Line Heater

The Line Heater calculator sizes an inline gas heater installed upstream of a pressure-reducing device such as a regulator. Given the gas flow rate, inlet and outlet pressures, current inlet temperature, and required outlet temperature, the calculator determines the heater rating needed to prevent hydrate formation downstream of the pressure reduction. It can also solve for volumetric flow rate, heater efficiency, heat required, fuel consumption, and heater outlet temperature depending on which value is designated as the unknown.

For detailed heater design and operating performance, the line heater manufacturer should be consulted.

Background

When a gas stream undergoes pressure reduction — for example, flowing through a regulator or an orifice — the gas temperature drops due to the Joule-Thomson effect. In long pipeline segments, the surrounding environment can partially offset this cooling. But across a short pressure-reducing device, the temperature drop is sharp and immediate, leaving no time for heat recovery from the surrounding soil or ambient air.

If water vapor is present in the gas stream, this rapid temperature drop creates a high risk of hydrate formation. Hydrates are ice-like solid compounds of water and hydrocarbon gas molecules that can form at temperatures above 32 °F (0 °C) when the pipeline is at elevated pressure. Hydrate build-up in equipment or instruments located downstream of a pressure reduction point can restrict or completely stop gas flow.

One of the most effective strategies to counteract this risk is to heat the gas stream immediately upstream of the pressure-reducing device using an inline line heater. The heater raises the gas temperature sufficiently so that, after the Joule-Thomson temperature drop across the regulator, the outlet temperature still remains above the hydrate formation temperature.

A common type of inline heater uses a shell-and-tube heat exchanger installed directly in the gas flow path, with pipeline gas as the fuel source. These heaters are typically rated in Btu/hr — the maximum rate at which heat can be transferred to the gas stream. Most pipeline pressure reduction processes are isenthalpic, meaning the energy state of the gas is conserved through the device, which allows the required heat input to be estimated from inlet and outlet conditions alone.

Equations

Heat Required

The input energy requirement for the heater is:

q = HR \times \eta_{th}

q = HR \times \eta_{th}

Where:
q − Heat required (Btu/hr)
HR − Line heater rating (Btu/hr)
η_th − Line heater thermodynamic efficiency

Fuel Consumption

Fuel consumption is calculated as:

FC = HR \times HV

FC = HR \times HV

Where:
FC − Fuel consumption (cfh)
HR − Line heater rating (Btu/hr)
HV − Heating value of fuel gas (Btu/cf)

Heater Outlet Temperature

The required temperature of the gas exiting the heater — accounting for the Joule-Thomson temperature drop across the downstream device — is:

T_{out} = \mu_{JT} \times (P_1 - P_2) + T_2

T_{out} = \mu_{JT} \times (P_1 – P_2) + T_2

Where:
T_out − Heater outlet gas temperature (Rankine)
μ_JT − Joule-Thomson coefficient (dimensionless)
P₁ − Device inlet pressure (psia)
P₂ − Device outlet pressure (psia)
T₂ − Required gas temperature at the device outlet (°R)

Heater Rating — Enthalpy Methods

When using either the Theoretic (Calculated Enthalpy) or Theoretic (Entered Enthalpy) method to solve for volumetric flow rate, heater rating, or efficiency, the following system of equations is solved simultaneously:

\frac{Q_F}{Q_B} = \frac{P_B}{P_F} \times \frac{T_F}{T_B} \times \frac{Z_F}{Z_B}

\frac{Q_F}{Q_B} = \frac{P_B}{P_F} \times \frac{T_F}{T_B} \times \frac{Z_F}{Z_B}

HR = \frac{\dot{M} \times (H_2 - H_1)}{\eta_{th}}

HR = \frac{\dot{M} \times (H_2 – H_1)}{\eta_{th}}

\dot{M} = Q_F \times \rho

\dot{M} = Q_F \times \rho

Where:
Q_F − Volumetric flow rate at flowing pressure and temperature (cfh)
Q_B − Volumetric flow rate at base conditions (cfh)
P_B − Base pressure (psia)
P_F − Average flowing pressure (psia)
T_F − Average flowing temperature (°R)
T_B − Base temperature (°R)
Z_F − Compressibility factor at P₁
Z_B − Compressibility factor at base conditions
HR − Line heater rating (Btu/hr)
Ṁ − Mass flow rate (lbm/hr)
H₁ − Heater inlet gas enthalpy (Btu/lbm)
H₂ − Heater outlet gas enthalpy (Btu/lbm)
η_th − Line heater thermodynamic efficiency
ρ − Gas density (lbm/cf)

