The term “demister” can be used in reference to several different types of devices which are used to remove mist or fog. The device being discussed is generally clear from the context of the conversation, as they are utilized in very different applications, from cars to scuba diving equipment. These devices are available from a number of different manufacturers, some of whom also develop custom products for specialty uses.
In the scuba sense, a demister is designed to remove fog from inside diving goggles. There are a number of tactics used by divers, snorklers, and other people who wear goggles to remove fog and to prevent the buildup of fog. In this case, the demister is actually a chemical compound which is swilled through the goggles to remove buildup and to prevent future buildup of fog. Demisting solutions are available from stores which sell dive supplies, as well as through diving catalogs.
Many cars have a feature known as a demister which is used to deal with condensation on the window glass. Since condensation can impair visibility, drivers want to be able to remove it as quickly as possible. The demister heats the glass, encouraging the condensation to dissipate and clearing the window. These devices are generally not designed to melt away frost, with frost requiring a scraper for successful removal to clear the window unless the frost is very light.
In the industrial manufacturing sense, a demister is a filter which is fitted into vapor processing units. As vapor passes through the demister, droplets adhere to the filter, pulling large droplets out of the vapor. The droplets can be filtered out to the bottom, allowing the vapor to rise and exit the device from the top, while the fluid collects at the bottom. Once passed through a vapor separator, the respective vapor and water can be utilized in a variety of ways.
Classically, the demister used in a vapor processing unit is a demister pad made from knitted metal mesh. The device is usually designed to be cleanable and replaceable so that it can be serviced as it wears down with use. One issue with mesh pads is that the demister tends to trap particulates, as droplets often form around particulates, and as a result it can become fouled over time. Thus, it is important that it be serviceable by technicians and users so that the vapor processing unit is kept in good working order.