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Filtration and Vacuum Specialists since 1976

A Scientific Review of Dust Collection - Part 11

Impact of Moisture in Dust Collectors

Reference material by: Scientific Dust Collectors

Because our newsletters are a service to our valued customers we have decided to share some important and educational information on Scientific Dust Collection. Over the next several months we will be focusing on the use of Dust Collectors. We felt that the extensive information and it's importance in the industry would be very useful in helping our customers make an informed decision on their needs for dust collectors in their businesses. Because the information is extensive we will be spreading it over several months.

Moisture in water in the liquid vapor solid state and it is very disruptive to the dust collection process. Basically, the moisture tends to cause the pores of the filter media to become plugged and, if the dust is hygroscopic, absorb water. The dust-liquid mixture can make an impermeable coating of "muck" that can resist most types of cleaning systems. In cartridge collectors., if the dust is not hygroscopic in nature, the moisture alone can ruin the cellulose media since it loses its permeability when it becomes wet and does not recover when it is dried. In a typical fabric and the bag is allowed to be dried. In many applications, the discovery of moisture in the dust collection system comes as a surprise and the damage has already occurred. In many cases, it results in plugged bags or cartridges, high pressure reading on the magnehelic differential pressure gage, and low flow rates of the process air stream resulting in poor vacuum.

Planning for Moisture

It is important to understand the conditions that can contribute to producing moisture and in having equipment or processes in place that can control it. Usually, the solution is to provide a mechanism to keep the moisture in a vapor form or to remove the excess vapor by using equipment to collect it before the water vapor can condense into its liquid state. These are some key factors that cause the moisture to be present:

  • The dust itself may be hygroscopic and / or contain moisture. Example: Sawdust usually has a moisture content of 19 percent.
  • Temperature differentials in the process air stream. Example: The air temperature at the collection source may be considerably hotter than in the dust collector.
  • Temperature differentials caused by seasonal changes. Example:  The air temperature at the collection source inside the manufacturing building may be hotter than the outside temperature where the dust collector is located and subject to the local weather conditions.
  • Humidity changes. Example: The process air can change its humidity due to wind directional change and other weather changes.
  • Wed compressed air for pulse cleaning may contain a high percent of moisture. Example: No upstream dryer in high humidity environment.
  • Mists or aerosol sprays that are intermittently or continuously added into the process air stream. Example: Coolant sprays for machine cutting tools.

Actually, it takes only one of these factors to cause a moisture problem in the dust collector.

How to Recognize a Moisture Problem in a Baghouse or Cartridge Collector System

Since moisture can arrive at the dust collector from a number of sources, the procedure is to identify the reasons for moisture and to divide the investigation into multiple steps. The first and most obvious step is to identify the presence of moisture in the collector which may occur during various times of the day, seasons of the year, or special events in the process air stream. At an opportune time when the possibility of moisture is high and the dust collector is not operating, check inside the dirty air chamber where the bags or cartridges are housed for signs of moisture past or present. A simple moisture test consist of inspecting the inside walls of the dust chamber for wearing appropriate protective clothing, carefully remove some dust and place the dust onto a paper towel. By squeezing the paper towel with the dust sample inside ti, see if any moisture or oils may have transferred to the paper towel. If any noticeable amount of moisture was transferred to the paper towel, a moisture problem is present and needs to be corrected.

If the dust collector is located outside and is subject to the seasons of the year, the inside of the collector may tend to gather moisture at the same time as outside dewpoint phenomena is occurring such as the formation of heavy water droplets on blades of grass or the actual time period of frost on the windshields of automobiles. Follow the same safety precautions as mentioned in the previous paragraph and inspect the inside of the dirty air chamber for the presence of moisture. Again, check a sample of the collected dust for moisture content by removing a small amount of dust, placing it carefully in a clean paper towel, and by squeezing the paper towel with the dust sample inside it. If moisture has been transferred to the paper towel, a moisture problem is present and it needs to be corrected. It is important to note that the presence of moisture may be intermittent and disappear before anyone has noticed it. However, the damage, especially to cellulose cartridge filters, results in plugged cartridge which can cause them to be replaced frequently.

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. The dewpoint temperature is a key concept and is defined as the temperature passes through the dewpoint temperature. The dewpoint temperature is a key concept and is defined as the temperature at which the air vapor mixture is saturated. As the temperature is lowered, some moisture will be release as droplets of water. There are many common examples of this dewpoint phenomena. A few examples are 100 percent humidity (fog), first formation of frost on the windows and surfaces of cars, or dew on blades of grass in the early morning.

For every given temperature, there is a corresponding dewpoint temperature which varies according to amount of water vapor that is present in the process air stream at that moment in time. 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 starts to add a cool air stream to the warmer main machine air flow. At these moments of change, temperature measurements at different points in the duct and in the collector should be taken in order to understand the effect of each part of the collection process. Large temperature differences of approximately 15 degrees Fahrenheit or more between the collection source and the dust collector can indicate a potential condensation of moisture. This temperature differential can vary depending on the relative humidity (amount of moisture vapor in the air) of the process air stream.

