referring to fig1 there is shown a steam generator 10 which is preferably a coal fired central power steam generating boiler producing high pressure superheated steam , for example , steam at 2400 psi and 1000 ° f . the steam from the boiler is fed to a back pressure turbine 11 which is coupled to a generator ( not shown ) for producing base load electricity . the back pressure turbine should be designed to exhause the steam at a pressure and temperature which will depend upon the desired pressure of the thermal recovery steam ( process use pressure ). the exhaust steam from the back pressure turbine is sent to a heat exchanger 12 which is preferably of the shell and tube type . the turbine may have multiple extraction points for supplying steam to the heat exchanges . this is particularly desirable when a plurality of heat exchangers connected in a series feed forward configurations are used . further , the turbine may include separate sections , for example , high , intermediate and low pressure sections . the steam flows in the shell section of the heat exchanger and is condensed with the condensate being returned to the steam generator 10 . the heat exchanger is supplied with feed water as shown and produces steam for the thermal recovery process at 1150 psi , 560 ° f . and 80 - 85 % steam quality . as explained above , in most areas where thermal recovery processes are utilized there is insufficient fresh water for use in the thermal recovery process and it has become a common practice to use brakish waters and low quality wet steam so that the dissolved minerals and salts will be retained in the liquid phase of the steam . this prevents the salts or minerals from being evaporated and forming a scale on the heat exchanger tubes . while the dissolved salts are retained in the liquid phase , it is preferable that the water be treated so that it is relatively soft . this will prevent the calcium from depositing as scale on the heat exchanger tubes . referring now to fig2 there is shown the heat exchanger for use in the present process , partially in elevation and partially in section . in particular , the shell 20 of the heat exchanger is provided with an inlet 21 for the turbine exhaust steam and an outlet 22 for the condensate . the heat exchanger is provided with two heat sections 25 and 26 which are coupled to the shell section by means of flange joints 27 and 28 . the head section 26 is also provided with an inlet 23 for the feed water and an outlet 24 for the low quality steam . in addition , the heat exchanger is provided with a baffle 41 which separates the inlet portion of the heater from the outlet portion . the shell 20 of the heater is provided with tube sheets 30 and 31 at each end . the tube sheet 30 has an opening 32 which forms the inlet to the first tube pass 33 through the heater . at the opposite end of the heater , the tube 33 is connected by a u - shaped tube 34 to the second pass tube 35 . similarly , tube 35 is connected by a u - shaped tube 36 to the third pass 37 of the heater which in turn is connected by u - shaped tube 38 to the final pass 39 . the final tube pass 39 opens into the space 40 where the steam can then flow out the outlet 24 . as explained above , the tube passes 33 , 35 , 37 and 39 may be progressively larger in cross sectional area to accommodate the larger volume fluid flow through the heat exchanger . u - bend portions 34 , 36 and 38 are shown as having flange connections so that they may be readily removed from the ends of the tubes . this will permit the removal of the u - bends so that the individual tubes 33 , 35 , 37 and 39 may be cleaned to remove scale deposits therefrom . while flange connections are preferred , obviously other removable connections such as unions or similar threaded connections may also be used . from the above description it is seen that a single continuous tube pass is provided from the inlet of the tube 32 to its outlet 40 . this ensures that the solids dissolved in the inlet water will remain in the liquid phase and be carried out with the steam through the outlet 24 . it can readily be appreciated that if a tube pass terminated in a header which then fed a plurality of tubes in the second pass , the possibility exists that some tubes in each pass would have substantially dry steam which would cause the solids to deposit a scale on the walls of the tubes . while only four passes are shown in fig2 obviously the heat exchanger may be provided with any number of passes in each individual flow path . for example , as described below with respect to fig3 six passes may be easily provided . referring now to fig3 there is shown an end view of the heat exchanger having six individual passes . each pass is indicated by the letters a , b and c with subscripts 1 , 2 , 3 , 4 , 5 and 6 indicating the particular pass . also , as shown , the cross section of each individual tube pass increases as the fluid flows through the heat exchanger . the dotted lines indicate the u - bends which forms the connection between the tube a 1 and the tube a 2 while the line 51 indicates a u - bend connecting tube a 2 to a 3 . the remaining u - bends are indicated by the numerals 52 , 53 and 54 . it can be readily appreciated that while only three continuous tube passes are shown in fig3 additional tube passes can be provided in the heat exchanger . the number of individual flow paths through the heat exchanger will of course , depend upon the overall diameter of the shell 20 and the size of the individual tubes . it is desirable to have as many tube passes as possible to increase tbe heat transfer area of the heat exchanger and increase it efficiency . from the above description , it is seen that the invention provides an efficient method by which coal may be used to supply low quality steam for use in a thermal recovery process . the invention burns the coal in an efficient manner in a large central steam - producing generator to produce high pressure and high temperature steam . the high pressure / high temperature steam is retained in a closed cycle and thus it is not contaminated by the water used for forming the low quality steam used in the thermal recovery . this ensures that the high pressure coal fired steam generator can be operated in a conventional manner as presently utilized in central generating plants . the use of the back pressure turbine provides a method by which part of the energy of the high pressure steam may be recovered while providing steam which can be used for producing the low quality thermal recovery steam . in some applications it may be desirable to use a number of heat exchangers connected in series to supply the thermal recovery steam . this would allow the use of multiple extraction points on turbines and improve the overall efficiency .