The present invention relates to a process and an apparatus for humidifying a combustible gas. The invention is employed, in particular, in the heat treatment of metallic products, particularly when the combustible gas is used for the heat treatment of metallic products.

In the heat treatment of metallic products, it is usual to heat the metallic products in a protective gas atmosphere composed of combustible gas in a furnace. When combustible gases, in particular gases having a low oxidation potential, are used, defects can arise on the metallic products. For example, residues such as lubricant residues or binders on the product surface and/or in the furnace space may be incompletely oxidized. These residues remain as cracking products on the product surface and/or on the interior surfaces of the furnace space and have to be removed, which costs money.

In the case of extremely clean and metallically pure product surfaces, welding of the metallic products to one another can occur. These unintentionally welded metallic products have to be separated from one another with an additional outlay and the resulting surface defects have to be eliminated. An unsatisfactorily low oxidation potential of the combustible gas occurs especially in the case of very dry combustible gases which are low in oxygen.

Matching the oxidation potential or the oxygen partial pressure of the protective gas atmosphere to the product and the furnace space is therefore known. To regulate the oxidation potential, the combustible gas is, inter alia, passed through a humidification apparatus, for example a heatable water bath. Since there is a risk of ignition of the combustible gas, the passage of the gas through the water bath has to be made safe by means of a very high technical outlay . It is therefore an object of the invention to at least partly alleviate the problems indicated in respect of the prior art and, in particular, to provide a process and an apparatus which makes it possible to humidify a combustible gas with a smaller technical outlay.

These objects are achieved by a process and an apparatus having the features of the independent claims. Further advantageous embodiments and also a preferred use of the invention are indicated in the dependent claims. It should be pointed out that the features indicated individually in the dependent claims can be combined in any technically meaningful way and define further embodiments of the invention. In addition, the features indicated in the claims are described in more detail and explained in the description, with further preferred embodiments of the invention being presented.

The objects are achieved, in particular, by a process for humidifying a combustible gas, which comprises at least the following steps:

provision of an inert gas,

humidification of the inert gas so that the inert gas has an inert gas humidity,

provision of a combustible gas,

combining of the humidified inert gas with the combustible gas to give a gas mixture having a dew point.

As combustible gas, it is possible to use hydrogen, carbon monoxide and/or hydrocarbons or mixtures of these. As inert gas, it is possible to use noble gases such as helium, argon and/or nitrogen. For the purposes of the present invention, provision means that the gases are supplied to the process from a suitable stock vessel. During humidification, water vapour is introduced into the inert gas by suitable measures. The proportion of water vapour per unit volume of inert gas [g/m ³ ] is referred to as inert gas humidity.

According to the invention, an inert gas humidified by means of suitable measures is thus mixed with a combustible gas . The gas mixture formed in this way has a defined dew point. The ratio of reducing gases (e.g. hydrogen) to oxidizing gases (e.g. water vapour) is a measure of the oxidation potential of the gas mixture.

According to the invention, the dew point of the gas mixture is thus set via the inert gas humidity. This is advantageous compared to the prior art since the inert gas can be humidified without a large technical outlay, especially with regard to suppression of ignitions and explosions of the gas. Accordingly, simple technical teaching which enables the dew point of the gas mixture and thus also the oxidation potential of a combustible gas to be set and/or regulated is provided. The gas mixture produced by the process of the invention is subsequently fed to a consuming apparatus, with the dew point preferably being matched to the requirements of the consuming apparatus. The consuming apparatus is preferably a furnace for the heat treatment of metals, with the dew point preferably being matched to the conditions in the furnace and the heat treatment to be carried out there. In particular, the dew point is matched to the type of heat treatment to be carried out, the temperature at which the heat treatment is carried out, the duration of the heat treatment, the desired result of the heat treatment, for example a particular surface structure and the like and/or the size of the surface to be treated. The dew point of the gas mixture is preferably regulated via the inert gas humidity. The regulation of the dew point can be effected via a dew point measurement in the gas mixture. If the actual value measured in the dew point measurement deviates from the intended value, the inert gas humidity is appropriately modified. The values matched to the consuming apparatus serve as intended values of the dew point. In the case of a heat treatment of metallic products, the values can be determined by the furnace geometry and/or the type of metallic products. The dew point is preferably selected so that carbon-containing residues on the product surface are completely oxidized or in the case of extremely clean and metallically pure surfaces of the metallic products, very thin, defined oxide layers which prevent welding together of the metal products are produced.

