ALARMS AND FAULTS
It is of critical importance to the long term health of the
compressor that EVERY time a chiller is visited, or contacted via
remote contact, there is a detailed review of the FAULTS and EVENTS
logs from the chiller controller.
Any ALARMS or FAULTS being displayed must be examined to determine
of the alarm or fault is the result of a routine event (such as a
power failure) or is being generated due to some abnormal operating
condition.
For most of the operating limits of the compressor, there is an ALARM limit and a FAULT limit. Normally, when the compressor exceeds an ALARM limit, the compressor controls will attempt to limit the compressor in some manner as to avoid getting to the FAULT limit and to return compressor operation to the point where it is no longer in ALARM.
Exceeding a FAULT limit will cause the compressor to shut down.
Of particular concern are FRONT RADIAL BEARING DISPLACEMENT FAULTS
that are not associated with a power failure. In the
case where there are repeated FRONT RADIAL BEARING DISPLACEMENT FAULTS
there is a significant probability that these faults were generated
as a result of the compressor entering a ¨surge¨ condition.
Any fault that is occurring repeatedly bears detailed investigation
to determine and eliminate the cause of the fault.
Clients should be routinely advised to report any ALARM or FAULT
condition to a trained Turbocor Service Technician.
Ignoring
ALARM and/or FAULT conditions is opening the door to a potential
compressor failure.
Tuesday, October 1, 2019
DISCHARGE CHECK VALVE OF COMPRESSOR
DISCHARGE CHECK VALVES
The installation of a check valve on the discharge of Turbocor compressor is mandatory to prevent the compressor rotating backwards on shutdown as well as pressure from the condenser flowing backward through the compressor to the evaporator when the compressor is shut down.
While there is little to guide the service technician regarding the maintenance/ replacement of this item, it is the experience/ recommendation of the author that anytime the Turbocor compressor operates in surge for any length of time, or has a failure due to a front radial bearing issue (which probably has been caused by surge), as a minimum, the discharge check valve should be inspected for possible damage and replaced as appropriate.
There are two types of check valves commonly used with the Turbocor compressor. One is a wafer type, swing check valve. The other is a plunger type combination check valve/ isolation valve.
The wafer type, swing check valve uses a ¨flapper¨ type disc with a spring to assist it in closing. It can be mounted with the valve opening horizontally or opening in an ¨up¨ position. This type of valve can´t be mounted such that it opens in a ¨down¨ orientation. Normally, this valve is used in conjunction with a ¨ball¨ valve for isolation purposes.
The swing check valve has advantages in that it takes up less space, is less expensive and is easier to replace when replacement is necessary.
The disadvantage of the swing check valve is that when it closes, it does so rather forcefully and seals with a metal on metal seal. While there is no proof of this, the observation of the author is that when the swing check valve closes, it creates a more abrupt disruption of the gas flow and possibly is more likely to cause a displacement of the compressor shaft which could lead to the shaft coming in contact with the touchdown bearing (and possible failure of the touchdown bearing). Additionally, the author has experienced several instances where the ¨flapper¨ of the swing check valve fractured, causing parts of the ¨flapper¨ to be sucked back into the compressor, damaging the compressor rotors.
The plunger type combination check valve/ isolation valve, there is a substantial Teflon seal that takes the ¨impact¨ of the closing force of the valve, plus the inertia of the plunger is much less than that of the swing check valve. The valve takes up more space in the installation, but does not require a separate isolation valve. However, in order to inspect the valve, the refrigerant charge must be removed from the condenser. There is a rebuild kit available for these valves.
In the opinion of the author, these valves are much less subject to a ¨catastrophic¨ failure that could also lead to compressor failure. On the other hand, damage to the valve could impair its ability to close properly, which would allow reverse refrigerant flow through the compressor and possible compressor damage. However, as with the swing check valve, in the case where the compressor has been subject to repeated episodes of surge or there has been a compressor failure due to front radial bearing issues, this valve should be torn down and inspected, as a minimum.
The installation of a check valve on the discharge of Turbocor compressor is mandatory to prevent the compressor rotating backwards on shutdown as well as pressure from the condenser flowing backward through the compressor to the evaporator when the compressor is shut down.
While there is little to guide the service technician regarding the maintenance/ replacement of this item, it is the experience/ recommendation of the author that anytime the Turbocor compressor operates in surge for any length of time, or has a failure due to a front radial bearing issue (which probably has been caused by surge), as a minimum, the discharge check valve should be inspected for possible damage and replaced as appropriate.
There are two types of check valves commonly used with the Turbocor compressor. One is a wafer type, swing check valve. The other is a plunger type combination check valve/ isolation valve.
The wafer type, swing check valve uses a ¨flapper¨ type disc with a spring to assist it in closing. It can be mounted with the valve opening horizontally or opening in an ¨up¨ position. This type of valve can´t be mounted such that it opens in a ¨down¨ orientation. Normally, this valve is used in conjunction with a ¨ball¨ valve for isolation purposes.
The swing check valve has advantages in that it takes up less space, is less expensive and is easier to replace when replacement is necessary.
The disadvantage of the swing check valve is that when it closes, it does so rather forcefully and seals with a metal on metal seal. While there is no proof of this, the observation of the author is that when the swing check valve closes, it creates a more abrupt disruption of the gas flow and possibly is more likely to cause a displacement of the compressor shaft which could lead to the shaft coming in contact with the touchdown bearing (and possible failure of the touchdown bearing). Additionally, the author has experienced several instances where the ¨flapper¨ of the swing check valve fractured, causing parts of the ¨flapper¨ to be sucked back into the compressor, damaging the compressor rotors.
The plunger type combination check valve/ isolation valve, there is a substantial Teflon seal that takes the ¨impact¨ of the closing force of the valve, plus the inertia of the plunger is much less than that of the swing check valve. The valve takes up more space in the installation, but does not require a separate isolation valve. However, in order to inspect the valve, the refrigerant charge must be removed from the condenser. There is a rebuild kit available for these valves.
