5 steps to Improve the Efficiency of Chillers
Preventive maintenance, including daily work logs, clean pipes and properly treated water, can extend equipment life and improve energy efficiency
water chiller is a huge capital investment and a major contributor to the operating costs of institutions and commercial facilities. For many organizations, chillers are the largest single energy user, and comprehensive maintenance is essential to ensure their reliability and efficient operation.
Although some organizations use predictive maintenance (including vibration analysis, infrared thermal imaging, and rotor rod testing) to diagnose problems in advance, a comprehensive preventive maintenance (PM) program is still the key to ensuring optimal cooler performance and efficiency.
Thanks to advances in control, refrigerant and equipment design, the efficiency of the cooler has steadily improved in the past decade. As a result, chillers now have tighter operating tolerances, and regular maintenance and maintenance are more important than ever. When developing a PM plan for refrigeration equipment, maintenance and engineering managers should consider five basic areas.
Step 1: Maintain daily operation log
The chiller operator should record the chiller performance with accurate and detailed logs every day, and compare this performance with the design and start-up data to detect problems or inefficient control set points. This process allows operators to summarize historical records of operating conditions, review and analyze them to determine trends, and provide advanced warnings of potential problems.
For example, if machine operators notice a gradual increase in the condensing pressure within a month, they can consult the daily operation log and systematically check and correct the possible causes of this situation, such as condenser tubes or non-condensate fouling.
The chiller manufacturer can provide a list of equipment-specific recommended data points upon request. The operator can take data readings every day, approximately once every shift. Today's chillers are controlled by microprocessor control, so managers can use microprocessor-controlled building automation systems to automate this process.
Step 2: Keep the test tube clean
One potential obstacle to the required refrigerator performance is heat transfer efficiency. The performance and efficiency of the chiller are directly related to its heat transfer capacity, which starts with clean evaporator and condenser tubes. There are miles of pipes in the heat exchanger of a large cooler, so keeping the large surface clean is essential to maintaining efficient performance.
When dirt, algae, sludge, scale or contaminants accumulate on the water side of the heat transfer surface, the efficiency of the cooling pipe will deteriorate as the pipe becomes fouled. The fouling rate depends on the type of system (open or closed) as well as water quality, cleanliness and temperature.
Most chiller manufacturers recommend cleaning the condenser tube every year because they are usually part of an open system. For a closed system, they recommend cleaning the evaporator tube every three years. However, if the evaporator is part of an open system, it is recommended that you perform regular inspection and cleaning.
Managers can consider two main methods of cleaning the pipeline:
· Mechanical cleaning can remove the mud, algae, sludge and loose materials in the smooth hole pipe, including removing the tank cover, brushing the pipe and rinsing it with clean water. For internally reinforced pipes, managers should consult the chiller manufacturer for mechanical cleaning recommendations.
·Chemical cleaning to remove scale. Most chiller manufacturers recommend consulting your local water treatment supplier to determine the appropriate chemical solution required. Thorough mechanical cleaning should always be performed after chemical cleaning.
The new type of chiller has an automatic pipe brushing system, which can be retrofitted on the existing chiller. These systems use small nylon bristles that flow through the tubes for cleaning. A custom-made four-way reversing valve is installed in the condenser water supply system. Every six hours, the system will automatically reverse the flow through the condenser tube for about 30 seconds.
Combined with proper water treatment, these systems actually eliminate fouling in the cooler and keep the temperature close to the design. These systems usually show a payback period of less than two years.
Step 3: Ensure that the device is leak-free
The manufacturer recommends that the compressor be checked for leakage every quarter. The refrigeration system part of the low-pressure cooler using the obsolete CFC-11 or HCFC-123 operates at sub-atmospheric pressure. Although these chillers are the most common chillers in today's facilities, it is difficult to make a completely sealed machine, and leaks can cause air and moisture (often referred to as non-condensable water) to enter the equipment.
After entering the cooler, the non-condensate is trapped in the condenser, which increases the condensing pressure and compressor power requirements, and reduces the efficiency and overall cooling capacity. The low-pressure chiller has an efficient purge device that can remove non-condensable gas to maintain the design condensation pressure and promote efficient operation. A chiller manufacturer estimates that 1 psi of air in the condenser equals a 3% chiller efficiency loss.
