combustion in condensing boilers

Agenda• What is a condensing boiler?• The Principles of Combustion• Turndown• Short Cycling• Multi-Boiler Operating Principle• Efficiency Standards• Patterson-Kelley Design• Things to Remember• Hot Water Seminar

What is a condensing boiler?

• Every boiler will condense when the return water temperature falls below the dew point

• But not every boiler is designed to condense

• Boilers designed to condense include heat exchangers constructed of specialized materials to capture latent heat

• Why condensing boilers?

EVERY BOILER IS A CONDENSING BOILER

How do we make it condense?• Principles of Combustion• Oxygen & Efficiency• Oxygen & CO2

• CO2 vs. Dew Point• Dew Point vs. Return Temperature

Natural Gas Combustion

1 part gas

10 parts air

Excess air

Stoichiometry

Combustion ProcessPERFECT Combustion/stoichiometric air-fuel ratio: The stoichiometric combustion or perfect combustion occurs when fuel is burned using only the theoretical amount of air. Theoretical amount of air is the amount of air used to achieve perfect combustion in a laboratory. When burned, all fuel and air is consumed without any excess left over.

COMPLETE Combustion: is combustion that occurs when all fuel is burned using the minimum amount of air above the theoretical amount of air required to burn the fuel. With complete combustion, fuel is burned at the highest combustion efficiency with minimum polluting emissions.

INCOMPLETE Combustion: If an insufficient amount of air is supplied to the burner, unburned fuel, soot, smoke, and carbon monoxide exhausts from the boiler - resulting in heat transfer surface fouling, pollution, lower combustion efficiency, flame instability and a potential for explosion. To avoid inefficient and unsafe conditions boilers normally operate at an excess air level

• if air content is higher than the stoichiometric ratio - the mixture is said to be fuel-lean • if air content is less than the stoichiometric ratio - the mixture is fuel-rich

Definitions of Efficiency

• Combustion Efficiency: 100 - flue loss. This is the effectiveness of the burner only and relates to it’s ability to completely burn fuel.

• Thermal Efficiency: Output/input. This is the effectiveness of the heat transfer in the heat exchanger.

• Seasonal efficiency/Part load efficiency: Overall effectiveness of boiler system throughout entire heating, season takes into account cycling losses. (No official test procedure).

• Turndown Efficiency: Efficiency of multi-boiler batteries throughout firing range.

8

Oxygen & Efficiency

Oxygen and Carbon Dioxide• The goal is to maximize

efficiency while ensuring reliability and safety

• Many manufacturers recommend tuning for:

- 5% oxygen- 9% carbon dioxide- 27% excess air

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Excess Air

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CO2

Patterson KelleyCombustion Efficiency

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80.085.090.095.0100.0105.0110.0115.0120.0125.0130.0135.0140.0145.0150.0155.0160.0165.0170.0175.0180.0

CO2 (%)

Dew Point

Carbon Dioxide vs. Dew Point• As carbon dioxide falls,

the dew point falls

• At 11.7% carbon dioxide (perfect stoichiometric combustion), the dew point is approximately 140⁰ F.

• At 9% carbon dioxide, the dew point is approximately 126⁰ F.

• At 6% carbon dioxide, the dew point is approximately 112⁰ F.

Dew Point vs. Return Temp.• Consider your system

design, supply water temp., and return water temp, given efficient, reliable, and safe combustion:

- 5% oxygen- 9% carbon dioxide- 27% excess air- 126⁰ F dew point

What is turndown?• Turndown is the ratio of Btus consumed at high fire

relative to the Btus consumed at low fire.

• A turndown of 2:1 indicates that a boiler is capable of firing at 50% of its maximum firing rate.

• Below 50% of its maximum firing rate, the boiler will shut off. Above 50%, it will modulate between 50% and 100% depending on load.

Why is turndown good?• Turndown allows the boiler or boilers to match the

load as it varies.

