Clean air — free from dirt, debris, and fibers, and closely maintained to fixed limits of temperature and humidity — is a vital necessity to the textile industry. Because of this need, the textile industry is one of the largest industrial users of air washing equipment. Similar equipment is found in other industries such as automotive industry spray paint booths, tobacco industry air washers, hospital surgery and nursery rooms, photographic film manufacturing plants, and aircraft industry clean rooms. Each of these industries uses water in a gas scrubbing device to clean and process air so that it meets their particular clean air standards.
The use of air-washing equipment in the textile industry is more difficult to understand than in most other industries since there are so many different textile processes, and different combinations of these processes that can be present in one plant. Air washers are utilized throughout the various processes in cotton mills where raw cotton is processed into woven cotton fabric.
They are also used in blending plants where raw cotton is processed and blended with synthetic staple into yarn and then woven into blended fabric. Man-made fiber plants producing nylon or polyester yarn also use washers. Fiberglass plants producing fiberglass yarn for tire cord and other industrial uses utilize air washers extensively.
Some of these plants have gas scrubber systems which double as plant air washers and which present some of the most difficult water treatment problems. Air washers are found extensively in knitting plants including ladies’ hosiery plants, and in carpet mills where carpet yarn is processed and dyed, as well as in the carpet-weaving plants.
Dyeing, finishing, and bleaching processes generally do not require air washer systems; but these processes are often located in the same plant as one or more of the previously described processes.
For example, a plant blending cotton with synthetic staple to produce drapery material may have another section of the plant where they dye this material, finish it, and possibly run it through a printing process. Although dyeing and finishing operations do not require air washers, they are starting to use various types of smoke abatement equipment to control vapors emanating from the plant from some of these finishing processes.
TYPICAL TEXTILE AIR CONDITIONING SYSTEM
Cooling capacity for a typical textile air conditioning system is provided by a standard Carrier, Trane, or Worthington refrigeration machine. Sizes run from 300 to 1200 tons or higher per machine, depending on the size of the plant. Single machine sizes up
to 2700 tons exist in the Southeastern United States in textile operations, but are rare. Total plant tonnage will run from
300 tons in a very small plant to 15,000 to 18,000 tons or higher for a very large textile plant. In large plants, the total tonnage may be designed with a single cooling tower circuit and a single chilled water system; but in most cases, there will be several smaller systems operating independently. The tonnage and number of air conditioning systems operating in a single plant depend on the combination of textile processes being accomplished and production capacity of the plant.
The use of air-washing equipment in the textile
industry is more difficult to understand than in most
other industries since there are so many different textile
processes, and different combinations of these
processes that can be present in one plant.
Cooling towers in textile plants are fairly standard, ranging from various types of old wooden field erected towers to modern metal package units. Some systems are designed with several package towers connected parallel to all of the refrigeration machines. In other cases, each refrigeration machine condenser unit is piped separately to its own cooling tower. Many textile processes require year-round refrigeration for proper temperature and humidity control, and these plants operate cooling towers 12 months of the year. Other plants vary cooling tower operations from six to eight months per year depending on the climate conditions of the plant.
New textile plants are being designed with as few chilled water systems as is practical to maximize efficiency and reduce maintenance. In older plants, air conditioning systems were added on a piecemeal basis, resulting in several small systems in the same plant. It is important to know the number and location of the various cooling towers and chilled water systems and their interconnections so that appropriate chemical feed points can be selected for proper water treatment. For example, several refrigeration machines, each with its own individual cooling tower system, can be operated on a single large chilled water system. Or each refrigeration machine can have its own individual chilled water circuit. These various combinations of equipment make feeding chemicals difficult and expensive in older plants since so many chemical feed points are necessary.
Chilled water systems are piped around the average textile plant for use in air washers, process heat exchangers, and small office air conditioning units. By far, the largest user of chilled water is the air washer unit. A small textile plant may have only one refrigeration machine, one chilled water system, and one air washer unit.
As plants get larger, more and more air washers are found. The average plant with 1200 to 2400 tons of air conditioning capacity may have six to eight air washers producing fairly low velocity air. Or the same plant may have two to four high velocity air washer units. In plants where refrigeration is not required during winter months, these washer units may operate autonomously, continually recirculating water from the sump up through the spray nozzles.
When refrigeration machines are operating, air washer units are supplied with 40 to 50°F chilled water from the system’s main chilled water sump. This sump is usually a large concrete structure containing from a few hundred gallons up to 50,000 gallons of chilled water. The total capacity of the chilled water system can vary between a few thousand gallons of water and over 100,000 gallons of water.
