Carryover occurs when liquid refrigerant droplets travel into the compressor from the cold, low-pressure vapor produced in the evaporator (Figure 1). Generally there are two types of carryover. The first type is by design and controlled with the intent of enhancing the lubrication process in the compressor. The second type is not by design and can be detrimental to the performance and reliability of a chiller. If excessive liquid droplets enter the compressor system, they vaporize on the metal internals, stripping away lubricant.
Ultimately this oil stripped from the compressor becomes entrained in the hot, high-pressure refrigerant vapor and enters the condenser. Once this happens, oil from the sump is used to replace the stripped oil, causing the oil level to drop in the compressor oil reserve.
Entrained oil travels with the refrigerant as it condenses from the hot, high-pressure vapor to the warm, high-pressure liquid in the condenser. The excessive oil in the warm liquid refrigerant then returns to the evaporator where the oil separates from the refrigerant.
Once separated, the oil can be removed and return to the compressor oil reserve. In severe cases, carryover also can cause excessive liquid refrigerant to accumulate (stack) in the condenser.
Stacking and carryover can occur under similar conditions in a chiller. They can develop together or separately.
CHILLER SUSCEPTIBILITY
Stacking and carryover can be rather common, affecting all types of water-cooled chillers. Low-pressure chillers, however, which operate the evaporator in a vacuum, are more susceptible.Compared to high-pressure chillers, they do not have a large difference in refrigerant pressure between the condenser and evaporator. Figure 2 illustrates the comparison of a low-pressure chiller using R-123 refrigerant and a high-pressure chiller using R-134a refrigerant. At 65°F entering condenser water temperature (ECWT) and 40° evaporator chill water temperature, the pressure difference in the R-123 machine is 4.5 psi.
The pressure difference in the R-134a machine is 28.9 psi. It is this lift in both chillers that can be beneficial or detrimental if not properly maintained. To be fair, the advantage of low-pressure chillers is the ability, in most cases, to achieve a lower full load design (FLD) kW/ton. Depending on plant design and operations, both types have their advantages.
THREE MAIN CAUSES
There are three problems that cause stacking and carryover: mechanical, maintenance, and, the most common, operational.Mechanical problems: There are three tower temperature control problems that can cause stacking and carryover in chillers. First is the location of the tower temperature control. This temperature should mirror the ECWT. Second, hysteresis is the delayed reaction time of the controller affecting the ability to maintain a tight temperature range. The third and most common problem is tower operations maintaining temperatures too cold for the design of the chiller.
To remedy these problems, make sure the tower temperature is controlled in the basin, mirroring the ECWT. Improve controls to keep a maximum 2° high-low temperature range in the tower basin. Maintain the design ECWT for the chiller. All will help contribute to a well-run, efficient plant.
A malfunctioning or noncalibrated refrigerant level control in the condenser can create stacking by not allowing refrigerant to flow back to the evaporator. Refrigerant builds up in the condenser, while at the same time lowering refrigerant levels in the evaporator due to an unbalanced refrigerant cycle. The chiller loses heat transfer efficiency and may shut down on low evaporator refrigerant temperature.
Maintenance problems: It can be difficult to add the proper amount of refrigerant to a chiller; therefore refrigerant overcharge may occur when the chiller is serviced. This additional refrigerant is added to compensate for the loss during normal operations or leaks (which have been repaired). As a result, stacking and carryover may occur.
When this happens, it is generally easy to identify. The obvious signs are higher-than-normal refrigerant pressures in both the evaporator and condenser after the chiller was serviced. Extreme overcharge will cause the chiller to run poorly or have difficulty staying online due to high condenser head pressure. A superheat test is an excellent tool for verifying and ensuring proper refrigerant levels.
Operational problems: The primary cause of stacking and carryover, as it relates to operations, is the operator running the ECWT too low for the chiller load conditions. This can bring about stacking, or in extreme cases, carryover with stacking. Both cause chiller inefficiency and damage over time.
ECWTs can have a good or bad impact on chillers. Lowering the ECWT can be a great energy management practice if done within the chiller design parameters. For example, every 1° drop in ECWT below FLD can improve a chiller's efficiency up to 1.5 percent, depending on the chiller design and motor type (constant-speed or variable-speed drives).
Lowering the ECWT reduces the lift, making it possible for the compressor to use less energy to produce the desired capacity (tonnage). This is extremely valuable, considering the cost of energy today and the fact that chillers are typically the largest energy consumer in most facilities. There is a fine line, however, between good energy management and initiating potential problems due to low ECWTs.
