Presentation on combustion in high efficiency condensing hot water boilers.

2. 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

3. What is a condensing boiler?
EVERY BOILER 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?

4. How do we make it condense?
•
•
•
•
•
Principles of Combustion
Oxygen & Efficiency
Oxygen & CO2
CO2 vs. Dew Point
Dew Point vs. Return Temperature

5. Natural Gas Combustion
Excess air
1 part gas
10 parts air

6. Stoichiometry

7. Combustion Process
PERFECT 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

8. 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

9. Oxygen & Efficiency

10. Oxygen and Carbon Dioxide
• The goal is to maximize
efficiency while ensuring
reliability and safety
30.0
- 5% oxygen
- 9% carbon dioxide
- 27% excess air
Excess Air
28.0
98.00
26.0
24.0
20.0
Percent (%)
18.0
96.00
Patterson
Kelley
Combustion
Efficiency
94.00
92.00
16.0
90.00
14.0
12.0
10.0
88.00
CO2
86.00
O2
84.00
8.0
6.0
4.0
82.00
2.0
0.0
80.00
Efficiency (%)
22.0
• Many manufacturers
recommend tuning for:
100.00

11. Carbon Dioxide vs. Dew Point
•
As carbon dioxide falls,
the dew point falls
13.0
180.0
175.0
170.0
165.0
160.0
155.0
150.0
145.0
140.0
135.0
130.0
125.0
120.0
115.0
110.0
105.0
100.0
95.0
90.0
85.0
80.0
12.0
•
•
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.
11.0
10.0
9.0
8.0
7.0
6.0
5.0
4.0
15.0
14.0
13.0
12.0
11.0
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
3.0
1.0
At 6% carbon dioxide,
the dew point is
approximately 112⁰ F.
0.0
•
CO2 (%)
Dew Point

12. 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

13. 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.

14. 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.

15. More Isn’t Always Better
• 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.

16. More Isn’t Always Better
Remember 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?

17. 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

19. HTD vs. LTD – 4,000,000 Btus

20. Typical Outdoor Air Reset
• Design for 180° F
(design day 0° F)
190
180
Supply Water
Temperature
(°F) - Standard
170
160
• ∆T at 20° F
150
140
• Dew point of 126°
F (9% CO2)
• Condensing
boilers operate
down to approx.
25° F
Return Water
Temperature
(°F) - Standard
130
120
110
100
90
80
70
70 65 60 55 50 45 40 35 30 25 20 15 10
5
0
-5 -10

21. 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
www.degreedays.net
Maintain Good Combustion Settings

22. HTD vs. LTD
97.0%
96.0%
95.0%
94.0%
Neither Boiler Condensing
93.0%
Combined Efficiency HTD
92.0%
91.0%
90.0%
Combined Efficiency LTD
89.0%
HTD Not Condensing
88.0%
87.0%
Output (Btus/Hr)
0
200,000
400,000
600,000
800,000
1,000,000
1,200,000
1,400,000
1,600,000
1,800,000
2,000,000
2,200,000
2,400,000
2,600,000
2,800,000
3,000,000
3,200,000
3,400,000
3,600,000
3,800,000
4,000,000
86.0%

23. HTD vs. LTD – 55% Load
97.0%
96.0%
95.0%
Combined Efficiency HTD
94.0%
Combined Efficiency LTD
93.0%
92.0%
91.0%
90.0%
89.0%
HTD Not Condensing
88.0%
87.0%
86.0%
Output (Btus/Hr)
84% of total
operating
hours in this
range

24. HTD vs. LTD – 20% Load
97.0%
Combined Efficiency HTD
96.0%
95.0%
Combined Efficiency LTD
94.0%
9% difference
93.0%
92.0%
91.0%
33% of total
operating
hours in this
range
90.0%
89.0%
88.0%
87.0%
86.0%
800,000
600,000
400,000
Output (Btus/Hr)
200,000
0

25. 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
190
185
180
175
170
165
160
155
150
145
140
135
130
125
120
115
110
105
100
95
90
85
80
75
70
Supply Water
Temperature
(°F) - Standard
Supply Water
Temperature
(°F) - Optimized
Return Water
Temperature
(°F) - Standard
Return Water
Temperature
(°F) - Optimized
70 65 60 55 50 45 40 35 30 25 20 15 10
5
0
-5 -10

26. 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

27. HTD vs. LTD – Optimized OAR
97.0%
96.0%
95.0%
Combined Efficiency HTD
Neither Boiler Condensing
94.0%
Combined Efficiency LTD
93.0%
92.0%
91.0%
90.0%
89.0%
HTD Not Condensing
88.0%
87.0%
0
200,000
400,000
600,000
800,000
1,000,000
1,200,000
1,400,000
1,600,000
1,800,000
2,000,000
2,200,000
2,400,000
2,600,000
2,800,000
3,000,000
3,200,000
3,400,000
3,600,000
3,800,000
4,000,000
86.0%

28. HTD vs. LTD – Optimized OAR
97.0%
96.0%
95.0%
Efficiency Gained By Optimized OAR
94.0%
93.0%
92.0%
Original OAR Curve
91.0%
90.0%
89.0%
88.0%
87.0%
86.0%

29. 0
200,000
400,000
600,000
800,000
1,000,000
1,200,000
1,400,000
1,600,000
1,800,000
2,000,000
2,200,000
2,400,000
2,600,000
2,800,000
3,000,000
3,200,000
3,400,000
3,600,000
3,800,000
4,000,000
Hybrid Systems
97.0%
96.0%
Noncondensing Boiler Here?
95.0%
Combined Efficiency HTD
94.0%
Combined Efficiency LTD
93.0%
92.0%
91.0%
90.0%
89.0%
88.0%
87.0%
86.0%

30. 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?

31. ANSI Z21.13 (CSA4.9) & BTS 2000
• Unrealistic conditions
• 180 supply temp
• 80 return temp
• 100% firing rate
• 30 minutes heat soak
• Take measurements

33. From our manual…

34. HTD Boiler – 20:1

35. From their manual…
HTD Section
What is the dew point at 1.1% CO2?

36. ANSI Flue Loss Calculator
At 1.1% CO2, the dew point is 77.9⁰ F

37. How is this possible?

39. 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 primarysecondary and variable primary systems
• On-board controls should be capable of sequencing,
system optimization, and outdoor air reset

40. 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

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

43. 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.

44. Hughes Debuts PK Stainless

45. 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

46. 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

47. Hughes Machinery Presents:
Hot Water Boiler Engineering Seminar
TBD, February 2014
8:00 A.M to 5:00 P.M.
6-1/2 PDHs
14400 College Blvd.
Lenexa, KS 66215