Heater Rating — Specific Heat Method

When using the Theoretic (Entered Specific Heat) method, the heater rating equation substitutes specific heat and temperature rise for the enthalpy difference:

\frac{Q_F}{Q_B} = \frac{P_B}{P_F} \times \frac{T_F}{T_B} \times \frac{Z_F}{Z_B}

\frac{Q_F}{Q_B} = \frac{P_B}{P_F} \times \frac{T_F}{T_B} \times \frac{Z_F}{Z_B}

HR = \frac{\dot{M} \times C_p \times (T_2 - T_1)}{\eta_{th}}

HR = \frac{\dot{M} \times C_p \times (T_2 – T_1)}{\eta_{th}}

\dot{M} = Q_F \times \rho

\dot{M} = Q_F \times \rho

Where:
C_p − Specific heat of gas at constant pressure (Btu/lbm·°F)
T₁ − Pipe inlet gas flowing temperature (°R)
T₂ − Pipe outlet gas flowing temperature (°R)
(all other variables as defined above)

Enthalpy Calculation

When the Theoretic (Calculated Enthalpy) method is selected, GASCalc computes the inlet and outlet gas enthalpies using the methods described in American Gas Association Report No. 10, “Speed of Sound In Natural Gas and Other Related Hydrocarbon Gases,” 2003. The Joule-Thomson coefficient used in that calculation is computed as described in the Thermodynamic Properties Calculation Reference. Alternatively, enthalpy values can be entered manually using the Theoretic (Entered Enthalpy) method; the Thermodynamic Properties calculator can be used to look up these values.

Calculation Methods

Three calculation methods are available from the Calculation Method list. The user must select the method appropriate for the available input data:

Solve-For Capability

The Line Heater calculator uses a solve-for interface. Red labels on screen identify the items that can be designated as the unknown to be calculated. To select the unknown, click the red label for that item until it is underlined. Only one item may be set as the unknown at a time; all other items must be provided as known inputs. The following items can be solved for:

• Heater Rating — the most common use case: find the required Btu/hr rating given a known flow and temperature rise requirement.
• Volumetric Flow Rate — find the maximum flow the heater can support given its rating and the temperature conditions.
• Efficiency — find the required heater efficiency to achieve the desired outlet temperature given a fixed heater rating and flow rate.

Case Guide

Part 1: Create Case

Input Parameters

Part 2: Outputs/Reports

Results

Notes & Considerations

• All pressure values displayed or entered on this screen are in gauge pressure. Internally, absolute pressure values are used in all calculations.
• The Volumetric Flow Rate is in standard volume units, adjusted to the base pressure and temperature specified in Base Conditions.
• The calculator assumes the gas flows through a pressure-reducing device (such as a regulator) immediately downstream of the heater. Device inlet pressure is the pressure just upstream of the device; device outlet pressure is just downstream.
• The Specific Heat value used in calculations represents heat capacity at constant pressure (C_p), not at constant volume.
• The Efficiency value represents the overall heat transfer efficiency of the heater — for example, if a heater rated at 100 MBtu/hr transfers only 80 MBtu/hr to the gas stream, the efficiency is 0.80 (80%).
• Enthalpy and Joule-Thomson values required for the enthalpy-based methods can be computed using GASCalc’s Thermodynamic Properties calculation routine.
• Red labels identify which items can be designated as the unknown. Click the label until it is underlined to select it. Only one item may be underlined (unknown) at a time.
• The Line Heater calculation is intended for preliminary sizing purposes. For detailed design and operating performance, consult the heater manufacturer.

References

• B3PE LLC — GASCalc™ 6.1 Calculation Reference: Line Heater Values, Revision 005, Copyright 2025
• American Gas Association — Measurement, GEOP Series Book M-1, 1993
• American Gas Association — Report No. 10, “Speed of Sound In Natural Gas and Other Related Hydrocarbon Gases,” 2003
• Van Wylen, Gordon J. and Sonntag, Richard E. — Fundamentals of Classical Thermodynamics, 2nd Edition, Revised Printing, 1978
• Holman, J.P. — Heat Transfer, 4th Edition, 1976

FAQ