low coolant charge, and the ability to handle the high pressures and close temperature approaches typical of direct-to-chip and immersion cooling. For very large flow rates or where serviceability is critical, a gasketed plate heat exchanger (PHE) or shell and tube heat exchanger (STHE) may be more suitable.
This guide explains exactly how to select the right heat exchanger for data center liquid cooling. You will learn:
The main liquid cooling approaches: direct-to-chip, immersion, and rear-door
Heat exchanger types used in coolant distribution units (CDUs)
Key selection parameters: flow rate, temperature approach, pressure drop, and coolant compatibility
How to calculate heat duty, LMTD, U-value, and required area
Material and gasket selection for water, glycol, and dielectric fluids
Step-by-step selection guide and pre-purchase checklist
Common mistakes that cause poor performance or downtime
Cost, lead time, and certification factors for global projects
Quick Answer
| Question | Short Answer |
|---|---|
| Best heat exchanger for data center liquid cooling? | Brazed plate heat exchanger (BPHE) |
| Typical coolant supply/return | 45 °C / 55 °C (direct-to-chip) |
| Facility water temperature | 30–40 °C |
| Temperature approach | 2–5 °C |
| Typical U-value | 3,000–6,000 W/m²·K (BPHE, water–water) |
| Allowable pressure drop | 20–50 kPa (coolant side) |
| Common materials | 316L stainless steel, copper, EPDM gaskets |
| Coolants | Water, glycol, dielectric fluids |
| Certification | PED, ASME, CE, UL where applicable |
Table of Contents
What Is Data Center Liquid Cooling Selection?
Types of Liquid Cooling Systems in Data Centers
Heat Exchanger Types for Data Center Liquid Cooling
Key Selection Parameters: Flow Rate, Temperature Approach, Pressure Drop
How to Calculate Heat Exchanger Capacity for Data Centers
Material and Coolant Compatibility
Step-by-Step Selection Guide
Maintenance and Fouling Prevention
Comparison Tables
Common Mistakes and How to Avoid Them
Pre-Purchase Checklist
Cost, Lead Time, and Certification Factors
FAQ
Key Takeaways
What Is Data Center Liquid Cooling Selection?
Data center liquid cooling selection is the process of choosing the right heat exchanger, coolant, and configuration to remove heat from IT equipment using liquid instead of air. It involves matching the heat exchanger to the cooling loop, facility water, and IT load requirements.
H3: Why Liquid Cooling?
Higher heat flux: CPUs and GPUs now exceed 500 W each; air cooling struggles.
Better efficiency: Liquid cooling can reduce PUE to 1.1–1.2.
Compact: Smaller footprint than air-cooled systems.
Heat reuse: Higher return temperatures enable waste heat recovery.
Lower noise: Fewer fans.
H3: Key Components
Cold plates: Attached to CPUs/GPUs; transfer heat to coolant.
CDU (Coolant Distribution Unit): Contains heat exchanger, pumps, controls.
Heat exchanger: Transfers heat from IT coolant loop to facility water.
Facility water system: Cooling tower, chiller, or dry cooler.
H3: Selection Goals
Meet IT heat load with margin
Maintain safe component temperatures
Minimize pressure drop and pump power
Ensure material compatibility with coolants
Provide reliability and serviceability
Meet certification and safety requirements
Types of Liquid Cooling Systems in Data Centers
The three main liquid cooling approaches are direct-to-chip, immersion, and rear-door. Each uses heat exchangers differently.
H3: Direct-to-Chip (D2C)
How it works: Cold plates mount directly on CPUs/GPUs. Coolant flows through microchannels to absorb heat.
Coolant: Water, glycol, or dielectric fluid.
Heat exchanger: BPHE or gasketed PHE in CDU transfers heat to facility water.
Typical temperatures: Supply 45 °C, return 55 °C.
Advantages: High heat flux capability, targeted cooling.
Limitations: Requires plumbing to each server.
H3: Immersion Cooling
How it works: Servers are immersed in dielectric fluid. Heat is transferred to the fluid, then to a heat exchanger.
Coolant: Single-phase or two-phase dielectric fluid.
Heat exchanger: BPHE or shell and tube transfers heat from dielectric to facility water.
Typical temperatures: Fluid 40–50 °C.
Advantages: Highest heat flux, no fans, uniform cooling.
Limitations: High cost, maintenance complexity.
