1. Steam Generation Efficiency
Efficiency Definition
Radiation and Convection losses – Shell losses
Blowdown losses
Stack losses

2. Steam Generation Efficiency
Efficiency Definition
Radiation and Convection losses – Shell losses
Blowdown losses
Stack losses
The boiler discussion is introduced with an example. This example serves as the foundation of an example system used throughout the course. The initial investigation is to point out the economic importance of proper boiler management.
The example boiler is a moderate (medium) size boiler. The operating conditions are not extreme. The example is intended to be “in-the-range” of understanding of all participants. A hospital facility with a 10,000 lbm/hr, 100 psig boiler can identify with this larger boiler. Alternately, a facility with a 400,000 lbm/hr, 1,500 psig boiler can relate to this example boiler.

3. Classic Boiler Efficiency
Steam generating unit efficiency is defined as the heat absorbed by the steam divided by the energy input with the fuel
Boiler efficiency is identified for a boiler operating under Steady-State, Steady-Flow (SSSF) conditions. In other words, the temperatures, pressures, flow rates, and other properties of the boiler are not changing with respect to time. The boiler is assumed to not change in total energy content. The amount of liquid water contained within the boiler is assumed to not change during the period of investigation.
Steam (and feedwater) “energy content” is described by the thermo-physical property enthalpy (h). This thermo-physical property is the most appropriate description of energy content for a system with mass flowing in and out.
The fuel energy content is described by the Higher Heating Value (HHV) of the fuel. Higher Heating Value is used as the convention for this course.

4. Operating Cost Determine the operating cost
Boiler fired with natural gas which has a higher heating value of 23,311 Btu/lbm
HHV is 1,000 Btu/sft³
Steam conditions: 400 psig, 700°F
Output: 100,000 lbm/hr (steady)
Rating: 120,000 lbm/hr (maximum continuous)
Feedwater: 600 psig, 242°F
Fuel supply: 149,000 sft³/hr (2,480 sft³/min)
Fuel cost: $10.00/10⁶Btu ($10.0/10³sft³)
Determine the operating cost
The fuel cost is set to $10.00/10⁶Btu not because it accurately reflects current natural gas prices but because it reflects a generic, realistic fuel price (or a nominal fuel cost). This fuel price choice opens the discussion to allow the audience to be engaged in identifying the characteristics of their boilers. They can reflect on how their fuel price compares to this price and how their steam production compares to the example. This value ($10.00/10⁶Btu) is also easily “scale-able”. In other words, if the actual current price of a participant’s fuel is $15.00/10⁶Btu then the operating cost is approximately 1.5 times the cost in the presentation. Or if the participants burn coal with a price of $2.00/10⁶Btu then the operating cost is approximately 1/5 of the noted cost.
This boiler is also easily “scale-able” from a capacity standpoint. The steam production is set to the round number of 100,000 lbm/hr for easy mental adjustment to a participant’s frame of reference.
In this example the fuel flow can be considered to have been measured by a traditional flow meter.

5. Boiler Operating Cost
The cost of fuel is very large for this medium size boiler operating with a moderate fuel cost. The reason the boiler is a focal point of investigation is that a tremendous amount of money is passed through the boiler.
The calculation presented here identifies the calculation path to determine the fuel-related operating cost of the boiler. The choice in calculation is based on the real-world information available.
Fuel represents the dominant cost of boiler operations. Other costs include:
Water treatment costs
Auxiliary component costs
Forced draft and induced draft fans,
Boiler feed pumps,
Flue gas conditioning,
Operating personnel,
Maintenance personnel,
Maintenance expenditures.
These additional costs typically represent minimal additional cost when compared to fuel cost. Often heavy fuel oil boilers are considered to have efficiency less than natural gas boilers because of the auxiliary losses. However, when the auxiliary losses are considered (fuel pumping, fuel heating, soot-blowing) they typically combine to be less than 2% of fuel input energy. Additionally, any thermal energy placed into the fuel is not lost. Any fuel must be heated to combustion temperature by the combustion of the fuel itself or by an external method. Therefore, the thermal energy placed into the fuel (as long as it is not lost to the environment) is essentially captured. As a result, the true additional auxiliary loss is in the 1% of fuel input energy range. It is similar for coal-fired boilers. The auxiliary loss for coal (even pulverized coal) is in the 2% of fuel input energy range.
Answer: Appendix

6. Steam Cost Indicator
This calculation is included to identify the gross fuel-related cost of steam. This steam cost is the steam produced at the boiler cost—not the steam distributed to the system cost. The steam system will require deaeration steam and other auxiliary requirements that reduce the amount of steam delivered to the process units.
Additionally, the cost noted is the fuel related cost only, it does not include any of the other cost factors. It is good to note that typically when improving boiler operations the auxiliary costs are not impacted. For example, when improving boiler efficiency through combustion tuning, the maintenance cost, operating (personnel) cost, and depreciation cost of the boiler will essentially not change—only the fuel consumption will change.
Answer: Appendix

