Finned Tube----High Pressure Air Cooler

Finned Tube----High Pressure Air Cooler

( A ) High-pressure air coolers work expertise and material selection

( 1 ) High-pressure air coolers function and work environment characteristics

After the hydrogenation reaction products by high-pressure heat exchanger 3-5 from the reactor effluent reaches the high pressure air cooler, where it is further cooled , the temperature is lowered to about 50 ℃. Then enters the high pressure separator liquid separation .

Work is characterized by high-pressure air cooler : normal operating temperature is not high (200 ℃ or less ) , but in cases of emergency venting , there will be a lot of hot gas without adequate heat (> 200 ℃) in a short time through the air cooler . Thus requiring finned tube air cooler has good impact resistance.

( 2 ) Material selection high-pressure air coolers

When the media into the air cooler temperatures generally has dropped below 200 ℃ . At this temperature , the hydrogen has no significant corrosion of carbon steel for general effect , so most of the high-pressure air cooler tubes are carbon steel timber . Hydrogen corrosion is hydrogen at high temperatures cause long-term damage accumulation process of steel. So even if there are short-term high-temperature ( > Red D ℃) and gas through the air cooler ( emergency venting ) , nor will it pose a threat to carbon steel tubes . But some hydrogenation process there will be some action in warm air cooler , its normal operating temperature greater than 240 ℃. At this point can not resist hydrogen corrosion of carbon steel . Thus should be selected anti- corrosive hydrogen to produce better CrMo steel tube box and base tube . But in the harsh conditions of the air-cooled tube boxes and tubes , some foreign companies , such as the use of duplex stainless steel or corrosion Incolloy825 stronger material.

(B ) The characteristics of finned tube air cooler and Scope

Five typical finned tube shown in Figure 5-4-6 . Due to the different processing methods of these five finned tube , so that the extent of snapping finned tubes with different groups , resulting in different gap thermal resistance , resistance to thermal fatigue is different , the ultimate long- term operation of the heat transfer performance is decreased after different degrees . To make the finned tube heat transfer performance can still maintain good long-term use , countries regulate temperature limit use different kinds of finned tubes made ​​provision . China's " air-cooled heat exchanger " standard GB 15386 finned tubes specified in the maximum temperature in Table 5-4-3 .

Table 5-4-3 maximum temperature of various forms of finned tubes 

The so-called thermal fatigue is what we often see the fins loose on the base tube . Air cooler fins to increase the surface area for heat transfer tubes and settings. Finned aluminum for good heat transfer performance is because of aluminum , and has good ductility , ease of molding . As the name suggests , wound finned tube is wrapped around the aluminum fins on the outside pipe. In order to further strengthen the snapping of the aluminum fins and base pipe and aluminum fins wrapped roots in extrusion knurled to form a knurled wound finned tubes (KL type ) . The aluminum cover on the base tube , aluminum tube from the outside with a mechanical compression methods , while forming fins , while the aluminum rolling on the base tube , rolling it into a bimetallic finned tubes . On the base of the groove pipe processing , and then the aluminum fins inserted and fixed in the recess , the finned tube can be obtained mosaic . This manufacturing process requires special equipment .

 

Aluminum fin molding process is relatively simple, cost is not expensive. However, due to the expansion coefficient of steel is larger than that of aluminum (150 ℃ , the carbon steel as 11.88x10-6mm/mm ℃; aluminum 23.94x10-6mm/mm ℃). When the heated aluminum fins , aluminum fins expanded volume, and a relatively small amount of expansion of steel , which makes the aluminum fins and a gap between the base steel pipe to increase . Ideally, after cooling aluminum fins and the gap between the steel substrate tube should be restored to its original working state. However, research and practice show that this gap can not be fully restored to its original working state. Especially after repeated thermal cycling , which is opened after several stoppages , aluminum fins with large gaps between the base steel pipe will be retained. Especially when an emergency venting device , there will be a lot of high temperatures without adequate heat and gas (> 200 ℃) quickly through the air cooler , the greater the impact of this heat to harm finned tubes , aluminum fins and steel the gap between the rapidly expanding base tube (resulting gap increased thermal resistance ) , and even loose , so that the heat transfer efficiency. If a finned tube after repeated thermal cycling , thermal gap decreased much , then it 's a good thermal fatigue resistance .

Thermal fatigue performance of the best mosaic finned tube , followed by : Bimetallic rolling finned tubes with knurled wound finned tubes, double L-shaped wound finned tubes, finned tubes ordinary wound . Due to the different processing techniques and costs, which are five common finned tubes finned tubes mosaic is also the most expensive , while the cheapest is the common wound finned tubes . Therefore, you should use the occasion , to select the appropriate operating temperature , more economical finned tubes .

Air-cooled with a high-pressure hydrogenation unit 's normal operating temperature is about 150 ℃. But when an emergency venting device , there will be a lot of high temperatures without adequate heat oil ( greater than 200 ℃) huge thermal shock quickly through the air cooler, to the finned tubes . Coupled with the annual open stoppage caused great impact on the gap finned tube heat resistance . If you use an ordinary wound finned tubes, after several cycles of operation , will produce the phenomenon of cold is not down , as well as affect the operation of downstream equipment . Therefore, the current high-pressure air coolers generally use double metal rolling finned tube . This is because the bimetallic finned tube rolling long-term operation in the following 200 ℃, maintain good heat transfer performance , but also short-term resistance higher than 200 ℃ thermal shock , and affordable , ranging from wound finned tubes and mosaic between finned tubes .

Tests show that the thermal fatigue properties of extruded bimetallic finned tube significantly better than LL -type wound finned tubes . That is, after a few stoppages or emergency vent opening , the contact resistance bimetallic finned tube rolling increment is small, the heat transfer performance degradation is small. Therefore , bi-metal rolling finned tubes in the reaction product of hydrogenation unit air cooler , you can make use of the device after a long period can still maintain good heat transfer performance.