Method and apparatus for preventing the occurrence of an explosive state in gas mixtures in confined spaces

In a method for preventing the occurrence of an explosive state in a gas mixture, especially of solvents in air, in an essentially confined space, virtually complete oxidation of at least part of the gas mixture taking place, either the temperature difference .DELTA.T between the temperatures of the gas mixture before and after the oxidation is determined, and if the temperature difference .DELTA.T is greater than a maximal permissible temperature increase .DELTA.T.sub.max, safety measures are taken, or the temperature T.sub.1 of the gas mixture before the oxidation and the temperature T.sub.2 of the gas mixture after the oxidation are measured, and the measured temperatures T.sub.1 and T.sub.2 are compared with a minimum temperature T.sub.min before oxidation of the gas mixture and a maximum temperature T.sub.max after oxidation of the gas mixture, the difference between T.sub.max and T.sub.min being less than, or equal to, the maximal permissible temperature increase .DELTA.T.sub.max, and if the measured temperatures T.sub.1 and T.sub.2 are outside the interval determined by the maximum and minimum temperatures T.sub.max and T.sub.min, safety measures are taken. In addition, the use of this method in drying webs printed with ink, and an appliance for this purpose are described.

BACKGROUND OF THE INVENTION 
The invention relates to a method for preventing the occurrence of an 
explosive state in gas mixtures, especially of solvents in air, in an 
essentially confined space, virtually complete oxidation of at least part 
of the gas mixture taking place. 
Such a method is disclosed, for example, by the Dutch Laid-Open Patent 
Application 8902754. In this known method, the concentration of solvents 
in a drying appliance for drying webs of base material is controlled, said 
webs being printed with an ink containing volatile solvent, a gas mixture 
containing evaporated solvents being combusted. This concentration 
monitoring comprises measuring the heat increase, caused by combustion, of 
the gas mixture in the combustion device, and the amount of heat supplied 
by the fuel, the heat of combustion of the solvent being determined from 
the difference between these, whereupon the concentration of the solvent 
is calculated. 
Monitoring of the concentration of the solvents which evaporate from the 
ink during drying, in the drying appliance is necessary, because this 
concentration should be kept below a certain value. This value is 
determined, in the first instance, by safety requirements which generally 
apply to rooms in which flammable substances are located. In the known 
drying appliance, this value is also defined by the fact that the 
circulating gas mixture must not become saturated with solvents, because 
otherwise no evaporation and therefore no drying can take place. If the 
concentration of the solvent exceeds the value defined by the safety 
standard, the appliance should be switched off. This safety standard is 
generally set to a certain percentage of the lower explosive limit of the 
solvent in air. 
A drawback of the known method is that the calculation of the concentration 
of the solvent requires a large number of measurements and thus a large 
number of measuring instruments and computation aids. In order to 
calculate, from an energy balance, the concentration of the solvent, the 
temperature and the flow rate of the gas mixture supplied to the 
combustion device need to be measured, as well as the temperature of the 
gas mixture after combustion and the flow rate of the fuel supplied to the 
combustion device. Based on the known heat of combustion of the fuel and 
the solvent, the concentration of the solvent in the combustion device is 
then calculated. Each solvent, however, has a specific heat of combustion, 
so that a general assumption is made for this purpose, which reduces the 
accuracy of the calculated concentration. In addition, the measuring 
instruments require regular calibration, are expensive and prone to 
faults. 
An alternative for preventing the occurrence of an explosive state in gas 
mixtures by monitoring the concentration of flammable substances in a gas 
mixture are direct concentration measurements. The measuring apparatus 
required for this purpose is, however, likewise very expensive and prone 
to faults, and requires regular calibration. 
SUMMARY OF THE INVENTION 
It has now been found that explosive states in gas mixtures can be 
prevented by means of a method in which only a temperature difference is 
measured or only two temperatures are measured. 
