Difference Between Fire Tube and Water Tube Boiler

Oscar Garner

By Oscar Garner

Last Updated on June, 2024

Both firetube and watertube boilers are heavyweights in the industrial market, but which one should you choose? 

There are some key differences to consider when choosing one or the other. In this article, we’ll examine both boiler types in detail, laying out their specs, applications, and pros and cons to help you make the right choice. 

You should always consult a boiler expert or professional architect before purchasing, but this article will be a good starting point for expanding your knowledge of the two boiler types. Let’s begin! 

Quick Summary

  • There are significant differences between firetube and watertube boilers in terms of their specs, applications, and pros and cons.
  • Firetube boilers work by passing hot flue gases through tubes surrounded by water, while watertube boilers use tubes to carry water and have hot gas fired externally.
  • Firetube boilers have advantages such as easy installation, low maintenance cost, and simple operation, while watertube boilers have benefits including higher efficiency, faster heating/response time, and a longer lifespan. The choice between the two will ultimately depend on the specific needs and priorities of the facility or industry.

Firetube Boilers Vs. Watertube Boilers: What’s the Difference?

a comparison picture of watertube boiler vs firetube boiler

The main difference between these two condensing boilers is how they produce steam. 

In firetube boilers, hot flue gases pass through tubes surrounded by water. In watertube boilers, the tubes carry water, while hot gas is fired externally.

Another difference is that firetube boilers are heavier, as they require a larger amount of water to function. Watertube boilers need a significantly smaller amount of water to function and are lighter as a result. 

How Does a Firetube Boiler Work? 

The hot gases produced during combustion travel internally in firetube condensing boilers. The gases are syphoned into tubes and carried through the boiler’s water tank. The resulting heat transfer converts the water outside the pipes into steam. 

How Does a Watertube Boiler Work?

A water tube boiler uses an inverse mechanism compared to its fire tube counterparts. In a water tube design, feed pumps supply water to the tubes while the combustion produces hot flue gases on the outside.

Heat transfer happens when the hot gas boils the water as it passes through the furnace, creating steam. 

Pros and Cons of Firetube Boilers

a picture of a firetube boiler

There are many reasons to select a firetube boiler, mainly due to its simplicity and ease of operation. Let’s look at the advantages. 

Pros

  • Easy Installation: A fire tube boiler has a simple construction that consists of a steel drum to store water, a shell configured with metal tubes, and a firebox that converts fuel into hot gas. As long as you have adequate space, good grounding for electricity, and a reliable fuel supply, installation should be straightforward. 
  • Easy And Cheap Maintenance: Tube cleaning and most other repairs can be done from outside the boiler unit instead of inside. Hence, the maintenance cost for firetube boilers is typically low. 
  • Simple Operation: These boilers do not have a steep learning curve, so operators will only require a little skill or training to become familiar with the control system.
  • Low Initial Price: Besides being easier to maintain, firetube boilers are less expensive. You can get them for half the cost of a watertube boiler.
  • Smaller Overall Footprint Relative To Capacity: A low-capacity fire tube boiler has a smaller footprint than a low-capacity water tube boiler. However, this only applies at the time of installation, as the capacity will have to be adjusted to allow for more water. 

Cons 

Despite the positives, there are also many areas where firetube boilers fall short of their water tube counterparts. 

  • Reduced Efficiency: Fire tube boilers are less efficient than water tube boilers. The fire tube design requires a full water tank to produce heat, and the increased water content gives these boilers a lower heat transfer coefficient. In other words, they use more fuel during the condensing process than water tube boilers. 
  • Takes More Time To Produce Steam: Since a larger volume of water surrounds the gas flow, firetube boilers may take over an hour to convert water into steam.
  • Reduced Capacity: These condensing boilers have smaller steam and hot water capacities. They typically have steam capacities of up to 25 tonnes/hr and hot water capacities between 2 million and 100 million BTU/hr. This also means they cannot tolerate pressure levels above 250 psig.
  • More Vulnerable To Tube Damage:  Unlike water tubes, these tubes come into direct contact with the hot gas flow from the combustion. This can alter their construction and cause the tubes to burst, resulting in leaks or system malfunctions. Flow rate and water temperature changes can also damage the tubes and heat exchanger. 
  • Delayed Response To Load Changes: Because a firetube boiler heats more water, it responds slower to load changes. As such, it is not equipped to handle increased steam demands. 

