Chiller surge is a problematic behaviour in buildings that are equipped with a chilled water system. Operators and engineers are having a hard time figuring out why chiller surge is happening. So, I did extensive research on the symptoms, causes and preventions of chiller surge.
Chiller surge occurs when the pressure of the refrigerant in the condenser exceeds the pressure limit of the centrifugal compressor, causing the refrigerant in the condenser to flow back to the centrifugal compressor.
Chilled water systems that use water-cooled centrifugal chillers are prone to chiller surge. Understanding why chiller surge occurs and how to prevent it can improve chiller efficiency.
Water-Cooled Centrifugal Chillers Surging
Water-cooled chillers are usually equipped with a centrifugal compressor and centrifugal compressors are prone to surging.
Unlike scroll compressors and screw compressors, centrifugal compressors are dynamic compressors. Basically, centrifugal compressors do not “touch” the refrigerant when compressing it while both the scroll compressors and screw compressors do physically “touch” the refrigerant during compression.
Centrifugal compressors spin quickly to produce kinetic energy that draws in the refrigerant from the suction line. Then, the refrigerant “flies” outward due to the centrifugal force and exits the compressor at a higher pressure through the discharge line.
Because centrifugal compressors do not physically “touch” and compress the refrigerant like scroll compressors and screw compressors do, the refrigerant from the discharge line (high pressure) can flow back into the compressor if the pressure at the discharge line exceeds the maximum pressure produced by the centrifugal compressor.
At the discharge line is the condenser. So, if the pressure of the refrigerant in the condenser is too high, the refrigerant can flow back into the centrifugal compressor and this phenomenon is known as chiller surge.
Chiller Surge Symptoms
Experienced operators and engineers know a chiller is surging by simply listening to the noise produced by the centrifugal compressor. Otherwise, they measure the amps of the compressor.
Normally, the noise produced by a chiller is consistent with the load. If a chiller is surging, the noise changes from normal to a high pitched sound that lasted for a few seconds once every few seconds.
The frequency of the noise change of small-capacity centrifugal chillers is greater than large-capacity centrifugal chillers.
Listen to an example of the noise change when a chiller is surging:
Another common symptom of chiller surge is the drastic change in electrical current (amperage) of the compressor motor.
Since the refrigerant is flowing backward, it is against the rotation of the compressor impeller. Such an opposite force put a significant amount of stress on the compressor motor and causes it to draw high amps for a few seconds before returning back to normal.
Sometimes, if the frequency of chiller surge is too great, the compressor motor amps may simply stay high and this may confuse the troubleshooters.
Chiller Surge Causes
Chiller surge is caused by the high refrigerant pressure in the condenser. So, identifying the cause of high condenser pressure can lead to the cause of chiller surge.
1. Dirty Condenser Tube
When the condenser tube is dirty, the heat exchange effectiveness drops thereby causing the refrigerant not able to be cooled by the condenser water effectively.
As a result, the refrigerant pressure in the condenser is unable to be reduced as intended. When it reaches the limit, the refrigerant can flow back to the compressor, causing chiller surge.
2. Low Condenser Water Flow
Faulty condenser pumps, dirty condenser water pipes and intentional condenser water flow reduction (to reduce power consumption) can cause low condenser water flow across the chiller condenser.
Consequently, less heat is being extracted from the refrigerant and thus, the pressure of the refrigerant stays higher than intended and eventually causes chiller surge.
3. Poor Heat Rejection
Undersized cooling towers, dirty cooling tower infills and intentional cooling tower fan speed reduction (to reduce power consumption) can result in poor heat rejection for the condenser water. Hence, the condenser water supply temperature (entering the chiller) may be higher than intended.
As a result, the higher-temperature condenser water has less capacity to absorb heat from the refrigerant and therefore, causing the refrigerant pressure to stay higher than intended and leading to chiller surge.
4. High Ambient Wet-Bulb Temperature
Unexpected weather conditions and miscalculated cooling tower design conditions can cause high condenser water supply temperature (entering the chiller). Cooling towers reject heat based on the ambient wet-bulb temperature. The lower it is, the better the heat rejection.
Relevant post: How Cooling Towers Work?
However, if the ambient wet-bulb temperature raises beyond expected, the condenser water supply temperature may also raises beyond expected, resulting in the refrigerant pressure staying higher than intended and eventually causing chiller surge.
Furthermore, if the design engineers did not select the cooling tower based on the highest ambient wet-bulb temperature, the chance of getting chiller surge is higher.
Chiller Surge Preventions
Chillers are not intended to surge. By knowing the causes of chiller surge, it can be prevented. The preventive measures for chiller surge can be carried out during the design stage and the operational stage.
Sometimes, unavoidable chiller surges are caused by poorly designed systems. Design engineers must understand why chillers surge and make sure they don’t when designing.
