What is Surge in Centrifugal Compressor

Summary

In this informative video, Holden Schamburger from Chiller Academy and HVAC Time explains the concept of centrifugal surge in chillers, a potentially damaging phenomenon that can cause significant issues if not addressed. He clarifies the difference between centrifugal and positive displacement compressors, describes the process of refrigerant compression, and details the causes and consequences of surge conditions, including high lift scenarios and the importance of preventing compressor damage. The video aims to educate viewers on recognizing and managing surge to protect chiller systems.

Takeaways

• 🛠️ Surge condition in a centrifugal chiller is a critical issue that can cause significant damage if not addressed promptly.
• 🔍 Understanding the difference between centrifugal and positive displacement compressors is crucial to grasping how surge conditions occur.
• 🌀 Centrifugal impellers work on the principle of velocity, using kinetic energy to compress refrigerant and generate pressure, unlike direct compression in positive displacement compressors.
• 💡 Surge is characterized by a reversal of refrigerant flow through the impeller, which can lead to axial thrust issues and overheating, potentially destroying the compressor.
• 🔧 High lift, the difference between condenser and evaporator pressures, is a primary cause of surge conditions, indicating the compressor's struggle to move refrigerant.
• 📈 Factors contributing to high lift include issues with the condenser, such as dirty tubes or high condenser water temperatures, and low load on the evaporator.
• 🚫 Stalls, which precede surges, are an early warning sign of potential surge conditions and should not be ignored as they indicate the compressor's inability to maintain normal operation.
• 📊 The range of 50 to 60 PSID is where compressors typically start to stall, with 65 to 80 PSID being the surge territory, indicating the severity of the lift condition.
• 🛑 It is essential to address surge conditions to prevent long-term damage to the compressor, as extended surging can lead to catastrophic failure.
• 🏭 Different compressor designs, such as VGD, multi-stage, or single stage, have varying capacities to manage lift values and may influence how surge conditions are handled.
• 📚 For further education on chiller systems, designs, and practices, the Chiller Academy offers resources to deepen understanding and improve industry knowledge.

Q & A

What is a surge condition in the context of a centrifugal chiller?

A surge condition in a centrifugal chiller refers to a reversal of refrigerant flow through the impeller, which can cause significant damage to the compressor if not addressed.

Why can a surge condition be a scary occurrence for someone on site?

A surge condition can be scary because it is accompanied by an unusual noise from the compressor, which can be alarming, especially for those not familiar with the operation of centrifugal chillers.

What is the role of a centrifugal impeller in a chiller system?

The centrifugal impeller's role is to move the refrigerant and create a pumping action by converting the refrigerant's kinetic energy into pressure through the use of velocity.

How does a centrifugal impeller differ from a positive displacement compressor?

A centrifugal impeller does not physically compress refrigerant molecules as they pass through; instead, it uses velocity to move the refrigerant, unlike positive displacement compressors, which force refrigerant through the compressor to create compression.

What is the purpose of the diffuser in a centrifugal compressor?

The diffuser is designed to convert the high-velocity refrigerant's kinetic energy into pressure, which can be used in the system, and is part of the process that creates the discharge pressure.

What causes a surge condition in a centrifugal chiller?

A surge condition is caused by high lift, which is the difference between the condenser pressure and evaporator pressure, leading to a reversal of refrigerant flow back through the impeller.

What are some factors that can lead to high lift and potentially a surge condition?

Factors that can lead to high lift include issues with the condenser, such as dirty tubes or high condenser approach values, cooling tower issues causing high condenser water temperatures, or a very low load on the evaporator.

What are the symptoms of a stall condition in a centrifugal compressor?

Stall conditions, which precede surge, can be indicated by a high-frequency sound and are typically experienced when the lift is around 50 to 60 PSID or higher.

How can surge conditions be prevented or managed in centrifugal chillers?

Surge conditions can be managed by addressing high lift conditions, which may involve cleaning condenser tubes, optimizing cooling tower performance, or adjusting the load on the evaporator to reduce the pressure differential.

What is the potential damage caused by surge conditions to a centrifugal chiller?

Surge conditions can cause axial thrust issues, overheating, and in severe cases, the destruction of bearings and other components, potentially leading to the complete failure of the chiller.

What is the Chiller Academy and how can it help someone learn more about chiller systems?

The Chiller Academy is a resource where individuals can learn the fundamentals of the chiller industry, including different chiller systems, designs, and best practices, to become well-versed and fully understanding of the field.

