GB2193304A

GB2193304A - Flow modifying inserts for tubes

Info

Publication number: GB2193304A
Application number: GB08613556A
Authority: GB
Inventors: John Edward Fletcher
Original assignee: Ambi Rad Ltd
Current assignee: Nortek Global HVAC UK Ltd
Priority date: 1986-06-04
Filing date: 1986-06-04
Publication date: 1988-02-03
Also published as: GB8613556D0

Abstract

Flow modifying insert 12, particularly a turbulator insert for heat exchanger tubes as used in radiant tube space heaters is formed as a strip of e.g. light gauge stainless steel which is first crimped along its length to form transverse corrugations. The strip can then readily be twisted about a longitudinal axis to form a helix prior to or during its insertion into the tube (10). <IMAGE>

Description

SPECIFICATION Flow modifying inserts for tubes This invention relates to inserts for modifying fluid flow within tubes and particularly but not exclusively to turbulators or other inserts for improving heat transfer between fluid flow within the tube and the tube walls, for example, in a radiant tube type heater where the heating effect is derived from a flow of hot gases through the tube.

Various types of turbulator inserts are known for improving heat transfer and other flow modifying purposes, for example wound and twisted wire inserts, or metal strip inserts shaped and deformed to complex bent and twisted shapes to cause changes in direction of the fluid flow within the tube. This type of turbulator acts largely by sideways buffeting of the fluid flow which causes high pressure resistance and increases the energy needed to maintain the through flow. Moreover they are effective only within the localised length of the duct in which they are installed and have little or no effect downstream thereof.

Another form of known turbulator or flow modifying insert consists of a flat strip of metal equal in width to the tube internal diameter and twisted to form a helix so that the through flow follows a spiral path improving contact with the tube wall and also providing an enhanced effect downstream of the insert in that the gases exiting therefrom will continue to spin giving some additional scrubbing effect and wall contact after their exit from the spiral. This type of insert gives relatively low pressure resistance, however the spirally shaped strip is difficult and expensive to manufacture, particularly where costly high grade heat resisting metals are needed for high temperature and/or corrosive or other aggressive gases or other fluids.

The object of the present invention is to provide a flow modifying insert, particularly for heat transfer applications, which is simple and economic to manufacture and install and particularly adaptable and effective in use.

According to the invention there is provided some one or more of the following: A. A flow modifying insert for a tubular duct consisting of or comprising an elongate strip of sheet material crimped to form transverse corrugations along its length and twisted about a longitudinal axis to form a helix; and/or B. A heat exchanger duct including an insert in the form of an elongate strip of sheet material crimped to form transverse corrugations along its length and twisted about a longitudinal axis to form a helix fitting within the duct to enhance heat transfer between a flow of fluid through the duct in use and the duct walls; and/or C.A method of forming a flow modifying insert for a tubular duct including the steps of crimping an elongate strip of sheet metal or other material to form transverse corrugations along its length and twisting the corrugated strip about a longitudinal axis to form a helix fitting within the duct.

The invention further resides in a radial tube heater or other space heating appliance incorporating one or more ducts carrying a flow or flows of heated gases or other heated fluid and incorporating one or more flow modifying inserts as defined in one or more of the preceding sub-paragraphs A-C.

When used in a radiant tube heater the invention may be particularly advantageously combined with a burner assembly as described in our co-pending British Patent Application No. 8613557 of even date herewith.

An example of the invention is now more particularly described with reference to the accompanying drawings wherein: Figure 1 is part sectional view of a duct having an insert positioned therein, and Figure 2 is a perspective view of an insert at one stage of its manufacture.

In this example the duct 10 forms part of a radiant tube space heater having a U-shaped tube with a burner assembly at the upstream end providing a through flow of hot gases which are drawn through the tube by an extraction fan at the downstream end. For maximum heating effect it is important that there is good heat transfer from the gas flow to the tube walls for radiation from the latter.