1. Minimum Load Condition
In large-scale projects, water-cooled centrifugal chillers are often deployed. Each of these chillers are usually large in capacity. However, the minimum load conditions must be determined and small-capacity chillers may be considered.
Large multi-stage projects usually have low loads during the first few stages. Chiller surge is inevitable if the chillers can’t handle the low load conditions.
Hence, it is a good idea to consider using small centrifugal chillers to cater to the initial low load conditions to prevent chiller surge.
2. High Ambient Wet-Bulb Temperature
Chiller surge may occur when the ambient wet-bulb temperature is higher than expected. Hence, it is feasible to consider exchanging chiller efficiency for higher condenser water supply temperature.
Therefore, it is worth selecting chillers with a condenser water supply temperature a few degrees above the designed condenser water supply temperatures to reduce the chance of getting chiller surge.
3. Cooling Tower and Condenser Water Pump
Chiller surge is caused by high condenser water temperature. So, proper selection of cooling towers and condenser water pumps ensures that there is sufficient condenser water flow and heat exchange to keep the refrigerant pressure in the condenser as intended and prevent chiller surge.
Furthermore, it is worth designing cooling towers based on the highest ambient wet-bulb temperature to prevent chiller surge during unusual weather conditions.
4. Chiller Feature
Manufacturers have preventive measures built within their water-cooled centrifugal chillers to prevent chiller surge. Features like a hot gas bypass can maintain minimum refrigerant flow rate at low load conditions to prevent chiller surge in exchange for lower efficiency.
Chiller surge is often associated with people trying to optimize the chiller efficiency by adjusting the water temperature and flow rate. See my post How Chilled Water Temperature Affects Chiller Performance? to learn why.
Even perfectly designed chillers can have chiller surge if they are not operated properly. Operators must understand why chillers surge and prevent it during operation.
1. Regular Maintenance
Maintenance is essential for chillers. One of the reasons why chiller surge occurs is due to dirty condenser tubes. Hence, regular cleaning of the condenser tube can help prevent chiller surge.
Besides, make sure that the water treatment system is working properly so that the evaporator and condenser tubes don’t get dirty so quickly and prevent chiller surge which also helps maintain the chiller efficiency.
Furthermore, dirty cooling tower infills and faulty cooling tower fans can also reduce the heat exchange efficiency thereby causing chiller surge. Hence, regular inspection and cleaning is a must to prevent chiller surge.
Moreover, ensure that condenser water pumps are delivering the condenser water as intended because low condenser water flow is one of the reasons why chillers surge.
2. Chiller Staging
Certain types of chillers (eg: VFD chillers) can perform well during low load conditions. However, there is a minimum refrigerant flow rate requirement that must be met.
Many operators tend to run too many chillers at low load conditions because of variable-speed chillers. However, if each chiller does not have the minimum required refrigerant flow rate, chiller surge can occur.
3. Proper Energy Optimization
Saving energy for the chilled water system is very motivating for operators. However, proper energy optimization activities must be carried out to avoid getting chiller surge.
- Minimum cooling tower condenser water flow rate must be maintained to prevent scaling on the infills which will decrease the heat exchange efficiency thereby increasing the condenser water supply temperature to the condenser and leading to chiller surge.
- Be careful when reducing condenser water pump speed to save energy when the ambient wet-bulb temperature is high. The condenser water supply temperature may raise beyond intended and eventually cause chiller surge.
- At low load conditions, increase the chilled water supply temperature and reduce the condenser water supply temperature when possible to reduce chiller lift and prevent chiller surge. Be cautious about the indoor humidity level as the chilled water supply temperature increases.
Relevant post: What is Chiller Lift?
4. Constant Monitoring
Chiller fouling (dirty evaporator and condenser tubes) can be monitored by comparing the chiller approach temperature. When the chiller approach temperature rises beyond acceptable, it indicates that the evaporator tubes or the condenser tubes are dirty and maintenance is needed.
With constant monitoring and maintenance, chillers can be kept at optimal performance and chiller surge can be prevented.
Relevant post: What is Chiller Approach?
Common Problems with Chiller Surge
- One of the chillers keeps surging while others do not – Possibly low refrigerant charge on the surging chiller since all chillers share the same condenser water.
- Incorrect cooling tower condenser water supply temperature – Cooling towers are not designed and selected properly for the local ambient wet-bulb temperature, causing high condenser water supply temperature and eventually chiller surge.
Chiller surge happens basically due to poor heat rejection. The ideas and preventive measures laid above can help reduce the chance of getting chiller surge. Nonetheless, it is best to be aware of chiller surge and then, design the chilled water system with prioritizing chiller surge in mind so that the system will not consume more energy than it needed.
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