BOILERS

BOILERS

FLAMEFLAMEBurner

CENTRAL HEATINGSystem Components

CENTRAL HEATING System

Combi-boiler

Radiators

Shower

Two-pipe systemThere is a flow and a return pipe.

Hydro-flow (or water circulation) Heating System

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.

CENTRAL HEATING

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.

Source: Fulton Boiler

Any boiler will be a condensing boiler when you bring cold water into it!

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

5 air = 1 Oxygen

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 combustion in a boiler, we must supply five units of air.

• 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

Air-fuel ratio AF (for the reactants)air

fuel

mm

= =

Stoichiometric → chemically correct

For the above reaction:

stoichAF 17.2air

fuel stoich

mm

⎛ ⎞= =⎜ ⎟⎜ ⎟⎝ ⎠

In practice, to overcome incomplete mixing of air with fuel, excess air, EA, is used.

,

,

EA 1air actual

air stoich

mm

= −

Insufficient air Unburned fuel in products

Reduced combustion efficiency

Increased hydrocarbon emissions

Too much air Reduced product temperature Reduced combustion efficiency

Fundamentals of Combustion

Percent excess air

Fundamentals of Combustion

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

4 – 5% oxygen (O2)21 – 28% excess air9,6 – 9,9% carbon dioxide (C O2)80 – 84% efficiency400 PPM carbon monoxide (CO)

• Fuel Oil4 – 5% oxygen (O2)22 – 29% excess air11,9 – 12,6% carbon dioxide (C O2)84 – 87% efficiency1 – 2 smoke test

Boiler EfficiencyChart

Data Needed:• Fuel type• % CO2

• % O2

• Stacktemperature

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:

• We can use this heat capacity to find answers to heating and cooling problems which involve only changing the temperature of liquid water.

C°kgkJ2.4

ExampleHow much heat is required to change the temperature of 3000 kg of 21°C water to 80°C water?

SolutionWater at 80°C is still liquid. Therefore,

= 206.5 kWh

kJ400,743=)°C2180(

kg°CkJ2.4kg3000

=kJX

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.

Calculate Steam Cost:1. Cost of Fuel =2. Efficiency of the Boiler =

Comments:

11. Use refuse as fuel / Install waste heat boiler

10. Keep boiler tube (water side) clean

9. Insulate bare steam lines

8. Heat recovery from boiler blowdown to pre-heat boiler feed water

7. Minimize boiler blowdown

6. Return steam condensate to boiler

5. Inspect & Repair Steam Traps

4. Eliminate Steam Leaks

3. Calculate the steam cost for different fuel & use lower fuel for combustion equipment

2. Install Stack Heat Exchanger (Economizer)

1. Adjust Air-fuel Ratio

FEASIBLE (Y/N)

Boiler Auditing Checklist

Boiler Savings Calculation

Data required:• Either - annual heating energy and cost/kWh

Or - annual heating costs

Combustion efficiency, η- Existing- Proposed

(energy)cost heatingAnnualSavingsAnnual ×−

=new

oldnew

ηηη