Control of the concentration of solvents in a dryer

The invention relates to a method and apparatus for drying lengths of carrier material which have been printed with ink comprising an evaporable solvent. To maintain the concentration of the solvents evaporating from the printing ink, this concentration has to be determined. As known apparatuses for measuring the concentration are expensive, prone to faults and need regular calibration, the present invention provides a method for calculating that concentration. According to said method the concentration of the evaporated solvents in the gas mixture is determined by measurement of the temperature and flow rate of a mixture to be supplied to the burner, and by measurement of the temperature of the gas mixture heated by the burner and by measurement of the flow rate of the fuel supplied to the burner. More precisely the concentration is determined by calculation of the increase in heat of the gas mixture in the burner, the amount of heat supplied by the burner, and in which from the difference thereof the burning heat of the solvents is determined, after which with the known burning value thereof the concentration is determined.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for drying of lengths of carrier 
material, which have been printed with an ink comprising an evaporable 
solvent. 
The present invention relates in particular to such a method, in which the 
lengths are guided through a chamber, a gas mixture heated by a burner is 
conveyed to that chamber, the gas mixture coming from such chamber is fed 
to the burner for heating, and a part of the gas mixture coming from the 
chamber is vented off. 
Such a method is known from the Dutch patent application 88.00226. 
The venting off of the gas mixture takes place to maintain the 
concentration of the solvents evaporating from the printing ink by the 
raised temperature beneath a certain value. Initially, this value is 
determined by the safety regulations, and secondly this value is 
determined by the fact, that the circulating gasses can of course not be 
saturated with solvents as otherwise no evaporation thereof can take 
place. 
The gasses thus vented off carry a considerable amount of heat. From an 
energetic point of view it is thus important to keep the amount of gasses 
vented off as small as possible. 
In such a method one aims for controlling the amount of gas mixture to be 
vented off such, that: 
the concentration of the evaporated solvents, generally oils, is kept 
sufficiently beneath the value, required for safety reasons; 
the concentration is kept on such a value, that printed matter of a good 
quality is obtained; and 
the amount of gasses vented off is as small as possible. 
Generally the second aim leads to a much lower value of the maximal 
allowable concentration, so that in practice consideration is made between 
the second and the third aim. 
To make this consideration it is necessary to determine the concentration. 
It is possible to measure the concentration. The known measuring equipment 
used therefor is expensive and prone to faults, and it has to be 
calibrated regularly. 
The aim of the present invention is to provide a method, in which the 
concentration of the evaporated solvents in the gasses is determined, and 
in which the disadvantages, related to direct measurement thereof, are 
avoided. 
SUMMARY OF THE INVENTION 
This aim is reached, in that the concentration of the evaporated solvents 
is determined by calculation. 
In the calculation of the concentration of the evaporated solvents one 
departs from some measurements; according to a preferred embodiment of the 
invention the following values are measured: the temperature and the flow 
rate of the gas mixture to be fed to the burner, the temperature of the 
gas mixture leaving the burner, and the flow rate of the fuel, feeding the 
burner. 
From the flow rate of the fuel the amount of heat developed by the burner 
is determined; the burning value of the fuel is known. The flow rate of 
the gasses coming from the chamber and the temperature rise of the gas 
mixture having passed the burner caused by the burner, leads to the total 
supply of heat. 
This value is compared with the amount of heat supplied by the burner. From 
the difference the heat value of the solvents can be determined, from 
which, with the help of calculated values for the burning value of the 
solvents, the concentration thereof can be determined. 
The invention relates also to an apparatus for executing the methods set 
out above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the embodiment shown in FIG. 1 the lengths of material to be dried are 
fed through a chamber 1. In this chamber 1 sprayheads, 1A are used, which 
make the carrier material dry. For supplying a heated gas mixture, 
commonly air, a burner or heated 2 has been provided, which is connected 
with the chamber 1 via a channel 5, in which a ventilator 3 has been 
located. The gas mixture emerging from the chamber 1 is fed to the burner 
through a channel 4. 
Also a by-pass 40 passing the burner has been provided for the gas mixture. 
To burn the solvents present in the gas mixture as far as possible in the 
burner, the temperature of the burner must be rather high, for instance 
about 800.degree. C. When a gas mixture with this temperature would be fed 
to the lengths to be dried, these would burn. To avoid this the by-pass 40 
with a controlling valve 41 has been provided, so that the heated air is 
mixed with cold air. 
Thus a closed system is present, within which the gas mixture travels, 
which gas mixture exerts its drying effect on the lengths of material fed 
through the chamber 1, and which thus cooled down is partly heated by the 
burner 2, is being mixed with the not-heated air, and is fed to the 
chamber 1 by means of the pump 3. 
Of course a fuel supply pipe 6 has been provided for supplying fuel to the 
burner 2. By evaporation of the solvents present in the ink, the 
concentration thereof in the circuit thus described is raised. For venting 
of gasses from the circuit a venting pipe 7 has been provided, which is 
connected to the channel 5 by means of a valve 8. 
