Whenever electrical power is being consumed in an Information Technology (IT) room or data center, heat is being generated. In the Data Center Environment, heat has the potential to create significant downtime, and therefore must be removed from the space. Data Center and IT room heat removal is one of the most essential yet least understood of all critical IT environment processes. Improper or inadequate cooling significantly detracts from the lifespan and availability of IT equipment.

A general understanding of the fundamental principles of air conditioning and the basic arrangement of precision cooling systems facilitates more precise communication among IT and cooling professionals when specifying, operating, or maintaining a cooling solution. Every Information Technology professional who is involved with the operation of computing equipment needs to understand the function of air conditioning in the data center or network room. In this article you will learn the basics of cooling and various terms used in cooling infrastructure which are the key things to provide a best in class support to any data center.

Despite revolutionary changes in IT technology and products over the past decades, the design of cooling infrastructure for data centers had changed very little since 1965. Although IT equipment has always required cooling, the requirements of today’s IT systems, combined with the way that those IT systems are deployed, has created the need for new cooling-related systems and strategies which were not foreseen when the cooling principles for the modern data center were developed over 30 years ago.

The Case for Data Center Cooling

A poorly maintained technology room environment will have a negative impact on data processing and storage operations. A high or low ambient temperature or rapid temperature swings can corrupt data processing and shut down an entire system. Temperature variations can alter the electrical and physical characteristics of electronic chips and other board components, causing faulty operation or failure. These problems may be transient or may last for days. Transient problems can be very hard to diagnose.

High Humidity – High humidity can result in tape and surface deterioration, condensation, corrosion, paper handling problems, and gold and silver migration leading to component and board failure.

Low Humidity – Low humidity increases the possibility of static electric discharges. Such static discharges can corrupt data and damage hardware.

Comfort vs Precision Cooling

Today’s technology rooms require precise, stable environments in order for sensitive electronics to operate at their peak. IT hardware produces an unusual, concentrated heat load, and at the same time, is very sensitive to changes in temperature or humidity. Most buildings are equipped with Comfort Air Conditioning units, which are designed for the comfort of people. When compared to computer room air conditioning systems, comfort systems typically remove an unacceptable amount of moisture from the space and generally do not have the capability to maintain the temperature and humidity parameters specified for IT rooms and data centers. Precision air systems are designed for close temperature and humidity control. They provide year-round operation, with the ease of service, system flexibility, and redundancy necessary to keep the technology room up and running.

As damaging as the wrong ambient conditions can be, rapid temperature swings can also have a negative effect on hardware operation. This is one of the reasons hardware is left powered up, even when not processing data. According to ASHRAE, the recommended upper limit temperature for data center environments is 81°F (27.22°C). Precision air conditioning is designed to constantly maintain temperature within 1°F (0.56°C). In contrast, comfort systems are unable to provide such precise temperature and humidity controls.

The Physics of Cooling

Let us see some of the physics terms that are used when we talk about the cooling infrastructure. We know that heat threatens availability of IT equipment, it’s important to understand the physics of cooling, and define some basic terminology.

First of all, What is Heat?

Heat is simply a form of energy that is transferred by a difference in temperature. It exists in all matter on earth, in varied quantities and intensities. Heat energy can be measured relative to any reference temperature, body or environment.

What is Temperature?

Temperature is most commonly thought of as how hot or cold something is. It is a measure of heat intensity based on three different scales: Celsius, Fahrenheit and Kelvin.

What is Pressure?

Pressure is a basic physical property of a gas. It is measured as the force exerted by the gas per unit area on surroundings.

What is Volume?

Volume is the amount of space taken up by matter. The example of a balloon illustrates the relationship between pressure and volume. As the pressure inside the balloon gets greater than the pressure outside of the balloon, the balloon will get larger. Therefore, as the pressure increases, the volume increases.

Ideal Gas Law

Now let us see how these terms are related to each other and its dependencies in data center cooling. The relation between pressure (P), volume (V) and temperature (T) is known as the Ideal Gas Law, which states PV/T= constant . In this equation, P = pressure of gas, V = volume occupied, and T = temperature. The ideal gas law states that the pressure, temperature, and volume of gas are related to each other. In simpler terms, if pressure is constant, an increase in temperature results in a proportional increase in volume. If volume is constant, an increase in temperature results in a proportional increase in pressure. Inversely, if volume is decreased and pressure remains constant, the temperature must decrease. Basically, pressure and volume are directly proportional to temperature and inversely proportional to each other.

Pressure and temperature are both controlled by the ideal gas law. However, because the volume is not held constant (that is, the atmosphere can expand and contract), the relationships between pressure and temperature are complex. Temperature decreases linearly with increasing altitude, whereas pressure decreases exponentially. For example, you may have experienced the outside of an aerosol can becoming colder as you spray it. This is because the can is a fixed volume, and as the pressure within the can decreases as it is sprayed, the temperature also decreases causing the can to feel cold.