In the opinion of the author, these valves are much less subject to a ¨catastrophic¨ failure that could also lead to compressor failure. On the other hand, damage to the valve could impair its ability to close properly, which would allow reverse refrigerant flow through the compressor and possible compressor damage. However, as with the swing check valve, in the case where the compressor has been subject to repeated episodes of surge or there has been a compressor failure due to front radial bearing issues, this valve should be torn down and inspected, as a minimum.
CONDENSATE ON EXTERIOR OF COMPRESSOR/ UNDER SIDE SERVICE COVER
CONDENSATE ON EXTERIOR OF COMPRESSOR/ UNDER SIDE SERVICE COVER
When doing service work on the Turbocor compressor, it is Always good to note the level of condensation near the rear bell housing and in the area of the cooling solenoids and rear bearing power pass through plug.
The amount of condensation experienced will depend to a large extent on the environment surrounding the compressor. In tropical areas where I am accustomed to working, there is always a significant amount of condensation.
I always remove the side service cover of the compressor and examine the area to the left as facing the compressor where the cooling solenoids are located (near the top left) and the rear bearing power pass through plug (near the bottom left).
On older compressor, there is a plug mounted directly on the pins of the Bering pass through plug. On newer versions, the pass through connections at the pass through plug are hermetically sealed and the connection is at the end of a short cable that is attached to the pass through plug.
You should take note if there is any obvious corrosion present in the area of the pass through plug. The compressor manufacturer recommends application of a non-conductive, lanolin based grease as a means of preventing corrosion in this area.
Spend some time with the compressor running, watching the inverter temperature and the motor cavity temprature. The cooling solenoid is activated by relays in the serial card. There is a max inverter temperature setting and a minimum setting. These are fixed in the compressor controls. As the inverter temperature increases, when it gets to the maximum temperature, the cooling solenoid will energize. With the cooling solenoid energized, the inverter temperature will fall to the minimum, at which the cooling solenoid will be de-energized. This cycle will continually repeat.
On the backplane board, near the top, just right of middle are the connection plugs for the cooling solenoids and there is a LED for each solenoid. You can note that the LED lights up whenever the cooling solenoid is activated.
One problem I have seen is that the relay contacts in the Serial Card ¨weld¨ and the cooling solenoid is permanently energized. This will result in the inverter temperature staying low and will result in more extensive condensation in the areas mentioned above.
While this situation is not immediately detrimental to the compressor operation, the problem should be corrected within a reasonable amount of time to prevent the excessive condensation resulting in corrosion.
When doing service work on the Turbocor compressor, it is Always good to note the level of condensation near the rear bell housing and in the area of the cooling solenoids and rear bearing power pass through plug.
The amount of condensation experienced will depend to a large extent on the environment surrounding the compressor. In tropical areas where I am accustomed to working, there is always a significant amount of condensation.
I always remove the side service cover of the compressor and examine the area to the left as facing the compressor where the cooling solenoids are located (near the top left) and the rear bearing power pass through plug (near the bottom left).
On older compressor, there is a plug mounted directly on the pins of the Bering pass through plug. On newer versions, the pass through connections at the pass through plug are hermetically sealed and the connection is at the end of a short cable that is attached to the pass through plug.
You should take note if there is any obvious corrosion present in the area of the pass through plug. The compressor manufacturer recommends application of a non-conductive, lanolin based grease as a means of preventing corrosion in this area.
Spend some time with the compressor running, watching the inverter temperature and the motor cavity temprature. The cooling solenoid is activated by relays in the serial card. There is a max inverter temperature setting and a minimum setting. These are fixed in the compressor controls. As the inverter temperature increases, when it gets to the maximum temperature, the cooling solenoid will energize. With the cooling solenoid energized, the inverter temperature will fall to the minimum, at which the cooling solenoid will be de-energized. This cycle will continually repeat.
On the backplane board, near the top, just right of middle are the connection plugs for the cooling solenoids and there is a LED for each solenoid. You can note that the LED lights up whenever the cooling solenoid is activated.
One problem I have seen is that the relay contacts in the Serial Card ¨weld¨ and the cooling solenoid is permanently energized. This will result in the inverter temperature staying low and will result in more extensive condensation in the areas mentioned above.
While this situation is not immediately detrimental to the compressor operation, the problem should be corrected within a reasonable amount of time to prevent the excessive condensation resulting in corrosion.
HIGH COMPRESSOR INVERTER TEMPERATURE
HIGH INVERTER TEMPERATURE
High inverter temperature alarms/ faults on an air cooled unit is frequently an indicator of low charge.
On an air cooled chiller, the control of the expansion valve attempts to maintain the liquid level in the evaporator in accordance with the liquid level setpoint. In the case where refrigerant charge is lost from the chiller, the liquid refrigerant ¨seal¨ in the liquid line will be lost and the compressor will begin to experience high inverter temperatures due to reduced liquid available in the compressor cooling connection.
In this case, bubbles will be observed in the motor cooling line sightglass.
This can be temporarily resolved by lowering the liquid level setpoint, which will back up liquid in the liquid line, restoring sufficient liquid to the compressor motor cooling line.
Other possible causes could be a problem with the temperature sensor itself, the cooling solenoid on the compressor or the relay in the Serial card that controls the cooling solenoid.
Trouble shooting on this depends a lot on if there are multiple compressor available, or only a single compressor. With multiple compressor, you can change out components to identify where the problem lies.
Under normal circumstances, with the compressor running, watching the reading from the inverter temperature sensor, you will see the temperature rise until the cooling solenoid opens, then the temperature will fall until the cooling solenoid closes. This cycle will repeat continuously.
There are LED´s on the backplane that indicate if the cooling solenoid is energized. These LED´s can give an indication to the service technician if the problem is with the solenoid if they are lighting up but the solenoid is not energizing. On the other hand, if the lights on the backplane are not lighting up, the problem can be with the relay in the Serial Card, or with the inverter temperature sensor itself.
High inverter temperature alarms/ faults on an air cooled unit is frequently an indicator of low charge.
