BOILERS
BOILERS
FLAME
Burner
CENTRAL HEATING
System Components
CENTRAL HEATING
System
Hydro-flow (or water circulation) Heating System
Combi-boiler
Radiators
Shower
Two-pipe system
There is a flow and a return pipe.
CENTRAL HEATING
Boilers
• Firetube
– Flame in tubes surrounded by water.
• Watertube
– Flame in chamber with water circulating
through tubes in the chamber.
• Electric
– Electrode submerged in water.
Condensing Boilers
• Condensing Boiler: A boiler designed to condense water vapor
in the flue gas on heat exchanger surfaces to capture latent
energy and drain away captured condensation.
Efficiencies up to 98%
– The more hours a condensing boiler
runs at low load with reduced inlet
water temperature, the greater its
seasonal efficiency.
Efficiency Depends on Operating
Conditions
–
–
–
–
–
Return (inlet) water temperatures
Hours at low loads
Boiler loads
Reset schedule
Boiler staging
Source: Fulton Boiler
¾ Any boiler will be a condensing boiler when you bring cold water into it!
Oxygen for the Combustion
Process
• Combustion – or burning – of fuels requires
•
•
•
•
the availability of oxygen, which in most cases
comes from the combustion air supply.
The chemical makeup of air is about 78%
Nitrogen, 21% Oxygen and trace percentages of
many other gases.
As a simplified model, we can roughly think of
air as 80% Nitrogen and 20% Oxygen.
Thus, to get one unit of Oxygen for fuel
en
g
xy
O
combustion
1
=
r
i
in a boiler, we must supply five units of air.
5a
This is a handy rule of thumb for combustion air.
Fundamentals of Combustion
Consider combustion of natural gas (methane) with air:
CH4 + 2O2 + 7.52 inerts → CO2 + 2H2O + 7.52 Inerts
mair
Air-fuel ratio = AF =
m fuel
(for the reactants)
Stoichiometric → chemically correct
For the above reaction:
AFstoich
⎛ mair
=⎜
⎜ m fuel
⎝
⎞
= 17.2
⎟⎟
⎠ stoich
Fundamentals of Combustion
In practice, to overcome incomplete mixing of air with
fuel, excess air, EA, is used.
EA =
mair ,actual
mair , stoich
−1
Insufficient air
Reduced combustion
efficiency
Too much air
Unburned fuel in products
Increased hydrocarbon
emissions
Reduced product temperature
Reduced combustion efficiency
Fundamentals of Combustion
Percent excess air
Some excess air is always desired for safety and for the problem of incomplete mixing, but
with natural gas, this amount of excess air should not need to be more than 15% to 20%.
This corresponds to a 3% to 4% level of excess Oxygen in the flue gas.
Combustion Efficiency Targets
• Natural Gas
ƒ
ƒ
ƒ
ƒ
ƒ
4 – 5% oxygen (O2)
21 – 28% excess air
9,6 – 9,9% carbon dioxide (C O2)
80 – 84% efficiency
400 PPM carbon monoxide (CO)
• Fuel Oil
ƒ
ƒ
ƒ
ƒ
ƒ
4 – 5% oxygen (O2)
22 – 29% excess air
11,9 – 12,6% carbon dioxide (C O2)
84 – 87% efficiency
1 – 2 smoke test
Boiler Efficiency
Chart
Data Needed:
• Fuel type
• % CO2
• % O2
• Stack
temperature
Boiler Stack Temperatures
• 135 ºC minimum to prevent
condensation
• Load plus 66 ºC target
• Consider economizer for well
tuned boiler over 205 ºC.
• Savings 1% per 5 ºC reduction
Heat Content of Liquid Water
• As long as water is liquid, its heat
capacity is almost exactly 4.2 kJ per kg
per degree Celsius.
• We express this heat capacity as:
4.2 kJ
kg °C
• We can use this heat capacity to find
answers to heating and cooling problems
which involve only changing the
temperature of liquid water.
Example
How much heat is required to change the
temperature of 3000 kg of 21°C water to
80°C water?
Solution
Water at 80°C is still liquid. Therefore,
3000 kg 4.2 kJ (80
X kJ =
kg°C
= 206.5 kWh
21)°C
= 743,400 kJ
Boiler Improvements
•
•
•
•
•
•
•
•
•
•
Keep excess air below 20%.
Keep waterside and fireside clean.
Preheat combustion air, feedwater, fuel oil.
Reduce blowdown losses.
Isolate off-line boilers.
Turn off during cooling season.
Reset boiler water temperature.
Install boiler stack economizer.
Lower boiler water temperature and pressure.
Insulate pipes, valves, boiler shell.
Boiler Auditing Checklist
FEASIBLE (Y/N)
1. Adjust Air-fuel Ratio
2. Install Stack Heat Exchanger
(Economizer)
3. Calculate the steam cost for different fuel
& use lower fuel for combustion equipment
4. Eliminate Steam Leaks
5. Inspect & Repair Steam Traps
6. Return steam condensate to boiler
7. Minimize boiler blowdown
8. Heat recovery from boiler blowdown to
pre-heat boiler feed water
9. Insulate bare steam lines
10. Keep boiler tube (water side) clean
11. Use refuse as fuel / Install waste heat
boiler
Calculate Steam Cost:
1. Cost of Fuel =
2. Efficiency of the Boiler =
Comments:
Boiler Savings Calculation
Data required:
• Either
Or
- annual heating energy and cost/kWh
- annual heating costs
Combustion efficiency, η
- Existing
- Proposed
η new − η old
Annual Savings =
× Annual heating cost (energy)
η new