By Phil Zito on Mar 29, 2021 10:29:18 AM
Topics: hydronic systems chilled water systems
Hey folks, Phil Zito here and welcome back! In this week's post, we will be talking about chilled water systems. We're continuing our series on HVAC Control and we have about two more posts left. So, this week, we're going to cover chilled water systems, how they work, how they function, and then we'll look at air-cooled and water-cooled. We'll also look at cooling towers, partial load chillers, and lead-lag.
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In last week's post, we covered hot water systems. Our primary focus was using some form of heat generation to transfer BTU's into the water stream so that the water could deliver those BTU's to the air stream within the building. With chilled water systems, we're looking at removing BTU's from the air stream within building.
We'll cover how we do that both with air-cooled chillers as well as with water-cooled chillers. You may be asking yourself, "What is new with chilled water systems?" Well, for starters, there's new terminology that we need to understand such as condenser side versus evaporator side, load versus lift, tonnage and full load amps.
These are all terms that you will hear utilized quite a bit as you work on chilled water systems both in their operation, as well as when you're working with mechanics during startup. So, you really want to understand these concepts and we will come back to those as we move through this post.
Now, good news, what we covered in our hot water post really applies to chilled water as well. You may be wondering, how does adding heat relate to chillers? Well, think about it. We're adding heat to hot water and then transferring that heat into a building. With chilled water, we're just doing the same thing, but reverse.
We're taking heat out of the building via chilled water coils that exist in air streams, typically on mixed air single path units, and occasionally, on outdoor air units where we're removing heat from an incoming outdoor air stream. We're removing heat either before it enters the building or removing it from the building as we recycle the air.
We need to focus in on that concept of heat removal, but we also need to focus in on the refrigeration cycle. The reason being because the refrigeration cycle is how chillers work. Chillers are essentially, giant compressors and evaporators using refrigerant to take advantage of different states of liquid to absorb heat and then transfer heat.
I have talked about the refrigeration cycle a bit in some of the past posts, and this is a little bit difficult to explain in a post because it's really something that is easier to understand graphically. With so many of us being visual learners, it would be easier if you had a resource to refer to.
So, visit Episode 188 to find the image that you can follow as we talk through this.
The Refrigeration Cycle
So, we have a refrigeration cycle for an air handling unit. You have an indoor unit, and you have heat going into it condensing the refrigerant. The refrigerant then goes into the expansion valve, then to the outdoor unit to the evaporator, and then back into the compressor.
What I really want you to understand is that on a chiller, there are two sides of the chiller: the condenser side and the evaporative side. The evaporative side is typically known as the chilled water side of the chiller, where the water is being chilled, and the heat is being absorbed. The condenser side is where we're removing that heat and transferring it outside.
It's important for us to understand this because once we start to learn about lift, about the difference between condenser, chilled water, and deltas, it will help us really understand why we control our chillers the way we do. It will help understand how efficiency works, why we may bring on one big chiller and a little chiller, or two big chillers. It'll help understand what we do when we have low outdoor air (low ambient), and we're trying to run a cooling tower.
Realize that we're utilizing refrigerant and we are transferring heat in the form of BTU's from that refrigerant, to another water source. So, in the case where we have a chilled water source, it is transferring the BTU's to the refrigerant, and then the refrigerant is transferring those BTU's back to the condenser water source. This then goes out to cooling towers which transfers the BTU's to the ambient air using evaporation.
With all of that said, how does this apply to chillers?
Well, you have a chilled water side that is absorbing BTU's and you have a condenser side that is exhausting BTU's. You need to understand that, and then understand how cooling towers work, because not all chillers are air-cooled. Some are water-cooled by using a cooling tower, and then we transfer those BTU's via the condenser water to the tower which then uses the evaporative effect.
As long as you understand that concept, then controlling chillers is not bad. It's not too difficult. So, let's talk about chillers and then we will dive through some sequences.
Chiller control Concepts
There are two types of chillers: air-cooled and water-cooled. Air-cooled chillers have a condenser coil and some fans, and use the evaporative effect for heat removal. The outside air absorbs the heat to the atmosphere directly from the chiller, and you can get away with this for some low tonnage chillers. However, when you get above like 150 tons, you start to not be able to exhaust enough of the heat from that air-cooled chiller and you have to start to use water-cooled chillers and cooling towers.
The reason for this is just the heat removal requirements and, as well as the way in which you process the refrigerant. Those have ramifications on the sizing tonnage, and tonnage is just simply a rating for the BTU removal capability of a chiller.
With air-cooled chillers, we need to be cognizant of two big things. Have you ever walked out to an air-cooled chiller, and seen it tucked behind a building, southern-exposed, with the sun beating down on it, and it's wrapped in a 12-foot fence with no ventilation?