H3: Rear-Door Heat Exchanger
How it works: A finned tube heat exchanger replaces the rear door of a rack. Air passes through, heat transfers to water.
Coolant: Water or glycol.
Heat exchanger: Finned tube coil.
Typical temperatures: Air 25–35 °C, water 30–40 °C.
Advantages: Retrofit for existing racks, no server modification.
Limitations: Lower heat rejection than D2C or immersion.
H3: Comparison of Liquid Cooling Approaches
| Parameter | Direct-to-Chip | Immersion | Rear-Door |
|---|---|---|---|
| Heat flux capability | High | Very high | Moderate |
| Coolant | Water/glycol/dielectric | Dielectric | Water/glycol |
| Heat exchanger type | BPHE, PHE | BPHE, STHE | Finned tube |
| Temperature approach | 2–5 °C | 3–6 °C | 5–10 °C |
| Retrofit | Moderate | Difficult | Easy |
| Cost | Moderate | High | Low–moderate |
| Best for | High-density racks | Extreme density | Existing racks |
Heat Exchanger Types for Data Center Liquid Cooling
The main heat exchanger types for data center liquid cooling are brazed plate, gasketed plate, and shell and tube. Brazed plate is most common due to compactness and high pressure capability.
H3: Brazed Plate Heat Exchanger (BPHE)
Construction: 316L stainless steel plates, copper or nickel braze.
Pressure: 30–45 bar.
Temperature: up to 200 °C.
Coolants: Water, glycol, dielectric fluids.
Advantages: Compact, low charge, high U-value, no gaskets.
Limitations: Not serviceable; requires clean fluids.
H3: Gasketed Plate Heat Exchanger (PHE)
Construction: 316L or titanium plates, EPDM/NBR gaskets.
Pressure: up to 25 bar.
Temperature: up to 150 °C (EPDM).
Coolants: Water, glycol.
Advantages: Serviceable, expandable, cleanable.
Limitations: Gasket temperature limit, larger than BPHE.
H3: Shell and Tube Heat Exchanger (STHE)
Construction: Carbon steel or stainless steel shell, copper or stainless tubes.
Pressure: 100+ bar.
Temperature: 600 °C+.
Coolants: Water, glycol, dielectric.
Advantages: Robust, serviceable, large capacity.
Limitations: Large, heavy, higher cost.
H3: Heat Exchanger Selection by Cooling Type
| Cooling Type | Recommended Heat Exchanger |
|---|---|
| Direct-to-chip | BPHE (preferred), gasketed PHE |
| Immersion (single-phase) | BPHE, STHE |
| Immersion (two-phase) | BPHE with special design |
| Rear-door | Finned tube coil |
| Large facility | STHE or gasketed PHE |
Key Selection Parameters: Flow Rate, Temperature Approach, Pressure Drop
The key parameters for data center liquid cooling heat exchanger selection are flow rate, temperature approach, pressure drop, coolant type, and heat load. Each must be calculated and checked.
H3: Flow Rate
Flow rate determines velocity and heat transfer. For water-based coolants:
Q = ṁ × cp × ΔT
Typical flow rates: 1–5 L/min per kW for D2C, higher for immersion.
H3: Temperature Approach
D2C: 2–5 °C between coolant return and facility water supply.
Immersion: 3–6 °C.
Rear-door: 5–10 °C.
Close approach requires more area and higher cost.
H3: Pressure Drop
| Loop | Typical Allowable ΔP |
|---|---|
| IT coolant loop | 20–50 kPa |
| Facility water loop | 30–80 kPa |
| Rear-door coil | 10–30 kPa |
Higher ΔP increases pump power. Optimize for total cost.
H3: Coolant Properties
| Coolant | Specific Heat (J/kg·K) | Density (kg/m³) | Notes |
|---|---|---|---|
| Water | 4,180 | 998 | Best heat transfer |
| 30% Glycol | 3,800 | 1,040 | Freeze protection |
| 50% Glycol | 3,600 | 1,070 | Lower heat transfer |
| Dielectric fluid | 1,500–2,500 | 1,600–1,800 | Immersion only |
H3: U-Values for Data Center Heat Exchangers
| Duty | BPHE U (W/m²·K) | PHE U (W/m²·K) | STHE U (W/m²·K) |
|---|---|---|---|
| Water–water | 3,000–6,000 | 3,000–7,000 | 500–1,500 |
| Glycol–water | 2,000–4,500 | 2,000–4,500 | 400–1,000 |
| Dielectric–water | 500–1,500 | N/A | 300–800 |
How to Calculate Heat Exchanger Capacity for Data Centers
Heat exchanger capacity for data centers is calculated from the IT load, coolant flow, and temperature difference. The same formulas apply as for other heat exchangers.
