Types of tubes employed in heat exchangers
Thursday, April 4, 2013
Types of tubes employed in heat exchangers
There are multiple different kinds of tubes used in the construction of a heat exchanger.
Type 1: Plain tube. There are several (3) forms of plain tubes which are used in
the construction of a heat exchanger: (a) Straight tubes; (b) U-tubes with U-bend; (c)
coiled tubes. Furthermore, plain tubes can be welded or seamless. The welded tubes are
made from strip material which is rolled into cylindrical shape and then automatically
welded per specification. The seamless tubes are subdivided into two types: extruded or
hot pierced. There are numerous materials which can be used in making the welded and
seamless tubes, however copper and copper alloys are used only in the manufacturing of
the seamless type. Because seamless tubes are considered to be of better eminence, they
are more costly than the welded type. One of the reasons who one may use plain tubes
in the making of the heat exchanger is that in comparison to other types of tubing it is
cheapest and easiest to handle.
Type 2: Finned tubes: These tubes get their name from the way they look. These
tubes in actuality have fins attached to their tubular surface. The fins can be attached
either on the outside or the inside of the tube. The fins are positioned in the following
positions: longitudinal, radial, or helical. Finned tubes are idea for use where at least one
of the fluids in the heat exchanger is gas.
Type 3: Duplex tubes aka bimetallic tubes—the bimetallic tubes are made of
two separate tubes which are made of different materials. One of the tubes is larger in
diameter than the other. The smaller tube through mechanical means is placed within the
larger tube ensuring that there is no gap between the tubes. To avoid the presence of a
gap, shrink fitting is used. The bimetallic tubes are often used in areas where other types
of tubes and metals are not compatible with the corrosive nature of the tube-side and
shell side fluids. Hence when selecting metals for the construction of the tubes one
should consider the rate of corrosion to ensure that both tubes will corrode at similar rate.
There are roughly over one hundred different types of material combinations available.
In addition to considering the material used to construct the bimetallic tube, one should
also pay close attention to selecting a tube with ideal and compatible thickness. Because
there is a perfect contact between the inner and the outer tubes, the thickness of the tubes
should be based on the basis of strength of the softer of the two materials.
Type 4: Enhanced surface tubes. These tubes increase the heat transfer coefficient
which is accomplished by two techniques:
a. The surface of the tube is contoured and or grooved so that ridges and
valleys are formed. Such tubes are often used in condensers. The increased heat
transfer coefficient results from the condensate floating into the valleys leaving
only a thin film on the ridges.
b. The surface is prepared from special coating providing large number of
nucleation sites which are to be used in boiling operations. For the maximum heat
transfer it is suggested that the tubes should have the largest possible surface area
per unit value. The professionals in the field of heat exchangers have determined
that cylindrical tubes are preferred to non cylindrical although some of the non-
cylindrical tubes have larger surface area per unit volume. Cylindrical tubes
are ideal at withstanding the fluid pressure. However, at low pressure settings,
especially in dealing with gases, non-cylindrical tubes are best to be used.
There are multiple different kinds of tubes used in the construction of a heat exchanger.
Type 1: Plain tube. There are several (3) forms of plain tubes which are used in
the construction of a heat exchanger: (a) Straight tubes; (b) U-tubes with U-bend; (c)
coiled tubes. Furthermore, plain tubes can be welded or seamless. The welded tubes are
made from strip material which is rolled into cylindrical shape and then automatically
welded per specification. The seamless tubes are subdivided into two types: extruded or
hot pierced. There are numerous materials which can be used in making the welded and
seamless tubes, however copper and copper alloys are used only in the manufacturing of
the seamless type. Because seamless tubes are considered to be of better eminence, they
are more costly than the welded type. One of the reasons who one may use plain tubes
in the making of the heat exchanger is that in comparison to other types of tubing it is
cheapest and easiest to handle.
Type 2: Finned tubes: These tubes get their name from the way they look. These
tubes in actuality have fins attached to their tubular surface. The fins can be attached
either on the outside or the inside of the tube. The fins are positioned in the following
positions: longitudinal, radial, or helical. Finned tubes are idea for use where at least one
of the fluids in the heat exchanger is gas.
Type 3: Duplex tubes aka bimetallic tubes—the bimetallic tubes are made of
two separate tubes which are made of different materials. One of the tubes is larger in
diameter than the other. The smaller tube through mechanical means is placed within the
larger tube ensuring that there is no gap between the tubes. To avoid the presence of a
gap, shrink fitting is used. The bimetallic tubes are often used in areas where other types
of tubes and metals are not compatible with the corrosive nature of the tube-side and
shell side fluids. Hence when selecting metals for the construction of the tubes one
should consider the rate of corrosion to ensure that both tubes will corrode at similar rate.
There are roughly over one hundred different types of material combinations available.
In addition to considering the material used to construct the bimetallic tube, one should
also pay close attention to selecting a tube with ideal and compatible thickness. Because
there is a perfect contact between the inner and the outer tubes, the thickness of the tubes
should be based on the basis of strength of the softer of the two materials.
Type 4: Enhanced surface tubes. These tubes increase the heat transfer coefficient
which is accomplished by two techniques:
a. The surface of the tube is contoured and or grooved so that ridges and
valleys are formed. Such tubes are often used in condensers. The increased heat
transfer coefficient results from the condensate floating into the valleys leaving
only a thin film on the ridges.
b. The surface is prepared from special coating providing large number of
nucleation sites which are to be used in boiling operations. For the maximum heat
transfer it is suggested that the tubes should have the largest possible surface area
per unit value. The professionals in the field of heat exchangers have determined
that cylindrical tubes are preferred to non cylindrical although some of the non-
cylindrical tubes have larger surface area per unit volume. Cylindrical tubes
are ideal at withstanding the fluid pressure. However, at low pressure settings,
especially in dealing with gases, non-cylindrical tubes are best to be used.
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