Episode Description:
Attention building automation professionals! Are you ready to deepen your expertise in hydronic systems? Episode 477 of the Smart Buildings Academy Podcast is here to guide you through the intricate world of water-cooled chilled water plants, cooling towers, and control strategies that drive system efficiency. If you’ve ever grappled with terms like "range," "approach," or "wet bulb," this episode is your ticket to clarity. It’s designed to provide actionable insights without overwhelming you with technical jargon, making it a must-listen for anyone in the field.
Key Topics Covered:
- Unpacking the components and control strategies of water-cooled chillers.
- The relationship between wet bulb, range, and approach—and why it matters.
- Essential considerations for effective cooling tower control.
- Understanding bypass valves, dual bypass setups, and low-temperature control.
- The role of partial load chillers in optimizing system efficiency.
Why It Matters:
Whether you're troubleshooting a chilled water system, optimizing performance, or designing your next sequence, this episode equips you with the concepts to elevate your projects. Dive in, connect the dots, and take the next step in mastering HVAC control sequences.
Click here to download or listen to this episode now.
Resources mentioned in this episode
- Free Technical Skill Assessments
- Smart Buildings Academy Workforce Development Program
- Smart Buildings Academy Courses
- Control Sequence Fundamentals HVAC Course
Podcast Video
Transcript
Phil Zito 0:00
This is the smart buildings Academy podcast with Phil Zito. Episode 477 Hey folks. Phil Zito, here, welcome to episode 477 of the smart buildings Academy podcast. In this episode, we are going to continue our series on hydronic systems and ultimately our series on HVAC. So we've got two more episodes. This one, we're going to be talking about chilled water plants. We're gonna be talking about water cooled chilled water plants. We're gonna be talking about cooling towers. So we're gonna dive into that. And then next episode, which will be 478, we will be wrapping up with pumping and piping. All right, so if you're trying to figure out how hydronic systems work. This is going to be the episode for you. As always, everything's available at podcast that's smart buildings. Academy.com forward slash 477 once again, that's podcast that's smart buildings. Academy.com forward slash 477 and if you're looking for all of the episodes in the series, just go to podcast smart buildings. Academy.com I do have a quick favor to ask of you. We provide these podcast episodes free every week. If you are finding them valuable, it literally takes a couple seconds of your time. If you could go on Spotify or on Apple podcast and leave us a five star review, it greatly impacts the amount of people we reach. If you're watching this on YouTube and you haven't subscribed, please consider hitting subscribe. Please Like and comment that helps the algorithm spread this to more people, and if you're on LinkedIn, please consider hitting the Share button or the Repost button, depending on your LinkedIn app, and sharing this with your network. That would mean the world to us. It really helps us spread this information that we're sharing freely every week. All right, with that being said, let's dive into the episode. So water cooled chillers. We talked about air cooled chillers in the previous episode. Now, what is the big difference between water cooled and air cooled chillers? You probably guessed it. They are cooled by water. Now, what does this exactly mean? And when you look at a condenser system, cooling tower system, the whole water cooled side of a chiller, it can be a little intimidating, because there's so many different moving parts. But don't worry, we're going to talk through all those parts and pieces. We're also going to go through a lot of different terms that you've probably heard utilized before, but you maybe didn't understand exactly what they meant. So the parts and pieces of the water cooled side of the chiller, also known as the condenser side of the chiller, are the cooling tower and its associated fans and valves and any low temperature control devices. So our primary focus as we move through the beginning of this episode is going to be on this control, or cooling tower control, bypass valve control and low temperature control conditions. So cooling tower control, when it comes to can really controlling your cooling tower the main objective, most of the cases, or most of the time, is to maintain a set point of 85 degrees using 95 degree water. This is a very common sequence. Now I know I'm going to get some of you who are going to email me and be like but my cooling tower could take lower or my chiller could take lower condenser water, my cooling tower doesn't operate at this realize there's ambient conditions, realize there's types of chillers. So don't get so hard set on these numbers. I really want you here to be grasping the concepts and not so much focus on the numbers. So for us, the numbers we're going to work with are 85 degree condenser water temperature set point using 95 degree water entering the cooling tower and 78 degree wet bulb conditions. So this would result in a 10 degree range, which is the difference between leaving and entering condenser water, and a seven degree