United States Patent: 3553461

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United States Patent	3,553,461
Siano , et al.	January 5, 1971

METHOD AND APPARATUS FOR DETECTING THE PRESENCE OF DANGEROUS CONCENTRATIONS OF COMBUSTIBLE GASES OR VAPORS IN THE AIR

Abstract

Method and apparatus for the continuous detection of relative amounts of combustible gases and vapors in ambient air and wherein the ambient air flows through a reaction chamber in contact with an oxidation catalyst in the presence of a plurality of detecting means sensitive to the varying radiant energy emitted along said catalyst in the direction of flow of the ambient air and means for receiving and comparing these different detected values.

Inventors:	Siano; Matteo (Bologna, IT), Guidelli; Guidi Guido (Bologna, IT)
Appl. No.:	04/680,558
Filed:	November 3, 1967

Current U.S. Class: 436/159 ; 250/565; 250/575; 340/633; 422/98; 436/164

Current International Class: G01N 21/71 (20060101); G01n 021/26 ()

Field of Search: 250/218 73/26 340/237(Inquired) 23/232E,254E,255E(Inquired)

References Cited [Referenced By]

U.S. Patent Documents


2878388	March 1959	Bergson
3312826	April 1967	Finkle
2879663	March 1959	Thomas
3238519	March 1966	Ramsey
3087795	April 1963	Ross

Primary Examiner: Lindquist; William F.
Assistant Examiner: Leedon; C. M.

Claims

We claim:

1. A method for continuously detecting the relative amounts of combustible gases or vapors in the ambient air comprising the steps of:

1. continuously flowing ambient air through a longitudinal chamber in contact with a longitudinally disposed oxidation catalyst;

2. continuously heating the oxidation catalyst to produce continuous and successive oxidation reactions along the longitudinally disposed catalyst whereby radiant energy is emitted from said reactions in response to the combustible gases and vapors in the ambient air at every point along the catalyst;

3. measuring the emitted radiant energy at a plurality of longitudinally disposed points along the path of the ambient air in the longitudinal chamber; and

4. determining the differences between measurements at the aforesaid points and applying said differences to prescribed standards to determine the relative amounts of combustible gases or vapors in the ambient air.

2. Apparatus for the continuous detection of the relative amounts of combustible gases and vapors in the ambient air said apparatus comprising a reaction vessel, said vessel having a longitudinally disposed reaction chamber, an oxidation catalyst longitudinally disposed in said chamber, means for activating the catalyst, means for flowing ambient air through the longitudinally disposed reaction chamber in contact with the oxidation catalyst thereby causing the catalyst to emit radiant energy in response to the combustible gases and vapors in the ambient air, two radiant energy detecting units positioned in longitudinal spaced relation adjacent the catalyst to sense radiant energy emitted from the catalyst, and means to receive detected values from the detecting units and compare the differences against a prescribed standard.

3. The invention as set forth in claim 2 and wherein an alarm is provided in the apparatus and is actuated when the detected differences exceed prescribed values.

4. The invention as set forth in claim 3 and wherein the reaction vessel is provided with a pair of longitudinally spaced pockets, each pocket housing one of the radiant energy detecting units.

5. The invention as set forth in claim 2 and wherein the means to receive the detected values and compare the differences against a prescribed standard includes a differential amplifier.

6. The invention as set forth in claim 2 and wherein the oxidation catalyst is a platinum wire.

7. The invention as set forth in claim 2 and wherein the catalyst is coated on a suitable support member.

8. The invention as set forth in the reaction vessel is provided with cooling fins.

Description

It is known that, when the concentration of a combustible gas or vapor in the ambient air attains a certain minimum value, called the "lower limit of inflammability" or LLI, a danger of spontaneous ignition of the air-combustible mixture exists; in like manner when the concentration of a toxic gas or vapor in the ambient air attains a certain maximum value, viz, the maximum tolerable limit (MTL) there exists a danger of intoxication. The ignition limits for the hydrocarbons are known, as well as MTL, and this particularly is true for the aromatic hydrocarbons.

