In particular the present invention has as its object the provision of a novel heat exchange apparatus wherein heat from a primary fluid contained in primary heat transfer tubes can be transferred to or from a secondary fluid contained in secondary heat transfer tubes by the use of an intermediate fluid flowing outside of, and in flow communication with, both primary and secondary tubes.

According to the invention a heat exchange apparatus includes a number of parallel primary heat transfer tubes, regularly arrayed, with clearance between each tube in two dimensions perpendicular to their length, with secondary heat transfer tubes arranged in successive rows passing between the primary tubes, each secondary tube row being arranged orthogonally one to the next, and each secondary tube row being arranged orthogonally to the primary tubes.

Further to this, the secondary heat recovery tubes may be arranged such that two or more adjacent secondary tube rows in a direction parallel to the primary tubes may be arranged in the same direction, upstream of successive groups of tube rows arranged orthogonally to them.

The primary fluid flows inside the primary tubes, whilst the intermediate fluid flows outside the tubes in a direction generally in parallel with the primary fluid, either in a co-current or counter-current direction.

The direction of flow of the secondary fluid is generally transverse to the direction of flow of the primary and intermediate fluids.

The distance between successive orthogonal rows of secondary tubes is preferably minimised whereby the intermediate fluid is caused to flow around the secondary tubes in a tortuous manner, thereby increasing its turbulence and local heat transfer coefficient against both primary and secondary tubes.

In this manner a heat exchanger can be produced which by virtue of high heat transfer coefficients can be made more compact than conventional heat exchangers without suffering from excessive fluid pressure drops.

Furthermore, the heat exchanger can be fabricated from standard manufactured components.

The spacing between the primary and secondary tubes is preferably small, generally between 0 and 0.5 times the secondary tube outside diameter, in order to minimise the superficial surface area available to the intermediate fluid and thereby to maximise its velocity and corresponding heat transfer coefficient.

Advantageously, the secondary tubes may be in close proximity or in contact with the primary tubes thereby to eliminate the possibility of tube vibration in the secondary tubes.

Alternatively, if necessary, the secondary tubes may be braced one to another or to additional members to prevent vibration.

Preferably the distance between each successive row of secondary tubes is minimised to provide as tortuous path as possible for the intermediate fluid and to minimise the size of the overall heat exchanger.

For example, the distance between successive rows may be in the range 0-0.5 times the secondary tube outside diameter.

The primary and secondary tubes may be profiled in order to modify the heat transfer characteristics. For example, the tubes may be finned, twisted, shaped, of varying diameters, or contain inserts.

Inert shapes may be placed between the secondary tubes and around the primary tubes in a regular or random manner to further promote turbulence and heat transfer.

A further variation of the invention may provide for heat transfer from the intermediate fluid to both the primary and secondary fluid, or from both the primary and secondary fluid to the intermediate fluid.

Additionally, the primary and secondary fluids may be the same fluid, whilst the primary and secondary tubes may also be connected in parallel or series.

For example, the primary and secondary fluids could be water and/or steam with the intermediate fluid being combustion gases for heating same.

A still further variation to the utilisation of the apparatus may be the use of different fluid streams within the secondary tubes, flowing either in the same, or reverse, or a transverse directions. In this way heat may be transferred to a number of different fluid streams from the same stream or streams within a single apparatus.

Overall, the provision of tubes in three dimensions serves to maximise the tube surface area per unit volume of heat exchanger, whilst maximising the tortuosity of the intermediate fluid thereby maximising the overall heat transfer coefficients in an efficient manner.

This results in a potentially more compact and cost-effective heat exchanger than has hitherto been achievable with conventional tubular heat exchangers.