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 to obtain m³/h. Use fluid density and specific heat at a representative operating temperature, not room-temperature water properties. The selected supply-to-return temperature difference must also be compatible with process control and heater design.
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.
2. Determine differential head from the hydraulic model
Add the pressure losses along the controlling circulation path: heater coil, process heat exchanger, supply and return piping, fittings, strainers, valves and control devices. Evaluate clean and credible fouled conditions where applicable. Convert pressure loss to fluid head consistently using the hot-fluid density.
In a properly filled closed loop, total building elevation is not simply added as continuous pump differential head because the rising and falling columns largely balance. Elevation still matters for filling, local pressure, venting, expansion-tank position and NPSH. Open systems and unusual process arrangements require separate review.
3. Check hot-fluid properties and NPSH
Density and viscosity change with temperature and influence hydraulic loss, pump power and curve correction. Compare the pump’s NPSH required with the NPSH available at the most demanding condition, allowing the project margin required by the responsible engineer and 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
Plot the system curve and check where it intersects the pump curve. Confirm efficiency, absorbed power, NPSH, minimum continuous flow and the allowable operating region. Avoid selecting a pump only because its nominal connection size matches the pipe.
A large arbitrary flow or head margin can move the operating point away from the preferred region, waste power and force throttling. Any engineering allowance should be traceable to uncertainty in the load, fouling, future branches or model tolerance.
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.
Trips and permissives commonly monitor pump status, flow or differential pressure and heater outlet temperature. The final logic follows the heater, pump and plant risk review.
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
Coordinate this work with the expansion tank sizing guide and the broader thermal oil heater selection guide. Final selection requires a qualified hydraulic calculation and an approved manufacturer curve.