approach. Now for some of you, you're going to be like, what is range, what is approach? What does mean? I've read these in sequences. I've heard these talked about before. I've heard about lowering range, like in the last episode, we talked about lift, and now we're talking about range and efficiency effects of range. What does all that mean? Okay, so first off, when the chiller supplies its condenser water to the tower, in our case, once again, I'm trying to avoid a bunch of emails from folks saying, but Phil, my chiller does 82 degrees or 93 or whatever. This is just a scenario here to to express a concept. All right, so with that being said, In our scenario, the condenser water being supplied to the tower is 60 or is 95 degrees. After running through the tower, we're. Bypassing it, depending on the water temperature, the water should return to the chiller around 85 degrees. So the difference between the entering and leaving condenser water is 95 minus 85 which is 10. So that is the range. It is simply the difference between entering and leaving condenser water. So you want to understand your range that way you are realizing kind of, hey, what can I do as far as relieving BTUs via the cooling tower from the primary chilled water loop? So we're taking BTUs and we're transferring them from the evaporative side the chiller to the condenser, side the chiller, and then out the atmosphere, very cooling towers. So what is this whole approach thing? Well, if you remember earlier in the podcast series, is why I talked about it being important to listen to all those episodes before we talked about psychrometrics, we talked about dry bulb and wet bulb and where dry bulb is our traditional temperature, wet bulb is actually the lowest temperature that can be reached through evaporative cooling when air is not dry. So wet bulb is actually temperature, but it's also considering the heat effect of moisture within the air, because, remember, we need to remove moisture from the air because that has actual heat in it. And here's the thing, with cooling towers, we're using, this thing called the evaporative effect, where we're actually evaporating the moisture, thus, through that phase change, causing BTUs to transfer from the water to the air, and thus cooling down the leaving water temperature. So the the water temperature that's leaving the tower All right now approach is quite simply the difference between leaving condenser water temperature and the wet bulb. So when we say we want a seven degree approach from a specific temperature, we're saying that we want the wet bulb to be seven degrees below that temperature. So if we're saying we want 85 degrees of our leaving water temperature, then we would be able to effectively do that as long as wet bulb is what's 85 minus seven, what that's 78 so long as the wet bulb is 78 degrees or lower. Now this matters quite a bit, because if the wet bulb, as we know it to be, is the lowest temperature that can be reached through evaporative cooling when air is not dry, then we need the temperature, that wet bulb temperature to be low enough to support evaporative cooling, because, after all, that's how cooling towers cool. So this is why, in really humid environments, you'll see that cooling towers aren't necessarily as effective. And in really dry environments where there's, you know, almost no humidity, cooling towers can be quite effective, because they can transfer so much moisture and that heat energy out into the atmosphere.
Phil Zito 8:10
So what does all this you know, dry bulb, wet bulb, approach range. What does all this mean to you? And why does it matter to you? Well, if you're controlling a cooling tower, you want to maintain as low a condenser water as your chiller can accept. This is where I was trying to stave off all those emails where you folks were gonna be like, hey, but my chiller can do X, Y, Z, condenser water temperature. Yes, you want to maintain as low a condenser water temperature as your chiller can accept. And the reason for this as we saw from the previous episode, with that concept of lift, is that the lower the condenser water temperature, the less your compressor has to work, thus more efficiency. Now, I mean, granted, there's all sorts of concepts, like, can your refrigerant tolerate those temperatures? Can your chiller work with this? There's a whole slew of things that you need to know from a mechanical perspective related to your chillers design, but the concepts remain the same in that we want to have a lower condenser water temperature, to have a lower lift, to have greater efficiency, to run the compressor not as hard. And also we have to realize that in order to do all of this. In order to do any of this, we have to have our cooling tower right that leaving water temperature has to be able to work within the wet bulb that we have. So with all that said, if the air cannot support proper evaporative cooling due to wet bulb temperature, then you're not going to be able to reach you're leaving temperature set point, right? It's pretty I'm hoping you all understand this by now, because if you don't, if it's not clicking, I really want you to dig into psychrometrics. Really understand wet bulb dry bulb. Understand the moisture content in. Air, and how that is affected by dry bulb temperature, how you know, you can have more grains of moisture