On an air cooled chiller, the control of the expansion valve attempts to maintain the liquid level in the evaporator in accordance with the liquid level setpoint. In the case where refrigerant charge is lost from the chiller, the liquid refrigerant ¨seal¨ in the liquid line will be lost and the compressor will begin to experience high inverter temperatures due to reduced liquid available in the compressor cooling connection.
In this case, bubbles will be observed in the motor cooling line sightglass.
This can be temporarily resolved by lowering the liquid level setpoint, which will back up liquid in the liquid line, restoring sufficient liquid to the compressor motor cooling line.
Other possible causes could be a problem with the temperature sensor itself, the cooling solenoid on the compressor or the relay in the Serial card that controls the cooling solenoid.
Trouble shooting on this depends a lot on if there are multiple compressor available, or only a single compressor. With multiple compressor, you can change out components to identify where the problem lies.
Under normal circumstances, with the compressor running, watching the reading from the inverter temperature sensor, you will see the temperature rise until the cooling solenoid opens, then the temperature will fall until the cooling solenoid closes. This cycle will repeat continuously.
There are LED´s on the backplane that indicate if the cooling solenoid is energized. These LED´s can give an indication to the service technician if the problem is with the solenoid if they are lighting up but the solenoid is not energizing. On the other hand, if the lights on the backplane are not lighting up, the problem can be with the relay in the Serial Card, or with the inverter temperature sensor itself.
CONDENSER APPROACH
CONDENSER APPROACH
On a chiller with water cooled condensing, this is the difference between the saturated temperature of the discharge pressure and the leaving condenser water temperature. On a high efficiency chiller with flooded type condenser, this should normally be no more than 1 to 2 degrees C (1.8 to 3.6 degrees F). Where historical information is available, it is always good to compare previous approach readings with the current Reading.
High condenser approach will cause loss of chiller efficiency and capacity.
The most likely case of increasing condenser approach is fouling of the waterside of the condensing tubes. Looking at the quality of the condenser water itself can give an indication of the probability of condenser tube waterside fouling.
High condenser approach may also be due to ¨drift¨ in the pressure reading of the compressor discharge pressure/ temperature sensor or the reading of the leaving condenser water temperature sensor. These items can be verified by comparing against a refrigerant manifold reading or temperature meter reading, as appropriate.
On a chiller with air cooled condenser, the approach is the difference between the saturated temperature of the discharge pressure and the ambient air temperature. Normally, this will be 10 to 15 degrees C.
Possible causes of high approach on air cooled chillers can be error in the compressor discharge pressure sensor Reading. A likely cause is dirty coils. This can easily be discerned with a visual inspection.
On a chiller with water cooled condensing, this is the difference between the saturated temperature of the discharge pressure and the leaving condenser water temperature. On a high efficiency chiller with flooded type condenser, this should normally be no more than 1 to 2 degrees C (1.8 to 3.6 degrees F). Where historical information is available, it is always good to compare previous approach readings with the current Reading.
High condenser approach will cause loss of chiller efficiency and capacity.
The most likely case of increasing condenser approach is fouling of the waterside of the condensing tubes. Looking at the quality of the condenser water itself can give an indication of the probability of condenser tube waterside fouling.
High condenser approach may also be due to ¨drift¨ in the pressure reading of the compressor discharge pressure/ temperature sensor or the reading of the leaving condenser water temperature sensor. These items can be verified by comparing against a refrigerant manifold reading or temperature meter reading, as appropriate.
On a chiller with air cooled condenser, the approach is the difference between the saturated temperature of the discharge pressure and the ambient air temperature. Normally, this will be 10 to 15 degrees C.
Possible causes of high approach on air cooled chillers can be error in the compressor discharge pressure sensor Reading. A likely cause is dirty coils. This can easily be discerned with a visual inspection.
EVAPORATOR APPROACH
EVAPORATOR APPROACH
Evaporator Approach is the difference between the saturated temperature of the suction pressure and the leaving chilled water temperature.
Normally, in a high efficiency chiller with a flooded type evaporator, the evaporator approach should be less than 2 degrees C. (3.6 degrees F.).
When historical information on a unit is available, it is always a good idea to compare historical evaporator approach readings with the current reading.
High evaporator approach results in a loss of efficiency and capacity in the chiller.
Potential Causes of High Evaporator Approach:
Loss of refrigerant charge in air cooled chiller – if the chiller experiences high inverter temperature alarms or faults and the evaporator approach is higher than normal, there is an excellent chance the chiller has lost charge. In this case, the actual liquid level in the evaporator will be less than the liquid level setpoint.
On a water cooled condenser chiller, condenser level is being controlled to a setpoint. Loss of refrigerant charge will result in lower liquid level in the evaporator. Verify visually the liquid level in the evaporator. If liquid level not visible, try lowering the condenser liquid level setpoint, which should raise the liquid level in the evaporator...... This also should result in some reduction of evaporator approach. Caution must be taken to not lower the condenser liquid level setpoint to the point where there is a loss of liquid ¨seal¨ on the compressor cooling connection. This will result in bubbles in the motor cooling line sightglass and increasing compressor inverter temperatures.
Fouling of the waterside of the evaporator tubes – This is not very likely that the evaporator tube watersides have fouling, but this is Always a possibility that must be considered.
Problem with the compressor suction pressure/temperature sensor – A small ¨drift¨ in the suction pressure sensor reading can dramatically affect the performance of the chiller. This can easily be verified by connecting a pressure manifold to the system and verifying the suction sensor pressure reading. I have actually seen this problem on several different installations.
Problem with the leaving chilled water temperature sensor – This can be verified by measuring chilled water temperature with a meter. If no meter available, turn of chiller and verify that entering and leaving chilled water readings are the same.
Evaporator Approach is the difference between the saturated temperature of the suction pressure and the leaving chilled water temperature.
Normally, in a high efficiency chiller with a flooded type evaporator, the evaporator approach should be less than 2 degrees C. (3.6 degrees F.).
When historical information on a unit is available, it is always a good idea to compare historical evaporator approach readings with the current reading.
High evaporator approach results in a loss of efficiency and capacity in the chiller.
