Boiler Trials — Factor of Evaporation (FoE) For practical boilers operating at pressures above atmospheric and with feedwater below saturation temperature, the factor of evaporation (FoE) is generally:

Introduction / Context:
The factor of evaporation (FoE) is used to convert actual steam generation under given conditions to the standardized “equivalent evaporation from and at 100°C.” It allows fair comparison of different boilers and operating states by normalizing to a common reference enthalpy rise.

Given Data / Assumptions:

• Operating pressure above atmosphere (so saturation temperature > 100°C).
• Feedwater temperature typically below 100°C.
• FoE = (actual enthalpy rise of feedwater to delivered steam) / (latent heat at 100°C).

Concept / Approach:
Because the actual boiler must raise feedwater from below 100°C to saturation at a higher temperature and perhaps superheat it, the enthalpy rise per kilogram is usually greater than the latent heat at 100°C. Therefore, when we divide this larger enthalpy rise by the 100°C latent heat, the resulting ratio (FoE) exceeds 1. Only in the special case where feedwater enters at 100°C and steam is produced as dry saturated at 100°C would FoE be exactly 1.

Step-by-Step Solution:
Define FoE = (h_steam_actual − h_feed_actual) / h_fg_100.For typical plants: h_steam_actual includes sensible heating to saturation at elevated pressure plus latent heat and perhaps superheat.Since h_feed_actual is below 100°C and h_steam_actual is at conditions hotter than 100°C, numerator > h_fg_100.Therefore, FoE > 1 in normal practice.

Verification / Alternative check:
Worked examples routinely yield FoE values like 1.05 to 1.3 or higher depending on pressure and feedwater heating. Only laboratory reference conditions make FoE approach unity.

Why Other Options Are Wrong:

• Equal to unity: true only for the special “from and at 100°C” case.
• Less than unity: would imply doing less enthalpy rise than the reference, contrary to most real operating states.

Common Pitfalls:
Confusing FoE with boiler efficiency; FoE is a normalization factor, not an efficiency measure by itself.

Final Answer:
greater than unity