Filtration in Freezing conditions

By Pertti Sundberg, Friendly Filter Guy at Eagle Filters

Typically, freezing problems are detected in power plants as an increase of filter dp causing direct power loss of a Gas Turbine (GT). In the worst case, the freezing of filters may be causing limitations of GT output due to restricted compressor air flow. Some GT′s have a bleed heating system redirecting part of the compressor hot air flow back to the inlet duct, while other GT′s only have high dp protection by shutting down a GT altogether.

Freezing requires two conditions to occur:

1.    High humidity

2.    Ambient temperatures a few centigrades above freezing

As temperatures drop the humidity decreases – the freezing problem is actually present within a narrow temperature and humidity window. This is not good news for a plant located north enough and close to the open sea. Luckily, understanding the root cause makes it easier to protect your GT against freezing.

Not all materials are as vulnerable to freezing. Many plant managers know that to be running effectively in freezing conditions, plants should avoid using cellulose as filtration material and instead use synthetic materials. Cellulose as a material is absorbing moisture and when subjected to freezing conditions icing will block the pores of the material. Synthetic materials with woven like material structure are not as vulnerable to icing.

The phasing of filtration will also help to cope with icing issues. Thick and course pre-filters have wider material structure and hydrophobic material properties can eliminate most of the moisture without freezing problems. Moreover, fine and EPA filters need to have similar material properties to make the combination work reliably under freezing conditions, and thus thin film filtration solutions should be avoided in such ice-cold environments.

For the coldest conditions there is the last resort - using heating before pre-filters. Heating can be achieved with hot water running in riveted coils or with infrared heating.

Hot water coils require more maintenance as they should be stand-alone systems, with non-freezing media (like glycol) with controls, pumps, valves. But if you have surplus low temperature heat available, running costs can be relatively low while there is also the loss of GT efficiency to be taken into account.

Infrared heating only requires a light installation in the inlet duct but also requires an electrical feed and may be limited by the available capacity of the current transformer as the need is approx. 1kw/m3/s of electricity. Infrared would not work if you have canister type filters with wraps as infrared will heat only the surfaces it can reach, and shape of canisters is not ideal for heating.

In some environments, we know that some plant managers accept freezing as a fact of life, and therefor also accept losses from freezing as a fact too. As this very short article states though, there are ways to combat freezing that are available in the market today.

Solutions avoiding Icing problems exists, and you can always just ask more advice from Friendly Filter Guy!