A plate heat exchanger (PHE) sits between two fluid streams — most commonly cooling tower water (CW) on one side, chilled water (CHW) on the other — transferring heat from one to the other. PHEs are the workhorse of waterside economisers, district cooling networks, and any plant where two water systems need to exchange heat without mixing.
This guide covers PHE selection: approach temperature targeting, NTU calculation, plate count sizing, fouling factor allowance, and the materials choice that determines longevity.
What approach temperature drives
The single most important PHE design parameter is approach temperature — the difference between the cold-side outlet temperature and the hot-side inlet temperature.
For waterside economiser application:
- Hot side (inlet): cooling tower water at, say, 11 °C (when outdoor WBT is 6 °C with 4 K approach)
- Cold side (target outlet): chilled water at 12 °C
- Required approach: 12 – 11 = 1 K
A 1 K approach is tight — requires a large PHE with many plates. A 3 K approach is easy — small PHE — but means the chilled water can only reach 14 °C, which may not be cool enough.
Typical approach temperatures:
- Free cooling PHE: 1-2 K (must be tight to maximize free-cooling hours)
- District cooling/heating PHE: 2-3 K
- Process heat exchanger: 3-5 K
- Waste heat recovery: 5-10 K
The Effectiveness-NTU method
NTU = UA / (C_min)
ε = (T_hot_in - T_hot_out) / (T_hot_in - T_cold_in) for C_min = hot side
ε ≈ NTU / (1 + NTU) for counter-flow with equal capacity rates
Where:
- UA = overall heat transfer coefficient × heat transfer area (W/K)
- C_min = minimum heat capacity rate of either side (W/K)
- ε = effectiveness (0 to 1)
For a typical PHE with overall U ≈ 6,000 W/m²·K and 50 m² of plate area:
- UA = 300,000 W/K
- For 50 L/s flow rate (Cp × ρ × Q = 4,180 × 1,000 × 0.05 = 209,000 W/K)
- NTU = 300,000 / 209,000 = 1.44
- ε = 1.44 / 2.44 = 0.59
A 59% effective PHE means: if hot side enters at 30 °C and cold side enters at 27 °C, hot side exits at 30 – 0.59 × 3 = 28.2 °C, cold side exits at 27 + 0.59 × 3 = 28.8 °C. Approach = 0.6 K.
Fouling factor — the design margin
Real PHEs degrade over time as deposits form on plate surfaces, reducing U:
| Application | Fouling factor (m²·K/W) |
|---|---|
| Clean cooling tower water (treated) | 0.0001 |
| Standard cooling tower water | 0.00018 |
| Untreated cooling tower water | 0.00035 |
| Closed circuit chilled water | 0.0001 |
| Industrial cooling tower (heavy) | 0.0005 |
Fouling factor adds resistance:
1/U_dirty = 1/U_clean + R_f_hot + R_f_cold
For a typical CW/CHW PHE with 5,000 W/m²·K clean U and 0.00018 + 0.0001 = 0.00028 fouling:
1/U_dirty = 1/5,000 + 0.00028 = 0.0002 + 0.00028 = 0.00048
U_dirty = 2,083 W/m²·K
This means the PHE loses 58% of its capacity as fouling builds. Designer must size PHE for dirty U at end-of-cycle, not clean. Practical implication: oversize plate area by 1.5-2× to maintain effectiveness through cleaning intervals.
Plate material selection
The plate material determines longevity and cost:
| Plate material | Cost factor | Typical service life | Application |
|---|---|---|---|
| Stainless steel 316L | 1.0× (baseline) | 10-15 years | Standard CW/CHW, treated |
| Stainless steel 304 | 0.85× | 8-12 years | Closed circuit (chilled water) |
| Titanium | 2.5-3.5× | 25+ years | Seawater, brackish water, high-chloride |
| Hastelloy C-276 | 4-5× | 20+ years | Aggressive industrial fluids |
| Copper | 0.7× | 5-8 years | NEVER for chlorinated CW (rapid corrosion) |
| Brazed plate (Cu-brazed SS) | 0.6× | 8-12 years | Small applications, glycol mixtures |
For Indian commercial applications:
- CHW side: SS 316L
- CW side (treated water): SS 316L
- CW side (untreated/borewell): titanium recommended; SS 316L will pit within 3-5 years
For seawater cooling (rare in commercial, common in marine/industrial): titanium mandatory.
PHE sizing example: 500 TR waterside economiser
Plant: 500 TR (1,758 kW) waterside economiser. Design conditions:
- CW supply 11 °C (cooling tower)
- CHW supply target 12 °C
- Approach target 1 K (tight)
NTU required for ε = 0.59 (rough): NTU = 1.44 → UA = 300 kW/K
For chilled water flow rate at 5 K cooling: Q = 1,758 / (4.18 × 5) = 84 kg/s = 84 L/s
Heat capacity rate: 84 × 4.18 = 351 kW/K
UA = 1.44 × 351 = 506 kW/K
For U_dirty = 2,083 W/m²·K (with fouling): Plate area = 506,000 / 2,083 = 243 m²
Selected unit: typical commercial PHE has plates of ~0.6 m² each. Plate count = 243 / 0.6 = ~400 plates.
Capex for 400-plate stainless 316L PHE: ~₹15-20 lakh.
Common selection mistakes
1. Using clean U for sizing. Plate area undersized by 50-60%; performance drops dramatically with fouling.
2. Ignoring approach in tight applications. A 3 K approach for waterside economiser kills 60% of free-cooling hours.
3. SS plates on untreated borewell water. 5 years of pitting corrosion; plate replacement.
4. PHE installation in line with no isolation valves. Cleaning requires plant outage; specify isolation valves on both sides.
5. No condensate drain on cold side. During tight-approach operation, vapor condenses on cold-side surfaces and pools; can corrode plates.
Quick checklist
- [ ] Approach temperature target consistent with downstream system requirement
- [ ] NTU and effectiveness computed for design point
- [ ] Fouling factor applied per fluid type
- [ ] Plate area sized for U_dirty (end-of-cycle)
- [ ] Plate material selected per fluid quality
- [ ] Isolation valves both sides for service
- [ ] Cleaning schedule documented (typical 2-5 years for treated systems)
- [ ] Spare gaskets stocked on site
References: ASHRAE Handbook HVAC Sys & Eqp 2024 Ch 48 (Heat Exchangers); TEMA Class R Standard; AHRI 400 (Plate Heat Exchanger Performance Rating); AHRI 1130 (Plate Heat Exchanger Performance for Free Cooling).
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