Background of the invention: Heat exchanger known in the prior art has used bags as heat exchange elements. Stacks of bags are made and the vapour is passed through the interior of bags and water is passed through the surface of the bags to transfer heat from vapour to water condensing the vapour. The bottom of the bags are kept inclined towards one end and a condensate outlet is attached to a tubular duct. In practice the narrow tubular duct complicates removal of uncondensed vapour. In another type of heat exchanger known in the art, a transverse strip with a duct is provided at the lower end of the stack of bags. By means of strips supporting one another, a stable structure is formed. However its heavy weight complicates installation.

In improved heat exchangers known in prior art using bag heat exchanging elements, the width of the condensate removal member is only a small fraction, about 10%, of the width of the bag bottom. Even this will result in restricting the liquid flow between the bags. The solids such as fibre present in the liquid accumulate on the condensate removal member causing a build up and restrict condensate flow in the bag. This can cause pressurisation of the bag resulting in mechanical and process failures. Since these bags are made with thin polymeric materials for effective heat transfer, they can get ruptured under these conditions.

In the heat exchangers known in the prior art the vapour and the liquid to which heat is transferred are fed from the same side from the top. Thus heat exchange between liquid and vapour takes place when liquid flow and vapour flow are parallel in all the heat exchangers known in the prior art.

Object of the invention: The invention provides a heat exchanger element which overcome the above mentioned problems. The heat exchanger element comprises a bag of a polymeric material in which the vapour flows transverse to the flow of the liquid to which heat is transferred from vapour.

The bag is held vertically from one of the sealed horizontal sides, with a strip attached outside the bag and provided with liquid inlet ducts so that the liquid flow over the surfaces of the bag vertically downwards. The vapour is fed from the vertical side of the bag through slightly inclined ducts formed by seams in the bag and the condensate and vapour is taken out from the other vertical side of the bag. A stack of such bags makes a heat exchanger with the liquid flowing vertically downwards and the vapour and condensate flowing in a transverse direction to the liquid flow. The bottom ends of the bags are not attached to each other and therefore the liquid flows freely through the surface of the bags. The liquid is fed from the top end through the liquid ducts provided in the liquid feeding strips attached to each bag. The bags are provided with slightly inclined conduits for passing vapour therethrough by seam welding along the horizontal sides. The inclination of the seam is such that the vapour entering transverse to the liquid flow, smoothly flows to the other end of the conduit to provide condensate and condensed vapour to an outlet.

In a heat exchanger comprising plurality of such heat exchanger elements stacked together, the liquid can be uniformly fed from top since there are no vapour entry or exit between the liquid inlets, as in the case of heat exchangers known in the prior art. Further the bottom of the bags are not attached to each other and therefore the liquid flow out easily without any obstructions.

Thus the invention provides a cross flow heat exchanger element comprising a bag of thin polymeric material with transverse seams forming inclined conduits for vapour to flow therethrough, the said bag being held vertically with a liquid feeding strip having vertical ducts attached outside the bag for feeding a liquid therethrough to flow over the surface of said bag and a vapour inlet plate with vapour ducts attached into the higher end of said inclined conduits in the bag and a vapour and condensate outlet plate with outlet ducts attached into the lower end of said transverse inclined conduits in the bag to facilitate transverse flow heat exchange between the liquid and vapour.

The invention also provides a heat exchanger comprising plurality of heat exchanger elements as described hereinabove, stacked together with a condensate receiver box attached to the stacked, vapour and condensate outlet plates, the said receiver box having a bottom outlet for the collected condensate.

Brief description of drawings : Figure 1 shows a schematic view of the heat exchanger element according to the invention.

Figure 2 shows a cross-sectional view along I-I in figure 1.

Figure 3 shows a cross-sectional view along II-II in figure 1.

Figure 4 shows an isometric view from one direction of a heat exchanger according to the invention.

Figure 5 shows another isometric view from opposite direction of a heat exchanger according to the invention.

Figure 6 shows a cross-sectional view of a heat exchanger according to the invention.

Heat exchanger element according to the invention is made of a bag (1) of thin polymeric material. The interior (3) of the bag (1) is divided by slightly inclined seams (8) to form inclined conduits (9) from one vertical side to the other vertical side of the bag (1). The bag (1) is held vertically from a liquid feeding strip (5) having vertical ducts (13) attached outside the bag for feeding liquid to the surface of the bag (1). The vertical sides of the bag (1) are held between a vapour inlet plate (6) with vapour inlet ducts (14) therein and a vapour and condensate outlet plate (7) with outlet ducts (15) attached into the bag. The higher end of the inclined ducts (9) in the bag (1) are alligned to the vapour inlet ducts (14) in the vapour inlet plate (6) and the lower end of the inclined ducts (9) are alligned to the outlet ducts (15) in the vapour and condensate outlet plate (7).

A stack of such heat exchanger elements forms a heat exchanger which allows uniform flow of liquid along the surfaces of the bags of heat exchanger elements. The vapour enters the conduits (9) in the bag (1) from one of the vertical sides of the bag and flows to the other vertical side of the bag. The vapour inlet plates (6) are stacked together and vapour is allowed to enter to the conduits (9) in the bags through vapour inlet ducts (14) in the vapour inlet plates (6).

The vapour and condensate outlet plates (7) are also stacked together and the vapour and condensate are lead through the outlet ducts (15) in it to a receiver box (10) provided with a bottom outlet (11) for collection of the condensate.

Preferably the receiver box (10) is also provided with an opening (12) on the top portion for vapour diversion to adjacent heat exchanger. The bags are stacked on a frame with vapour inlet plates (6) on one side and vapour and condensate outlet plates (7) on the other side. Therefore loading on the material of the bag is negligibly small allowing use of very thin polymeric material for the bag and more effective heat transfer.

The bottom ends of the bags in the heat exchanger are left free so that the liquid flows freely even with solids such as fibres and other matters in the liquid.

The heat exchanger elements and the heat exchanger assembled form the heat exchanger elements are efficient and very useful for evaporation of water and waste water which contain solid fibers in industries such as paper mill, textile plants and distillery.