the subcooling and superheat

[trace]

the chiller composed by a screw compressor, a water-cooled shell-and-tube condenser, a thermal expansion valve and a dry-expansion shell-and-tube evaporator, from the ARI 550/590-98, the standard inlet and outlet cooling temperature are 85F and 95F, the chilled water ranged from 54F to 44F. then I have some questions as follow:
(1) what should be appropriate sub-cooling for this style of unit?
(2) what should be appropriate approach for the DX shell-and-tube evaporator?
(3) what should be the super-heat values for the evaporator and the system(super-heat of suction line)
in addition, the approach for flooded evaporator is
Approach_flooded=Tout_chilled_water-Tsat, but DX shell-and-tube evaporator always equipped an external balance thermal expansion valve, so how to define the approach of DX evaporators. in my mind, Approach_DX=Tout_chilled_water-Tout_refrigerant, where Tout_refrigerant is the actual refrigerant outlet temperature of the DX evaporator, I don't know is it correct.

[moideen]

the chiller composed by a screw compressor, a water-cooled shell-and-tube condenser, a thermal expansion valve and a dry-expansion shell-and-tube evaporator, from the ARI 550/590-98, the standard inlet and outlet cooling temperature are 85F and 95F, the chilled water ranged from 54F to 44F. then I have some questions as follow:
(1) what should be appropriate sub-cooling for this style of unit?
(2) what should be appropriate approach for the DX shell-and-tube evaporator?
(3) what should be the super-heat values for the evaporator and the system(super-heat of suction line)
in addition, the approach for flooded evaporator is
Approach_flooded=Tout_chilled_water-Tsat, but DX shell-and-tube evaporator always equipped an external balance thermal expansion valve, so how to define the approach of DX evaporators. in my mind, Approach_DX=Tout_chilled_water-Tout_refrigerant, where Tout_refrigerant is the actual refrigerant outlet temperature of the DX evaporator, I don't know is it correct.
The exact technical form is LTD(leaving temperature difference).The difference between chilled water outlet and saturated suction temperature is the LTD or define as approach,pinch or split. Whether flooded or dx, this is the method.

[trace]

Thanks moideen for your reply. The saturated suction temperature is decided by the boiling pressure and the pressure drops of the DX evaporator, since the pressure drops is not a constant for different manufacturer, the LTD must vary in a certain range, I wonder the LTD range.