H3: Heat Duty (Q)
Q = ṁ × cp × ΔT
For IT load, Q is the total heat generated by servers, typically 10–50 kW per rack, up to 100 kW for high-density racks.
H3: LMTD
ΔTlm = (ΔT1 − ΔT2) / ln(ΔT1 / ΔT2)
For D2C: ΔT1 = coolant return − facility supply; ΔT2 = coolant supply − facility return.
H3: Required Area
A = Q / (U × LMTD × F)
Add 10–20% margin for fouling and future load.
H3: Example Calculation
IT load: 100 kW
Coolant: water
Coolant supply: 45 °C, return: 55 °C
Facility water: 30 °C supply, 40 °C return
ΔT1 = 55 − 40 = 15 K, ΔT2 = 45 − 30 = 15 K
ΔTlm = 15 K
U = 4,500 W/m²·K (BPHE)
A = 100,000 / (4,500 × 15 × 1.0) = 1.48 m²
Add 15% margin → 1.70 m²
Select BPHE with 1.7 m² and verify ΔP ≤ 50 kPa.
Material and Coolant Compatibility
Material selection for data center liquid cooling depends on coolant type, water quality, and pressure. 316L stainless steel and copper are standard; EPDM gaskets are common.
H3: Material Selection
| Coolant | Recommended Material | Gasket |
|---|---|---|
| Water | 316L, copper | EPDM, NBR |
| Glycol | 316L, copper | EPDM |
| Dielectric fluid | 316L, copper | EPDM, FKM |
| Facility water (treated) | 316L | EPDM |
| Facility water (cooling tower) | 316L, titanium | EPDM |
H3: Coolant Compatibility
Water: Excellent heat transfer; requires treatment to prevent scale and corrosion.
Glycol: Freeze protection; lower heat transfer; monitor concentration.
Dielectric fluids: For immersion; low heat transfer; compatible with 316L and copper.
Two-phase dielectric: Requires special heat exchanger design and materials.
H3: Gasket Selection (Gasketed PHE)
| Gasket | Temp Range | Best For |
|---|---|---|
| NBR | −20 to 110 °C | Chilled water |
| EPDM | −35 to 150 °C | Hot water, glycol |
| FKM | −20 to 200 °C | High temperature |
| PTFE-encapsulated | −20 to 180 °C | Aggressive fluids |
H3: Water Quality Guidelines
| Parameter | Recommended Limit |
|---|---|
| pH | 7.5–8.5 |
| Chlorides | < 200 ppm (316L) |
| Hardness | < 100 ppm as CaCO₃ |
| Conductivity | < 500 µS/cm |
| Suspended solids | < 10 ppm |
Step-by-Step Selection Guide
Follow these steps to select the right heat exchanger for a data center liquid cooling project.
Step 1 — Determine IT heat load.
Calculate total heat load in kW from server power ratings.
Step 2 — Select cooling approach.
Choose direct-to-chip, immersion, or rear-door.
Step 3 — Define coolant and temperatures.
Specify coolant type, supply/return temperatures, and facility water temperatures.
Step 4 — Calculate flow rate.
ṁ = Q / (cp × ΔT). Confirm flow rate in L/min.
Step 5 — Choose heat exchanger type.
BPHE for compact, high-pressure. Gasketed PHE for serviceability. STHE for large capacity.
Step 6 — Calculate LMTD and required area.
A = Q / (U × LMTD × F). Add 10–20% margin.
Step 7 — Check pressure drop.
Verify ΔP on both sides within allowable limits.
Step 8 — Select materials and gaskets.
316L for water/glycol; titanium for seawater; EPDM for most coolants.
Step 9 — Verify certification.
PED, ASME, CE, UL as required by market.
Step 10 — Request vendor calculation.
Ask for thermal design, drawing, pressure test, and material certificates.
Maintenance and Fouling Prevention
Maintenance and fouling prevention for data center liquid cooling focus on water quality, filtration, cleaning schedules, and leak detection.
