A chilled water system lives or dies by its heat exchangers.
Pick the wrong plate heat exchanger and you get low delta-T, high pump energy, chiller short cycling, fouling, gasket leaks, or freeze damage. Pick the right one and the system runs quietly for years with predictable maintenance.
This guide covers what matters: type, duty, approach temperature, pressure drop, materials, and the mistakes that cost the most.
Quick answer
For most HVAC chilled water duties, a gasketed plate heat exchanger is the default. It is easy to open, clean, and re-gasket. Use a brazed plate heat exchanger for small, clean, sealed duties where field cleaning is not required. Use a semi-welded or welded plate heat exchanger for high pressure, aggressive fluids, or where gaskets are not acceptable.
The type is only half the decision. The sizing must match the real flow, temperature, and pressure drop — not just the peak load.
1. Know your duty before you look at a catalog
You cannot select a plate heat exchanger from cooling capacity alone. Gather these data points:
Chilled water supply and return temperatures
Secondary side supply and return temperatures
Design flow rates on both sides
Heat load in tons, kW, or Btu/h
Allowable pressure drop on each side
Design pressure and temperature
Fluid type: water, glycol mix, brine, or process fluid
Water quality: chloride, hardness, suspended solids
Cleaning access and maintenance plan
A typical chilled water loop might run 44°F supply and 54°F return. The secondary side might be 45°F to 55°F. That 1°F approach changes the size, cost, and pressure drop more than most people expect.
2. Choose the right plate heat exchanger type
| Type | Best for | Watch for |
|---|---|---|
| Gasketed | HVAC chilled water, primary/secondary loops, district cooling, clean fluids | Gasket temperature limits, leakage, re-gasketing cost |
| Brazed | Small chillers, heat pumps, compact skids, clean fluids | Cannot be mechanically cleaned, limited repair |
| Semi-welded | High pressure, ammonia, aggressive refrigerants, some process duties | Higher cost, limited plate access |
| Welded | High pressure, high temperature, aggressive fluids, no gasket compatibility | No field cleaning, usually replace or chemical clean |
For most building chilled water systems, gasketed units win. They can be opened for inspection and cleaning. Plates can be added or removed to adjust capacity. That flexibility matters when the load changes.
Brazed units are common in packaged chillers and small heat pump systems. They are compact and cost-effective. But if the fluid is dirty or the unit fouls, you cannot open it. Chemical cleaning is the only option, and replacement is often cheaper.
3. Size the unit correctly
Heat load
Use the basic heat balance:
Q = 500 × gpm × ΔT (imperial, Btu/h)
Q = 4.18 × m³/h × ΔT (metric, kW)
Where ΔT is the temperature difference on the chilled water side.
LMTD and approach
The log mean temperature difference (LMTD) drives the surface area. A tighter approach — meaning the hot outlet and cold outlet temperatures are close — requires more plates. More plates mean higher cost and higher pressure drop.
A 1°F to 3°F approach is possible in plate heat exchangers. But do not specify 1°F unless you truly need it. It can double the surface area and pressure drop compared with a 3°F approach.
Overall heat transfer coefficient
For water-to-water plate heat exchangers, U values are typically in the range of 3,000–7,000 W/m²K, depending on velocity, plate pattern, and fouling. Use the manufacturer’s selection software for the final number. Do not guess.
Pressure drop
Pressure drop is not a side issue. It is a major operating cost.
Typical allowable pressure drop for chilled water plate heat exchangers is 10–20 psi (70–140 kPa) per side, but this depends on the system. If you allow too little, the unit becomes large and expensive. If you allow too much, the pump energy penalty can wipe out the savings.
Check both sides. The secondary side often has a different allowable pressure drop than the primary side.
Velocity and shear
Plate heat exchangers rely on turbulent flow to achieve high heat transfer. Too low a velocity and the unit fouls. Too high a velocity and erosion and pressure drop rise. The manufacturer’s software will balance this. Give it real flow rates, not just design peak.
4. Materials and gaskets
Plates: 316L stainless steel is standard for chilled water. For chloride-rich water or aggressive fluids, consider higher alloys such as titanium or SMO 254.
