in an electrographic printer , paper sheets pass various stations one after another , specifically an exposure station , a developing station and a fixing station . referring now to the figures of the drawing and , first , particularly to fig1 thereof , there is shown therein two successive paper sheets 1 passing the fixing station , the sheets 1 lying on a level section of a transport belt 12 running to the lefthand side in the figure . the transport belt 12 has an upper side 16 and a lower side 18 ( see fig2 ). the fixing station essentially contains a cuboidal housing 2 extending over the width of the sheet and having an open side facing towards the paper sheets 1 . the distance between the side walls of the housing 2 and the paper sheets 1 is made as small as possible , so that an essentially closed space 3 is formed in the interior of the housing 2 . for a given spacing between the side walls of the housing 2 , on the one hand , and the paper sheets 1 , on the other hand , which only just permits the contact - free paper transport to pass the housing 2 , it is possible for the gastightness of the space 3 to be improved with the aid of seals 10 provided on the undersides of the housing , opposite the paper sheets 1 , as is shown diagrammatically in fig1 and 2 . the housing 2 is subdivided by a dividing wall or partition 4 into a lower subspace 3a and an upper subspace 3b , which are connected to one another at the left - hand side of fig1 so that the space 3 has a u - shaped cross section . at the right - hand side of fig1 gas inlet lines 5 open into the lower subspace 3a and gas outlet lines 6 open into the upper subspace 3b . hot steam at a temperature of , for example , 300 ° c . is fed out of the gas inlet lines 5 , passes through the space 3 in the direction of the appertaining arrows , and leaves the space 3 via the gas outlet lines 6 . the hot steam sweeps over the paper sheets 1 while the latter are passing the fixing station , as a result of which the toner images applied to the paper sheets 1 in the non - illustrated developing station disposed upline of the fixing station are caused to melt . after the sheets 1 have left the fixing station , the toner images and paper sheets 1 , respectively , are cooled by a cooling device 14 , so that the toner images are permanently joined to the paper . fig2 shows a different embodiment of the fixing station according to fig1 in which two housing halves 7a and 7b , which are each similar to the housing 2 of fig1 are located with the open sides thereof disposed opposite one another , by which a substantially closed space 8 with a lower subspace 8a and an upper subspace 8b is formed . the side walls of the housing halves 7a and 7b are at a small distance from one another , in order to form elongate , narrow openings 9a and 9b , through which the paper sheets 1 are transported to the lefthand side or to the righthand side of the figure . the housing halves 7a and 7b , respectively , are connected to gas inlet and gas outlet lines 5 and 6 , as in fig1 . in this exemplary embodiment of fig2 however , the housing halves 7a and 7b do not have any dividing walls like the housing 2 of fig1 instead the hot vapor moves within each subspace 8a and 8b essentially as a result of convection when the vapor cools down at the paper surface , as indicated by the arcuate arrows . in the exemplary embodiment of fig2 it is not only the upper side of the sheet , which has the toner images thereon , that is swept by hot steam , but also the underside of the sheet . this very reliably counteracts any drying - out of the paper . it is possible in many ways to ensure that the steam will also reach the underside of the sheet . for example , the sheet can be held at the sides thereof while it is passing the space 8 , or a gas - permeable transport belt can be used therefor . fig3 shows an estimate of the available energy content u of the steam , when the latter is cooled from a temperature t to 100 ° c . while passing through the housing 2 or the housing halves 7a and 7b . there follows a closer investigation of the energy balance for the convection fixing of toner in a toner fixing station for electrographic printing systems , the functional principle of which is based upon blowing hot gas into an oven chamber , convection fixing as shown in fig2 being assumed . in contrast with other contact - free fixing methods known to date , for example radiation fixing , in the method according to the invention of the instant application , the energy which is incident in the form of heat radiation from the heating elements is not used directly for the fixing . with reference to two examples , specifically the introduction either of hot air or of hot steam , the following simple estimate shows that the concept of heat transport by heated steam is feasible . the variables and constants used for the estimate are : ______________________________________molar internal energy [ jmol . sup .- 1 ] umolar heat capacity [ jk . sup .- 1 mol . sup .