15 Ways to Increase Boiler Efficiency

From Rasmussen Mechanical Services

One of the easiest ways for a business to reduce operating expenses is to increase boiler efficiency. A great place for a business to start lowering their bills is to look at recently performed boiler maintenance and discover how efficient your boiler is.

Before we get to our efficiency tips, we need to understand boiler efficiency. The majority of heat lost in a boiler is in the stack or boiler water. The goal is to create conditions that generate the smallest possible amount of flue gas at the lowest possible temperature. This results in increased boiler efficiency.

Think about it; the boiler draws in cool air, heats it up, and sends it out the stack. A lower stack gas temperature is ideal because the higher the temperature, the more energy leaves with the flue gas. On the other hand, the boiler system takes in cold water, heats it to steam, and uses the heat. Anywhere we are losing heat, steam, condensate or hot water, we are losing valuable BTU’s.

Here are 15 easy tips to help make a boiler system more efficient and save your customers money on their monthly bills:

1. Lower the Stack Temperature

Lowering stack temperatures may be as simple as a day/night set back. This lowers the operating pressure for steam boilers and the operating temperature for hydronic boilers when idling at night or mild days.

2. Install an Economizer

An economizer uses the wasted hot flue gas to heat feed water on its way to the boiler. If the steam boiler doesn’t have an economizer or the economizer isn’t working, this should be the first priority. Economizers save fuel and prevent damaging effects of feeding the boiler with cold water. For serious savings, see if a Heatmizer® is right for the boiler or domestic hot water application.

3. Tune the Burner Regularly

Another common issue occurs when it comes to boiler efficiency is not getting the right amount of air. For proper combustion of fuel inside the boiler, a certain amount of oxygen is required. If too little air is present, the carbon in the fuel will be oxidized, making carbon monoxide. This causes less heat to release because the fuel isn’t completely burned, which lowers fuel use efficiency. Low air generates soot, smoke and carbon monoxide, all of which are dangerous. Too much air also reduces efficiency. The extra air comes in cold and sent out the stack hot, wasting heat.

An optimal process provides just enough air for the fuel to burn safely. To achieve this, measure the amount of air needed with an O2 probe. Insert the probe into the stack while tuning the burner for optimum efficiency. However, in some facilities, the temperature of the air drawn into the burner varies with the seasons. This requires the tuning of the burner more frequently for maximum savings.

4. Install a Variable Frequency Drive

Not many burner fans or pumps exist out there today without VFD’s. However, if you haven’t heard of VFD’s or have a system that doesn’t utilize them, take note. The unbelievable power savings come from a concept known as the affinity laws for pumps and fans. If your system has a fan or circulation pump controlled by a damper or valve, your system is wasting electricity at partial loads. A VFD lets your system control flow with fan or pump speed instead, and that’s where the magic happens.

5. Insulate Your Valves

Many plants remove the insulation on valves in the boiler room for maintenance and never put it back because it is a hassle. However, exposing these large valves to the air causes a lot of heat loss and can make the boiler room unbearably HOT. Insulating these valves (with, for example, a Heatmizer® Removable Blanket) can save significantly, improve boiler room comfort. The blankets also reduce the risk of burns while still allowing easy access for maintenance.

6. Clean the Fireside

Over time, soot may build up on the fireside of the boiler tubes, especially with older equipment. This layer of soot acts as an insulator, bringing down the heat transfer rate and increasing fuel use. Because of the lower heat transfer rate, the hot gases pass through without transferring the heat to the water, increasing your stack temperature. Cleaning and inspecting the boiler tubes as part of regular boiler maintenance ensures that the soot remains minimal. This does improve the overall boiler efficiency.

7. Preheat Combustion Air

The burner has to heat incoming combustion air with the flame. If the air introduced to the burner is warmer, it requires less fuel to make the same amount of steam in the boiler. A modest 40 °F increase in fresh air temperature can save 1% of the fuel bill. If your client runs large boilers around the clock, this can really add up even with our historically low gas prices. In some cases, an air preheater can pay for itself in under a year.

8. Clean the Water Side

Keeping the waterside of your boiler clean and free of leaks requires diligent water treatment. Inspect the boiler's waterside regularly. Clean out any mud legs or mud drums to ensure good heat transfer from the metal to the water. Scale will accumulate on heat transfer surfaces because of high water hardness, improper chemicals, and not blowing down the boiler regularly. This scale will impede the heat transfer, reducing boiler efficiency. The scale will also keep the water from cooling these heat transfer surfaces. If left untreated, the scale can cause the boiler to overheat, leading to costly boiler repairs and leaks.

