FORM 2
THE PATENTS ACT, 1970 (39 of 1970)
As amended by the Patents (Amendment) Act, 2005
& The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
Horizontal box type heat recovery unit
APPLICANTS
Larsen & Toubro Limited, Engineering and Construction Division, Powai Campus (W), Saki Vihar Road, Mumbai 400 072, Maharashtra, India, an Indian company
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed :

FIELD OF THE INVENTION
This invention relates to a horizontal box type heat recovery unit.
BACKGROUND OF THE INVENTION
Heat Recovery Units (HRUs) are devices used at the downstream of equipments like diesel generators, gas turbines, furnaces, cracking furnaces, process heaters, reformers or gasifiers in order to recover heat from hot gas like process and / or flue gases emanating from such equipments. Heat recovery units may be shell and tube type heat exchangers which are governed by the Tubular Exchanger Manufacturers Association Inc, USA or TEMA standards or box type (non-TEMA type) heat exchangers. Generally, box type heat exchangers are used in most of the heat recovery applications.
A typical box type heat recovery unit comprises a box type body having a pyramidal hot gas inlet at one end thereof and a pyramidal hot gas outlet at other end thereof and atleast one tube bank located in the box type body. The heat recovery unit may comprise guide vanes located in the hot gas inlet thereof. The tube bank has a working fluid (heat receiving or absorbing fluid) inlet and outlet. Hot gas enters the heat recovery unit via the hot gas inlet thereof, travels through the heat recovery unit over the tube bank and exits the heat recovery unit through the hot gas outlet thereof. Simultaneously, a working fluid such as oil to be heated up enters the tube bank via the working fluid inlet thereof, flows through the tube bank and exits the tube bank via the working fluid outlet thereof. While

flowing though the tube bank, the working fluid receives or absorbs the heat of the hot gas flowing over the tube bank and heat exchange or heat transfer between the hot gas and the working fluid takes place and the working fluid gets heated up.
In order to achieve uniform distribution of the hot gas across the tube bank, the heat recovery unit may comprise a perforated distributor plate ahead of the tube bank. However, the distributor plate causes the flow pressure of the hot gas to drop thereby increasing the energy demand of the induced draft fan located in the downstream of the heat recovery unit and/or the forced draft fan located in the upstream of the heat recovery unit. Further in the case of a horizontal heat recovery unit, the hot gas flowing through the heat recovery unit tend to bias towards the top of the unit owing to thermal buoyancy. As a result of this, the heat recovery by the heat recovery unit is reduced as the amount of hot gas flowing in the lower portion of the heat recovery unit reduces.
DETAILED DESCRIPTION OF INVENTION
According to the invention there is provided a horizontal box type heat recovery unit comprising a box type body having a pyramidal hot gas inlet at one end thereof and a pyramidal hot gas outlet at the other end thereof and atleast one tube bank located in the box type body, the tube bank having a working fluid inlet and outlet, the heat recovery unit further comprising guide vanes located in the hot gas inlet thereof, wherein the heat recovery unit also comprises a horizontal gas flow partition plate disposed horizontally in

the hot gas inlet thereof and extending axially upto the tube bank, the guide vanes being equidistantly located with respect to the inner wall of the hot gas inlet and the partition plate being equidistantly located with respect to the guide vanes.
The following is a detailed description of the invention with reference to the schematic accompanying drawings, in which:
Fig 1 is a plan view of a horizontal box type heat recovery unit according to an embodiment of the invention; and
Fig 2 is a cross-sectional view of the heat recovery unit of Fig 1 along the length thereof.
As illustrated in Figs 1 and 2 of the accompanying drawings, the heat recovery unit 1 comprises a box type body 2 having a pyramidal hot gas inlet 3 at one end thereof and a pyramidal hot gas outlet 4 at the other end thereof. The heat recovery unit also comprises guide vanes 5 located in the hot gas inlet 3 thereof for guiding the hot gas entering the heat recovery unit through the inlet. The heat recovery unit also comprises two tube banks 6 and 7 located in the box type body in spaced apart relationship with each other. The tube bank has a working fluid inlet 8 and outlet 9. The tube banks are connected to each other through connector 10. The heat recovery unit also comprises a horizontal gas flow partition plate 11 disposed horizontally in the hot gas inlet thereof and extending axially along the centre of the hot gas inlet upto the tube bank 6. The guide vanes are located equidistantly with respect to the inner wall of the hot gas inlet and the partition plate is located equidistantly with respect to the guide vanes. The guide vanes are fixed