The moisture in the cooler will also produce acid, which will corrode the windings and bearings of the motor, and cause rust inside the housing. Small rust particles called fine powder float in the container and are trapped in the heat exchanger tubes. The fine powder on the tube will reduce the heat transfer efficiency and overall efficiency of the equipment. If left unchecked, they can lead to expensive pipe repairs.
The best way to monitor leakage in the low-pressure cooler is to track the operating time of the purge unit and the amount of moisture accumulated in the purge unit. If any of these numbers is too high, it indicates that the equipment is leaking. Other indications of air in the system include increased head pressure and condensation temperature.
High pressure coolers using CFC-12, HFC-134a or HCFC-22 operate at pressures much higher than atmospheric pressure. Leaks in these types of coolers release potentially dangerous refrigerants into the environment. Environmental regulations limit the annual refrigerant leakage.
Leakage also leads to a decrease in refrigerant charge and other operational problems, such as a decrease in evaporator pressure, which may cause the compressor to work harder, resulting in lower cooling capacity. For positive pressure coolers, the technician should monitor the refrigerant charge and evaporator pressure to detect leaks.
Step 4: Maintain proper water treatment
Most coolers use water for heat transfer, so the water must be properly treated to prevent scale, corrosion and biological growth. The closed water system requires a one-time chemical treatment, which is a typical feature of the cooling water system connected to the cooler evaporator.
Open system is usually used for condenser-water system connected to cooler condenser. Condenser systems that use water sources such as cooling towers require continuous chemical water treatment. Managers should cooperate with chemical processing suppliers who are familiar with the local water supply, and can provide all-round maintenance for the water supply system of all facilities.
If the supplier conducts appropriate chemical treatment on the evaporator and condenser-water system, scaling will not be a problem. Scale in the condenser or evaporator tube indicates improper water treatment. The supplier needs to test the water quality and correct the water treatment program every three months, which should help clean the cooler tube.
In addition, all system filters should be cleaned every three months. If properly maintained, sand filters and side-flow filters used in condensate systems are very effective in maintaining clean water. In order to determine when cleaning is required, the technician should monitor the pressure drop on the filter and refer to the manufacturer's recommendations for cleaning. The filter should be cleaned every quarter, regardless of the pressure drop.
The filter and the maintenance of the filter limit the corrosion of the cooling pipe caused by the high-speed movement of sand or other small particles. Corrosion and pipe pitting will reduce overall heat transfer efficiency and reduce efficiency. If left uncorrected, these conditions can lead to pipeline blockages or catastrophic pipeline failures.
The technician should check the cooling water and condenser water piping system for signs of corrosion and erosion every year. Most manufacturers recommend eddy current inspections of heat exchange tubes every five years, including the use of electromagnetic procedures to evaluate tube wall thickness.
Step 5: Analyze oil and refrigerant
The annual chemical analysis of oil and refrigerant can help detect them before serious chiller contamination problems. Testing includes spectrochemical analysis to identify contaminants that can affect performance and efficiency, including moisture, acids, and metals. The analysis must be performed by a qualified chemical laboratory specializing in HVAC equipment. Most manufacturers provide annual oil and refrigerant analysis services.
During the operation of the cooler, the technician should take oil samples. The oil should only be changed when the oil analysis indicates that there is oil. The technician should also monitor the pressure drop of the oil filter and replace it during the recommended oil change or if the pressure drop is out of tolerance.
Oil analysis can help detect other cooler problems. For example, high moisture content in the oil may indicate a problem with the purge unit, and changes in the oil characteristics may indicate unacceptable compressor wear.
Managers use refrigerant testing to identify contaminants that can cause reliability and efficiency issues. One major pollutant is oil that migrates into the refrigerant. A refrigerator manufacturer estimates that for every 1% of oil found in the refrigerant, the efficiency of the refrigerator decreases by 2%. It is not uncommon to find 10% of oil in the refrigerant of older refrigerators. According to this estimate, this pollution will result in a significant drop in efficiency of 20%. The most important thing is-testing can bring great benefits.