GB1589262A

GB1589262A - Tubular heat exchangers

Info

Publication number: GB1589262A
Application number: GB3276576A
Authority: GB
Original assignee: Babcock International Ltd
Current assignee: Babcock International Ltd
Priority date: 1976-08-06
Filing date: 1976-08-06
Publication date: 1981-05-07
Also published as: FR2360864A1; NL7708697A; JPS5358843A; DE2735618A1

Description

(54) IMPROVEMENTS IN OR RELATING TO TUIBULAR HEAT EXCHANGERS (71) 'We, BABCOCK INTERNATIONAL LIMITED) (formerly known as Babcock & BR< Wilcox Limited), a British Company of Cleveland House, 19, St. James's Square, London SWiY 4LN, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to tubular heat exchangers.

It is known to form a heat exchanger from a plurality of U-tubes extending into a drum from a tube sheet forming the closure lid of the drum. Each leg of each Utube penetrates the tube sheet and a rigid seal is formed between the leg and the tube sheet. The drum is provided with an inlet and an outlet for fluid to be heated and a baffle means disposed within the drum define a path such that the fluid to be heated flows first over one leg of each tube and then over the other. Heating fluid is passed through the U-tubes to flow first through the legs over which the fluid to be heated flows last.

To prevent the tubes from vibrating or coming into contact with each other, means is provided that serve to maintain each leg in a predetermined position during operation of the heat exchanger. Such means may include a plurality of guides by which any leg is closely encircled in such a way as to allow longitudinal expansion of the leg but prevent any noticeable lateral movement.

Now with such an arrangement, since one limb of each U-tube will be heated to a higher temperature than the other in use of the heat exchanger, the relative expansions of the two limbs of any tube will strain the return bend and the bend will thus be under strain when the conditions in the heat exchanger are the most onerous.

An object of the invention is to provide an improved heat exchanger in which Utubes are used.

According to the present invention, tliereis provided a heat exchanger comprising a U-tube disposed in a drum so that a fluid can be passed through the U-tube in heat exchange with a fluid flowing through the drum, so that in operation one limb will become hotter than the other, in which each limb is anchored at one location such that movement of the locations relatively to each other in the longitudinal direction of the limbs is prevented . during changes of temperature in the heat exchanger between the operational and the ambient, guide means disposed between these locations and the return bend of the U-tube serve to permit longitudinal thermal movement of the limbs whilst preventing lateral movement, and the relative lengths of the limbs are such as to establish a stress in the return bend at ambient temperature that will be relieved. as the temperature in the heat exchanger rises with the relatively shorter limb becoming hotter than tlie relatively longer limb.

According to the present invention there is also pro-iided a heat exchanger comprising a U-tube disposed in a drum and a fluid flows through the U-tube in heat exchange with a fluid flowing through the drum so that one limb is hotter than the other, in which each limb is anchored at one location such that movement of the locations relatively to each other in the longitudinal direction of the limbs is prevented during changes of temperature in the heat exchanger between the operational and the ambient, guide means disposed between these locations and the return bend of the U-tube serve to permit longitudinal thermal movement 6f the limbs whilst preventing lateral movement, and the length of the hotter limb would be less than that of the cooler limb were the heat exchanger at ambient temperature so that the stress in the return bend is not greater than would, be the stress if the heat exchanger were at ambient temperature.

By way of example, an embodiment' 'of the invention will now be described with reference to the 'accompanying drawing which illustrates, somewhat diagrammatically and in vertical axial cross-section a heat exchanger in operation.

The heat exchanger illustrated includes a pressure vessel comprising a drum 1 closed by a closure lid 2. A plurality of Utubes, of which one is indicated at 3, penetrate the lid 2 with their ends connected outside the pressure vessel to inlet and outlet headers (not shown). Each limb of the Utube, where it penetrates the lid 2, is rigidly connected to the lid 2 at 4. Heating fluid flows from the inlet header through limb 3a to the outlet header through limb 3b.

A baffle 5 depends from the lid 2 so that one limb of the U-tube lies on one side of the baffle 5 and the other limb of the U-tube lies on the other side of the baffle 5. Guide means 7 are mounted on the baffle 5 and are in the form of plates having openings through which the limbs of the U-tube extend; thus, the limbs are maintained in the same position during changes in the temperature of the heat exchanger, the openings permitting longitudinal thermal movement of the limbs but preventing lateral movement thereof.

The drum 1 is provided with an inlet 8 and an outlet 9, fluid being heated by entering the drum 1 through the inlet 8, flowing over the limbs 3b, when under the baffle 5 and then over the limbs 3a to the outlet 9.

The heat exchanger that is illustrated is in operation with little or no stress in the return bend of the tube as a result of relatime thermal expansion of the limbs of the U-tube, even through the limb 3a is hotter than the limb 3b. When taken out of operation and allowed to cool it will revert to its original state, with stress in the Ubend tending to deflect it in the direction of the limb 3a i.e. the limb that in operation is the hotter. This condition is established in constructing the heat exchanger by making the limb 3a shorter than the limb 3b by an amount that will be compensated for by thermal expansion as the heat exchanger is brought to its operating temperature.The U-tube is initially symmetrical but in manufacture, when the connections 4 are established, the limb 3a is "pulled", or the limb 3b is "pushed", to establish the stress in the return bend before the corresponding connection 4 is made. Consequently, when the heat exchanger is at ambient temperature there will be stress in the return bend but the stress in the return bend when the heat exchanger is operating, and conditions are at their most onerous, will be less.