9. Return Condensate to the Boiler

Condensate forms as the steam transfers its heat and condenses. It is irresponsible to waste this by product. The clean water is without dissolved solids or gasses that are ready for use again in your boiler. The water is already hot and therefore requires significantly less fuel to make it into steam again. Reusing the condensate also reduces how much cold makeup water, chemicals, and treatment is required for your boiler. Lastly rerouting condensate back into the feed water system can reduce wastewater treatment and sewer costs.

For an even larger boiler efficiency increase, consider a high-pressure condensate return system on your largest steam users. This keeps your condensate at a higher pressure. The condensate does not flash, so you return more water at a significantly higher temperature directly to the boiler.

10. Recover Heat from Boiler Blowdown

Much like the return of condensate to the boiler, recovering the heat from the boiler blowdown can increase boiler efficiency. The blowdown valve is used to remove boiler water which contains soluble and insoluble solids. It helps reduce the level of dissolved solids in the boiler water to prevent the boiler scale. Unfortunately, when it removes hot water, it also wastes energy. Installing a blowdown heat exchanger, flash tank or combination of the two can help recover some of this energy for your boiler system. Using heat recovery to cool down the blowdown and heat up your make-up water will improve energy efficiency.

11. Control Blowdown Rate

Blowdown removes impurities, like water hardness, from the boiler and is required to keep the boiler surfaces clean. However, blowdown also removes heat from the system. Water enters the system cold, is heated up to the boiler temperature, and leaves through the blowdown. Some boiler systems have continuous blowdown that does not change with boiler load. To control the heat sent down the drain, blowdown should be limited only to the amount necessary to control the dissolved solids. For serious savings, control dissolved solids with an automatic blowdown valve. If you blow down the boiler when it regularly, you can save a lot of energy. This also reduces risking damage to your boiler from the scale.

12. Reduce Excess Air

Boilers require excess air in order to complete combustion. Although necessary, the amount of excess air can result in totally different efficiencies for your boiler. Too little excess air and the boiler will build up soot and dangerous carbon monoxide, while too much excess air reduces efficiency. Fortunately, there are automatic combustion control systems that can intelligently monitor required air quantities for your combustion systems. As discussed above, tuning can keep your burner running at its best, but it is limited to the best your old burner can offer.

Upgrading to a high-efficiency burner will save a significant amount of fuel and pay for itself many times over. If your burner is more than 15 years old, uses linkages, or runs above 3% O2, consider a burner retrofit. In most cases, a burner upgrade can save over 20% on the gas bill!

13. Reduce Carry-over

Carry-over is boiler water that leaves the boiler in the steam but is still water. It carries impurities such as dissolved solids with it. These impurities leave deposits around the steam system. They get caught inside intricate devices like control valves and pressure regulators. This causes a lot of damage and increased maintenance.

As for efficiency, this moisture reduces the BTU content of the steam at the end-use. Essentially, this is more water that was heated in the boiler but did not give off useful heat before going to the condensate system. Carry-over happens because of a number of things. The solution depends on the cause. Suspect operating practices such as bringing loads on fast, high TDS, or poor separation equipment as a culprit.

14. Survey Steam Traps

Stuck, worn out, or just broken steam traps can stick open, allowing valuable steam to blow right through into the condensate system. If you want to ensure you are running at peak efficiency, survey your steam traps regularly and replace broken or sticking traps.

15. Reduce Steam Usage

The best way to save on fuel and electricity to your boiler is to reduce steam usage in your processes. Insulating piping and tanks heated by steam, for example, can significantly reduce steam usage, and therefore, fuel usage. High-pressure condensate systems can reduce steam usage at the deaerator AND fuel usage in the boiler.

Efficiency Formula

In an article by Forbes Marshall, they discuss how to find the direct efficiency of a boiler. The formula is:

η=(Energy output)/(Energy input) X 100

In order to calculate boiler efficiency, we divide the total energy output by total energy input, multiplied by hundred.

This formula breaks down to:

E= [Q (H-h)/q*GCV]*100

Where,

Q= Quantity of steam generated (kg/hr)

H= Enthalpy of steam (Kcal/kg)

h= Enthalpy of water (kcal/kg)

GCV= Gross calorific value of the fuel.

Rassmusen Mechanical is a full-service mechanical contractor with locations in Iowa, Nebraska, South Dakota, Colorado.