The object of the invention is to implement, in a simple manner, a method 
for preventing the occurrence of an explosive state in gas mixtures in an 
essentially confined space by solely carrying out temperature 
measurements, avoiding the abovementioned drawbacks. 
It has further been found that there is a relationship between the maximal 
permitted concentration of a flammable substance in a gas mixture and the 
temperature increase during oxidation thereof, which is independent of the 
type of the flammable substance. 
A further object of the invention is to provide a method for preventing the 
occurrence of an explosive state in gas mixtures in an essentially 
confined space, which method makes use of this relationship. 
Another object of the invention is to provide such a method which is 
independent of the type of the flammable substance. 
A further object of the invention is to provide a method for drying webs 
printed with ink containing solvent, in which method the method for 
preventing the occurrence of an explosive state in a gas mixture of 
solvent and air can be used. 
Yet another object of the invention is to provide a drying appliance for 
implementing the drying method according to the invention. 
The method of the abovementioned type according to the invention is 
characterized in that the temperature difference .DELTA.T between the 
temperatures of the gas mixture before and after the oxidation is 
determined, and if the temperature difference .DELTA.T is greater than a 
maximal permissible temperature increase .DELTA.T.sub.max, safety measures 
are taken. 
It was found that, if the temperature difference .DELTA.T between the 
temperatures of the gas mixture to be oxidized and the oxidized gas 
mixture is smaller than the previously set maximally permissible 
temperature difference .DELTA.T.sub.max, the concentration of the 
flammable substance in the gas mixture is below the maximally permitted 
concentration on the basis of the safety standard. The maximally 
permissible temperature difference .DELTA.T.sub.max, for most of the 
flammable substances used, depends solely on the safety standard and can 
be determined empirically for each standard. It was also found that here 
the type of the flammable substance and the type of the oxidation cell are 
not important, so that, once the correct maximal permissible temperature 
difference .DELTA.T.sub.max has been determined, rooms in which different 
flammable substances are present alternately can be safeguarded without 
the need to adjust .DELTA.T.sub.max. In order to safeguard the confined 
space, it is therefore sufficient to measure the temperature difference 
between the temperature of the gas mixture supplied to the oxidation cell 
and the temperature of the gas stream formed after the oxidation. If the 
measured temperature difference is greater than the maximal permissible 
temperature difference .DELTA.T.sub.max, suitable safety measures, for 
example switching off the oxidation cell and/or ventilation of the 
confined space, should be taken. Generally, these safety measures are 
stipulated. 
Oxidation of the gas mixture can be performed, for example, by combustion 
thereof with the aid of an auxiliary fuel in a combustion device or by 
catalytic reaction of the gas mixture with the aid of a suitable catalyst. 
Other advantages of the method according to the invention are the obviation 
of regular calibration of the measuring instruments, the measuring 
instruments being stable in time and requiring no maintenance. 
For the purpose of measuring the temperature difference or the real 
temperatures, simple measuring instruments such as thermocouples can be 
used.

DETAILED DESCRIPTION OF THE INVENTION 
To supplement the temperature difference measurement, according to a 
preferred embodiment of the method according to the invention, the 
temperature T.sub.2 of the gas mixture after the oxidation is also 
measured, and, if the measured temperature T.sub.2 exceeds a certain value 
or is outside a certain temperature range, safety measures are taken. 
Thus, an additional safeguard is incorporated. It is within the scope of 
the invention to measure, instead of or in addition to the temperature of 
the gas mixture after the oxidation, the temperature T.sub.1 of the gas 
mixture before the oxidation and to monitor this temperature T.sub.1. 
The invention also relates to a method of the type mentioned at the outset, 
in which, instead of a temperature difference measurement, the actual 
temperatures before and after the oxidation are measured. This method 
according to the invention is characterized in that the temperature 
T.sub.1 of the gas mixture before the oxidation and the temperature 
T.sub.2 of the gas mixture after the oxidation are measured, and the 
measured temperatures T.sub.1 and T.sub.2 are compared with a minimum 
temperature T.sub.min before oxidation of the gas mixture and a maximum 
temperature T.sub.max after oxidation of the gas mixture, the difference 
between T.sub.max and T.sub.min being less than, or equal to, the maximal 
permissible temperature increase .DELTA.T.sub.max, and if the measured 
temperatures T.sub.1 and T.sub.2 are outside the interval determined by 
the maximum and minimum temperatures T.sub.max and T.sub.min, safety 
measures are taken. 
