Data Center Cooling, Efficient Cooling is Cost Effective Cooling.

This brief article won't make you a mechanical engineer, but it will give you the common sense knowledge needed to make wise planning decisions!



Let's be clear on what's being cooled. We define a data center as any room with at least one intermediate data frame (IDF) or rack as they're often called, that has operating electronic equipment in it. However, if you read all of this article, you'll have the basic knowledge to plan the cooling for numerous rows of racks with hundreds of electronic devices in them. You'll also be able to confidently direct and manage the consultants you hire to prepare your data center construction documents.

Fork in the road! Which is the correct objective? It's really important that you start down the right road here. The objective of data center cooling is NOT to cool the room. The correct objective is to cool the electronic equipment in the room. When it comes to efficient cooling there is a huge difference. With the correct objective we can achieve cost effective cooling - without it we can't.

Data center cooling protects the equipment in it from damage caused by overheating. For many businesses, overheating data processing equipment can have far reaching cost and operational impacts. That means it must be dependable, but it doens't mean it has to be expensive.

Efficient cooling = Cost Effective cooling! Efficient cooling plays a major role in value engineering and can be worth $millions, but for you to know how to get it you first need to understand how cooling really works.


Let's start by answering just 3 fun questions!
Question 1
How does most of the heat in a data center get there?
  1. IT professionals carry it in with their bodies.
  2. Tiny stoves used by data mongers to keep their food hot.
  3. Soldering irons used by cable junkies.
  4. Through the electrical wires that bring electricity into the data center and to the electronic equipment that use it.
Answer to Question 1 is "D"

Surprised!  I was too. The reason is because I always thought in terms of something hot like hot equipment or hot air. Not what actually makes it hot. So let's start thinking in terms of what makes things hot.

The thing that makes anything hot (such as equipment and air) is called thermal energy or heat energy. To make sure we differentiate between heat energy and objects that are hot, we will italicize the term heat energy for the remainder of this article. So now let's see how heat energy makes a data center hot.

First heat energy rides electrical current that travels through electrical wire into the data center and to the electronic equipment (such as data servers) that the electrical current powers. When heat energy reaches the electronic equipment it is resisted by the hundreds of tiny components they're made of. The heat energy then builds-up in the components making them and the equipment as a whole hot.

The more electrical current that goes to the equipment means the more heat energy that's going to it. The more heat energy that goes to it means the hotter the equipment will get. When the equipment gets to hot it will be damaged or "burn up" as it is often phrased.

Question 2
How is heat energy removed from electronic equipment so it doesn't burn up?
  1. Dichlorodifluoromethane (freon-12) is sprayed on the components.
  2. Heat energy eating nanobots are time released.
  3. The heat energy jumps from the hot components onto cold air.
  4. Micro thermostats turn on heat energy filters.
  5. Flying heat sinks carry it away.
Answer to Question 2 is "C"

We are talking atoms and molecules here. Heat energy can literally jump from one molecules to another. We will talk more about that in a bit, but for now it's important to remember that as long as electrical current is bringing heat energy into the data center equipment the air around it will get hotter and hotter. The hotter the air gets the hotter the equipment gets. So to keep the equipment from getting to hot we need to cool the air around it. That will also be explained in more detail in a bit so let's move on to question 3.

Question 3
How is data center air cooled?
  1. Reverse osmosis.
  2. The heat energy on the hot air inside the data center is transferred to outside air making the data center air cold again.
  3. Electromagnetic Induction.
  4. Photosynthesis.
  5. Flux capacitors capture the heat energy and take it back to the future.
Answer to Question 3 is "B"

No matter how you look at it, to cool a data center you have to get the heat energy out. There are several different ways to do that and some are better than others as you'll soon see.


First understand how cold air is made. I think the easiest way to do that is to know what hot air is made of. Hot air is the combination of two things: 1) the air its self, which is an object or substance (gas), and 2) heat energy. When heat energy attaches to air it makes it hot. Hint: Air without heat energy attached to it is naturally cold. So the way to make cold air, is to remove the heat energy from hot air.
Yikes, do I have to be a scientist to do that! No, you just have to know one easy thing about physics. That's the law of Conservation of Energy. It states that energy cannot be created nor destroyed. It's the “cannot be . . . destroyed” part that is really important here.

If we can't destroy the heat energy attached to air, how do we get rid of it? Simple! The law of Conservation of Energy also says it can be transferred from one object to another, so we transfer it!!!!

