US3875997A - Tubular heat transfer members - Google Patents

Abstract

A vertical heat transfer tube for condensation of vapor in a condenser is of substantially uniform wall thickness and grooved so as to impart turbulence to coolant passing through the tube and to assist in runoff of condensate on the tube surface. The hand of the part-helical lands and grooves changes at intervals along the tube before the helix has completed a complete turn. The pitch of the grooves (determined by half a wave length of the helix) is more than 0.5 but less than 5 times the inside minimum diameter of the tube.

Description

United States Patent [1 1 Newson et a1.

1 1 TUBULAR HEAT TRANSFER MEMBERS [75] Inventors: lvan Henry Newson; Thomas David Hodgson, both of Didcot, England [73] Assignee: United Kingdom Atomic Energy Authority, London. England [22] Filed: Aug. 8, 1973 21 App1.No.:386,6l6

Related U.S. Application Data [63] Continuation-in-part of Ser. No, 156.713. June 25.

1971, abandoned.

2.864.591 12/1958 Frink 165/177 [451 Apr. 8, 1975 2.396.426 7/1959 Ayling 165/184 x 3.212.992 10/1965 Salesse et a1 138/42 X 3.358.749 12/1967 Chisholm et a1..... 165/177 X 3,612,175 10/1971 Ford et a1. 165/179 3,762,468 10/1973 Newson et a1. 165/110 X FOREIGN PATENTS OR APPLICATIONS 947,327 1/1964 United Kingdom 165/184 521,548 5/1940 United Kingdom 138/38 Primary E.\'aminerAlbert W. Davis. Jr. Assistant ExaminerSheld0n Richter Attorney, Agent, or Firm-Larson. Taylor & Hinds [57] ABSTRACT A vertical heat transfer tube for condensation of vapor in a condenser is of substantially uniform wall thickness and grooved so as to impart turbulence to coolant passing through the tube and to assist in runoff of condensate on the tube surface. The hand of the parthclical lands and grooves changes at intervals along the tube before the helix has completed a complete turn. The pitch of the grooves (determined by half a wave length of the helix) is more than 0.5 but less than 5 times the inside minimum diameter of the tube.

4 Claims, 5 Drawing Figures TUBULAR HEAT TRANSFER MEMBERS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of copending application Ser. No. l56,7l3, filed June 25. I971, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to improvements in heat transfer tubes which are in the main suitable for heat exchange in condensing vapour/non boiling liquid and condensing vapour/evaporating liquids.

Various proposals have been made hitherto for enhancing the heat transfer between fluids passed along opposite sides of the tube wall but the special case of a tube for a condensing/evaporating system presents problems which are not easily answered. The virtue of a uniform wall thickness tube having a special profile on the tube bore comprising regular helical grooves with complementary profile on the exterior is recognised. It has now been appreciated that the different requirements of the condensing/evaporating functions can be met advantageously by complementary profiles of a different nature. This is especially so where the tube is employed in the vertical mode and this has a distinct bearing on the optimum shape.

SUMMARY OF THE INVENTION It has now been found that provided the pitch of the grooves in a helically grooved condensing/evaporating tube are within a certain range, the dual function of such a tube may be performed in an advantageous manner by reversing the hand of the helix along the length of the tube before the helix has completed a complete turn. The preferred range is that the pitch lies between one and five times the inside, ie minimum tube diameter. Pitch on such tube is equal to one half a wave length of the part helical grooves.

Thus, according to the invention a heat transfer eondensing/cvaporating tube for operation in a vertical attitude has its wall shaped to impart turbulance to the coolant flowing through the tube bore by part helical parallel grooves and lands in the bore and complementary lands and grooves along the tube surface to assist in run-off of condensate. the pitch of the grooves being between point live and five times the internal, i.e., minimum. tube diameter and the hand of the helix changing along the length of the tube before completing one whole turn. The preferred pitch is preferably about two tube internal or minimum diameters. The cooling, or non-boiling liquid flows through the tube bore.

The relatively slow helix of the helical grooves gives, on the outside of the tube, swift run-off of condensate which is desirable whilst, within the tube where the same helix is reproduced, adequate turbulence is imparted to the coolant by the change in hand of the helix; such a turbulence not being achievable by a continuous helix of that pitch. Within this limitation of tube profile, a further preferred limitation may be put upon groove depth in that it should depend upon tube inside, ie minimum diameter. The groove depth should be between 0.02 and 0.2, and preferably about 0.15, times, the tube inside, minimum diameter. Such a depth of groove is one which will co-operate with adjacent lands to increase heat transfer on the evaporatorating tube, is readily obtainable by swaging a plain walled tube and contributes to turbulence in the tube bore.

DESCRIPTION OF THE DRAWING One tube which embodies the invention will now be described with reference to the accompanying drawing in which:

FIG. 1 is a plan view of one end of a tube,

FIG. 2 is an axial cross-section through a profiled portion of FIG. 1.

