Maintenance and Efficiency of Evap Condensers

November 8, 2000
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OCEAN CITY, MD — All too often, the evaporative condenser is treated like “the forgotten step-system,” according to Mike Silverstein, advanced marketing engineer of evaporative condenser products for Baltimore Aircoil Co. (BAC), Baltimore, MD.

For many owners, the evaporative condenser is just some part of the system “somewhere up on the roof,” Silverstein said. But in order for the entire refrigeration system not to suffer in terms of performance and efficiency, the evap condenser must be maintained for reliability, with as little unscheduled downtime as possible.

The most critical concern, and an area which Silverstein focused on at the 91st annual convention of the Refrigerating Engineers and Technicians Association (RETA) here, is performing maintenance to avoid high head pressure.

High head pressure can lead to:

  • Less product out the door;
  • Less throughput; and/or
  • Harder work for the rest of the system, including the compressor.
  • Silversteinsaid that refrigeration service technicians should focus on four main areas of evaporative condenser maintenance:

    1.Routine maintenance;

    2.Water treatment;

    3.Purging; and

    4.Piping.



    Regular Maintenance

    Silverstein said that maintenance should be performed according to a regular schedule (see Table 1) on the following sections of the evaporative condenser.

    The water system includes:

  • Strainers, some of which are easy to open while others need to be reached through an access door, so consult your manufacturer;
  • Makeup and bleed valves; and
  • Nozzles, which should be checked monthly for spray patterns and distribution; make sure they’re not clogged and that the water is flowing evenly.
  • The drive system (mechanicals) includes:

  • Bearings (ball bearings should be lubricated every 2,000 hrs or six months with water-resistant inhibited grease);
  • Belts (check the tension once a year, perhaps at the seasonal system startup); listen for “chirps” and “squeals” when the fan motor starts, Silverstein said; if you hear them, they could be an early warning sign of premature belt deterioration (see Figure 1); and
  • Sheaves. (Figure 2 shows the four points of contact for proper sheave alignment.)
  • A session participant offered this advice on bearing lubrication: “Don’t get too aggressive with your grease gun when you’re purging out the old grease. As soon as you see old grease come out, stop or you could break the seal.”

    The evaporative condenser, as its name implies, evaporates water in order for heat transfer to occur. However, this process leaves behind impurities.



    Water Treatment; Let It Bleed

    “You must bleed the system occasionally so not all impurities are left behind,” said Silverstein.

    In cases where performing a regular water bleed (blowdown) isn’t enough to maintain an acceptable level of water quality, technicians need to look at chemical treatments such as rust and scale inhibitors, passivation of zinc, and control of biological contamination.

    Calcium carbonate (CaCO3) scale may be the most common water-related problem a tech encounters. This hard, granular, white coating forms when solids fall out of solution. It is especially troublesome in hard-water areas, and tends to form in the middle of the coil first, Silverstein said.

    Calcium carbonate scale not only robs system performance (see Table 2), it also reduces overall system life. The best way to deal with it, said Silverstein, is to prevent its accumulation in the first place.

    Water that’s too hard is commonly talked about. However, were you aware that water that’s too soft can also lead to trouble?

    It’s true, said Silverstein and other techs in his session. Don’t rely absolutely on the boiler water treatment to take care of the evap condenser. “Boiler treatment people don’t understand the condenser,” said one tech. “They push for a softener.” But if calcium carbonate hardness falls below 30 ppm, this may create a leaching condition for typical galvanized steel (see Table 3).



    Scale Prevention

    Water quality varies across the country. Silverstein pointed out that it’s imperative to know the quality of the water you’re dealing with, in order to know how to treat it within your refrigeration system.

    To prevent scale:

  • Ensure proper water coverage; wetting and drying help form scale.

  • Avoid dry spots.

  • Avoid pump cycling; some technicians use this for capacity control, but it aids scale formation by promoting wetting-drying.