Outlines

00:00

😨 Understanding Centrifugal Chiller Surge

This paragraph introduces the concept of surge in centrifugal chillers, a potentially alarming event for those unfamiliar with this type of compressor. Holden Schamburger, from Chiller Academy and HVAC Time, explains that surge is not like the operation of typical positive displacement compressors and emphasizes the importance of not getting hung up on technical terms. He provides a brief overview of how a centrifugal impeller works, comparing it to a water pump and a blower motor, using velocity to compress refrigerant and create pressure. The paragraph concludes with an introduction to the surge condition, which is a reversal of refrigerant flow that can cause significant damage to the compressor, including axial thrust issues and overheating.

05:00

🔍 Causes and Consequences of Surge in Centrifugal Chillers

The second paragraph delves into the causes of surge in centrifugal chillers, which is linked to high lift conditions resulting from the difference between condenser and evaporator pressures. High lift can be caused by issues such as poor heat exchange in the condenser, high condenser water temperature, or low load on the evaporator. The paragraph explains how these conditions can lead to the compressor's inability to move refrigerant, resulting in stalling and eventually surging. Stalls are described as a precursor to surge, with the latter being a more severe and damaging state. The importance of addressing surging conditions promptly is stressed, as allowing a chiller to surge for an extended period can lead to catastrophic damage. The paragraph concludes with a reminder to check out Chiller Academy for more training and to prioritize family time.

Keywords

💡Surge condition

A surge condition in the context of a centrifugal chiller refers to an undesirable situation where the flow of refrigerant reverses, moving back towards the suction side instead of being compressed and discharged. This is a critical issue as it can lead to significant mechanical stress, overheating, and potential damage to the compressor. The video emphasizes the importance of understanding and addressing surge conditions to prevent equipment failure, as it can be 'quite terrifying' when experienced on-site.

💡Centrifugal chiller

A centrifugal chiller is a type of cooling system that uses a centrifugal compressor to increase the pressure and temperature of a refrigerant by imparting kinetic energy to it through a spinning impeller. The video script explains that these chillers operate differently from positive displacement compressors and that understanding their unique characteristics is crucial for proper maintenance and operation.

💡Impeller

The impeller is a key component in a centrifugal chiller, responsible for the movement of refrigerant by converting its kinetic energy into pressure. The script describes the impeller as similar to a water pump, using centrifugal force to 'sling' the refrigerant through the compressor, which is essential for creating the pumping action that drives the cooling process.

💡Positive displacement compressor

A positive displacement compressor is contrasted with a centrifugal compressor in the script. Unlike centrifugal compressors, positive displacement compressors physically compress refrigerant molecules as they are forced through the compressor, such as in scroll, reciprocating, or screw compressors. The video aims to clarify the differences in operation between these types of compressors.

💡Diffuser

The diffuser is a part of the centrifugal compressor that takes the high-velocity refrigerant from the impeller and converts its kinetic energy into pressure. The script explains that the diffuser is designed to manage the velocity and pressure of the refrigerant, which is a critical step in the compression process.

💡Lift

Lift, in the context of the video, refers to the difference between the condenser pressure and evaporator pressure, measured in PSID. High lift indicates that the impeller has to work harder to push the refrigerant through the system, which can lead to surge conditions if the pressure differential is too great.

💡Condenser

The condenser is a heat exchanger in the chiller system responsible for releasing heat from the refrigerant to the surrounding environment. Issues with the condenser, such as dirty tubes or high condenser water temperatures, can contribute to high lift and potentially cause a surge condition, as mentioned in the script.

💡Evaporator

The evaporator is another heat exchanger in the chiller system where the refrigerant absorbs heat from the space being cooled. A low load on the evaporator, as described in the script, can result in a large pressure differential with the condenser, leading to a high lift and the risk of surge.

💡Stall

A stall in a centrifugal compressor is a precursor to a surge condition, where the compressor begins to lose efficiency and the flow of refrigerant becomes unstable. The script mentions that stalls typically occur at a lift of 50 to 60 PSID and are an important indicator that surging may soon follow if conditions are not addressed.

💡Axial thrust

Axial thrust is the force that acts along the axis of the compressor, which can become an issue during a surge condition. The script warns that surge can cause significant axial thrust, potentially leading to the destruction of bearings and other mechanical failures within the compressor.