Most textile chilled water systems have high level float switches on the chilled water sump. This
switch controls a solenoid valve on interconnecting piping between the chilled water system and the cooling tower water
circuit. During summer operation, when the chilled water is dehumidifying plant air, the volume of
water in the chilled water system increases. When this occurs, the level in the sump necessarily rises until it hits the limit switch. Excess chilled water then flows to the cooling tower system as makeup. This procedure conserves both water and energy since it avoids wasting excess chilled water and increases the efficiency of the condenser unit.
Chilled water sumps also contain, in many cases, some type of filter for suspended solids removal. These filters may be a simple wire mesh screen on a moving drum and a water spray device to remove suspended matter from the screen. Many potential fouling problems in the air washer systems can be avoided by removing suspended solids in the chilled water sump in this manner.
Most textile plants use compressed air for controls, water atomizers, and other uses and, therefore, must deal with the problem of supplying cooling water to the air compressor heads,
New textile plants are being designed with as few
chilled water systems as is practical to maximize
efficiency and reduce maintenance.
oil coolers, and after coolers. The cooling water may come from the main cooling tower system in larger plants where there are several air conditioning units, or there may be a separate small tower provided for these units. Over the years small textile plants have used once-through cooling water, but this practice is declining due to economics and discharge limitations.
A few plants that are set up for once-through cooling water on air compressor systems conserve this water by sending it to one of the cooling towers during summer operation as makeup. If the cooling tower is not run during the winter months, this water again becomes once-through. When air compressor cooling water is utilized as makeup for a cooling tower system, a second sump float level control in the cooling tower sump controls the makeup from the air compressor units to avoid disturbing tower water concentrations and treatment chemical residuals. This second float is set at a higher level than the main makeup water valve so that it operates first when makeup is required. Water that is not needed when this float closes moves vertically a few feet from the float valve and then out a vented overflow line to waste.
TYPICAL AIR WASHER UNIT
Just a few years ago, textile air washers were designed primarily as low-air-velocity, nonchilled water systems. They were so large that air-washer rooms were an integral part of the building housing the textile operation. Lint and fiber screens ahead of the washers were nonexistent in many cases, and the washerrooms were typically filled with cotton lint. The warm water in the washer was a perfect environment for bacteria. Lint, dirt, and other suspended matter in the water continually plugged air washer spray nozzles, reducing the efficiency of these units. As the textile industry began to install air conditioning equipment, these units were upgraded and modified so that they would perform properly with chilled water.
Air cleaning units found on textile chilled water systems today consist of two basic types: packaged air washer units and rotor spray units.
Air entering either unit from the inside of the plant first passes through a fine mesh screen or drum roll filter. The function of these filters is to remove lint, dust, oil in some cases, and other debris before they get into the water in the air washer units. Some of these units have moving paper media, while others have semipermanent synthetic media that is replaced two or three times per year. Large rotating drum filters with vacuum attachments to continually remove lint are another type of filter performing the same function. This type of filter would be found primarily in a cotton or blending plant that had a great deal of lint in the air.
A knitting plant or a package dyeing plant with winding operations would use the vertically mounted corrugated synthetic media for oil removal. Instead of a lint problem, this type of plant generally has oil in the air. Between the filter and the air washer unit, oil is kept to an acceptable level in these plants. Drip pans under the filter collect oil that drops slowly by gravity.
Most textile air washer systems are designed with temperature and humidity controls to automatically blend outside air with the in-plant air appropriate to the needs of the particular textile process being run in the plant. This is done through electrical or air-controlled louvers opening the intake of the washer to the outside air. The specific temperature and humidity may vary from one department to another in the same plant. This is another reason for having separate chilled water systems for different processes in a plant.
The mixture of air enters the actual washing section of the air washer unit after passing through solid cartridge type filters. Several vertical headers with spray nozzles are spaced evenly across the area of air flow. The nozzles spray water against each other so that the incoming air must pass through a wall of water two or
Most textile air washer systems are designed with
temperature and humidity controls to automatically blend
outside air with the in-plant air appropriate to the needs
of the particular textile process being run in the plant.
three feet thick. This does three things to the air:
1. Washes and cleans the air
2. Saturates it with moisture
3. Controls the temperature of the air
The spray nozzle headers are connected to a recirculating pump that continually picks up water from the air washer sump and pushes it through the nozzles. The capacity of this recirculating pump ranges from 250 to 1200 gallons per minute. The main chilled water pump located at the chilled water sump continually supplies chilled water to each individual air washer sump. Overflow and gravity take the chilled water back to the main refrigeration machine and chilled water sump. Capacities of air washer sump pans range from 600 to 2500 gallons.
