a generalized system utilizing post load mixing ( plm ) is shown in fig1 , to which reference is now made . the thermal control system 10 or “ tcu ” is consistent with the tdsf concept but differentiated by incorporating the plm approach , and forms a closed loop that encompasses an active thermal control system ( tcu ) 10 and a thermal load 30 . the thermal load 30 is typically a heat exchanger that functions with a processing unit ( not shown ), such as a chuck for processing semiconductors . in the thermal control system 10 a refrigerant comprising a medium such as r - 507 is input to a compressor 12 in gaseous form and a pressurized output is provided therefrom into a main line 13 . one branch from the main line 13 includes an air cooled ( in this example ) condenser 14 having an external air - cooled fin structure 15 engaged by flow from a fan 16 shown only symbolically . the condenser 14 provides a fully or substantially liquefied output of refrigerant at an essentially ambient temperature in a first output path 20 . a separate branch from the compressor 12 output 13 is taken from a junction before the condenser 14 to direct pressurized hot gas from the compressor 12 into a second flow path 22 . this second flow path 22 includes a proportional valve 24 that is operated by a controller 18 so as to adjust the proportion ( in mass flow rate ) or hot gas that is to be used out of the compressor 12 output . this adjustment modulates the two flows and ultimately determines the proportion of hot gas to be employed in the consequent mixture of the two flows , as described below . the adjustment consequently sets the target temperature for the thermal load 30 . in the first branch 20 the output from the condenser 14 is applied to a thermo - expansion valve txv 26 , this output being dependent on and determined by the differential temperature between the superheated gas as sensed a proximate by bulb 35 and the temperature of output fluid from the second mixer 32 a point in line 51 adjacent where the bulb 35 is located . the thermo - expansion valve 26 thus senses the pressure difference between liquid contained within bulb 35 and the pressure sensed by a line 48 connected to externally equalized txv 26 . the output flow from the txv 26 is here coupled to the thermal load 30 , which is depicted only generally . said output flow from the txv 26 travels through a delta p valve 49 which valve performs the same function as disclosed in u . s . pat . no . 7 , 178 , 353 . after passing through valve 49 the expanded cooled output from the txv 26 mixes with some of the hot gas in the first mixing tee 50 . the output 31 from the load 30 is , in accordance with the plm approach , returned to the input of the compressor 12 via one input of a second mixing tee 32 , which also receives , at a separate input , some of the output from the proportional valve 24 . the output line from the second mixing tee 32 returns to the compressor 12 , but the input pressure of this return flow is sensed on route to the compressor 12 input by the external equalization bulb 35 which is coupled into the txv 26 via the line 36 . this connection also provides the known external equalization feature disclosed in the patents referred to above and in other patents and applications on the tdsf system , so that it need not be described in further detail . in addition , the controller 18 for the proportional valve 24 receives a temperature input from a sensor 38 that is responsive to the temperature level at the thermal load 30 . alternatively , said temperature sensor 38 may be mounted so as to sense any other location that is desired to regulate . the plm dual flow , dual mixing system , has other features and advantages . a solenoid valve , labeled sxv 54 is in the path from the proportional valve 24 to the second mixer 32 . the sxv 54 is controlled by the controller 18 , so it can be shut off whenever the system is programmed to make a change in the target temperature from one level to a higher level . shutting off this path at the sxv 54 assures that all hot gases flow to the input of the first mixer 50 , and more rapidly increase the temperature of the flow into the thermal load 30 . in the input to the sxv 54 , a settable impedance , shown symbolically , constituting a controllable orifice 78 is included , in parallel to a comparable settable impedance or controllable orifice 79 in the direct path to the first mixer 50 . by the use of these control orifices 78 and 79 , the two separate flows of pressurized gas fed into the first mixer 50 and second mixer 32 can be proportioned and balanced as desired . the system also includes , as shown , a heater 117 in the input to the compressor 12 , which heater 117 may be activated by the controller 18 to convert a liquid containing mixture returning from the second mixer 32 to the wholly gaseous phase for proper operation of the compressor 12 . mixing the hot gas from the proportional valve 24 with the cooled expanded flow from the txv 26 after the thermal load 30 retains the essential benefits of the tdsf system , but offers particular added benefits . these are particularly applicable where substantial pressure drops or differentials in heat transfer coefficients may be encountered or exist within thermal load 30 . the mass flow from the proportional valve 24 , when combined with the system flow at the second mixing tee 32 and also with the txv 26 output to the first