Is It Worth Replacing Old Air Preheaters?
By Bob Dubil
We recently were asked our thoughts regarding whether it was worthwhile to consider replacing an old air preheater with one of today's new designs. The focus of the question revolves around how much improvement in preheater efficiency can be expected. The following summarizes items to consider.
- If you have a fired heater or boiler without an air preheater, the unit efficiency can usually be increased to 90-92% by installing an air preheater.
- If you already have a furnace or boiler with an older air preheater, and are looking for improvement, you may have some air preheater options.
- Rotary regenerative. This is the Ljungstrom type, which has a slow-turning rotor that is packed with closely spaced heat transfer surfaces, similar to an automobile radiator. The rotor turns in the gas streams, picking up heat from the flue gas and transferring it to the combustion air. The element temperatures generally operate about halfway between the flue gas and air temperatures.
- The advantages of the rotary regenerative air preheater are:
- Smaller footprint and lighter structure, which usually results in lower installed cost.
- Replaceable heat transfer surfaces. Elements are designed in pie-shaped “baskets” which are relatively easy and economic to replace and maintain performance.
- Disadvantages of the rotary regenerative air preheater are:
- Has seals that leak, and leakage generally increases with time. Normal, as-built leakage to be expected is about 15%. This can limit the lowest cold-end temperature achievable, especially if cold-end corrosion is a concern due to firing sulfur-bearing fuels. Enamel coating of the cold-end metallic surfaces can prolong element life.
- Due to air-leakage, the potential for fires may be greater. If poor combustion occurs in the fired heater, unburned combustibles may accumulate in the air preheater. The possibility of fire in an air preheater increases if one air preheater is shared with multiple furnaces.
- Fixed recuperative. These are generally called static air preheaters. Their designs vary from a plate-frame type to a cast-iron “tubular” type.
- The cast-iron type generally is larger and heavier, such that the foundations and structures become costly. The newer plate-frame type (sometimes called the Open Channel Air Preheater, "OCAP") are lighter, and often can fit in a similar footprint as the rotary-regenerative type. This makes them a good choice for replacing a rotary type.
- The cast-iron type has finned elements at the cold-end, which can be biased to the flue gas side, thus keeping the metal temperature above the average of the flue gas and air temperatures. This reduces the tendency for cold-end corrosion, especially when firing sulfur-bearing fuels. In addition, glass-tube bundles have been used downstream of cast-iron air preheaters to maximize efficiency.
- Like the rotary type, the plate-frame APH metal temperature will be the average temperature of the two streams.
- The cast-iron type air preheaters are generally bolted and sealed such that leakage can be considered nil. The plate-frame type sealing method differs depending upon manufacturer. Some use spring seals, which have not been 100% successful. Others use a welded seal construction, which provides a better, but more costly seal.
- The OCAP heat transfer surfaces are not designed for easy replacement. Instead, entire replacement of the air preheater would be necessary.
The bottom line is that both types of air preheater have offsetting advantages and disadvantages. There have been little design changes in recent years, which have increased air preheater efficiency. However, most efforts seemed aimed at reducing manufacturing costs. It seems that most recent installations have been fixed-recuperative type, rather than rotary-regenerative type. I believe that this is due to lower first-cost and a willingness to operate the air preheater to failure, rather than spend more initially, and perform the continued maintenance necessary to maximize unit life.