The focal point of every economizer is the heat exchanger, which uses waste heat from the boiler exhaust to preheat incoming cold water. By transferring heat from the exhaust to the incoming boiler water or water used for secondary processes, an economizer can allow a processor to reduce overall energy costs.
Newer designs of economizers have smaller heat exchangers with more plate fins per inch and boilers fired with natural gas. This winning combination makes new and existing boilers operate even more efficiently. In addition, advanced controls for economizers can reduce emissions from boilers, bringing them into compliance with environmental standards.
Economizers are used with boilers in pulp and paper production, food processing, electric power generation and other process applications as well as HVAC equipment. Some are used with high-pressure fluids pumped through the tubes in heat exchangers and some are used for low-pressure fluids. Many are used in dirty-air environments and some for corrosive atmospheres or liquids.
Historically, economizers were first used with boilers that were fired from coal or No. 6 diesel oil. Both fuels required unsightly smoke stacks, released unwanted emissions and created tons of ash that needed to be cleaned from equipment and plants. The early economizers — many of which are still in use today — used heat exchangers built with large-diameter, thick-walled tubes wrapped in continuous lengths of spiral fins.
Typically, spiral-fin heat exchangers were used to ensure that water temperatures would never fall below 270°F (132°C). Exceeding this temperature helped minimize condensation that could create sulfuric acid when mixed with sulfur dioxide from burning diesel fuel. Wider fin spacing was needed to make cleaning out the ash easier and to prevent clogging. In addition, the older model standard economizers often weighed in at 25,000 to 30,000 lb and stood 6’ tall or more.
Today, clean burning, low-cost natural gas is the energy source of choice for firing boilers. In addition, burning gas allows the use of more plate fins per inch on heat exchanger tubes in an economizer, which in turn provides more heat transfer. These more efficient economizers with plate-fin heat exchangers weigh less and are smaller in size than spiral-fin designs. The compact size allows them to be more easily retrofitted to older boilers.
When selecting the type of economizer and heat exchanger for your operation, it is important to consider the application carefully. Fluid flow rates on packaged boilers are usually not excessive and do not require large heat exchanger tubes in an economizer. Two important factors to consider include:
• The size of the tube, which is dependent upon the fluid flow rate. Tubes with a 0.625" outside diameter (OD) can handle flow rates well over 100 gal/min with the correct tube circuiting.
• The walls of the tubes in the coil are dependent on the pressure of the fluid and internal erosion of the tubes. Normally, the latter is not an issue for economizers because the fluid is clean and treated. Carbon steel tubes with a 0.625" OD and a wall thickness of 0.049" can handle pressures higher than 1,500 psi with a safety factor of 4.0.
Smaller sized tubes allow more tubes to be used in a heat exchanger and provide more internal surface than fewer large tubes. A greater number of smaller tubes mean more primary surface for heat transfer. Smaller tubes also mean higher velocities of the fluid passing through the tubes. More primary surface and increased tube velocities increase heat transfer performance and result in a more efficient, less costly economizer.
Economizers with plate-fin heat exchangers are becoming the norm. Besides all of the reasons already mentioned, other reasons to use plate-fin heat exchangers for economizers include:
Economizers with plate-fin heat exchangers are becoming the norm.
• The number of fins per inch can be tightly controlled and specified to custom dimensions such as 5.5 or 6.5 fins per inch. This allows the heat exchanger to meet the heat load without being oversized (or overpriced).
• Manufacturers have a range of tube sizes and patterns, and selection programs can be used to design on exact requirements.
• There is more cooling surface on a plate fin and they are more efficient.
• They offer close fins per inch to dissipate more heat in the same amount of time than spiral-fin designs.
• The heat exchangers can be removed from the economizers for cleaning and inspecting.
• The heat exchangers can be circuited or designed for maximum performance.
Modern high efficiency heat exchangers are transforming economizers. If your process could benefit from capturing and reusing waste heat to preheat incoming cold water or other processes, give plate-fin heat exchanger economizers another look.
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