mixing tee 50 , modulates the pressure within the load 30 . this variation affects the temperature within the circuit and thereby controls the temperature of the load . with plm , the temperature level across a thermal load , such as a semiconductor chuck can be contained within tolerances that are more precise than previously expected . tests of a practical system show a reduction in temperature differential to 3 ° c . from a prior 10 ° c . differential . the media fed into the thermal load 30 is stabilized in temperature throughout its flow path in the heat exchanger therein because of the total pressure of the refrigerant fluid , which pressure is controlled by the proportion of hot gas propagated into the circuit . the pressure of the refrigerant in the suction line to the compressor 12 is influenced by the mass passed into the compressor , which compressor 12 processes a fixed volume per unit of time . because of these interrelated factors , the thermal load 30 is more tightly temperature controlled than in non - plm based systems . in the system shown , the flow through the thermal load 30 is generally restricted so as to be completely or almost completely that refrigerant that flows through the thermo - expansion valve 26 . by so limiting the flow , the pressure drop undergone by the refrigerant passing through the load is lessened . also , since the hot gas is mixed at the second mixer 32 with the two - phase output of the txv 26 after the output has passed through the load 30 there is a greater percentage of liquid in the mix at this point . thus the heat transfer coefficient is maintained high throughout the thermal load 30 . therefore , adjustments in the two flows can also be made after sensing the thermal load temperature , in order to anticipate temperature differentials . reference should now be made to the mollier diagram of fig5 which depicts the thermodynamic variations in enthalpy ( abscissa ) vs . pressure ( ordinate ) in a complete cycle for the system of fig1 . the pressure - enthalpy points in fig5 are identified by numbers in parentheses to correspond to the similarly identified numbers in brackets positioned around the block diagram of fig1 . thus the input at point ( 1 ) to the compressor 12 is , as seen in fig5 increased by the compressor in pressure and enthalpy to point ( 2 ) before some of it is liquefied in condenser 14 to point ( 3 ). after controlled expansion to point ( 4 ) in the txv 26 , then consequently mixing some hot gas from the proportional valve 24 at point ( 6 ) in the first mixer 50 ( see also fig1 ) results in an increase in enthalpy to point ( 4a ). this interchange is illustrated in fig5 by the dotted line 57 between points ( 6 ) and ( 4 ). passage of the refrigerant through the thermal load 30 absorbs heat from thermal load 30 and shifts the enthalpy to the point ( 5 ). the injection of pressurized hot gas at the input to the second mixing tee 32 of fig1 , as also shown at point ( 6 ), and depicted by dotted line 58 on fig5 . this input adjusts the heat and enthalpy from point ( 5 ) to point ( 1 ). the addition of hot gas at the mixing tees 50 and 32 also adjusts the pressure of the throughput flow , thus further and more precisely adjusting the temperature of the refrigerant at the thermal load 30 . consequently , the controller 18 may set the proportional valve 24 to vary the hot gas mass flow , and responsively , the cooled expanded flow from the txv 26 , to create pressure and enthalpy parameters at the operative levels needed to achieve a target temperature at the thermal load 30 . in this system , the restriction of the direct flow through the load 30 reduces the pressure drop through the load 30 to a minimum . also , the heat transfer coefficient within the load 30 is maintained at a maximum . accordingly , the system provides superior results in achieving and maintaining target temperature . this conclusion is exemplified by factual results achieved in the use of the plm concept in controlling the temperature of an electrostatic chuck used in semiconductor processing . in prior systems , temperature control units have used a liquid mix of thermal exchange fluid , and provided temperature differentials of the fluid through the chuck typically averaging 10 ° c . (± 5 ° c .). using post load mixing , however , the temperature differential through the entire area of the chuck was reduced to no more than about ± 3 ° c . fig2 and 3 disclose an alternative which resolves problems of unwanted liquefaction in transporting a two - phase medium in a long line system employing the tdsf concept . for completeness , the system diagram of fig2 partially repeats the principal elements of fig1 , placing the principal subsystems that provide phase conversion or energy consumption in a single block labeled “ tdsf system ” 10 . from this system , a hot gas line 63 controlled by a proportional valve 24 , a cooled liquid flow line 64 from the condenser 14 and a return line 65 to the compressor 12 are all coupled to a remote control box here termed a plm line box ( or lb ) 70 . the energy converting units in the tdsf system 10 are not attempted to be depicted to scale , in the interest of clarity and understanding , since the line box 70 is exaggerated , as the subsystems of interest . the system of fig2 solves a problem which may arise because of the manipulation , in the tdsf system , of gas and liquid phases of refrigerant , in an