Chiller manufacturers today not only build chillers with more efficient FLD kW/ton, but they can also design chillers to maximize efficiency at part loads. Since most chillers run in part loads 98 percent of the time, it is up to the facility to decide which chiller best fits their needs and to master the operation of these chillers to achieve the lowest kW/ton under all conditions. They must also operate in a manner that protects this very expensive equipment.
In terms of potential savings, a well-run chiller should achieve an overall operating kW/ton below FLD kW/ton by 5-15 percent for constant-speed drives, and by 20-30 percent for variable-speed drives. How can plants achieve this efficiency? Primarily by maintaining the design ECWT for the chiller load and using the operator's knowledge of the chiller to maximize its performance.
There is no substitute for well-trained operating engineers; however, they need the required data and analysis to make informed decisions when operating a chiller to achieve its best efficiency.
IDENTIFICATION AND PREVENTION
Both stacking and carryover affect chiller performance and are not easily detected from a single sensor. In fact, it requires multiple sensors, experienced operators, and data analysis to positively identify. Therefore, it is typically only diagnosed in severe cases, usually when the chiller shuts down or after an oil or refrigerant analysis is performed.Furthermore, misunderstanding of these conditions may lead to false assumptions like low oil and refrigerant levels, resulting in excess oil being added to the chiller or refrigerant overcharge.
Compressor oil levels drop: When oil levels drop in the compressor sump due to low ECWT and excessive carryover, it can prompt the operator to add additional oil. Later, when the ECWT returns to normal, excess oil separates in the evaporator, where it is removed and returns to fill the compressor oil sump with surplus oil. This surplus oil may need to be manually removed.
If the chiller continues to run with a low ECWT, however, low oil levels may persist and even more oil may migrate. This makes it possible for this excess oil to emulsify in the refrigerant and inhibit heat transfer on the evaporator tubes.
Keep detailed records of oil additions between oil changes. When the additions are made, check for leaks. If no leaks exist, examine operations. Look for foaming in the evaporator refrigerant sight gauge as a sign of high oil levels in the refrigerant. Look for refrigerant boiling in the compressor oil sight gauge as a sign of stacking and/or carryover.
Evaporator liquid refrigerant levels drop: This can happen very quickly or slowly depending on the disparity between the ECWT and chiller load. When severe, the refrigerant is rapidly pulled out of the evaporator and stacked in the condenser. This may cause the chiller to shut down on low refrigerant temperature. The refrigerant cycle is broken due to low lift.
Increasing ECWT restores the lift and the refrigerant will go back to a balanced cycle between the condenser and evaporator. Seasonal cold weather conditions can make tower water basin temperatures difficult to control and the potential for stacking and carryover greater.
During startup, lowering the demand limit on the chiller and/or reducing condenser flow may be the easiest solution for providing the desired condenser water temperature.
Oil and refrigerant analyses: It is best practice to perform quarterly oil and refrigerant analyses when problems are suspected. Abnormally high metal content in the oil analysis can be an indicator of past refrigerant carryover and bearing wear caused by increased friction from stripping the lubrication. The chiller representative and a physical inspection of the compressor bearings can confirm this.
There is an acceptable level of oil that can be in a refrigerant analysis and not impede chiller heat transfer. When refrigerant carryover is chronic, the percentage of oil in the refrigerant will increase, causing heat transfer problems.
DIAGNOSTIC SOFTWARE
Investing in a chiller diagnostics program, such as EffTrackâ„¢, can make it easy to identify stacking and carryover. The program collects, stores, and analyzes chiller operating data to determine performance, diagnose causes of inefficiency, and recommend corrective action.The monitoring service notifies plant contacts if problems occur. Plant operators and facility managers can review the hourly updated information by logging in to the program from any computer with Internet access. By following the recommendations for improvement, plant operators can eliminate chiller faults and significantly lower the chiller kW/ton and plant kWh consumption.
These savings are identified and measured in daily reports. In addition to stacking and carryover, such programs can identify high or low refrigerant levels; compressor problems; water flow rate problems, including plugged or restricted water flow; fouling and scaling; noncondensable gases in low-pressure chillers; cycles of concentration problems; and sensor calibration problems or bad data.
KNOW YOUR CHILLERS
Every chiller has its own unique circumstances that create stacking and carryover. Ask your chiller manufacturer what is the best operating condition for ECWT and the matching part-load values.Then implement a program for monitoring efficiency and identifying chiller problems. Once achieved, the facility can reap the rewards of peak efficiency and performance at the lowest possible expense.
Don Clark is president of Efficiency Technologies Inc. He may be contacted at 866-333-8321.
Publication date: 07/03/2006
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