Evaporative condensers are commonly used for heat rejection in low temperature ammonia refrigeration applications in cold storage and food processing plants. Although often confused with standard cooling towers, there are some key differences between evaporative condensers and the cooling tower systems used in HVAC applications. Understanding these differences and the challenges they present is important for water treatment program success.
Evaporative Ammonia Condenser |
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Bridging Deposit Caused by Poor
Flow and/or Blocked Nozzles |
Design Differences
Both cooling towers and evaporative condensers reject heat by evaporative cooling. As fresh makeup water is added to replace evaporation losses, the dissolved solids in the recirculating water concentrate. If the dissolved solids are allowed to over-concentrate, scale deposits will form.
In cooling tower systems, the cooling water is circulated from the basin through extensive piping to heat exchange equipment and returned to the top of the tower to be cooled again. In an evaporative condenser, the cooling water is recirculated from the basin to the top of the unit to be sprayed over the condenser coil containing the refrigerant (e.g., ammonia).
Importance of Good Flow
In evaporative condensers, good flow through the spray nozzles that distribute cooling water over the densely packed condenser tube bundle is critical for deposit control. If flow is inconsistent, uneven, or blocked, dry spots and bridging deposits will occur that can dramatically reduce refrigeration efficiencies and increase operational costs.
It is the owner’s responsibility to routinely inspect the spray nozzles and tube bundle with the pumps on for signs of dry spots, blockages, or missing nozzles. More frequent inspections may be required in response to seasonal issues related to plants and bugs. Once bridging deposits form, condenser tube bundles are nearly impossible to clean.
If the supply water for the water treatment control loop is taken off the spray pump discharge, care must be taken to ensure that the reduced flow does not cause any dry spots on the condenser tubes. It may be necessary to install an inline circulation pump or an independent circulation line to maintain flow to the water treatment control loop. The goal is to ensure that whenever the system is operating, there will be flow across the spray nozzles and to the chemical feed controller.
System Water Volume
Evaporative condensers operate as both the heat exchanger and cooling tower, and they often contain a much smaller volume of water than HVAC cooling tower systems. The smaller water volume means that the dissolved solids concentration can change very quickly in an evaporative condenser, and better control is needed to prevent waterside problems due to over-concentration or loss of treatment. PTSA traced inhibitor feed is recommended to ensure consistent residuals. Bleed lockouts are generally avoided since low volume systems can quickly over-concentrate resulting in scale deposits.
Prefabricated Forced Common Sump |
Common Sumps
In plants with multiple evaporative condensers, a common sump design can greatly reduce the potential for upsets and improve control. In a true common sump,water from the individual evaporative condensers drain directly into a remote sump with either individual or common recirculating pumps feeding the spray nozzles in each unit. When the spray pumps are off, all the water drains into the common sump leaving the outside basins dry. True common sump systems are standard in cold weather climates to address freeze protection concerns.
Plants with multiple, standalone evaporative condensers can often be retrofitted with a forced common sump system to help simplify water treatment and improve control. In a forced common sump system, a sump tank with a dedicated recirculating pump is used to force the water in the individual evaporative condensers to mix. This increases the water volume and eliminates the need to install chemical feed and control equipment on each evaporative condenser. It is important that forced common sump systems be properly designed, sized, and balanced to provide consistent mixing. Pre-fabricated forced common sump systems are available.
White Rust
In contrast to HVAC cooling towers, evaporative condensers typically employ galvanized steel coils for heat transfer surfaces. Under the right conditions, the shiny zinc coating on new galvanized steel will form a non-porous, protective film. However, if the proper water chemistry in not maintained, especially during startup, a white, porous deposit called white rust can form that makes galvanized steel less corrosion resistant. White rust is an especially serious concern in evaporative condensers since condenser tube corrosion can lead to ammonia leaks and production losses
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New galvanized steel surfaces must be properly passivated to reduce the potential for white rust to form. This requires maintaining the water quality within tight parameters during the initial 8 to 12 weeks, which can be difficult to accomplish. Good pH control during passivation is the most critical factor. Where possible, the target pH range should be maintained by operating at reduced cycles rather than acid feed. Softened makeup should also be blended to maintain > 50 ppm of calcium hardness during passivation.
Regular visual inspections are important to detect signs of white rust. Re-passivation of galvanized surfaces may be necessary following disinfections, pH excursions, chemical cleanings, and startups.
Depending upon the makeup water source, manufacturer water quality guidelines for passivation may be difficult, or impossible, to consistently maintain. Extreme caution with additional control measures are necessary when using acid or acid-based inhibitors for pH control. A single acid overfeed event can cause permanent damage to the galvanized coating and significantly reduce equipment life. A well designed acid feed and control system with daily pH monitoring by site personnel is required at a minimum. A web enabled water treatment controller is highly recommended to provide remote monitoring and automatic alarms.
Conclusion
Evaporative condensers present a number of unique challenges when it comes to water treatment. Variations in makeup water quality and the impact of operating conditions beyond the control of the water treatment program can make white rust difficult to prevent. Chem-Aqua has a unique combination of expertise and specialized products to help you address these challenges. We have the ability to provide chemistry and equipment solutions for any application. Contact Chem-Aqua today to have a water treatment professional survey your facility and provide our expertise.
Written by: Tim Daniels