FIELD OF THE INVENTION The present invention generally relates to a horizontal condensing type duplex or multiplex heater in which at least two low pressure heaters of different stages and pressures are accommodated in a single casing and used in steam power plants for heating up water for water/steam circuit of coal based thermal power plants by exchange of heat between tapped steam from steam turbines and water flowing in heat exchanger tubes. More particularly, the invention relates to an integrated horizontal condensing Duplex heater for heating up water for water and steam circuit of coal based thermal power plants. BACKGROUND OF THE INVENTION A Duplex Heater consists of two heat exchanger module, and two Low Pressure (LP) Heater disposed in one common casing. It is located in the condenser neck. The Two heater spaces are defined through a partition plate in the shell and steam of different pressure and temperature extracted from the turbine is fed via inlet nozzles. The water to be heated flows from the water box through the U-tubes of the first heat exchanger module while the extracted steam with a first pressure condenses on the outer surface of the tubes. The water heated in the first heater flows through the U-tubes of the second heat exchanger module and is further heated through the extracted steam with a second pressure and flows again into the water box to the outlet nozzle. The second pressure of the extracted steam has a higher value than that of the first extracted pressure of the steam. The process of non-condensing gases are removed by a plurality of vent pipes which are positioned at lowest pressure zone is known as air venting. Air Venting plays a very important role in efficient functioning of the Duplex feed water heater. Improper venting arrangement may result in following problems in the feed water heaters:-a) Accumulation of non-condensable gases/air, which will cause a drop in partial pressure of the steam vapour leading to the depression of saturation temperature. This results in the lowering temperature difference between the hot and cold medium which adversely alters the temperature gradient required for heat flow. b) blanketing of air/non condensable gases/steam over the tube bundle of the heater. Thus, any condensation which takes place has to cross the vapour layer which increases the fouling factor, thus resulting in a poor heat transfer coefficient. c) presence of non-condensable gas in the heaters, mainly Oxygen and Carbon-di¬Oxide increases the corrosion of various components of the heater. d) presence of pockets/blankets of non-condensable gases results in an uneven distribution of steam inside the heater. This decreases the efficiency of the heater. e) Undesired vibration components of the tube bundle generates which may lead to higher stress values and stress induced corrosion. According to the known process, venting extraction is taken from the first pass (water side) as it is the coolest part of the heater. In a two pass (water side) heater, it is easier to do so as one complete half of the heater is available for venting arrangement. As per prior art, in Duplex feed water heater venting arrangement (refer figure 1), air extraction manifold (vent pipe) is disposed ex-centric to the tube bundle, which results in skewed flow of steam vapour (refer figure 2) and thereby leading to a lesser efficient heat transfer between the hot and cold medium. Further, multiple LP heater spaces in a common outer shell are separated through thermally insulated chambers using thermal insulation which requires use of costly material and with the passage of time, tends to deteriorate since exposed to elevated temperature. As may be observed in figure 2, this type of prior art venting arrangement has unequal steam distribution resulting in a lesser efficiency heater. OBJECTS OF THE INVENTION It is therefore an object of the present invention to propose an integrated horizontal condensing Duplex heater for heating-up water for water and steam circuit of coal based thermal power plants. Another object of the invention is to propose an integrated horizontal condensing Duplex heater for heating-up water for water and steam circuit of coal based thermal power plants in which the air venting arrangement has a venting manifold disposed in the center of the tube layout for uniform distribution of steam. A still another object of the invention is to propose an integrated horizontal condensing Duplex heater for heating-up water for water and steam circuit of coal based thermal power plants in which a plurality of air baffles of the air venting arrangement are disposed in the tube-layout in such a way that, it envelopes the first pass on water side so that there is no direct steam entry into the air vent pipe. Yet another object of the invention is to propose an integrated horizontal condensing Duplex heater for heating-up water for water and steam circuit of coal based thermal power plants the venting orifice in the manifold having an opening in the first pass only to allow the steam to travel maximum possible path before reaching the vent orifices to ensure complete condensation of the steam. A further object of the invention is to propose an integrated horizontal condensing Duplex heater for heating-up water for water and steam circuit of coal based thermal power plants in which the size of the vent orifice is selected to allow the critical flow of non-condensable gases to be uniform in the manifold. