Boiler Efficiency - Kentuckydca.ky.gov/Training Document Library/4_Decker_BoilerEfficiency.pdfBoiler...

@2016 TRSA - 1800 DIAGONAL ROAD, SUITE 200, ALEXANDRIA, VA 22314 – 877.770.9274 - TRSA.ORG@2016 TRSA - 1800 DIAGONAL ROAD, SUITE 200, ALEXANDRIA, VA 22314 – 877.770.9274 - TRSA.ORG

Boiler Efficiency

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®

918 W WALNUT ST, DANVILLE, KENTUCKY 40422-0048 PHONE (859) 236-3181 WWW.SELLERSMFG.COM918 W WALNUT ST | DANVILLE | KENTUCKY 40422-0048 | PHONE (859) 236-3181 | WWW.SELLERSMFG.COM

@2016 TRSA - 1800 DIAGONAL ROAD, SUITE 200, ALEXANDRIA, VA 22314 – 877.770.9274 - TRSA.ORG

Boiler Room Efficiency

• Boiler efficiency is far too complicated to allow any single measure to identify the most efficient boiler for any real-world application.

• Site-specific variables, which are generally overlooked or assumed-away in published efficiency data, must be taken into consideration and assessed.

• For this presentation, we will look at typical 4 pass wetback and Immersion design firetube boilers.

• We will try to put into perspective, real world applications of both “Combustion & Boiler” efficiency.


Typical Steam System

Cyclonic blow down

separator


Fuel Types

• Standard Fuels:• Natural Gas, Propane, Propane/Air• #2-#6 Oil

• Non-standard Fuels: (Must have fuel analysis)

• Bio Gas, Digester, Landfill, Hydrogen• Waste Oil, Bio Diesel, Ethanol, Talo

• Solid Fuels:• Coal, wood, Animal waste• Bio mass crops: corn, trees, sugar cane


Natural Gas

• Natural gas is emerging as an ideal package of cost-effective, reliable, widely-available and reduced-emissions fuel.

• It is also significant that the USA has become the largest natural gas producer in the world.

• For this presentation, we will be using Natural Gas to base our discussion.


Boiler Designs

Cast Iron Sectional

Firebox

Commercial Watertube

Industrial Watertube


Firetube Boilers

7

• Used for process and heating applications

• Hot water or steam (15 to 300 psi)• Higher efficiency than an IWT• Less expensive than an IWT• Usually sent as a package• Typical inputs between 30-1,000

BHP

Firetube Boiler


Boiler Room Efficiency

• Combustion Efficiency – Burner• Boiler Efficiency – Boiler Design• Ancillary – Boiler Support Systems


Burner Efficiency

• How Efficient is the Burner?• Burners are typically 99.5%

efficient in the job of burning the natural gas.

• Unburned Fuel = CO.• Typical CO levels at <50 ppm.

• Too low to have any effect on efficiency or performance


Combustion Efficiency

• Excess Air levels (O2)• Combustion Air Temperature (air density)

• Unburnt Fuel (CO)• Turndown• Control Systems



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Multiply this by 40 years (expected life of boiler) = $386,120 savings at today’s fuel cost

Savings of $9,653 by increasing efficiency by 1.1%

82.3% efficiency at $1.00 per therm = $712,550 per year

400 HP input running for 4,380 hours a year

81.2% at $1.00 per therm = $722,203 per year

400 HP input running for 4,380 hours a year (total per year 8,760 hours) 50% firing rate

A gain of 1.1% combustion efficiency

Boiler runs at 3% O2 with a stack temp of 400 f = 82.3% combustion efficiency

Boiler runs at 5.5% O2 with a stack temp of 400 f = 81.2% combustion efficiency


Bureau of Labor Statistics for the Baltimore area $1.39 per therm June 2017



3D Surface of Stack O2, Net Stack Temp, and


Low excess air = higher combustion efficiency. Excess air absorbs heat.

Typical low fire setting is 6% O2 or 40% excess air.

Typical high fire setting is 3% O2 or 15% excess air.

CO and the ability to control the combustion mixing is a limiting factor.

UL limits are 400 ppm CO. Burners today run between 0-50 ppm CO in actual practice.



Typical Combustion Curve for a 5:1 Turndown Burner


Setting O2 Levels WithIndustrial Combustion Air Dampers

14


Typical Combustion Curve for a 10:1 Turndown Burner



Typical boiler combustion efficiency based on load. Efficiency increases as load increases due to

drop in excess air levels.

At higher loads the efficiency begins to decrease due to higher stack temperatures.

Given the lower efficiency in low fire, it may be more efficient to shut the boiler off if the boiler has a rapid response to load demands.



Control Systems

• Single point modulation system with linkage.• Requires all fuels to be set for one air curve.

• Must be set for the lowest turndown of the fuels. (Natural Gas 5:1, #2 Oil can be as low as 1.8:1 turndown)

• Typically, the technician will set the unit up for good low and high fire O2 with a lean condition in mid fire lowering combustion efficiency.