This invention has as its object a method and apparatus of detecting in the ambient air the presence of a gas or vapor which is combustible, such as a hydrocarbon, or toxic, such as carbon monoxide, by the emission of a signal when the concentration of the gas or vapor reaches a certain safety value.

This value is lower than the lower inflammability limit (or LLI) of the gas or vapor on the basis of a safety coefficient, which is determined in advance.

In the ambient air and in presence of a gas concentration (or vapor concentration) which is equal or greater than the lower limit of inflammability (LLI) there exists a danger of ignition, from any heat source whatsoever (lighted cigarette, strongly heated metallic parts, and so on) when it brings the temperature of the mixture to the spontaneous ignition temperature (SIT) of the fuel. This will immediately promote the ignition of the mixture and the propagation of the flame with consequent swift increase of the pressure and explosion.

On the other hand, there is no ignition danger when the concentration of the gas or vapor is lower than the LLI, because, whatever the heat source may be, no ignition reaction will be possible, due to the lack of the necessary kindling energy for obtaining the reaction, in view of the insufficient fuel mass.

The problem of the emission of a signal when the concentration of the gas or vapor attains a certain safety value is therefore equivalent to solving the problem of rendering possible the continuous observation of the variations of the activation energy of the oxidation reactions of the mixture, prior to its ignition.

These reactions do not generate any light and only a small amount of heat. Generally it might be said that, up-to-date, there is no apparatus which performs a direct and continuous observation.

The Wheatstone-bridge devices, for example, indicate the electrical effect (resistance variation) within the catalyst filament due to the heat generated by the reaction; but this effect does not measure this amount and the indications are indirect and rough.

Other apparatus, which are usually influenced by the relative humidity of the ambient air, present considerable inconveniences and limitations.

This invention aims to overcome the drawbacks of the known devices by means of a method and apparatus permitting the continuous observation of the variations of the activation energy of the oxidation reactions of combustible gases or vapors, within the concentration limits of safety within the ambient air, independently of the variations of the humidity and in a very accurate manner.

In practice, the oxidation reactions take place within a catalytic reaction chamber having an almost stationary thermodynamic behavior and the stationary energy status (relative variation of activation energy) which corresponds to the safety concentration, is signaled by photosensitive devices which are suitably arranged and connected to the reaction tube.

The peculiar characteristic features and advantages of the invention will be apparent from the following specification of an embodiment of the invention, described with reference to the accompanying drawing, wherein:

FIG. 1 shows diagrammatically an apparatus according to the invention; and

FIG. 2 shows, in cross section, a detail of the reaction chamber.

Referring particularly to the drawing, 1 denotes the reaction tube which, in the example as shown, is made of glass. One end of the tube 1 is connected to a feed pump P for the air under examination, while the opposite end opens to the atmosphere. The tube 1 is enclosed in a suitable envelope 2 made of good heat-conductive material, for example aluminum, and is provided with fins 3 for heat dissipation.

In the envelope 2 two small chambers or pockets 4 and 5 open towards the bore 6 of said tube 1. In each of said pockets 4 and 5 a photosensitive cell, 7, 107, is mounted. Coaxially within tube 1 a thin platinum helical filament 8 is mounted. The helical filament 8 has such a length as to extend in front of the photosensitive elements 7 and 107. B is an electric battery for feeding current to the filament 8.

The photosensitive elements 7, 107 are connected to a suitable differential amplifier AD, the output of which is utilized for controlling a relay R which, upon being sufficiently energized, closes the usually open contact 9 of an alarm circuit comprising, for example, a battery B1 and the bell 10.

To the output of the amplifier AD some recording, measuring, indicating or other apparatus M may be connected.

It is to be noted that within the tube 1, which contains a heat source, a fuel-air mixture flowing therethrough under the action of the heat forms a thermodynamic reaction system which is almost stationary. Experience has shown that the partial concentrations and pressures of the system are not equal in the different points within the tube 1. This is in contrast with an approximately insulated system in which to each temperature value there corresponds an almost uniform distribution of energy (thermodynamic equilibrium). Within tube 1 there is no such uniform energy or temperature distribution.

