Gas Heater Pressure Drop & Flow Velocity Design Guide

INTRODUCTION

Pressure drop is one of the most important mechanical and process constraints in an electric gas heater. A heater must provide sufficient heat-transfer area without creating excessive resistance to the gas flow.

The correct design depends on actual gas density, actual volumetric flow, flow velocity, heater free-flow area, element arrangement, internal supports, bends, nozzles, ducts, baffles and allowable system pressure loss.

This guide explains the main engineering relationships used during preliminary gas-heater design.

1. Why Pressure Drop Matters

Excessive pressure drop can:

• reduce process flow
• increase fan or compressor load
• reduce downstream pressure
• disturb control-valve performance
• limit plant capacity
• increase operating cost

Very low gas velocity can also be undesirable because it may reduce convective heat transfer around the heating elements and create less uniform local cooling.

The design objective is therefore not “minimum velocity” or “maximum velocity.” It is a balanced flow path that supports heat transfer, acceptable pressure loss, temperature uniformity and element life.

2. Use Actual Flow for Velocity Calculations

Gas velocity is calculated from:

Where:

Afree is not always equal to the pipe or duct cross-sectional area.

Heating elements, support plates, baffles, internal guides and structural members reduce the open area. The local velocity between elements can therefore be higher than the velocity calculated only from the inlet pipe diameter.

3. Normalized Flow Must Be Converted Before Velocity Is Calculated

Nm³/h or Sm³/h is useful for process mass-flow reporting, but it is not the actual physical volume inside a pressurized heater.

For an ideal-gas approximation, actual volume changes with absolute pressure and absolute temperature. The conversion must use the project’s defined normal or standard reference condition.

For higher accuracy, especially at higher pressure or for non-ideal gas mixtures, use verified gas-property data and an appropriate compressibility factor.

Engineering rule:
Use mass flow for heat-duty calculation and actual local conditions for velocity and pressure-drop calculation.

4. Main Sources of Pressure Drop

Total heater pressure loss may include:

• inlet nozzle or duct transition
• expansion or contraction
• heating-element bank
• internal support plates
• flow guides or baffles
• straight shell or duct friction
• outlet transition
• elbows or connected fittings included in the supplier scope

A simplified engineering representation is:

Straight-section friction is commonly estimated with the Darcy-Weisbach relationship:

Where:
f = Darcy friction factor
L = flow length
Dh = hydraulic diameter
rho = local gas density

Local losses may be represented as:

Where K is the loss coefficient for a specific geometry.

In a real electric gas heater, the heating-element bank and internal geometry often require test data, detailed engineering correlations, numerical analysis or CFD rather than a simple empty-pipe equation.

5. Compressible Gas Requires Additional Care

For a gas heater with a significant:

• pressure change
• temperature rise
• density change
• high velocity
• high operating pressure

a constant-density calculation may not be accurate enough.

The heater may need to be divided into calculation sections so that density, velocity and gas properties are updated along the flow path.

For demanding projects, pressure-drop and temperature-distribution review may require detailed calculation or CFD.

6. Example — Why Actual Volume Matters

Assume an actual gas flow at the heater inlet is:
1000 m³/h actual

Convert to m³/s:

If the available free-flow area is:

Then:

If internal elements reduce the local free area to 0.050 m²:

This simple example shows why heater-internal free area must be considered.

It does not provide the final pressure drop. The final pressure loss also depends on density, geometry, surface obstruction, element-bank arrangement, supports, transitions and gas-property changes.

7. Element Arrangement and Pressure Drop

Important variables include:

• element diameter
• number of elements
• element pitch
• staggered or aligned arrangement
• finned or plain tubular elements
• heated length
• number of element rows
• shell or duct cross section
• baffle arrangement
• support plates

Increasing element quantity can increase heat-transfer area but may also reduce free-flow area.

A successful design must review total power, element surface loading and pressure drop together.

8. Duct Heater vs Pressure-Bearing Inline Gas Heater

Electric Duct Heater:
Typically used for larger-volume, lower-pressure airflow in round or rectangular ductwork.

Main design inputs include:

• duct dimensions
• airflow
• air velocity
• outlet temperature
• fan capability
• allowable pressure drop

Electric Inline Process Gas Heater:
Used for closed pipelines and pressure-bearing gas systems.

Main design inputs include:

• gas composition
• mass or normalized flow
• actual operating pressure
• inlet and outlet temperature
• pipeline size
• design pressure
• allowable pressure drop
• flange standard
• pressure-bearing material

Do not select the heater type from power alone.

9. Pressure Drop and Safety Interlocks

Depending on the application, the control system may monitor:

• flow switch
• differential pressure switch
• differential pressure transmitter
• fan running status
• compressor or upstream equipment permissive
• pipeline pressure
• heater outlet temperature
• element or sheath temperature

A low-flow condition can reduce element cooling and increase element temperature. For this reason, heater energization is commonly interlocked with confirmed gas flow or related process permissives where required by the application.

10. Data Required for Pressure-Drop Review

Provide:

1. Gas type and composition
2. Flow rate and unit
3. Flow reference condition
4. Inlet pressure
5. Outlet or downstream pressure requirement
6. Allowable heater pressure drop
7. Inlet temperature
8. Required outlet temperature
9. Pipe or duct size
10. Flange or duct connection
11. Heater installation direction
12. Fan or compressor information where relevant
13. Dust, moisture, oil mist or corrosive constituents
14. Required heater power
15. Space limitations

11. Related Xiangyuan Products

Electric Air & Process Gas Heaters

Electric Inline Process Gas Heater

Electric Duct Heater

Electric Compressed Air Heater

12. FAQ