H3: Common Fouling Issues
Scale from hard water
Biological growth in open loops
Particulate from piping
Corrosion products
Dielectric fluid degradation
H3: Prevention Measures
Water treatment (softening, pH control, biocide)
Filtration (50–200 micron)
Closed-loop with inhibitors
Regular coolant testing
Leak detection and monitoring
H3: Cleaning Methods
| Component | Cleaning Method |
|---|---|
| BPHE | CIP with compatible chemicals; not serviceable |
| Gasketed PHE | CIP or manual plate opening |
| STHE | Tube brushing, hydrojetting |
| Finned tube | Compressed air, water spray |
H3: Monitoring
Track U-value and pressure drop
Clean when U drops 15% or ΔP rises 30%
Inspect gaskets and plates during maintenance
Keep spare gaskets and plates on site
Comparison Tables
Table 1: Heat Exchanger Types for Data Center Liquid Cooling
| Parameter | Brazed Plate | Gasketed Plate | Shell and Tube |
|---|---|---|---|
| Max pressure | 30–45 bar | ~25 bar | 100+ bar |
| Max temperature | 200 °C | 150 °C (EPDM) | 600 °C+ |
| Temperature approach | 1–2 °C | 1–3 °C | 5–10 °C |
| Serviceable | No | Yes | Yes |
| Expandable | No | Yes | No |
| Footprint | Very small | Small | Large |
| Coolant charge | Low | N/A | High |
| Fouling tolerance | Low | Low | High |
| Best for | D2C, immersion | Large facility | Large capacity |
| Relative cost | Low | Low–moderate | Moderate–high |
Table 2: Comparison of Liquid Cooling Approaches
| Parameter | Direct-to-Chip | Immersion | Rear-Door |
|---|---|---|---|
| Heat flux capability | High | Very high | Moderate |
| Coolant | Water/glycol/dielectric | Dielectric | Water/glycol |
| Heat exchanger type | BPHE, PHE | BPHE, STHE | Finned tube |
| Temperature approach | 2–5 °C | 3–6 °C | 5–10 °C |
| Retrofit | Moderate | Difficult | Easy |
| Cost | Moderate | High | Low–moderate |
| Best for | High-density racks | Extreme density | Existing racks |
Table 3: Typical Selection Parameters for Data Center Liquid Cooling
| Parameter | Typical Value |
|---|---|
| IT load per rack | 10–100 kW |
| Coolant supply/return | 45/55 °C |
| Facility water supply | 30–40 °C |
| Flow rate | 1–5 L/min per kW |
| Allowable ΔP (IT loop) | 20–50 kPa |
| U-value (BPHE) | 3,000–6,000 W/m²·K |
| Temperature approach | 2–5 °C |
| Design margin | 10–20% |
Common Mistakes and How to Avoid Them
1. Undersizing for future load.
Data center loads grow. Add 20–30% margin or select expandable PHE.
2. Ignoring water quality.
Poor water quality causes scaling and corrosion. Implement treatment and monitoring.
3. Choosing BPHE for dirty fluids.
BPHE is not serviceable. Use gasketed PHE or STHE if fouling is likely.
4. Using wrong gasket material.
Glycol and high temperatures require EPDM or FKM. Match to coolant.
5. Overlooking pressure drop.
High ΔP increases pump power and reduces flow. Verify ΔP on both sides.
6. Not planning for leak detection.
Leaks in data centers are critical. Install leak detection and use reliable heat exchangers.
7. Forgetting certification.
PED, ASME, UL may be required. Plan early to avoid delays.
8. Using incompatible materials with dielectric fluids.
Some dielectric fluids require specific materials. Verify compatibility.
9. Skipping maintenance.
Regular cleaning and coolant testing prevent downtime.
10. Buying on price alone.
Cheapest unit may have higher operating cost or no support. Evaluate lifecycle cost.
Pre-Purchase Checklist
- □
IT heat load calculated and verified
- □
Cooling approach selected (D2C, immersion, rear-door)
- □
Coolant type and temperatures defined
- □
Flow rate calculated
- □
Heat exchanger type selected (BPHE, PHE, STHE, finned tube)
- □
LMTD and required area calculated with margin
- □
Pressure drop checked on both sides
- □
Materials and gaskets selected
- □
Water quality and treatment planned
- □
Certification requirements identified (PED, ASME, UL)
- □
Footprint and access verified
- □
Maintenance and cleaning method planned
- □
Spare parts strategy defined
- □
Vendor thermal calculation and drawing requested
- □
Test certificates and traceability confirmed
- □
Lead time and delivery terms agreed
Cost, Lead Time, and Certification Factors
Cost, lead time, and certification for data center heat exchangers depend on type, material, and market. BPHE is usually lowest cost; STHE is higher but more robust.