Older textile plants control implant air temperature by regulating the temperature of the chilled water at the refrigeration machine. They control humidity by the
use of water and compressed air atomizers located in the overheads of the department requiring humidity.
Some newer designs have automatic controls in each washer that automatically adjust the temperature of air washer sump water by letting more or less chilled water in. Automatic control valves are connected to the temperature and humidity sensing devices. This means that the amount of chilled water makeup can vary from one washer
to another in a plant, but this
variation is not large enough to disrupt adequate recirculation of treatment chemical being fed to the main chilled water sump.
Air washer units have lint screens located across the front of the sump in addition to the chilled water sump screens and filters. The purpose of the washer lint screens is to prevent fouling of the centrifugal spray pump. There are generally two banks of screens so that one can be left in place while the other is cleaned. The size of wire mesh in these
screens will vary from 1/4 to 1/8 inch mesh. Construction in new units is stainless steel, while old plants may be galvanized wire or copper. Some old washers have a small-scale drum filter inside the washer sump. The purpose of this filter is primarily to filter out lint during winter
operation when some air washers operate autonomously without chilled water makeup.
After the air passes through the spray section of the washer unit, it passes through mist eliminator blades that function to remove condensed moisture. These eliminator blades are constructed of stainless steel in newer units, and galvanized metal in older ones. After the mist eliminator section, some washer units are designed with steam reheat coils to temper the cooled air to suit a particular textile process. Bypass
duct work is sometimes designed into a unit to allow for simple heating of the air without washing. Large fans or blowers takes the air from the end of the
washer unit and force it out through the plant duct work.
The metal of construction is an important factor to be considered when recommending water treatment for textile air conditioner systems. On newer air washer units, areas that are regularly in contact with water are constructed of stainless steel, except for piping,
which is generally galvanized. Nozzles are generally brass with rubber inserts. The body of the packaged air washer unit is generally galvanized steel. Plant duct work is either galvanized metal or aluminum. Some plants have the air conditioning duct work cast into the concrete structure of the building under the floor.
There are some air washer units, particularly in synthetic plants, that do not use chilled water for the washing process. They continually recirculate the same water through the spray nozzles, except for a small amount of makeup and blowdown. Permanent cartridge type metal filters both before and after the washing section are used to help keep the unit clean. These cartridge filters must be regularly steam cleaned. The eliminator sections of these washers are followed by steam reheat coils and chilled water coils to adequately control temperature and humidity.
Roto-spray systems are similar in principle to the packaged air washer units, but are thought to be more efficient. These systems are housed in a cylindrical
housing slightly larger than the supply duct work, and are generally located on the roof of the textile plant. They have stationary spray nozzles and rotating eliminator blades that look like the compressor stages in a gas turbine engine. There is very little water in these units, and they are somewhat more difficult to treat from a water treatment standpoint because the same amount of dirt is concentrated in much less water. They are also not as accessible as a conventional air washer system because the access doors or manholes are bolted shut during operation. Roto-spray systems can be found on the same chilled water system as air washer units. Both require the same basic water treatment application technology. Unlike an air washer system that can function autonomously, roto-spray units operate from a central sump both during humidification and dehumidification seasons of the year.
OPERATION AND PROBLEMS ASSOCIATED WITH AIR WASHER SYSTEMS
There are fundamental differences between summer and winter operation of textile air washer systems. A basic understanding of these differences is necessary in order to fully comprehend
the water treatment problem presented.
Summer Operation
During summer operation, most textile air washer systems dehumidify plant air. The condensation from this dehumidification process drops into the air washer sump and enters the chilled water system. In one day, a typical textile plant may add condensation in quantities equal to or, in some cases, greater than the volume of the entire chilled water system. This means that there will be no raw water makeup to the chilled water system. Instead, overflow from the
chilled water sump goes either to the sewer or to the cooling tower, as previously discussed, through a solenoid valve.
Condensation of this magnitude directly affects the parameters to be considered for proper water treatment. For example, a textile plant with 30 ppm total hardness in raw water makeup may find chilled water hardness on a hot day to be 5 to 6 ppm or lower. The pH will generally drop to 6.5 to 7.0, and alkalinity will similarly drop. Dissolved solids tend to stay the same or to increase slightly depending upon the degree of process contamination.