💡Suction superheat

Suction superheat refers to the temperature difference between the refrigerant entering the compressor and the saturation temperature corresponding to the pressure at the suction point. The script explains that normally, this is very low, helping to keep the compressor cool. However, during a surge, the discharge gas can backflow, causing the suction superheat to increase dramatically, which is detrimental to the compressor's operation.

Highlights

Understanding what a surge condition is for a centrifugal chiller, which can be a scary thing on site if unfamiliar.

Centrifugal chillers differ from regular positive displacement compressors, so don't get hung up on the terms.

A surge can be quite terrifying and cause confusion about what is happening and why that noise is being made.

Holden Schamburger from Chiller Academy and HVAC Time specializes in chillers and discusses centrifugal surge today.

Centrifugal impellers create a pumping action for refrigerant by using velocity, similar to a water pump or blower motor.

In a surge condition, refrigerant flow reverses, which is very bad for the compressor and can cause axial thrust issues.

High lift, the difference between condenser and evaporator pressure, can cause a surge condition.

Problems with the condenser, such as dirty tubes or high condenser approach values, can lead to high lift and surge.

Low load on the evaporator with high condenser water temperature can also create a high lift condition and surge.

Most centrifugal compressors start showing symptoms of stalling at around 50 to 60 PSID before surging.

Stalls precede the surge condition, so understanding stalls is important to prevent surge.

High lift conditions that overcome the compressor's ability to move refrigerant can lead to surging.

Surging creates a high-frequency swooshing sound that can be very terrifying, especially for those not used to it.

Surging is very bad for the compressor and can lead to its destruction if allowed to continue for a long time.

Different compressor designs, like VGD or multi-stage, can influence how lift value is managed and surge is prevented.

Between 60 to 80 PSID, you are in surging territory and need to tread carefully to prevent compressor damage.

ChillerAcademy.com offers training to learn the fundamentals of the chiller industry and develop expertise.

Make time for family, as they really need you, in addition to focusing on professional development.

Transcripts

So you will come out the other side of this  video understanding what a surge condition is  

for a centrifugal chiller. And this is kind of  a... it can be a scary thing when you're on site  

and if you're not working... used to working on  centrifugals and their differences and how they're  

not like a lot of regular positive displacement  compressors, don't get hung up on the terms,  

like it's okay. And I have talked about  these things in other videos. We'll...  

I'll recap it here in a minute. So if you're not familiar with all that  

and then you hear a surge happen and  it can be quite terrifying...  

well what do you do? What is it? What's even  happening? Why is that noise being made? Like  

that is an odd noise for a compressor to make.  Let's talk about that today. How you doing? I hope  

you're having a great day. I am Holden Schamburger  with Chiller Academy and HVAC time. I specialize  

in chillers. We're going to talk about centrifugal  surge today and with that let me grab my model  

real quick. So this is a centrifugal impeller.  This is what makes the refrigerant move and you  

know creates the pumping action. The... this  looks very similar to a water pump because what  

we're using the same theory and these centrifugal  impellers, I mean they... it's centrifugal theory.  

Same thing our blower motors also use to make air  move. So how we're doing that is through velocity.  

These are your veins of... this is where the  refrigerant comes in on your suction. This is your  

discharge side here and as that refrigerant moves  through we are able to create a compression. Now  

this is not a positive displacement. It's not a  direct compression. So a scroll compressor for an  

example or a re-sip or screws. Those are positive  displacement. The refrigerant molecules are being  

physically compressed as they get forced through  the compressor and that's how we create that. This  

is not doing that. This is actually creating  a... it's taking velocity. So essentially the  

refrigerant comes in and as this spins it gets  thrown through these louvers or these veins  

just like a blower motor would do, right? So it  gets thrown out and it just slings out into the  

diffuser section of the compressor and as it goes  flying we hit the diffuser and the diffuser is  

designed to take all that high velocity, you know,  kinetic energy and actually convert it into a  

pressure. So pressure we can work with so  that that pressure becomes your discharge  

pressure which is what your canister pressure  is. That's the base summary of the centrifugal.  