Did you stop and wonder why that chiller is not operating as efficiently as one that has a chain link fence and is northern exposed?
Solar gain has significant effects because it will heat the machine and the ground around the machine, as well as the ambient air around the machine. When you couple that with a tightly, non-ventilated fence, where there's no airflow, and you've got a recipe for disaster. Ambient is going to heat up, so you'll want to be aware of location.
I have realized as a BAS professional, there's not too much you could do about the location of an air-cooled chiller. What we can do, however, is we can be aware of these effects so that if our chiller is not performing optimally during design-day conditions, and we get in a room with an engineer, we can understand and explain how environmental effects could be impacting the performance of that air-cooled chiller.
Water-cooled chillers work by discharging heat from refrigerant into condenser water. The condenser water is pumped out to cooling towers and these cooling towers have fans with spray nozzles. They utilize the effect of drawing air across the surface of the water or using these spray nozzles to create a mist, and then draw the water through the mist which creates evaporation, causing BTU's to be transferred into the air stream.
This is one of our main ways of removing heat from buildings with large centrifugal chillers that are water-cooled and are using big cooling towers.
With Air Cooled Chillers building automation system will usually control a primary pump, isolation valve, an air-cooled chiller enable command, and potentially a temperature setpoint.
When we're working with water-cooled chillers, we're usually also controlling cooling tower fans, cooling towers, bypass valves, spray, VSD's, heat trace, we're adding levels, etc. All of these new variables need to be considered in our programming.
With cooling towers, there are primarily three ways that we could remove heat from our condenser water with flow through cooling towers being the main one. The three ways we can remove heat are:
Flow-through is the airflow going across the surface of the water as it flows through the tower.
Flow-over is where a valve opens and allows the water to flow from the top of the tower and drip or spray down, and the fan will use the evaporative effect.
Then in low ambient conditions, there is a plate heat exchanger within a building, and often you can use this as an auxiliary form of heat.
Cooling tower fans exist to bring the air through the cooling tower. Bypass valves will allow us to utilize flow through the tower or flow over the tower. And then heat trace is an electrical heat that exists on the condenser piping that's exposed to ambient and will keep the piping from freezing. Please note that it's a hedge against freezing; it's not necessarily going to keep things from freezing.
Let's talk about lift. Lift is the difference between condenser refrigerant pressure and evaporator refrigerant pressure. Why does this matter? Well, in an ideal world, the lower the lift, the less the compressor has to work. So, if there is less of a difference between condenser and evaporative pressure, then it's not going to have to compress as much which, as it compresses it uses energy, and energy costs money.
To be efficient, we keep this lift low. To do that, you want to have the lowest entering chilled water temperature, also known as chilled water return temperature. By having that be lower, the compressor doesn't have to transfer as much heat from the water to cool the leaving water.
Now, let's cover load. This is something you definitely need to understand because this is our staging parameter, typically for switching and bringing on a secondary chiller. Load is a percentage representation of the performance of the chiller.
Load can be calculated manually, but most of the time, it's FLA (full load amps), or it's a tonnage calculation based on temperature and GPM. So, these calculations will then be fed to your building automation system, which then based on set points, will make determinations of bringing on the lag chiller to back up the lead chiller, or bringing on a pony chiller, etc. It's based on your sequencing.
Now, that is known as staging and typically our stage parameters are somewhere between 90-95% load or full load amps. You'll then bring on a second chiller, and then 30% or less is your stage down. That's our typical settings.
We'll use partial-load chillers, which is typically a smaller tonnage chiller, to handle these loads below 30%. If you look at an efficiency curve with a centrifugal chiller, and you look in that 30% or lower load requirement, you'll start to see that the efficiency drops pretty low. So, you will want to bring on an air-cooled chiller, or something similar, and that air-cooled chiller has a better efficiency curve for those lower tonnage requirements.
By running that, it's a way of saving energy and gives you a greater degree of control of granularity of control for temperature. This is so you're not removing a ton of BTU's from spaces and actually sub cooling areas, which can be very detrimental once you realize the effects of sub cooling on humidity and can actually dry out air too much.
Air Cooled Chiller Sequencing
So, that is our introduction to chillers. Now let's dive deeper into the air-cooled chiller, but first recap a few details. We want to understand what we're doing when we're controlling these, so I have a concept that I call the flow of the sequence.
What is the purpose of our chiller? Its purpose is to chill water, remove BTU's from the water stream and reject those BTU's either into the atmosphere or into the condenser side of your system. In order to do that, we have to make sure that we have water flowing, systems are enabled, and pumping is enabled.