In this use of the relationship found between the maximal permitted 
concentration and the temperature increase during oxidation to safeguard a 
confined space, the actual temperatures before and after the oxidation are 
measured and compared with a predetermined maximum temperature T.sub.max 
after oxidation and a minimum temperature T.sub.min before oxidation, the 
constraint applying that the difference between the maximum and minimum 
temperatures be equal to or smaller than the maximal permissible 
temperature increase .DELTA.T.sub.max. In this case, it is therefore not 
the temperature difference which is monitored, but the temperature T.sub.1 
before the oxidation and the temperature T.sub.2 after the oxidation. 
Preferably, the maximal temperature increase is constrained on the basis of 
the maximal permitted concentration of the flammable substances in the gas 
mixture. Consequently, reliable safeguarding can be effected. 
For the most commonly used solvents and other flammable gases, the maximal 
permissible temperature increase .DELTA.T.sub.max is within a narrow 
range. For a customary safety standard of 25% of the concentration of the 
flammable substance at the lower explosive limit, the maximal permissible 
temperature increase .DELTA.T.sub.max is in the range of 
300.degree.-400.degree. C. It should be noted that, if a different 
standard applies, the maximal temperature increase should be adjusted 
accordingly. This can be determined empirically or be calculated 
accurately from the safety standard in a manner as described hereinafter. 
In a preferred embodiment of the method according to the invention, the 
maximal permissible temperature increase .DELTA.T.sub.max is determined 
from the temperature increase during the oxidation of the gas mixture 
according to the equation .DELTA.T.sub.max =k*dT*C.sub.LEL, where 
.DELTA.T.sub.max is the maximal permissible temperature increase 
.degree.C.!, k is a factor which depends on the safety standard, dT is 
the specific temperature increase .degree.C./(g/Nm.sup.3)! and C.sub.LEL 
is the concentration g/Nm.sup.3 ! of the flammable substance at the lower 
explosive limit. The values of dT and C.sub.LEL are reported in handbooks 
for the most common substances or can be calculated therefrom, while the 
factor which depends on a safety standard is prescribed by government or 
other safety authorities. Thus it is possible to calculate accurately, for 
every substance in the gas mixture, the associated maximal permissible 
temperature increase .DELTA.T.sub.max. 
The factor depending on the safety standard will characteristically be in 
the range of 0.15-0.99. 
It is possible to derive, from the maximal permissible temperature increase 
.DELTA.T.sub.max and the properties of the device in which the oxidation 
of the gas mixture takes place, the maximum temperature T.sub.max after 
the oxidation and the minimum temperature T.sub.min before the oxidation. 
Advantageously, the maximum temperature T.sub.max is the maximal 
temperature of the discharged gases. Thus, efficient, good oxidation is 
effected, while the concentration is kept below the safety standard. 
In order to utilize the heat produced during oxidation, it is possible to 
employ heat exchange between the gas mixture before the oxidation and at 
least part of the oxidized gas mixture. In so doing, there are various 
alternatives for the point where the temperature difference or the 
different temperatures can be measured. Depending on the position, the 
maximum and minimum temperatures need or need not be adjusted. 
If the temperature difference is monitored, advantageously the temperature 
difference .DELTA.T between the temperature of the gas mixture before the 
oxidation at a point between the heat exchange and the oxidation, and the 
temperature of the gas mixture after the oxidation at a point between the 
oxidation and the heat exchange is determined. 