Potential Causes of High Evaporator Approach:
Loss of refrigerant charge in air cooled chiller – if the chiller experiences high inverter temperature alarms or faults and the evaporator approach is higher than normal, there is an excellent chance the chiller has lost charge. In this case, the actual liquid level in the evaporator will be less than the liquid level setpoint.
On a water cooled condenser chiller, condenser level is being controlled to a setpoint. Loss of refrigerant charge will result in lower liquid level in the evaporator. Verify visually the liquid level in the evaporator. If liquid level not visible, try lowering the condenser liquid level setpoint, which should raise the liquid level in the evaporator...... This also should result in some reduction of evaporator approach. Caution must be taken to not lower the condenser liquid level setpoint to the point where there is a loss of liquid ¨seal¨ on the compressor cooling connection. This will result in bubbles in the motor cooling line sightglass and increasing compressor inverter temperatures.
Fouling of the waterside of the evaporator tubes – This is not very likely that the evaporator tube watersides have fouling, but this is Always a possibility that must be considered.
Problem with the compressor suction pressure/temperature sensor – A small ¨drift¨ in the suction pressure sensor reading can dramatically affect the performance of the chiller. This can easily be verified by connecting a pressure manifold to the system and verifying the suction sensor pressure reading. I have actually seen this problem on several different installations.
Problem with the leaving chilled water temperature sensor – This can be verified by measuring chilled water temperature with a meter. If no meter available, turn of chiller and verify that entering and leaving chilled water readings are the same.
CORRECT CHARGE LEVEL IN CHILLER WITH FLOODED EVAPORATOR
DETERMINING CORRECT CHARGE IN A CHILLER
WITH FLOODED EVAPORATOR
Determining when a Turbocor compressor based chiller is very
different as compared to a DX type system.All units should have a nameplate which should include the design charge of the system. On smaller systems, if there is doubt about the charge, system charge can be removed and the correct charge weighed back in.
However on larger systems, this may not be practical. The typical Turbocor compressor based chiller will have something on the order of 1 to 1.5 kilos of charge per TR.
With a DX system, if it is not practical to remove the charge and weigh in the correct charge, charge can be added until suction superheat stabilizes and until the discharge pressure starts to increase (indicating liquid is backing up in the condenser).
However, in a Turbocor compressor based chiller with flooded evaporator, determining correct charge is completely different.
On a water cooled chiller with flooded type evaporator and condenser you must determine the correct charge level in the condenser and the evaporator.
Normally, the water cooled condenser will have a subcooling circuit at the bottom of the condenser tube bundle that designed to be immersed in liquid during normal operation. In most cases, there will be a sightglass mounted low on the condenser such that, when condenser charge is correct, a liquid level will be visible in this sightglass. If there is no sightglass, depending on the size of the chiller, you will need to estimate the height of the sub-cooling circuit. The sub-cooling circuit will be the bottom 6 to 12 rows of tubes (6 on smaller capacity chillers, 12 on larger capacity chillers) with tube spacing about 2/3 of an inch.
In the case of the air cooled condenser, correct level of liquid in the condenser is somewhat more difficult to determine. Normally, the air-cooled coils will have a sub-cooling circuit near the bottom of the coil. This can only be determined through visual inspection of the header end of the coil. If the air cooled coil has a sub-cooling circuit, correct charge of the condenser can be determined by the fact that the liquid header of the air cooled coil will have a lower temperature in the area of the sub-cooling circuit as compared to the area of the liquid header in the area where only hot gas is present
Normally, the evaporator will have a sightglass mounted such that the top row of evaporator tubes is visible through this sightglass. Evaporator charge is correct when during full load operation, the liquid refrigerant in the evaporator is visible just barely covering the top row of evaporator tubes.
To determine correct charge in a chiller with flooded water cooled condenser, adjust the setpoint on the liquid level sensor (which will be measuring the liquid level in the condenser) until the liquid level in the condenser is at the desired level.
Once the liquid level in the condenser is correctly established, with the chiller fully loaded, the liquid level in the evaporator should be just covering the evaporator tubes. If the liquid level in the evaporator is not visible, add charge until the liquid becomes visible.
HINT FROM THE AUTHOR - When I have the correct level in the condenser established, I reduce the condenser liquid level setpoint by 5%. Then when I see the evaporator liquid level come up over the top evaporator tubes, I can make a fine adjustment to the evaporator level by increasing the condenser setpoint slightly.
With the flooded evaporator in an air cooled chiller, the liquid level sensor will be directly in the evaporator. To determine the correct charge in a chiller with air cooled condenser coils, you will need to adjust liquid level setpoint until the liquid level in the evaporator just covers the top tubes in the evaporator. Then, sufficient charge must be added to back liquid refrigerant up into the liquid line and up into the very bottom part of the air cooled coils.
On the Turbocor compressor based air cooled chiller, there is a motor cooling connection on the condenser liquid line and a sightglass on the cooling line going to the compressor. As a minimum, when the chiller is correctly charged, there will be no bubbles in the motor cooling lone sightglass during compressor operation.
Absolute correct charge on the air cooled unit will be when liquid is backed up into the sub-cooling circuit of the air cooled coil during full load operation.
As with air-cooled DX systems, excessive charge in the unit will result in an increase in the discharge pressure of the air cooled chiller.
EXV FUNCTION IN FLOODED EVAPORATOR
FUNCTION OF EXV in a system with flooded evaporator.
Most technicians are familiar with the application of an expansion valve (mechanical or electronic) where the expansion valve is controlling the quality of the suction gas to the compressor. Nominally, the expansion valve is set to maintain a certain level of superheat in the suction gas. The valve monitors suction gas temperature and compressor suction pressure, using this relationship to adjust expansion valve opening as appropriate to maintain proper suction gas conditions at the compressor.