Pros and Cons of Watertube Boilers

While both boiler designs have been used for centuries, watertube boilers have recently gained popularity due to their emphasis on performance and efficiency. Let’s look at the advantages.

a picture of a watertube boiler

Pros

  • More Efficient: Water tube boilers are more efficient than fire tube boilers. Because water flows through channels, water tube boilers do not need a large volume of it to function. They require 90% less water than their fire tube counterparts. As a result, they have a higher heat transfer coefficient. 
  • Faster Heating And Response Time: Because the water flow is surrounded by hot gases, watertube boilers can produce steam in about 5 minutes. They are also better at adapting to shifting loads and can quickly generate more steam if needed. 
  • Increased Capacity: A water tube boiler can have a steam capacity of up to 120 tonnes/hr. This gives them a pressure limit of up to 5,000 psig and the ability to handle very high temperatures. 
  • Smaller Footprint For Higher Capacities: Low-capacity firetube boilers may have a smaller footprint than low-capacity watertube boilers, but the latter still has an advantage regarding high capacities. This is because firetube boilers need larger tanks to accommodate more water. On the other hand, you can improve the surface area in a watertube boiler by configuring more tubes. 
  • Boiler Safety: A watertube boiler has better safety measures than a fire tube design. It uses less water, which protects it from flow rate fluctuations and temperature changes. Water travels internally, and the tubes are made of more robust materials, greatly reducing the risk of leaks or explosions. 
  • Longer Lifespan: A water tube condensing boiler can tolerate high pressures, adapt to flow rate changes, and better manage temperature spikes to protect its heat exchanger. With proper maintenance, these boilers can last 30 years or more, while fire tube boilers last between 20 and 25 years. 

Cons 

While water tube boilers have many advantages, they are not without their drawbacks. 

  • Harder To Install: Water tube boilers are not as simple as fire tube designs, making installation and assembly more difficult and time-consuming. 
  • Complicated Operation: This type of condensing boiler also has a more complex control system. Operating it will require a certain level of skill and training.
  • High Initial Price: When factoring in installation, a water tube boiler can cost twice as much as a fire tube boiler. 
  • High Maintenance Cost: Unlike fire tube boilers, most water tube boiler repairs must be performed inside the unit. This makes maintenance more difficult and drives up the cost. 

What is the Right Boiler for You? 

There are some essential factors to consider when choosing between a fire tube and a water tube design. Your choice will ultimately depend on your facility’s demands and priorities. 

Because of their limited performance, fire tube boilers are better suited to providing heat to commercial buildings or powering small industrial processes. They typically work best for low-pressure applications that do not demand large amounts of steam to be generated quickly.

They also have a larger footprint and are not ideal for facilities with limited space. 

On the other hand, water tube boilers are well-equipped to handle high pressures and can quickly create large amounts of steam. This makes them suitable for more demanding applications such as power plants and steam turbines. 

In addition to having a smaller footprint, water tube boilers are also more efficient with their resources. Due to their design, these condensing boilers need less water to function, allowing them to conserve more fuel. 

So, if going green is a top priority for your facility, a water tube boiler is the best option. 

However, if your primary concern is reducing costs, a firetube boiler is the more attractive choice. 

Final Thoughts 

Fire tube and water tube boilers have distinct applications; understanding them is critical to making an informed decision. 

Fire tube boilers are still extremely popular among smaller industries despite their shortcomings. They are more affordable, easier to maintain, and dependable for low-pressure tasks. 

That said, if you work in an industry that requires higher performance standards, you should consider investing in a water tube boiler because of its design, efficiency, and energy-saving capabilities.

Before making a decision, ensure that you do proper research and get the opinion of an architect or boiler expert to give you the best judgement going forward.

Good luck!

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