In "counter-flow" heat exchangers the fluids enter the exchanger from opposite ends. Microchannel heat exchangers can be made from metal or ceramics. The heat exchangers in direct-combustion furnaces, typical in many residences, are not 'coils'. Tubular heat exchangers and plate heat exchangers are still the most widely applied product types. Double pipe heat exchangers are the simplest exchangers used in industries. Lubricants can also clog heat exchangers, especially the regenerator. Flippers act as heat exchangers, warming or cooling the seal as needed. Large heat exchangers entail high investment costs. Particularly exothermic fermentations may require the use of external heat exchangers. In the hot, dry ground, earth tubes or downhole heat exchangers can collect the heat. Further information about copper alloy heat exchangers for industrial facilities is available. It has applications in piping systems, pollution control equipment, heat exchangers, and bleaching systems. Small heat and moisture exchangers, or humidifying/bacterial filters, can be used. For sludge treatment, SHEs are generally smaller than other types of heat exchangers. Heat transfer enhancement is the process of increasing the effectiveness of heat exchangers.