Conventional direct expansion systems must be set up to maintain something like 7 to 10 degrees C (13 to 18 degrees F) of superheat in the suction gas, in order to make absolutly sure that no liquid enters the compressor. The superheat level is that heat above the saturated temperature of the gas at the working suction pressure. More superheat reduces capacity and efficiency of the system. On an operating system, due to variations in load on the evaporator a superheat of the level indicated is necessary to make sure that, as the load varies and the valve modulates to adjust to the varying load, the superheat does not go to ¨0 ¨ and incur the risk of a liquid slug to the compressor which could, in turn, cause serious damage to the compressor. The design of the system is normally such that, once the liquid passes through the expansion valve, the volume enclosing the 1iquid is increasing, causing the liquid to expand, changing to a gas and absorbing heat in the evaporator coil. This expansion process takes place in the evaporator, absorbing heat from the air (or liquid as the case may be).
In the case of a Turbocor compressor chiller with a flooded evaporator, the function of the expansion valve (normally an electronic expansion valve) is to maintain the correct liquid level in the evaporator.
In the case of an air cooled unit, there is a liquid level sensor on the evaporator and the controls use a liquid level setpoint in combination with the actual liquid level being measured to operate the expansion valve. Actual liquid level higher than the setpoint results in closing the valve to reduce liquid flow into the evaporator, lowering the liquid level. Actual liquid level lower than the setpoint results in opening the valve to increase liquid flow to the evaporator, raising the liquid level. The controller normally has a control algorithm that the action of the expansion valve is ¨smoothed¨ to prevent large swings in the liquid level.
In the case of a water cooled unit, the expansion valve controls the level of the liquid in the condenser which, indirectly controls the liquid level in the evaporator. When the chiller is correctly charged and the liquid level setpoint of the condenser correctly set and maintained, the liquid level in the evaporator will be correct for the operation of the system.
With a flooded evaporator, the heat exchanger tubes are only in the bottom half of the shell. The top half of the shell is open, frequently with baffles of some type. In operation, the liquid refrigerant fills the bottom half of the shell, just covering the heat exchanger tubes. This design is such that liquid refrigerant droplets will not be pulled off the surface of the liquid refrigerant and into the suction of the compressor. The change of the liquid refrigerant to a gas occurs at the tube surface as heat is transferred from the liquid being cooled (normally the chilled water), to the refrigerant. The gas bubbles formed at the tube surface rise within the liquid refrigerant in the evaporator, leaving the surface of the liquid refrigerant as gas.
In the Turbocor compressor based chiller, the suction superheat is precisely controlled by the compressor itself, by adjusting compressor rotation speed. In this manner, suction superheat can be maintained at less than 1 degree C, significantly increasing both capacity and efficiency of the system.
Most technicians are familiar with the application of an expansion valve (mechanical or electronic) where the expansion valve is controlling the quality of the suction gas to the compressor. Nominally, the expansion valve is set to maintain a certain level of superheat in the suction gas. The valve monitors suction gas temperature and compressor suction pressure, using this relationship to adjust expansion valve opening as appropriate to maintain proper suction gas conditions at the compressor.
Conventional direct expansion systems must be set up to maintain something like 7 to 10 degrees C (13 to 18 degrees F) of superheat in the suction gas, in order to make absolutly sure that no liquid enters the compressor. The superheat level is that heat above the saturated temperature of the gas at the working suction pressure. More superheat reduces capacity and efficiency of the system. On an operating system, due to variations in load on the evaporator a superheat of the level indicated is necessary to make sure that, as the load varies and the valve modulates to adjust to the varying load, the superheat does not go to ¨0 ¨ and incur the risk of a liquid slug to the compressor which could, in turn, cause serious damage to the compressor. The design of the system is normally such that, once the liquid passes through the expansion valve, the volume enclosing the 1iquid is increasing, causing the liquid to expand, changing to a gas and absorbing heat in the evaporator coil. This expansion process takes place in the evaporator, absorbing heat from the air (or liquid as the case may be).
In the case of a Turbocor compressor chiller with a flooded evaporator, the function of the expansion valve (normally an electronic expansion valve) is to maintain the correct liquid level in the evaporator.
In the case of an air cooled unit, there is a liquid level sensor on the evaporator and the controls use a liquid level setpoint in combination with the actual liquid level being measured to operate the expansion valve. Actual liquid level higher than the setpoint results in closing the valve to reduce liquid flow into the evaporator, lowering the liquid level. Actual liquid level lower than the setpoint results in opening the valve to increase liquid flow to the evaporator, raising the liquid level. The controller normally has a control algorithm that the action of the expansion valve is ¨smoothed¨ to prevent large swings in the liquid level.
In the case of a water cooled unit, the expansion valve controls the level of the liquid in the condenser which, indirectly controls the liquid level in the evaporator. When the chiller is correctly charged and the liquid level setpoint of the condenser correctly set and maintained, the liquid level in the evaporator will be correct for the operation of the system.
With a flooded evaporator, the heat exchanger tubes are only in the bottom half of the shell. The top half of the shell is open, frequently with baffles of some type. In operation, the liquid refrigerant fills the bottom half of the shell, just covering the heat exchanger tubes. This design is such that liquid refrigerant droplets will not be pulled off the surface of the liquid refrigerant and into the suction of the compressor. The change of the liquid refrigerant to a gas occurs at the tube surface as heat is transferred from the liquid being cooled (normally the chilled water), to the refrigerant. The gas bubbles formed at the tube surface rise within the liquid refrigerant in the evaporator, leaving the surface of the liquid refrigerant as gas.
In the Turbocor compressor based chiller, the suction superheat is precisely controlled by the compressor itself, by adjusting compressor rotation speed. In this manner, suction superheat can be maintained at less than 1 degree C, significantly increasing both capacity and efficiency of the system.
DETERMINE CAUSE OF FAILURE - A MUST
COMPLETE SERVICE MUST INCLUDE IDENTIFYING THE CAUSE
OF ANY FAILURE -
Training courses offered by Danfoss Turbocor and the various OEM manufactruers that use the Turbocor compressor focus on how to identify and correct problems with the Turbocor compressor.
I have frequently seen situations where there is one type or another of a failure, the technician comes in and identifies the problem then orders whatever parts are necessary (up to and including replacing the complete compressor) and finally puts the compressor back into service.
