Evaporative Cooling for Data Centers — Pros and Cons

Selecting the right cooling system for your data center can be a daunting task. Among the various options available, evaporative cooling stands out as an innovative and efficient solution. 

As the name suggests, evaporative cooling uses the evaporation process to lower temperatures, similar to how we sweat on a hot day.

While the concept might sound straightforward, there are various approaches to evaporative cooling, each with its own set of benefits and drawbacks. Could one of these be the right fit for your data center? Let’s explore.

First, What is Evaporative Cooling?

Evaporative cooling, also known as adiabatic cooling, is a ventilation technique that leverages the natural process of water evaporation to cool air. Unlike traditional air conditioning systems that rely on chemical vapor compression, evaporative cooling uses water as its green refrigerant. 

The process works by passing hot outside air through absorbent wet cooling pads. These pads are kept constantly moistened by a water pump that supplies water. When the hot air interacts with the water, the heat (latent heat) causes the water to evaporate, transitioning from liquid to gas (water vapor). This evaporation cools down the air, which is then blown into the space. The water that drips from the pads is collected at the bottom of the cooler and sent back to the top of the pads, creating a continuous cycle. As the role of cooling in data centers continues to grow, it becomes imperative to utilize an efficient, sustainable, and reliable cooling system, and evaporative cooling could meet these criteria.

The evaporative cooling process can be utilized in three primary ways: direct evaporative cooling, indirect evaporative cooling, and two-stage evaporative cooling. Let’s examine each of these approaches. 

Direct Evaporative Cooling

Direct evaporative cooling, also known as direct adiabatic cooling, involves direct contact between water and the air stream that cools the data center. Specifically, hot air passes through the evaporative media, contacts water, evaporates, and cools the air directly into the server room. This method is the most widely used form of evaporative cooling, particularly in dry regions.

The Advantages of Direct Evaporative Cooling:

• Energy Efficiency: Consumes only 1/4 of the power used by traditional mechanical air conditioning units.
• Rapid Cooling: Capable of quickly reducing air temperature by up to 30°F.
• Humidity Control: Adds moisture to the air, which is especially beneficial in dry climates.
• Fresh Air Supply: Constantly supplies fresh air from outdoors.
• Health and Safety: Using only water as the refrigerant makes direct evaporative cooling safe for human health and environmentally friendly.

The Disadvantages of Direct Evaporative Cooling:

• Excess Moisture: It always adds moisture to the air, even if it exceeds the ideal humidity level.
• Cooling Limitations: Unable to cool air below a certain temperature, dependent on initial air temperature and humidity.
• Contamination Risk: Mixing cooled air with the air in the data center can introduce contamination and impair air quality.
• Climate Dependency: Less efficient in high-humidity regions (mostly the US South-East).

Indirect Evaporative Cooling

Indirect evaporative cooling operates on the same principle as direct evaporative cooling, namely lowering air temperature through water evaporation. The critical distinction lies in the incorporation of a heat exchanger as a middle stage, with its role to isolate the cooling process from the air supplied to the space. This design aims to minimize the humidity levels generated and prevent the mixing of outside hot air with the cooled airstream entering the data center. However, this improved control comes at a cost: indirect evaporative cooling requires two fans instead of one, leading to higher electricity consumption than direct evaporative cooling.

The Advantages of Indirect Evaporative Cooling:

• Achieves Lower Temperatures: Can reduce ambient temperatures by 10-15 °F in summer conditions, surpassing those achieved by direct evaporative cooling.
• Better Moisture Control: Provides cooler air with less humidity, making it suitable for areas where additional humidity is undesirable.
• Air Isolation: Keeps room air purified, preventing the mixture of outside hot air with the cooled air stream.
• Cost-Efficiency: Offers significant energy savings compared to conventional and air-cooled free cooling

The Disadvantages of Indirect Evaporative Cooling:

• Higher Electricity Consumption: Requires two fans rather than one, leading to higher electricity consumption than direct evaporative cooling.
• Setup Complexity: The system setup may involve exhausting or re-cooling the warmed air, depending on the configuration.
• Increased Maintenance Requirements: Regular upkeep is necessary to maintain the system’s efficiency, such as replacing evaporative media and maintaining circulating pumps.

Two-Stage Evaporative Cooling

Two-stage evaporative cooling, also known as hybrid or indirect/direct cooling, combines the advantages of direct and indirect approaches to offer advanced cooling solutions for data centers. These systems typically use an indirect stage followed by a direct evaporative cooling stage. First, hot outside air passes through a heat exchanger cooled by external evaporation, avoiding added moisture. In the second stage, the air stream moves through a water-soaked pad for further cooling, picking up some humidity. This two-stage approach allows for more precise control over temperature, humidity levels, and air quality, making it a versatile solution for various environmental conditions and data center requirements.

The Advantages of Two-Stage Evaporative Cooling:

• Energy Efficiency: According to the American Society of Heating and Engineers (ASHRAE), two-stage evaporative cooling uses 60%-75% less electricity than conventional refrigerant-based AC systems.
• Lower Humidity Levels: Achieves indoor humidity levels of 50%-65%, compared to 70% with direct evaporative cooling systems.
• Effective Cooling: Can reduce temperatures by up to 44.6°F compared to evaporative cooling systems.
• Reduced Water Consumption: Consumes up to 30% less water.
• Technologically Advanced: Features pre-coolers, more effective pads, and efficient motors.

The Disadvantages of Two-Stage Evaporative Cooling:

• Costs: Higher initial costs compared to single-stage systems and potential for increased operational costs due to more sophisticated technology.
• Maintenance Requirements: Requires regular maintenance to ensure effectiveness and longevity.
• Complexity: More complex system setup compared to single-stage cooling methods.

Is Evaporative Cooling Suitable for Your Data Center?

Several key factors come into play when considering evaporative cooling for your data center. These include your facility’s geographical location, the specific temperature and humidity requirements of your equipment, access to maintenance support, and budget constraints.

At AIRSYS, we understand that choosing the right data center cooling solution is a complex decision, so we can guide you through it. Our professional team, backed with decades of experience and a commitment to innovation and sustainability, is ready to learn your needs and tailor the best cooling strategy for your data center. Explore our data center cooling solutions and reach out for a free consultation. Let’s work together to find the perfect balance of efficiency, performance, and cost-effectiveness for your facility.