1、 Tom Warnert is the Vice President of the Heat Transfer and Industrial Division(s) at LAKOS Separators and Filtration Solutions. Prashant Joshi is the Business Development Manager for the Heat Transfer Division at LAKOS in Fresno, California. Filtration Selection for Cooling Tower Water Tom Warnert
2、Prashant Joshi ASHRAE Member ASHRAE Member FILTRATION DEBATE HVAC water systems (Cooling Tower/Condenser water) are frequently operated without filtration leading to reduced efficiency. Filtration selection for Cooling Tower water is accomplished by determining the type of solids present in the wate
3、r, water quality requirements, physical space availability, weighing the pros and cons of the various filtration options, and budget constraints. Selecting the wrong type of filtration is akin to taking vitamins for pain relief. Good product but wrong application. This paper compares the differences
4、 between barrier and non-barrier filtration options and when to apply each option. Either type of filtration, when correctly applied, assists HVAC equipment to operate at design efficiency. INTRODUCTION It is well documented (Oranski 2012) that there are many benefits to continuously removing dirt f
5、rom HVAC water systems. However, the debate on how to apply, how much to apply, and what type of filtration continues. This paper will compare on a broad scale the advantages and limitations of barrier filtration vs. non-barrier filtration. Also, the comparison brings into discussion the types of so
6、lids typically present in HVAC water systems and filtration types best suited to remove them. The results of this comparison will be applied to the specific application types on HVAC water systems so the reader can quickly determine which technology will help achieve energy and water savings within
7、a framework of physical space and lifecycle costs. BACKGROUND: BASICS OF COOLING TOWER OPERATION AND CONTAMINENT SOURCES Cooling towers, in their operation as heat exchangers, scrub the air of airborne particulates and accumulate dirt. If this dirt is not removed from the cooling towers it often end
8、s up inside other heat transfer equipment dramatically reducing operating efficiencies, and decreasing water treatment efficacy. There are several sources of dirt in HVAC water systems. The contaminants are introduced in two fundamental ways: 1) Dirt from external sources and, 2) System generated co
9、ntaminants. EXTERNAL SOURCES OF DIRT IN COOLING SYSTEMS During normal operation dirt comes in contact with the water - causing it to become suspended in the circulating water of the tower. It is estimated that a typical 200 ton cooling tower, operating 1000 hours, may accumulate upwards of 600 lbs.
10、(272 kg) of particulate matter from airborne dust and makeup water supply (HVAC Systems and Equipment 2012). While not as common or concerning in typical systems, make-up water that is continuously introduced can contain dirt. This dirt then settles out in the basins and downstream system areas wher
11、e the water velocities are lower. SYSTEM GENERATED SOURCES OF DIRT AND FOULING System generated contaminants are present and common in evaporative and closed loop cooling systems. Cooling towers, interconnecting piping, and condensers are often made from iron, steel, and other metals. Due to oxidati
12、on and dirt accumulation corrosion occurs - and the by-products of corrosion end up in the cooling water and system. In the evaporative cooling process, water lost through evaporation results in dissolved minerals precipitating out. These minerals settle inside cooling tower surfaces, heat exchanger
13、s, low flow areas, and areas that are alternatively wet/dry. These precipitated minerals have to be removed from the system to maintain peak system efficiency. Regardless of the source, particles are continuously introduced to the system and if they are not continuously removed they will adversely i
14、mpact energy efficiency, system reliability, operating costs, and reduce the effects of water treatment. Water treatment efficacy, critical in maintaining good water chemistry, is negatively affected by the presence of suspended solids. Ineffective water treatment can lead to biological growth. This
15、 further contributes to fouling of heat transfer surfaces, under-deposit corrosion, and contributes to the increasing risk of transmitting infections such as legionella. BENEFITS OF FILTRATION 1. Maintain design heat transfer efficiencies of chillers, heat exchangers, and cooling towers2. Improve wa
16、ter treatment efficacy3. Reduce risk of Legionnaires Disease and other safety concernsIncreased emphasis is placed on the cleanliness of cooling tower systems for the purpose of managing the risk of Legionellosis. The recently published ASHRAE STD 188-20154 Legionellosis: Risk Management for Buildin
17、g Water Systems calls for not only inspection of general system cleanliness, but also for a schedule of basin or remote sump cleaning. Dr. Barry Fields, Chief of Respiratory Disease Control for the U.S. Centers for Disease Control and Prevention confirmed that buildup of as little as 1/16th of an in