However, rarely do I see a concerted effort to determine what originally caused the failure and action taken to correct the root cause of the failure.
In truth, the only way a technician has to understand how to determine the cause of a failure in these systems is EXPERIENCE and/ or through conversations with other technicians with some level of experience. Frequently, the only individual available with some higher level of experience are the application engineers in the service department of the various OEM manufacturers.
My purpose with this blog is to provide a reference to technicians working on Turbocor compressor based chillers where they might find relevant information when addressing a problem they have not previously encountered.
In truth, it is to all of our best interests to improve the level of service to these compressor, and thus improve the reputation of the compressor itself.
I have heard more than once complaints about the fact that the Turbocor compressor are ¨fragile¨ .
I have to completely disagree with this complaint. In my opinion, anybody that makes this type of complaint is woefully ignorant of how to properly service the Turbocor compressor based chiller.
The Turbocor compressor is an extremely sophisticated piece of equipment. It has many built-in protections against entering into operation in a manner potentially dangerous to the longevity of the compressor. The compressor also has detailed records in its memory that can be accessed to analyze the historical operation of the compressor. All the ¨tools¨ are available to research the cause of a failure, but also to monitor situations that could lead to failure.
Proper maintenance of the Turbocor compressor based chiller by a well qualified technician can virtually guarantee against unexpected compressor failures.
Training courses offered by Danfoss Turbocor and the various OEM manufactruers that use the Turbocor compressor focus on how to identify and correct problems with the Turbocor compressor.
I have frequently seen situations where there is one type or another of a failure, the technician comes in and identifies the problem then orders whatever parts are necessary (up to and including replacing the complete compressor) and finally puts the compressor back into service.
However, rarely do I see a concerted effort to determine what originally caused the failure and action taken to correct the root cause of the failure.
In truth, the only way a technician has to understand how to determine the cause of a failure in these systems is EXPERIENCE and/ or through conversations with other technicians with some level of experience. Frequently, the only individual available with some higher level of experience are the application engineers in the service department of the various OEM manufacturers.
My purpose with this blog is to provide a reference to technicians working on Turbocor compressor based chillers where they might find relevant information when addressing a problem they have not previously encountered.
In truth, it is to all of our best interests to improve the level of service to these compressor, and thus improve the reputation of the compressor itself.
I have heard more than once complaints about the fact that the Turbocor compressor are ¨fragile¨ .
I have to completely disagree with this complaint. In my opinion, anybody that makes this type of complaint is woefully ignorant of how to properly service the Turbocor compressor based chiller.
The Turbocor compressor is an extremely sophisticated piece of equipment. It has many built-in protections against entering into operation in a manner potentially dangerous to the longevity of the compressor. The compressor also has detailed records in its memory that can be accessed to analyze the historical operation of the compressor. All the ¨tools¨ are available to research the cause of a failure, but also to monitor situations that could lead to failure.
Proper maintenance of the Turbocor compressor based chiller by a well qualified technician can virtually guarantee against unexpected compressor failures.
Thursday, September 19, 2019
PART LOAD OPERATION
PART
LOAD OPERATION ISSUES:
One
of the very strong features of the Turbocor compressor is its ability
to operate at part load conditions. This is particularly true in
multiple compressor units where, as the load reduces, one or more
compressors can be staged off.
It
is always preferable for the Turbocor compressors to run part loaded
as much as possible as opposed to cycling on and off. When reviewing
operating logs, anytime it is noticed that one or more compressors
are cycling more than once per hour, attention should be given to
what is causing this cycling.
Also,
in multiple compressor units, attention should be given as to the
part load operating characteristics of the various compressors in
the system such that they all operate the same under part load
conditions..
The
most difficult situation for part load operation is when the system
has high discharge pressures. Normally, part load operation in
strict A/C applications will be during night time hours when the
discharge pressure will be lower, due to lower outside ambient
temperatures.
In
cases where a low load condition exists on a single compressor unit
at the same time that discharge ´pressures are high, the compressor
will be typically near the surge limit of the compressor and,
particularly if cycling occurs, may enter into surge while cycling
off.
Their
is also the circumstance where load on the chilled water system is
suddenly reduced and the chiller controller holds the compressor on
at very low load/ chilled water temperature due to delays built into
the chiller control algorithm. During this time, the compressor may
enter surge.
The
parameters of the chiller control algorithm are defaulted to values
that serve for the majority of installations. However, in the case
of an installations subject to sudden changes in the chilled water
loop and/or part load operation combined with high discharge pressure
attention must be given to the modifying these parameters to prevent
the compressor from going into surge during these conditions. There
also may the possibility to put an offset into the controls such that
the minimum rpm of the compressor is set at a fixed offset from the
calculated minimum rpm (surge limit) for the compressors
In
multiple compressor units, particularly an older unit where changes
have been made to one or more compressors, it is necessary to look at
the BMCC program revision level. Different BMCC programs have
different part load operating characteristics.
One
compressor may run fine at part load conditions for that particular
installation, while another tends to cycle.
In
the case of multiple compressor units the optimal solution is to make
sure all compressors have the same BMCC program revision. If this is
not practical, it may be possible to make the compressor that
operates well at part load conditions, the permanent lead
compressor. This means that when the chiller unloads, this
particular compressor will always be the last compressor running.
Given
that, mechanically, the Turbocor compressors do not experience
¨wear¨, there is no need to be overly concerned about equalizing
run hours between compressors. In fact, it would be better to run
the compressor that does not cycle, always as the last compressor
running rather than put a different compressor having lower run hours
online when that compressor is going to cycle.
In
the case of multiple compressor units with all compressors having the
same BMCC revision level and one of the compressors runs differently
under part load conditions, investigation must be made as to why this
difference exists.
SURGE - WHAT CAUSES IT - HOW TO PREVENT
PREVENTING
SURGE:
Surge
typically occurs when the compressor is operating with a high
pressure ratio (PR), or when either the suction or discharge pressure
change more rapidly than the compressor controls can adjust the
compressor speed (RPM´s) and/ or inlet guide vane (IGV) position.