In another embodiment of this method according to the invention, 
advantageously the temperature difference .DELTA.T between the temperature 
of the gas mixture before the oxidation at a point upstream of the heat 
exchange, and the temperature of the oxidized gas mixture at a point 
downstream of the heat exchange with the gas mixture before the oxidation 
is determined. 
If, instead of the temperature difference, the temperatures T.sub.1 and 
T.sub.2 are monitored, there are again a number of possibilities for so 
doing. 
In the first version, advantageously the temperature T.sub.1 of the gas 
mixture before the oxidation is measured at a point between the heat 
exchange and the oxidation, and the temperature T.sub.2 of the gas mixture 
after the oxidation is measured at a point between the oxidation and the 
heat exchange. 
According to the second version, the temperature T.sub.1 of the gas mixture 
before the oxidation is measured at a point upstream of the heat exchange, 
and the temperature T.sub.2 of the oxidized gas mixture is measured at a 
point downstream of the heat exchange with the gas mixture before the 
oxidation. In so doing, the minimum and maximum temperatures, T.sub.min 
and T.sub.max, respectively, need to be adjusted to the temperature 
increase and decrease, respectively, caused by the heat transfer. 
The method for preventing the explosives if an explosive situation in gas 
mixtures according to the invention can be applied over a wide range of 
industrial fields, in which the concentration of flammable substances 
needs to be kept below a certain value in order to avoid dangerous 
situations. A non-limiting listing of examples comprises, inter alia, the 
safeguarding of storerooms, coating lines, combustion installations, 
pipeline systems and the like. A particular field of application of the 
method according to the invention is the graphic industry. 
The invention also relates to a method for drying webs printed with ink 
containing solvents, using a safeguarding method according to the 
invention. 
In addition, the invention relates to a drying appliance for drying the 
webs printed with ink containing solvents, in which appliance a 
safeguarding method according to the invention is used. 
Employing the monitoring method according to the invention ensures safety, 
in a simple, inexpensive and reliable manner, during drying of the printed 
webs and oxidation of the evaporated solvents. 
FIG. 1 is a graph in which, for a large number of solvents and the like, 
the specific temperature increase dT .degree.C./(g/Nm.sup.3)! as a 
function of the concentration at the lower explosive limit C.sub.LEL 
g/Nm.sup.3 ! is depicted. These values are indicated with a "+" sign. As 
can be seen, this temperature increase for the most common, flammable and 
environmentally damaging substances is in a narrow range whose boundaries 
are depicted as continuous curves. By taking into account the factor k 
determined by the safety standard, it is possible to determine therefrom 
the maximal permissible temperature increase .DELTA.T.sub.max and/or the 
minimum temperature T.sub.min before oxidation and the maximum temperature 
T.sub.max after oxidation. 
FIG. 2 shows a graph in which, according to one embodiment of the method 
according to the invention, the temperature difference .DELTA.T has been 
measured as a function of time. The maximal permissible temperature 
difference .DELTA.Tm.sub.max has been set to 350.degree. C. in this 
example. The variation of .DELTA.T with time is shown as a continuous 
curve. In this example, the measured temperature difference .DELTA.T after 
some time becomes greater than the maximally permissible temperature 
difference .DELTA.T.sub.max, at which time an alarm will become active. 
In FIG. 3, a graph is drawn in which, according to another embodiment of 
the method according to the invention, the temperature T.sub.1 before the 
oxidation of the gas mixture and the temperature T.sub.2 after the 
oxidation of the gas mixture are measured against time. The maximum 
temperature T.sub.max derived from the maximally permissible temperature 
increase .DELTA.T.sub.max (in this example likewise 350.degree. C.) has 
been set to 800.degree. C., and the minimum temperature T.sub.min has been 
set to 450.degree. C. This graph shows two different situations, A and B, 
respectively. In situation A, the measured temperature before oxidation of 
the gas mixture T.sub.1A becomes lower than T.sub.min after some time, so 
that an alarm will become active, while in situation B the measured 
temperature after oxidation of the gas mixture T.sub.2B becomes larger 
than T.sub.max, so that an alarm will become active and suitable safety 
measures will be taken. This graph likewise shows that in situation B the 
difference between T.sub.2B and T.sub.1 remains smaller than the maximally 
permissible temperature increase .DELTA.T.sub.max of 350.degree. C., while 
nevertheless an alarm becomes active. 