in air as temperature gets hotter, and what that has to do with BTUs and heat and phase changes, all of that, really explore that, because if you don't understand those concepts, you can really spend a lot of time troubleshooting issues that are design and ambient condition issues that you cannot fix. There is nothing you can do. From a controls perspective. I mean, I remember I got asked to go work on a central plant in Louisiana, and they were having issues during the summer in Louisiana, like the city I grew up in, in Houston. In Houston, Texas is a very humid climate, and because of this, the approach did not let us optimally use that chiller. Thus, when it hit design day conditions, there were issues, and they were screaming its controls issue. But after showing them a psychrometric chart and explaining how all this stuff works, we're able to get off the job, because otherwise we would have been stuck there forever, troubleshooting an issue that we could not have fixed. I really hope that's sinking in All right, so continuing on. So how do we do the control? I mean, I'm not going to spend a whole lot of time on the evaporative side of chillers. We will be talking in the next episode about pumping and piping and all that jazz. So I'm primarily going to be focused on the condenser side. When we're dealing with Tower Control, there's really three steps right when the condenser water is at or below set point, you can bypass the tower using a bypass valve and disable the fans. Potentially, this means you literally just cycle the water in the loop and bypass the tower. You can also flow through the basin. So that is as the temperature rises, you'll typically adjust the bypass to flow through the tower, not over the top, but through the tower basin, and then from there, as temperature continues to increase, you can turn on the fans or send water over the tower. So basically, the water is raining down, and then the fans will draw through the air, which will cause evaporation to occur. So in in that scenario, as I just described, temperatures at set point, we're just flowing through the loop temperatures a little bit above set point. We're just bringing air across the surface of the water in the basin. We're flowing through the tower on the basin and causing evaporative effect across the surface of the water, still not really doing what we need, then we start bringing water over the tower, and we start evaporating water as it falls, thus making even more time where we're bringing air flow over the surface of the water as it's falling through. So that is essentially how we do cooling tower control. Pretty straightforward, pretty simple. You can have cooling towers that are on you can have ones that are driven by VSDs and the fans adjust. You can have cooling towers that flow through the top or flow through the sides. There's a variety of different cooling tower architectures, but these main concepts remain the same across cooling towers. So as I mentioned, bypass control is you're either going to bypass the tower completely or simply bypass the top of the tower and run through the basin. So there's really that's something I wanted to point out, because when you're dealing with bypass control, oftentimes you'll read bypass the tower in the sequence. But you need to clarify, am I bypassing the tower through the basin? Am I bypassing the tower completely? And based on understanding the sequencing, you'll be able to start to see where bypass valves are placed. And you'll I've seen several times where the bypass valve was improperly installed, and because of that, could not effectively bypass through the basin, and was just purely bypassing the tower. So you need to understand this in order to make sure that things are properly installed. Now, in addition to bypass right, you can actually have dual bypasses. So I mentioned that you can either bypass the tower or bypass the space, and there are scenarios where you can actually do both, where you can bypass the basin and completely bypass the tower. Now this is less common as it tends to leave to water starvation issues, potentially, especially if there's not proper water balancing and makeup and level switches and all of that jazz. But it is a possible scenario, So be cognizant of that. And finally, let's talk about low temperature control. So in a low temperature control scenario, we have really three things that happen. One, the bypass valve can be 100% bypass when the outdoor air drops below a specific temperature. That's That's one, two. What we can do is actually bypass the condensing water. Basically will start to bypass the tower, and we'll start to turn off the fans. So that is a scenario where it's not so much outdoor air temperatures driving it. It's just the condenser water is dropping too low. So you have these kind of low limit scenarios, one being based on ambient, and the other being based on maybe low load conditions in the building. It's night time, something like that, and you're starting to drop the condenser water temperature too low, lower than you want it to be. But it's not being driven off ambient. It's actually being driven off the low condenser water temperature. So you start to do the sequence a little bit different, whereas you may not bypass the tower, you may and you may just turn off the fans, because, remember, fans are going