• Hysteresis results in fluctuating O2 levels when modulating up and down the firing rate.



Control Systems

• Although an extreme case, it is not uncommon to see a similar combustion curve in the field, when utilizing linkage arms and flat blade air inlet dampers.

• Technician will try to overcome an “air spike” coming off of low fire. This will result in a lean condition in most of the combustion curve.Each additional 1% of O2 required represents

approximately ½% reduction in efficiency



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Control Systems

• Parallel Positioning Burner Controls• Provides the ability to set separate low fire

settings per fuel.

• Capable of setting separate air curves per fuel.

• Allows the technician to set multiple points throughout the firing range for each fuel.

• Eliminates hysteresis so that the combustion curve is the same at each set point when increasing or decreasing the firing rate.

• Typical fuel savings of 3-5%.



Combustion Air Temperature

Air Temp.

Barometric

Pressure

Resulting %

Excess Air

Resulting

% of O2

20 29 27.5% 5.0%

40 29 23.0% 4.4%

60 29 18.8% 3.7%

80 29 15.0% 3.0% Set Up

100 29 11.9% 2.5%

120 29 8.1% 2.0%

Control Systems

• Effects of air temperature on combustion• Air density can adversely affect the excess air on a

burner.

• Technicians will set the burner slightly lean to make up for these conditions so that the burner does not soot the boiler.

• Soot has five times the insulating value of asbestos. 1/8” of soot results in a heat loss of 47% increasing fuel consumption by 8.5%.



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Control Systems

• O2 Trim Option• Continuously monitors the excess

air levels in the stack and provides a signal back to the main controller.

• Main controller adjusts the fuel servo so that the curve set by the technician is maintained.

• Allows the Service technician to set the burner at minimal combustion air settings.

• Typical fuel savings is an additional 2-3%.


Digital Connectivity

• Communication via BMS and/or Process Control System.• Non-proprietary system provides the facility owner with

low-cost and seamless communication capability with existing BMS protocols. (Eliminates the high cost of unique controls that provide limited flexibility.

• Common protocols• BACnet MS/TP & IP• Modbus TCP/IP & RTU• Metasys N2• LonWorks

• Remote longer-term trending service• 6” or 10” touchscreens allow real-time wireless remote

monitoring of equipment with text or email alarm alerts to your tablet or cellphone.

• Can eliminate or reduce down time.

Control Systems


Boiler Efficiency

• Boiler efficiency is measured by fuel-to-steam efficiency, basically BTU input versus BTU output, also ultimately reflecting the amount of heat transfer from the fuel to the steam or hot water.

• Fuel to steam efficiency is a better measure of overall efficiency at it takes into consideration the effectiveness of the heat transfer as well as the radiation and convection losses in the boiler shell.

• Published efficiency is based on 100% of its rated capacity in a steady state.

• Does not take into consideration cycling, and purge losses, along with other operating variables by application that will reduce actual efficiency.

• Stated boiler efficiency will be less than combustion efficiency.


Boiler Efficiency

• Boiler Standby losses

• This is the loss of energy when a

boiler is in the off mode. This will

need to be made up when the

boiler is back online.

• Efficiency losses between 0.3%

– 2.0% are typical.

• Many variables can contribute to

the different efficiency losses

such as the temperature and

even the air movement in the

boiler room. Standby losses


Boiler Efficiency

• Boiler Standby losses• The larger the pressure vessel the larger

the standby loss due to the surface area of the boiler.

• Can also be affected by the boiler design. A dryback boiler with the large refractory rear door will lose more energy than an immersion or wetback boiler.

300 bhp Immersion-Fired Boiler

769 ft3, drastically reduced volume

and footprint

Typical 300 bhp Boiler 1,435 ft3,

very large volume and footprint

160 ft2

Standby losses


Boiler Efficiency

• Purge Losses• This is the loss associated with the pre and post purge

of the boiler.• As per UL, the boiler must move 4 passes of clean air

through the boiler before ignition.

• Typically: For each 1 bhp, the purge cycle is losing 42.58 btu/min. • Multiple pass boilers run 60 or 90 second purge times.

Immersion boilers can run as little as 15 seconds.


Boiler Efficiency• Purge loss examples: 200 bhp x 42.58= 8,517 btu/min loss during purge.

• Multi-pass boiler (3 or 4 pass design)• 200 bhp cycling 6 times per hour.• Each purge is 90 seconds in duration.• 8,516 btu/min @ 90 seconds X 6 = 76,644 btu lost per

hour.• 40 hour week = 3,065,760 btu lost per week.• 52 weeks = 159+ mmbtu total purge losses.

• Immersion boiler (single-pass design)• 200 bhp cycling 6 times per hour.• Each purge is 15 seconds in duration.• 8,516 btu/min @ 15 seconds X 6 = 12,778 btu lost per

hour.• 40 hour week = 511,140 btu lost per week.• 52 weeks = 26+ mmbtu total purge losses.

