Air preheater

What is air preheater?

 

Air preheater, also known as APH, is a gas-air heat exchanger to preheat combustion air for power plant steam boiler, refinery fired heater or industrial furnace. It is a critical equipment designed to improve thermal efficiency by flue gas heat recovery.

 

 

Function of air preheater:

 

1. Reduce exhaust temperature, enhance boiler efficiency, save fuel consumption. Air preheater could extract heat from flue gas to preheat cold ambient air. Flue gas temperature is reduced accordingly. Data shows that when exhaust temperature goes down by every 10℃, fuel consumption would go down by 1%.

2. Increase air temperature, enhance combustion condition, reduce incomplete combustion loss. Combustion air at elevated temperature could accelerate fuel drying, ignition and burning process. It enables a stable combustion and improves combustion efficiency.

3. Increase furnace temperature to intensify heat transfer by radiation. Under certain boiler capacity, air at higher temperature means smaller heating surface inside furnace and less metal usage.

4. Lower flue gas inlet temperature creates better working condition for induced draft fan.

5. For coal fired power plant, hot air is also medium for drying and conveying of pulverized coal.

 

Types of air preheater

 

APH could be classified into basically two types: recuperative air preheater and regenerative air preheater. Recuperative air preheater includes tubular air preheater, plate type air preheater, heat pipe air preheater. Regenerative air preheater is also called rotary air preheater.

 

Tubular air preheater uses thin wall tubes as heat transfer element, including acid-resistant steel tube (Corten steel, ND steel), enamel coated steel tube, borosilicate glass tube. To improve heat transfer efficiency, turbulator would be used if necessary.

 

 

For acid-resistant steel tube air preheater, parallel tubes are welded to tube sheet at two ends. Flue gas passes through tubes vertically, while combustion air passes crosswise.

 

For enameled tube air preheater, combustion air passes through tubes, while flue gas passes crosswise. And it is featured with smooth surface, erosion & corrosion resistant.

 

Glass tube air preheater has some unparallel advantages, except all the features mentioned above as enameled tube APH. For a metal tube/plate APH, metal temperature should be higher than flue gas dew point. Thermal recovery is limited considering dew point corrosion. But glass tube air preheater could work safety under 100℃ for a long term due to its excellent resistance to acid. Moreover, it has good thermal ability and resistance to thermal shock at 400℃. Although glass tube is fragile, borosilicate glass gives tubes a high elasticity. And online washing could take away soot deposit to keep its heat transfer efficiency.

 

 

Although tubular air preheater has advantages like simple structure, easy fabrication, but it is no more suitable for units with large capacity because of its large size. Following APHs will give you more options.

 

Flue gas from fired heaters in refinery and petrochemical plant at 250-400℃ is a huge potential for heat recovery.

 

Due to heat transfer by evaporation and condensation, heat pipe has high thermal efficiency. And fins on heat pipe could spread heating surface. So, heat pipe air preheater is compact in size. If certain heat pipes are broken, partition plate in the middle could avoid flue gas or combustion air leakage into the other side, operation of whole APH is not influenced. By changing heat transfer area in hot/cold end, we can get controllable temperature to avoid dew point corrosion.

 

 

Plate type air preheater has typical structure of plate heat exchanger. Material selection of metal plate could be different in different situation. When flue gas temperature is higher than 250℃, smooth or corrugated plate made of high nickel alloy is an option. All-welded structure gives it a good sealing performance. When flue gas temperature is lower than 250℃, finned plate made of grey cast iron is a better choice. Gilled heating surface gives it higher heat transfer ability. Cast iron has advantages when used for heat exchanger.

 

1. Inert to acid corrosion.
2. Good resistance to fly ash erosion.
3. High thermal conductivity.
4. Good ability to bear high pressure.
5. Good elasticity.

 

So, cast iron air preheater with external fins air side and internal fins flue gas side achieved remarkable success in flue gas heat recovery in refinery & petrochemical industry. It is featured with long service life, low pressure drop, compact structure, high efficiency, etc. In practice, flue gas temperature could be reduced to 90℃. Compared with other APHs, its thermal efficiency is 15-20% higher. It could run reliably for 10 years with flue gas temperature range 140-650℃.

 

 

Along with growing capacity of power plant boiler, heating surface of traditional tubular air preheater is getting bigger and bigger. Arrangement of APH is becoming difficult because of its large size. Rotary air preheater with compact structure is more popular for large capacity boiler. Heating surface density of rotary APH is up to 500m2/m3, but its size is only 1/10 of tubular APH of the same capacity, and metal usage of rotary APH is only 1/3.

 

 

We have two, three or four sector design with rotary air preheater. Flue gas and combustion air pass through heating elements alternately. Heat from flue gas is stored in heating elements, and then passed to combustion air. Fabricated with leading technology in the world, our rotary air preheater with diameter up to 30 meters could serve 50MW~1500MW boiler unit. Now we have more than 2000 sets of rotary APH in operation.

 

Dew point corrosion in air preheater

 

Although APH could reduce flue gas temperature to save energy, special attention must be paid to dew point corrosion when fuel contains sulfur. Fuel combustion would produce sulfur dioxide, and by SCR catalyst is turned into sulfur trioxide, which will react with steam and change into acid vapor. When flue gas temperature is above dew point, acid vapor has barely no damage to heat transfer element. Otherwise, dew point corrosion happens. A steam air preheater can be put into operation for preheating of air before entering APH in winter season. Or, some corrosion resistant materials could be used against this problem, such as enamel, glass, Corten steel, ND steel (09CrCuSb), cast iron, etc.

 

Air preheater cleaning methods

 

Air preheating cleaning is a periodical job in power plant or refinery. Fouling on APH would cause degraded heat transfer, bigger pressure drop, more power consumption on ID & FD fan, or even unscheduled shutdown. APH fouling has two types: loose fly ash deposit or hard, tenacious buildups. When temperature is lower than 280℃, ammonia escaped from SCR process will react with SO3 and form NH4HSO4, which is liquid phase under temperature 146～207℃. Liquid NH4HSO4 is sticky and has high ability to catch fly ashes. They will bond together and form molten-salt deposit in air preheater. In industrial practice, various APH cleaning methods are used:

 

• Sonic horn
• Online washing
• Chemical cleaning
• Steam soot blower
• Shock wave soot blower
• Pneumatic pulse soot blower
• Hydro jet cleaning with pressurized water

 

Each cleaning method has its features and application. Pls choose a method based on deposit and APH types.

 

Advantages of rotary air preheater:

 

1. Low energy consumption.
2. High reliability.
3. Compact structure, less metal usage.
4. Excellent leakage control.
5. Corrugated, enameled heating elements.
6. Excellent adaptability for SCR at full load.

 

Air preheater manufacturing

 

GeckoIntech have a long history in the design and fabrication of air preheater for boiler and petrochemical fired heater. Extensive product range gives us capability to deliver various APH for different types of boiler. Our expertise and experience will make your equipment running at a higher efficiency.