How to keep hvac condensing water systems free from contamination

June 1, 2000
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Mechanical refrigeration systems provide chilled water for building air conditioning and industrial process cooling applications. A common system includes a cooling tower, chiller, and compressor.

Keeping this equipment in good working order and operating at maximum efficiency through the use of chemical treatment and mechanical filtration is an on-going challenge to all operating engineers and service-maintenance staff.

Water treatment and sources of contamination

Water treatment is an extension of a preventive maintenance program. For optimum efficiency, it must control scale, corrosion, biological growth, and suspended solids.

These components are interrelated and to have an effective program, must be viewed in a holistic manner. The holistic approach, encompassing chemical treatment and mechanical filtration, will extend the life expectancy of operating equipment.

Scale, for example, results from precipitation of dissolved solids from water onto heat transfer surfaces. Chemical treatment proves to be most successful for handling this problem.

Suspended particles in the water, however, adsorb treatment chemicals and make it difficult to maintain proper residuals. Therefore, viewing the water treatment program as a process, suspended solids must be removed to maximize chemical efficiency.

Mechanical filtration is the most efficient and cost effective method of removing these solids. This idea will surface again as we discuss control of corrosion and biological growth.

Corrosion is caused by electrochemical reactions between system materials and the water. Products of corrosion are usually oxides of the system metals. Water treatment chemicals address corrosion problems.

Once again, the presence of suspended solids not only decreases chemical effectiveness, but also contributes to accelerated corrosion, via different mechanisms, as described below.

First, abrasion from the continual bombardment of metal surfaces by particles results in erosion.

Second, the particles may also scratch away the protective metal-oxide or other passivating surface films causing localized corrosion. Similarly, particles settling in low-flow areas may enhance the process of under- deposit corrosion.

In this situation, the particles act as a barrier between the metal surface and the corrosion-control chemicals applied to the system. The particles prevent the chemicals from reaching the metal surfaces and create corrosive microenvironments in the water, which are trapped in the pore spaces of the metal.

Biological problems occur from the growth of bacteria, algae, and fungi in the water system. Uncontrolled growth of these organisms results in microbiological slime or biofilm-biomass throughout the system, as well as health threats from biological pathogens (Legionella and others). Biological control is maintained by using broad-spectrum microbial biocides.

Remember that water treatment is a process. Suspended contaminants in the water provide surfaces for bacterial growth, as well as a source of dissolved nutrients.

In addition to enhancing growth, suspended solids adsorb treatment chemicals and decrease their effectiveness. Just as important, if biomass accumulation is not properly handled, system congestion and under-deposit corrosion may result.

Biological slime, reinforced by trapped particles, is the basic fouling material. Biocide use, coupled with the removal of fine particles by filtration, reduces fouling by controlling its basic components.

Suspended solids

The third and final component of water treatment is suspended solids. Suspended solids affect scale, corrosion, and biological growth, and can enter the system water stream from a number of sources.

In an open cooling tower, airborne debris is entrained in the airstream and can be both organic and inorganic. Organic debris enhances biofilm growth. Inorganic debris includes such contaminants as silt, earth, and dust particles.

In closed systems, suspended solids are typically corrosion products. By definition, a closed system does not employ evaporation and does not use more than 5% of its volume in makeup in a year’s time. However, in actual practice, water losses at pump seals, packing, expansion tanks, valves, and pipe connections are common.

For example, if there is leakage of 1 gal/hr (approximately 30 drops/min), there will be enough dissolved oxygen in the makeup water to form a sufficient amount of rust to clog 14 ft of a 1-in. pipe.

Dissolved oxygen also can enter the closed system during shutdowns and due to temperature changes, which further exacerbate corrosion.

In summary, both chemical treatment and mechanical filtration are needed to address hvac system problems. While there is a great deal of material written about chemical treatment, there is little written about the impact of suspended solids on hvac systems.

Hvac problems and their causes

Scale, corrosion, biofilm, and suspended solids are interactive with each other and can result in serious operational problems for hvac systems. These are briefly summarized below:
  • Decrease in heat transfer efficiency, resulting in wasted energy and higher operating costs; as solids build up on heat exchange surfaces, they produce a fouling layer resulting in a measurable foul factor. (Foul factors are further discussed in the next section.) For a build-up of solids equal to the thickness of a sheet of paper, the resulting energy penalty is more than 20%.

