When upgrading or selecting a new boiler, it’s important to consider how burner and boiler turndown will impact the system’s overall efficiency. Boiler capacity and the range of load demand variation for the process are key factors to choosing the right system for the application.
What is Boiler Turndown?
Boiler turndown is the ratio between a boiler’s maximum and minimum output. Depending on the burner’s design, it may have a turndown ratio between 4:1 and 10:1 or even higher. A 4:1 turndown means the boiler’s minimum operating load is 25% of the boiler’s full capacity (100% capacity divided by 4). A 10:1 turndown means the minimum operating load is 10% of the full load capacity (100% capacity divided by 10).
Why is Boiler Turndown Ratio Important?
Turndown ratio is important for boilers that are required to operate at a wide variation of capacities. A boiler with a higher turndown ratio will usually handle fluctuating loads more efficiently than one with a low turndown. Here’s why:
A boiler’s burner will modulate or “turn down” as the demand for hot water or steam decreases, in an attempt to meet only the required load. The turndown ratio tells you the minimum output the boiler can handle before turning off and then cycling on and off frequently.
If the demand goes below the burner’s minimum turndown, the boiler will cycle off until there’s more demand. This results in the system cycling on, running for a brief period and then cycling off again. Every time this happens, the boiler must go through a pre-purge, a firing interval and then a post-purge.
For example, a 500-horsepower boiler with 10:1 turndown will modulate down to 50 horsepower before turning off and then cycling on and off. At a 4:1 turndown, the same capacity boiler would only modulate down to 125 horsepower. If the application has a minimum load of 50 horsepower, the 10:1 boiler could accommodate that without shutting off, but the 4:1 boiler would have to use multiple short cycles to maintain the load.
How Turndown Ratio Affects Boiler Efficiency
For applications with a low load demand, a low turndown ratio will generally result in lower boiler efficiency and frequent on-off cycling. When the system has to cycle on and off frequently, it can also cause a number of costly issues:
Energy Loss and Waste
Every time a boiler starts and stops, it has to perform a “purge” to clear the fire box of any combustible gases. Fresh air is blown through the system and up out of the stack. This cold air picks up any residual heat in the boiler and carries it outside through the stack. Not only is that heat energy wasted, but the process makes boilers less efficient at the beginning of a cycle. And, when the boiler starts again, it has to make up the lost energy plus any additional amount to meet the demand. Even if a boiler only cycles three or four times per hour, the energy loss adds up over the course of days and weeks of operation.
Inconsistent Steam Pressure
This can be a critical issue for process applications. If the boiler and burner continually short-cycle, the steam pressure will rise and fall. There is a required starting sequence that has to run for safety before the burner can fire, so there is a delay in response time when demand goes up. In contrast, if the system stays on even during periods of low load needs, it can maintain constant steam pressure and respond quickly to meet higher demands.
Excessive Wear on Components
When a boiler and burner cycle on-off frequently, components like the combustion fan motors and the control valves will wear out prematurely. Not only does this lead to additional repair and maintenance costs, if the condition deteriorates far enough it can shut down the entire boiler system. If no redundant boiler system is in place, this could leave a building without heat or even halt production.
Premature Equipment Failure
Over time, frequent cycling on and off can shorten the lifespan of boiler equipment. The refractory, steel, stainless steel and other components in the areas of the boiler subjected to the highest temperatures are designed to handle extreme heat for long periods of time. However, drastic temperature changes over short periods of time can cause thermal shock. Going from cold pre-purge air to 2,500°F flame temperature within a few seconds creates excessive stress and will cause the parts to fail sooner than expected.
Selecting the Right Turndown Ratio
Correct boiler sizing and turndown ratio go hand in hand. In many cases, facilities may opt for a single over-sized boiler to accommodate the maximum load for the application and any expansion in the future. However, if load demand fluctuates widely, a high-capacity system may not be able to handle the minimum load without frequent cycling. On the other hand, it wouldn’t make sense to choose a boiler with a high turndown if it’s not sized to meet the highest load demand.
At Superior Boiler, we recommend considering both turndown and capacity when selecting a new boiler and burner. A modular or hybrid system can be a good solution for meeting varying load demands. Using a mix of different types and sizes of boilers allows for a larger boiler to handle high load needs while a smaller, more efficient boiler covers lower load demands. And, each boiler can be turned down when not in use.
Contact your local sales rep to discuss the optimal boiler system for your application.