Chillers Performance: Challenges and Solutions
As efficiency demands mount, refrigerant management and preventive maintenance take center stage
Chillers are among the most complex and energy-intensive pieces of equipment in institutional and commercial facilities. Because of their complexity, they offer maintenance and engineering managers big opportunities to improve energy efficiency, minimize repairs, and increase reliability.
To take advantage of these opportunities, managers often focus on two key areas — refrigerant management and preventive maintenance. Using a strategic approach in these activities, managers can deliver benefits to facilities in terms of reliability and energy efficiency.
Strategic refrigerant management is one of the most effective tools managers have to improve chiller efficiency. Unfortunately, most managers think of refrigerant management in terms of regulatory compliance, refrigerant inventories, and chiller-refrigerant conversions.
In fact, refrigerant management is a time-tested strategy that ensures equipment operates at its maximum efficiency. During operation, chillers develop small leaks in their sealed refrigerant systems. As the system loses refrigerant, the unit’s operating efficiency declines.
Technicians add the required amount and enter it in the chiller log when they notice the loss, but as long as the loss is below the action level required by regulations, they don’t take additional action, and the leak continues.
Technicians need to closely monitor the addition of refrigerant to any chiller to improve its efficiency. For their part, managers need to review their operation regularly to detect changes indicating leaks that inhibit performance.
The most effective method of finding refrigerant leaks is with a refrigerant-management program that tracks refrigerant use by each refrigerant-using system. Older chillers can lose refrigerant at 2-15 percent annually. Newer chillers equipped with specially designed pressure vessels and high-performance seals can cut the loss rate to 0.1 percent per year.
To further enhance efficiency, managers have several options. They can establish a refrigerant-use log for each piece of refrigerant-using equipment.
They also can establish a baseline of refrigerant use for that equipment and compare it to manufacturer specifications. If a piece of equipment uses more refrigerant than the manufacturer suggests, technicians can use a hand-held detector to find the leak.
Finally, managers can instruct technicians to track refrigerant use to identify leaks as they occur. Timely detection and correction will reduce investments in replacement refrigerants, and it will keep chillers operating at the highest possible level of efficiency.
Advances in Efficiency
Contaminants in a chiller’s refrigerant circuit can reduce its operating efficiency. Low-pressure chillers are more susceptible to contaminants because any leak allows air and moisture to enter the system.
To minimize the effects of contaminants, manufacturers initially added purge systems that separate air and moisture from refrigerants. The systems worked but weren’t very efficient. They tended not to remove all of the contaminants, and their use resulted in lost refrigerant.
Newer, high-efficiency purge systems are both efficient and effective. Besides removing most air and moisture from refrigerant, they operate without allowing refrigerant to escape. As a result, refrigerants are practically free of contaminants.
High-efficiency purge systems are available on new chillers, or managers can specify them for existing chillers. In both cases, technicians should regularly monitor the unit’s run time because any changes in operation can indicate a developing leak.
Another feature that has helped chillers’ energy efficiency is improved heat-transfer surfaces. To perform as designed, these surfaces must be clean and free of buildup, such as sludge, corrosion and scale. Even a thin layer on the surface can mean decreased efficiency. How quickly they become fouled depend on the system’s water quality, the chiller’s operating temperature, and the condition of condenser and evaporator loops.
Technicians should clean condenser tubes annually to keep these surfaces clean. Evaporator tubes need to be cleaned only every three years or so because they operate in a closed system. While technicians often can use brushes for the cleaning, badly fouled systems might call for the use of chemicals.
Eye on Maintenance
Operating logs are one of the most important, yet often overlooked, tools in chiller maintenance. The log tracks data related to equipment operation. Technicians and operators use the log’s data to evaluate performance.
By reviewing the data, they can spot slowly developing performance trends that might go unnoticed in daily operations. Technicians also can use log data to diagnose specific chiller problems. To be most effective, though, operators must enter data at least once daily, and someone must review the data regularly.
Information in the log varies by chiller type. At a minimum, information should include time, date, drive-motor currents and voltage, supply and return temperatures of condenser water, supply and return temperatures of chilled water, evaporator and condenser pressure, and oil temperature and pressure. Technicians also need to record any maintenance activities they perform.
Scheduled inspections also are essential for proper maintenance. Depending on the components technicians inspect, the procedures typically can take place daily, weekly, monthly, annually or biannually. Some facilities — particularly those with multiple, large centrifugal chillers — schedule inspections once per shift. Items in the inspection also vary by chiller type.
Generally, inspections require only a few minutes to complete and don’t interfere with operation. But in other cases, such as tube inspection, the chiller must be shut down and partially disassembled to conduct the inspection.
In both cases, inspections are designed to maintain efficiency and identify minor problems so technicians can correct them before they become more costly and disruptive.
Chillers present a challenge in determining operating efficiency. Manufacturers rate efficiency at full load, even though few chillers regularly operate at full load. Part-load efficiencies are lower and vary based on several parameters, such as supply and return chilled-water temperatures, entering condenser-water temperature, and condenser and chilled-water flow rates.
Adding to the challenge is the level of precision required to measure each parameter to calculate efficiency accurately. An error of as little as 1 degree in water-temperature measurements can cause an error in the efficiency calculation of 1-2 percent. So it is essential to install high-quality flow meters and temperature sensors and maintain them properly.
Any program to monitor efficiency must be ongoing to be effective. As technicians amass data under different operating conditions, managers will be able to develop an efficiency-performance baseline for the chiller. Technicians then can compare this baseline to the published performance curve to evaluate whether the chiller is performing as efficiently as possible.
As data continues to pile up, technicians will be able to monitor performance trends. Managers can expect a slight drop in performance because of normal wear and tear, but they should look for trends indicating a need for maintenance, such as cleaning the chiller’s tubes, replacing refrigerant, or overhauling the chiller.
Fortunately, new monitoring equipment can help managers monitor chiller performance. This equipment collects the necessary data and automatically calculates chiller efficiency.
The equipment then compares the operating parameters and the calculated efficiency to past performance values, and it sends an alert if a value falls outside the expected range. By continuously monitoring chiller operation, the systems help managers ensure chillers continue to operate at peak efficiency.
James Piper is a national facilities management consultant based in Bowie, Md. He has more than 25 years of experience in facilities management and maintenance issues.
The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) is the trade association representing manufacturers of air conditioning, heating and commercial refrigeration equipment. AHRI is an internationally recognized advocate for the industry and develops standards for and certifies the performance of many of these products.
AHRI’s 370 member companies account for more than 90 percent of the residential and commercial air conditioning, space heating, water heating and commercial refrigeration equipment manufactured and sold in North America.
For more information, visit www.ahrinet.org