Boilers cycle on when there is a demand for steam and off when that steam demand is met. When a boiler is able to quickly satisfy the demand for process or space heating, this can create a problem known as short cycling. The boiler must cycle on and off very quickly which results in cycling losses. In this article we cover the main causes of boiler short cycling and how you can avoid it.
Losses From Boiler Short Cycling
A typical boiler goes through a lengthy start up and shut down sequence that includes:
- Call For Heat.
- Safety Check.
- Pre-Purge Or Blower Start Up.
- Modulate Combustion Air Damper From Low To High and High To Low.
- Low Fire Start Switch Mode.
- Pilot Ignition.
- Prove Pilot Using Flame Sensor (usually around 10 seconds.)
- Ignition Drops Out.
- Main Fuel Valve Energized.
- Main Flame Proven.
- Release to Modulate.
- Demand or Call For Heat Satisfied.
- Post Purge Or Blower Start Up (Usually 60 seconds.)
Doing these steps in rapid succession hurts the overall efficiency of a boiler. This is because fixed losses are amplified when there is a light load on the boiler. When a boiler is short cycling, you can generally see upwards of 15 percentage points less than the lowest efficiency achieved at low fire. That can take an 85% efficient boiler and drop it to 70% efficient which will spike operation costs.
Consider this example from the U.S. Department of Energy: “if the radiation loss from the boiler enclosure is 1% of the total heat input at full-load, at half-load the losses increase to 2%, while at one-quarter load the loss is 4%. In addition to radiation losses, pre and post-purge losses occur. In the pre-purge, the fan operates to force air through the boiler to flush out any combustible gas mixture that may have accumulated. The post-purge performs a similar function. During purging, heat is removed from the boiler as the purged air is heated.”
Additional Problems Caused By Short Cycling
Short cycling also causes issues with mechanical components. Rapid heating and cooling can cause corrosion issues on burner components (also known as cyclic stress). It also causes gas valves to experience significantly more wear and tear from frequent on and off operation.
Short cycling can also cause problems with water chemistry. Hot water and chemical additives need time to disperse throughout the system. If this isn’t achieved before another heating cycle begins; then those chemicals can remain in one area of the boiler. This can create conditions which result in corrosion of the boiler’s internal components.
What are the Causes Of Short Cycling?
1. Oversized Boiler
The most common cause of boiler short cycling is an oversized boiler. This commonly occurs because a boiler is sized for a planned plant expansion that never happens.
2. Energy Recovery Measures
Energy recovery systems have made the boiler more efficient, resulting in less steam being needed to meet the demand.
3. Process Changes
Process that was using steam may have changed, dropping demand. In any case, this causes the boiler to quickly cycle on and off.
4. Incorrectly Configured Set Points
Short cycling can also occur when control limit set points are incorrectly configured. This may look like forcing the temperature or pressure to stay within a few degrees or PSI.
5. Incorrect Heat Load
When we use boilers to heat a space, sometimes thermal assumptions are incorrect. This often includes neglecting to factor in body heat, lighting heat and equipment heat into the boiler size calculation. This can result in an oversized boiler that quickly meets demand.
6. Multiple Boilers Operating At Low Fire
Short cycling is also an issue when you have multiple boilers operating at low fire to meet steam demand.
How To Prevent Boiler Short Cycling
Each situation is different and will require a different solution but below are a few options.
1. Install A High Turndown Burner
To start, you can consider installing a burner that has a high turndown ratio. This can lower the burners low-fire rate. This will allow it to run at a lower load without short cycling.
2. Install A Smaller Boiler
This allows the smaller boiler to meet general loads and the larger boiler to kick in when demand increases. With a multiple boiler set up, you can manage load swings more easily. This reduces the risks of short cycling.
3. Linkage Less System
According to an article written by ABMA, a linkage-less system “eliminates the problem of hysteresis. This happens when the system is unable to repeat a set performance. It also often occurs on a linkage system after combustion is set on low-fire. When the jackshaft drives to high fire and comes back down, the O2 levels can be off by 2% or more. This causes technicians to set the O2 levels too high throughout the firing range. This results in too much air in the combustion and high flue gas losses. Using a linkage-less burner control system, O2 levels will stay below 3% throughout the firing range. This can achieve fuel savings of 12-15% over a linkage burner operating at an average 6% O2 level.”
A Real World Example
The Department of Energy gives a good example:
A 1,500 horsepower (hp) (1 hp = 33,475 Btu/hr) boiler with a cycle efficiency of 72.7% (E1 ) is replaced with a 600 hp boiler with a cycle efficiency of 78.8% (E2 ).
Calculate the annual cost savings.
Fractional Fuel Savings
= (1 – E1 /E2 )
= (1 – 72.7/78.8) x 100
If the original boiler used 200,000 MMBtu of fuel annually, the savings from switching to the smaller boiler (given a fuel cost of $8.00/MMBtu) are:
= 200,000 MMBtu x 0.077 x $8.00/MMBtu