Waterside Economizer for Air-Cooled Chillers


Author’s note: This article discusses using devices known as “waterside economizers” and “dry coolers” as means to achieve “free-cooling”. Free cooling (sometimes referred to as a “free cooling system”) can reduce energy consumption and operating costs by using cold ambient air in lieu of running chiller compressors for cooling loads.

While it may not seem obvious, in many HVAC applications there’s a need to provide cooling even in colder months because of internal loads driven by people, computers, machinery and lighting.

Running a chiller year-round is one way to meet that need but doing so can be costly and inefficient. Another solution is to operate a waterside economizer, sometimes known as a free cooling system. There are a few reasons a free cooling system might be used.

One reason to run a free cooling system is to meet ASHRAE Code 90.1 2016 requirements. ASHRAE breaks the country into different climate zones. If your local building code has adopted ASHRAE Code 90.1 2016, any location in Zone 4 (Fig. 1) or above needs to have either an air or waterside economizer to meet the code.

Figure 1 (Source: www.ashrae.org)

Another reason to use a free cooling system is to take advantage of colder weather to meet the building’s setpoint, saving energy. 

Facilities such as data centers are particularly interested in these types of free cooling systems because of the vast amount of heat they generate in a 24/7 operation. This means they will use free cooling a good portion of the year in almost any climate.

When using an air-cooled chiller for mechanical cooling, all methods of obtaining free cooling utilize some type of “dry cooler,” the only difference is where it’s located.

There are various types of Dry Coolers as seen in Figure 2.

Remote Dry Cooler: a general-purpose cooler that sits apart from the air-cooled chiller. The two are piped together.

Stacked Coil Dry Cooler: free-cooling coils are mounted (or “stacked”) next to the chiller’s existing refrigerant condenser coils.

Bolt-On Dry Cooler (also known as modular design): This attaches directly to the air-cooled chiller. Available in a host of sizes, the modular design can provide the appropriate amount of free cooling for the application. 

Figure 2

Stacked or Modular dry coolers are preferred in many cases because they are designed to integrate directly with specific chillers from the factory, improving performance. Where there is space available, the modular option can offer superior performance and convenience. Let’s examine why:

A free cooling system will run in one of three modes: 

  • Mechanical Cooling only 
  • Free Cooling only 
  • Hybrid Mode where Mechanical Cooling and Free Cooling occur simultaneously


Mechanical Cooling Only 

When the ambient temperature is too high to provide free cooling, the chiller operates in mechanical cooling only mode.

In this case, the fans on the stacked coil design need to force air through both the chiller condenser coils and the unused free cooling coils, which ends up wasting energy.

The modular design is different.

When it’s in mechanical cooling mode, the air flows only over the mechanical cooling coils that are in use, while the fans for free cooling are turned off, thus, saving energy. (See Figure 3 for air flow diagrams.)

Figure 3

Free Cooling Only

When the ambient temperature is low enough, mechanical cooling can shut off completely, and the setpoint can be met by free cooling mode alone.

With the stacked coil design, air again needs to flow over both sets of coils when only one is active, which wastes energy.
With the modular design, only the free cooling section will be active, maximizing free cooling, while the fans for mechanical cooling will turn off. Once again, saving energy 

Hybrid Cooling (Mechanical and Free Cooling simultaneously)

Even when the ambient temperature isn’t low enough for free cooling only, some free cooling is still possible. This requires hybrid cooling mode, where both sections of the chiller work in tandem.

With the stacked coil design, air needs to flow over both sets of coils. This presents a problem for the fan control system.

The free cooling coils want as much air as possible to maximize free-cooling performance. Conversely, the mechanical cooling coils want to lower the fan speed to ensure the refrigerant doesn’t get too cold, missing the setpoint and shutting down the chiller. As a result, the system can become less dependable with frequent nuisance trips. In addition, the system fails to make the most of the free cooling opportunity.

In the modular design, the mechanical and free cooling sections are able to operate independently.

So, in hybrid cooling mode, the air flows over the mechanical and free cooling coils separately. This allows the free cooling fans to speed up, extracting every bit of free cooling possible. At the same time, the mechanical cooling fans can slow down, to maintain setpoint and protect the system, again, saving energy (Figure 3).

It’s important to note that when ambient temperatures allow a system to take advantage of free cooling, a large majority of the hours occur in the hybrid mode. For example, in data center applications it can range from 70 to 100% of the operating hours. Therefore, maximizing the free cooling capacity, especially in the high-hour hybrid mode operation, will be key to optimizing your energy savings goal.

While the modular design is engineered and built to optimize its free cooling performance, one of the most desirable features of this design is its customizable nature. Instead of requiring customers to install an “all or nothing” option, they can customize how much free cooling they would like for each job. The modular design allows for many different configurations. While some jobs may only desire a limited amount of free cooling, others may want substantially more. With the modular system, the amount is completely customizable. 

The modular design not only offers superior performance and customization, it offers significant service advantages in all operating modes.

With the stacked coil design, as the air flows over two closely coupled coils one of them essentially becomes a filter, trapping dirt and debris between the coils. This can make cleaning and coil replacement extremely difficult (Figure 4).

With the modular design, there’s only one coil, so most dirt is expelled by the existing fans. A normal coil wash can eliminate remaining dirt. In addition, service can be done on either the mechanical or free cooling side of the system, without taking the other side of the system offline.


Figure 4

Choosing between the two options can depend on a number of factors including desired free cooling performance, ease of service and space constraints.

Free cooling is a great opportunity to generate savings, but the choice of the type of waterside economizer may be crucial to maximize the benefits.

About the Author
Frank Silva is a Product Manager of Air-Cooled Products for Carrier Commercial HVAC North America. With over twenty-five years of experience in industrial sales and marketing management, Silva is responsible for development and implementation of marketing strategies and plans for new products and technology innovation. He works out of Carrier North America’s Chiller manufacturing plant in Charlotte, North Carolina.

About Carrier
Founded by the inventor of modern air conditioning, Carrier is a world leader in high-technology heating, air-conditioning and refrigeration solutions. Carrier experts provide sustainable solutions, integrating energy-efficient products, building controls and energy services for residential, commercial, retail, transport and food service customers. Carrier’s HVAC business is a part of Carrier Global Corporation, a leading global provider of healthy, safe and sustainable building and cold chain solutions. For more information, visit www.carrier.com/commercial.

To read similar Chiller Technology articles visit https://coolingbestpractices.com/technology/dry-coolers.