BOILER-EFFICIENCY AND PERFORMANCE TIPS

A boiler consumes a large amount of a facility's energy budget. Even a small decrease in a boiler's efficiency can cause a sharp increase in energy costs. To improve the efficiency of boiler heat generation:

• Optimize air-to-fuel ratio. A boiler requires just the right amount of oxygen to ensure an appropriate air-to-fuel ratio. Air consumes energy as it is heated. Thus, excess air/oxygen wastes energy, as heated air is released up the stack. If air/oxygen is insufficient, not all fuel will burn. The unburned fuel will move through the system, leaving behind soot. Additionally, too little air may cause a buildup of carbon monoxide and smoke.

By analyzing flue gas, one can measure oxygen and stack gas temperature and calculate boiler efficiency. Adjustments then can be made to optimize the level of excess air and the temperature of incoming air.

To optimize air-to-fuel ratio, one can use a computer-based distributed control system, which automatically controls a fuel burner to reduce oxygen levels as needed.

• Optimize water treatment. Before being pumped into a boiler, feedwater is treated to remove dissolved oxygen and other impurities that might cause corrosion or buildup of sediment, both of which reduce boiler efficiency. These impurities waste energy, as they necessitate routine boiler blowdowns. The cleaner the water going into a unit, the fewer boiler blowdowns that will be necessary.

• Clean heat-transfer surfaces. Soot buildup on boiler heat-transfer surfaces acts like an insulator and diminishes the efficiency of heat transfer between combustion gas and steam generation.

• Minimize heat loss. To recover waste heat from a stack, install an economizer. Heat then can be directed to boiler feedwater for preheating. Before installing an economizer, be sure a boiler system is cleaned and tuned so that an accurate measurement of stack gas temperature can be taken. Additional heat can be extracted from flue gas (below 300 F) using a condensing economizer. When a condensing economizer is used, caution must be exercised because a reduction in flue gas below the dew point will cause condensation, which can contain sulfuric or hydrochloric acid because of the sulphur, hydrogen, and chlorine in the fuel. These acids can significantly corrode the surfaces with which they come in contact.

• Install a stack-temperature gauge. A stack-temperature gauge indicates the temperature of flue gas leaving a boiler. The lower the temperature of flue gas, the more efficient the system. A high stack temperature indicates soot or scale may be building up in tubes or the baffling inside of the boiler may have deteriorated or burned through, allowing gases to bypass heat-transfer surfaces. These conditions generally develop slowly and unbeknownst to operators. Approximately 1 percent in boiler thermal efficiency is lost per 40 F increase in stack temperature.

• Recover condensate. Condensate drained from steam traps can be collected and used as boiler feedwater. This reduces boiler operating costs and usually is more cost-effective than using fresh utility water. Recovered condensate takes less fuel to convert into steam than fresh utility water does. The temperature of recovered condensate is 160 F to 200 F, while the temperature of fresh utility water usually does not exceed 80 F. The proper maintenance of steam traps throughout a distribution system can maximize the amount of condensate returned to a boiler, minimizing energy waste associated with feedwater heating.

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Sidebar: General Tips for Optimal Safety

• Test the functioning of all boiler controls.
• Test all drains and blow-offs to ensure they are functioning properly.
• Check the fuel system for leaks, and ensure all fuel filters and strainers are replaced regularly.
• Ensure all shut-off valves are leak-tight.
• Ensure all valves in instrument lines are functioning properly.
• Open the boiler vent valve or top tri-cock to vent air, and fill the boiler to the proper level with treated water. Check that the expansion tank is filled properly and the air-cushion volume is correct.
• Verify that the vent valve on gas-fired boilers is operating as required and the vent is not clogged.
• Verify that the flame scanner or sensors are connected and functioning properly.
• Check the settings of all instruments and safety devices. Ensure the water-pressure regulator functions as required.
• Ensure the recirculation pump works as required.
• Confirm there are no signs of overheating, corrosion, or erosion.
• Ensure all heating-system isolation valves are functioning properly.
• Verify there are no leaks from any part of a boiler or piping external to the boiler. Have any cracked surfaces repaired by a qualified repair company immediately.
• Test safety valves regularly. Replace leaking safety valves.
• Verify the low-water fuel cutoff works as required. Inspect it for proper sequencing.

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Stephen Kleva is president of Insparisk (www.insparisk.com), a national safety-inspection company and the parent company of City Spec Inc. City Spec inspects low-pressure boilers in New York City.

 

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