H3: Cost Factors
| Factor | Impact |
|---|---|
| Heat transfer area | Direct cost driver |
| Material (316L vs titanium) | 1.5–4× cost difference |
| Pressure rating | Higher pressure = thicker plates |
| Coolant type | Dielectric requires special design |
| Certification | Adds engineering and inspection cost |
| Custom vs standard | Custom adds 20–50% |
H3: Lead Time Factors
| Item | Typical Lead Time |
|---|---|
| Standard BPHE | 4–6 weeks |
| Custom BPHE | 6–8 weeks |
| Standard gasketed PHE | 2–4 weeks |
| Standard STHE | 6–10 weeks |
| Custom STHE | 10–16 weeks |
| Finned tube coil | 4–8 weeks |
| Certification (PED, ASME, UL) | +2–6 weeks |
H3: Certification Requirements by Market
| Market | Requirement |
|---|---|
| Europe | PED 2014/68/EU, CE marking |
| North America | ASME Section VIII, U-stamp, UL |
| Middle East | ASME, PED, local authority |
| Southeast Asia | ASME, PED, national standards |
| Data center | PED, ASME, UL, or local code |
FAQ
1. What is the best heat exchanger for data center liquid cooling?
Brazed plate heat exchangers are best for most data center liquid cooling applications due to compact size, high pressure capability, and low coolant charge. For large capacity or serviceability, gasketed plate or shell and tube may be used.
2. What coolant is used in data center liquid cooling?
Water, water-glycol mixtures, and dielectric fluids. Water is most common for direct-to-chip; dielectric fluids are used for immersion cooling.
3. What is the typical temperature approach for data center heat exchangers?
2–5 °C for direct-to-chip, 3–6 °C for immersion, and 5–10 °C for rear-door heat exchangers.
4. How do I calculate the heat exchanger size for a data center?
Calculate Q = ṁ × cp × ΔT, then A = Q / (U × LMTD × F). Add 10–20% margin. Verify pressure drop and flow rate.
5. What is the typical flow rate for data center liquid cooling?
1–5 L/min per kW of IT load for direct-to-chip. Higher for immersion.
6. What materials are used in data center heat exchangers?
316L stainless steel plates, copper or nickel braze, EPDM or NBR gaskets. Titanium for seawater or high-chloride facility water.
7. Can I use a gasketed plate heat exchanger in a data center?
Yes, for larger capacities or where serviceability is required. EPDM gaskets are suitable for water and glycol up to 150 °C.
8. How often should I clean a data center heat exchanger?
Clean when U drops 15% or ΔP rises 30% from baseline. Typical cleaning interval is 6–24 months depending on water quality.
9. What certifications are required for data center heat exchangers?
PED and CE in Europe, ASME and UL in North America, and local codes elsewhere. Plan early to avoid delays.
10. What is the typical pressure drop for a data center heat exchanger?
20–50 kPa on the IT coolant loop, 30–80 kPa on the facility water loop. Verify with vendor software.
11. Can I use a brazed plate heat exchanger with dielectric fluid?
Yes, but dielectric fluids have lower heat transfer coefficients. Verify material compatibility and size accordingly.
12. How do I prevent fouling in a data center heat exchanger?
Use water treatment, filtration, closed-loop with inhibitors, and regular monitoring. Clean at the trigger point.
Key Takeaways
Brazed plate heat exchangers are the best choice for most data center liquid cooling applications due to compact size, high pressure capability, and low coolant charge.
Gasketed plate and shell and tube exchangers are used for larger capacities or where serviceability is required.
Key selection parameters are heat load, flow rate, temperature approach, pressure drop, and coolant compatibility.
Calculate heat duty Q = ṁ × cp × ΔT, then required area A = Q / (U × LMTD × F). Add 10–20% margin.
Materials: 316L stainless steel for water and glycol; titanium for high-chloride water; EPDM gaskets for most coolants.
Monitor U-value and pressure drop, and clean when performance drops.
Certification (PED, ASME, UL) affects design, cost, and lead time — plan early.
Always request a vendor thermal calculation and drawing before finalizing selection.