to the inner wall of the hot gas inlet. The partition plate is also fixed to the inner wall of the hot gas inlet.
The partition plate helps to reduce the swirling motion of the incoming gas and acts as a flow straightener. As a result, the hot gas bias towards the top of the heat recovery unit owing to thermal buoyancy is reduced. Thus the partition plate helps to distribute the hot gas uniformly across the top and bottom of the heat recovery unit. It also reduces the fluid gas flow pressure drop in the heat recovery unit. As a result of all this, the heat recovery by the heat recovery unit is increased. Further the energy demand of the induced draft fan in the downstream of the heat recovery unit and/or the forced draft fan in the upstream of the heat recovery unit is reduced. The heat recovery unit of the invention otherwise functions in the usual manner.
The following comparative experimental example is illustrative of the invention but not limitative of the scope thereof.
Example 1
Comparative simulation studies were conducted with a horizontal box type heat recovery unit (HRU) of Figs 1 and 2 designed for heat recovery of 2.2Gcal/h. The HRU was placed downstream of a gas turbine. Flue gas at 445°C was passed through the HRU. Oil (Hytherm - 500 of Hindustan Petroleum, India) at 140°C was passed through the tube banks of the HRU. Mass flow rates of flue gas and oil were 15.51 kg/s and 8.42 kg/s, respectively.

Using CFD (Computational fluid dynamics) software Fluent of ANSYS Inc, USA, gas velocity in the HRU was measured with the horizontal partition plate (HRU of the invention) and without the horizontal partition plate (conventional HRU) and the results were as shown in the following Table:
Table

Gas velocity across the HRU close to the first tube bank 6 Gas velocity across the HRU close to second tube bank 7 Gas velocity at the centre of the HRU along the gas flow direction

HRU
without the
partition plate HRU with the partition plate HRU without the . partition plate HRU with the partition plate HRU
without
the
partition
plate HRU with the partition plate
Max velocity (m/s) 26.06 21.65- 22.35 18.89 43.18 38.88
Min velocity (m/s) 0.20 0.30 0.16 0.46 0.20 0.30
Standard Deviation 6.45 6.00 5.30 4.08 8.92 6.42
It is evident from the Table that in the case of the invention the standard deviation and difference between maximum and minimum gas velocities reduced thereby improving the gas velocity field inside the HRU. For the flue gas flow rate of 15.51 kg/s, pressure drop in the case of the conventional HRU was 1233 Pa, whereas in the case of the invention the pressure drop was only 1200 pa. Thus, the reduction in hot gas pressure drop according to the invention was 33 Pa. The above findings clearly show that in the HRU

of the invention hot gas bias towards the top of the HRU due to thermal buoyancy is
reduced.
Because of the reduction in pressure drop, the energy demand of the induced draft fan at the down stream of the heat recovery unit and/or the forced draft fan at the upstream of the heat recovery unit is also reduced. Uniformity of hot gas distribution is improved.
Using the ASPEN-EDR software of ASPENTech, USA, the heat recovery was calculated. Heat recovery in the conventional HRU was 1.97 Gcal/h andin the HRU of the invention was 2.2 Gcal/h. Thus the heat recovery in the case of the HRU of the invention increased by 10% and was equal to the designed heat recovery of the HRU.
The oil temperature increased to 246°C in the case of the conventional HRU and in the case of the HRU of the invention the oil temperature increased to 2580C. While operating the conventional HRU, the temperature of the exit flue gas dropped to 312°C and while operating the HRU of the invention, the temperature of the exit flue gas dropped to 297 °C. The increase in the oil temperature and the drop in the exit flue gas in the case of the HRU of the invention validate the simulated improved heat recovery value by the HRU of the invention.

We Claim:
1. A horizontal box type heat recovery unit comprising a box type body having a
pyramidal hot gas inlet at one end thereof and a pyramidal hot gas outlet at the other end thereof and atleast one tube bank located in the box type body, the tube bank having a working fluid inlet and outlet, the heat recovery unit further comprising guide vanes located in the hot gas inlet thereof, wherein the heat recovery unit also comprises a horizontal gas flow partition plate disposed horizontally in the hot gas inlet thereof and extending axially upto the tube bank, the guide vanes being equidistantly located with respect to the inner wall of the hot gas inlet and the partition plate being equidistantly located with respect to the guide vanes.
2. The heat recovery unit as claimed in claim 1, which comprises two tube banks located in the box type body in spaced apart relationship with each other and connected to each other through a connector and the partition plate extends upto the first tube bank from the hot gas inlet of the heat recovery unit.