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Moisture is very disruptive to the dust collection process, causing plugged bags or cartridges, high pressure readings on the magnehelic differential pressure gage, and low flow rates of the process air stream. If the dust is hygroscopic (highly absorbent), it will absorb the water, and the resulting dust-liquid mixture can make an impermeable coating of “muck” that usually resists most types of cleaning systems. In cartridge collectors, if the dust is not hygroscopic, the moisture alone can ruin the cellulose media, since the media loses its permeability when it becomes wet and does not recover when it is dried. In a typical fabric filter in a baghouse, an intermittent moisture occurrence can sometimes be resolved when the moisture is removed and the bag is allowed to dry. In many cases, however, the damage to the dust collection system has already occurred by the time the moisture has been discovered.
Preventing damage from moisture requires periodic inspections of the dust collection system, as well as an understanding of potential moisture sources and solutions.
Identifying a Moisture ProblemSince moisture can arrive at the dust collector from a number of sources, preventing a moisture problem involves identifying the reasons for moisture and taking the necessary steps to correct the problem. First, identify the presence of moisture in the collector, which can occur during various times of the day, seasons of the year, or special events in the process air stream. Choose a time when the possibility of moisture is high and the dust collector is not operating, and check inside the dirty air chamber where the bags or cartridges are housed for signs that moisture is or has been present. A simple moisture test consists of inspecting the inside walls of the dust chamber for drops of moisture, wet dust clumps, and/or areas of rust formation. If the dust is non-hazardous, carefully remove some of it and place it into a paper towel. Squeeze the towel with the dust sample inside it and check to see whether any moisture or oils may have transferred to the paper towel. If any noticeable amount of moisture is present, a moisture problem exists in the dust collection system and needs to be corrected.
If the dust collector is located outside and is subject to seasonal variations, the inside of the collector might tend to gather moisture at the same time outside dewpoint phenomena are occurring. The dewpoint temperature is the temperature at which the air-vapor mixture is saturated. As the temperature is lowered, some moisture will be released as water droplets. Common examples of this dewpoint phenomenon include 100% humidity (fog), the formation of frost on windows and car surfaces, and dew on blades of grass. Follow the same test procedure described above to check for the presence of moisture.
It is important to note that moisture can be intermittent and can disappear before anyone has noticed it. However, even intermittent moisture can cause damage, especially to cellulose cartridge filters. If cartridges are often plugged and have to be replaced frequently, this is a good indication that a moisture problem is present.
Recognizing the SourcesTo correct moisture problems, it is important to understand the conditions that can contribute to producing moisture. Temperature differentials in the process air stream and wet compressed air used for pulse cleaning are two common sources of moisture.
Process Air Stream Moisture. The quantity of water vapor is affected by the change in temperature in the process air stream. Hot air can hold more water in the form of water vapor than cooler air. If the dust collection point is over the heat source and the process air stream travels far enough to cause significant cooling of the air stream, the water vapor in the air stream can condense when the temperature passes through the dewpoint temperature.
For every given temperature, there is a corresponding dewpoint temperature that varies according to the amount of water vapor that is present in the process air stream at that moment. The process air stream needs to be monitored for times or events that cause sudden changes in the process air stream. For example, a second factory machine might add a cool air stream to the warmer main machine air flow. At these moments of change, the temperature should be measured at different points in the duct and in the collector to understand the effect of each part of the collection process. Large temperature differences of approximately 15ºF or more between the collection source and the dust collector can indicate a potential condensation of moisture. The temperature differential can vary depending on the relative humidity (amount of moisture vapor in the air) of the process air stream.
There are multiple ways to control moisture in the process air stream and each solution is application driven.
Case 1: When condensation occurs intermittently inside the dust collector, some action needs to occur to correct this phenomenon. The moisture in the process air stream must stay in vapor form during the entire time it takes to pass through the dust collector. The main actions to accomplish this include the following:
- Locate the dust collector close to the dust source so that the temperature duct losses are at a minimum. The higher temperature through the dust collector will help the moisture stay in vapor form.
- Add an auxiliary source of hot process air into the main process air stream. In existing installations, the present fan air flow capacity may limit the auxiliary hot air fan capacity. Therefore, heat flow calculations are usually required to determine the best combination original and auxiliary fan rates and the desired temperature of this arrangement.
- Insulate the process air stream duct to help retain its original source temperature. By retaining the existing heat and keeping the moisture in vapor form, additional capital and operating expenses may be reduced or eliminated.
To help eliminate the present moisture, a basic cyclone collector can be placed upstream from the main dust collector. The cyclonic shape uses centrifugal forces to wring the moisture and heavy dust out of the air stream. It collects on the side walls and travels downward into a container. However, an auxiliary source of hot process air is still needed to keep the remainder of the moisture in vapor form and away from the dew point temperature. Heat calculations can help determine the auxiliary fan flow rate and related air stream temperature.
Case 3: Some dusts exhibit hydroscopic properties and may require limits to the amount of moisture or relative humidity in the process air stream. Various dusts can become “tacky” or “mucky” when a critical relative humidity value or temperature is reached.
With instrumentation, an existing process air stream can be tested for a “safe” relative humidity value, related dry bulb temperature, and corresponding dew point. Since each type of dust is unique, some experimentation is also needed to observe how the moisture interacts with dust particles. If the moisture is quickly absorbed into the dust, precautions must be taken to keep the relative humidity low so that the dust stays as dust.
Compressed Air Moisture. Wet compressed air can cause moisture problems for both bags and cartridge filter media. Typically, when the cleaning air pulses moist air in the inside of the cartridge or bag, the filter media becomes soaked from the inside, and the wetness extends to the outside surface of the filter media. Moisture also wets the layer of dust cake that clings to the filter media, and the wet dust acts like an impermeable coating. The process air or fan air flows with more difficulty through this clogged filter media, causing a pressure drop across the media, with a corresponding reduction in the processed air flow rate.
Clean, dry, compressed air is required for successful long-term clean air pulsing of the filter media and extended filter life. The standard method of dry air cleaning is to use air dryers and filters. Filters offer some protection but are not nearly as effective as air dryers.
Desiccant dryers remove moisture down to approximately -20ºF. Instead of using a refrigeration cycle, desiccant dryers use chemicals to absorb the moisture in the air stream. When the desiccant is saturated with moisture, a standby tube of desiccant is employed, and the water-saturated desiccant tube is regenerated through a drying process that removes the captured water. The advantage of this system is that no secondary system is needed to remove the moisture from the compressed air system.
Some companies use both a refrigerant and a desiccant dryer arrangement. The refrigerant dryer is used first to remove the moisture down to 35ºF, and a smaller desiccant dryer system is added to remove the remaining moisture down to an approximate -20ºF.
Other Moisture Sources. Other key factors that can introduce moisture into the system include:
- Hygroscopic or moisture-containing dust
- Temperature differentials caused by seasonal changes
- Humidity changes
- Mists or aerosol sprays that are intermittently or continuously added into the process air stream, such as coolant sprays used for machine cutting tools