Chillers often represent a plant’s biggest electric load. But when you factor in leaking refrigerant, fouled tubes and a dozen other factors, the operating costs can increase by as much as 10%. A modern high-efficiency centrifugal chiller is designed to minimize power consumption but even so, the major components (tubes, refrigerant, condenser, compressor, starting equipment, etc.) require regular maintenance to ensure maximum output.
Although a large number of industrial units use predictive maintenance to diagnose common problems (e.g., rotor bar testing, vibration analysis, infrared thermography, etc.), the best way to promote performance and efficiency is to establish a comprehensive maintenance plan.
Chillers have improved over the last decade thanks to major advances in controls, equipment and refrigerants; and because of this, new models come with tighter operational tolerance. However, some components within the unit are more susceptible to damage.
These are the more delicate components:
· Controls- they need regular inspection and calibration;
· Tubes- they should be cleaned regularly and repaired immediately in case of damage;
· Oil- the oil should be changed regularly, and a schedule should be made for annual oil inspection.
The most commonly overlooked aspect of chiller maintenance is taking daily or weekly readings. If the system appears to be operational and with no apparent defect, the staff are likely to ignore the readings. But this doesn’t allow them to watch for trends or catch problems before they lead to equipment damage or unexpected down time.
Remote monitoring has brought about a few changes in the way organizations plan a maintenance schedule, and more users are moving away from a pre-determined maintenance plan. So how often do you need to service your chiller system? Although annual inspection is considered the standard for many chiller systems, it cannot guarantee good performance or prevent unit damage the way a comprehensive, segmented service plan can.
A number of factors come in to play when designing the best maintenance plan for specific chiller systems, but there are proven methods to prevent incidents of breakage or high power consumption- and maximize performance.
Whichever type of system your building uses, the following should be observed when performing maintenance for a chiller:
· Set up a daily operating log. The operators should document daily performance on a detailed log and compare this information with design and startup data to check for problems. A daily log allows the operator to record a history of the system’s operating conditions, which can then be analyzed to determine existing trends and give early warning in case of potential problems.
If for instance, machine operators discover a gradual increase in condensing pressure over the course of a month, they can refer to the operating log to find out what the right levels should be and in the process correct the cause of the issue (e.g., fouled condenser tubes).
In the case of faulty unit, manufacturers can provide a list of recommended data points for each model upon request.
· Tube cleaning and heat transfer. One common hindrance to optimum chiller performance is heat-transfer efficiency. On average, large chillers contain miles of tubing– and the system’s capacity to transfer heat can be hampered by fouled tubes. Performance drops as mud, algae, sludge, or other contaminants accumulate in the system, and the rate at which the tubes clog up largely depends on the type of system (open, closed) and the quality of water.
Manufactures recommend cleaning the tubes once every year and the evaporator tubes at least once every three years (for closed systems).
Machine operators can use two primary methods to clean the tubes:
1. Mechanical cleaning: by using a special brush, this method can remove mud, sludge, algae, and a host of other foreign materials from the tubes. For a more complex tubing system, operators should contact the manufacturer for cleaning recommendations.
2. Chemical cleaning: this method is used to remove scale- but operators should consult with a water-treatment supplier to find the right chemical solution. For a more effective job, use both methods.
Bear in mind the condition of local water has a direct impact on the frequency of tube cleaning and inspection. The cooler tubes should be inspected at the end of the first operating season. Because these tubes are designed with internal ridges, they require a rotary-type thorough cleaning to get rid of pollutants.
· Watch out for leaks. Annual inspections cannot guarantee the compressor is free of leaks. Older, low pressure chillers contain a section in the system that operates at sub-atmospheric pressure. Most manufacturers recommend quarterly tests to check for leaks, which allow air and moisture (referred to as non-condensables) to enter the unit. When these non-condensable materials get inside the chiller, they become trapped in the condenser, which raises condensing pressure and affects the way the condenser uses power. This in turn reduces efficiency and the system’s overall cooling capacity.
According to one chiller manufacturer, just 1 psi of air inside a condenser translates to a 3% loss in operational efficiency. Moisture can also cause acids to build up, corroding he bearings and motor windings to create rust inside the shell.
· Controls need regular service. The starting equipment is often overlooked but it is critical to precise operation. A typical chiller control test is meant to facilitate proper operation by checking the temperature sensors, water pumps, oil pumps, pressure transducers, and a variety of on/off outputs.
The mechanical and electrical components of a chiller are designed to work together, which means that proper servicing must include the controls- that is the only way to maintain good operation.
If the chiller is installed as part of a central plant control or integrated into a building energy management system, a complete system evaluation should be done to make certain that performance is optimized. Trend reports are one of the ways machine operators assemble data to get a complete picture of a chiller plant or an entire building’s HVAC system.
Chiller maintenance involves several other areas, but the most important part is to perform regular inspections on all crucial components and to check the system’s vital signs (pressure and temperature) every day.