GB2352803A

GB2352803A - Air pre heater for fired process heater

Info

Publication number: GB2352803A
Application number: GB9917010A
Authority: GB
Inventors: Gary Prosser
Original assignee: Foster Wheeler Energy Ltd
Current assignee: Amec Foster Wheeler Energy Ltd
Priority date: 1999-07-20
Filing date: 1999-07-20
Publication date: 2001-02-07
Anticipated expiration: 2019-07-20
Also published as: GB2352803B; GB9917010D0; EG23052A

Abstract

The fired process heater has a number of natural draught burners 2 or banks of burners, and a plurality of air heat exchangers 6 for preheating the combustion air, each individual heat exchanger being connected to one or a small group of burners by a relatively short duct 4 such that the natural draught of the burners is greater than frictional losses in the supply of the preheated air.

Description

2352803 AIR PREHEATER FOR FIRED PROCESS HEATER This invention relates to a

combustion air preheat scheme using saturated steam as a heating medium in a fired process heater.

Where a fired process heater containing multiple burners employs a combustion air preheat scheme to increase the overall thermal efficiency, significant capital cost savings can be made over a classical forced draught steam to air preheat scheme by individually preheating the air to each burner.

The frictional losses as the air passes through the steam to air exchanger and short section of duct work can be overcome by the negative pressure inside the furnace allowing natural draught burners to be used, removing the need for a forced draught fan.

In a process heater or furnace, energy is transferred from hot combustion gases to the charge increasing the enthalpy of the charge. The input is provided by burning fuel, usually gas or oil, in a combustion chamber to produce hot combustion gas. Heat is transferred from the hot combustion gases to the feed material contained in tubes. The fuel-oxidizing medium is typically air.

Raising the temperature of the fuel oxidising medium increases the overall thermal efficiency of the process heater reducing the fuel consumption rate. This process is known as combustion air preheating. Significant fuel cost savings can be made by preheating the combustion air.

To achieve this when using steam as the heating medium, a single steam to air heat exchanger is used to heat all the combustion air and the hot air is ducted to each burner. This results in large lengths of combustion air ductwork. Also, to overcome the frictional pressure drop, as the air flows through the ductwork, a forced draught fan has to be employed to blow the air through the heat exchanger and ductwork to the burners. This system results in a large capital outlay for:

2 force draught fan(s), combustion air ductwork, foundations to support fan and ductwork.

There is also the running cost associated with the forced draught fan.

Natural draught burners rely on the negative pressure inside the process heater or furnace, generated by either stack buoyancy or an induced draught fan, to induce the combustion air into, and through the burner. With this method of air preheat this is not possible as the negative pressure inside the process heater or furnace is not sufficient to overcome the frictional losses associated with the long run of combustion air ductwork and heat exchanger, and a forced draught combustion air fan must be employed.

It is an object of the invention to avoid the need for this additional forced draught fan and to save on the cluctwork required.

According to the invention there is provided a fired process heater having a number of natural draught burners or banks of burners, and a plurality of air heat exchangers for preheating the combustion air, each individual heat exchanger being connected to one or a small group of burners by a relatively short duct such that the natural draught of the burners is greater than frictional losses in the supply of the preheated air.

The proposed preheat scheme overcomes the need for a forced draught fan, resulting in a significant capital outlay, by individually preheating the combustion air to each burner or bank of burners rather than all of the burners. The arrangement incorporates a single steam to air heat exchanger and a short combustion air duct to each burner or each group of natural draught burners. The frictional losses are sufficiently low such that the negative pressure inside the furnace is sufficient to overcome these frictional losses and so allow the natural draught burners to operate without the need of an extra forced draught fan.

3 The air-heating medium is desirably saturated steam which passes through a series of coils, one in each air heat exchanger, with the combustion air flowing over the coils.

Thus, saturated steam passes through the series of tubes and air passes over the tubes and is heated by the steam.

Typically there would be as many as 36 or 48 burners in a fired process heater. According to the invention, an individual air preheater is provided for each of these or alternatively a group of 2, 3 or 4 of these. It is unlikely that more than 5 burners would be supplied from an individual air heater otherwise the length of ducting required would become too long and the frictional losses too great.