Individual Pre-

Mixed High Velocity

Flames Directly

Into Each Tube

3-Pass Firetube

Single Pass

Immersion Firetube


Boiler Efficiency

• Warm-up Losses• This is the loss associated with the

time and fuel required to heat a boiler from cold start to steam.

• Boiler design is the largest contributing factor due to thermal stresses.

• Labor hours must also be considered.


Boiler Efficiency

• Thermal shock/stress is the #1 cause of firetube boiler damage and repair usually caused by inappropriate start-up measures.

• Temperature differences on common heating surfaces (tube sheets).

• Expansion and contraction of heating surfaces.• Conventional boiler performance is limited by

thermal stress resulting in inefficiency by requiring slow boiler start-up (up to 8 hours) and continuous idling operation.

Single Pass Immersion Tube Pattern Typical 3 Pass tube Pattern

Thermal Stress Points

Even Heat Distribution with a single pass is not subject to

thermal shock

Large Temperature drops with each pass are prone to

thermal shock

350° F 350° F

1,800° F

Traditional Multi-Pass Tube Sheets


• Warm-up Losses• Furnace is ½” thick and absorbs half of

the heat. Most of this is high temp radiant heat.

• Due to the high temperature of the furnace, overall size and thickness, it will expand at a higher rate than the .095 tubes which are absorbing cooler convection heat.

• Tube sheet is being expanded by the furnace which causes stress on the thinner tubes.

• Results can be cracked tubes and/or tube sheets.

• Boiler must be warmed up slowly to overcome this quick thermal stress.


Traditional Multi-Pass Shell Design BoilersAlways Under Thermal Stress

Single Pass Immersion Firing Shell DesignEven Heat Distribution Eliminates Thermal Stress

Each tube absorbs the same amount of radiant heat


Warm up time to steam from 50°f water 4

to 8 hours depending on the size of boiler

Warm up time to steam from 50°f water

is less than 20 mins for all sizes of boilers


Comparison Warm-up Calculation

Bureau of Labor Statistics for the Baltimore area $1.39 per therm June 2017


Ancillary Products

• Support equipment that can increase overall boiler efficiency.• Deaerators.

• Increases efficiency and protects the boiler against oxygen.

• Heated Condensate Tanks• Increases efficiency by heating the

condensate being returned to the boiler.

• Economizers.• Increases efficiency by utilizing

stack losses.


What is a Deaerator?

A system to preheat boiler feed water and drive off free oxygen, carbon dioxide and other non-condensable gases.

Consists of:

Storage Tank.

Heating Apparatus.

Feed Pump(s).

Water Level Controls.


Why use a Deaerator?

Lower operating costsMechanical deaeration provides a fixed method of eliminating oxygen.

The only other method is to inject high volumes of expensive oxygen scavenging chemicals into the boiler feed water supply.

=


How does a Deaerator Work?

Raise the water temperature above the zero saturation temperature.

Agitate to overcome surface tension.

Atomize water to smallest possible droplets.

Vent the gases from the system.

Heating the feedwater to 227° f increases boiler efficiency 1% for every 10° f.

Typical condensate at 180° f temp is increased 47° = 4.7% efficiency gain.


Ma

ke

up

& C

on

de

nsate

Spray

Nozzle

Stainless Steel

Steam Chamber

Steam

Pressure

Reducing

Valve

Steam

Scrubber

Ve

nt

P5 Single Tank

Deaerator Flow


THERMA-FEED BOILER FEED SYSTEM

• Atmospheric Tank• Up to 90% Deaeration

• Lowers the amount of oxygen scavenger chemicals required.

• Epoxy Lined• Stainless Heater Tube• Heats the feedwater to 208° f.


Stack Economizers

• Fuel Savings Up to 8%• Fits any Manufacturers Boiler

• Is a boiler efficiency equalizer• Conventional & Condensing designs• Average Payback time is Less than 18 Months• Takes advantage of lost flue gases and raises boiler

feedwater temps.


Efficiency Review

• Maintenance is 10% of the total cost.• Staff should be trained and licensed for the

equipment so it can be operated in the most efficient manner.

• Lack of maintenance can lead to failed products as well as dangerous conditions.

• Different boiler designs can have different maintenance requirements.


Efficiency Review

• Efficiency can not be measured just by looking at published efficiency data

which is measured at 100% firing rate in a steady state.

• Site-specific variables must be considered.

• Combustion efficiency is not the same as Boiler efficiency.

• 1% efficiency gain can provide large savings.

• Control Systems can provide increased efficiency.

• Different Firetube boiler designs can also play into the equation.

• Stand-by, Purge and Warm-up losses must be considered.

• Radiant compared to Convection heat transfer.

• Ancillary products can provide increased efficiency and less maintenance.

• Equipment is only 9.75% of the total cost of ownership compared to 74% for fuel.

• Maintenance considerations should be reviewed when considering total boiler

room cost.

Boiler Efficiency - Kentuckydca.ky.gov/Training Document Library/4_Decker_BoilerEfficiency.pdfBoiler...

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