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 filer media becomes soaked from the inside and the wetness extends to the outside surfaces of the filter media. Also, moisture wets the layer of dust cake that clings to the filter media and acts like an impermeable coating. Furthermore, the process air on fan air flows with more difficulty through the filter media causes the pressure drop across the filter media to rise with a corresponding reduction in the processed air flow rate.

It must be emphasized that clean, dry compressed air is required for successful long term clean air pulsing of the filter media and will extend filter life. The standard methods to achieve this is by using air dryers and air filters. Filters offer some protection but are nearly as effective as air dryers. There are two main types of dryers. These are refrigerant dryers and desiccant dryers. Each system has its own advantages and disadvantages. For the purpose of this chapter, only a few important criteria are presented. First, the refrigerant dryers can typically take the dewpoint temperature to 35 degrees Fahrenheit. If temperature conditions cause the compressed air to reach 35 degrees Fahrenheit or below, the remaining water vapor will condense and potentially cause problems to the filter media. If the compressor and the collector are both mounted indoors, there is only a small change that the collector would see the 35 degree temperature. However, if the dust collector is located outside and the outside temperature drops below 35 degrees Fahrenheit, there is a good possibility the moisture can be sent into the dust collector filters by the cleaning air pulses. However, if the dust collector is equipped with an automatic drain arrangement that frequently drains the compressed air manifold on the dust collector such as illustrated in Figure 11-1, the moisture problem can usually be eliminated.

The second alternative which actually removes the moisture down to approximately minus 20 degrees Fahrenheit is a desiccant dryer. Instead of using the refrigeration cycle, the desiccant uses the chemicals to absorb the moisture in the air stream. When the desiccant is saturated with moisture, a standby tube of dry desiccant is employed and the water saturated desiccant tube is regenerated by a drying process which removes the captured water from it. The advantage with this system is that no secondary system is needed to remove the moisture from the compressed air system. In fact, some companies use both a refrigerant and a desiccant dryer arrangement. The refrigerant dryness is employed first to remove the moisture down to a 35 degree Fahrenheit dewpoint and then a smaller desiccant dryer system is added to remove remaining moisture down to an approximate minus 20 degrees Fahrenheit. Both types of dryer systems function very well, but it is important to review your requirements with qualified personnel who can assist you in specifying your present and future needs.

There is another important moisture situation that can develop which involves both the warm process air stream and the clean, dry, compressed air pulsing. The mixture of the warm process air stream with the cooler, clean, dry air can potentially lower the overall temperature of this mixture. If the warm process air stream is carrying significant moisture (the air is almost saturated with moisture or closer to its dewpoint), then any significant cooling by the clean air jet could lower the temperature of the mixture and cause condensation to appear inside the collector. For example, in some parts of the USA, winter can mean temperatures of below zero degrees Fahrenheit. The cleaning air manifold will tend to cool the compressed air differential is greater in the winter months and this greater temperature differential may cause more moisture to condensate. It is important to take periodic temperature readings between the collection source and the dirty side chamber of the dust collector during different times of the day and seasons of the year. These temperature differentials will help to anticipate potential condensation problems.

Refrigeration Cycle in Compressed Air Expansion

There is another seldom considered aspect of the reverse jet characteristics in all pulse jet dust collector designs that may be important in some special applications. This characteristic is that the compressed air expands and cools as it leaves the orifice of the pulse pipe. The cleaning jet air draws in process air which moderates the temperature of the overall cleaning jet. Inside the pulse pipe during the initial part of the pulse, we will assume the following conditions:

Compressed air Pressure = 100 pounds per sq. in. absolute or 85 psig
Temperature, Ti = 530 degrees absolute or 70 degrees F.

As the air expands through the orifice, it cools. This cooler temperature, To, can be calculated by the following equation:

To = (Ti) (0.528) (k-1) / k where K is a gas constant for air, 1.4
To = (530) (0.833) = 441 degrees Rankine = -19 degrees F.

Since the jet grows by drawing in at least 4 times the air by inducing cleaned process air into the jet, the mixture temperature is:
Heat Lost = Heat Gained
1 cfm (Tm - 441 = 4 cfm (530 - Tm)
Tm = 512 degrees absolute or 52 degrees F.

There is some heat regained because of turbulence in the air jet and the net effect is that the jet cools off by 5 to 10 degrees F. If the cooling effect drops the jet below the dew point, plugging of the bags from mud and forming may occur.

This can be compensated by heating of the compressed air before it enters the collector. A proprietary pulse jet manifold heater (see Figure 11-2) is available to be installed on existing collectors or on new collectors. The air jet temperature can be raised higher than the process temperature to automatically eliminate this cooling effect. This equipment requires frequent monitoring but can be a benefit in a difficult application.

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