A timeless debate many design teams encounter:
Should a project use air-cooled chillers or water-cooled chillers?
It’s truly a question as old as time itself (mainly to MEP engineers but go with it). Well, it’s as old as 1922, when Willis Carrier first introduced the concept of a centrifugal chiller and people undoubtedly asked, “Should that thing be air-cooled or water-cooled?” Unfortunately, like most engineering questions, the answer is: “it depends.” This strategic decision shapes efficiency, costs, and environmental impact. Picture the scene: climate as the lead, efficiency as the hero, spatial dynamics, financial narrative, and environmental subplot. So, who wins this age-old war? As with any worthy conflict, the answer lies not in brute force, but in cunning strategy.
The FUNdamentals
As the name suggests, a chiller makes something cold. That something is typically water, or it could be a water-glycol mix called a brine. All chillers do that, whether they’re air-cooled or water-cooled. The biggest difference is how they reject heat, and you’ve probably already figured out that it can be done via air or water. Air-cooled chillers reject their heat to the ambient air, so they are located outdoors. On the other hand, water-cooled chillers reject their heat to water, and that water is then cooled by another piece of equipment such as a cooling tower.
Now, I’m assuming that if you’re reading an article on the differences between air-cooled and water-cooled chillers on an engineering company’s website, you already understand those basics. So let’s just get right into the real meat of this discussion: which chiller type is better, and what does that mean for the chiller plant? I’ll be crowning a winner for each topic of discussion so we can keep track of which chiller type is better. (Spoiler alert: I’ve already said the answer in the first paragraph.)
Quick disclaimer: There are many topics below that are not discussed thoroughly in this article, such as psychrometrics, the vapor-compression cycle, chiller lift, compressor technologies, etc. However, I will do my best to explain enough of each topic so that the concepts make sense. If you have additional questions, I’d love to hear from you so check down below on how to contact us.
Chiller Energy Efficiency
We’ll start with this one, because this is typically what people always want to know. How efficient is each type of machine? To fully understand the answer to this question, we first need to think about how the heat rejection happens. On an air-cooled chiller, it uses the ambient air around the chiller to cool the refrigerant. Depending on the location, the ambient air temperature could be 95°F or higher. We typically design air-cooled chillers to operate at 105°F in Austin. Since we’re mechanical engineers, I should specify that this is dry bulb temperature. For the sake of comparison, I will use the ANSI/AHRI Standard 550/590 conditions that chillers are rated for, which is 95°F for air-cooled chillers. For all chiller types, the rating condition is to make 44°F chilled water.
On a water-cooled chiller, the water used to cool the chiller is also being cooled by something, such as a cooling tower. The cooling tower uses the ambient air and evaporative cooling to cool the water, but instead of the dry bulb temperature being the operational factor, it uses the wet bulb temperature. This is important because the wet bulb temperature is never greater than the dry bulb temperature. In Austin, for example, we design for a dry bulb temperature of 105°F for air-cooled compressorized equipment, but we design cooling towers for a wet bulb temperature of 78°F. With a wet bulb temperature of 78°F, a cooling tower can make 85°F water consistently, and that 85°F water is what’s used to cool a water-cooled chiller. The ANSI/AHRI Standard 550/590 condition for a water-cooled chiller is 85°F condenser water.
You may have already figured out the answer to this one: water-cooled chillers are more efficient. If we just look at the heat rejection temperatures, we can see that air-cooled chillers are tasked with making 44°F chilled water by using 95°F air, and a water-cooled chiller must make 44°F chilled water by using 85°F condenser water. This means that a water-cooled chiller doesn’t have to work as hard to make 44°F chilled water, simply because it is being cooled with a “colder” fluid.
Now for some actual examples of the efficiencies. Keep in mind that these are all efficiencies based on the ANSI/AHRI Standard 550/590 conditions, so your mileage may vary if your project conditions are different. EER is an industry-standard efficiency rating, and it is a ratio of the input energy (watts) to the output cooling (BTUs). The higher the EER, the more efficient the equipment is. On the other hand, the other common efficiency rating for chillers is kW/ton (input kilowatts per output ton of cooling). The lower this number is, the more efficient. It should be noted that both EER and kW/ton provide the same information, and are related with the following equation:
At the time of this article, air-cooled centrifugal chillers and water-cooled scroll chillers are rarely used, so they are not included in the table below.
If chiller efficiency is the only thing that matters on a project, then water-cooled chillers are the way to go. But read on, because there are plenty more factors to evaluate when deciding which type of chiller to use!
Plant Energy Efficiency
Did you think we were done with energy efficiency? We here at EEA are way into energy and engineering (it’s literally in our name). Don’t worry, this will be a much shorter discussion than the chiller efficiency section.