That seems hard! Fortunately it's not because heat energy is attracted to cold objects (objects with less heat energy attached to them). So much so that if you simply put a cold object next to or on a hot object, the heat energy will jump from the hot object onto the cold object. Heat energy will jump from hot metal to cold metal, hot air to cold metal, hot metal to cold air, and so on.

The perfect example is electronic equipment. As we discussed earlier, heat energy is transferred onto components in the electronic equipment when electrical current is run through them and is resisted by the components. To prevent the equipment from burning up, the heat energy is transferred to cold air. That's done in two steps: 1) For efficient cooling the cold air that is supplied to the data center by an air conditioner is directed toward the cold side of the equipment - not just dumped into the room (details and diagrams on that to follow) and 2) the equipment draws it in, moves it past its hot components so the heat energy can jump onto it, and then pushes the now hot air out the other side (the hot side). Step 2 is done with the use of small fans inside the equipment. Be cautioned though. . . . .

CAUTION! If the hot air that comes out on the hot side isn’t taken away the cooling process will dwindle! This is because of hot air pressure that builds up, like in a hot air balloon, and starts blocking the cold air from passing through. Eventually the hot air will fill not only the equipment, but the whole room. Then the hot air pressure will become so great that it will even prevent cold air from entering the room. Now the entire cooling process is stopped. Finally the worst of all scenarios can happen. Unless power is turned off, heat energy will continue to enter through the electrical wiring and build-up in the equipment until it burns them up.

How Can I Be a Cooling Hero? If you do the following five things you are sure to make a positive difference and be recognized for it. 1) Always remember that to cool electronic equipment you MUST get the heat energy out of it and the best way to do that is by running cold air through it. 2) Also remember that electronic equipment has a cold side and a hot side. The cold side is where air is sucked into the quipment and the hot side is where it's pushed out. 3) The most efficient use of cold air and the air conditioner that makes it is to direct the cold air so it only goes to the cold side of the equipment - not the whole room - and of course not the hot side. You'll see two ways to do that shortly. 4) Use the most efficient method to get the heat energy out of the now hot air. More on that to follow as well. 5) When planning a data center, YOU direct your consultants and contractors - don't let them direct you. Do not accept any consultant's or contractor's plan until they have adequately explained in understandable terms how it's going to achieve the cooling process you want. Also, be convinced that the plan they propose is the most efficient possible and be sure to factor in all the pros, cons, and arguments that we will be going over below.

Depending on who you might have to contend with (architect, engineer, salesman, or even your boss) things can get a bit touchy here because of the differing knowledge and opinions on what the best method is or political posturing that might be involved. It's been my experience that architects and engineers often look for the easiest way to engineer a set of plans, salesmen are looking for a commission, and bosses often feel a stronger need to report construction savings now as opposed to bigger savings on operational costs later.

When I was faced with such situations and had to debate the latest so called engineering marvels, I found it helpful to fall back on good old mother nature. You see the fundamentals of cooling were not created by man. They are basic laws of physics that are not going to be changed by the latest innovation. If heat energy is brought in, it has to be taken out. Water short circuits and corrodes electronics and air does not. The list goes on. It's that simple.

Achieving low post-construction operating costs needs to be a strong consideration in your planning. It's important to determine the most efficient method for getting heat energy out and cold air in during the planning and design process. Value engineering may show that you'll have to spend a little more on construction in order to achieve big savings in post-construction operating costs later. With that in mind let's look at the three most common methods used to remove heat energy from a data center.


The three most common methods to get heat energy out of a data center are:

1. AC (All-out). A machine called an Air Conditioner (AC) located completely outside of the data center. With the use of its air handler an AC sucks hot air (air with heat energy attached to it) out of the data center through air ducts. Then with the use of its condenser it gets the heat energy out of the air. Its air handler then pushes the now cold air back into the data center through other ducts. All work to move/circulate the air and to remove the heat energy from it is done outside of the data center. More on how air handlers and condensers work a little later.

2. CRAC (Half-in Half-out). A machine called a Computer Room Air Conditioner (CRAC) that does half of the cooling work inside the data center and half outside of it (the air handler is inside the data center and the condenser is on the outside). In general it does this by piping cool liquid into the data center where heat energy transfers from the hot air to it in the CRAC's air handler. The now hot liquid is piped outside where the heat energy transfers to the outside air in the CRAC's condenser. The now cool liquid circulates back into the data center to get more heat energy from hot air and so on. This type of CRAC is usually considered a permanent installation. Also see Illustration-1 below.

