By Mike Neagle
| A digital multimeter enables technicians to test the output voltage of the variable frequency drive. |
Many building owners are opting for high efficiency chillers when the time comes to replace older systems, which is a win-win decision for the contractor and the customer. New chillers help customers save on energy costs and provide improved comfort, and the contractor has the pleasure of maintaining state-of-the-art equipment.
And, while it’s convenient to think that all chillers — whether old or new — are basically alike, they’re not, and we should consider the differences.
Most new chillers have high efficiency purge units, microprocessor controls, enhanced tubes, electronic expansion valves (EXVs), and in some cases, variable frequency drives (VFDs). Each of these components requires a different approach to maintenance as compared to older machines.
The technology used in new chiller equipment, changes how technicians perform annual maintenance on those systems. The following is a description of some differences you may encounter when performing maintenance on this equipment.
Purge Units Easier to Maintain
Many new chillers incorporate high-pressure refrigerants, and don’t require purge units. Those that do require purge units have undergone many changes. Older units use oil-lubricated pumpout compressors, and an oil separator. Annual maintenance included an oil change, compressor efficiency test, and oil separator cleaning. The refrigerant purge drum would require extensive maintenance that included purge chamber cleaning, new drain valve seats, and sight glasses.
| The internal spiraling of high efficiency chiller tubes improves the mixing of water and the heat transfer process. |
New chiller systems use small refrigeration systems that separate non-condensable gases from the refrigerant. This type of unit requires little more than a filter drier change, periodic carbon canister change-out and a functional test to assure the unit is operating satisfactorily. Because the newer, high efficiency chillers rely on low, non-condensable levels to achieve their rated efficiency, purge maintenance is a must.
Microprocessor Controls Provide More Information on System Performance
In years gone by, technicians would spend considerable time testing the accuracy and functionality of operating and safety devices. This would include checking motor overcurrent devices, low temperature refrigerant safeties, motor temperature, purge compressors, and system high pressure cut-outs, just to name a few. They nearly always found one or two that were out of calibration or that had failed completely. The latter was always a concern, because it could go undetected and leave the entire unit exposed to a major failure.
In the mid 1980’s, chiller manufacturers’ started using microprocessor-based controls, which brought improvements in operating and safety reliability, and accuracy. As technology matured, improvements in motor control and monitoring, refrigerant temperature, and water flows were incorporated into chiller control panel logic, which has had a significant impact on overall equipment reliability, including reduced motor failures.
As this technology advances, it opens up the opportunity to increase chiller efficiencies by varying water flow rates in both the condenser and evaporators.
Newer units now store the original chiller design parameters in the controller. This allows service technicians quick access to critical information regarding water flow rates, chiller kW rating, design refrigerant and water temperatures, maximum motor amps, and vessel pressure drops — all of which help technicians perform scheduled maintenance.
The days of checking and testing mechanical temperature and pressure devices have been replaced with scrolling through onboard diagnostics. We no longer rely only on a fluid thermometer and an ohm meter to check and test these devices. Gone are the days of 2- to 4-degree dial resolutions; we can now set and test critical devices to within plus or minus .5 degrees. Additionally, many of the newer control systems include adaptive logic, such as the ability to monitor motor temperature and adjust the restart time based on actual motor winding temperature.
Manufacturers now have the means to make changes in the refrigerant-towater ratios that were once considered not feasible. Since most of the new high efficiency chillers have a low water-torefrigerant ratio, technicians must be very careful to check and verify temperature and pressure overrides and shutdown settings. Water flowing at just 10% above or below design can affect the chiller’s stated efficiency considerably. Therefore, checking and adjusting flow rates should be performed at least annually.
Enhanced Tubes Improve Heat Transfer
Electronics can do much, but have little to do with the actual business of transferring heat. Heat transfer has everything to do with type and number of tubes, the refrigerant selection, and the temperature and type of medium being conditioned. Nearly all manufacturers use enhanced tubes to improve the transfer process. The enhanced tubes all have thinner walls, they generally have internal spiraling, and the external fin system has been modified, which improves heat transfer from the copper to the refrigerant.
Spiraling — also known as rifling — causes the water passing through the tube to rotate. Older tubes without this enhancement will pass through the tube much like water out of a garden hose, and only the surface of the water will be in direct contact with the tube.
