INTRODUCTION
Electric liquid-heater sizing starts with the actual heating duty. The required power depends on whether the application is continuous-flow heating, tank heat-up, temperature holding, circulation heating or a combination of these duties.
For continuous-flow service, the preliminary process heat duty is normally calculated from mass flow, average specific heat and required temperature rise. External heat loss should then be added before the electrical input is corrected for the assumed overall efficiency.
For tank heating, the fluid mass, required heat-up time and heat loss are equally important. Fluid viscosity, pressure, fouling tendency, boiling or decomposition risk, material compatibility, allowable surface watt density and safety requirements must also be reviewed before the final heater is selected.
1. Identify the Heating Duty
A. Continuous-flow heating
Liquid enters the heater at one temperature and leaves at a higher controlled temperature.
B. Tank heat-up
A known liquid mass must be heated from an initial temperature to a target temperature within a specified time.
C. Temperature holding
The liquid is already near the required temperature and the heater mainly replaces ongoing heat loss.
D. Circulation heating
Liquid is circulated through an external inline or circulation heater until the tank or process loop reaches the required temperature.
2. Continuous-Flow Heater Calculation
For a single-phase liquid with no phase change:
Where:
If actual volumetric flow is provided:
Where:
A more complete preliminary electrical input is:
Where:
Do not automatically add a fixed “design margin” to every project. Installed power should be reviewed against startup duty, maximum flow, minimum inlet temperature, heat loss, control range, power staging, element surface watt density and the actual operating envelope.
3. Worked Example — Continuous Water Heating
Fluid: water
Flow: 2.0 m³/h
Inlet temperature: 20°C
Required outlet temperature: 60°C
Representative average density: approximately 990 kg/m³
Representative average Cp: approximately 4.18 kJ/(kg·K)
If, only for this calculation example, additional external heat loss is neglected and an overall efficiency of 97% is assumed:
The 97% value is only an example assumption. It is not a published design efficiency or selection guarantee.
For an actual project, use:
and review actual fluid properties, working and design pressure, pressure drop, element surface watt density, material, control method and safety protection.
4. Tank Heat-Up Calculation
Where:
m = liquid mass, kg
If the required heat-up time is t hours:
Where:
Then:
This definition avoids double counting: Pideal is the useful energy rate absorbed by the liquid, Ploss is external heat loss, and eta corrects the total thermal requirement to estimated electrical input.
5. Worked Example — Tank Heat-Up
Fluid mass: 2000 kg
Average Cp: 2.0 kJ/(kg·K)
Initial temperature: 20°C
Target temperature: 80°C
Heat-up time: 4 h
If average external heat loss is estimated at 3 kW and, only for this example, eta = 0.95:
6. Property Data Should Match the Heating Range
Density and specific heat can change with temperature. For preliminary calculations over a moderate temperature range, representative average values may be used. For a wide temperature range or temperature-sensitive liquid, use verified temperature-dependent property data from the customer, fluid supplier or recognized engineering data source.
7. Viscosity Changes the Heater Design
High viscosity can reduce convection, increase liquid-film temperature and pressure drop, reduce natural circulation and increase coking or thermal-degradation risk. Crude oil, heavy oil and viscous fluids should be evaluated with viscosity data at actual operating temperatures.
8. Phase Change and Decomposition
The simple sensible-heat equation is not sufficient for boiling, evaporation, vaporization, melting, crystallization, chemical reaction or thermal decomposition.
9. Total Heater Power Is Not Surface Watt Density
Total heater power determines the total process heat available. Surface watt density determines the electrical heat released per unit active heated surface area.
10. Data Required
Fluid name and composition; flow or tank volume; density; specific heat; viscosity versus temperature; inlet/initial temperature; outlet/final temperature; heat-up time; working and design pressure; allowable pressure drop; tank dimensions or pipe size; material compatibility; electrical supply; operating mode; hazardous-area requirement; required interlocks.
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FAQ
Can liquid-heater power be calculated only from tank volume?
Why does heavy oil often need a larger heated surface than water?
Should heat loss always be estimated as a fixed percentage?
Can the same calculation be used for boiling or vaporization?
Engineering Note
The formulas, examples and values in this guide are for technical explanation and preliminary engineering reference only. They do not constitute a final heater selection, guaranteed performance value or project commitment. Final design must be confirmed from the actual medium, composition, temperature, pressure, flow, hazardous-area requirements, material compatibility, applicable standards and agreed supply scope.
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