Fix VLE.set_PH silently ignoring H spec for locked-phase-only streams

tests/test_stream.py

@@ -563,7 +563,41 @@ def test_vle_critical_pure_component():
     assert s.phase == 'g'
     s.vle(P=N2.Pc, S=s.S + 10)
     assert s.phase == 'g'
-
+
+def test_vle_PH_locked_phase_only():
+    # A stream composed entirely of locked-phase chemicals (e.g. a permanent
+    # gas modeled with a fixed phase) has no chemicals eligible for VLE, so
+    # `VLE._setup` raises `NoEquilibrium` before `VLE._N` is (re)assigned.
+    # `VLE.set_PH`'s own `self._N == 0` branch already handles this case
+    # correctly (solving for T via a direct single-phase enthalpy balance),
+    # but was unreachable through this path -- `set_PH` used to silently
+    # leave the stream's temperature completely unchanged instead of
+    # raising or solving for the requested duty. Fixed in VLE.set_PH by
+    # catching NoEquilibrium from _setup and falling back to the same
+    # xsolve_T_at_HP call as the self._N == 0 branch.
+    H2 = tmo.Chemical('H2', phase='g', cache=True)
+    tmo.settings.set_thermo(
+        [H2, 'Water', 'Ethanol', 'Octane', 'Nonane', 'Decane',
+         'Dodecane', 'Hexadecane', 'Octadecane'], cache=True,
+    )
+    s = tmo.Stream(None, H2=1, units='kg/hr', T=638.15, P=7134008.172, phase='g')
+    s.vle(T=s.T, P=s.P)
+    H_start = s.H
+    T_start = s.T
+
+    # A modest, unambiguous heating request (T should increase).
+    target_H = H_start + 3705.25
+    s.vle(H=target_H, P=s.P)
+    assert_allclose(s.H, target_H, rtol=0, atol=1.)
+    assert s.T > T_start
+
+    # A modest cooling request too, for symmetry.
+    s.vle(T=T_start, P=s.P)
+    target_H = s.H - 3705.25
+    s.vle(H=target_H, P=s.P)
+    assert_allclose(s.H, target_H, rtol=0, atol=1.)
+    assert s.T < T_start
+
 if __name__ == '__main__':
     test_empty_chemical()
     test_registration_bypass()
@@ -577,6 +611,7 @@ def test_vle_critical_pure_component():
     stream_methods()
     test_stream_property_cache()
     test_vle_critical_pure_component()
+    test_vle_PH_locked_phase_only()
     test_critical()
     test_mixture()
     test_mixing_phases()

thermosteam/equilibrium/vle.py

@@ -1133,10 +1133,26 @@ def set_PS(self, P, S, gas_conversion=None, liquid_conversion=None):
         self._S_hat = S_hat
 
     def set_PH(self, P, H, gas_conversion=None, liquid_conversion=None):
-        self._setup(gas_conversion, liquid_conversion)
+        try:
+            self._setup(gas_conversion, liquid_conversion)
+        except NoEquilibrium:
+            # `_setup` raises before `self._N` is (re)assigned whenever there
+            # is flow but none of it belongs to VLE-eligible chemicals (e.g.,
+            # a stream composed entirely of locked-phase chemicals), so the
+            # `self._N == 0` branch below is unreachable through the normal
+            # path for that case even though it already handles it correctly.
+            # `self._phase_data` is set unconditionally at the top of
+            # `_setup`, before either of its two `NoEquilibrium` raises, so
+            # it's safe to use here.
+            thermal_condition = self._thermal_condition
+            thermal_condition.P = self._P = P
+            thermal_condition.T = self.mixture.xsolve_T_at_HP(
+                self._phase_data, H, thermal_condition.T, P
+            )
+            return
         thermal_condition = self._thermal_condition
         thermal_condition.P = self._P = P
-        if self._N == 0: 
+        if self._N == 0:
             thermal_condition.T = self.mixture.xsolve_T_at_HP(
                 self._phase_data, H, thermal_condition.T, P
             )