In an embodiment of the invention, it is proposed that the proportion of humidified inert gas in the gas mixture be not more than 10% by volume, in particular not more than 5% by volume. Such a low proportion of humidified inert gas is sufficient to set the dew point of the gas mixture and at the same time to make a sufficiently large amount of combustible gas available to a consuming apparatus.

It is advantageous for the inert gas to be humidified in water bath and the inert gas humidity to be set by means o at least one of the following parameters:

- the temperature of the water bath and

- the flow rate of the inert gas .

In particular, the inert gas is fed into a lower region of the water bath, preferably via a nozzle, particularly preferably a sintered nozzle having a maximum porosity of 1.0 μιη, and taken off again above the water bath. The temperature of the water bath can be regulated, in particular, by means of a heating element in the water bath. The flow rate can, in particular, be regulated via first valves through which the inert gas is passed. Such a process allows particularly simple setting of the dew point of the gas mixture. Furthermore, it is advantageous to feed the gas mixture as protective gas into a furnace suitable for heat treatment. The high proportion of combustible gas in the gas mixture provides a sufficient protective atmosphere, while the proportion of humidified inert gas allows setting of the dew point or the oxidation potential of the gas mixture. The properties of the gas mixture can thus be matched to the requirements of the furnace and the product to be heat- treated .

The objects of the invention are also achieved by an apparatus for humidifying a combustible gas, which comprises at least one fuel gas feed line, at least one inert gas feed line with at least one humidification apparatus and at least one feed line for a gas mixture, wherein the at least one feed line for a gas mixture can be supplied with a combustible gas by the at least one fuel gas feed line and an inert gas which can be humidified in the at least one humidification apparatus via the at least one inert gas feed line to produce a gas mixture having a dew point. The apparatus of the invention is particularly suitable and intended for carrying out the process of the invention.

The fuel gas feed line, the inert gas feed line and the feed line for a gas mixture can be configured as pipes which are suitable for conveying the corresponding gases. At the connection between inert gas feed line and feed line for a gas mixture, a safety device, in particular, which prevents backflow of the gas mixture and/or of the combustible gas into the inert gas feed line is provided. Such a safety device is preferably configured as a nonreturn valve for gas. The humidification apparatus serves to humidify the inert gas, i.e. to mix the inert gas with water vapour. The apparatus of the invention allows setting of the dew point or the oxidation potential of a combustible gas without the latter having to be humidified directly in a complicated fashion . It is advantageous for a measurement device, in particular a lambda sensor, for determining the dew point of the gas mixture to be assigned to the at least one feed line for a gas mixture. Thus, the actual value of the dew point or of the oxidation potential of the gas mixture can be determined or checked at any time using the measurement device. When a lambda sensor is used, for example, the oxygen content of the gas mixture is determined and the oxidation potential or the dew point are concluded therefrom.

In addition, it is advantageous for the at least one humidification apparatus to comprise at least one of the following elements:

- at least one water bath;

- at least one water bath having at least one heating element and

- at least one first valve, in particular an adjustable valve, in the inert gas feed line.

The water bath preferably comprises a sintered nozzle through which the inert gas is fed in in a lower region of the water bath. The water bath preferably further comprises a heating element by means of which the temperature of the water bath can be set. The inert gas humidity of the inert gas passed through the water bath can be set via the temperature of the water bath. The flow rate of the inert gas through the water bath can be set via the first valve and this is therefore likewise suitable for setting the inert gas humidity of the inert gas.

Furthermore, it is advantageous for a regulating unit for regulating the dew point of the gas mixture and/or an inert gas humidity of the inert gas to be provided, with this regulating unit being connected to at least two of the following elements :

- the at least one heating element of the water bath,

- the at least one first valve in the inert gas feed line,

- at least one second valve in the fuel gas feed line, - the at least one measurement device assigned to the feed line for a gas mixture.

The connections are preferably data connections via which data can be transmitted from and/or setting commands can be transmitted to the individual elements. The data connections can be wireless or via wires.

