What are extended surface tubes?
Extended surface tubes, also known as finned tubes, are heat transfer devices used in various applications such as heat exchangers, air coolers, and boilers. They consist of a tube with extended surfaces or fins attached to the outer surface.
The fins increase the surface area of the tube, which improves the transfer of heat between the tube and the surrounding fluid or gas. This increased surface area allows for more efficient heat transfer and can reduce the overall size and weight of the heat exchanger.
Extended surface tubes generally and regularly used for efficient cooling or heating. These extended surface tubes are used in air-cooled heat exchangers, shell and tube heat exchangers, Evaporator, condensers even used in furnaces. Extended surface tubes also called finned tube.
Fins are basically external or inserted on outside of tubes in order to increase the total surface area and increase thermal efficiency of compact heat transfer equipment. The method of connect the fins over the tube is very important, since any slightness, air gap between the tube and fins may create an additional resistance to heat flow. When the heat transfer coefficient on the outside of the tube is significantly lower than the heat transfer coefficient on the inside of the tube, there is a major advantage to incorporate fins on the outside tube surface to take full advantage of the high heat transfer rate on the inside of the tube.
If the fin and tube have different thermal coefficients of expansion, the bond may loosen due to a differential movement between them. The loosening effect will increase as the bond temperature increases and consideration must be given to the bond temperature which be attained should the fan fail. The bond may loosen due to the weakening effect of thermal and pressure cycling and also because of corrosion at the fin-base tube junction. Operating temperature limits for the various fin-tubes attachments that our company can offer are given in each section.
Extended surface tubes include Extruded, Integral, Bi-metallic Fin tube
Extruded extended surface tubes are formed in a cold rolling extrusion process, where continuous helical fins are radically extruded from a thick aluminum muff over a liner tube. The extruded fin is formed from an outer aluminum tube with a large wall thickness (muff), which is aligned over an inner base tube. The two tubes are pushed through three arbors with rotating discs that literally squeeze or extrude the aluminum fins up and out of the muff material in a spiral shape in one operation. The process of extrusion hardens the fins and prevents dissimilar metal contacts at the fin root. Compared to other products, fins do not break, unwind or become loose which affects greatly thermal efficiency. During the finning process a tight mechanical bond is made between the finned aluminum outer tube and inner base tube of the required metal, which is usually steel. The exposed external surface is aluminum and there are no minute gaps between adjacent fins where moisture can penetrate. As the outer tube and fins are integral with one on the temperature, lack of contact between outer and inner tubes can occur to create an additional resistance to heat transfer. The applicable temperature is up to 540 F degree (280 C degree)
Extruded extended surface tubes Advantages:
High-efficiency fins for higher temperature applications giving complete and permanent atmospheric corrosion protection of the base tube.
The extended fin is strong enough and resistant to mechanical damage and stand the handling shocks better, compared to other types extended surface tubess can be cleaned without damage using either steam or high-pressure water. The bonding of the outer and inner tubes removes the risk of loss of contact with the aluminium due to thermal stress.
The transfer coefficient remains steady, As the outer sleeve is continuous no electrolytic couple is created and no galvanic corrosion at the base of the fins can be accrued higher heat transfer performance for the life of the unit, No need to fix the fins at ends.
Extruded extended surface tubes weakness:
This is a costly manufacturing method because of needing a considerable amount of finning material. Total weight of equipment which use this kind of extended surface tubes will be bigger.
Maximum operating temperature limits to 280°C.
Extruded extended surface tubes application:
1. Oil and gas refineries
2. Petroleum, chemical and petrochemical industries
3. Natural gas treatment
4. Steel making industry
5. Power plants
6. Air conditioning
7. Compressor coolers
Embedded extended surface tubes or called G-type finned tube
Embedded extended surface tubes geometry:
A helical groove 0.2-0.3 mm (0.008-0.012 in) is ploughed into the outside of the base-tube wall such that metal is only displaced, not removed. The aluminium fin is wound mechanically into the groove under tension, after which the displace metal is rolled back on each side of the fin to hold it in place. That is why this type is also called embedded extended surface tubes. The affective thickness of the base-tube wall is the thickness beneath the groove . This type provides excellent contact, both thermal and mechanical, between fin and groove. Although the base-tube metal is exposed to the atmosphere, tests under server conditions showed that corrosion over a long period is required before any bond weakness occurs.
The G-type extended surface tubes is applicable to high temperature up to 150 F degree (400 C degree)
Embedded extended surface tubes main Application:
1. Oil and gas refineries
2. Petroleum, chemical and petrochemical industries
3. Natural gas treatment
4. Steel making industry
5. Power plants
6. Air conditioning
7. Compressor coolers
Embedded extended surface tubes advantages:
The fin/tube wall contact is constant because of the setting and makes it possible to use a wall temperature of up to 400°C.
The fin is set throughout its length and consequently does not unwind even when partially uprooted.
This kind of extended surface tubes is one of optimum choices for having good effectiveness/cost ratio.
Embedded extended surface tubes weakness:
The fin is not so strong to resist mechanical damage when external forces applied on fin area
Handling shall be done with care to avoid any damage.
Extended surface tubes may be damaged while using either steam or high-pressure water for cleaning
As the fins are helically wrapped in grouves, un-finned area is not covered which can be exposed to corrosive media and galvanic corrosion at the base of the fins can be accrued
Tube shall be straight with smooth out side area to make a good extended surface tubes
It is hard to use core tube again when finning is failed.
Fins should be fixed at both ends avoiding un-wrapping
L-Footed shape extended surface tubes
L-Footed shape extended surface tubes geometry:
The L-shaped aluminum extended surface tubes are standard fin for low-temperature application where some degree of tube wall protection is required. The fin is wound mechanically under tension on to the base tube. The short horizontal limb of each L is in contact with the base-tube surface to cover it completely. This construction provides greatly improved fin-tube contact over the knurled base tube. It has been demonstrated by tests that a greatly superior fin-tube contact is achieved by knurling the outside of the base tube and foot of the L-shaped fin. In both designs the whole of the exposed external surface is aluminum. L Foot or shoulder tension construction assures uniform fin spacing, rigidly interlocked by the fin radius overlap over the exposed shoulder edge. The applicable temperature is up to 270 F degree (130 C degree)
L-Footed shape extended surface tubes advantages:
Suitable for low-temperature application
The finning type is the most cost effective method compared to the others.
The failed fins can be easily removed during production and bare tube can be used again for finning.
There is no need for make a helical grove over the core tube.
It is possible to place the fins on a very thin-walled tube with is particularly desirable when using expensive alloys as core tube.
The tube can withstand a temperature of up to 130°C without the risk of atmospheric corrosion or thermal stress.
L-Footed shape extended surface tubes weakness:
The fin is not so strong to resist mechanical damage when external forces applied on fin area
Handling shall be done with care to avoid any damage.
Extended surface tubess may be damaged while using either steam or high-pressure water for cleaning
As the fins are helically wrapped in grooves, un-finned area is not covered which can be exposed to corrosive media and galvanic corrosion at the base of the fins can be accrued
Tube shall be straight with smooth out side area to make a good extended surface tubes
Fins should be fixed at both ends avoiding un-wrapping
L-Footed shape extended surface tubes application:
1. Oil and gas refineries
2. Petroleum, chemical and petrochemical industries
3. Natural gas treatment
4. Steel making industry
5. Power plants
6. Air conditioning
7. Compressor coolers
Extended surface tubes |
