How can two-pipe or double-pipe condensers be identified?

Two-pipe or double-pipe condensers are identified primarily based on the flow direction of the cooling water and refrigerant, which move in a counterflow direction. In this configuration, the refrigerant flows through one pipe while the cooling water flows through another pipe adjacent to it, but in the opposite direction. This arrangement enhances the heat transfer efficiency because the temperature difference between the refrigerant and cooling water is greater along the length of the pipes. As the refrigerant cools down, it encounters progressively cooler water, which optimizes heat exchange.

The other options do not correctly describe the key identifying characteristics of double-pipe condensers. For instance, while compact size might be a feature of specific designs, it is not a defining quality of all double-pipe condensers. Similarly, the notion of requiring more maintenance may vary based on specific models or installation conditions rather than being a general attribute of double-pipe designs. Lastly, having parallel flow directions would actually characterize a different arrangement, which does not maximize the heat exchange as effectively as counterflow systems do.