Actual Steam Capacity vs From & At 100°C

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Actual Steam Capacity vs From & At 100°C: Understanding Boiler Steam Capacity

Actual Steam Capacity vs From & At 100°C: What's the Difference?

When comparing steam boilers, the capacity shown in a boiler catalogue may not be the same as the actual amount of steam produced under your plant's operating conditions.

The reason is simple: steam capacity can be expressed based on actual operating conditions or converted to a standardized “From & At 100°C” basis.

This article explains the difference, why the numbers can vary, and how engineers use the Factor of Evaporation (FE) to convert between the two.

1. What Is Actual Steam Capacity?

Actual steam capacity refers to the amount of steam a boiler produces under its specific operating conditions.

These conditions can include:

Operating pressure
Saturated steam temperature
Feedwater inlet temperature
Actual plant operating conditions

For example, saturated steam produced at a higher operating pressure has a corresponding saturation temperature that is higher than 100°C.

Therefore, the amount of energy required to produce that steam is different from the energy required to produce steam at atmospheric pressure.

Actual steam capacity is therefore useful when assessing real-world boiler performance at a specific plant.

2. What Is Steam Capacity From & At 100°C?

“From & At 100°C” steam capacity is a standardized basis used to compare boiler capacities under the same reference conditions.

The concept assumes:

Feedwater starts at 100°C
The water is converted into saturated steam at 100°C
The reference condition is approximately atmospheric pressure

This provides a common basis for comparing boilers operating at different pressures and feedwater temperatures.

In other words:

Actual capacity tells you what the boiler produces under its operating conditions.

From & At 100°C capacity gives you a standardized basis for comparison.

The use of a standardized basis for comparison is also reflected in published boiler engineering calculations and specifications.

3. Actual Steam Capacity vs From & At 100°C

Feature	Actual Steam Capacity	From & At 100°C Capacity
Basis	Actual operating conditions	Standardized reference conditions
Operating pressure	Based on actual boiler pressure	Standardized to atmospheric reference conditions
Steam temperature	Saturation temperature corresponding to operating pressure	Standardized to 100°C
Feedwater	Based on actual inlet temperature	Assumes water starts at 100°C
Purpose	Assess real-world plant performance	Compare boiler capacities on a common basis

The key difference:

Actual steam capacity reflects the boiler's real operating conditions, while From & At 100°C provides a standardized equivalent capacity.

4. Why Is the Actual Steam Output Lower?

This is where the difference becomes easier to understand.

In real operation, a boiler has to perform two energy-intensive steps:

Step 1 — Heat the feedwater

The boiler first heats the incoming water from its actual feedwater temperature to the boiling/saturation temperature.

This requires sensible heat.

Cold feedwater → Heated water → Boiling point

Step 2 — Convert water into steam

Once the water reaches its boiling point, additional energy is required to convert the water into steam.

This is the latent heat of vaporization.

Boiling water → Steam

So, under actual operating conditions, part of the energy supplied by the boiler is used to heat the water before steam is produced.

As a result, the actual steam output can be lower than the equivalent From & At 100°C capacity.

5. Why Does From & At 100°C Show a Higher Capacity?

The “From & At 100°C” basis starts with a different assumption.

Instead of starting with cold feedwater, it assumes the water is already at 100°C.

Therefore, the calculation only considers the energy required to convert that water into steam at the standardized reference condition.

This produces a higher equivalent capacity figure because the energy required to heat the water from its actual inlet temperature to 100°C is not included in the same way.

Simply put:

Actual operation

Cold feedwater

↓

Heat water

↓

Reach saturation temperature

↓

Produce steam

From & At 100°C

Water already at 100°C

↓

Produce steam

That's why the two capacity figures should not be treated as identical numbers.

6. How Is Steam Capacity Converted?

Engineers use the Factor of Evaporation (FE) to convert actual steam capacity into its equivalent From & At 100°C capacity.

Formula

Steam Capacity (From & At 100°C) = Actual Steam Capacity × Factor of Evaporation (FE)

The Factor of Evaporation is calculated as:

FE = (hᵍ − hᶠ) / 2257

Where:

hᵍ = Enthalpy of saturated steam at the actual operating pressure (kJ/kg)

hᶠ = Enthalpy of feedwater at the actual inlet temperature (kJ/kg)

2257 = Latent heat of vaporization of water at 100°C (kJ/kg)

This relationship is consistent with standard equivalent-evaporation calculations, where 2257 kJ/kg represents the reference latent heat at 100°C.