to be driving that evaporative effect. And sometimes turning off the fans is enough if you have a process load in the building to actually go and heat up the condenser water all right, and then low temperature control, as I mentioned, you're going to have the bypass valve bypassing the tower, and you're also potentially going to have this thing called Heat trace, which is electrical wiring, and it is going to go and allow you to essentially heat that piping that's exposed to ambience, so that you don't go and ice the water that is actually in that condenser loop. Now there's a final other thing, which is heat exchangers. Sometimes in certain conditions where you can have a persistent process load, you're gonna go and have a heat exchanger. This will be a plate heat exchanger, and it will either plate itself to ambient, or it will plate itself to domestic hot water. And you will be transferring the heat from the condenser water to, you know, maybe ambient within the mechanical room, or to the domestic hot water as an additional heat source. So you've got a couple different ways of exhausting heat from the condenser side in low load conditions. All right, so let's take a look at the sequence of operations. Then we'll talk through multi chiller control, and then we'll wrap this up, and we'll be almost done with this HVAC series, which, quite honestly, I'm ready to be done with. I'm ready to talk about something else. So in this case, we have a chilled water or a chiller that is using water cooling. And what's going to happen? I'm going to skip through this sequence all the way to the condenser side. So let me go down here to the condenser side and keep scrolling, scrolling, scrolling. So cooling towers will operate with their associated condenser water pumps, and they will run based on if they are the lead tower, and when they're the lead tower, they are going to have a reset schedule that is going to be driving their set point
Phil Zito 18:12
is going to be the outside air wet bulb, plus seven degrees. So they're going to look at wet bulb and drive their entering chilled water set point. So basically, the chilled water set point that's going to be entering the chiller is going to be driven based off a reset schedule with wet bulb. Hence why I taught you about wet bulb so far. If this sequence is or this temperature set point is not being able to make then they are going to gradually begin to stage on cooling towers, and the bypass valves are going to close, the normally closed cooling tower. Valves gonna open, and water is going to start going over the tower, and then the fans going to start to increase, right? So we're seeing we were kind of bypassing and now we're not. Now we're going over the tower, and now we're ramping up our fans to take advantage of that evaporative effect. This is a common sequence. I would say. The part that's not common is resetting the set point based off outdoor wet pulp. I don't see that as much. I normally see a static condenser water set point, but the rest of this, how the staging works is pretty much how things will happen. All right, so let's shift real quick to lead lag and partial load chillers. So we know lead lag from covering hot water systems, so I'm not going to beat that to death. But what we are going to look at is partial load systems. Why would we want these? Well, partial load systems are independent, typically, of the lead lag sequence, and they exist typically to fill two scenarios when the load is lower than a range that a single primary chiller could efficiently handle. I really want to emphasize. Size, the word efficiently because I mean the primary chiller can handle that load. It's just not efficient. There's an efficiency curve, and in which case we bring on a smaller chiller that has a different efficiency curve to handle that lower loads. Now there's also a reason we may bring on a secondary chiller, which would be when the load is higher than all the primary chillers can handle. The key point I want you to take away is that partial load systems are independent of the primary system, so they do not operate as part of the lead lag cycle. They are often driven based off of a load percentage, and looking at both of the primary chillers being on, this is the case when a tertiary or partial load chiller would be brought on, because load is not being met by the primary chillers, or the primary chillers are at A low load scenario and it's inefficient to run them. So I really want you to understand that pretty straightforward, pretty easy sequence. If you've got the chilled water sequence and you're running a primary or you're running a water cooled chiller sequence with lead lag, and you're able to figure that out and get that running, then running a partial load chilled water sequence is going to be pretty straightforward as well. So let's read through this here. All right, so the BAS lead lag selector is going to allow any of the chillers to act as lead machine. I already don't like this because the sequence expressed earlier in the sequence that there was a partial load chiller that would not act as the lead machine. So I would fix that. But whatever. That's just me being nuanced. The chiller is going to be enabled to operate in auto when it's scheduled on by the BAS, and that is when outdoor air temp is above 55 degrees. There's no active