Winter Operation
Winter operation of air washer systems is just the reverse. Humidification of plant air generally takes place, and the air washer evaporates water just like a cooling tower in summer. Air washer sumps are each equipped with a float-controlled makeup line so that they can be operated independently during this humidification time of the year. During this time, each washer
must be bled independently to deconcentrate suspended solids. It should be pointed out that although the water in the air washer during winter operation becomes highly concentrated, the major problem is still fouling and not corrosion or scale formation. There is a current movement in the textile industry to install bypass piping around refrigeration machines so that the main chilled water pump and sump filter can be utilized all year long. The benefits of this procedure are:
· A severe fouling problem in one washer is diluted around the plant
· Chemical feed can be done from one feed point
· Sump screen filter can be used
· Standing water in idle chilled water lines is kept recirculating
Air washers are periodically shut down and washed out manually to help control the severe fouling and deposition problems that
occur. The frequency of shutdown and washout depends on the type of textile process being run and the severity of the problem. It may be done every week in some plants, while in other plants five to six week intervals between washouts may be standard. In some cases where fouling potential is minimal, washers operate three or more months between washouts. Most textile plants do this maintenance during scheduled shutdown periods. The actual cleaning is difficult to do while the unit is in operation, except for daily screen cleaning. Some deposits will not come off wet. They must be
allowed to dry, after which high pressure water or steam easily removes them. Physical inspection of the air washer unit during operation is the only way to determine the effectiveness of the chemical treatment program, and the point at which a shutdown for cleaning will be required.
Problems Encountered in Air Washer Operations
Microbiological Growth
Air washers harbor an ideal environment for the growth of microorganisms because of the process contaminants and soluble oils that feed them. After lab analysis, most deposits from air washer units will be shown to contain dirt and debris from the process involved, corrosion products, and some crystalline particulate matter.
The most important part of the deposit is the microbio growth or slime masses; and these are the most difficult to measure. They result in the very sticky slime that combines with the dirt and debris, corrosion products, and crystalline matter to form hard encrusted deposits above the water level and thick slimy masses below the water on metal surfaces inside the washer. When microbio growths combine with process contamination, the resulting deposit in some cases is like a separate organic chemical that is almost impossible to remove.
Controlling the growth of microorganisms in a chilled water or air washer system is the key to an effective treatment program. Microorganism growth can cause odors,
carryover by blocking air passages, encrustation, and corrosion under deposits. In addition, they can cause air washer sump screens to plug, which results in overflow of solids into the sump recirculating pump and the subsequent plugging of spray nozzles.
Oil
Oil and other materials picked up from the plant air can be tremendous nutrients to feed microorganism growth. Even though preremoval of oil may be accomplished with the filters, some oil will be present in the washer and will cause sticky surfaces and increased microbiological growth, and will generally increase the fouling tendency of the unit. Additionally, if certain amines are used in the microbicide program, oil can be coagulated in the washers and chilled water sump to form an extremely sticky slime that will quickly collect fibers and other suspended solids.
Foam
This problem is generally caused by the chemicals being added for treatment, but may be the result of impurities being cleaned from plant air or of high solids in the washer water. Severe foam can overflow the sump pan of the washer onto the floor, spreading slime and dirt, and producing hazardous walking conditions. It can also be sucked out of the eliminator section of the washer
and into the fan. Deposits occur inside the fan shroud, disturbing air flow. The resulting solids carried through the fan deposit in the duct work. In extreme cases, deposits develop on the fan blade itself, unbalancing the fan so that it must be shut down and cleaned.
Carryover
Carryover is caused both by microorganism growth on eliminator blades, which disrupts air flow and allows solids to pass into plant duct work, and by foaming, which also sends solids from the washer sump into the plant. Extremely high levels of dissolved solids in the washer sump water can also result in carryover. These solids can cause a variety of problems in the plant, from spotting of product to disrupting the temperature and humidity controls. Their worst damage is done in the plant duct work. Most textile plant duct work contains a mat of lint and fiber on its inside surfaces. This mat builds up over a period of time and is removed on a regular basis. When this mat becomes wet with solids from the washer, it becomes encrusted with the dirt and salts that are present in the washer water. Severe corrosion results, particularly if the ducts are aluminum and one of the treatment chemicals in the water is phosphate.
Corrosion
Corrosion potential is most
severe in textile air washer and chilled water systems during summer operation. This is the time when the water in the systems deconcentrates and creates corrosive conditions. Chiller tube sheets and heads are the most vulnerable areas. Idle chilled water lines in winter are also vulnerable if they are left full of water. Some plants have been known to allow the chiller to stand full of water all winter long, and this has aggravated their corrosion problem. Lint, dirt, fiber, oil, and microbiological deposits from the air washers cause most of the corrosion problems on the chiller tube sheets. They combine in some cases to totally slime areas of the tube sheet. Severe pitting occurs underneath deposits of this
nature. A standard Nalco recommendation to help control this problem is to clean both the tube sheet and the heads thoroughly and to coat them with several coats of a suitable epoxy paint.