FIG. 3 is a developed view of part of FIG. 1,

FIGS. 4A and 4B are graphs showing the performance of the tube shown in FIG. 1 as compared with a smooth walled tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawing, FIG. 1 shows the tube with a plain end portion 1 suitable for fitting into a conventional tube plate and a specially profiled portion 2. The latter commences at the plain portion with eight start helical grooves 3, with intervening lands 4 which are initially right-handed. These grooves proceed helically along the tube wall for about a quarter of a wavelength and then smoothly reverse hand, to proceed left handed for a further quarter of a wavelength reverse hand again and so on. The wavelength of the part-helical turns is about four tube diameters. A wave-length may be varied between 1 and 0 tube diameters without departing from the invention. The groove depth d is also related to the tube inside diameter Die the minimum diameter and it is preferred that a ratio of groove depth d to tube inside diameter equal to 0.15 be adhered to.

With the tube employed with its axis vertical, the pitch of the grooves must not be longer than specified, or else a condensate drainage path on the groove would build-up a condensate film to an unwelcome thickness. The reversal of the helical grooves on a short pitch breaks up the continuous path length into a number of short lengths thus, condensate draining down any land on the outside of the tube will intersect a groove sooner or later. However, the pitch is not too short such that it will impede run-off of condensate.

It will be seen from FIG. 3 that the fluid flowing near the bore wall will follow a very confused part helical flow pattern, the fluid near the tube wall continually meeting helical grooves of changing hand so that a high degree of turbulence results, yielding a good heat trans fer characteristic. The outside of the tube has a continuous longitudinal peaks and troughs 6 complementary to those in the bore; the outside troughs 6 provide channels for the run off of condensate with gathers in the grooves leaving the peaks exposed to ambient vapour.

In practice the groove depth is linked to the inside diameter of the tube owing to the fact that the greater the groove depth the greater the pressure drop and this becomes the limiting factor in case of the water flowing through the tube bore. To this extent the ratio between groove depth and tube internal diameter lies between 0.02 and 0.2 and is preferably equal to or less than 0.15.

The pattern of grooves shown in the drawing is formed preferably by using a planetary swage tool, the tool comprising a swage .ring from whose inner diameter protrude eight hard steel balls at equal spacing around the ring. The swage ring is engaged with the tube periphery and drawn axially along it; at the same time the required degree of rotary oscillation is imparted to the swage (or the work) to produce helical grooves as shown. This will produce the characteristic wrinkled tube wall as opposed a ribbed wall.

The respective dimensions of a typical tube were as follows:

Helix lead 3.5 inches Starts 8 Of course the form of the grooves may be altered within the ambit of the invention to suit different applications.

The performance of the above tube for heat transfer between water flowing in the tube bore and steam condensing on the tube periphery was as shown on the graphs FIGS. 4A, 4B. The invented tube is indicated in full iine and the equivalent performance of a copper smooth-walled tube is shown in dotted line. The high degree of turbulence imparted to the fluid in the tube bore makes the tube of the invention especially suited to pass a highly viscous fluid such as crude oil. In this case. the oil within the tube would be receiving heat from a heating fluid passed over the tube surface. The tube is equally suited to use with less viscous oils and liquors used in industrial plant which are required to be preheated, prior to chemical treatment. by gas or other heating fluid passed or flowed over the tube surface. Of course the helix lead may be varied to give required dcgree of turbulence. Generally the more viscous the fluid the shorter the helix lead and/or the greater the depth of the grooves.

Condensing surfaces in accordance with the invention are amenable to use in a wide range of applications, notably in evaporator plants as used for desalination and in the food and chemical industries. ln the case of desalination such condensing surfaces may be employed with advantage for the condenser tubes of a plant of the flash type and for the preheater tubes in plants of the multi-effect type. In the food and chemical industries such condensing surfaces may serve in similar roles in the concentration of solutions, residues and similar products.

We claim:

1. A condenser having at least one vertical tube for condensing vapor on its outer surface. the improvement comprising the tube wall being of substantially uniform thickness and shaped so as to exhibit a number of parallel part-helical continuous lands and grooves in the tube bore to impart turbulence to the coolant, and grooves and lands on the tube surface to assist in run off of condensate, the hand of the helix changing at intervals along the tube before the helix has completed a complete turn, the pitch of the grooves (determined by half a wavelength of the helix) being more than 0.5 but less than 5 times the inside, i.e. minimum, diameter of the tube.

2. A condenser as claimed in claim 1 in which the pitch is about twice the tube internal diameter.

3. A condenser as claimed in claim I in which the ratio of groove depth to the minimum inside diameter of the tube lies between 0.02 and 0.2.

4. A condenser as claimed in claim 1 in which the ratio of groove depth to the inside diameter of the tube is equal to 0.15.

Claims (4)

1. A condenser having at least one vertical tube for condensing vapor on its outer surface, the improvement comprising the tube wall being of substantially uniform thickness and shaped so as to exhibit a number of parallel part-helical continuous lands and grooves in the tube bore to impart turbulence to the coolant, and grooves and lands on the tube surface to assist in run off of condensate, the hand of the helix changing at intervals along the tube before the helix has completed a complete turn, the pitch of the grooves (determined by half a wavelength of the helix) being more than 0.5 but less than 5 times the inside, i.e. minimum, diameter of the tube.

2. A condenser as claimed in claim 1 in which the pitch is about twice the tube internal diameter.

3. A condenser as claimed in claim 1 in which the ratio of groove depth to the minimum inside diameter of the tube lies between 0.02 and 0.2.

4. A condenser as claimed in claim 1 in which the ratio of groove depth to the inside diameter of the tube is equal to 0.15.

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