Direct Contact Heat Exchangers / Condensers transfer heat from hot gasses to a liquid, by passing the gas through a shower of liquid. The liquid is often water but could also be other substances such as an oil.
The equipment tends to be relatively low cost and where both fluids are the same, and condensing is involved, a high heat transfer can be achieved between the condensing vapour in the gas and the liquid. A closer temperature approaches (high efficiency) can often be obtained compared to non-contact (indirect contact) heat exchangers.
By condensing say water vapour from the hot gas using water as the coolant, we not only get a reasonable heat transfer coefficient, but we sometimes unlock a large amount of heat that is in the gas stream as latent heat.
Another advantage is that this type of cooler is that it can be designed to tolerate (and remove) a quantity of dust in the hot gas.