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- EXTRA EDITION
In a previous article on chilled water systems we focused on maintenance of the “heart” of the system, the chiller. (See “Keep Chillers Operating At Peak Efficiency.”) Larger buildings, process facilities, and other applications use chilled water cooling systems for its many advantages including refrigerant and maintenance containment, energy efficiency, and lower first cost.
While the chiller is typically the most expensive, most energy consuming, and most visible part of the system, other key components include chilled and condenser water pumps, cooling towers, heat exchangers, and hydronic specialties such as water pressure regulators, air separators, and chemical feed pots. Controls are also critical to the system because they operate water valves, control setpoints, and schedule equipment. All these components and capabilities support the delivery of cooling water to the airside or process loads and carry unwanted heat to sources of heat rejection. This article will focus on maintaining these components, “beyond the flange,” as found in most typical building comfort cooling applications.
Mechanical water chillers are available in many types and configurations typically categorized by compressor type — reciprocating, scroll, screw, and centrifugal. These types are further segmented by air-cooled or water-cooled designations. Most air-cooled chillers are “packaged” units which come as complete assemblies. Some air-cooled chillers are even available with integral pumping packages. Water-cooled chillers have condensers that are cooled by cooling towers or other sources to reject the heat. This article will focus on the water-cooled chiller system.
Service on the chiller itself has been covered previously in an article by this author. As we review maintenance issues for the larger chiller system, we need to recognize that other system component issues may in fact manifest themselves at the chiller. Yet we need to keep in mind that the true causes of these symptoms can be based elsewhere in the system. Real detective work may be necessary to identify the real cause somewhere in the ancillary components of the chilled water system.
Chilled water pumps carry the heated (54°F) water from the airside coils back to the chiller for re-cooling to 44°. Condenser water pumps deliver the 95° water rejected by the chiller to the cooling tower for cooling back down to 85°. The principle maintenance issues are a result of chilled water loops being typically a closed system, while condenser water loops are open. These differences are important maintenance factors we will discuss later.
Common centrifugal impeller pump types include end-suction and split case construction. Service considerations include pump and motor bearing lubrication and water seal cooling on larger pumps. Motor-pump shaft alignment is important and should be checked periodically as heavy piping and supports may shift over time. Providing positive suction pressure is important to prevent cavitation and air erosion. Pressure regulator stations maintain water loop pressure and air separators remove unwanted air from the chilled water.
Condenser water carries the unwanted heat load removed by the chiller and the chiller’s compressor work (heat of compression) to the cooling towers. These towers come in several common types: forced or induced draft and single and cross draft. Typically towers are constructed of steel, fiberglass reinforced plastic, wood, or concrete. Service requirements across all types of cooling towers are consistent. Fan motors, gear drives, fan belts, and water make-up float assemblies all require routine maintenance and inspection. Tower basins, fill and distribution pans all need periodic cleaning.
This is where the heat load is transferred to the chilled water loop via a chilled water coil. Coils are part of an air handler unit, which also contains air filters, fans, mixing boxes/dampers, and other air handling devices. Coils are commonly constructed of copper tubes and aluminum fins, requiring routine service such as air filter replacement and fin cleaning. Drain pans and lines also need to be cleaned of accumulated biological growth and dirt to maintain proper indoor air quality. Dirty coils can account for significant reductions in heat transfer and increased energy use since operators typically must lower chilled water temperatures to overcome the reduction in heat transfer caused by dirty coils.
Pressure water feed and relief stations should be checked periodically to ensure proper water loop pressure. Pressure that’s too low may prevent circulation to high level air handler coils or cause pump cavitation. Expansion tanks and air separators which require minimal attention should also be checked. Chemical feed pots are used to introduce chemicals or glycol to closed loops. Heat exchangers are used to isolate different loops and used in economizer systems. Larger heat exchangers are field cleanable, yet that can be a time-consuming task due to the complexity of the procedure.
Both loops require treatment for the prevention and control of corrosion, scale, and biological growth. Closed chilled water system loops are not exposed to the atmosphere, but still need inhibiters to control corrosion. Open cooling tower systems are more demanding. Cooling towers act like a large air washer that requires regular maintenance to combat corrosion problems. Many water treatment approaches are successfully used in systems today, including chemical, magnetic, and ozone types. Fouled water or scaled pipe inhibits heat transfer at the chiller and cooling coils. A miscue in water treatment can quickly cause major damage to the chiller’s tubes. Therefore, regular eddy current testing of tubes is critical combined with consistent, effective water treatment. Because cooling towers evaporate large amounts of water with some drift to the atmosphere, control of biological matter is an important health issue. Several antimicrobial growth products are available that will minimize biological growth in the cooling tower basin.
New digital-based controls are fairly low maintenance other then occasional software updates and control device calibration. Older pneumatic systems employ air compressor/driers which require specific routine service. Moisture in a pneumatic system can be detrimental to proper operation causing expensive clean-up costs. Dampers and water control valves also should be checked for operation and lubed where necessary. Controlling the chiller plant pump sequence, air handler scheduling, and exhaust fan operation can all impact chiller operation and performance. Chilled water temperature pull down rates need to be slow and steady. Fast temperature and/or flow changes can cause erratic and inefficient chiller operation. On variable flow systems, minimum flows should be confirmed.
We have covered only a few of the most important tasks. Like all equipment and components, the manufacturers’ operation and maintenance manuals should be consulted for specific service tasks and frequency. As illustrated, many components contribute directly to a properly operating and efficient chilled water system. Developing a service schedule plan and executing it will help minimize unscheduled and costly shutdowns, while protecting the investment in equipment.
Many factors impact the chiller and are not always immediately apparent without further investigation. Pumps not sequencing properly or low flow conditions may fault a chiller and not be understood until operation is restored. All system factors must then be thoroughly examined and confirmed.
There are many variations and types of chilled water systems. Understanding the one you have and how it operates will make identifying and implementing the right service procedures more effective, ensuring the full life of the equipment. Proper commissioning and establishing an energy baseline can also help in noting any service trends that require attention.