7. Typical Boiler Efficiency
A typical boiler will have an efficiency of ----?

8. Typical Boiler Efficiency
A typical boiler will have an efficiency of ----?
75% to 82% to 90%
Wood Natural Gas Oil and Coal
Efficiency is dependent on the type of fuel and the installed equipment.
The reference for this information is our combined experience of evaluating boiler efficiency on thousands of boilers with various types of fuel. In other words, it is based in real-world data. This data range is substantiated by information from the boiler manufacturers for the specific boilers and fuel types. It is also important to note that stack loss dominates boiler efficiency. It is difficult to attain stack loss less than 9% and it is difficult to achieve stack loss greater than 25% (for the fuels noted).
As a point of note, condensing economizers are not considered “typical” boiler components. This is because they are limited to natural gas (and light fuel oils). Also, they are not universally applicable even in natural gas boilers. The steam system must have a low temperature resource (less than 120°F) that requires heating and the amount of heat needed must be significant.
Most people are under the impression that natural gas provides the highest efficiency operation. This is a good opportunity to note that this is not the case for most industrial boilers. The main reason for this is the amount of H₂O formed in the combustion process. This water exits the boiler as steam in the exhaust gases. Unless a condensing economizer is in operation (in this case natural gas efficiency can exceed 92%). Condensing economizer performance is excellent because the water formed in the combustion process is cooled to condensate and the energy is recovered.
Answer: Appendix

9. Boiler Efficiency
For the example boiler determine the steam generation efficiency.
This is also called
Boiler efficiency
First law efficiency
Fuel to steam energy conversion efficiency
Typically, the participants are not asked to perform any calculations for this example. However, it is left to the instructor to decide.

10. Efficiency Example Steam Properties
Properties 
Location
Temp P
Specific
Enthalpy
Entropy
Quality
[°F]
[psia]
Volume
[Btu/lbm]
[Btu/lbm°R]
[%]
[psig]
[ft³/lbm]
Boiler outlet
700
414.7
1,361.88
****
400
Saturated vapor
448
1,204.62
100.0
Saturated liquid
428.04
0.0
Deaerator storage
239
24.7
207.74 10
Feed pump exit
242
614.7
210.42
600
Condensate
180
14.7
147.91
Makeup water 75
43.04
In order to complete the classic (direct) boiler efficiency evaluation steam and water property data is required.

11. Direct (Classic) Efficiency Calculation
This boiler is operating with somewhat low efficiency. It may be beneficial to note that this is a “real boiler”. This boiler was evaluated during a steam assessment and it serves as a classic example of the common improvement opportunities.
This evaluation is also known as direct efficiency

12. Efficiency Calculation
Boiler efficiency (77% in this case) indicates that the fuel resource was converted into a steam resource. However, only part of the fuel resource became steam energy. The question could be asked; where did the other 23% of the fuel go?
Why is the efficiency not 100%?

13. Efficiency Calculation
Why is the efficiency not 100%? losses
Answer: Appendix

14. Boiler Losses Identify the Boiler losses Exhaust Gases Feedwater Inlet
Steam Outlet
This “question to the audience” is an opportunity to re-engage the audience. Let the audience identify the losses as they see them.
Fuel and Air

15. Boiler Losses Identify the Boiler losses Combustion and Temperature
Feedwater Inlet
Steam Outlet
Exhaust Gases
Radiation and
Convection
These losses are the most common for all boilers. The goal of a boiler assessment is to identify the losses, quantify the losses, and set a strategy to best manage the losses. The primary tool used for this is the indirect boiler efficiency evaluation.
Fly Ash
Blowdown
Fuel and Air
Answer: Appendix
Bottom Ash

16. Indirect Efficiency
Boiler efficiency can also be determined in an indirect manner by determining the magnitude of the losses
Primary losses are typically
Shell loss
Blowdown loss
Stack loss
Indirect boiler efficiency is developed to make the same determination as direct (classic) boiler efficiency. In other words, if the measurements were perfect a direct efficiency evaluation and an indirect efficiency evaluation would yield identical results. In the real-world the two evaluations never yield identical results. However, the results should be comparable. The evaluation differences result from measurement inaccuracies, estimation of losses, and the fact that the indirect method often does not include all of the boiler losses. It is good to note that the indirect method and the direct method use different measurements to complete their evaluations. Therefore, if both methods can be accomplished on a boiler they can be compared.
The indirect efficiency evaluation is the most often used evaluation in the field. The field measurements are relatively easy to obtain. No fuel measurements are required other than to know the composition of the fuel. Additionally, once the individual loss components are determined a strategy for improvement can be established. For example, if shell loss, blowdown loss, and stack loss are 1%, 2%, and 15% (respectively) and if shell loss is difficult to change, blowdown loss can be “eliminated”, and stack loss can be reduced to 12%, then a path forward can begin to be established.
Another point of note is the fact that auxiliary losses are not addressed. Auxiliary losses are most prominently fan energy, flue gas treatment energy, feedpump energy, and fuel conditioning energy.

17. ASME Boiler Efficiency
American Society of Mechanical Engineers (ASME) has established a comprehensive testing standard for fired boilers
ASME Power Test Code 4 (ASME PTC–4 )
Fuel efficiency (the same as the classic equation)
Gross efficiency (includes auxiliary input streams)
ASME PTC–4 describes two investigation methods
Input/output (direct method)
Energy balance (indirect method)
This slide is just to note that the definitive evaluation technique for boiler efficiency is the ASME Power Test Code 4. The simplified calculations presented in this course are fundamental and appropriate. It is also good to note that the foundational principles of the ASME PTC-4 are the direct and indirect efficiency evaluations presented here. There is much more detail in the ASME PTC-4.
As a note to the instructor it should be identified that ASME PTC-4 identifies blowdown energy as a “desired boiler output” rather than a loss. This arrangement has not been adopted for the presentation because the blowdown energy is a direct target to control.