HPAC Magazine

Safety, Savings and Sustainability

Simple test offers several benefits.

December 1, 2014   By RON AUVIL

Many commercial buildings have boilers, heat exchangers and appliances that burn fossil fuel. Combustion analysis helps ensure that these devices operate in both a safe and efficient manner.

The top three goals of combustion analysis are:

1. Fuel Efficiency: Keeping fossil fuel burning equipment operating at peak efficiency reduces energy consumption and saves money. Tens of thousands of dollars in savings are possible using combustion analysis to correct problems and increase combustion efficiency.

2. Safety: Combustion problems are one of the main causes of boiler explosions. Combustion analysis can help identify combustion problems and correct them before they damage life or property.

3. Emissions:Combustion analysis helps reduce emissions that may be harmful to the environment.

COMBUSTION BASICS

Combustion occurs when a fossil fuel such as natural gas, oil, coal, or propane comes into contact with oxygen in the air in the presence of heat to ignite. The heat energy released from combustion is used to heat water, generate steam, or heat air in the facility.

Chemically, combustion can be understood by carbon, hydrogen, oxygen, nitrogen, and other compounds such as sulfur entering the combustion process. Heat, carbon monoxide (CO) carbon dioxide (CO2), water vapour (H2O), and various possible emissions such as nitrogen oxide (NO) are leaving the combustion process. Combustion itself depends on the ‘3 Ts’ of time, temperature and turbulence to permit good combustion.

Categories of combustion

There are three categories of combustion. These categories are known as incomplete, perfect, and complete combustion. These categories depend on the amount of air combined with the fuel.

1. Incomplete: Not enough air is present to combine with the fuel, causing carbon monoxide to be present in the exhaust. Carbon monoxide is highly toxic to people and also leads to energy waste.

2. Perfect: The exact amount of air needed to burn the fuel is provided. This is known as the theoretical amount of air.

3.Complete: Extra air is provided above the theoretical amount to burn the fuel. This extra air is known as excess air. In this instance, carbon dioxide (CO2) is present in the exhaust instead of CO. The downside of excess air is that it leads to a decrease in efficiency.

Fuel Types

Fossil fuel types include natural gas, fuel oil, propane and coal. Today, natural gas is the most common fossil fuel. Coal is in decline due to stricter emissions regulations. Fuel oil is used in some locations due to ease of transport and pricing. In addition, fuel oil is often used as a backup in some boiler plants as well. In some locations propane may be the favoured fuel of choice, especially in smaller applications. Each fuel has different composition and combustion parameters.

Air Types

Air used in the combustion process can be categorized as primary, secondary, excess and dilution. Primary air is that which is introduced with the fuel at the burner. Secondary air is air which enters the combustion process by entering the boiler through cleanout doors and other areas of the boiler. Excess air is that which is used to obtain complete combustion. Dilution air is introduced at the draft hood and is not part of the combustion process. Its purpose is to control the draft at the stack and reduce condensation.

Draft

Draft is the measurement of gas flow through the combustion process. Draft can be separated into different categories as well. Natural draft refers to the fact that hot gases rise through a chimney or flue. Natural draft does not use a fan. Forced draft and induced draft both use fans. A forced draft is the use of a fan at the burner itself while induced draft is the use of a fan in the stack to pull the hot gases through the combustion chamber. Draft is measured as a pressure at the stack immediately at the boiler or other fossil fuel burning device.

Tune Up Tips:

1. Always follow all safety precautions

2. Warm instruments up before using them.

3. Make sure instruments are calibrated as required.

4. Allow unit to start and warm up before taking measurements. Allow a minimum of 15 minutes before taking measurements.

5. Take measurements across different firing rates over a period of time.

6. Change burner adjustments and linkages to obtain correct excess air amount, usually around 15 per cent for gaseous fuels but different for other fuels.

7. Watch for poor flame or producing smoke, may indicate not enough excess air.

8. Always use manufacturer literature to make proper adjustments.

9. Record combustion efficiency values and adjustments to evaluate the boiler for the next combustion
analysis test.

Combustion Analysis Instruments

Combustion analysis measuring devices can be portable or permanent. Smaller and mid-size boilers will use a portable “kit” of measuring instruments designed for combustion analysis. Larger applications will have permanently mounted instruments that give continuous readouts of the different measurements. In addition, the ability to record and print out data for documentation and evaluation is available for both.

It should be noted that the instruments may require calibration at intervals to maintain their accuracy. Often this is done with portable calibration instruments, or in some cases by returning the instrument to the factory for calibration.

Measurement Do’s and Don’ts:

Do’s:

1. Select an instrument with accuracy less than one per cent of the range of the sensor for that condition

2. Select a flue gas temperature instrument with a range maximum of more than 600F 

3. Maintain documentation of efficiency of each boiler

4. Maintain calibration documentation

Don’ts

1. Try to save money by purchasing a low quality instrument. ‘You get what you pay for’ is true here. Inaccurate instruments can return either a false negative – not detecting inefficiencies – or a false positive, instigating un-necessary repairs.

2. Abuse the equipment. Store instruments in a clean, dry place when not in use. This will prolong the life of the measuring equipment.

Typical Measurements

CO: Measuring ambient and exhaust carbon monoxide indicates the type of combustion and the presence of CO above and beyond the a
mbient level. Ambient CO measurements can be taken with either a CO-specific measurement tool or a multi-function Air Meter.

CO2: The CO2 level in the exhaust is measured to indicate complete combustion, efficiency and the amount of
excess air.

O2: Measuring oxygen in the flue gas indicates the amount of excess air. The findings are used to adjust excess air amounts and increase boiler efficiency. A measurement of six per cent O2 indicates 40 per cent excess air.

Flue Temp: The temperature of the flue gas is usually taken with a thermocouple. It is taken right at the flue before any draft diverters and after it has passed through all the heat exchangers. Use a high quality thermocouple thermometer to check both the ambient inlet and flue temperature.

Inlet Air Temp: The inlet air temperature at the burner is taken as well. The difference between inlet and flue temperature is used in efficiency and combustion calculations.

Draft Pressure: One of the most important measurements is the draft pressure in inches of WC. This is taken at the flue close to the device. This measurement, when compared to manufacturer’s specifications, helps establish proper combustion rates. Use a handheld micromanometer to measure draft pressure.

NO and NO2: These compounds are considered pollutants and can cause environmental problems such as acid rain. They are very tightly regulated by federal, provincial, and local agencies.

SO2: Sulfur dioxide can be in the exhaust gases as well. It can form sulfuric acid when combined with water vapour. It is considered a pollutant and is tightly regulated as well.

MULTIPLE PAYBACK

Unfortunately, combustion analysis is ignored in many facilities. Combustion analysis can save tens of thousands of dollars in fuel. It can also lead to a safer combustion process and help the environment by reducing air pollutants. In addition, it can reduce the likelihood of violating environmental laws with steep fines. The money saved will pay for the instruments in a short period of time. <>

Ron Auvil has more than 30 years of experience in the classroom where he has taught everything from basic electricity and refrigeration to steam plant licensing and HVAC Controls. Auvil is a technical contributor to Fluke Corporation. 
www.fluke.com/fluke/caen/home

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