18、ch (1.59 mm) can provide a breeding habitat for bacteria in a cooling tower environment. (Jessup M.D., 2007) BASICS OF HVAC WATER FILTRATION HVAC water filtration is the process of separating suspended solids from water. This is accomplished by flowing water mixture (water with suspended solids) thr
19、ough a Barrier Filter or Non-Barrier Filter to separate water from solids. BARRIER FILTERS Barrier filters use a physical barrier like metal screen, paper, cloth, sand, or other porous material to separate solids from water. As the water mixture flows through a barrier filter solids larger than the
20、pores in the barrier are held back while water flows through. The separated solids are then manually or automatically removed. Barrier filters can remove all solids regardless of their floating property and some even have the ability to remove solids smaller than 5 microns. The disadvantage of barri
21、er filters is that they require maintenance, replacement filters (or media), and use large amounts of water during backwash. Once barrier filters reach their collection capacity, pressure requirements increase, and water flow is reduced. Equipment upstream and downstream of the barrier filter have b
22、e sized accordingly to accommodate for the fluctuation in flow and pressure, and backwash requirements. Also, automatic barrier filters (during backwash cycles) require backwash water holding tanks or larger drains as flows can overwhelm available piping. TYPES OF BARRIER FILTERS Sand Filters - Sand
23、 Filters are comprised of layers of sand and other suitable granular material. As the water mixture flows through the Sand filter, solids larger than the gaps between the sand granules are captured. Solids are then removed by flowing clean water in the reverse direction, also known as backwashing. A
24、utomatic Self-Cleaning Screen Filters - Automatic self-cleaning screens utilize a metal or plastic mesh to remove solids from water. Solids larger than the mesh openings are captured. These captured solids are either pulled off the screen or dislodged as a result of reverse water flow. Disc Filters
25、- Disc filters use plastic discs that are grooved on both sides. These discs are stacked together inside a housing. The water mixture is forced through the narrow groove openings between the discs. Solids larger than the gaps between the discs are captured. Solids are then removed by reversing the f
26、low of water. Cartridge/Bag Filters and Strainer Baskets - Cartridge filters use paper or cloth to remove solids from water. As the water mixture flows through the paper or fabric filter solids larger than the porous openings are captured. Captured solids and the filter are disposed and a new filter
27、 is installed. Strainer Baskets operate in a similar manner except that they are usually made of metal and can be easily cleaned. Both product types require operator intervention. NON-BARRIER FILTERS Centrifugal separators (non-barrier filters) achieve filtration through centrifugal action. Pressuri
28、zed water mixture enters the separator tangentially near the top. Centrifugal forces act on the water mixture as it rotates inside the separator. Suspended solids, that are heavier than water, are drawn outwards towards the wall of separator. As the water moves downward, separated solids fall to the
29、 bottom of the cylinder and are removed from the main flow. Clean water exits upward through the center of the separator. Collected solids are periodically purged from the separator. Due to the lack of a barrier inside centrifugal separators filtration efficiencies are dependent on internal water ve
30、locities and maintaining consistent acceleration. Centrifugal Separators have no internal moving parts, require very little maintenance, have a constant pressure loss, and do not require backwash holding tanks or upsizing the drains. The limitation of centrifugal separators is that they only remove
31、solids that normally settle within 3 minutes and heavier than water. TYPES OF CENTRIFUGAL FILTERS Unlike barrier filters, most centrifugal separators look very similar on the outside, therefore it is very difficult to tell them apart. However, internally they are built very differently and this has
32、a tremendous impact on filtration performance. Single Barrel Centrifugal Separators Separators are built with one internal barrel (sometimes none) inside the separator shell. On impact from tangential entry, the water mixture experiences centrifugal forces. However, this centrifugal force slows down
33、 due to lack of additional acceleration and a wide turning radius. This decrease in water velocity reduces solids removal capabilities. Also, separated solids are re-entrenched into the exit flow due to high turbulence (caused by slower velocities) near the bottom of the separator. Multi Barrel Cent