Three Properties of Heat Energy

Now that we know the key terms related to the physics of cooling, we can now explore the 3 properties of heat energy.

1. A unique property of heat energy is that it can only flow in one direction, from hot to cold. For example if an ice cube is placed on a hot surface, it cannot drop in temperature; it can only gain heat energy and rise in temperature, thereby causing it to melt.

2. A second property of heat transfer is that Heat energy cannot be destroyed.

3. The third property is that heat energy can be transferred from one object to another object. When considering the ice cube placed on a hot surface again, the heat from the surface is not destroyed, rather it is transferred to the ice cube which causes it to melt. The property of heat transfer from one object to another object is a wide topic and as below.

Heat Transfer Methods

There are three methods of heat transfer: conduction, convection and radiation.

Conduction is the process of transferring heat through a solid material. Heat transfers through various materials are different. Some substances conduct heat more easily than others. Solids are better conductors than liquids and liquids are better conductors than gases. Metals are very good conductors of heat, while air is a very poor conductor of heat. An example of conduction heat transfer is two bodies at different temperatures kept in contact with each other. When we touch handle (shown in the above image) heat from the handle is transferred to our hand because our hand is cold medium.

Convection is the result of transferring heat through the movement of a liquid or gas. Or we can say that the transfer of heat between the solid surface and the liquid is called as convection heat transfer. Let us considering a vessel of water being heated, in this case heating of water due to transfer of heat from the vessel to liquid.

Radiation related to heat transfer is the process of transferring heat by means of electromagnetic waves, emitted due to the temperature difference between two objects. Or this process can be also explained as when two bodies are at different temperatures and separated by distance, the heat transfers between them and this is called radiation heat transfer. In the case of the conduction and convection heat transfer there is a media to transfer the heat, but in the case of the radiation heat transfer, there is no media. The radiation heat transfer occurs due to the electromagnetic waves that exist in the atmosphere. In the above example, heat from the heater coil is transferred to the nearest atmosphere so that when you bring your hands near to the heater coil you will feel the heat.

There is no such thing as cold — only the absence of heat.

Lastly but the most important thing that you need to understand this quote “There is no such thing as cold — only the absence of heat”. When an air conditioner or refrigerator is cooling a space, do not think about it as adding cold air into space. The purpose of the refrigeration cycle is to remove the heat in a given area and reject it outside. Less heat means a colder room!

Air Flow in IT Spaces and Heat Generation

We have seen that naturally, heat energy can only flow from hot to cold. For this reason, we have air conditioners and refrigerators. They use electrical or mechanical energy to pump heat energy from one place to another and are even capable of pumping heat from a cooler space to a warmer space. The ability to pump heat to the outdoors, even when it is hotter outside than it is in the data center, is a critical function that allows high-power computing equipment to operate in an enclosed space. Understanding how this is possible is a foundation to understanding the design and operation of cooling systems for IT installations.

Now let us say that the heat is generated at various areas in a data center and especially high density of heat from white space areas. Heat generation in white space area occurs at various levels throughout the data center, including the chip level, server level, rack level, and room level. With a few exceptions, over 99% of the electricity used to power IT equipment is converted into heat. Unless the excess heat energy is removed, the room temperature will rise until IT equipment shuts down or potentially even fails. Hence the most efficient way for heat removal is to remove it from the closest area of the heat source itself.

Let’s take a closer look at heat generation at the server level. Approximately 50% of the heat energy released by servers originates in the microprocessor. A fan moves a stream of cold air across the chip assembly. The server or rack-mounted blade assembly containing the microprocessors usually draws cold air into the front of the chassis and exhausts it out of the rear. The amount of heat generated by servers is on a rising trend. A single blade server chassis can release 4 Kilowatts (kW) or more of heat energy into the IT room or data center. Such a heat output is equivalent to the heat released by forty 100-Watt light bulbs and is actually more heat energy than the capacity of the heating element in many residential cooking ovens.

The Refrigeration Cycle

The refrigeration cycle is a closed cycle of evaporation, compression, condensation and expansion, that has the net effect of moving heat energy away from an environment and into another environment, in this case, from inside the data center, to the outdoors. Before going further I want you to understand these four terms as simple as possible that is,

• Evaporation process is performed by the evaporator components which is having the functionality of heat absorption.
• Compression process is performed by a compressor having the function to increase pressure.
• Condensation process is performed by condenser components having the function of heat rejection.
• Expansion process is done by an expansion valve(metering device) having the function of pressure dropper(decrease).

Now that we know the basic principles related to cooling and we can talk about how the refrigeration cycle works in HVAC(heating, ventilation, and air conditioning).