  • Use a colder spray water temperature; it keeps solids in solution better than warmer water.

  • Use an appropriate water treatment program.

    To chemically remove scale:

  • Clean the interior of the unit.

  • Add a 10% inhibited sulphamic acid solution; do not use sulphuric acid on galvanized steel, Silverstein warned.

  • Initially, the pH level will equal 1. Circulate condenser water 1 to 2 hrs until the pH level reaches 7, which is neutral.

  • Drain the evaporative condenser immediately; clean and inspect the unit.

    Silverstein pointed out that “Continuously low pH will strip zinc, so don’t recirculate the water too long.”



    White Rust, Biologicals

    White rust, a waxy, white coating that you can wipe away (one participant said it looks like “white tips of a feather”), occurs in higher pH water (greater than 8.2 pH). Zinc “sacrifices” itself to white rust, but does not create a protective barrier. In fact, white rust can quickly strip away a zinc coating.

    To prevent this, Silverstein advised passivating the system periodically; that is, bringing the pH level below 8 for a couple of hours. “Try to do it on startup,” he said.

    Biological contamination, including algae, slime, bacteria, and under-deposit corrosion, has more serious implications because of Legionella, which has the potential to make people ill and may even lead to death.

    Some end-users need to be aware of the fact that their systems have the potential to breed and feed Legionella, which feeds on the biofilm on the bottom of the tower, pointed out an attendee.

    In addition to using the correct biocide, systems need to be tested regularly for the presence or, preferably, the absence of Legion-ella. “The only way to know is to get tested,” he said. Then, keep the paperwork on file. ASHRAE and Cooling Tower Institute websites (www.ashrae.org and www.cti. org) have additional information.

    Other sources of contamination include airborne stack gases, wind-blown debris, and ammonia leaks, which strip zinc.

    “Evaporative coolers are excellent air scrubbers,” pointed out Silverstein. “You have to be careful of what’s being pulled in. It can clog the strainers more quickly.”



    Purging Noncondensibles

    Noncondensibles such as air can leak into the system low side if operation is at a pressure below atmospheric, Silverstein said.

    They can also enter due to:

  • Poor evacuation of a new system prior to charging at startup;
  • Failure to evacuate completely after part of a system has been open for repair; and
  • The chemical breakdown of oil and/or refrigerant.
  • Noncondensibles in the system can raise head pressure, waste energy (and money), and reduce system capacity. Air itself acts as an insulator (see Figure 3). One solution is to purge noncondensibles from the system.

    Purge points should be located where the air will collect, said Silverstein: the lowest temperature areas, and the lowest velocity areas. Techs need to make sure the liquid is not drawn into the purger, and need to use traps to create a liquid seal to gather air at the purge point.

    Silverstein said that purging can be done in one of two ways:

    1. Start the purge while the system is down. All the air will rise to the top, so purge from the top (see Figure 4 for proper top and bottom purge locations).

    2. Purge while the system is in operation (auto purge); in this case, purge lower, where temperature and pressure are low. Make sure when you’re doing an auto purge, that you only purge from one point at a time.

    As for piping, concentrate your maintenance efforts on the drop legs, traps, equalizers, or alternate “sewer flow.” “It’s important to use drop legs,” said Silverstein. “And they must be sized correctly. For ammonia, you need a minimum 3-foot drop leg. For R-22, use a minimum 8-foot drop leg.”

    With regular maintenance, an evaporative condenser can have a long, healthy, efficient, and productive life.



    Sidebar: Troubleshooting Tips

    Symptom: Low airflow What to check:

  • Belts slipping?
  • Low fan motor voltage?
  • Obstructions in the air inlet or discharge?
  • Direction of fan rotation (miswired)?
  • Scale build-up on condensing coil?
  • Symptom: Low water flow What to check:
  • Clogged strainer?
  • Clogged nozzles?
  • Direction of pump rotation (miswired)?
  • Worn impeller?
  • Publication date: 11/13/2000

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