  • Decreased effectiveness of the chemical treatment program as the solids form a barrier to the metal surface as well as adsorb chemicals;

  • Damage to pumps, seals, piping, spray nozzles, and other equipment;

  • Pitting and clogging of chiller tubes;

  • Under-deposit corrosion; and

  • Nutrients and particle surfaces for bacterial growth.


Foul factors and energy efficiency

Condenser and chiller tube fouling due to suspended solids in a circulating water stream results in higher energy costs and increased maintenance costs. Suspended solids impact heat transfer coefficients and power consumption.

The foul factor of 0.00025 sq ft/hr/°F/Btu is used to define the extra resistance to heat transfer caused by solids on heat transfer surfaces. This resistance can result in up to a 60% loss of heat transfer efficiency from a fouling layer of 0.0625 in.

Looking at it another way, based upon information from The Trane Company, for every 0.00001-sq ft/hr/°F/Btu increase in the fouling factor, there is a 1.5% to 2% increase in approach temperature.

For example, if the design approach is 5°F with a fouling factor of 0.00025 sq ft/hr/°F/Btu, then the approach temperature would change to 7.5° with a fouling factor of 0.00050 sq ft/hr/°F/Btu. This change in approach temperature would translate into about a 4% increase in power consumption (1.5%/°F).

Finally, in the Carrier Corp. Heat Exchanger Physical Data and Selection Guide, fouling factors of 0.003 to 0.006 sq ft/hr/°F/Btu are shown for untreated cooling water. Using these foul factors in Carrier’s System Design Manual, there are significant increases in the required hp/ton and the aerodynamic lift with centrifugal compressors. This translates into higher energy and operating costs for the end-user.

Loss of heat transfer efficiency and increase in power consumption will raise energy costs and reduce productivity. Removing the suspended solids from the water provides immediate benefits in terms of building air conditioning and process cooling.

Filter selection

The selection of the correct solid-liquid separation device depends upon the size and type of solids to be removed. The important word is “size” of the solids.

Particle sizes are measured in microns. A micron (µm) is 10-6 meters, which is 4 x 10-5 in. A grain of table salt is 100 microns across; a strand of hair is about 40 microns thick. The smallest particle you can see with the naked eye is approximately 5 microns.

Figure 1 illustrates why particle size is important, not their weight. Both 1-liter barrels contain 12.6 grams of solids. One 1/2-in. marble weighs 12.6 grams — so does 256 billion 2-µm particles.

The latter, with the much greater surface area and number of particles, would result in severe operational and fouling problems; the one marble would have no impact at all. The larger surface area of the small particles results in an increase in chemical adsorption and decrease of chemical effectiveness. Furthermore, more particles create more sites and nutrients for bacterial growth.

Finally, the large number of small particles creates a layer of dirt, which increases fouling and the foul factor, as previously described. Therefore, when evaluating and selecting filtration equipment, removal by particle size is the correct specification.

What size particles are in the system? In closed systems, most of the contaminants are corrosion products. These particles are less than 10 µm and typically are in the 1- to 5-µm range. Hence, the filtration equipment must be able to remove this size of particles.

In open systems, samples of water in the cooling tower basins have been analyzed and the reports show that the largest quantity of contaminants are between 1 and 10 µm. Figure 2 shows typical particle size distribution (PSD). This PSD is further supported by filtration theory.

Cooling towers scrub the particles from an airstream. Large particles (greater than 25 µm) settle out before they reach the tower by diffusional interception. Small particles (less than 1 µm) settle out of an airstream by inertial impaction. The particles in the 1- to 10-µm range stay with the airstream, make their way into the tower, then finally into the tower basin, where they must be removed.

Filtration choices

Many types of particle-removal systems have been used in the hvac industry, with varying success. These include centrifugal separators, screens, bag-and-cartridge filters, and permanent media filters.

Selection must be based on the system’s ability to remove the specific size of suspended solids.

Centrifugal separators work on the principle of separation by weight. The liquids enter tangentially into the system and centrifugal forces separate the particles that are significantly more dense than water. Removal efficiency is, therefore, related to the specific gravity of the solids (the more dense the solids, the more efficient the separation).

For solids with a specific gravity of 2.6 (i.e., gravel, sand, and silica), removal efficiency is 98% by weight of particles greater than 74 µm. For solids with lower specific gravity (such as earth, silt, and soil), the removal efficiency is lower. As most of the particles are below 10 µm in cooling tower and closed-loop water systems, centrifugal separators are not recommended.

Screens and strainers are typically made of stainless steel and come is a variety of configurations and removal efficiencies. They generally have limited surface areas and may blind off prematurely if low-micron mesh is used.