Let’s start with air-cooled chillers again. In addition to the chiller, there will be a set of pumps for the chilled water. Depending on the system configuration, this could be just one set of pumps (such as variable-primary design) or two sets of pumps (such as primary/secondary design). Those pumps contribute to the overall energy usage of the chiller plant. However, both air-cooled and water-cooled plants will have these chilled water pumps, so they are a wash for the comparison. No extra equipment is required for heat rejection since that is built into the air-cooled chiller, so the only auxiliary equipment to consider here is the chilled water pumps.
On the water-cooled side, they will require some form of air-to-water heat exchanger, such as a cooling tower. They also need a set of pumps to send condenser water from the chiller to the cooling tower. This means that even though water-cooled chillers are more efficient by themselves, they require more energy-consuming equipment that air-cooled chillers don’t need (the condenser water pumps and cooling towers).
As an example, the table below shows some typical efficiency values for the major pieces of equipment in each type of plant. These are rough numbers, and there are many factors that go into determining how efficient each component will be. All of the efficiency values are in kW/ton to keep things consistent.
While the condenser water pumps and cooling towers do consume additional energy, it’s not enough to offset the difference in chiller efficiency. But, if you’re thinking that a water-cooled chiller plant sounds a lot more complicated because of the extra equipment, keep reading!
Plant Complexity
As I noted above, an air-cooled chiller plant is basically just chillers and chilled water pumps. A water-cooled chiller plant needs extra pumps and cooling towers to make everything work. The additional equipment on a water-cooled plant makes them inherently more complex. But wait, there’s more!
I mentioned that the cooling towers on a water-cooled plant cool the condenser water by using the ambient air. That means that the condenser water is exposed to the atmosphere and can get all sorts of junk in the water. In order to keep the condenser water clean, a water treatment system is needed. This adds another set of equipment that must be operational to keep the plant running smoothly.
An air-cooled chiller plant is much simpler. There are only the chillers and the chilled water pumps to operate. With a water-cooled plant, there are many more pieces of equipment that have to operate to keep the plant running: the condenser water pumps, cooling towers, and the chemical treatment system. All that additional equipment on a water-cooled plant sounds like it could be expensive…
Plant First Cost
Sometimes just as important (or more important) than efficiency, the project budget can determine which chiller type to use before anything else is figured out. This one is pretty easy to figure out (sorry, water-cooled chillers), but there are some interesting nuances.
We know that a water-cooled plant will have additional equipment to operate. However, there are two things to keep in mind. Air-cooled chillers are more expensive than water-cooled chillers. This is because the air-cooled chiller has its heat rejection built into the chiller, but the water-cooled chiller needs a separate piece of equipment for that. So, the air-cooled chiller will be more expensive, until you compare it to a water-cooled chiller with a cooling tower and condenser pump.
As an example, typical costs are shown below for the various components of each type of plant. Keep in mind that these are only the costs of the materials; no labor is included for installation. The water-cooled plant will have higher labor costs, simply because there are more pieces of equipment to install.
The real kicker is the additional piping and wiring needed for the additional equipment on a water-cooled plant. Based on how the plant is laid out, this extra cost could be as much as another chiller (or more) in special situations. If first cost is king, then air-cooled chillers are the way to go.
Plant Size
The required plant tonnage could be a major factor in which type of chiller to use, as each type has limitations. Air-cooled chillers typically start around 20 tons and can max out around 500 tons per chiller. On the other hand, water-cooled chillers typically start around 200 tons and can go upwards of 3,000 tons per chiller.
In general, if the plant has a huge cooling load, it makes sense to use water-cooled chillers. For example, if the total cooling load was 4,000 tons, it typically makes more sense to use 2 – 4 water-cooled chillers instead of 8+ air-cooled chillers. On the other hand, if the total plant capacity needs to be 300 tons, it typically makes more sense to use a couple of air-cooled chillers instead of using 1 – 2 water-cooled chillers (especially when you think about the additional complexity that a water-cooled plant brings).
It’s hard to crown a winner for this section since this is very project dependent. Air-cooled chillers are the winner for small plants, and water-cooled chillers are the winner for large plants.
Building Space Requirements
The two different types of plants have very different requirements for how much building space is needed. As I’ve noted earlier, air-cooled chillers need to go outside in order to use the ambient air for heat rejection. The chilled water pumps will typically go inside the building in a mechanical room, but pumps can be placed outdoors if the building space is tight.
If you’re thinking that water-cooled plants require more space because there’s more equipment, you’re right. The chillers themselves need to go indoors in a mechanical room. The chilled water pumps and condenser water pumps can also go in the mechanical room, but they can go outside if there isn’t enough space in the mechanical room. The cooling towers have to go outside because they use the ambient air. Finally, the chemical treatment system needs to go in the mechanical room
Plant Service and Maintenance
Does this sound like an easy win for the air-cooled chillers because they have less equipment? Well, it is. But that doesn’t mean we can just skip over this section.
