Four Tips for Trouble-Free Digester Heater and Heat Exchanger Operation

Digester heater and heat Exchanger maintenance is often overlooked and can lead to decreased performance and component failure. To prevent downtime, the following are four tips for keeping your heater and heat exchanger operating at peak performance.

Perform General Routine Maintenance

Routine maintenance is integral to long-lasting performance of mechanical components. At a minimum, the following should be performed on a monthly basis to ensure proper and safe operation of the unit.  

Grease blower bearings

• There are two bearings supporting the blower shaft on top of the heater/heat exchanger. Both bearings should be greased on a monthly basis per the manufacturer’s recommendation. Always use high temperature grease and consult vendor data in the O&M manual provided with the equipment.

Check blower belt(s)

• Inspect belt(s) for signs of wear or deterioration. Belts that are frayed, cracked or broken should be replaced.

Check each main gas line shut-off valve

• Ensure gas shut-off valves are operating properly. These are an important safety device and must be checked regularly. Consult vendor data for testing procedures.

Visual inspection for water leakage

• Visually inspect the unit for water leakage, which can occur at any gasketed joint including sludge tubes, hand holes or waterbacks.  If a leak is repaired, use touch-up paint to prevent corrosion.      

Clean Fire Tubes and Combustion Chamber 

Fireside corrosion occurs during operation when large quantities of sulfur are present in digester gas. If flue gas temperature is not kept above 350°F (177°C) or if the water bath temperature is not kept above 145°F (63°C), water vapor present in the flue gas will condense and sulfur based acids will form causing corrosion. The fireside of the fire tubes and combustion chamber should be brushed to remove all soot and deposits yearly or as determined by operating conditions. Cleaning of fire tubes and the combustion chamber minimizes downtime for the heating process.

Firing heating boilers intermittently during the shut-down period is not recommended. If extended shut-down is necessary, refer to the operations manual supplied with the equipment for the detailed procedure. Boiler tubes will provide many years of service when properly maintained. If severe scale and/or corrosion are encountered, consider seeking the assistance of a boiler maintenance or water treatment specialist to inspect equipment and make recommendations for repair or rehabilitation.

Clean Sludge Tubes  

Sludge tubes should typically be removed from service to clean the exterior every 3 to 4 years.  The condition of boiler water is paramount to the duration of this interval and boiler feed water should be treated as appropriate for local conditions. At a minimum the water should be slightly alkaline. If excessive amounts of make-up water are used on a regular basis, the exterior condition of the sludge tubes should be checked frequently.

The interior of the sludge tubes will also gradually become fouled and require cleaning.  This occurs when the operating temperature of the water bath reaches a maximum of 190°F (88°C) and the unit no longer meets the heating requirements.  To clean, sludge tubes can remain in place with no water draining required from the unit.  It is recommended that the interior of the sludge tubes are cleaned on a yearly basis to ensure the efficient operation of the fire tubes. 

Analyze Flue Gas 

Flue gas should be analyzed at least once per year, at any time combustion irregularities exist or if the quantity or quality of fuel-air mixtures change.  Potential problems that indicate a need for analysis include: the unit not delivering desired heat transfer, sporadic flame as viewed from the furnace tube sight glass, or the flame not staying lit.

The analysis should be performed using a commercially available direct reading flue gas analyzer under conditions where the water bath temperature is between 160°F (71°C) and 180°F (82°C). A well-performing system meets the following performance criteria: 

• Combustion efficiency in excess of 80%
• An O2 reading of less than 5%
• A CO reading of less than 1% or 0.

The air damper settings and the gas flow should be adjusted until these readings are obtained. If the desired performance cannot be achieved, the unit will most likely need to be taken out of service for cleaning.