Gaskets: EPDM is common for chilled water and glycol. NBR is used for some oils and refrigerants. HNBR and Viton are used for higher temperatures or chemical resistance. Check the temperature and fluid compatibility.
Brazing: Copper-brazed units are common for HVAC. Nickel-brazed units are used for ammonia and some process duties.
Connections: Flanged, threaded, or grooved. Match the port size to the system piping. Undersized ports create unnecessary pressure drop.
5. Chilled water system specific issues
Low delta-T syndrome
This is the most common chilled water problem. The heat exchanger is oversized or the flow is too high on one side. The result is a small temperature difference across the loop. Pumps move more water than needed. Chillers run inefficiently.
Fix it by selecting the heat exchanger for the real design delta-T, not a conservative guess. Check control valve authority and balancing. Do not use a plate heat exchanger to compensate for poor hydronic design.
Freeze protection
Chilled water is above freezing, but glycol systems, low-load conditions, and outdoor equipment can freeze. Use glycol where needed. Add freeze stats. Provide drain-down or heat tracing for outdoor units. A frozen plate heat exchanger can rupture plates and gaskets.
Fouling and water quality
Chilled water loops are usually closed and clean. But open cooling towers, glycol breakdown, and corrosion products can foul plates. Specify a fouling factor that matches reality. Too high a fouling factor oversizes the unit and lowers efficiency. Too low and the unit fouls quickly.
Strainers and proper water treatment protect the heat exchanger. So does a maintenance plan.
District cooling and thermal storage
District cooling and chilled water storage often use plate heat exchangers to separate primary and secondary loops. These units must handle large flows and tight approaches. Gasketed units are common. Welded or semi-welded units may be used for higher pressure or where gaskets are a concern.
6. Common selection mistakes
Sizing on peak load only, ignoring part-load performance.
Specifying a 1°F approach when 3°F would work.
Ignoring pressure drop and pump energy.
Using a brazed unit where cleaning is needed.
Choosing the wrong gasket material for glycol or water quality.
Forgetting freeze protection.
Using a plate heat exchanger to fix low delta-T caused by control valves.
Not checking port sizes and connection pressure ratings.
Buying on purchase price instead of total cost.
7. Maintenance and lifespan
Gasketed units: Open, clean, inspect plates, replace gaskets. Plates can be replaced individually. Lifespan is typically 10–20 years with good maintenance.
Brazed units: Cannot be opened. Chemical cleaning only. Often replaced when fouled or plugged. Lifespan is typically 10–15 years in clean service.
Welded units: No gaskets, but no easy access. Chemical cleaning or replacement. Lifespan can be 15–20 years in aggressive service.
Keep spare gaskets and plates for critical units. A gasket leak during peak cooling season can shut down a chiller plant.
FAQ
What type of plate heat exchanger is best for chilled water?
Gasketed plate heat exchangers are the best fit for most HVAC chilled water systems. They are cleanable, flexible, and cost-effective.
Can I use a brazed plate heat exchanger for chilled water?
Yes, for small, clean, sealed duties such as packaged chillers and heat pumps. If the fluid is dirty or the unit needs frequent cleaning, use a gasketed unit instead.
How do I avoid low delta-T syndrome?
Select the heat exchanger for the real design delta-T. Check control valves, balancing, and pump curves. Do not oversize the heat exchanger to cover up a hydronic problem.
What pressure drop should I allow?
Typical chilled water plate heat exchangers use 10–20 psi (70–140 kPa) per side. The right number depends on pump energy, pipe size, and the rest of the system.
What approach temperature is practical?
1°F to 3°F is possible. A 2°F to 3°F approach is usually a better balance of cost and performance. A 1°F approach increases surface area and pressure drop significantly.
How do I prevent freezing?
Use glycol where required. Add freeze protection controls. Drain outdoor units. Do not rely on the heat exchanger alone to prevent freezing.
What plate material should I use?
316L stainless steel is standard. Use titanium or higher alloys for chloride-rich or aggressive fluids.