You check the chiller screen and see that the condenser approach is higher than normal.
Your first thought may be:
The condenser tubes are dirty.
Maybe.
But if you clean the tubes and the number does not improve, you just spent time and money fixing the wrong problem.
High condenser approach can come from dirty tubes, but it can also come from low condenser water flow, bad temperature or pressure readings, air inside the refrigerant system, or water bypassing part of the tube bundle.
The number is trying to tell you something.
The trick is figuring out what.
Condenser approach shows the symptom. It does not name the cause.
The quick answer
High condenser approach usually means heat is not moving from the refrigerant into the condenser water as easily as it should.
The most common causes are:
- Dirty condenser tubes
- Low condenser water flow
- Incorrect pressure or temperature readings
- Air or noncondensables inside the condenser
- Water bypassing part of the tube bundle
Before cleaning tubes, verify the number, check the water flow, and determine whether the approach increased gradually or suddenly.
What is condenser approach?
A water-cooled chiller removes heat from the building and sends that heat into the condenser water.
Inside the condenser, hot refrigerant surrounds the outside of the condenser tubes. Water flows through the inside of those tubes and carries the heat away to the cooling tower.
Condenser approach compares two temperatures:
- The refrigerant temperature inside the condenser
- The condenser water temperature leaving the chiller
The calculation is:
Condenser Approach = Refrigerant Condensing Temperature − Leaving Condenser Water Temperature
Example
- Refrigerant condensing temperature: 100°F
- Leaving condenser water temperature: 94°F
- Condenser approach: 6°F
That 6°F difference is the condenser approach.
The refrigerant condensing temperature may also be called the condenser saturation temperature. The chiller usually calculates it from condenser pressure.
What does high condenser approach mean?
Heat naturally moves from something hotter to something cooler.
Inside the condenser, the refrigerant has to be warmer than the condenser water before heat can move into the water.
When the condenser is clean and operating correctly, it should not need an unusually large temperature difference to transfer that heat.
When condenser approach increases, the refrigerant is staying hotter than normal compared with the leaving condenser water.
In plain language:
The refrigerant is hot, but the condenser water is not picking up the heat as easily as it should.
That can raise condenser pressure, increase compressor lift, make the compressor work harder, and reduce chiller efficiency.
But the approach number does not tell you why the heat transfer became worse.
That is where troubleshooting begins.
Does high condenser approach mean dirty tubes?
Sometimes.
Dirty condenser tubes are one of the most common causes, especially when the approach has slowly increased over weeks or months.
Dirt, scale, biological growth, sediment, and corrosion products can build up inside the tubes and act like insulation.
The refrigerant then has to become hotter to move heat through the tube wall and into the water.
But one high reading does not prove that the tubes are dirty.
A pressure transducer can drift.
A water-temperature sensor can read incorrectly.
A pump can slow down.
A valve can move.
Air can enter the refrigerant system.
A damaged waterbox gasket can allow water to bypass part of the condenser.
All of those problems can produce a similar symptom.
A useful clue
- Approach increasing slowly: Fouling becomes more likely.
- Approach increasing suddenly: Check readings, flow, valves, pumps, purge operation, and controls first.
Condenser tubes normally do not become heavily fouled overnight.
What is a normal condenser approach?
There is no single condenser approach that is normal for every chiller.
The expected number depends on:
- Chiller model
- Condenser design
- Tube material
- Number of tube passes
- Refrigerant type
- Condenser water flow
- Chiller load
- Entering condenser water temperature
- Sensor accuracy
- Original equipment selection
One chiller may normally operate with a lower approach than another, even when both are working correctly.
The best references are:
- Manufacturer design information
- Startup or commissioning data
- Previous readings taken when the condenser was known to be clean
- Readings from the same chiller under similar operating conditions
The most useful question is usually not:
Is this approach high compared with a generic number?
It is:
Has this chiller’s condenser approach increased compared with how it normally operates?
A chiller running lightly loaded with cool condenser water may not have the same approach as the same chiller running near full load on a hot afternoon.
The comparison has to be fair.
