How to Size an Electric Process Gas Heater: Flow Rate, Temperature Rise and Power

INTRODUCTION

Correct heater sizing starts with the actual gas duty, not with a nominal heater power. Gas type, flow rate, pressure, inlet temperature, target outlet temperature, operating mode and heat loss all affect the required electrical heating capacity.

For most process-gas applications, the first thermal estimate can be made from mass flow, average specific heat and required temperature rise. The result is then reviewed against startup conditions, heat loss, maximum flow, control method, allowable element watt density, pressure drop and safety margin.

This guide explains the preliminary sizing method for electric air heaters, inline process gas heaters, compressed-air heaters and nitrogen heaters. Final heater design should always be based on confirmed process data and project requirements.

1. Start With the Correct Gas Flow Basis

Gas flow may be given as:

• kg/h or kg/s — mass flow
• Nm³/h — normalized volumetric flow
• Sm³/h — standard volumetric flow
• m³/h — actual volumetric flow at stated pressure and temperature

Mass flow is the most direct basis for heater-duty calculation.

If the flow is given in Nm³/h or Sm³/h, the reference pressure and temperature used for the “normal” or “standard” condition must be confirmed. Different projects and standards may use different reference conditions.

For a normalized volumetric flow:

Where:

If actual volumetric flow is provided instead, the density must correspond to the actual operating pressure and temperature.

Engineering note:
Do not mix Nm³/h with density at actual process conditions. That can create a significant power-calculation error.

2. Calculate the Required Temperature Rise

Where:

Tin = inlet gas temperature
Tout = required outlet gas temperature

The design should consider the minimum expected inlet temperature and the maximum expected gas flow if the heater must maintain outlet temperature under worst-case operation.

For a wide temperature range, gas heat capacity changes with temperature. Use an appropriate average heat capacity or temperature-dependent property data instead of one room-temperature value.

3. Calculate the Preliminary Heating Duty

A common preliminary calculation is:

Where:

Cp,avg = average specific heat over the heating range, kJ/(kg·K)

Because 1 kJ/s = 1 kW, the equation directly gives thermal duty in kW when the above units are used.

The required electrical input can then be estimated as:

Where:

eta = overall thermal efficiency expressed as a decimal

The final selected electrical power should also consider:

• heat loss from the heater body and connected piping or duct
• startup and warm-up requirement
• maximum process flow
• minimum inlet temperature
• operating pressure and gas-property variation
• control turndown
• number of heating stages
• allowable element surface loading
• project design margin

Do not apply a fixed oversizing percentage to every project. Excessive installed power may make temperature control more difficult and may increase element surface temperature if the heater geometry is not designed accordingly.

4. Worked Example — Preliminary Air-Heater Calculation

The following example is only to demonstrate the calculation method.

Assumptions:

Gas: clean air
Flow: 1000 Nm³/h
Normal condition for this example: 0°C and 101.325 kPa absolute
Air density at the stated normal condition: approximately 1.293 kg/Nm³
Inlet temperature: 20°C
Outlet temperature: 100°C
Average Cp used for preliminary estimate: approximately 1.005 kJ/(kg·K)

Step 1 — mass flow:

Step 2 — temperature rise:

Step 3 — thermal duty:

If an overall preliminary thermal efficiency of 95% is assumed only for this example:

A practical heater selection would not be finalized from this number alone. The engineer must still review heat loss, actual operating density, pressure, allowable pressure drop, element layout, surface loading, control method and safety interlocks.

5. Normal Flow vs Actual Flow in Pressurized Gas Systems

This distinction is especially important for compressed air, nitrogen and other pressure-bearing gas systems.

A gas stream reported as 1000 Nm³/h does not physically occupy 1000 m³/h inside a pressurized pipeline. The actual volumetric flow in the heater depends on operating pressure and temperature.

Thermal duty is best calculated from mass flow.

Flow velocity and pressure drop, however, must be checked using the actual gas volume and density inside the heater at the relevant operating condition.

Therefore the same RFQ should normally confirm:

• normal or standard flow
• normal-condition definition
• operating pressure
• inlet temperature
• target outlet temperature
• maximum flow
• allowable pressure drop

6. Total Heater Power Is Not the Same as Element Watt Density

Total heater power answers:
“How many kilowatts must be transferred to the gas?”

Element watt density answers:
“How much electrical power is released per unit heated surface area of the element?”

Two heaters may both be rated at 100 kW but use very different element quantities, element diameters, heated lengths and surface loads.

Element watt density affects:

• element sheath temperature
• local gas-film temperature
• element life
• fouling or deposit risk
• temperature uniformity
• heater size
• required gas velocity around the elements

High gas outlet temperature does not automatically mean that a high watt density is acceptable. Element design should be checked against gas composition, gas velocity, surface temperature and material limits.

7. Power Control and Turndown

A correctly sized heater still needs suitable power control.

Common options include:

• contactor-based staged control
• PID temperature control
• SCR / thyristor power regulation
• PLC-based sequence control
• multiple heating banks or zones

Continuous SCR control can provide fine modulation where outlet-temperature stability is important. Staged control may be suitable where a simpler control architecture is acceptable.

The selected control method should match:

• installed heater power
• process thermal inertia
• gas-flow variation
• required temperature tolerance
• electrical supply
• number of heater stages
• upstream and downstream interlocks

8. Data Required for a Gas-Heater Sizing Check

For preliminary sizing, provide:

1. Gas name and composition
2. Normal and maximum gas flow
3. Flow unit and reference condition
4. Inlet temperature
5. Required outlet temperature
6. Normal working pressure
7. Maximum or design pressure
8. Allowable pressure drop
9. Pipeline or duct size
10. Voltage, phase and frequency
11. Continuous or intermittent operation
12. Ambient and installation conditions
13. Hazardous-area requirement, if any
14. Control method
15. Required safety interlocks
16. Heater-only, heater-plus-control, or complete skid scope

9. Related Xiangyuan Products

Electric Air & Process Gas Heaters

Electric Inline Process Gas Heater

Electric Air Heater

Electric Compressed Air Heater

Electric Nitrogen Gas Heater

Electric Duct Heater

10. FAQ