7. Why Does the Factor of Evaporation Matter?

The Factor of Evaporation allows engineers to compare steam production on a common reference basis even when boilers operate under different conditions.

For example, two boilers may operate at different pressures or receive feedwater at different temperatures.

Simply comparing their actual kg/hr steam output may not provide a like-for-like comparison of the energy involved.

The Factor of Evaporation accounts for the difference in energy required under those operating conditions.

This is particularly useful when:

Comparing boiler specifications
Reviewing boiler performance
Evaluating steam requirements
Assessing boiler capacity
Comparing different boiler models
Reviewing existing plant performance

8. Which Steam Capacity Should You Use?

It depends on what you are trying to determine.

If you are comparing boiler specifications:

Use the From & At 100°C capacity.

It provides a standardized basis for comparison.

If you are evaluating your existing plant:

Look at the actual steam capacity under your operating conditions.

This gives a more practical picture of what the boiler is producing in the plant.

Remember:

From & At 100°C = standardized comparison

Actual Steam Capacity = real operating conditions

9. Why This Matters When Selecting a Boiler

Understanding the difference between these two capacity figures can help prevent misunderstandings when reviewing boiler specifications.

A boiler catalogue may state a capacity based on From & At 100°C, while the actual plant operates at a specific pressure and feedwater temperature.

If these two figures are compared directly without considering their different bases, the expected steam production can be misunderstood.

For a proper boiler assessment, engineers should consider the actual operating pressure, feedwater temperature and required steam conditions rather than looking at the capacity number alone.

10. A Simple Way to Remember It

ACTUAL STEAM CAPACITY

What is the boiler producing under actual operating conditions?

⬇️

Based on actual:

Pressure + Temperature + Feedwater Conditions

FROM & AT 100°C

What would that steam production be on a standardized reference basis?

⬇️

Based on:

100°C reference condition

The Key Takeaway

The catalogue number and the actual steam output are not necessarily the same thing.

From & At 100°C capacity is a standardized reference used to compare boiler capacities.

Actual steam capacity reflects the boiler's real operating conditions.

Understanding the difference helps engineers and plant operators make a more accurate assessment of boiler capacity, steam requirements and expected plant performance.

Frequently Asked Questions

What is actual steam capacity?

Actual steam capacity is the amount of steam a boiler produces under its specific operating conditions, including its operating pressure and feedwater conditions.

What does From & At 100°C mean?

From & At 100°C is a standardized basis for expressing steam capacity. It provides a common reference for comparing boilers operating under different conditions.

Why is actual steam capacity lower than From & At 100°C capacity?

Under actual conditions, the boiler must first use energy to heat the feedwater to its boiling point before converting it into steam. The From & At 100°C basis assumes the water is already at 100°C.

What is the Factor of Evaporation?

The Factor of Evaporation (FE) is used to convert actual steam generation into an equivalent steam capacity on a From & At 100°C basis.

Why is From & At 100°C used for boiler comparison?

It provides a common reference condition, making it easier to compare steam capacities between boilers operating at different pressures and feedwater temperatures.

Can Mechmar help determine the actual steam capacity required for my plant?

Yes. Boiler capacity requirements depend on the application's operating conditions and steam demand. Contact Mechmar's boiler specialists to discuss your steam requirements and application.

Need Help With Your Boiler Capacity?

Choosing the right boiler capacity is more than simply looking at the number in a catalogue.

Your operating pressure, feedwater temperature, steam demand and plant conditions all need to be considered.

Talk to Mechmar's boiler specialists to discuss your application and steam requirements.

Call / WhatsApp: +60 18-281 1234

Email: meg@mechmar.com.my

[ENQUIRE ABOUT YOUR BOILER REQUIREMENTS]

Articles

Actual Steam Capacity vs From & At 100°C: What's the Difference?

1. What Is Actual Steam Capacity?

2. What Is Steam Capacity From & At 100°C?

3. Actual Steam Capacity vs From & At 100°C

4. Why Is the Actual Steam Output Lower?

Step 1 — Heat the feedwater

Step 2 — Convert water into steam

5. Why Does From & At 100°C Show a Higher Capacity?

6. How Is Steam Capacity Converted?

7. Why Does the Factor of Evaporation Matter?

8. Which Steam Capacity Should You Use?

9. Why This Matters When Selecting a Boiler

10. A Simple Way to Remember It

The Key Takeaway

The catalogue number and the actual steam output are not necessarily the same thing.

Frequently Asked Questions

Need Help With Your Boiler Capacity?

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