• Turndown prevents the boilers from turning on and off frequently (short-cycling) in low load conditions.

• Turndown can improve system efficiency.

• Remember: As long as good combustion is maintained, condensing boilers are most efficient at low fire.

• Why?

1. Mechanical limitations of standard gas valves

2. Incomplete mixing of air and gas inside the pre-mix burner

3. Flame stability

• The current practical limit to turndown is approximately 5:1.

• “High turndown” boilers (10:1 and greater) are very inefficient at low fire.

More Isn’t Always Better

More Isn’t Always BetterRemember high excess air = inefficiency. More specifically:

High excess air = High oxygen =

Low carbon dioxide = Low dew point =

Boiler never condenses = High turndown boilers are inefficient at low fire

If the dew point is low, does it matter what the return water temperature is?

Turndown Optimization• Turndown across multiple boilers is additive: Four boilers, each

with 5:1 turndown, have a system turndown of 20:1

• To maximize boiler system efficiency:

- Select multiple boilers with 5:1 turndowns

- Operate the boilers such that the maximum number of boilers is firing at any given load

- Assuming good combustion, condensing boilers are most efficient at low fire

HTD vs. LTD – 4,000,000 Btus

Typical Outdoor Air Reset• Design for 180° F

(design day 0° F)

• ∆T at 20° F

• Dew point of 126° F (9% CO2)

• Condensing boilers operate down to approx. 25° F

70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5 -1070

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190

Supply Wa-ter Tempera-ture (°F) - Standard

Return Wa-ter Tempera-ture (°F) - Standard

Hot Water Boiler Laws

• 1) Maintain 140 F Return Temperature to the Boiler (Unless Condensing Boiler)

• 2) We Must Have Flow, Turbulent Flow• 3) Maintain Proper Temperature profiles

Across the Boiler• 4) Do Not Enable/Disable the Boiler Plant• 5) Prevent Boiler Short Cycling• 6) Provide Adequate Combustion Air and

Maintain Good Combustion Settingswww.degreedays.net

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Combined Efficiency - HTD

Combined Efficiency - LTD

Output (Btus/Hr)

HTD vs. LTD

Neither Boiler Condensing

HTD Not Condensing

2,000,000

1,800,000

1,600,000

1,400,000

1,200,000

1,000,000

800,000

600,000

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200,000 086.0%

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Combined Efficiency - HTD

Combined Efficiency - LTD

Output (Btus/Hr)

HTD vs. LTD – 55% Load

HTD Not Condensing84% of total operating hours in this range

800,000 600,000 400,000 200,000 086.0%

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97.0%

Combined Efficiency - HTD

Combined Efficiency - LTD

Output (Btus/Hr)

HTD vs. LTD – 20% Load

33% of total operating hours in this range

9% difference

Outdoor Air Reset - Optimized• Rule of thumb: Every 4⁰ F

decrease in supply water temp. results in a 1% savings

• Increase ∆T from 20° to 40° F

• Design for 180° F at design day (-10 ° F)

• Dew point of 126° F (9% CO2)

• Condensing boilers operate down to approx. 3° F

70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5 -10707580859095

100105110115120125130135140145150155160165170175180185190

Supply Wa-ter Tem-perature (°F) - Stan-dard

Supply Wa-ter Tem-perature (°F) - Opti-mized

Return Wa-ter Tem-perature (°F) - Stan-dard

Return Wa-ter Tem-perature (°F) - Opti-mized

Hot Water Boiler Laws

• 1) Maintain 140 F Return Temperature to the Boiler (Unless Condensing Boiler)

• 2) We Must Have Flow, Turbulent Flow• 3) Maintain Proper Temperature profiles

Across the Boiler• 4) Do Not Enable/Disable the Boiler Plant• 5) Prevent Boiler Short Cycling• 6) Provide Adequate Combustion Air and

