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

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

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

Answer: (A)

In a boiler, the energy put into the fluid goes into increasing its specific enthalpy as it changes from state 1 to state 2, with no shaft work and negligible changes in kinetic or potential energy. The steady-flow energy balance (per unit mass) is q = h2 − h1, so the heat added to the fluid equals the enthalpy rise. This makes q = h2 − h1 the correct form. The other options would either add the enthalpies together or put the opposite sign, which doesn’t reflect the positive heat input that raises the fluid’s enthalpy.

In a boiler, the energy put into the fluid goes into increasing its specific enthalpy as it changes from state 1 to state 2, with no shaft work and negligible changes in kinetic or potential energy. The steady-flow energy balance (per unit mass) is q = h2 − h1, so the heat added to the fluid equals the enthalpy rise. This makes q = h2 − h1 the correct form. The other options would either add the enthalpies together or put the opposite sign, which doesn’t reflect the positive heat input that raises the fluid’s enthalpy.