alarms, right? The cooling tower, sump temperature is above its minimum. That's that low condenser water temperature control we talked about earlier, and there's a call for cooling, and the chilled water and condenser water flows are verified. So everything we've talked about up to this point, right? We have flow. We've got calls for cooling. Condenser water temperature is not too low, ambient is not too low. Chillers turned on. And in this case, chilled water pumps are going to start to turn on. Valves are going to start to open, actually, not in that order, the valves are going to open, then the pumps are going to turn on, they're going to prove flow, and then the cooling towers and the chillers are going to start to run lead chillers going to start it's going to start to open its veins to allow frigid to flow through. This is where you have variable vein control and chillers, which we're not really going to get into here, and we're not also going to talk about VSD control of chillers and compressors, just realize that you can actually have chillers that can control themselves variably, rather than just simply on off sequencing. So the chiller is going to maintain its temperature, which is going to range based on a reset schedule, and it's going to modulate the chiller capacity, so it's going to range between 42 to 48 degrees. And this reset schedule, although it is not clear what the reset schedule is based on, in this sequence, which is kind of weird that it doesn't tell you what it's based on, I would assume it's based on either a load calculation in the secondary loop, which is basically building load, or it's looking at average age, you valve positions. Those are various reset parameters that would drive a primary chilled water temp reset. So then from here we're going to drive the chilled water, and it is going to then go and the chiller is going to maintain the chilled water temperature. From here, though, if we do not maintain our chilled water temperature, and we are starting to have issues where we are having all our valves open up, temperature is just driving above our set point, we're going to start to stage on other chillers. How this is determined is it's looking at 95% load on the lead chiller and looking at secondary flow exceeding primary flow, which means that there is most likely a decoupler in the sequence, which we're not going to talk about decouplers In this episode, we will talk about them in next episode. But essentially, a decoupler is a way of changing the direction of flow by either having the primary or. Secondary pumps overpower and draw water through based on their needs. So with that being said, when both of these scenarios occur, the secondary flow is exceeding the primary flow, and the lead chiller is 95% loaded, then a second chiller will stage on. Now here's the deal. If we continue in this scenario for a long enough time, we're gonna and we continue to be in a scenario where we are not meeting the secondary building load with our two chillers. This is where the third chiller would stage on and it would stage on as a partial load chiller. Now there's also the reverse, which is if we drop below our FL a rating, our percent loaded rating, or fully loaded amps rating, and our primary pump flow is above secondary flow for a certain amount of time, and we stage down using that sequence, and we stage down till there only being one primary chiller running. The next step after that is to stage on our alternate partial load chiller, which will run accordingly. And then after that, if we continue to have a no load scenario, we can stage the chilled water plant off. All right, everybody, I hope you found this episode valuable. I really hope you took a lot out of it. This is critical. So there's three kind of pillars of building, automation, electrical, it and HVAC that you need to know to really be able to go on your journey with building automation. And I find people tend to be lacking in all three. We've had our skill assessment running for several years now. Had 24,000 people go through it, it and HVAC are usually our lowest score, our lowest scored sections for our skill assessment, and that is because people just aren't being taught how these systems work, and that's what we're trying to cover in these episodes. Like I mentioned earlier in the episode, if you haven't heard the whole series, you can go to podcast at smart buildings academy.com forward slash, actually. Forward slash, nothing, just podcast at smart buildings academy.com and you'll be able to see the series If, however, you have listened to the series and you just want to dive more into maybe a skill assessment for your team, figure out the gaps, maybe some training services, maybe some of our webinars or free mini courses. Simply go to podcasts at smart buildings academy.com forward, slash 477, once, again, that's podcast at smart buildings. Academy.com forward slash 477
Phil Zito 27:47
and once again, if you find this valuable, I really would appreciate it. If you'd go to Apple podcasts Spotify, leave us a five star review if you're on LinkedIn, share or comment this with your network, and if you're on YouTube watching us, please consider hitting the like the subscribe button and commenting. It really helps. Really appreciate that. Thanks so much everybody, and I hope you all have an awesome week. Take care.
