What Is a Pressure Vessel?

According to the ASME Boiler and Pressure Vessel Code (BPVC Section VIII) and China’s TSG 21-2023: Supervision Regulation on Safety Technology for Stationary Pressure Vessels, a pressure vessel is defined as:

A sealed container designed to hold gases or liquids at a pressure significantly higher or lower than atmospheric pressure. If certain thresholds of pressure, volume, and fluid type are exceeded, the equipment must be classified and managed as a pressure vessel.

Key classification criteria:

• Design pressure: typically ≥ 0.1 MPa (gauge)

• Internal volume: typically ≥ 25 liters (per Chinese regulations)

• Fluid type: hazardous, toxic, or flammable media

• Fully sealed construction: the unit must be enclosed and not easily opened during normal operation

Types and Structure of Plate Heat Exchangers

A plate heat exchanger transfers heat between fluids through multiple corrugated metal plates stacked to form alternating channels. PHEs are categorized based on their assembly method:

Are Plate Heat Exchangers Classified as Pressure Vessels?

The answer depends on several factors including design pressure, volume, structural sealing, and application.

Gasketed Plate Heat Exchangers – Usually Not Pressure Vessels

• Not fully sealed (removable construction)

• Common design pressure < 1.6 MPa

• Typically regulated under GB/T 16409 or NB/T 47004, not under pressure vessel codes

• Managed as mechanical equipment, not pressure vessels

Brazed and Welded Plate Heat Exchangers – May Be Pressure Vessels

• Fully sealed by brazing or welding

• Higher pressure ratings and compact volume may meet pressure vessel thresholds

• If working pressure ≥ 0.1 MPa and volume exceeds regulatory limits, they must comply with ASME Section VIII, TSG 21, or PED (EU Pressure Equipment Directive)

• Commonly used for refrigerants, steam, oil, or corrosive fluids

Regulatory and Standard Reference

In some cases, even small-volume exchangers are regulated as pressure vessels if the medium is hazardous or flammable.

Practical Engineering Examples

• Gasketed PHEs in HVAC: Operate below 1 MPa, not sealed, not classified as pressure vessels

• Brazed PHEs in refrigeration: Compact and sealed; pressure and refrigerant type determine classification

• Welded PHEs in chemical plants: Fully enclosed, high pressure; classified and managed as pressure vessels

Conclusion

Not all plate heat exchangers are pressure vessels. The classification depends on:

• Whether the unit is fully sealed

• Working pressure and internal volume

• Type of working fluid (hazardous or non-hazardous)

• Compliance with local or international codes

In general:

• Gasketed PHEs for water or low-risk fluids are not pressure vessels

• Welded or brazed PHEs used in industrial settings with high pressure or hazardous fluids are typically regulated as pressure vessels

Before purchase or commissioning, engineering teams should verify design conditions and consult applicable standards to determine if a PHE falls under pressure vessel regulations.