Data center Cooling Infrastructure

The high-density deployments of modern data centers have intensive power demands, but much of that power doesn’t actually go to the servers and computing equipment itself. Instead, it goes to the cooling equipment that prevents those systems from overheating. Data center cooling refers to a collective equipment, tools, techniques and process that ensures an idle operating temperature within a data center facility. In general, temperatures should not fall below 50°F or rise above 82°F in a server room(10°C -  27.7°C) whereas optimal temperatures range between 68° and 71° Fahrenheit(20°C - 21.6°C). To ensure reliable operation and the longest possible life from components you need to ensure that the temperature stays within that band. Even a few degrees too hot can blow a server chip. The cost of a catastrophic server failure can be considerable. Think how much money you would lose if your servers went down. There is the cost of replacement, but think also of lost e-commerce business, lost customer details, wasted staff time, and all the other associated costs.

IT equipment consumes electricity and leaves behind heat as its “waste product”. On my previous articlewe have seen that power is the biggest part in a data center. Do you know that your cooling infrastructure is the biggest consumer of this power. This may take up to 40 percent of all the power going into your data center and you know the heat generated by servers in data centers is currently ten times greater than the heat generated by them 10 years ago. So it’s huge , you can imagine, right. 

The simplest way to define cold is nothing the absence of heat. Generally there are two types of heat removal methods used in a data center environment,

·      Air Cooling

·      Liquid Cooling(typically water or some form of refrigerant)

Air cooling is a standard method of system cooling used to method of dissipate heat. The object being cooled will have a flow of air moving over its surface. Most air cooling systems use a combination of fans and heat sinks, which exchanges heat with air.

Liquid Cooling is the first method within liquid-based cooling which is “water-cooled racks.” This method uses water to cool along the hot side of the cabinet bringing the temperature down. The water is confined within basins and flows from tower pumps through pipes alongside the servers, but does not touch the components of the servers. The water-cooled racks system works very well, but still has the dangerous possibilities of leaks. We can say that this is not a largely adopted solution for cooling do you know why?

The answer is pretty simple: Water + Electricity = Disaster.

However this is being used on some industry level and we will look into those details in next article.

Immersion Cooling

The last method we will go over is “liquid immersion cooling” system. It is the practice of submerging computer components (or full servers) in a thermally, but not electrically, conductive liquid (dielectric coolant). Green revolution cooling(GRC) has patented this method of cooling technology. 

Before proceeding further I want you to understand some of the fundamental principles of cooling which are as below,

   Compression

·      Gases are compressible - liquids are not

·      Gases get hot when compressed

    Heat flow

·      Heat flows from high temperature to low

·      When a liquid is heated it vaporizes into a gas

·      When a gas is cooled it condenses into a liquid

·      Lowering the pressure above a liquid reduces its boiling point, and increasing the pressure raises it

Fluid flow

·      Fluids flow from high pressure to low

Cooling System Components in a data center

As you know the output from a machine is heat and we need to supply cold as the input for these machines. Let us see and understand the equipment used in a chiller based cooling systems and understand the flow.

The Chiller

Chillers are the cooling machines used to lower the temperature of equipment in data centre environments (uses the basic refrigerant cycle operation). A chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air or equipment as required. The function of a chiller is to produce chilled water (water refrigerated to about 8 -15°C [46 - 59°F]). Chilled water is pumped in pipes from the chiller to the Computer Room Air Handlers (CRAH) units located in the IT environment.

Chillers use either a vapor-compression or absorption refrigerant cycle to cool a fluid for heat transfer. The working principles are relying on three basic principles:

      I.         First - When a liquid is heated, it vaporizes into a gas, and when a gas is cooled, it condenses into a liquid

    II.         Second - Lowering the pressure above a liquid reduces its boiling point and increasing the pressure raises it

  III.         Third - Heat always flows from hot to cold

There are three main types of chillers distinguished by their use of water or air to reject heat:

·      Water-cooled chillers – This model is have a cooling tower, thus they feature higher efficiency than air-cooled chillers. Water-cooled chillers are more efficient because they condense depending on the ambient temperature bulb temperature, which is lower than the ambient dry bulb temperature. The lower a chiller condenses, the more efficient it is.

·      Glycol-cooled chillers - Glycol chillers are industrial refrigeration systems that use a type of antifreeze called glycol, mixed with water, to lower the freezing point in the application of the chilling system.

·      Air-cooled chillers - Air cooled and water cooled and chillers work in a rather similar manner. They both have an evaporator, compressor, condenser and an expansion valve. The main difference is that one uses air to fuel condenser cooling and the other uses water.

If anyone interested to understand more in this concept please review the article by Carrier

Cooling tower

A cooling tower is a heat rejection device that rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature(atmosphere).  Cooling tower heat rejection process is termed “evaporative cooling”. Cooling towers minimize the thermal pollution of natural water heat sinks and allow the reuse of circulating water. The heat from the water stream transferred to the air stream raises the air's temperature and it relative humidity to 100%: this air is then discharged to the atmosphere.

Cooling towers minimize the thermal pollution of natural water heat sinks and allow the reuse of circulating water. A cooling tower extracts waste heat from the condenser water loop to the atmosphere through the cooling of a water stream to a lower temperature.

The cooler water at the bottom of the tower is collected and sent back into the condenser water loop via a pump package. Evaporative heat rejection devices such as cooling towers are commonly used to provide significantly lower water temperatures than achievable with "air cooled" or "dry" heat rejection devices, thereby achieving more cost-effective and energy efficient system operation.