The five most likely causes of high condenser approach
1. Dirty condenser tubes
This is the obvious suspect, and it is often the correct one.
Dirt, scale, biological growth, and sediment build up inside the condenser tubes and make it harder for heat to move into the water.
The refrigerant has to stay hotter to keep rejecting the same amount of heat.
That raises condenser approach.
Usually develops: Gradually
Strong clues:
- Approach has slowly increased over time
- Condenser pressure is higher under similar conditions
- Compressor power or kW/ton has increased
- Condenser water flow appears close to normal
- Water treatment has been poor
- Deposits are visible during inspection
- Approach improves after tube cleaning
The trend matters more than one isolated reading.
A condenser approach that rises from 3°F to 6°F over several comparable inspections means more than one random 6°F reading.
2. Low condenser water flow
The condenser water carries heat away from the chiller.
When flow drops, each gallon of water has to carry more heat. The lower water velocity can also reduce heat transfer through the tubes.
Possible causes include:
- A partially closed valve
- A dirty strainer
- A weak or damaged pump
- Low pump speed
- Air trapped in the piping
- A restricted piping path
- A bypass valve in the wrong position
Can happen: Gradually or suddenly
First things to check:
- Entering condenser water temperature
- Leaving condenser water temperature
- Condenser water temperature difference
- Pressure drop through the condenser
- Pump operation
- Pump speed
- Valve positions
- Strainer condition
- Chiller load
- Design flow and pressure drop
A large condenser water temperature difference combined with lower-than-expected pressure drop may support a low-flow condition.
Do not diagnose low flow from condenser approach alone.
Look at the entire water side.
3. Incorrect pressure or temperature readings
Before opening the condenser, make sure the number is real.
Condenser approach depends on:
- Condenser pressure
- Refrigerant type
- Leaving condenser water temperature
If any of those inputs are wrong, the calculated approach will also be wrong.
Possible problems include:
- A drifting condenser pressure transducer
- A biased water-temperature sensor
- A sensor installed in the wrong location
- Poor sensor contact
- The wrong refrigerant selected in a calculator
- A BAS point mapped incorrectly
- Incorrect units
- A display or control-board problem
Usually appears: Suddenly
First thing to verify: Compare the chiller display with calibrated field instruments.
Check the actual condenser pressure and actual leaving condenser water temperature.
If the approach suddenly jumps but the rest of the machine does not appear to have changed, question the readings before blaming the tubes.
4. Air or noncondensables inside the condenser
Air and other gases that do not condense normally are called noncondensables.
These gases can take up space inside the condenser and increase condenser pressure.
Because the chiller calculates refrigerant condensing temperature from that pressure, the displayed condensing temperature can also rise.
That can increase the calculated condenser approach.
This problem is especially important on low-pressure chillers, where air can enter through leaks.
Strong clues:
- Purge operation has increased
- The purge cannot keep up
- The chiller has known refrigerant leaks
- Condenser pressure appears too high for the operating condition
- Tube condition and water flow have already been checked
High purge time does not prove that air is the only problem, but it is a strong reason to investigate further.
5. Water bypassing the condenser tubes
Condenser water is supposed to follow a specific path through the tube bundle.
Gaskets and divider plates inside the waterbox force the water through each pass.
If one of those parts is damaged or installed incorrectly, water can take a shortcut through the waterbox instead of flowing through the full tube bundle.
The pump may still be moving plenty of water.
The water is just not moving through the condenser correctly.
Often appears: After waterbox service or gasket work
Strong clues:
- Approach remains high after tube cleaning
- Condenser pressure drop is lower than expected
- Water temperatures are uneven
- The problem began after waterbox service
- A pass gasket is damaged or installed incorrectly
- Pump operation appears normal
Cleaning the tubes will not repair a damaged gasket or divider plate.
A simple example
A chiller is operating with a 7°F condenser approach.
The tubes are assumed to be dirty, so the condenser is opened and cleaned.
The chiller is restarted under similar conditions, but the approach is still close to 7°F.
The condenser pressure drop is then checked and found to be lower than expected.
At that point, the problem is no longer pointing strongly toward tube fouling. The next inspection finds a damaged pass gasket allowing water to bypass part of the tube bundle.
