Constant-Temperature Lines. For ASHRAE’s chart, since enthalpy is one of the coordinates, only the 120°F constant-temperature T line is a true vertical. All the other constant-temperature lines incline slightly to the left at the top. From Eq. (2.27), T ha / cpd; therefore, one end of the T line in ASHRAE’S chart can be determined from the enthalpy scale at w 0. The other end can be determined by locating the saturated humidity ratio ws on the saturation curve.