If desired, the measuring principles shown in FIGS. 2 and 3 may be 
combined, in part or as a whole, so that, in addition to a measurement of 
the temperature difference .DELTA.T, the temperature T.sub.2 of the gas 
mixture after oxidation and/or the temperature T.sub.1 of the gas mixture 
before oxidation are also measured. 
In FIG. 4, an embodiment of the drying appliance according to the invention 
is indicated in its entirety by the reference numeral 1. Through of the 
appliance 1, a web 2 is guided via suitable conveying means (not shown), 
the web being printed with an ink containing solvents. In the embodiment 
shown, the drying appliance 1 comprises a drying chamber 3 in which, both 
above and below the web 2, a plurality of blowing devices 4, provided with 
a large number of nozzles (not shown) are set up, through which a heated 
gaseous medium, generally air, is blown onto the web 2 in order to 
evaporate the solvents from the ink. After the web 2 has been dried, it is 
passed through a cooling chamber 5, in which the web 2 is cooled with the 
aid of cold air which is blown onto the dried web 2 with the aid of 
blowing devices 6. Part of the gaseous medium, which is loaded with 
evaporated solvents, is passed, via fans 7 and 8 and heat exchangers 9 and 
10, to a combustion chamber 11 arranged above the web 2 and a combustion 
chamber 12 arranged below the web 2, respectively, in which chambers, with 
the aid of burners 21 and 22, the gas mixture is combusted. In the heat 
exchangers 9 and 10, heat is transferred from a branch stream of the 
combusted gas mixture to the gas mixtures supplied to the combustion 
chambers 11, 12. The branch streams leave the drying appliance 1 via the 
discharge ducts 13 and 14. The remaining part of the combusted gas mixture 
is passed into the drying chamber 3 via supply valves 15 and 16, whereupon 
it can be reused for drying the web 2. 
In order to prevent the occurrence of an explosive state of the solvent in 
air in the drying appliance 1, so that safety measures are taken if the 
maximally permissible concentration is exceeded, the temperature T.sub.1 
of the gas mixture supplied to the combustion chambers 11 and 12 is 
measured with the aid of thermocouples 17 and 18. The temperature T.sub.2 
of the combusted gas mixtures is measured with the aid of thermocouples 19 
and 20. The temperature monitoring is thus carried out in accordance with 
FIG. 3. If the temperature T.sub.1 of the incoming gas mixture is too low 
(&lt;T.sub.min ), or the temperature T.sub.2 of the combusted gas mixture is 
too high (&gt;T.sub.max), suitable safety measures are taken. 
It is found that, if the actual measured temperatures T.sub.1 and T.sub.2 
are within the temperature interval defined by the minimum temperature 
T.sub.min and maximum temperature T.sub.max, the concentrations of the 
solvents, irrespective of the type of solvent, are below the maximally 
permitted concentrations. 
If the heat exchanger does not function correctly, the temperature T.sub.1 
of the gas mixture supplied to the combustion chamber will be too low. By 
measuring the temperature of the gas mixture supplied to the combustion 
chamber at a point between the heat exchanger and the combustion chamber, 
as in the set-up of the temperature measuring elements in this figure, a 
check can be carried out at the same time regarding the performance of the 
heat exchanger. 
The safety of the appliance amply meets the standards set down, because the 
maximally permissible temperature difference .DELTA.T.sub.max is 
predetermined on the basis of the maximally permitted concentration, 
whereas in reality, owing to the contribution of the heat exchange and the 
combustion of the auxiliary fuel in order to combust the solvents, the 
temperature range remaining for the contribution of the solvents is 
smaller than the maximally permissible temperature difference 
.DELTA.T.sub.max.