The regulating unit can thus continually determine the dew point or the oxidation potential of the gas mixture. If required, the regulating unit can adjust the inert gas humidity by controlling the heating element or the first valve in such a way that a prescribed intended value of the dew point is achieved. Furthermore, the composition and the flow rate of the gas mixture can be regulated via the first valve and/or the second valve. The regulating unit thus allows the parameters of the gas mixture which can be prescribed by the consuming apparatus to be continually maintained .

Furthermore, it is advantageous for the at least one inert gas feed line to be at least partly protected against precipitation of condensation. For the purposes of the present invention, precipitation of condensation is condensation of the gaseous water in the inert gas or in the gas mixture on the surfaces of the feed lines. Such protection against precipitation of condensation can be afforded, for example, by heating of the feed lines. Furthermore, it is conceivable for the gases to be passed through the inner tube of an insulated double tube.

According to a further aspect of the invention, the use of the apparatus of the invention for operating a furnace using a process according to the invention is proposed. If the furnace is intended for heat treatment of metallic products, the formation of cracking products due to carbon-containing residues on the products and welding together of the metallic products when extremely clean and metallically pure metal surfaces can be avoided by means of the use according to the invention .

The details and advantages disclosed for the process of the invention can also be applied and employed to the apparatus of the invention and vice versa. The invention and the technical field to which it relates will be illustrated by way of example below with the aid of the figure. It should be noted that the figure shows a particularly preferred variant of the invention but the invention is not restricted thereto.

The figure schematically shows a variant of the apparatus 1 according to the invention for humidifying a combustible gas. The apparatus 1 comprises a fuel gas feed line 2, an inert gas feed line 3 and a feed line 4 for a gas mixture. A humidification apparatus 5 having a water bath 7 and a heating element 8 is arranged in the inert gas feed line 3. A first valve 9 is present in the inert gas feed line 3 and a second valve 13 is present in the fuel gas feed line 2. A measurement device 6 is arranged in the feed line 4 for a gas mixture. As an alternative or in addition, a measurement apparatus (not shown) can be arranged between the water bath 7 and the valve 9 in order to determine the inert gas humidity. The heating element 8, the first valve 9, the second valve 13 and the measurement apparatus 6 are connected via lines 12 to a regulating unit 10. The feed line 4 for a gas mixture is connected to a furnace 11.

In operation of the apparatus 1, combustible gas is fed via the fuel gas feed line 2 to the feed line 4 for a gas mixture. Inert gas which is firstly passed through the humidification apparatus 5 is fed via the inert gas feed line 3 to the feed line 4 for a gas mixture. As a result of the inert gas being passed through the water bath 7 , the inert gas is loaded with water vapour so that the inert gas has an inert gas humidity. The inert gas which has been humidified in this way is fed to the feed line 4 for a gas mixture. In the feed line 4 for a gas mixture, the combustible gas and the humidified inert gas mix to form a gas mixture. It is ensured by suitable means that the gas mixture does not go back into the inert gas feed line 3.

The gas mixture formed in this way has a dew point which correlates with the oxidation potential of the gas mixture. In operation, the dew point or the oxidation potential of the gas mixture is measured by means of the measurement device 6 and evaluated by the regulating unit 10. The regulating unit 10 compares the actual value of the dew point with an intended value which can be prescribed. The regulating unit 10 can regulate both the temperature of the water bath 7 and the flow rate of the inert gas through the water bath 7 by means of the first valve 9. In this way, the inert gas humidity is altered and the dew point of the gas mixture is thus also altered. The regulating unit 10 can also regulate the flow rate of the combustible gas by means of the second valve 13, so that the proportion of inert gas in the gas mixture can be regulated. Thus, a gas mixture matched in terms of the dew point to the requirements of the furnace 11 is applied at all times.

The present invention makes it possible to humidify a combustible gas by setting a dew point without the combustible gas having to be humidified in a complicated manner directly in a water bath.

List of reference numerals

1 apparatus

2 fuel gas feed line

3 inert gas feed line

4 feed line for a gas mixture

5 humidification apparatus

6 measurement device

7 water bath

8 heating element

9 valve

10 regulating unit

11 furnace

12 lines