3. CRAC (All-in). A CRAC that does all the cooling work inside of the data center (both air handler and condenser are inside the data center in the equipment). This type of CRAC does something a little out of the ordinary though. It actually sucks outside air into it through one duct, transfers the heat energy to the outside air, and then push it back outside through another duct, Go Figure! You'll soon see a picture of one of these gems below. This type of CRAC is usually considered portable and is often on wheels.

Quick side note on cooling towers, evaporative coolers, and chillers. These devices serve the same purpose as a condesner - they transfer heat energy to outside air. They are sometimes used to suplement a condenser and other times instead of a condenser. It really depends on the size, type, and load of the facility being cooled. For the purpose of keeping things simple, just a condenser is being used in this article. Also see the following videos: How A Cooling Tower Works, How An Evaporative Cooler Works, and How A Chiller Works.

OK, before you decide which makes the most sense for you, consider the following pros and cons.
1. AC (All-out)
  • Pro- Machine itself generates hot air, but not inside data center.
  • Pro- Machine itself doesn't take up any room inside data center.
  • Pro- Machine itself easiest to repair/maintenance without impacting data center because it's outside.
  • Pro- Ductwork easiest to position for most efficient air distribution (more on this below).
  • Con- Could initially cost more than using a CRAC if significant upgrades to an existing system are required (AC/Air Handler, duct system, building structure, power, etc) or if a new dedicated AC system needs to be retrofitted.
Click on figure to enlarge
2. CRAC (Half-in Half-out)
  • Pro: Could initially cost less than using an AC if significant upgrades to an existing system are required (AC/Air Handler, duct system, building structure, power, etc) or if a new dedicated AC system needs to be retrofitted.
  • Con: Machine itself generates hot air inside data center.
  • Con: Machine itself takes up room inside data center.
  • Con: Machine itself difficult to repair/maintenance without impacting data center.
  • Con: Machine itself makes it difficult to distribute air efficiently (see Illustration-1 below).
  • Con: Machine must bring undesirable liquid into data center in order to get heat energy out.
  • Con: If retrofit, piping must be installed from CRAC inside data center to condenser on outside of building. See "Arguments For and Against CRACs" below.
Click on figure to enlarge
3. CRAC (All-in)
  • Pro-Could initially cost less than using an AC if significant upgrades to an existing system are required (AC/Air Handler, duct system, building structure, power, etc) or if a new dedicated AC system needs to be retrofitted.
  • Con- Machine itself generates hot air inside data center.
  • Con- Machine itself takes up room inside data center.
  • Con- Machine itself difficult to repair/maintenance without impacting data center .
  • Con- Machine itself makes it difficult to distribute air efficiently. This type of CRAC only cools the data center as a whole. It is not designed to direct cold air to the cold side of the electronic equipment and remove hot air from the hot side specifically.
  • Con- Machine must bring outside air into data center in order to get heat energy out.
  • Con: If retrofit, air ducts must be installed from CRAC inside data center to outside of building or existing ducts must be modified to accommodate the CRAC. See "Arguments For and Against CRACs" below.
Click on figure to enlarge


Argument 1: CRAC's are more dependable than standard AC systems.

This implies that there are statistics to that effect. Beware of any statistics you might be given.

I spent most of my career managing the design and construction of hospitals and research labs. Because of their size and complexity I was often involved with the facilities they were associated with (such as medical centers and educational institutions) and saw firsthand how their AC systems worked and held up. Both types of facilities had highly critical spaces that relied on continuous air circulation and temperature control.

They never used CRACs, yet never was there a time where AC downtime was an issue. This was because they used quality AC equipment, included some redundancy, continuously monitored the equipment (didn't wait for it to break to check it), and frequently serviced the equipment, which included replacing parts before they broke.

Argument 2: CRACs don't require as much common area (area outside of tenant's space).

Except for the air handler, a CRAC has many of the same components that a standard AC system has that must be located in a common area. They can include condensers, cooling towers, chillers, related piping, power boxes, power lines, power disconnects, and so on. If the facility has a utility area on the side of the building, that is the common area often used to locate them. Otherwise the common area most often used is the roof. If any of the equipment is dedicated (installed to serve a specific space by the tenant), the common areas used to locate them are usually negotiated into the lease.

Argument 3: CRAC's give data center owners/operators 100% control of the equipment whereas a standard building AC system is normally controlled by the building owner or property manager, which can make things difficult for the tenant.

I can see where this would be a concern if you are a tenant, but again if you are leasing space for a sizable data center, control of the associated AC system can usually be negotiated into the lease. After all, the building owner wants a happy tenant and usually recognizes the need for the tenant to have control.