It is still to be noted that from an electrical view point, and as experience has shown, the above-described tube is intrinsically safe, as in it no ignition of combustible mixture can take place, whatever the concentration of the fuel may be.

It is thus possible to effect along the metallic catalyst and upon a very thin skin thereof taken as support, different oxidation reactions from one spot to another, as the respective concentrations and temperatures are different. It is possible to detect these AIR of radiating energy generated by said reactions by means of photosensitive cells 7 and 107.

Said phenomena take place as follows:

UNPOLLUTED AIR

The unpolluted air, i.e. the air which does not contain a combustible gas or vapor, is filtered before being sucked by the pump P and is blown continuously from this pump (which may be a membrane pump having an adjustable output) through tube 1 at a slightly superatmospherical pressure. The metallic catalyst 8, being energized by its own energy source B is rendered incandescent or, in other words, is caused to emit, due to the activation of the valence electrons of the metal, luminous radiations within the visible spectrum with a wave length .lambda. of 0.8 to 0.4 .mu. (micron). These radiations, through energy quanta a quantity of energy (heat) which increases with the frequency .nu. and is reduced with the increase of the wave length of said radiations and the heat quantity thus set free (metal incandescence status) are proportional to the energy furnished to the metal.

As the amount of this energy is small, the luminous emission, during the normal operation of the apparatus, is in the band of the red. But .lambda. depends upon the composition of the atmosphere (c is light propagation velocity in the vacuum); whereas the ambient air is a mixture of nitrogen (about 78 percent by volume), oxygen (about 20 percent) and a certain amount of water vapor and consequently, within the tube 1 there is an interaction between the radiations (photons) emitted by the metal and the corpuscles of the ambient air (electrons loosely bound to the atoms), which has for its effect to reduce the frequency (and to increase the wave length) of the radiations.

The radiations (generally within the short infrared band) striking against 7 and 107 are the result of this interaction and represent the status of the energy taken as starting level of the observation.

POLLUTED AIR (ambient air containing some combustible gas or vapor).

The gas or vapor and the air react along the catalyst 8 and the values of fuel concentration, temperature of the mixture and reaction heat are different from place to place. Namely, they are greater in front of 7 than in front of 107 and the different energy values (with respect to one another and to the starting level) in front of 7 and 107 depend upon the fact of the different transitions of the atoms (their valence electrons) during the respective reactions, viz. they depend upon the different frequency of the emitted radiations.

By the photosensitive devices 7 and 107 the radiations striking against them are transduced to electric values, magnified by AD and, upon the relative energy attaining a certain (adjustment) value which corresponds to the safety concentration of the polluting gas or vapor, relay R closes the switch G and the alarm bell rings.

The catalyst may be made either of a transition metal, such as platinum, palladium, nickel or cobalt, having a compact structure and a large active surface area and in form of helical or coiled filament, or by two or more of said components, forming an alloy, or by one of them acting as catalyst and another substance (for example, thorine) acting as "kindler", whose function is to enhance the catalytic action, even if the kindler is, per se, inactive; or also be a mixture of substances producing together an effect of synergism. The catalyst, with or without kindler or synergistic effect may be made in a variety of manners, for example, upon a support which may be either metallic (such as nickel-chrome) or of suitable ceramic material (such as aluminosilicates).

The photosensitive elements 7 and 107 may be constituted by photovoltaic cells made of semiconductors (silicium, germanium and so on), said semiconductors being chosen so as to give a suitable spectral reaction in a determined band, or else by other devices for the direct transformation of luminous energy into electric energy.

Finally it may be noted that a rougher detection may be effected by means of thermocouples welded to the catalyst filament or by thermistors suspended within the tube.

* * * * *

Current U.S. Class:	436/159 ; 250/565; 250/575; 340/633; 422/98; 436/164
Current International Class:	G01N 21/71 (20060101); G01n 021/26 ()
Field of Search:	250/218 73/26 340/237(Inquired) 23/232E,254E,255E(Inquired)