What effect does a decrease in boiler pressure have?

A decrease in boiler pressure typically leads to a reduction in the efficiency of steam generation. As the pressure in a boiler decreases, the boiling point of water also decreases, meaning that the steam produced will have a lower temperature compared to steam generated at higher pressure. This drop in temperature can result in less efficient energy transfer when the steam is utilized, leading to a decrease in overall thermal efficiency in the system.

Lower boiler pressure can also impact the potential for steam to do work in turbines or other applications, as steam at reduced pressures does not contain as much energy as steam generated at higher pressures. The work output from the steam is thus affected, further contributing to a decrease in efficiency.

In contrast, the other options suggest effects that do not directly correlate with a decrease in boiler pressure. For example, higher pressures would typically increase steam temperature, and a decrease in pressure does not necessarily improve combustion speed or water chemistry directly. Overall, the relationship between boiler pressure and steam efficiency is well-established, making it clear that a decrease in boiler pressure leads to reduced steam efficiency.