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Moisture in Dust Collection
Water in vapor or liquid state is moisture. Moisture
can seriously affect dust collection efficiency in many ways.
- by its tendency to plug the filter media. Sometimes
dust particles have a
hygroscopic nature (tendency to absorb water). The mixture
of dust and water then coats the filter media. This mixture is impermeable and
cannot be dislodged by many cleaning systems.
- Moisture damages cellulose media in cartridge
collectors because cellulose becomes impermeable when wet. However
moisture in baghouses is less dangerous because the moisture can be taken out and bag can be
dried.
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By
its nature moisture damage goes unnoticed until the system starts becoming
inefficient with plugged bags and cartridges, high pressure and low flow rates.
It is necessary therefore to check for moisture frequently before serious damage
could be caused.
Keep reading for more
dust collector
information on how to detect and control different kinds of moisture
in different situations.
Moisture Control
Keeping moisture in vapor from is the best way to control moisture damage.
Excess vapor must be removed before it can condense to liquid.
Moisture could enter the system due to the following reasons:
- Dust characteristic: The hygroscopic nature of dust
could absorb moisture. Sometimes the dust itself may contain moisture (saw dust
contains 19% moisture)
- Temperature (differential in process flow): The
difference in temperature at different places in the air stream could cause
moisture
- Temperature (seasonal): Seasonal temperature could
cause moisture in cases where manufacturing facility is indoors and dust
collector is outdoors. Exposed to climatic changes the dust collector may
attract some moisture.
- Humidity: Weather changes could cause moisture in
the system
Wet compressed air: This is a process-induced moisture and could be aggravated
by not having an upstream dryer in high humidity conditions.
- Sprays: Coolant and other sprays that are used on
other machines could enter the system and cause moisture
How to Detect Moisture
It is essential to identify moisture proactively because it normally goes
unnoticed otherwise.
- Actively identify presence of moisture. Moisture
could occur at anytime and by a variety of factors
- Check for moisture at times when possibility of
moisture is high (inside dirty air chamber where bags are housed)
- Conduct an inspection of walls of dust chamber for
drops of moisture, wet dust clumps or rust
- If non-hazardous, dust could be removed in a paper
tissue and squeezed for traces of moisture
- Moisture may gather in places where the
dust collector is outdoors and exposed to outside environment
When presence of moisture is detected, corrective action must be taken. A
feature of moisture is that it could be intermittent and may disappear before
coming to notice. Since damage to certain dust collectors such as the cellulose in
cartridge collectors could be rapid, it is best to check frequently.
Process Air Moisture
Temperature in the process air affects the amount of water vapor carried in
the air. Hot air holds more water in vapor state than cold air. If air at dust
collection point is hot and it travels long enough in the ducts to cool down,
water vapor could condense when temperature falls to dew point. Dew point is the
temperature at which air vapor mixture is condensed. As temperature lowers,
vapor forms water droplets. Dew point temperature varies according to water
vapor present in the air stream. By monitoring process stream, this can be
controlled.
If cases where one machine adds a cool air stream to the warm air process flow,
it would be best to measure temperature at various points in the system. A large
temperature difference (15 degree F or more) indicates condensation. The
difference of temperature varies on humidity of the process air stream
Compressed Air Moisture
Wet compressed air affects bags and cartridge filter media. When cleaning
air is moist, filter media becomes saturated with water from the inside and
leaks to the outer surface of the filter media. This exposes it to the dust cake
and the dust water mixture forms an impermeable coating on the filter media
causing an increase in pressure drop across the filter media. This also blocks
out fan air from passing through. It is therefore essential to maintain a clean
and dry compressed air flow, which contributes to long term, benefits and
extends filter life.
Clean and dry compressed air can be achieved by the use of air filters and air
dryers. Air dryers are more efficient than air filters and come in two types –
refrigerant and desiccant dryers.
Refrigerant dryers use refrigeration cycle and can take dew point temperature to
35 degree F. If compressed air temperature reaches 35 degree F water vapor
condenses and forms moisture. The efficiency of refrigerant dryers can be
increased by keeping both compressor and dust collector in factory area where
temperature is normally over 35 degree F. When dust collector is outdoor and
temperature dips below 35 degree F, moisture could seep into filter media
(pushed by cleaning air pulse). In such cases an automatic drain can drain the
compressed air manifold.
Desiccant dryers remove moisture by using chemicals that absorb moisture. They
can remove moisture below 20 degree F. When desiccant gets saturated, a standby
dry desiccant replaces it while the saturated tube is drained. This system has
the advantage of not having a separate drain system.
In some cases both procedures are used in combination – refrigerant system
brings moisture down to a 35 degree F dew point and then a desiccant dryer
brings the moisture down to less than 20 degree F. Both types of dryers have
specific advantages and disadvantages and must be used keeping specific
requirements in view.
The mixing of warm process air stream and cool, clean compressed air can
sometimes lower the overall temperature. Warm process air carrying moisture
could, after cooling, cause condensation inside the collector.
For example, in areas where temperatures fall below zero degree F, the cleaning
air manifold cools the compressed air to the temperature outdoors. This
temperature differential could cause condensation. It is a good practice to
measure temperature at the collection source and dirty side chamber to be geared
up for moisture condensation.
Refrigeration
Cycle in Compressed Air Expansion
The characteristic of compressed air as it leaves the pipe orifice is to
expand and cool. The cleaning air jet draws in process air thereby moderating
temperature of the cleaning jet. Assuming the pulse pipe characteristics during
initial pulse as follows
Compressed air pressure – 100 pounds per square inch absolute or 85 psig
Temperature T1 = 530 degree absolute or 70 degree F
As air expands through the orifice it cools. To the cooler temperature can be
calculated thus:
To = (t1) (0.058) (k-1)k where k is a gas constant for air, 1.4
To = (530) (0.833) = 441 degrees Rankine = - 19 degrees F
Since jet grows by drawing at least 4 times the air by inducing cleaned process
air into the jet, the mixture temperature can be found as:
Heat lost = heat gained
1 cfm ( T m – 441) = 4 cfm (530 – Tm)
Tm = 512 degrees absolute or 52 degrees F
Due to instability in the air jet, some heat is regained. However the jet is
cooler by 5-10 degree F overall. When cooling effect goes below dew point on the
air jet, plugging occurs.
Heating compressed air with a pulse jet manifold heater tackles this problem by
increasing air jet temperature above process air temperature. Temperature can be
regulated by thermostats.
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