- 1 ] c . sub . pthermodynamic temperature [ k ] tmolar volume [ m . sup . 3 mol . sup .- 1 ] vmenergy per unit volume [ jm . sup .- 3 ] e . sub . vpower [ w ] ppressure [ pa ] pvolume flow [ m . sup . 3 s . sup .- 1 ] i . sub . vgeneral gas constant [ jk . sup .- 1 mol . sup .- 1 ] r = 8 . 3144 jk . sup .- 1 mol . sup .- 1______________________________________ the internal energy of a gas is the product of heat capacity and temperature : dividing by the molar volume v m yields the energy per unit volume e v , ## equ1 ## if the molar volume of ideal gases v m = rt / p is used as an approximation , this gives ## equ2 ## or , in integral form , ## equ3 ## the available heat power p of the air then results from multiplication by the volume flow i v : ## equ4 ## after taking into account a conversion factor for the fixing efficiency f e , the fixing power p f is obtained as ## equ5 ## solving for t 2 results in ## equ6 ## estimating the energy needed for fixing both the main constituents of air , namely n 2 ( 78 %) and o 2 ( 21 %) have a molar heat capacity of c p = 29 j k - 1 mol - 1 . therefore , this value should also be used for dry air . the pressure in the fixing chamber is p = 1 · 10 5 pa ( 1 bar ) the final temperature of the air must not lie below the softening temperature of normal toner . this is set at 127 ° c ., therefore t 1 = 400 k . according to g . goldmann , technologie der ops - hochleistungs - drucker [ the technology of the ops high capacity printer ], in das druckerbuch ( oce printing systems , 1992 , pp . 3 - 16 ), the energy demand q for fixing toner is essentially given by the heat capacity of the paper and the heat of evaporation of the water stored in the paper . at a water content of 5 %, the energy demand q = 236 j / cm 3 . the amount of energy taken up by the toner is negligible because of the small amount of toner . ______________________________________paper weight g = 0 . 15 kg / m . sup . 2printing speed v = 0 . 3 m / sprinting width l = 0 . 3 mdensity r = 700 kg / m . sup . 3______________________________________ hence , in the printing process , the paper volume throughput per unit time is ## equ7 ## and the power needed for fixing is given as ## equ8 ## the efficiency of the fixing is f e = 0 . 1 , and thus 10 %. substituting the values for t 1 , c p , p , p f and f e from assumptions 1 to 5 into eq . ( 8 ) gives ## equ9 ## according to this equation , for example for an air delivery capacity of 4 · 10 - 3 m 3 s - 1 (= 15 m 3 / h , which corresponds to the delivery capacity of conventional pumps with a 1 kw power consumption ), a temperature of 5 · 10 16 k is calculated , i . e ., an impracticably high value . if eq . ( 12 ) is solved for the volume flow i v : ## equ10 ## it is then possible to calculate the necessary volume flow of the air for a given air temperature . in the case of the maximum possible temperature of the air , it must be taken into account that paper ignites at about 233 ° c . ( 506 k ). if this temperature for the hot air is inserted , a volume flow of 0 . 55 m 3 s - 1 is calculated . however , at 300 ° c ., 0 . 36 m 3 s - 1 is still obtained , and 0 . 25 m 3 s - 1 at 400 ° c . such high volume flows can be realized only with a considerable outlay for engineering . equation ( 8 ) describes the relationship between the gas temperature used for fixing and the necessary volume flow ## equ11 ## in order to change the system so that it becomes technically feasible , it is necessary for the exponent in eq . ( 8 ) to be reduced . this can be effected , for example , by increasing the efficiency f e or increasing the pressure p . in each case , the aim is to attain the highest technically feasible efficiency . whether it is possible to attain a value significantly above 10 %, or whether this value is reached at all , remains to be tested . increasing the pressure is possible only with a high outlay for engineering , and is therefore ruled out . options which remain are reducing the power p f needed for fixing , and increasing the heat capacity c p of the gas . both are possible by using steam as the heat transport medium . p f is reduced , because driving water out of the paper is prevented or at least retarded in a steam atmosphere , and thus , according to goldmann ( cf . above ), only about 50 % of the energy is still needed for fixing . on the other hand , c p is increased , because steam makes a significant difference with respect to air , with 33 . 6 jk - 1 mol - 1 instead of 29 jk - 1 mol - 1 . inserting these values into eq . ( 8 ), given otherwise unchanged conditions , yields , in a manner similar to eq . ( 12 ) and ( 13 ): ## equ12 ## at a gas temperature of 300 ° c ., the computation now yields a volume flow of 0 . 16 m 3 s - 1 , which corresponds to an improvement of about 66 % over hot air . these relationships are illustrated by fig4 which shows the necessary gas temperature as a function of the volume flow in the case of the convection fixing of toners , for hot air and steam as the heat transport medium , in accordance with equations ( 12 ) and ( 14 ).