Pressure
Ratio (PR) is a number that is calculated based on the relationship
between the discharge pressure and the suction pressure. Pressure
Ratio for water cooled chillers typically will be on the order of 2.0
to 2.5. Due to the higher discharge pressure experienced by Air
Cooled units, Pressure Ratio for Air cooled chillers will typically
be on the order of 2.5 to 3.0. Higher condenser water temperature or
higher ambient temperatures will result in higher Pressure Ratio.
It
is important to recognize that the compressor is experiencing surge
and take action to prevent this from occurring. This action may be
the repair of a defective component of the chiller or an adjustment
to the chiller control loop algorithm parameters.
Routine
review of the chiller controller logs and the compressor histories is
important to assist in recognizing that the compressor is entering
surge.
Anytime
there is a Front Radial Bearing Displacement Fault, the cause must be
investigated. Particularly if this Fault is displayed repeatedly.
As
an overall rule, the compressor should not cycle more than three to
four times per hour under normal operating conditions.
There
are many possible causes of surge, however, following are some of the
more common ones encountered:
Chilled
Water Flow Switch inoperable in the closed position or jumped out
– This condition is particularly dangerous for the compressor. If
the compressor starts with no chilled water flow or the chilled water
flow is reduced/ eliminated during compressor operation, the suction
pressure will rapidly drop. The compressor controls can´t adjust
rapidly enough to prevent the compressor entering into a surge
condition. Eventually, the compressor shaft displacement will exceed
the safety limits of the controls due to the surge and the compressor
will shut down, typically with a Front Radial Bearing Displacement
fault.
The
continued operation of the chiller under this condition, with
repeated Front Radial Bearing Displacement faults has a high
probability of eventually resulting in damage to the touchdown
bearing.
Lack
of Water Flow proof or inoperative flow proof on condenser of water
cooled unit – If there is no water flow proof installed to
protect the chiller against operating with low water flow, a
condenser pump becoming inoperative will force the compressor into a
surge condition.
High
Ambient Conditions in combination with lower chilled water setpoint
– In cases where the client is operating the chiller at low load
with a chilled water setpoint below 4.5 degrees C (40 degrees F.),
during high ambient conditions, it is imperative that proper
adjustments be made to the chiller control parameters to prevent the
compressor entering in surge.
Under
these conditions, the compressor will likely be operating close to
the surge rpm limit. As the the chiller controls attempt to reduce
compressor capacity to meet the low load at the same time maintaining
enough compressor rpm to overcome the difference in suction and
discharge pressures, surge is a risk.
High
Evaporator Approach – High evaporator approach (more than 8
degrees C or 14.4 degrees F.) will result in a lower than normal
suction pressure. This, in combination with higher ambient
conditions can also result in the compressor operating close to and
occassionally entering into surge condition.
Failure
of Inverter Temperature Sensor or Motor Cavity Sensor – If
either of these sensors fails such that it is giving a higher than
normal reading, the compressor controls will automatically reduce
compressor rpm in order to limit the amount of heat being generated
by the inverter or the compressor motor itself and forcing the
chiller into a surge condition.
Chiller
Operation with rapidly changing load – In cases where the
chilled water loop is extremely small or the load on the chilled
water loop suddenly drops off, the compressor will tend to cycle
frequently and has risk of entering surge as the load disappears and
the compressor attempts to reduce compressor capacity in response to
the changing load.
In
this case, the compressor may enter surge before the chiller control
algorithm reaches the point where the compressor will be cycled off.
Repeatedly cycling the compressor under these conditions can result
in surge and compressor damage.
Failure
of the Compressor Suction Pressure Sensor – The compressor is
equipped with a pressure/ temperature sensor on the compressor
suction and discharge. If the suction pressure sensor begins to read
high, the compressor controls can drive the compressor into surge
repeatedly due to the fact that the compressor does not really know
where it is operating on it internal compressor map.
It
is important to routinely verify the suction pressure sensor accuracy.
There is no calibration possible on this sensor. Sensors reading
improperly must be replaced.
SURGE - WHAT IS IT?
Historically, the author has worked with technicians from Danfoss Turbocor, Multistack and Smardt, Canada, Smardt Australia and Smardt, Germany. The author is aware that there is a higher than normal percentage of compressor failures at certain sites and has discussed these failures with technicians and engineers from various organizations.
The conclusion drawn from this set of information is that allowing Turbocor to routinely enter Surge condition on a continuous basis will eventually lead to a bearing failure. Occasional Compressor surges can be expected during chilled water pump exchange or unusual operating conditions. However, the routine occurrence of surge is dangerous to the long term health of the compressor.
What is Surge?
This is the condition in which the compressor enters an operating condition where the differential pressure generated by the centrifugal action of the compressor impeller is not sufficient to overcome the difference between the suction pressure and the compressor discharge pressure. When this occurs, there is an instantaneous reversal of refrigerant flow through the compressor. In the case of Turbocor application, there is a discharge check valve. When this instantaneous inversion occurs, the check valve closes abruptly. When this occurs, the pressure differential between suction and compressor discharge disappears. Flow through the compressor returns to normal, the check valve opens and the original pressure differential returns, causing the cycle to repeat. All of this happens very quickly. This phenomenon is accompanied by a ¨clacking¨ from the check valve in the compressor discharge and vibrations in the discharge line of the compressor. Normally, this Surge condition will last 5 to 10 seconds as the compressor controls adjust compressor speed to get the compressor out of the Surge condition.
