Mastering Data Center Air Conditioning: Principles of Refrigeration, Air-Cooled, and Water-Cooled Systems (40 Illustrated Diagrams)

> Source: Technical standards reference

> Category: Data Center Engineering

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Data center air conditioning is a type of precision air conditioning, designed to meet the special process and environmental requirements of precision equipment. Its purpose is to precisely control temperature, humidity, and other parameters within a specific range.

Data center air conditioners feature a high sensible heat ratio and require large air volumes. To achieve the required air parameters, the air conditioning system consists of two cycles: the refrigeration cycle and the air cycle. The refrigeration cycle is mainly divided into water-cooled and air-cooled types. Below, through a series of animated diagrams, we will explore the refrigeration cycle and air cycle of data center air conditioning.

Part 1: Refrigeration Cycle Principles

The refrigeration cycle consists of four processes: compression, condensation, expansion, and evaporation. It uses a limited amount of refrigerant in a closed refrigeration system, repeatedly compressing, condensing, expanding, and evaporating the refrigerant. The refrigerant continuously absorbs heat and vaporizes at the evaporator, providing cooling and transferring heat from indoors to outdoors.

The difference between water-cooled and air-cooled systems essentially lies in whether heat exchange occurs with water or with air.

Refrigeration Cycle

Part 2: Air Cycle

2.1 Air Supply Methods

Conventional terminal air supply methods include overhead air supply, ducted air supply, and underfloor air supply.

Overhead Air Supply

Ducted Air Supply

Underfloor Air Supply

2.2 Typical Layouts

To optimize airflow and further improve cooling, contained air supply is a common method for creating cold aisles by separating hot and cold air paths, as shown below.

Hot and Cold Aisle Separation

In addition, to reduce airflow distribution distances, in-row air conditioners and rack-level air conditioners are also available.

Evolution from Traditional Room-Level Air Conditioning to Micro-Modules

Some data centers also use overhead air conditioners with enclosed hot aisle methods to further shorten the airflow cycle distance. Overhead air conditioners can be installed horizontally or vertically.

Horizontal Overhead Air Conditioner

Vertical Overhead Air Conditioner

To further reduce airflow distribution distance, some data centers adopt rack-level cooling methods such as heat pipe backplanes.

Rack-Level Air Conditioner

Part 3: Air-Cooled Systems for Data Centers

This is the most traditional cooling method. The air conditioner consists of an indoor unit and an outdoor unit connected by refrigerant pipes. The indoor unit comprises a compressor, expansion valve, and evaporator, providing cooling and airflow delivery functions, while the outdoor unit is used for heat dissipation.

Air-Cooled Refrigeration Principle

Conventional systems use fixed-speed scroll compressors, with a small number using digital scroll or variable-frequency scroll compressors. Air-cooled outdoor units are installed outdoors or on rooftops. There are restrictions on the distance between indoor and outdoor units: typically no more than 20 meters above the indoor unit and no less than 5 meters below. The recommended refrigerant pipe length is less than 60 meters; exceeding this requires extension components and measures.

Typical Structure of Air-Cooled Data Center Air Conditioner

3.1 Suitable Scenarios

Air-cooled air conditioners are independent of each other with no single point of failure, making them particularly suitable for small and medium-sized data centers. When the airflow distribution distance is short, single-sided layout can be used; when the distance is longer, double-sided layout is employed, as shown below.

Air-Cooled Air Conditioner Layout in Data Centers

3.2 Indoor Unit Configurations

In addition to room-level air conditioners, there are also air-cooled in-row and rack-level air conditioners. These configurations can reduce airflow distribution distance and lower fan power consumption. In-row air conditioners combined with cold/hot aisle containment can achieve good cooling effects. Figure shows contained configuration.

Typical Airflow of Room-Level Air Conditioner

In-Row Air Conditioner with Hot Aisle Containment

Air-Cooled In-Row Air Conditioner

3.3 Pros and Cons of Air-Cooled Systems

Advantages: Simple system, completely independent, low investment, easy staged construction, no single point of failure, high reliability.

Disadvantages: Large number of outdoor units; scroll compressor energy efficiency lower than centrifugal compressors; outdoor unit air-cooled heat dissipation unsuitable for large-scale deployment, prone to high pressure in summer; overall energy efficiency not high.

Suitable for: Small and medium-sized data centers.

Part 4: Water-Cooled Systems for Data Centers

Similar to traditional commercial water-cooled systems, water-cooled chilled water systems include two water systems: the cooling water system and the chilled water system. The system mainly consists of water-cooled chillers, chilled water pumps, cooling towers, cooling water pumps, water treatment equipment, constant pressure water makeup systems, chilled water terminal air conditioning units, and piping with valves.

Water-Cooled Chilled Water Air Conditioning

4.1 Compressors

Water-cooled chilled water systems generally use large chillers. Those with larger cooling capacities typically use centrifugal units for better energy efficiency, while some medium-sized data centers may also use screw units.

Centrifugal Unit Working Principle

Centrifugal Compressor Working Principle

Refrigerant Pressure Change in Centrifugal Unit

Centrifugal Unit Speed Range

Screw Compressor Working Principle

4.2 Plate Heat Exchangers

To further improve energy efficiency and reduce energy consumption, plate heat exchanger systems can also be optionally configured. When the ambient temperature is low, the plate heat exchanger is activated for natural heat exchange.

Plate Heat Exchanger Working Principle

4.3 Terminal Configurations

Chilled water terminal air conditioning units are also divided into room-level, in-row, and rack-level types.

Chilled Water Room-Level Architecture

Chilled Water In-Row Architecture

4.4 Cooling Towers

Cooling towers can be open or closed, crossflow or counterflow. Data center cooling towers typically use steel towers. The diagram below shows a crossflow open-type steel tower.

Crossflow Open Cooling Tower

Crossflow Tower

Counterflow Tower

4.5 Circulating Water Pumps

A water pump is a device that converts mechanical energy into liquid energy, enabling water to circulate within the water system. Both chilled water and cooling water circulation are performed by pumps. Common circulating water pumps include end-suction pumps and double-suction pumps.

Circulating Water Pump

4.6 Chilled Water Storage Tanks

Considering that after a utility power interruption, the chiller requires a certain amount of time from startup to normal operation, a certain volume of chilled water reserve must be maintained. This is achieved through the installation of chilled water storage tanks or reservoirs to ensure safe operation of data center equipment. Storage tanks can be open or closed, with closed tanks further divided into vertical and horizontal types.

Horizontal Chilled Water Storage Tank

Chilled Water Storage Tank Working Principle

4.7 System Characteristics

Advantages: Uses cooling towers for heat dissipation, small footprint with good heat dissipation; high centrifugal compression efficiency; free cooling can be achieved using plate heat exchangers in winter conditions.

Disadvantages: Complex system structure, high investment, involves both cooling and chilled water systems, complex construction and maintenance; lower water system reliability with potential risk of water flooding the equipment room; single point of failure exists, requiring equipment redundancy, ring networks, or dual systems to address reliability issues.

Common Layout: Chillers offer high energy efficiency, and equipping terminal load with plate heat exchanger free cooling can achieve excellent energy efficiency — this is currently the widely adopted solution for large data centers.

4.8 Energy-Saving Methods for Refrigeration Units

Water-cooled refrigeration units can use variable frequency drives and other methods to regulate flow, achieving energy savings.

Fixed Orifice Plate vs. Adjustable Orifice Plate

Chiller VFD: Adjusting Flow Based on Load

VFD Limitations: Compressor Surge at Low Flow

Chiller Startup: VFD Startup is More Energy-Efficient

Flooded Evaporator