So, I have this sequence that I've used that has been very beneficial for me and has helped me to ensure my chilled water sequences and programming operate optimally, and are easy to troubleshoot. It starts with system enable. We want to enable our chilled water system, usually based on ambient conditions or sometimes based on a call for cooling. This is just a system enable, this is not chiller enable or pump enable, its system enable. It simply allows our logic to start controlling to enable chilled water systems.
Now, if you've followed my posts for a while or you've been through our BAS Programming Bootcamp, then you know I'm a big fan of state-based control. I believe you should utilize state-based control and you should have logic that is commanded based on state. You should have a cooling-enabled state, a heating-enabled state, and your logic should do specific things based on the state it's in. This makes it easier to troubleshoot because you know what state you're in, you know what things should be doing, and it keeps things from just being intermixed and confusing.
So, our system enable is our state-enable for chilled water. From that point, we move on to valve control. This is where we enable our valve and then we go to pump control flow status, chiller-enable, and temperature control.
So, as I mentioned, system enable is typically brought about by either schedule, call for cooling or load, or outdoor air temperature. From there we have our valve control, which is optional, but it's not. What I mean by that is, some chillers will command their own valve, but I'm highly hesitant to let chillers command their own valves and pumps.
Once the chilled water system is enabled, the isolation valves on the chiller should be commanded open by your control system. That should be the first thing you do. From here, you want to turn on your pump, typically your primary pump.
Unlike boilers, you typically don't have circulation pumps with chillers. You typically have a primary pump, and a secondary pump. A secondary pump is hit or miss, but with air-cooled, you don't always have secondary pumps with air-cooled chillers.
Like hot water, there are various pumping setups, and you want to make sure that you're controlling based on how that works. Most of the time with air-cooled, you're only going to have primary pumps, usually constant volume.
From here, we have flow status. There are internal flow sensors that automatically enable the chiller or an external flow sensor that, once proven, will allow your control system to send an enable command to the chiller. I prefer the external flow sensor so that I know its proven status, and the chiller has its own flow switch. That's a secondary safety that I don't want to rely on, I want my primary, which is my flow switch or flow sensor, and it tells me when the pump is on.
From there, I have my isolation valves open, my pump is running, and flow is proven. Now there's three ways the chiller will enable:
It'll self-enable based on an internal flow switch.
It will be enabled via a set of contacts, typically a RIBU1C.
It will be enabled via a network command.
My personal approach is to use the RIBU1C and hardwire as it's easier to troubleshoot. You eliminate the variable of networks and you eliminate the variable of, "Is the controller inside the chiller working?" By having a RIBU1C and some dry contacts, you directly know whether or not you're commanding that chiller to enable.
From here, we move on to temperature control. Temperature control is pretty much the same for air-cooled as it is for water-cooled in that we either have an internal setpoint that's configured during startup, or via some touchscreen display, or we have a control voltage being set to a set of contacts via a remote BAS controller, typically 0-10 volts and doing a 10-degree reset. We could even have a network set point command, usually via BACnet or LAN card.
Now, as you know, I prefer hard-wired. When it comes to critical systems, and I have an option to hardwire, I'm going to utilize that option. Now that doesn't mean that you can't have a BACnet card for secondary monitoring, but for my primary source of control, I'd like it to be hardwired. I tend to find that from a troubleshooting perspective, as well as from just a peace of mind and stability perspective, hardwired wins the day.
So, from here we're going to move on to our air-cooled chiller sequence of operations, and in this sequence of operations, our air-cooled liquid chiller and chilled water pump make up this chilled water system. It's in operation year-round in this sequence because maybe they just want to have that chilled water, and the pump is running constantly.
Now, via some sort of chilled water flow switch, the chiller is enabled to operate. Via its own micro process controls, the chiller is going to operate to maintain leaving chilled water temperature of 55-degrees adjustable, which is really high. That tells me this is most likely dealing with some sort of process load because ideally, you would not want that controlling spaces that have tremendous heat loads. You would be looking more for 42-degrees.
Alright, so the chilled water leaving temperature 55 degrees adjustable, the pressure, then there's pressure control because this is a constant volume pump, and it talks about the chilled water two-way valve bypass. It's located across the chilled water supply and return lines at the end of the main and is essentially acting as a way of balancing the pressure in the piping since we're using constant volume piping. We'll cover this more in depth in a future post when we talk about pumping piping, decouplers, bypass valves, mixing valves, etc.
That is air cooled chillers. Now I know we didn't get to water-cooled chillers and voltage chiller control, but we will get to it in the next few posts. So I look forward to hearing your thoughts and questions.
If any of this is confusing you and you're just not picking up HVAC as quickly as you'd like, I can't encourage you enough to pick up a copy of our Control Sequence Fundamentals course. It will be a game changer to your Building Automation career.
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Once again, thanks a ton and take care!
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