It is envisaged that the invention may be used with a wide range of heating fluid and temperatures and may be incorporated in nuclear applications, when high integrity is desirable.

In the heat exchange operation that has been described, the fluids are in counterflow and the tendency for thermallyoriginating strains to develop in the return bend are greater than with the otherwise less desirable parallel flow. Whilst as described, the heating fluid flows in the Utubes and the fluid being heated flows over them, in other embodiments it could be the fluid being heated that flows through the tubes.

WHAT WE CLAIM IS: - 1. A heat exchanger comprising a U-tube disposed in a drum so that a fluid can be passed through the U-tube in heat exchange with a fluid flowing through the drum, so that in operation one limb will become hotter than the other, in which each limb is anchored at one location such that movement of the locations relatively to each other in the longitudinal direction of the limbs is prevented during changes of temperature in the heat exchanger between the operational and the ambient, guide means disposed between these locations and the return bend of the U-tube serve to permit longitudinal thermal movement of the limbs whilst preventing lateral movement, and the relative lengths of the limbs are such as to establish a stress in the return bend at ambient temperature that will be relieved as the temperature in the heat exchanger rises with the relatively shorter limb becoming hotter than the relatively longer limb.

2. A heat exchanger comprising a U-tube disposed in a drum and a fluid flows through the U-tube in heat exchange with a fluid flowing through the drum so that one limb is hotter than the other, in which each limb is anchored at one location such that movement of the locations relatively to each other in the longitudinal direction of the limbs is prevented during change of temperature in the heat exchanges between the operational and the ambient, guide means disposed between these locations and the return bend of the U-tube serve to permit longitudinal thermal movement of the limbs whilst preventing lateral movement, and the length of the hotter limb would be less than that of the cooler limb were the heat exchanger at ambient temperature so that the stress in the return bend is not greater than would be the stress if the heat exchanger were at ambient temperature.

3. A heat exchanger substantially as described with reference to and as illustrated by the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. and in vertical axial cross-section a heat exchanger in operation. The heat exchanger illustrated includes a pressure vessel comprising a drum 1 closed by a closure lid 2. A plurality of Utubes, of which one is indicated at 3, penetrate the lid 2 with their ends connected outside the pressure vessel to inlet and outlet headers (not shown). Each limb of the Utube, where it penetrates the lid 2, is rigidly connected to the lid 2 at 4. Heating fluid flows from the inlet header through limb 3a to the outlet header through limb 3b. A baffle 5 depends from the lid 2 so that one limb of the U-tube lies on one side of the baffle 5 and the other limb of the U-tube lies on the other side of the baffle 5. Guide means 7 are mounted on the baffle 5 and are in the form of plates having openings through which the limbs of the U-tube extend; thus, the limbs are maintained in the same position during changes in the temperature of the heat exchanger, the openings permitting longitudinal thermal movement of the limbs but preventing lateral movement thereof. The drum 1 is provided with an inlet 8 and an outlet 9, fluid being heated by entering the drum 1 through the inlet 8, flowing over the limbs 3b, when under the baffle 5 and then over the limbs 3a to the outlet 9. The heat exchanger that is illustrated is in operation with little or no stress in the return bend of the tube as a result of relatime thermal expansion of the limbs of the U-tube, even through the limb 3a is hotter than the limb 3b. When taken out of operation and allowed to cool it will revert to its original state, with stress in the Ubend tending to deflect it in the direction of the limb 3a i.e. the limb that in operation is the hotter. This condition is established in constructing the heat exchanger by making the limb 3a shorter than the limb 3b by an amount that will be compensated for by thermal expansion as the heat exchanger is brought to its operating temperature.The U-tube is initially symmetrical but in manufacture, when the connections 4 are established, the limb 3a is "pulled", or the limb 3b is "pushed", to establish the stress in the return bend before the corresponding connection 4 is made. Consequently, when the heat exchanger is at ambient temperature there will be stress in the return bend but the stress in the return bend when the heat exchanger is operating, and conditions are at their most onerous, will be less. It is envisaged that the invention may be used with a wide range of heating fluid and temperatures and may be incorporated in nuclear applications, when high integrity is desirable. In the heat exchange operation that has been described, the fluids are in counterflow and the tendency for thermallyoriginating strains to develop in the return bend are greater than with the otherwise less desirable parallel flow. Whilst as described, the heating fluid flows in the Utubes and the fluid being heated flows over them, in other embodiments it could be the fluid being heated that flows through the tubes. WHAT WE CLAIM IS: -

1. A heat exchanger comprising a U-tube disposed in a drum so that a fluid can be passed through the U-tube in heat exchange with a fluid flowing through the drum, so that in operation one limb will become hotter than the other, in which each limb is anchored at one location such that movement of the locations relatively to each other in the longitudinal direction of the limbs is prevented during changes of temperature in the heat exchanger between the operational and the ambient, guide means disposed between these locations and the return bend of the U-tube serve to permit longitudinal thermal movement of the limbs whilst preventing lateral movement, and the relative lengths of the limbs are such as to establish a stress in the return bend at ambient temperature that will be relieved as the temperature in the heat exchanger rises with the relatively shorter limb becoming hotter than the relatively longer limb.