The stated removal efficiency is most often in the 75- to 100-µm range. While these systems remove particles by size, they are not efficient in the micron range for cooling tower and hvac applications.

Bag-and-cartridge systems remove particles by size and are efficient in the 1- to 10-micron range. In this respect, they are well suited for cooling tower and hvac applications. However, they have limited dirt-holding capacity. Therefore, on large and dirty systems, the operation and maintenance costs are very high. For small systems, however, they are a cost-effective alternative.

In most cooling tower and hvac applications, permanent media filters are the best alternative. Media filters remove particles by size and are depth filters such that removal efficiencies are in the 1- to 5-micron range and smaller.

These high removal efficiencies are due to the use of specialized “round” media at a specifically calculated operating flux rate. Media filters are self-cleaning and therefore require little operator attention.

Sizing, installation of fine media filters

Sidestream filtration extracts a portion of the total system volume for filtration on a continuous basis. After several circulation cycles, the total system volume has passed through the sidestream filter.

Field studies have shown that filtration of the system water once per hour will provide for high-quality water in an open system. For a closed system, because the filter is generally only removing system-generated corrosion products, a total volume turnover of once every 4 to 6 hrs is all that is required.

In an open system, there are two ways to install a filter: in the basin-sump of the cooling tower, or on a tap off of the water flow line. While both methods are acceptable, the preferred method is a sidestream from the basin. This produces no disruption to the process water flow and, more importantly, results in cleaning of the basin.

The water from the basin is pumped through the filter and the clean water is supplied back through PSJ sweeper jets in such a way as to sweep the dirt across the basin-sump floor and back to the suction side of the filter. Figure 3 shows a typical installation photograph.

For a closed system, filter installation is shown in Figure 4. The inlet to the filter is from the high-pressure side of the pump, while the clean water is returned to the low-pressure side of the system. Figure 5 shows the closed-loop media filter with double actuators to isolate the filter from the closed loop during backwash.

In some cases, it may be necessary to use a portable sand filter (PSF) in order to clean up a closed-loop system, as shown in Figure 6. One typical application is at a facility with multiple systems, where a continuous preventive maintenance program is employed.

Here, the engineers tie the PSF into each closed-loop system for a period of one to three months. The water in the closed loop is sampled periodically, and the PSF is moved when the dirt load in the system is reduced by greater than 95%.

The PSF has its own pump and controls and is easily connected, as shown in Figure 4. It is an economical approach for a large facility where the PSF can be moved from building to building.

The portable PSF filter is also used as a temporary filter for cleaning up a new closed-loop system prior to tying it into the main loop. This is an important application so as not to contaminate an existing system with additional solids. The PSF is available on a rental basis, which makes it cost-effective for mechanical contractors.

Payback analysis

The installation of a fine media filter has many benefits in both open- and closed-loop applications. The benefits are energy savings, maintenance savings, chemical savings, and equipment and/or operating improvements.

Figure 7 shows before-and-after water reports conducted by an independent laboratory. This manufacturing company realized energy savings of more than $22,000 from the installation of a media filter.

Clean water protected the heat exchange equipment from fouling with solids. The filter reduced the dirt load from 97 to 1.6 lb of dirt in the tower within four weeks, and reduced the foul factor from an initial 0.0007 down to lower than design levels.

A similar result was realized at a cogeneration facility. Here, the media filter reduced the particle loading by 94% in the tower and produced a ten-fold reduction in the foul factor — to 0.0001 — for energy savings of $8,900 per year. This foul factor is less than the current design fouling factor from ARI of 0.00025.

In another example, the Birmingham Airport Authority realized the following benefits with the installation of filters for their hvac systems for the airport terminals:

  • Tower cleaning reduced by 90%;

  • Basin cleaning time reduced to 1.5 workhours;

  • Chiller efficiency improved;

  • Chiller cleaning reduced to every two years;

  • Biocide usage decreased;

  • Conductivity setpoint increased seven-fold; and

  • Setpoint increase resulted in less blowdown, less chemical usage, and greater water savings.

The payback for this application was approximately one year.

Conclusion

Water treatment is a continuing process, not an event. Many variables must be taken into account to have an effective program.

The combination of the correct chemicals and the correct mechanical filtration system will result in an optimized open- or closed-loop system.

The operating benefits include improved heat transfer and energy efficiency, maximized efficiency of the towers and chillers, simplified and reduced-cost chemical treatment program and overall enhanced system cleanliness.

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