GB2166546A

GB2166546A - Apparatus for flue gas analysis

Info

Publication number: GB2166546A
Application number: GB08527126A
Authority: GB
Inventors: John Paul Christopher Fenton
Original assignee: BESTOBELL
Current assignee: BESTOBELL
Priority date: 1984-11-06
Filing date: 1985-11-04
Publication date: 1986-05-08
Also published as: GB8428026D0; GB2166546B; GB8527126D0

Abstract

A sample of hot gas is channeled from a flue (1), via a flow pipe (3) to a gas analyser unit (5) having a thermally insulating casing (13). Upon entering the analyser unit (5), the gas sample is cooled as it passes through a heat exchanger section (16), from which heat is extracted by a Peltier cell (20) and then disipated by a finned radiator (21). The gas sample passes from the heat exchanger section (16) to a gas analysing section (17), and is then returned to the flue (1) via a return pipe (6). The relative positioning of the Peltier cell (20), heat exchanger section (16) and gas analysing section (17) is such that the gas analysing section (17) is at a higher temperature than the heat exchanger section (16). This prevents water condensing from the gas sample after it has left the heat exchanger section (16) and thereby ensures that the atmosphere in the gas analysing section (17) is not saturated with water. <IMAGE>

Description

SPECIFICATION Apparatus for flue gas analysis The invention relates to apparatus for monitoring at least one component, such as oxygen or carbon monoxide, in gas passing along a flue from a burner, such as a burner associated with a boiler. The usual reason for monitoring one or more components of the gas is to provide a feed back for automatic or manual trimming of the burner, e.g. adjusting the combustion air supply, to avoid a dangerous or inefficient oxygen rich or oxygen lean mixture of fuel and oxygen at the burner. The apparatus has a flow pipe which leads from within the flue to a unit positioned externally of the flue and incorporating a cell for measuring the concentration of the component of interest. Such an apparatus is described for example in our GB-A-2097531.

Typical measuring cells are electrochemical in operation and are sensitive to temperature and humidity, and must be maintained below a maximum working temperature, of say 35"C for an oxygen cell and 45"C for a carbon monoxide cell, and in a non water-saturated atmosphere. Typical flue gas temperature is in excess of 200"C and the sample of gas must be cooled to the working temperature of the cell before coming into contact with the cell.

Even if the rate of flow of the gas sample along the flow pipe frqm the flue is very slow, so that the gas is cooled substantially to ambient temperature before it reaches the cell, this will usually be insufficient as ambient temperature in the vicinity of a boiler and flue pipe may be between 60 and 80"C, and further cooling is therefore necessary. This may be achieved by means of a heat exchanger through which the gas passes upstream of the unit containing the cell. This in turn introduces the problem that flue gas has a high water vapour content and it is to be expected that the gas sample will be cooled below the dew point temperature of the flue gas so that the gas sample leaving the heat exchanger will be saturated and, upon any further cooling in the unit will cause condensation at the cell.It has therefore been proposed that, after cooling of the gas sample to a first temperature in a primary heat exchanger, the gas is heated slightly and then cooled adjacent to the inlet of the cell by a secondary heat exchanger to a second temperature which is slightly higher then the first temperature, thereby ensuring that the gas entering the cell is not saturated.

This solution is extravagant in requiring two heat exchangers and a heating means between the heat exchangers, and is therefore cumbersome and expensive, and is difficult to control owing to the operation of three separate temperature-controlling devices.

In accordance with the present invention, apparatus for analysing a gas passing through a flue comprises a unit which is arranged to be positioned outside the flue and to receive a gas sample via a flow pipe from within the flue to the unit; the unit comprising an externally insulated, thermally conductive, housing containing a heat exchanger section for cooling the gas sample and condensing water vapour in the sample and, downstream of the heat exchanger section, a gas analysing section, and the unit being provided with a heat pump for withdrawing heat from the heat exchanger section and for creating a temperature gradient in the housing in a direction such that the temperature at the gas analysing section is higher than that in the heat exchanger section.

This arrangement of uniting the heat exchanger and gas analysing cell in a common thermally conductive housing enables the use of a single heat pump to cool the gas in the heat exchanger and, in providing a temperature gradient in the appropriate direction, ensures that the saturated gas in the heat exchanger will positively be raised to a slightly higher temperature, at which is not saturated, as it enters the analysing cell.

Most conveniently the housing is a metal block, usually of stainless steel, accomodating the heat exchanger and analysing cell alongside one another. The heat pump may be provided by flowing cold water , or by a compression/expansion refrigeration system, but is preferably provided by a Peltier cell, in thermal contact with a wall of the block to the side of the heat exchanger remote from the gas analysing cell. The heat output side of the Peltier cell may be coupled to a finned metal radiator.

