01Why velocity matters
Velocity controls how quickly the fluid moves past the heated surface. Too little circulation can create hot boundary layers, poor temperature uniformity and excessive element sheath temperature. Higher velocity can improve heat transfer, but it also raises pressure drop and may increase erosion or vibration risk in some services.
The acceptable range depends on fluid properties, heater geometry, element spacing, viscosity, temperature, pressure and materials. A value copied from another project may be wrong even when the nominal pipe size is the same.
02Calculate from flow and open area
For a single flow passage, average velocity is v = Q/A, where Q is the volumetric flow at operating conditions and A is the net flow area. For mass flow, first convert using the fluid density at the relevant temperature and pressure: Q = ṁ/ρ. Use the actual flow area around the elements and baffles, not simply the pipe cross-section when the internal geometry differs.
For gases, density changes with temperature and pressure, so evaluate velocity through the heater using the local operating conditions. For liquids with large viscosity changes, check the cold-start case as well as normal hot operation.
03Review the full operating envelope
- Minimum, normal and maximum flow, including startup and upset cases
- Pressure drop across the heater at each important operating point
- Element sheath or film temperature at minimum flow
- Fluid viscosity, density, fouling tendency and solids content
- Flow distribution through parallel passes or element bundles
The thermal calculation and hydraulic calculation must agree. If the pressure drop is too high, review the flow path, vessel diameter, number of passes or element arrangement instead of simply accepting a pump that cannot deliver the required flow.
04Interlocks matter as much as nominal velocity
Where low flow can overheat the element, provide a suitable flow or differential-pressure permissive and a high-temperature trip. The heater should not energize until circulation is proven, and loss of circulation should remove heater power through a defined control sequence. Sensor placement and trip response should be reviewed against the heater’s thermal inertia.
For procurement, provide fluid properties, operating pressure and temperature, flow range, allowable pressure drop, connection size, heater duty and the pump curve if available. The supplier can then check the design point and the low-flow boundary together.
↗Related guide
Related guide: Electric Circulation Heater Pressure Drop: Checks Before Changing the Pump