What is a disadvantage of an open feed-water heater?

An open feed-water heater operates by directly mixing the feed water with steam, which can result in some limitations regarding its effectiveness at removing dissolved gases such as oxygen and carbon dioxide. This interaction may not allow for efficient degassing compared to other systems like closed feed-water heaters, which typically provide better separation of water and steam and allow for more effective gas removal. Therefore, the statement that it is less effective in removing oxygen and CO2 correctly highlights this characteristic of open feed-water heaters, indicating a significant disadvantage in certain applications.

The other potential answers discuss attributes that do not necessarily pertain only to open feed-water heaters. For example, heating the feed water to very high temperatures may not inherently be a disadvantage, depending on the specific design and requirements of the steam cycle. Additionally, the operating conditions of open feed-water heaters are generally flexible, allowing them to function under normal atmospheric pressures and a variety of environmental conditions rather than requiring specific weather or high pressure to operate. Thus, the choice regarding inefficiency in removing gases encapsulates a clear drawback pertinent to the design and operational limits of open feed-water heaters.