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout, and in which : Figure 1 shows a cross-sectional view through a Duplex Heater, as per prior art, to illustrate an asymmetric placement of vent pipes. Figure 2 shows skewed steam flow, as per prior art, and unequal steam distribution which results in poor heat transfer between cold and hot medium. Figure 3 shows basic details of a Duplex Heater according to the invention in a cross-sectional view. Figure 4 shows an outer side of a Duplex Heater with water chambers, an outer casing for two heat exchangers and a cross-sectional view through the water chamber of the Duplex Heater of the invention. Figure 5 shows a cross-sectional view of the Duplex Heater of the invention depicting a single wall partition. Figure 6 shows uniform and equal steam distribution which results in better heat transfer between cold and hot medium. SUMMARY OF THE INVENTION According to the present invention, there is provided an integrated horizontal condensing Duplex heater for heating-up water for water and steam circuit of coal based thermal power plants. A central venting in the Duplex heater is applied for removal of non-condensable gases. In order to remove non-condensable gases (air) from the heater, the vent tubes via which the gases are sucked away, are mounted in the bundle lanes inside the tube bundles. The inventive duplex heater ensures location of the vent pipe in the center of a first and second space within the heater. DETAILED DESCRIPTION OF THE INVENTION For better understanding of the proposed duplex/multiple heater, reference numerals for important features are being linked with like parts in relevant drawings wherein : Item 1 is a steam jacket (outer casing) Item 2 is a water-chamber casing Item 3 is plurality of steam inlet connection piece (turbine tapped steam) Item 4 is plurality of steam inlet connection piece (turbine tapped steam) Item 5 is water inlet connection piece Item 6 is water outlet connection piece Item 7 is condensate outlet connection piece from preheater S1 Item 8 is condensate outlet connection piece from preheater S2 Item 9 is tube-bundle leg Item 9’ is water inlet zone preheater S1 Item 10’ is water outlet zone from preheater S1 Item 11’ is water inlet zone from preheater S2 Item 12’ is water outlet zone from preheater S2 Item 10 is tube-bundle leg similar to item (9) Item 11 is tube-bundle leg similar to item (9) Item 12 is tube-bundle leg similar to item (9) Item 13 is tube plate Item 14 is the water inlet shroud (part water chamber) in water chamber Item 15 is the water outlet shroud (part water chamber) in water chamber Item 16 is plurality of tie rods and spacers Item 17 is plurality of bundle supporting plates Item 18 is the partition plate between heat exchange spaces S1 and S2 Item 19 is plurality of angles Item 20 is baffle plate Item 21 is seal Bar Item 22 is vent Pipe Item 23 represents welds Item 24 represents welds Figure 1 shows the exterior of a Duplex Heater with an outer casing or a steam jacket 1, attached to the steam jacket 1, acting as steam inlet connection pieces (3, 4). Steam, for example from a first tapping stage of a low-pressure turbine, is conducted via a plurality of steam inlet connection piece (3) into a first preheater space (S1) inside the jacket 1, wherein the steam from a second tapping stage of a low-pressure turbine is connected via the steam inlet connection piece (4) into a second heater space (S2) within the jacket (1). Arranged so as to adjoin the steam jacket (1) is a tube plate (13) in which the tubes of U-shaped tube bundles for the two heat exchangers are anchored. Connected to the tube plate (13) is a water chamber (2) with at least four part water chambers (9’, 10’, 11’, 12’). A water inlet connection piece (5) and a water outlet connection piece (6). The water to be heated up passes through the water inlet connection piece (5) into a first part of the water chamber (9’), the water inlet zone for the first-stage heater, and from there through the tube bundles arranged in series. (The direction of flow of the water from the part water chambers into the tube bundles and from these into the part chambers is indicated by crosses and circles). The tube bundles are connected to one another via a second part of the water chamber (10’), the water outlet zone of the first heater, and a third part water chamber (11’), and the Water inlet zone of the second heater. After flowing through the two tube bundles, the water flows into a fourth part of the water chamber 12’ the water outlet zone of the second heater, which finally emerges from the duplex heater via a water outlet connection piece (6). The part water chambers (10’ and 11’) being formed by the outer walls of the shrouds (14 and 15) and the inner wall of the water chamber (2) and being connected to one another. Figure 2 shows the interior of the duplex heater with a partition plate (18) according to the invention for subdividing and separating the pressure regions of the two heat exchangers from supporting plates (17) for the tube bundles (9-12) for implementing the transmission of the pressure force to the steam jacket (1). Shown again on the steam jacket (1) are the steam inlet connection piece (3 and 4), via which steam at a first pressure (P1) is conducted into the first heat exchange space (S1), or first-stage heater, and steam at a second pressure (P2) is conducted nto a second heat exchange space (S2), or the second-stage heater, in which case P1