Boiler Blog | Nationwide Boiler Inc.

Last month on the Boiler Blog, we focused on increased efficiency through the use of O2 trim. This is an easy, cost-effective addition to a boiler system with multiple added benefits. There are, however, additional ways to increase the efficiency of your steam plant even further. A Variable Frequency Drive (VFD) controls a motor’s speed by varying the frequency supplied to it, and VFD’s can help achieve significant electrical power savings when added to your boiler.

To illustrate the benefits of VFDs, take the power usage of the fan. A combustion air fan on a boiler typically uses a large amount of energy. For example, a 125,000 pph boiler can have a fan motor as large as 300 hp. While the actual power usage would typically be less than the rated size of the motor, when operating 24/7/365 at full load and assuming an electricity cost of 8 cents/KW, the cost of electricity can be upwards of $150,000 - just for the combustion air fan! 

The use of VFDs will provide the most savings for boilers that have an average annual operational load of less than 100%. In fact, if your average boiler load throughout the year is 50%, or half load, you could save ⅞ ^th the fan power. This means that with the use of a VFD, the fan would require a fraction of the typical amount of energy used when running your boiler at full load. Generally speaking, if your boiler is operating at half load the fan will also operate at half speed.  According to the fan laws, fan power is related to change in fan speed to the 3^rd power.  When operating the fan at half speed, the change in power is (½)³ or ¹/₈^th the power!  This is where the power savings would come from and why it would be most beneficial to utilize a VFD for scenarios where the boiler system operates more consistently at half load.

Let’s look further into the reason behind using 50% fan speed for 50% boiler load. When running your boiler at half load, the air flow requirement will also be reduced by half (assuming the burner excess air stays the same).  Since the fan laws state that air flow changes linearly with fan speed, that means that at 50% fan speed (or RPM), the flow would be 50% of full load.  For the static pressure requirement, the fan pressure is closely related to the square of the change in boiler load.  So, at 50% load, the static pressure change would be (½)² which also matches the fan laws which state change in fan speed changes fan static by the square.  You’ll notice that if you multiply the flow and static changes together (i.e. ½ * (½)²) you get ⅛ ^th which is the same number for the power savings.

If a VFD is not being used, the alternate device is likely a line motor starter. With a motor starter, the fan is always running at full speed. At 50% load, the air flow is about half but the static pressure requirement typically increases due to the closing of the air dampers (which are used instead of a VFD to control the flow).  That said, with a standard motor started, the overall fan power requirement stays about the same regardless of whether the boiler is operating at half or full load.

Stay tuned for our next Boiler Blog for additional educational topics, Nationwide Boiler news, and more!

Given today’s awareness to the advantages of minimizing energy usage and carbon footprint, boiler operators and plant managers are always on the lookout for ways to improve boiler efficiency and ensure emissions compliance. With improved efficiency, fuel usage is minimized which in turn reduces the carbon footprint (i.e. reduces CO2 emissions from the boiler) and reduces issues around emissions compliance. One way to increase boiler efficiency is to use oxygen trimming, or O2 Trim, at the stack.

A typical burner will operate from 3 to 4% O2 at 50% boiler load and higher. This stack O2 concentration corresponds to the amount of excess air at the burner, and excess air is required for burner operation to assure complete combustion of the fuel.  For example, for natural gas firing, 3% O2 corresponds to 15% excess air. During commissioning, the burner service engineer will set the fuel / air ratio so that there is always excess air over the firing range of the burner.  The service engineer must also keep in mind that ambient conditions (mainly air temperature changes) will affect air density which will affect the burner fan air flow output.  On cold days the fan will flow more air due to a higher air density, and on hotter days the flow will be less. Varying air flow conditions can adversely affect burner operation.

Boiler efficiency is affected by the excess air concentration in the flue gas. The rule of thumb is that for every 5% more excess air, boiler efficiency decreases by 0.5%. If not adjusted, the boiler stack can vary by at least 2% O2 (i.e., if normal operation is 3% O2, it can increase to 5% O2 on a cold day). That corresponds to about 1% boiler efficiency loss. Saving 1% efficiency over a year operation can save big on fuel costs. If the normal fuel bill is $10,000,000 per year, you would save $100,000. Adding an O2 Trim system would cost a fraction of that amount (assuming a 150,000 lb/hr steam boiler or smaller), providing a quick and worthwhile ROI. So, what exactly is an O2 Trim System?

Many burners use a control system where the fan air flow does not vary based on air temperature.  As explained above, the air flow can vary based on ambient conditions causing the stack O2 to vary; this can be solved by adding O2 Trim to the control system. O2 Trim is an air flow trimming system where stack O2 is measured (using an O2 probe) and the air flow is adjusted (trimmed) based on the reading. It’s a closed loop control system since changes in air flow will directly affect the stack O2 reading (assuming fuel flow is the same). By maintaining a consistent air flow rate, O2 trim reduces fuel usage in turn increasing boiler efficiency.

In addition to increased boiler efficiency, utilizing O2 Trim will ensure stable and safe O2 levels. On hot days with reduced fan air flow, the stack O2 level can drop to dangerously low levels and boiler emissions can go out of compliance. With O2 monitoring, alarms can be created to alert the boiler operator to either reduce fuel flow or increase air flow, to return to safe operating conditions.  

O2 Trim isn’t ideal for every boiler, though. Due to the residence time in the boiler and ducting, it takes time for the changes in burner fan flow to reach the stack. This causes a time delay with an O2 Trim system, which works well for boilers with slow load changes. However, for systems with rapid boiler load changes, the O2 Trim system typically can’t keep up easily and it is often “hunting” for the optimum air flow.

If you are interested in learning more about whether an O2 Trim System will benefit your operations, reach out to one of our qualified parts specialists or call 800-227-1966. Check out other articles on Nationwide’s Boiler Blog for more tips and tricks for improved boiler efficiency, routine maintenance, and more!