To enhance such transfer, particularly in the downstream limb of the tube, a spiral turbulator or insert 12 is positioned in a portion of tube 10.

Insert 12 is formed from a strip or sheet metal, for example stainless steel of 0.3 mm gauge, the strip having a width slightly less than the internal diameter of tube 10, for example 70 mm width for use in 75 mm internal diameter tube.

The strip is subjected to a crimping process by passing it through a pair of meshing toothed rollers to form transverse corrugations along its full length. This is a very simple process which merely involves bending the light gauge metal without any actual stretching or other deformation and the axial length of the rollers used can be sufficient to accommodate a wide range of strip widths for making inserts for tubes of varying diameters. The pitch of the corrugations may, for example, be 12 mm and their trough to crest height 4 mm for 70 mm width strip.

This stage of manufacture is shown in Figure 2 and the strip so formed can readily be stored or transported as a coil until required for use.

Corrugations 14 enable the strip to be readily twisted about a longitudinal angle to form a helix, and the pitch of the turns thereof can very readily be varied according to require ments . This process can readily be carried out by hand or using simple tools either before or at the time of the insertion of the strip into tube 10, indeed if an insert has to be positioned in the tube in a confined space, for example positioning a long insert in a tube where there is restricted clearance at the tube mouth, the flexibility of the strip prior o or during twisting will be found to be particularly convenient. There is also the possibility of feeding the strip around a bend in the tube while forming the twist.

In forming a helix in a flat metal strip the outer borders of the strip have to be stretched relative to the centre area which necessitates special tools and processes and also means that heavier gauge material must be used which can be stretched without tearing. On the other hand, the corrugations 14 of strip 12 permit its outer borders to expand longitudinally relative to its central area without difficulty and the rigidity provided by the lateral corrugations also facilitate easy and even twisting. Typically the strip 12 is twisted to a pitch of from 250 to 350 mm using 70 mm wide strip.

Corrugations 14 themselves provide a slight increase in gas turbulence in the region of tube 10 occupied by insert 12 as well as the improved heat transfer provided by the scrubbing effect of the spiral gas flow on th wall of tube 10 due to the helical shaping of insert 12. The twisting impetus imparted to the gases as they exit from the downstream end of the insert 12 continues this scrubbing effect giving increased heat transfer in regions of tube 10 downstream of the insert again giving increased efficiency of operation.

As light gauge strip material can be used there is considerable economy of material (e.g.

of costly corrosion and heat resistant stainless steel) and a possible reduction in weight as well as the econonlies of manufacture and assembly referred to above.

Claims

1. A flow modifying insert for a tubular duct consisting of or comprising an elongate strip of sheet material crimped along its length to form transverse corrugations and twisted about a longitudinal axis to form a helix.

2. An insert as in claim 1 wherein the sheet material is metal.

3. An insert as in claim 2 wherein the metal is stainless steel.

4. An insert as in claim 2 or three wherein the gauge of the metal is 0.3mm or some other light gauge.

5. A flow modifying insert substantially as herein before described with reference to and as shown in the accompanying drawings.

6. A heat exchanger duct including an insert as in any preceding claim.

7. A radiant tube heater or other space heating appliance incorporating one or more ducts as in claim 6 carrying a flow or flows of heated gases or other heated fluid in use.

8. A radiant tube space heating appliance substantially as herein before described.

9. A method of forming a flow modifying insert for a tubular duct including the steps of crimping an elongate strip of sheet metal or other material along its length to form transverse corrugations and twisting the corrugated strip about a longitudinal axis to form a helix fitting within the duct.

10. A method as in claim 9 wherein the crimping is effected by passing a strip through a pair of meshing toothed rollers.

11. A method as in claim 9 or 10 wherein the twisting is effected before the strip is inserted into the duct.

12. A method as in claim 9 or 10 wherein the twisting is effected while the strip is being fed into the duct.

13. A method of forming a flow modifying insert for a tubular duct substantially as hereinbefore described with reference to the accompanying drawings.