Why is this bad for the compressor? When the pulse occurs, there are rapid pressure changes within the compression area on the compressor. Typically, the compressor shaft rotates between 20,000 and 30,000 revolutions per minute. The compressor bearing control acts to keep the shaft precisely located radially and axially. Movement of the shaft at a distance less than the thickness of a sheet of paper during operation may result in the shaft entering contact with the contact touchdown bearings at a high rotational speed. Touchdown bearings are designed to support the shaft once it is completely stopped. The shaft that contacts the landing bearings at normal operating speed has a high chance to destroy the touchdown bearings. The rapid changes in pressure that occur during surge act to shift the axis from its normal operating position. Magnetic bearing control measures shaft position 10,000 times per second and adjusts for any shaft movement. However, under surge conditions, with rapidly changing pressures and related forces on the shaft, there is an internal safety limit to the ―orbital displacement‖ of the shaft. Under Surge conditions, when these internal forces cause shaft displacement from its normal position, if the displacement exceeds the ―orbital displacement‖ limit, the compressor will shut down automatically and normally this will be accompanied by a message ¨FRONT RADIAL BEARING DISPLACEMENT FAULT¨. In certain instances, the internal forces are such that the shaft enters in contact with the touchdown bearing at a high rotation speed, possibly causing and damage the touchdown bearing.
The touchdown bearing is a standard type roller bearing which is part of the magnetic bearing assembly, specifically designed to support the shaft when the shaft is de-levitated. The touchdown bearing is not designed to support contact with the shaft at operating speeds.
Under these circumstances (damaged touchdown bearing), the compressor must be returned to the compressor manufacturer (Danfoss Turbocor) for repair. Repair of the touchdown bearing locally is expressly prohibited by the compressor manufacturer.
The conclusion drawn from this set of information is that allowing Turbocor to routinely enter Surge condition on a continuous basis will eventually lead to a bearing failure. Occasional Compressor surges can be expected during chilled water pump exchange or unusual operating conditions. However, the routine occurrence of surge is dangerous to the long term health of the compressor.
What is Surge?
This is the condition in which the compressor enters an operating condition where the differential pressure generated by the centrifugal action of the compressor impeller is not sufficient to overcome the difference between the suction pressure and the compressor discharge pressure. When this occurs, there is an instantaneous reversal of refrigerant flow through the compressor. In the case of Turbocor application, there is a discharge check valve. When this instantaneous inversion occurs, the check valve closes abruptly. When this occurs, the pressure differential between suction and compressor discharge disappears. Flow through the compressor returns to normal, the check valve opens and the original pressure differential returns, causing the cycle to repeat. All of this happens very quickly. This phenomenon is accompanied by a ¨clacking¨ from the check valve in the compressor discharge and vibrations in the discharge line of the compressor. Normally, this Surge condition will last 5 to 10 seconds as the compressor controls adjust compressor speed to get the compressor out of the Surge condition.
Why is this bad for the compressor? When the pulse occurs, there are rapid pressure changes within the compression area on the compressor. Typically, the compressor shaft rotates between 20,000 and 30,000 revolutions per minute. The compressor bearing control acts to keep the shaft precisely located radially and axially. Movement of the shaft at a distance less than the thickness of a sheet of paper during operation may result in the shaft entering contact with the contact touchdown bearings at a high rotational speed. Touchdown bearings are designed to support the shaft once it is completely stopped. The shaft that contacts the landing bearings at normal operating speed has a high chance to destroy the touchdown bearings. The rapid changes in pressure that occur during surge act to shift the axis from its normal operating position. Magnetic bearing control measures shaft position 10,000 times per second and adjusts for any shaft movement. However, under surge conditions, with rapidly changing pressures and related forces on the shaft, there is an internal safety limit to the ―orbital displacement‖ of the shaft. Under Surge conditions, when these internal forces cause shaft displacement from its normal position, if the displacement exceeds the ―orbital displacement‖ limit, the compressor will shut down automatically and normally this will be accompanied by a message ¨FRONT RADIAL BEARING DISPLACEMENT FAULT¨. In certain instances, the internal forces are such that the shaft enters in contact with the touchdown bearing at a high rotation speed, possibly causing and damage the touchdown bearing.
The touchdown bearing is a standard type roller bearing which is part of the magnetic bearing assembly, specifically designed to support the shaft when the shaft is de-levitated. The touchdown bearing is not designed to support contact with the shaft at operating speeds.
Under these circumstances (damaged touchdown bearing), the compressor must be returned to the compressor manufacturer (Danfoss Turbocor) for repair. Repair of the touchdown bearing locally is expressly prohibited by the compressor manufacturer.
PURPOSE OF BLOG
PURPOSE
OF BLOG -
As
technicians that work on a highly sophisticated and complex
compressor, in the opinion of this blogger, we are severely
handicapped by the lack of really in depth information in the area
of determining causes of failures and indicators of potential
failures.
The
Danfoss Turbocor training course is aimed at how to locate, isolate
and repair a compressor failure. The Turbocor OEM manufacturer´s
training courses (such as Multistck, Smardt and Arctic) essentially
copy the Danfoss Turbocor training course (in fact, they use many of
the same visuals) with added information regarding each
manufacturer´s chiller control.
There
are two very important areas not addressed by any of these training
courses.
First
of all, and most important, any time there is a compressor failure,
all possible efforts must be made to determine why the failure
occurred. This is absolutely critical to prevent repeat failures.
These efforts include detailed review of the chiller controller
historical logs as well as the faults and events logs from the
compressor itself. I personally know of various installations with
multiple compressor failures where the cause of the failures has
never been addressed.
Secondly,
any time we have contractual responsibility for maintenance on a
chiller, we, as the responsible technicians, must continually review
the operation of the chiller using the same information indicated
above, looking for potential causes of future failures. This is
extremely important to protect the client against a potential
compressor failure.
In
my experience, the application engineers and technical support people
from Danfoss Turbocor and the various OEM manufacturers have
extensive levels of experience and are good reference sources.
However, they typically have a great deal of priorities occupying
their time and it is difficult to get real time help when you are in
the field working on a unit and need assistance. Added to this is
the fact that the most experienced engineers are the Danfoss Turbocor
application engineers and they are not directly available to the
technicians/ engineers working directly with the units. They are
only available to the application engineers of the OEMs.
The
information we need to properly support the Tubocor Compressor based
chillers in the field is only available through experience. There is
no reference manual that contains this information.
The
purpose of this blog is to begin to record this body of experience in
a manner that it is readily available directly to personnel working
directly with the Turbocor technology.
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