Argument 4: Cost of construction using CRACs is less than the upgrades required to use the standard AC system.

Good value engineering analysis that applies all the considerations mentioned in this article during the design process may show that, in the long run, it's more cost effective to adapt the existing building AC system or add a dedicated AC system, and negotiate control of the equipment in the lease than using CRACs.


From a technical perspective, we recommend method number one (AC) first because of the pros listed for it and the Cons listed for the CRACs. As it relates to risk management we also strongly recommend against anything that circulates significant amounts of liquid in and out of a data center. Method number two does just that. Liquids and moisture can more easily damage electronic components than heat.

Also, except for unique or temporary situations, we don't recommend bringing additional heat energy into a data center in the form of cooling equipment solely for the purpose of getting heat energy out, which is what both types of CRACs do. Aside from the fact that it normally is not as efficient and cost effective as a standard AC system over the long term, it just doesn't seem as sensible.

From a cost perspective, we recommend value engineering over cost reduction. Greater cooling efficiency that results in long term cost effectiveness usually wins out over meeting immediate cost goals.

ILLUSTRATION-1: Click on illustration to enlarge


Ok, what is the most efficient way to cool a data center? Glad you asked because efficiency usually results in lower operating cost.

For efficient, cost effective cooling use the 5 guidelines below and the diagrams that follow them:

1. Air Weight: Cold air is heavy and wants to drop (settle), so have it enter the room at or near the floor where it's coolest. If it enters from the ceiling it will usually pass through hot air and pick-up heat energy as it drops toward the floor. Keep in mind that the more heat energy it picks up before it gets to the equipment it's intended to cool, the less it will pick up when it gets to the equipment, making the process less efficient.

Hot air is light and wants to rise, so have it leave the room at the ceiling (highest point).

2. Air Pressure: Air pressure means there is a lot of air packed into a space. Air under pressure, like in a supply air duct, wants to go where there is less pressure like at the ceiling near a return air duct where there is low pressure and even better, inside a return air duct, where there is negative pressure (vacuum).

3. Air Flow: Position ducts and vents so that the cold air takes the most direct route to the hot electronic equipment and then from there so that the now hot air takes the most direct route to the return air vents.

4. Air Location: Position vents so that cold air is least likely to pick up heat energy before it reaches the hot electronic equipment.

5. Air Space: Limit the space (cubic feet) to be cooled. By doing this you limit the amount of air to be cooled, which in turn means less heat energy that has to be removed, and less building operating cost to accomplish that.


The following diagrams illustrates these guidelines . . .

DIAGRAM-1: Click on diagram to enlarge
DIAGRAM-2: Click on diagram to enlarge
DIAGRAM-3: Click on diagram to enlarge
DIAGRAM-4: Click on diagram to enlarge
WAIT! You never actually said how an air conditioner removes heat energy from the hot air after it's removed from the room.

Ok, generally speaking there are two main parts to an air conditioning system. One is the air handler and the other is the condenser.

An air handler's primary job is to circulate air. A condenser's primary job is to circulate liquid (coolant). Together they are good at getting heat energy out of a any space.

Both have an air coil and both have a fan for pushing or pulling air through them. Cars have an air coil, but they're usually referred to as a radiator. To save time we are just going to call it a coil.

The primary job of the air handler's coil is to transfer the heat energy from the hot air that came from the room to the liquid circulating through it. The primary job of the condenser's coil is to transfer the heat energy from the liquid circulating through it to outside air.

The general process goes like this. From the room (in our case the data center), the hot air travels through "return air ducts". They're called that because they return air to the air handler that supplied it before it was hot. There, hot air passes through the air handler's coil. As it does, the heat energy jumps from the air onto the coil's metal tubing and then onto the liquid that runs through the tubing. From there the liquid travels through a pipe to the condenser's coil, which is always outside. There the heat energy jumps onto the condenser coil's metal tubing and finaly from there to the outside air, causing the liquid in the condenser's coil to cool as it circulates through. The now cool liquid travels back to the air handler's coil through another pipe to get more heat energy.

In short, getting heat energy off of air is really just a series of heat energy transfers or "heat transfers" as they are often called.

To wrap up the air cooling process, when air comes through the other side of the air handler coil it is now cool. The duct on the cold air side is called a "supply air duct" because it supplies the data center with cold air.

What about Back Draw? Excellent question! You must have noticed it in Illustration-1 above that shows the inefficient air distribution of CRACs.