34、rifugal Separators These separators contain two internal barrels inside the separator shell. Multiple barrels increase acceleration through the use of slots and a smaller turning radius. This results in higher velocities throughout the separator - thus greatly improving separation efficiencies. A WO
35、RD ABOUT PARTICLE SIZE In most HVAC water systems smaller particles dominate the quantity counts as compared to larger particles (results from water samples tested by independent labs). This leads to general belief that since there are so many small particles they can create problems and need be rem
36、oved through filtration. However, on second look, these smaller particles, though large in quantity, have a very small footprint due to their miniscule size. Larger particles, on the other hand, though fewer in quantity counts, have a much larger footprint. Larger particles, due to their weight and
37、mass, settle on the bottom of basins, in finned condenser tubes, and in low-flow areas. To provide some perspective to micron and particle sizes: Red blood cells are 5 to 10 microns, human hair is approximately 70 microns, and typical sand is 100 microns (Jessup, 2007). FILTRATION APPLICATIONS Barri
38、er filters or a non-barrier filters are applied in the following methods: Full Flow Filtration, Side Stream Filtration, or Basin Cleaning Filtration. Full Stream Filtration This application calls for the installation of the filter at discharge of the system supply pump (from the tower basin or remot
39、e sump) prior to the heat exchangers/chillers. The filter is sized to match the full flow of the pump, filtering 100% of the water that passes on to the heat exchangers/chillers. The primary value of this approach is protection of the heat exchangers/chillers. It should be noted, this application do
40、es not address the solids accumulation in the cooling tower basin or remote sump. Side Stream Filtration A common industry practice is to divert 15-20% of the full-stream flow through the filter and back into the system flow between the system pump and the heat exchangers. For this application to be
41、 effective solids must be filtered at a rate greater than the anticipated input of solids. Less than 15% (as a percentage of full flow) is not recommended because solids are continuously introduced to the system. A lower percentage also means a high probability that most solids will pass the filtrat
42、ion system entirely. This application is commonly employed when Full Stream filtration or Basin Cleaning is cost prohibitive. Similar to Full Stream filtration, this application does not address the problem of solids accumulation in the tower basin or remote sump. Basin Cleaning Water is drawn from
43、the tower basin or remote sump to the filter package, filtered, and then returned back to the tower basin or remote sump. Return water is reintroduced via a pattern of specialized nozzles to direct any suspended dirt particles toward the filtration package pump intake. The size of the filter package
44、 is based on the square footage size of the cooling towers basin or remote sump. Unlike other applications listed above, this application directly addresses dirt accumulation in the basin (2000). SELECTING THE RIGHT FILTER AND APPLICATION When deciding on a filtration solution, a determination must
45、be made about the type and quantity of dirt present in the water. For new design applications, using ASHRAE recommendations (REF: 2012 ASHRAE Handbook HVAC Systems and Equipment. Page: 40.16) is a good measure. Selecting the wrong filtration for the application is similar to taking vitamins for pain
46、 instead of pain medication. Good product but wrong application. Equally important is to determine if the filtration solution will help maintain original design efficiencies and help reduce operating costs for the building owner/operator. In most cases a maintenance free Non-Barrier or Barrier Filte
47、r with a good water treatment program will achieve the desired goals. See Table 1 for a quick reference of non-barrier vs barrier filtration. Table 1: Filtration Type Comparison Table Filtration Type Filter Characteristics Pressure Fluctuations Flow Fluctuations Filtration Applications Maintenance S
48、pace Required Barrier Dirt larger than filter opening is captured from flow. Requires disposal through backwash or changing filter media Dependent on solids loading. Increases as solids buildup Flow reduces as solids buildup Side Stream, Closed Loop, and Basin Cleaning Maintenance on moving parts an
49、d replacement of bags, cartridges, or media. Medium to large footprint Non-Barrier Dirt heavier than filter is separated from flow and collected. Disposal is through purging. Steady pressure loss Steady flow Full Stream, Side Stream, Closed Loop, and Basin Cleaning Near zero maintenance Small to medium footprint CONCLUSION The benefits of filtration for water based HVAC systems are well documented and offer excellent payback. Either type of filtration (barrier or non-barrier) offers solutions to various issues resulting from the presence of suspended solids in cooling
