1. Calculate preliminary circulation flow
For a single liquid-phase loop, the energy balance can be written as:
Flow (m³/s) = heat duty (kW) ÷ [density (kg/m³) × specific heat (kJ/kg·K) × temperature difference (K)]
Multiply m³/s by 3,600 for m³/h. Use thermal-fluid density and specific heat at operating temperature. Check the chosen temperature difference against process control and minimum heater flow.
Illustrative calculation: for 2,000 kW, density 780 kg/m³, specific heat 2.3 kJ/kg·K and a 30 K difference, flow is about 0.0372 m³/s, or 134 m³/h. This is a heat-balance result, not a final pump size.
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Thermal oil flow and pipe velocity calculator
Estimate circulation flow from useful heat duty, operating fluid properties and supply-to-return temperature difference. The optional pipe inside diameter converts that flow into average line velocity.
Calculated circulation flow
133.8 m³/hAverage pipe velocity
1.18 m/sFlow = heat duty × 3,600 ÷ (density × specific heat × temperature difference). Velocity uses the entered inside diameter.Preliminary heat-balance result only. Final pump and pipe selection requires the complete hydraulic model, hot-fluid viscosity and density, pressure-loss schedule, NPSH review, pump curve, minimum heater flow, materials and operating range.
2. Determine differential head from the hydraulic model
Sum losses through the controlling circuit: heater coil, heat exchanger, pipes, fittings, strainers and valves. Check clean and fouled conditions. Convert pressure loss to head with the fluid density at the same condition.
In a filled closed loop, the rising and falling liquid columns balance; building height is not added to continuous pump differential head. Elevation still affects local pressure, filling, venting and NPSH.
3. Check hot-fluid properties and NPSH
Check density and viscosity at both cold startup and hot operation. Correct the pump curve where needed. NPSH available must exceed NPSH required by the margin agreed with the pump supplier.
- Fluid vapor pressure and temperature at pump suction
- Expansion-tank level and elevation relative to the pump
- Suction-line loss, strainer condition and local fittings
- Pump casing, shaft, bearings and seal compatibility with hot fluid
- Cold-start viscosity and the start-up operating procedure
- Motor power at the full reviewed operating range
4. Place the operating point on the pump curve
The pump and system curves determine the operating point. Check efficiency, absorbed power, NPSH and the allowable operating region. Connection size alone does not select a pump.
Justify each flow or head allowance. Excess margin can cause throttling, wasted power and operation outside the preferred range.
5. Duty, standby and control philosophy
Continuous processes often evaluate one duty and one standby pump, with automatic changeover and clear isolation for maintenance. Parallel operation requires curve and control review so two pumps share load without unstable interaction. Variable-speed control may be useful, but the heater’s minimum circulation and every user’s required flow must remain protected.
Link loss of flow to heat-input shutdown. Check pump status, flow or differential pressure, and outlet temperature in the trip logic.
6. Data to include in a pump inquiry
- Required normal, minimum and maximum flow
- Calculated differential head and a pressure-loss schedule
- Fluid name and property curve across the temperature range
- Suction pressure, expansion-tank elevation and NPSH calculation
- Operating temperature, ambient, altitude and electrical supply
- Seal, material, motor, control and standby requirements
Check the expansion tank sizing guide and thermal oil heater selection guide together with the pump curve.