Dropwise and Filmwise Condensation

When the heat from steam is transferred either to the product or to the surrounding, the steam condenses on the cold surfaces. This condensation of steam can take place in two ways- drop wise or film wise. Drop wise condensation is always preferred over the film type as it offers a better rate of heat transfer.

Film wise condensation
In film wise condensation, the surface over which the steam condenses is wet-able and hence, as the steam condenses, a film of condensate is formed.  Generally, film wise condensation results in low heat transfer rates as the film of condensate impedes the heat transfer. The thickness of the film formed depends on many parameters including orientation of the surface, viscosity, rate of condensation etc.

Drop-wise condensation
Drop-wise condensation takes place when the surface over which condensation takes place is non wet-able.  In this mode, when steam condenses, the droplets are formed. When the drops become bigger, they simply fall under the gravity. In drop wise condensation, high heat transfer rates are achieved and hence, many times, chemicals are used to ensure that condensation takes place drop wise.