Like other components in a system, evaporators are normally quite reliable. However, periodic inspections and proper maintenance are essential to help sustain system design capacity, dependability, and long service life.
Most service contractors are well informed about water quality-related problems that can affect condensers. This is due in part to the fact that condenser water is normally circulated in an open system with cooling towers — or even pond or river water in some instances.
As such they are, in general, more susceptible to corrosion, fouling, and mineral deposition than chiller barrels. Minerals, slime, sludge, and microorganisms are often inherent in water, in reasonably acceptable concentrations. However, these contaminants can often be concentrated to unacceptable levels due to insufficient tower water bleed and inadequate fresh water makeup.
While water problems of chillers are generally not as dramatic as those for condensers, they do demand attention.
Chilled water for air conditioning, which normally circulates in a closed-loop system, should be monitored closely. For chilled water in process cooling systems, it is essential.
Depending upon the application, chiller barrel performance can be highly prone to poor water quality. Many process water cooling systems have large reservoirs, auxiliary sumps, cooling troughs, spray systems, and large piping systems.
When compared to air conditioning, chilled water for process cooling can create extraordinary water quality problems, including high exposure to airborne dust, dirt, and other particulate matter.
Here are some vital chiller barrel inspection and service procedures to help keep customers and their products cool during peak loads. For obvious reasons, air conditioning system inspections are best performed at the end of the cooling season.
But, first, a little background on water quality, coolant problems, and other potentially destructive operating conditions that can result in lost chiller barrel performance and reduced service life.
Destructive conditions“Early retirement” of key system components, such as chiller barrels or condensers, is not an option for most building owners due to costly repairs and downtime.
Remember, there are no short cuts to proper chiller barrel inspections. Periodic maintenance is the only way to ensure peak heat transfer, energy efficiency, and longer operating life for the entire packaged chiller.
Make sure there’s cool, clean water. Fouling due to poor water quality can result in gradual but significant reductions in chiller barrel performance and/or reduced service life.
While a small amount of scaling is normal and will even help protect the tubes from corrosion, excessive hard scale from water with high mineral content can severely inhibit heat transfer and increase pressure drop.
While scale can be removed with acid, improper acid cleaning can shorten the life of the chiller barrel. So having good water quality early on can be vital to chiller barrel performance and service life.
Sediment from airborne matter, heavy rust, and other system-related sources can also result in significant performance losses — even freeze-ups if build-up severely restricts water flow. Sediment problems are easily remedied by installing and maintaining strainers or filters on chiller barrel inlet water.
Large accumulations of sediment can be readily removed by backflushing or reverse flow. Sediment agitation and removal can be enhanced by injecting low-pressure air at 25 to 50 psig with a nozzle into the coupling on the flange where water is being pumped through.
For air conditioning, the best time to perform this work is after seasonal shutdown. Maintenance for process cooling is generally required during line shutdown.
Be alert to changes in chilled water acidity. Chilled water or ethylene glycol solutions do not necessarily remain chemically stable and neutral throughout the system’s lifecycle. Recirculated chilled water and other brines can become corrosive over time, and attack and destroy internal baffles.
Performance reductions of 50% to 60% can occur due to the fact that the baffles, designed to promote turbulent fluid flow to the tube surface for optimum heat transfer, will no longer function properly.
Additionally, corrosive water can attack copper tubes and create leaks. Proper monitoring of water conditions is essential, and is best performed by laboratory analysis.
Your water samples should be run for Langelier and Ryznar indexes at professional local labs. Depending upon test results, it may be necessary to use corrosion inhibitors, or drain and flush the entire system and add fresh, clean water.
Some manufacturers now offer test kits for mail order analysis. Specially designed fluid-testing packages are available for a wide range of heat transfer fluids analyses, including water, oil, and refrigerants. (Standard Refrigeration offers standard fluid analysis [SFA] test kits. For more information, use the phone number or website listed at the end of this article.)
Water velocity is the chiller killer. Chiller barrel service life can be cut short by high water velocity. Excessive flow will result in velocities that exceed intended design conditions.
This should be checked at start-up and periodically to ensure that flow does not exceed values recommended by the manufacturer. Use a properly calibrated flow meter.
Do not use pressure drop to calculate flow. Pressure drop will rise after a unit is commissioned, and is therefore an inadequate method of determining flow. The rise in pressure drop is due to deposits and fouling that accumulate during the evaporator’s lifecycle. The pressure drop rise can be several psi above that measured at start-up.
Maintain vital control settings. Proper maintenance of chiller barrels is more than just monitoring fluid quality. Many chillers are needlessly frozen due to control failure or improperly adjusted controls that should also be protected from tampering.
The low-pressure cutout and flow switch deserve special attention. Both controls should be periodically inspected for functionality and proper setpoint to trip the compressor if limits are exceeded.
Low-pressure cutouts should trip slightly above the freezing point of your chiller’s recirculating fluid. Lower settings minimize nuisance tripping, but the trade-off is increased risk of freeze-ups. Additionally, be sure to account for the glide in the new zeotropic refrigerants when determining the proper setting for the low-pressure cutout.
Flow switch settings are equally important to help prevent needless freeze-ups. Settings should be adjusted to trip not only when water flow is completely stopped, but also to monitor any appreciable flow reduction. Settings for either control should always be verified through several operating cycles, using only the most accurate gauges.
Watch out for big chiller barrel troubles. Chiller barrels with excessive performance loss and poor maintenance records can often be restored to useful service life. Depending upon the degree of damage, these units can sometimes be restored to useful service during seasonal shutdown.
If they have accumulated excessive scale formation or have sediment problems, the problem can often be resolved by backflushing or reverse flow.
Tubes damaged by freeze-up or corrosive attacks can be replaced. Flooded chillers, with end caps removed, can be cleaned readily. And DX chillers can be cleaned chemically by using the proper procedures.
Chiller barrels should only be reassembled following the manufacturer’s specifications and put back in service after pressure testing. However, these procedures are costly and time consuming. What better reason for regular inspections and maintenance?
If chiller barrel inspections are performed at least once a year, there will be fewer surprises or needless service interruptions. Experience proves that most potentially damaging operating conditions can be revealed in time for corrective maintenance that is essential to efficient heat exchange and long service life.
Standard Refrigeration is a manufacturer of water-cooled condensers, chiller barrels, subcoolers, receivers, and suction line heat exchangers. For more information, contact the company at 2050 N. Ruby St., Melrose Park, Ill. 60160-1133; 708-345-5400; 708-345-3513 (fax); www. stanref.com (website).