Fired process heaters tend to be constructed with external upright steel supports spaced along the side edges of the structure. These supports therefore tend to divide the sides of the structure into convenient portions each containing one or a group of a few burners. Therefore, it is convenient to provide a separate air preheater for each of these portions and to position it closely alongside that portion. Then the length of ductwork required to join such an air preheater to one or a group of burners can be short, e.g. about 2m, and the frictional losses in the movement of the air can be overcome by the natural draught of the burner or burners.

Preferably each air preheater includes a first portion containing the heating coil and a second mixing portion which allows for thorough mixing of the heated air and one or more short ducts then joins that second portion to an individual burner. Desirably there is an individual duct from that mixing portion direct to an individual burner so that the frictional losses to each burner are largely identical.

4 The invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure I shows a known form of preheater arrangement for a fired process heater; Figure 2 is an enlarged detail showing an arrangement according to the invention; and Figure 3 is a side view of Figure 2 taken in the direction of the arrow A.

The known form of a fired heater shown in Figure 1 includes a process fired heater 1 shown schematically. It includes a number of natural draught burners 2 in the base of the furnace and a process stream passes through suitable heating tubes suspended within the heater.

The outlet gasses are drawn from the furnace through an induced draught fan 12 and sent to an exhaust stack 13.

Input air passes initially through a forced draught fan 14 to an air preheater 15. In the air preheater are heating coils 16 fed with saturated steam to heat the air. The resulting heated air then passes through a duct 17 before passing to individual short branch lengths of ducting 18 for each burner 2.

In a typical configuration the fired heater 1 could have 36 to 48 burners and the length of ducting 17 could be of the order of a minimum 1 Om since the air preheater 15 will be relatively large to deal with all of the combustion air and since the burners might be positioned at a far corner of the heater, these might be positioned much further away than others. Because of this relatively long length of ducting, the forced draught fan 14 is required.

Figures 2 and 3 show an arrangement according to the invention and the same reference numerals are used as appropriate.

In this case, atmospheric combustion air enters through an inlet 9 having an optional rain hood 10 to the heat exchanger 6 and flows down over one or more steam coils 12. The size and number of steam coils is dependant on the degree of preheat and the steam conditions.

I Air flows over the coils and is heated and the heat exchanger 6 is sized to give a desired combustion air temperature at its outlet.

The preheated combustion air passes into a distribution box 5 which forms a second portion of the exchanger where the flow is distributed and mixed so that it can flow evenly to each burner 2. One or more air ducts 4 leave the distributor and the combustion air flows down the ducts 4 to an individual burner 2. The duct 4 is sized to fit into the specific application. This duct 4 is, however, quite short, e.g. about 2m in length, and will be the same for each burner.

The combustion air duct 4 is attached to the burner 2 via an adapter section 3 to fit the inlet of the burner. The negative pressure inside the fired heater 1 induces the flow of combustion air through the system removing the need for a forced draught fan.

Although a number of steam coils and heat exchangers are required their cost is less than the cost of a large coil in a single heat exchanger plus the extra forced draught fan and the relatively long lengths of air ducting required with prior arrangements.

6

Claims

CLAIMS:

I. A fired process heater having a number of natural draught burners or banks of burners, and a plurality of air heat exchangers for preheating the combustion air, each individual heat exchanger being connected to one or a small group of burners by a relatively short duct such that the natural draught of the burners is greater than frictional losses in the supply of the preheated air.

2. A fired process heater as claimed in Claim 1 in which the air-heating medium is designed to be saturated steam which passes through a series of coils, one in each air heat exchanger, with the combustion air flowing over the coils.

3. A fired process heater as claimed in either preceding claim in which there are 36 to 48 burners, and an individual air preheater is provided for each of these or alternatively a group of 2, 3 or 4 of these.

4. A fired process heater as claimed in any preceding claim which is constructed with external upright steel supports spaced along the side edges of the structure, these supports dividing the sides of the structure into convenient portions each containing one or a group of a few burners, and a separate air preheater is provided for each of these portions and is positioned closely alongside that portion.

5. A fired process heater as claimed in any preceding claim in which each air preheater includes a first portion containing the heating coil and a second mixing portion which allows for thorough mixing of the heated air and in which one or more short ducts then joins that second portion to an individual burner.

7 6. A fired process heater as claimed in Claim 5 in which there is an individual duct from that mixing portion direct to an individual burner so that the frictional losses to each burner are largely identical.

7. A fired process heater substantially as herein described with reference to Figures 2 and 3 of the accompanying drawings.