advantageous manner for temperature control . concurrent modulation can introduce undesired liquefaction as in the transport of the two - phase medium along a long path . the system of fig2 addresses this problem effectively , and details of a specific implementation further confirming this result are shown in fig3 , to which reference should also be made . in order efficiently to utilize the thermal and fluid pressure energy in the lines 63 and 64 in propagating fluids to and from the physically well separated tdsf system 10 , the operative elements for mixing and control are principally located relatively remotely in what is here called a “ plm line box ” 70 , as shown in both fig2 and 3 . in this practical example , the line box 70 is very small in volume by comparison to the energy generating subsystems . the example shown in fig3 is 12 ″× 12 ″× 6 ″, or 864 in 3 , and it is typically located within about 1 meter or less from the thermal load 30 input and output points . in the lb 70 , the condensate line 64 is directed to a thermo - expansion valve 26 the output of which is applied to a δp valve 76 for pressure reduction , as is well known in tdsf systems . the thermo - expansion valve ( txv ) 26 is externally equalized by pressure transmitted from a point in return line 65 via line 36 . consistent with the system diagram of fig1 , at a suitable point in line 65 a sensor bulb 35 is disposed in thermal communication with the return line 65 to sense the temperature of flow returning to the tdsf system 10 . the output from the δp valve 49 is combined with a portion of the high pressure hot gas flow from the line 63 that is transmitted through a check valve 52 to one input of a first mixer 50 , which also receives a separate input from the δp valve 76 . the output from the first mixer 50 is , as is disclosed above in relation to fig1 , applied to the input of the thermal load 30 . also consistent with the arrangement of fig1 , the output of the thermal load 30 is coupled to one input of a second mixer 32 having a second input ultimately receiving the flow of pressurized hot gas from the line 63 . this bypass flow is , consistent with fig1 , directed through a solenoid valve , ( designated sxv ) 54 that is operated by signals from the controller 18 . the input to the sxv 54 is applied via the flow control orifice 78 , inserted to balance flows between the bypass path and the separate path to the thermal load 30 . from the flow balancing or control orifice 78 the flow is directed to the second input of the second mixer 32 that is in circuit with the return line 65 to the compressor 12 input . the arrangement of elements inside the plm remote box 70 is shown three dimensionally in fig3 , with the depicted elements being numbers correspondingly to the elements in fig2 . although the volumetric size , as set forth above , is very compact by comparison to the compressor and condenser units , it is fully functional for the semiconductor chuck installation . the unit can be further compacted as desired . incorporating the operative control elements for unification and mixing of the two flows of refrigerant in the very small volume illustrated in fig2 and 3 resolves the problem of unwanted temperature variations and accumulation of liquid in the return line , all while retaining the benefits of the plm approach . the plm flow balance orifices 78 and 79 control the flow proportions both before and after the thermal load 30 . furthermore , the added line in the tdsf system 10 provided by the plm remote box 70 directs the bulk of hot gas around the load so that it can unite with the two - phase liquid after the load 30 . consequently the “ quality ” of the fluid that is fed to control the thermal load 30 is lowered , while still operating in the plm mode . in effect , there is an increase in the liquid content in the two - phase mixture that is supplied to the load , which enhances the cooling efficacy of the two - phase liquid . the advantages of employing the plm mode in conjunction with long line installations , are made evident in an objective way by the comparison of performance characteristics in fig6 , to which reference is now made . this comparison is between a conventional chiller , such as an advanced thermal sciences , mp40c , and a “ direct chiller ” of the tdsf type that incorporates the present post - load mixing long line improvement . in all individual parameters that are significant to throughput and uniformity the chiller disclosure herein confirms the significant improvement in performance over a commercially state - of - the - art unit . care was taken to ensure test conditions were comparable in all respects . as a qualitatively limited alternative , when substantial line lengths might introduce problems with liquid puddling within transport lines , unstable temperature changes due to puddling can be limited or avoided using the insulation technique depicted in fig4 . the supply line 22 for cooled expanded flow and the output line 25 from the proportional valve 24 ( both as shown in fig1 and 2 ) are insulated from each other within a jacket 66 until they reach the near vicinity of the load 30 , as at the mixer 50 . although there have been described above and illustrated in the drawings various forms and expedients for post load mixing , the invention is not limited thereto but incorporates all features and alternatives within the coverage of the appended claims .