---
title: "Common Fired Heater Calculations: Understanding Duty and Fired Duty"
description: "In our latest Tulsa Heaters Midstream video, we're going back to the whiteboard to explain one of the most common heat transfer formulas used when working with fired heaters:\nDuty = Mass Flow Rate × Specific Heat × Delta T\nUnderstanding this basic calculation can help explain what's happening inside a fired heater, how much heat is being absorbed by the process fluid, and why the burner must produce more heat than the process ultimately receives."
---

[Midstream & More](https://blog.tulsaheatersmidstream.com)

# [Common Fired Heater Calculations: Understanding Duty and Fired Duty](https://blog.tulsaheatersmidstream.com/common-fired-heater-calculations-understanding-duty-and-fired-duty)

 Written by [Justin Forth](https://blog.tulsaheatersmidstream.com/author/justin-forth) | Oct 7, 2026, 3:00:00 PM

Fired heater engineering can involve complex calculations, but some formulas come up again and again.

In our latest Tulsa Heaters Midstream video, we're going back to the whiteboard to explain one of the most common heat transfer formulas used when working with fired heaters:

**Duty = Mass Flow Rate × Specific Heat × Delta T**

Understanding this basic calculation can help explain what's happening inside a fired heater, how much heat is being absorbed by the process fluid, and why the burner must produce more heat than the process ultimately receives.

## **Understanding Fired Heater Duty**

Consider a typical system where a fired heater is heating hot oil or another heat medium.

The fluid enters the fired heater at a lower temperature, absorbs heat, leaves the heater at a higher temperature, travels to a heat exchanger, and eventually returns to the heater to repeat the process.

To determine how much heat is being added to that fluid, we can use:

**Duty = ṁ × Cp × ΔT**

Where:

- **ṁ (M dot)** = Mass flow rate, typically expressed in pounds per hour
- **Cp** = Specific heat of the process fluid, expressed in BTU per pound per degree Fahrenheit
- **ΔT** = Change in temperature across the heater

Together, these values tell us the amount of heat being absorbed by the fluid.

## **Which Specific Heat Should You Use?**

Specific heat is a physical property of the fluid, and it can change slightly as the fluid temperature changes.

For example, if a heat medium enters a fired heater at 300°F and exits at 350°F, its specific heat at the inlet may be slightly different from its specific heat at the outlet.

So which value should you use?

For this calculation, use the **average specific heat between the inlet and outlet conditions**.

## **Calculating Delta T**

Delta T is simply the difference between the outlet and inlet temperatures.

Using our example:

**350°F - 300°F = 50°F ΔT**

That temperature change, combined with mass flow rate and average specific heat, allows us to calculate the absorbed duty of the fired heater.

## **Where Does the Absorbed Heat Go?**

The heat added to the fluid in the fired heater doesn't simply disappear.

In a heat medium system, the heated fluid travels to a heat exchanger where that energy is transferred into another process.

In simple terms, the heat absorbed at the heater should approximately equal the heat delivered at the heat exchanger, with some allowance for heat loss through piping along the way.

The video uses the example of potentially losing a few degrees of temperature as the fluid circulates through the system.

## **Absorbed Duty vs. Fired Duty**

This brings us to another important fired heater calculation.

**Absorbed duty and fired duty are not the same thing.**

Absorbed duty is the amount of heat actually transferred into the process fluid.

Fired duty is the amount of heat the burner must generate to achieve that absorbed duty.

Why is fired duty higher?

Because the fired heater isn't 100% efficient. Some energy leaves the system with the hot flue gases exiting the stack.

The relationship can be expressed as:

**Fired Duty = Absorbed Duty ÷ Heater Efficiency**

For example, if a fired heater operates at approximately 85% efficiency, the absorbed duty is divided by 0.85 to determine the required fired duty.

In other words, **you have to fire more heat than the process fluid ultimately absorbs.**

## **Simple Formulas, Important Fired Heater Fundamentals**

These calculations may be straightforward, but they describe some of the most fundamental relationships in fired heater engineering.

Understanding mass flow rate, specific heat, temperature change, absorbed duty, and fired duty can provide a much clearer picture of how energy moves through a fired heater and the overall process.

At Tulsa Heaters Midstream, fired heater design is about understanding those relationships and applying them to real-world operating conditions.

**Watch our latest Common Calculations video for a whiteboard walkthrough of duty, fired duty, heat transfer, and fired heater efficiency.**

 

[View full post](https://blog.tulsaheatersmidstream.com/common-fired-heater-calculations-understanding-duty-and-fired-duty)

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