Back draw is hot air that comes out of the hot sided of a rack and curls back over the top and into the cool side as depicted in the diagram below. The term "draw" is used because of the slight vacuum condition that develops from the cold air flowing through the rack.

DIAGRAM-5: Click on diagram to enlarge
How do we prevent Back Draw? To do that we'll just add thin walls between the ceiling and the top of the racks as shown in the diagram below. These thin walls are often referred to as "aprons".

In addition to preventing back draw, aprons provide two other benefits. 1) They contain the cold air on the cold side of the racks and the hot air on the hot side. 2) Because of the first benefit they allow cold air to get from vents in the ceiling to equipment in the racks without mixing with hot air on the way (not shown).

Remember, hot air is light and rises while cold air is heavy and drops. For that reason we normally don't like to deliver cold air from the ceiling because as it drops through rising hot air on it's way to the equipment it picks up heat energy. That reduces it's equipment cooling ability. However, with aprons that doesn't happen because they block hot air from going to where cold air drops from the ceiling. That allows the cold air to stay cold until it gets to the hot equipment in the racks and significantly increases cooling efficiency .

DIAGRAM-6: Click on diagram to enlarge
One other thing! For maximum efficiency it's best to completely enclose the cold air side of the rows with similar walls and a door at each end shown below. The idea is to contain every bit of cold air and keep it cold for cooling hot equipment - not lose/waste any of it, which reduces the cost of cooling.

IS THAT THE BEST YOU CAN DO?    NO!  To get the absolute most efficient data center cooling system, we need to continue delivering the cold air from floor vents as we've been showning all along, but instead of using aprons, we will use a row lid as shown below. A row lid will eliminate cooling yet another space that doesn't need to be cooled, the space above the racks and it will even better direct the cold air to the electronic equipment in the racks.

DIAGRAM-7: Click on diagram to enlarge
Now just close off the ends with some doors and the cool side is completely contained. The picture below is very close to our most efficient design. As can be seen through the plastic floor strip there is no underfloor duct supplying the cold air. This means that at this data center, instead of using ducts to deliver the cold air directly to the floor vents, the entire subfloor space is pressurized with cold air so that it will push up through the floor vents. Unless there is a need to cool the subfloor space, it just adds more load to the AC system, which in turn adds more operating cost. So to make it the most efficient design, just add underfloor ducts.


Row cooling is what we've been describing is called because the racks are arranged rows. Whether you have just two or three racks or hundreds of them, the goal is to have your air conditioners doing the least amount of work possible to keep your electronic equipment in them at the manufacturers recommended temperature. Row cooling is the most efficient way to accomplish that.

You now know the basics necessary to make intelligent data center cooling decisions. As part of your planning and decision making process be sure to do some value engineering analysis. I promise, you'll be glad you did.


Cost Effective Cooling
  1. The less an air conditioner has to work the longer it lasts, the less it breaks, and the less it costs to run.
  2. The more efficiently a data center is cooled the less the AC will have to work.
  3. Make sure your data center cooling system design is efficient.
  4. To know if it is, you must first understand how cooling really works.
Heat Energy Fundamentals
  1. Heat energy on air is what makes air hot.
  2. Heat energy cannot be destroyed, only transferred per the Law of Conservation of Energy.
  3. Heat energy is attracted to things that are cool (things that have less heat energy than where they are).
  4. Heat energy transfers by jumping from the thing it's on to something that has less heat energy.
  5. Heat energy gets into a data center by riding electrical current that runs in electrical wires coming from outside the data center.
  6. Heat energy makes a data center hot by jumping from the electrical wires to the electronic equipment they serve then from there onto available cold air around it making it hot.
Air Fundamentals
  1. Cold air tends to drop and hot air tends to rise.
  2. Don't drop cold air through hot air on its way to hot equipment because the cold air will then pick up heat energy, which reduces it's ability to cool electronic equipment.
  3. Keep the cold air in the cold aisle side separate from the hot air in the hot aisle side with barrier walls.
  4. Contain and direct cold air so it only goes to the electronic equipment.
  5. Keep hot air moving away from the electronic equipment so cold air can continually move into its place.
  6. If value engineering justifies it, use outside AC systems instead of CRACs.
Row Cooling
  1. Row cooling is the term given for the data center rack and cooling system configuration described and illustrated in this article.
  2. Row cooling using completely contained cool aisles and ducts that deliver the cold air all the way to the cool aisle floor vents provides the most efficient and cost effective data center cooling.
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