Maintain Good Combustion Settings

4,000,000

3,800,000

3,600,000

3,400,000

3,200,000

3,000,000

2,800,000

2,600,000

2,400,000

2,200,000

2,000,000

1,800,000

1,600,000

1,400,000

1,200,000

1,000,000

800,000

600,000

400,000

200,000 086.0%

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97.0%

Combined Efficiency - HTD

Combined Efficiency - LTD

HTD Not Condensing

HTD vs. LTD – Optimized OAR

Neither Boiler Condensing

HTD vs. LTD – Optimized OAR

4,000,000

3,800,000

3,600,000

3,400,000

3,200,000

3,000,000

2,800,000

2,600,000

2,400,000

2,200,000

2,000,000

1,800,000

1,600,000

1,400,000

1,200,000

1,000,000

800,000

600,000

400,000

200,000 086.0%

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97.0%

Original OAR Curve

Efficiency Gained By Optimized OAR

4,000,000

3,800,000

3,600,000

3,400,000

3,200,000

3,000,000

2,800,000

2,600,000

2,400,000

2,200,000

2,000,000

1,800,000

1,600,000

1,400,000

1,200,000

1,000,000

800,000

600,000

400,000

200,000 086.0%

87.0%

88.0%

89.0%

90.0%

91.0%

92.0%

93.0%

94.0%

95.0%

96.0%

97.0%

Combined Efficiency - HTD

Combined Efficiency - LTD

Hybrid Systems

Noncondensing Boiler Here?

Efficiency Standards• No (realistic) standard testing variables established

• No testing for multi-boiler batteries (additive turndown)

• AHRI is the best source we have

• Combustion efficiency vs. thermal efficiency

• CO2 and efficiency

• How do you read AHRI’s Certificates?

• How do these ratings compare to manufacturers’ manuals?

ANSI Z21.13 (CSA4.9) & BTS 2000

• Unrealistic conditions

• 180 supply temp

• 80 return temp

• 100% firing rate

• 30 minutes heat soak

• Take measurements

From our manual…

HTD Boiler – 20:1

From their manual…

What is the dew point at 1.1% CO2?

HTD Section

ANSI Flue Loss Calculator

At 1.1% CO2, the dew point is 77.9⁰ F

How is this possible?

PK Approach to Boiler Design• Only aluminum and stainless steel are approved by

ASHRAE

• Entire heat exchanger must be designed to condense

• Heat exchanger design must work in primary-secondary and variable primary systems

• On-board controls should be capable of sequencing, system optimization, and outdoor air reset

Why does PK perform better?• Unique pressure vessel design allows for unparalleled

heating surface to water volume

• Single cast aluminum alloy heat exchanger

• Aluminum is 10x more conductive than stainless steel

• Casting allows us to control velocity and turbulence on the fire and water sides

• Counter flow for maximum heat transfer

View of the gas side section. Flow remains turbulent across the burner’s entire modulation range.

Cut-away view of the waterside heat exchanger. Note six passes, each one smaller than the one before. Top mounted burner fires down as the water flows up through the heat exchanger.

Hughes Debuts PK Stainless

Remember These Key Points• 9% CO2 is practical

• RWT must be below 126⁰ F

• HTD boilers may not condense below 5:1 regardless of RWT

• Select multiple boilers with 5:1 turndowns (additive)

• Compare AHRI CO2 (and efficiency) to manufacturer’s manual

• Look for 95% or better at 9% CO2

• Per ASHRAE, only AI and SS are approved for condensing boilers

• Entire heat exchanger must be designed to condense

Additional Presentation Modules

• Hybrid Systems: Condensing & Non-Condensing Boilers in New and Existing Projects

• System Design: Primary/Secondary vs. Variable Primary & Preventing Short-Cycling

• Domestic Hot Water Priority in Condensing Boiler Applications

Hughes Machinery Presents:

Hot Water Boiler Engineering Seminar

TBD, February 20148:00 A.M to 5:00 P.M.

6-1/2 PDHs

14400 College Blvd.Lenexa, KS 66215