This decreases the amount of circulating gas, so that also means have to be 
provided for the supply of new gas. Therefore it is possible to provide 
the supply channel not depicted in the drawing for supplying gas, for 
instance air from outside; it is also possible to supply a part of the 
burned gasses of the burner to the gas circuit. This considerably enlarges 
the energetic efficiency. 
To control the amount of gasses to be vented as accurate as possible, it is 
of importance, that the concentration of the evaporated solvences present 
in the gasses is determined very accurately. 
In the present invention this is provided by the application of a 
temperature measuring element 9, which measures the temperature of the gas 
mixture to be fed to the burner 1. Further a flow rate measuring element 
10 has been provided from measuring the flow rate of the gas mixture to be 
supplied to the burner 1. 
Further a temperature measuring element 11 is present for measuring the 
temperature of the gas mixture leaving the burner 2, and in the fuel 
supply pipe 6 a flow rate meter 12 has been provided for measuring the 
flow rate of the fuel. The signals from these measuring elements are fed 
to a control element 13, which supplies a control signal to the 
controllable valve 8 through a signal lead 14. 
The calculation of the concentration of the evaporated solvents takes place 
as follows: from the fuel flow rate the amount of heat which is supplied 
by the burner 2 to the passing gasses, is determined. 
Then, by measurement of the flow rate of the gas mixture and of the 
temperature rise thereof, the increase of heat content of the gasses in 
the burner is determined. This value is compared with the heat supplied by 
the burner, which can be calculated from the flow rate of the fuel and the 
burning value thereof. From the difference of these values and the known 
burning value of the solvents, the concentration thereof can be 
calculated. The assumption is made that the solvents burn completely. 
With the help of this concentration, the position of the valve 8, and thus 
the amount of gasses to be vented is determined. When instead of a burner 
an electric heating element is provided, it is also possible to measure 
the heat developed by such an element, and to execute an equivalent 
calculation. 
In the embodiment depicted in FIG. 2, the invention is integrated in a 
dryer with a complicated configuration. This dryer comprises three zones, 
first zone 14, in which the carrier material is pre-heated, a second zone 
15, in which the carrier material is dried; and a third zone 16, in which 
the carrier material is cooled down. 
The features of the invention are in particular applicable to the first and 
the second zone. The gas mixture emerging from the burner 17 is supplied 
to a first zone 14 via a first chamber 18 and a valve 19. A part of this 
gas mixture is supplied to a heat exchanger 21 via a second chamber 20, 
and is subsequently vented outwardly. In the heat exchanger 21 the gas 
mixture fed to the burner 17 is heated to obtain an efficiency as high as 
possible. 
The gas mixture arriving in the second zone 14 is fed to sprayheads 23 with 
the help of a first ventilator 22 to heat the carrier material. Also fresh 
air is fed to said first zone via the entrance slit 24 for the carrier 
material. 
A part of the gas mixture just developed goes to the second zone, from 
which it is guided to a second array of sprayheads 26 via a second 
ventilator 25. The gas mixture emerging from this zone is partially 
supplied via a third ventilator 28 to a heat exchanger 21 and subsequently 
to the burner 17. 
In the third zone 16 fresh air from outside is supplied with the help of a 
fourth ventilator 25, which is sprayed to the carrier material by means of 
an array of sprayheads 30, which makes the carrier material cool down. 
Extra air from the third zone 16 is supplied to second zone 15, and indeed 
at the entrance at the second ventilator 25. 
Thus the circuit of the gas mixture is closed. As appears from the diagram 
above, the venting of the gas from the system can be controlled through 
the second chamber 20 and the heat exchanger 21, which is controlled by 
the valve 19 together with the flow rate of the ventilator 28. 
According to the invention the flow rate of the ventilator 28 is set such, 
that in the gas circuit a desired concentration of evaporated solvents is 
maintained. Also in this case this concentration is determined by 
measurement of the temperature of the gasses supplied to the burner by the 
temperature measuring element 31, the measurement of the gasses having 
left the burner 17 in the first chamber 18 by means of the temperature 
measuring element 32, the measuring of the flow rate of the fuel by means 
of the flow rate meter 33, and the measurement of the flow rate of the 
ventilator 28 with the flow rate meter 34. 
Also the temperature of the gasses which are supplied to the heat exchanger 
21 is measured with the help of a temperature meter 35. This temperature 
is used for measuring the flow rate of the mass of the gas mixture from 
the flow rate of the volume thereof; this specific heat is reversed 
proportional with the temperature. The signals coming from these measuring 
elements are supplied to a control element 36. This control element 
determines the concentration of the evaporated solvents from the measured 
values on a equivalent way as in the first embodiment. 
The control element 36 supplies a signal to a steering element 37, which 
supplies signals to the control valve 19 for determining the temperature 
of the dryer, the motor 38 of the ventilator 28 and the motor of the fuel 
valve 39. Thus the relevant values can be adapted, so that an energy 
system is obtained with the correct properties. Of course the values thus 
obtained can be applied to control all the parameters in the system. This 
allows to control the supply of fuel just as the flow rate of the 
ventilator 28.