This limits the heat transfer. Today’s internally enhanced tubes cause the water to rotate, which improves the mixing of the water while it’s still in the tubes and improves the heat transfer process.
Therefore, tube inspection and cleaning has become one of today’s most important maintenance procedures. Just .00025-in. of buildup on the interior of a tube wall will degrade the chiller efficiency.
Over the years, water system treatment has changed considerably, and water chemistry has a paramount effect on any chiller’s efficiency. Any change in chemistry affects the tubes’ condition, and technicians must carefully clean and inspect tubes. Poor water quality may require tubes be cleaned more than once each year. Maintain accurate operating logs throughout the year, as a way to monitor tube condition. Many of the newer control systems include trending data for just this reason.
Be Certain EXV is Adjusted Properly
Another component crucial to improved chiller efficiency is the electronic expansion valve (EXV). The EXV, when coupled with a microprocessor, controls refrigerant levels to very strict values, and helps design engineers maximize the “net” chilling affect.
As with any automatic valve-controlled refrigeration system, out-of-adjustment EXV’s can cause serious compressor damage. Most control systems depend on the valve travel to be calibrated and associated to the number of steps in the stepper motor. By doing so, the control system “knows” where the valve is at all times.
Another trap associated with electronic expansion valves is the issue of testing and adjusting the machine for a proper refrigerant charge. An EXV that’s out of adjustment can be misleading, therefore it’s necessary to test and verify the EXV prior to testing the unit for a proper charge.
The name of the game is heat transfer, so if you can maintain constant refrigerant levels, you can predict how many tubes in a tube bundle are actively absorbing or rejecting heat. Since most manufacturers use a stepper motor, testing and calibrating the EXV hould be part of annual maintenance.
Variable Frequency Drives
I know of no comfort cooling application that runs at design conditions 100% of the time. Therefore, how a chiller operates at less than design conditions can result in big efficiency gains. Initial valves should always be set per the manufacturer’s recommendations. However, in some cases, applications will dictate minor changes to achieve desired results. These changes must be done correctly. Incorrect settings can result in poor operating efficiency, higher than normal noise levels, hunting, and in certain cases, premature motor or bearing failure(s).
It’s imperative to know how to test and set the many adjustable parameters in a VFD. Testing the carrier frequency and tuning a VFD can extend bearing service life and minimize noise levels. Having a working knowledge of how to use and read an oscilloscope is a must for today’s high-efficiency chiller technician. The oscilloscope is used to “see” the quality of the electricity the VFD produces and supplies to the motor in a graph form. Always research each manufacturer’s recommendations when testing VFD’s. All VFDs are not alike, and most require detailed adjustment to the oscilloscope prior to testing.
Evaluating Refrigerant Charge
With new, more efficient chillers, the refrigerant charge plays a larger role in achieving the stated efficiency, and technicians must follow accepted practices when evaluating charges. Knowing the manufacturer’s nameplate charge will only help when the charge has been removed from the unit and weighed. In some cases, even the nameplate charge may not be 100% accurate, especially if the jobsite conditions have changed.
Knowing how to log a unit for proper refrigerant levels is the most effective tool for ensuring maximum operating efficiency, and knowing the design temperatures will help the technician determine if a unit has a proper charge. Since all manufacturer design submittal data is based on 100% load, one has to know to interpret part load data. All readings are not always linear. The most reliable way to ensure a proper charge is to operate the unit at design conditions, but as most of us know, that’s nearly impossible. This is why we recommend trend logging. The sooner you start collecting data, the more data you have to track changes with.
All manufacturers have improved the static sealing processes of their systems. These improvements have resulted in lower leak potential and improved internal chemistry. However, leak testing should be performed at least annually. Leak testing equipment must be rated to detect the type refrigerant in use and detect at very low levels, typically 12 oz. per year or less. As with any vibrating device, mechanical joints are subject to fail. Early detection will prevent possible heat stress or refrigerant side contamination.
Remember that a chiller is just one component of a chilled water system, and auxiliary support equipment must also be properly inspected and maintained. A perfect chiller connected to a neglected cooling tower will not produce the desired results.