The major substance which is involved in refrigeration cycle is refrigerant. A refrigerant is a substance, often a fluid, used in a refrigeration cycle to cool a space. Refrigerants captures heat and then release it to another space by using the thermodynamic phenomena of phase changes, in which a fluid changes to a gas or vice-versa. In earlier days the fluid which used as refrigerant was air, water or CO2. Modern systems primarily use fluorinated hydrocarbons that are nonflammable, non-corrosive, nontoxic, and non-explosive. Refrigerants are commonly referred to by their ASHRAE numerical designation. Environmental concerns of ozone depletion may lead to legislation increasing or requiring the use of alternate refrigerants like R-134a. Additional legislation related to the use of alternate refrigerants is under consideration.

Refrigerant changes its physical state from liquid to gas and back to liquid again each time it traverses the various components of the refrigeration cycle. As the refrigerant changes state from liquid to gas, heat energy flows into the refrigerant from the area to be cooled (the IT environment for example). Conversely, as the refrigerant changes state from gas to liquid, heat energy flows away from the refrigerant to a different environment (outdoors or to a water source). In the refrigeration cycles, the refrigerant is pressurized(compressor) to increase the temperature so that the heat can be ejected. Depressurized(metering device) to decrease the heat so that the heat is going to be absorb into refrigerant.

We will see the refrigeration cycles that draws the warm air from inside the data center, to the outdoors and how the cooling process occurs.

STEP 1. Evaporation

This the first step in removing heat energy from a computer room, and is the first step in the Refrigeration Cycle. The evaporator coil acts as an automobile radiator operating in reverse. Warm air from the computer room is blown across the evaporator coil by a fan, while the tubes comprising the coil are supplied with the refrigerant exiting the expansion valve. When the warm computer room air passes through the cold evaporator coil it is cooled and this cool air is delivered back to the computer room. Even though the evaporator coil is cold, at approximately 46°F (7.8°C), the refrigerant inside is evaporating, or boiling, changing from liquid to a gaseous state. It is the heat from the computer room that is boiling the refrigerant, passing heat energy to the refrigerant in the process. The refrigerant at this point is a cool gas in a small pipe that is carrying the heat energy away from the computer room. The vaporized but cool refrigerant carrying the heat from the evaporator(data center area) is drawn into a compressor.

STEP 2. Compression

The compressor is widely considered as the engine of the refrigeration cycle. It consumes the most power out of the HVAC system’s components and forces the refrigerant through the system. In the process of being compressed the cool, gaseous refrigerant is turned to a very hot and high-pressure vapor. This compressor has two important functions: It pushes the refrigerant carrying the heat energy around the refrigeration loop and it compresses the gaseous refrigerant from the evaporator coil, over 200 psi. It is a fundamental property of gases that the compression of a gas causes its measured temperature to rise. Therefore, the moving gaseous refrigerant exiting the compressor is hot, over 125°F (52°C), as well as compressed. This temperature rise due to compression is the key to the ability of the refrigeration cycle to eject heat into the outdoor environment.

STEP 3. Condensation

The next stage of the refrigeration cycle is Condensation. In this stage, the hot compressed refrigerant carries the computer room heat energy from the compressor to the Condenser Coil. The coil is made of small tubes “coiled” up into a block of metal fins and resembles an automobile radiator. This coil transfers heat to the air and operates at a temperature HIGHER than the air. This means that the air flowing across the coil is heated by the coil, and that the hot gaseous refrigerant flowing through the coil is conversely cooled. Heat is flowing from the refrigerant to the air. The air is typically blown across the hot coil by a fan which exhausts the hot air to the outdoors. In this way the heat energy from the computer room has been transferred to the outdoors. The Condenser coil acts similarly to the radiator in a car, in that it carries heat from the engine to the air outside the car.

STEP 4. Expansion

In the last stage, the expansion stage, the refrigerant exits the Condenser Coil as a high-pressure liquid, although at a lower temperature. The refrigerant then passes through an expansion valve(metering agent) which has two key functions that are critical to the refrigeration cycle:

• It precisely regulates the flow of high-pressure refrigerant at a rate that maintains an optimal difference in pressure to ensure efficient cooling.

• Secondly, the refrigerant escapes the expansion valve as a cooled refrigerant.

Once this cooled refrigerant has passed through the evaporator coil, it is changed to a gas. This is because the boiling point of the liquid refrigerant is extremely low. Therefore as the warm air from the computer room blows across the coils of the evaporator, the refrigerant that enters the coil gets heated and starts boiling. Thus it changes to a gas. In this way, the cold refrigerant absorbs the heat energy from the air and carries it away from the data center. At this stage, the refrigeration cycle is repeated, and the net result of the process is that heat is continuously flowing into the Evaporator Coil and continuously flowing out of the Condenser Coil.

To summarize the refrigeration cycle process— heat is absorbed by the refrigerant (cooling the air) in the evaporator and expelled from the refrigerant to the outdoor air in the condenser. Simultaneously, the expansion device and compressor help us manipulate the pressure of the refrigerant to make the cycle possible.

Here we have covered all the basic concepts and principles related to cooling infrastructure. More of the detailed portions are covered in various articles.

Knowledge Credits: Energy University by Schneider Electric

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