For a condenser, the general energy equation relating heat transfer to enthalpy change is which?

For a condenser, the general energy equation relating heat transfer to enthalpy change is which?

• A. q = h1 - h2
• B. q = h2 - h1
• C. q = h1 + h2
• D. q = -(h2 - h1)

Answer: (A)

In a condenser, the vapor enters with a high enthalpy and leaves as condensate with a lower enthalpy, so the heat transfer is the drop in the fluid’s enthalpy. Per unit mass, that change is q = h_in − h_out. With h_in labeled as h1 and h_out as h2, the heat transfer becomes q = h1 − h2. Since h1 > h2, q is positive, meaning heat is released to the surroundings. Writing it as q = h2 − h1 would imply heat is added to the fluid, which isn’t the case here. The form q = h1 + h2 doesn’t represent an enthalpy change at all. The expression q = −(h2 − h1) is algebraically the same as q = h1 − h2, just written differently.

In a condenser, the vapor enters with a high enthalpy and leaves as condensate with a lower enthalpy, so the heat transfer is the drop in the fluid’s enthalpy. Per unit mass, that change is q = h_in − h_out. With h_in labeled as h1 and h_out as h2, the heat transfer becomes q = h1 − h2. Since h1 > h2, q is positive, meaning heat is released to the surroundings.

Writing it as q = h2 − h1 would imply heat is added to the fluid, which isn’t the case here. The form q = h1 + h2 doesn’t represent an enthalpy change at all. The expression q = −(h2 − h1) is algebraically the same as q = h1 − h2, just written differently.