With an air-cooled plant, there are only the chillers and chilled water pumps to service and maintain. Pumps can be serviced by most technicians without needing a manufacturer’s representative. Parts of the air-cooled chiller can also be serviced by most techs, such as cleaning the condenser coils. Depending on the type of air-cooled chiller, there may be some service that must be performed by a manufacturer’s technician.
On the water-cooled plant, we already know there are the additional condenser water pumps, which will require the same types of service as the chilled water pumps. The cooling towers will also require regular maintenance, such as basin cleaning, which can be done by most technicians. The chillers are a different story. Most water-cooled chillers will require a manufacturer’s technician to perform any service on them.
The water treatment system on a water-cooled plant will need to be regularly maintained to keep the water quality at the appropriate level. This also means that the condenser on the chiller is more likely to experience fouling, which is junk coating the heat transfer surfaces inside the chiller. The condenser may need to have regular eddy current testing or tube cleaning to ensure it is functioning properly. Typically, these services would be performed by a specialty contractor.
Some people say that more pieces of equipment means more possible points of failure. I like to think of it as more opportunities to see my favorite service tech. Whichever way you look at it, a water-cooled plant not only has more equipment to run, but also more equipment that will need to be serviced, maintained, and eventually replaced.
Noise and Acoustics
On most projects, there is some level of consideration given to how loud the equipment will be (or at least, there should be). As engineers, we want to keep the equipment noise levels in check for occupant comfort and for human safety. Noise on chillers typically comes from the compressor/motor, gear box (if the chiller has one), and condenser fans (air-cooled chillers only). For this discussion, we’ll assume that none of the chillers have a gear box as manufacturers have moved away from using them. Also, I’ll be referring to the A-weighted decibel levels (dBA) of the chillers. The higher the dBA, the louder the noise.
We’ll start with water-cooled chillers this time. These can have dBA levels in the low 90s, and can go down to the 70s depending on the chiller configuration. Many older chillers are in the upper 80s/lower 90s range, so hearing protection is required when working in those mechanical rooms. However, newer chillers tend to be much quieter, and some mechanical rooms don’t even require hearing protection. For reference, OSHA requires hearing protection if the noise is at or above 85 dBA. The cooling towers should also be considered. These typically have dBA levels in the mid-80s, but can range between the low 70s to the low 90s.
Air-cooled chillers can be loud. Really loud. It’s not uncommon for an air-cooled chiller to have dBA levels over 100. A typical air-cooled chiller will likely have dBA levels in the upper 80s to low 100s. The main reason is because the condenser fans on air-cooled chillers make a huge difference in the sound levels. It is possible to get a very quiet air-cooled chiller through modifications and careful engineering, but remember that I said air-cooled chillers are sometimes chosen due to the lower first cost. It’s unlikely that an air-cooled chiller project would have the budget for sound abatement to get the chiller down to water-cooled chiller levels.
Equipment Life
Before we can crown a winner, we need to talk about equipment lifespan. This can be a major determining factor when looking at plant longevity and total lifecycle cost. For reference, the equipment lifespans we will compare are taken from the ASHRAE 2023 HVAC Applications handbook for equipment median service life.
The table below shows the various components of each type of plant and their median service lifespans.
Let’s talk about that water-cooled chiller lifespan for a moment. This is for a water-cooled centrifugal chiller, which is the most common type of large, water-cooled chiller. The service life is shown as >25 years, but the ASHRAE handbook has a chart for how long a centrifugal chiller can last (Fig. 1 Survival Curve for Centrifugal Chillers, page 38.3). The chart shows that about 50% of centrifugal chillers last for 30 years, and 25% can last 40 years.
This can have a huge impact on the plant lifecycle cost. As the other sections have shown, we know that air-cooled chiller plants have a lower first cost, but water-cooled chiller plants are more efficient. If we factor in that water-cooled chillers don’t have to be replaced as often, it makes water-cooled plants more attractive. If we imagine a project with a 50 year life, an air-cooled chiller would need to be replaced twice (at the 20 year and 40 year marks), but a water-cooled chiller would only need to be replaced once (at the 25 year mark). Water-cooled chillers are the winner here for their longer lifespan.
Conclusion
If you’re keeping track of the score, it’s clear that the winner is… oh, it’s a tie? That can’t be right. I was pulling for water-cooled chillers, and I’m writing this article so my favorite should win, right? Or maybe the best chiller is the friends we made along the way?
Well, if you remember back at the very beginning, I said the answer was “it depends.” As we’ve seen here, there are many reasons to choose one type over the other. There isn’t a clear-cut rule that we can apply to every project to determine which type to use. But, by knowing the differences between air-cooled chillers and water-cooled chillers as we’ve discussed here, we can make an informed decision on which chiller type would be best suited for the project.
If you have a project and would like assistance in evaluating your air-cooled vs. water-cooled chillers, feel free to contact us in the form below!
