Posts Tagged 'Crac'

March 9, 2011

Building a Data Center | Part 2: The Absence of Heat

As you walk down the cold aisle in a data center, you might be curious about how all that cold air gets there. Like the electrical system, data center cooling travels a path through the data center that relies on many integrated systems working together to achieve the desired result.

To start, I should give a crash course in Heating, Ventilating, and Air Conditioning (HVAC). The most important thing to understand in HVAC theory is that cold is the absence of heat. When you say you're cooling a space, you're not adding cold air, rather you are removing heat. Heat is removed in a cycle called the refrigerant cycle. The refrigerant cycle is present in all air conditioning systems and is made up of four main components:

  • Refrigerant: Refrigerants are engineered chemicals developed to have very specific boiling and condensation temperatures. They come in many different flavors with cryptic names like 410a, R22, and water.
  • Compressor: Compresses refrigerant, turning it from a warm liquid to hot gas. This compression assists in the movement of heat and refrigerant within the system.
  • Evaporator: Evaporators are heat exchangers (devices built for efficient heat transfer from one medium to another), so an evaporator passes heat from the air to the refrigerant.
  • Condenser: Condensers are also heat exchangers. The condenser releases trapped heat in refrigerant outside the space being cooled.

This is a very simplified explanation of the refrigerant cycle components, and I only mention these four components or steps because they are common to all HVAC systems regardless of size or type, and you can apply them to any data center cooling system (or even a residential system).

Cooling System

Which came first - the chicken or the egg? Like the old analogy, figuring out a starting point for our cooling cycle isn't easy to do, so let's start at the source of heat. Your server uses electrical energy to process the information in the CPU, turn the spindle of hard drives, and light up the pretty little LEDs on the chassis. All that conversion of electrical energy to useful work creates heat as a side effect. Remember that we have to remove heat to cool something, so a server's cooling system acts like a heat exchanger, extracting heat from its components and passing that heat to cooler air entering the front of the server. That heat is rejected from the back of the servers into the hot aisle.

When the heat is exhausted into the hot aisle, it is pulled to the evaporator, which we call different things depending on how they perform their function. CRACs – Computer Room Air Conditioners – and AHU – Air Handling Units – are a few of the common terms we use. Regardless of what they are called, they perform the same function by removing heat from the hot aisle (called return air) and supply cooled air to the cold aisle. This completes the first step in the cycle.

Now that the heat was removed from the server room and passed to the refrigerant, it must go somewhere and that is where the compressor comes in. Warm liquid refrigerant is compressed into a hot gas and this compression of refrigerant forces it to travel to the condenser where the heat is absorbed into the outside air. This allows the cooled refrigerant to condense and return to the evaporator to start the process all over again. And again, this part of the cycle is accomplished different ways depending on the type of equipment installed.

If a CRAC unit is installed, the evaporator and compressor are on the computer room floor, a remote condenser will in placed outside, and fans will extract the heat from the refrigerant. In areas where AHUs are used, only the evaporator will be typically be on the raised floor. These systems use a remote compressor and condenser to send chilled water to the AHU’s on the raised floor. Also, these chilled water systems actually have two separate refrigerant systems in place, isolating the inside and outside portions of the refrigerant cycle. They are used in larger denser data centers because they allow for more efficient control of temperatures in the data center.

Like I said, this is a simplified explanation of the cooling of a data center but it lays the ground work for a more in-depth look at your specific systems.

-John

February 24, 2011

A Crash Course in CRAC Units - Data Center Cooling

In the past few weeks, we've fielded a few questions from our Twitter followers about temperatures in our data center and how CRAC units work. John mentioned in the "Building a Data Center" series that his next post would be about keeping the data center cool, so I'll try not to steal too much thunder from him by posting a basic CRAC unit explanation to answer those questions.

To record this video, we made the long walk (~2 minutes) downstairs to Pod 1 of SoftLayer's DAL05 facility to give you a first-hand look at the star of the show: the DC Computer Room Air Conditioning Unit. Because this was recorded on a "Truck Day" at SoftLayer, the pod was bustling with activity, so we found a "quiet" open area in a section of the pod that will soon be filled with new servers to record the video.

Due to the ambient noise in the data center, my explanation had to be "yelled," so please forgive the volume.

What else do you want to see/learn about in SoftLayer's data centers?

-@khazard

July 22, 2008

Always Awake, Cool and Dry

As I turn on to the main road after leaving my Kumdo dojang (Korean fencing school), I glance at the rear view mirror down the street, in the direction of SoftLayer's new east coast datacenter. The strangely cool, red light from the setting sun fills the mirror and signals the end of this long, hot day. My mind briefly escapes the fading heat by recalling the cool temperature and humidity regulated environs within the datacenter.

Ever wonder how to keep thousands of servers cool? In a word: CRAC - Computer Room Air Conditioning. These giants sit throughout the datacenter pumping cool air up through ventilated floors. The cool air blows up in front of the server racks, gets sucked in through the front of the servers, over the drives, past the CPU heat sinks and RAM, then out the back of the server. The warm air exits, rises, and returns to the CRACs where the humidity and temperature are adjusted, and the cycle continues. Just like you learned in science class.

So it must be a serene, sterile environment - like those IBM commercials? That would be nice, but the reality is : computers need fans. One or two fans wouldn't bother anyone when they kick in on your gaming pc, but multiply 4 or 5 fans (do you like RAID arrays? You get extra fans!) by one thousand, or more and the decibels add up. Solid state hard drives - when they become available - might help with the noise (and also with power consumption), but it is mostly from the server fans. Liquid cooling works, but I think most people would prefer not to have fluid of any sort circulating over their motherboard. Zane (resident Linux guru) extols the benefits of passive cooling. Whatever cooling solutions arise in the future, you can be sure SoftLayer will be leading in technology implementation.

My attention returns to the road ahead and the pale blue of the evening sky. I hope to get a few hours of shut-eye before returning for my shift. Because SoftLayer doesn't sleep. Always awake, cool and dry.

-Philip

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