In this case the heat exchanger may be provided by a number of parallel ducts through the block, separated by narrow webs and all adjacent to the wall of the block to which the Peltier cell is coupled.

The gas sample may be caused to flow through a flow pipe to the unit, and back from the unit to the flue via a return pipe, under a pressure differential at the ends of the flow and return pipes in the flue, by appropriate shaping, positioning and orientation of the ends of the flow and return pipes in the flue.

One example of apparatus constructed in accordance with the present invention is illustrated diagrammatically in the accompanying drawing.

The apparatus is located to one side of a flue 1, up which hot flue gas passes. An end portion 2 of a flow pipe 3, which has a low point 4 and which leads to the inlet of a gas analyser unit 5, projects into the flue 1. A return pipe 6 leads from an outlet of the unit 5 and has an outlet end portion 7 also projecting into the flue 1. The end of the portion 2 is cut obliquely at 8 and the end of the portion 7 is cut straight across at 9, and this ensures a sufficient pressure difference to cause a gentle flow of flue gas through the flow pipe 3 for analysis in the unit 5 and back to the flue.

In use, water condensing in the flow pipe 3 drains back into the flue via the low point 4 and through an S-shaped drainpipe 10, which has an end portion 11 also projecting into the flue. In the steady state water will collect in the drain pipe as shown at 12. The end portion 11 may be shaped as shown in dashed lines at 11 a, so that, like the end portions 2 and 7, it projects perpendicularly through the wall of the flue, enabling simple assembly with the flue. However, in both cases, the outlet opening of the portion 11 or 11 a faces in the upstream direction of the flue, and is positioned closer to the wall of the flue than the opening at the end of the portion 2, to provide the necessary pressure differentials upon startup.

The unit 5 comprises a thermally insulating casing 13, containing a stainless steel housing block 14 with a removable cover 15. The block 15 incorporates a heat exchanger section 16 and a gas analysing section 17. In the section 16, the flue gas supplied to the unit inlet through the flow pipe 3 is fed up through a number of parallel ducts 18, separated by narrow webs 19. Heat exchange takes place with the walls of the ducts and the heat is transferred by a Peltier cell 20, fixed to the wall of the housing 14, to a finned radiator 21 which dissipates the heat.

The cooled flue gas leaving the upper ends of the ducts 19 passes down past an analyser housing 22 and into a groove 25. At the bottom of one end of the groove 25 is an opening 26 of the return pipe 6 along which the gas is eventually returned to the flue 1. The gas, therefore, flows along the groove 25 and down through the opening 26. The analyser housing 22 stradles the groove 25 above the opening 26. This arrangement enables some of the gas to enter the analyser housing 22, through a small hole (not shown) in its base, where appropriate analysis is carried out. Electrical signals are conducted away from the cell for processing, and the Peltier cell 20 is energised, via terminal blocks 23 and leads 24.

As the Peltier cell 20 is on that wall of the block 14 which is to the side of the heat exchanger 16 remote from the analysing section 17, the thermal gradient through the block will be such that it will be slightly warmer in the section 17 than at the outlet from the ducts 18. The effect of this is that it is to be expected that water in the flue gas will condense in the section 16, and run back through the pipe 3 to the low point 4, but that the saturated flue gas leaving the section 16 will be slightly heated as it enters the section 17, so that no further condensation will occur to interfere with the gas analysis in the section 17.

Claims

1. An apparatus for analysing a gas passing through a flue comprises a unit which is arranged to be positioned outside the flue and to receive a gas sample via a flow pipe from within the flue to the unit, the unit comprising an externally insulated, thermally conductive, housing containing a heat exchanger section for cooling the gas sample and condensing water vapour in the sample and, downstream of the heat exchanger section, a gas analysing section, and the unit being provided with a heat pump for withdrawing heat from the heat exchanger section and for creating a temperature gradient in the housing in a direction such that the temperature at the gas analysing section is higher than that in the heat exchanger section.

2. An apparatus according to claim 1, in which the heat pump is a Peltier cell in thermal contact with a wall of the housing to the side of the heat exchanger section remote from the gas analysing section.

3. An apparatus according to claim 2, in which the heat exchanger section comprises a number of parallel ducts through the housing, separated by narrow webs and all adjacent to the wall of the housing to which the Peltier cell is coupled.

4. An apparatus substantially as described with reference to the accompanying drawings.