Now what's happening in a surge condition is, in  simple, we're reversing flow. So quite literally  

what's happening refrigerant is supposed to come  this way but because of our surge conditions and  

I'll get into that a little more in a second,  the refrigerant ends up reversing back through  

this into our suction side and a very bad thing  to happen. You get a lot of axial thrust issues  

because it's just it's a lot of force and  this will destroy your bearings. All of  

this will get hot at that point because a lot  of the cooling for this impeller in the system  

comes from that suction gas and it's used to  seeing, you know, suction superheats of say  

half a degree to one or two degrees. Very, very  low suction superheats and that helps keep all  

of this cool and not to overheat. Well if all  of a sudden our discharge gas is back flowing,  

that suction superheat, it turns into discharge  superheat. Now quite literally like it's it's  

very dramatic event and it's bad. That is  that will just there are plenty of chillers,  

plenty of centrifugals who have utterly ripped  themselves apart in their life because they were  

allowed to just constantly surge and surge and  surge and nobody ever did anything to address  

it. So that is what a surge is, a reversal of  refrigerant through the impeller. Now what causes  

that? Well it comes back to high lift. So lift  is your difference between condenser pressure and  

evaporator pressure and by doing that subtraction,  it's a PSID, we are able to determine  

how hard that impeller is having to push  refrigerant through it. What causes high  

lift? Well it depends. So you could have a high  lift condition if you have say something wrong  

with the condenser. You're not exchanging heat,  dirty tubes, whatever. Even if you've got proper  

condenser water, you could be running really  elevated condenser approach values. So whole  

separate videos I've done on condenser approach.  I highly recommend you go check that out. But  

if your condenser approach starts running  high, then that could push you into a surge  

condition. If you say you're having a cooling  tower issue and your condenser water begins  

to run really high and you can't keep it down,  that's going to push you into a surge condition.  

Or let's say you have a really low load on  the evaporator. Say we've only got three,  

maybe four degrees of difference from entering  to leaving water. And we're making set point,  

but we are, we're not able to get the condenser  water temperature down. So let's say you've got  

that low of a load going through the evaporator,  but you've still got you know 85 to 80 degrees of  

condenser water and you can't back that off for  whatever your reasons are. A lot of valid ones,  

so don't take that the wrong way. But anyway, it  creates a lot of lift. That difference between  

those two things stays really high and that lift  will overcome what the impeller is able to force  

through it. Most centrifugal compressors, they  usually start to show some symptoms. You know,  

get into stalling at around 50 to 60 PSID. Now if  you don't know what stalls are, I've done a whole  

video on stalls. I actually should have posted  yesterday, so quite literally the video before  

this if you go to my feed, whatever else. Go  check that out if you need to know what stall. So  

stalls prelude or precede, precede, maybe I don't  have that. I guess I don't realize what prelude,  

prelude, prelude. It doesn't matter. They precede  the surge, the surge condition. So it's important  

to understand what the stall is and for the same  reasons we get into a stall state, we get into a  

surge state. Just the surge is the extreme climax  of what happens. When we have those really high  

lift conditions, we overcome the compressor's  ability to continue moving that refrigerant.  

And it creates this really high pitch or higher  frequency. It's not always high pitch, but it's  

really high frequency just swooshing sound that  is, it can be very terrifying, especially if  

you're not used to dealing with that condition.  And it is very bad for the compressor. I cannot  

stress that enough. We've gotten much better with  our compressor designs and their ability to take  

extended surging, but gee whiz, please don't let  that continue for a very long time. That is the  

fastest way to terra-pus centrifugal is allow  it to just surge itself into oblivion. So while  

we may start dealing with stall conditions at that  50 to 60 going into 65, the higher that lift gets,  

the more the stalling becomes prevalent. And  eventually, once you start hitting 65 to 80 PSID,  

you're flat out in surging territory.  And it depends on your compressor,  

whether or not you have a VGD video coming, or  whether or not you are a multi-stage, like a CVH,  

where you've got two stages or three stages or  a turbo core has got a couple of stages.  

Or whether you're a single stage. So there's  variables and design on how we can increase or  

manage that lift value. But ultimately, they're  all kind of about the same ballpark. Essentially,  

between 60 to 80 PSID, you're looking, you're in  the middle of surging territory and you need to  

tread very carefully, very lightly. So just bear  that in mind as you're moving forward working on  

these chiller's and just understanding how they  function, what they're doing, and why they're  

doing it. With that, if this was any great benefit  to you and if you really enjoyed this training,  

go check out chilleracademy.com. It's a place  where you can come to and learn the fundamentals  

of the chiller industry and developing yourself  and come out the other side well-versed and fully  

understanding exactly what the different  chiller systems, designs, fundamentals,  

and practices are that we need to follow.  With that, MTT. Make the time for your family,  

for your spouse, for your kids. They  really need you. I'll see you around.