The tubes may have needed cleaning.
They were not the full problem.
That is why the follow-up reading matters.
Less-common causes
If the common causes have been checked, other possibilities include:
- Refrigerant level that is too high
- A sticking refrigerant-level valve
- A failed valve actuator
- Too many plugged condenser tubes
- Oil coating the refrigerant side of the tubes
- Internal refrigerant-distribution problems
These conditions are possible, but condenser approach alone cannot confirm them.
Investigate them only after checking the readings, water flow, tube condition, purge operation, and basic condenser controls.
The cooling tower trap
High condenser pressure does not automatically mean high condenser approach.
This is one of the easiest mistakes to make.
A weak cooling tower can send hot water into the chiller. That raises condenser pressure and makes the compressor work harder.
But the condenser approach may still remain normal.
The condenser may be transferring heat correctly. It is simply receiving water that is already too warm.
Hot entering water with normal condenser approach
The condenser may be operating normally.
Look more closely at:
- Cooling tower fans
- Tower water distribution
- Outdoor wet-bulb temperature
- Tower approach
- Tower bypass valve
- Airflow through the tower
- Tower staging
- Condenser water setpoints
Normal entering water with high condenser approach
Look more closely at:
- Dirty condenser tubes
- Low condenser water flow
- Bad pressure or temperature readings
- Air or noncondensables
- Waterbox gaskets
- Refrigerant-side problems
Hot entering water and high condenser approach
There may be more than one problem.
The tower can be underperforming while the condenser also has dirty tubes, low water flow, or another internal problem.
Condenser approach is not tower approach
The names sound similar, but they measure two different heat exchangers.
Condenser approach compares:
- Refrigerant condensing temperature
- Leaving condenser water temperature
Cooling tower approach compares:
- Water leaving the cooling tower
- Outdoor wet-bulb temperature
Condenser approach tells you about heat transfer inside the chiller condenser.
Tower approach tells you how well the cooling tower is cooling the water.
Mixing them together can send the troubleshooting in the wrong direction.
What should you check first?
Before cleaning tubes, work through the problem in this order.
Step 1: Verify the number
Confirm condenser pressure and leaving condenser water temperature with reliable instruments.
Step 2: Record the operating condition
Record:
- Chiller load
- Entering condenser water temperature
- Leaving condenser water temperature
- Condenser pressure
- Condenser approach
- Compressor power
- Condenser water pressure drop
- Pump status
- Valve positions
- Purge operation
Step 3: Compare it fairly
Compare the current reading with design information, startup data, or previous readings taken under similar load and water conditions.
Do not compare a lightly loaded chiller on a cool morning with the same chiller near full load on a hot afternoon.
Step 4: Separate the tower from the condenser
Determine whether the chiller is receiving water that is too warm or whether heat transfer inside the condenser has changed.
Step 5: Verify condenser water flow
Check:
- Pump speed
- Valve positions
- Strainers
- Condenser pressure drop
- Water temperature difference
- Design data
Step 6: Inspect the condenser when the evidence points there
Then evaluate:
- Tube condition
- Waterbox gaskets
- Divider plates
- Plugged tubes
- Internal water distribution
Step 7: Verify the result after the repair
After cleaning, adjusting, or repairing the system, collect another reading under similar operating conditions.
The repair being completed does not prove that the condition improved.
The follow-up reading does.
The most important clue is how the number changed
A slowly increasing approach points more strongly toward tube fouling.
A sudden increase points more strongly toward a sensor, pressure transducer, flow change, pump problem, valve position, purge problem, or refrigerant-control issue.
A high approach that remains after tube cleaning tells you the tubes were not the entire story.
The number matters.
How the number got there matters more.
Calculate condenser approach
The calculator gives you the difference between refrigerant condensing temperature and leaving condenser water temperature.
Calculate Condenser Approach →
One reading can show that something deserves attention.
Comparable readings show whether the chiller actually changed.
Before you call for tube cleaning, verify the number, check the flow, and ask whether the change happened gradually or suddenly.
That one question can keep you from fixing the wrong problem.