Flat plate heat exchangers work similarly to older types of heat exchangers, such as tube and shell heat exchangers, only in that they are used to exchange heat between two liquids. The difference is that a flat plate heat exchanger uses flat metal plates, typically stainless steel, to exchange the heat energy. They are used to in water heaters and even in heated swimming pools. Stainless steel is the metal of choice simply because is less susceptible to corrosion and it can withstand higher temperatures.
These plates are welded or brazed together, or in larger versions they are held together by gaskets. They are then placed in a frame in a way that facilitates the flow of hot and cold liquids through different chambers. One reason flat plate heat exchangers work so much more efficiently that the older types is that the fluids touch a larger surface area, therefore the heat is exchanged more quickly and efficiently. Another reason is that the fluid touches the actual heat conductor rather than simply the outer shell, which also contributes to faster and more even heat exchange.
While it is typical for heat exchangers to be used for heating fluids, the also work the other way, meaning they can be used to cool fluids down. They do this by pulling the heat out of the liquid in one chamber into the liquid in another, thus exchanging the heat energy. They come in various sizes from very small and compact to much larger, and are used for all different purposes.
It would be untrue to say that tube and shell exchangers are obsolete, but they are certainly on the decline. This is due to the extreme efficiency and speed of the flat plate variety in comparison. Certainly they have made a splash in the world of domestic water heater sand heated pools of all kinds, and with new technology great things are being done in the flat plate heat exchanger industry.