There is little corrosion potential inside the air washer units themselves, except for the mild steel or galvanized piping and any other mild steel structures present. Except for the insides of piping, the best preventive measure for corrosion remains to be a good coating of paint.
Encrustation
This is the name given to a
deposit when it occurs above the water level in a dry area of an air passage in an air washer unit.
This is a very difficult deposit to remove. Neither strong acids nor caustic seem to be effective as cleaning agents. In some cases, a chisel is the only way to remove these deposits. As mentioned previously, these deposits are composed of lint, dirt and
debris, fiber, and process contamination. They are held together by the organic slime from the microbiological
growth. When a deposit of this nature dries out, it adheres to air washer surfaces like epoxy glue.
Odors
Most odors in textile plant air washer systems result from the chemicals that are used in treating the system. For example, chlorine compounds, polychlorophenates, and some organic sulfur compounds, when fed in quantities sufficient to kill bacteria, result in severe odor problems in plant air. Odor problems in air washer systems generally do not result from gases inside the textile plant. Most of these gases are dealt with using some form of smoke abatement.
INSPECTING AIR WASHER SYSTEMS
Physical inspection of an air washer unit while it is in operation is the only way to determine if any of the above problems exists.
Checklist for inspecting
an air washer unit
1. Secure necktie and other
loose clothing prior to
entering air washer unit.
2. Check sump screens for fouling.
3. Check spray nozzle section for fouled and inoperative nozzles.
4. Observe suspended solids in washer sump water.
5. Reach into washer sump with your hand and note any deposits or slime that may be present below the water level.
6. Check for foam.
7. On discharge side of washer, check for foam in the eliminator section.
8. Note any obvious carryover and deposits in the air chamber and fan shroud.
9. Using your hand, reach into the eliminator section a few inches and check for any deposits that might be present.
10. Note any slime growth on the floor in this section of the washer.
11. When walking through the plant, observe air conditioning duct work, and look for small spots on the bottom surfaces that may be indicative of corrosion.
WATER TREATMENT OF AIR WASHER SYSTEMS
A complete equipment survey, line diagram, and a thorough understanding of equipment operation are necessary to recommend effective water treatment programs for air
washer and chilled water systems. Treatment problems must be categorized relative to their severity. In most cases, treatment problems will be present in the order noted under the previous section, or they will relate specifically to operating conditions in the plant. In all cases, factors that limit the treatment selection must be discovered and considered.
The selection of an effective microbicide and dispersant is generally the best place to start in developing a water treatment program for air washers and chilled water systems. Choose one that will kill the organisms that predominate in your particular system, one that will minimize foaming, one that is stable at the temperatures of the chilled water in the system, and one that will not coagulate or precipitate with other solids or oils in the system water. Feed rates for the selected microbicide must be below the threshold at which odor problems begin. In some cases, it is desirable to feed microbicide continuously at low levels rather than slug dosages at high levels. The dispersant selected should be able to handle primarily microbiological deposits, but in systems with heavy oil concentrations, a strong oil dispersant may be appropriate. The oil dispersant should not foam excessively or present odor problems.
In most cases, present product technology will allow the feed of treatment chemicals to air
washer systems at recommended use levels without a foaming problem. Continuous automatic feeding of treatment chemicals is one of the reasons for this fact. Whenever foaming from a microbicide or dispersant is a consistent problem, an appropriate antifoam should be considered. Generally, an antifoam for use in this type of environment should have the following properties:
· Emulsifiable product stable to any temperature to be encountered in the chilled water system
· Free of odor-causing materials
· Effective for the pH range found in the system
· Has been evaluated and found to be effective in combatting the type of foam encountered
Periodic slug feeding of the antifoam in small dosages will, in most cases, adequately control foam in chilled water and air washer systems.
Corrosion control, as previously discussed, is closely related to the effectiveness of the microbicide and dispersant. Choose a corrosion inhibitor that is effective for the type of corrosion problems you anticipate, and compatible with the biocide selected, system metals, and any anticipated process contamination.
There are no easy answers to the selection of a corrosion inhibitor for air washer and chilled water systems due to the great number of differences between textile
plants. By far, the most important part of controlling corrosion is keeping the system clean. Molybdate, zinc, and phosphates are among the materials used for corrosion control in textile systems. Application technology for each of these materials should be considered in relation to the operating parameters of the washer or chilled water system to be treated.
Scale control is not a factor, except in the cooling tower system. This is because of the tremendous dilution of chilled water during summer operation and the fact that there are no heat transfer surfaces for the concentrated air washer water during winter operation.