Computer room air conditioning (CRAC)

A computer room air conditioning (CRAC) unit is a device that monitors and maintains the temperature, air distribution and humidity in a network room or data center. CRAC is  also known as Precision Air Conditioner or Close Control Air Conditioner or Close Control Unit (CCU) or server room air conditioner. A CRAC unit is exactly like the air conditioner at your home. It has  direct expansion(DX) refrigeration cycle built into the unit. This means that the compressors required to power the refrigeration cycle are also located within the unit. The temperature of the coil is maintained with the help of a refrigerant. A CRAC is often thought of as having an internal compressor, thus not needing the support of a centralized chilled water system.

There are a variety of ways that the CRAC units can be situated. One CRAC setup that has been successful is the process of cooling air and having it dispensed through an elevated floor. The air rises through the perforated sections, forming cold aisles. The cold air flows through the racks where it picks up heat before exiting from the rear of the racks. The warm exit air forms hot aisles behind the racks, and the hot air returns to the CRAC intakes, which are positioned above the floor.

Computer room air handler(CRAH)

A computer room air handler (CRAH) is a device used frequently in data centers to deal with the heat produced by equipment. Unlike a computer room air conditioning (CRAC) unit that uses mechanical refrigeration to cool the air introduced to a data center, a CRAH uses fans, cooling coils and a water-chiller system to remove heat. A CRAH is similar to a chilled water air handling unit found in most of these high rise commercial office buildings. Here, the cooling is accomplished by blowing air over the cooling coil filled with the chilled water. These chilled water in turn, are supplied to the CRAHs by a chilled water plant, otherwise called as, the chiller. These CRAHs can have VFDs that modulate fan speed to maintain a set static pressure either under floor or in the overhead ducts.

Now that we understand how air circulates in the data center, we must understand the difference between air handling and air conditioning. Anyone who pays their power-bill at home knows that the cost of cooling/heating a home is one of the highest costs associated with power. The same holds true for data centers. Cooling a data center, which houses equipment that generates substantial heat, is very expensive. Because of this, data centers have turned to what is call air-side economization. What that means is that when the ambient outside temperature is within a certain threshold, the air is then ‘handled’ and pulled into the facility and pushed down in the pressurized sub-floor. The handled air flows through the data center as traditional air conditioned air would. However, in this scenario, the facilities with CRAH units are actually using ‘free’ air and the cost to cool the facility significantly drops.

The efficiency of this method obviously depends on climate, In many Europe and US  market we can use this technology about 60% of the year(during the winter months). This saves immense power cost, which in turn lowers PUE and drives the cost per kilowatt down for you. Overall, the utilization of air-handling and air-side economization rather than traditional air-conditioning is an all-around innovation win for the industry and its clients.

Water Side Economizer

Using this economizer, there are no noticeable changes on the data center floor. The same collection of air handlers, raised floors, and fans move air as they normally would. The change occurs behind the scenes in the production of the chilled air and the removal of waste heat. Users with an existing chilled water infrastructure can accomplish “free-cooling” via a supplemental heat exchanger called a Water-Side Economizer.

For data centers with water- or air-cooled chilled water plants, a water-side economizer uses the evaporative cooling capacity of a cooling tower to produce chilled water and can be used instead of the chiller during the winter months. Water-side economizer operation depends on ambient conditions. The outside air must sufficiently cool the condenser water to allow for proper heat exchange between the two loops.

Water-side economizers can be integrated with the chiller or non-integrated. Integrated water-side economizers are the better option because they can pre-cool water before it reaches the chiller. Non-integrated water-side economizers run in place of the chiller when conditions allow. Water-side economizers offer cooling redundancy because they can provide chilled water in the event that a chiller goes offline. This can reduce the risk of data center down time. During water-side economizer operation, costs of a chilled water plant are reduced by up to 70%.

Air-Side Economizer

An air-side economizer(see below picture) brings outside air into a building and distributes it to the servers. Instead of being re-circulated and cooled, the exhaust air from the servers is simply directed outside. If the outside air is particularly cold, the economizer may mix it with the exhaust air so its temperature and humidity fall within the desired range for the equipment. The air-side economizer is integrated into a central air handling system with ducting for both intake and exhaust; its filters reduce the amount of particulate matter, or contaminants, that are brought into the data center.

Because data centers must be cooled 24/7, 365 days per year, air-side economizers may even make sense in hot climates, where they can take advantage of cooler evening or winter air temperatures. For various regions of the United States, Figure 14 shows the number of hours per year with ideal conditions for an air-side economizer.

Intel IT conducted a proof-of-concept test that used an air-side economizer to cool servers with 100% outside air at temperatures of up to 90°F. Intel estimates that a 500kW facility will save $144,000 annually and that a 10MW facility will save $2.87 million annually. Also, the company found no significant difference between failure rates using outside air and an HVAC system. Some of the things has to be considered is control systems are very important to the operation of the air-side economizer and must be properly maintained. Excessive humidity control can cut into the savings achieved by the economizer. In certain geographic locations, for example, air can be very cool but very dry, and the system may spend excessive energy humidifying the air. Users will need to consider ASHRAE's recommendations, studies of their ambient climate, and their humidity preferences before considering implementation. If desired humidity ranges are too restrictive, net energy savings from an economizer can be limited. Proper management and controls are imperative to ensure that correct air volume, temperature and humidity are introduced.

Air-side and water-side economizers offer the potential for significant energy savings by using the ambient environment to augment or replace mechanical air conditioning. Depending on your location, environment, data center design and existing infrastructure, implementing a “free cooling” strategy can be extremely challenging. And the objective of economization is to run those less or run those with less load.

What allows more economizer hours?

The basic function of a chiller is to remove heat energy from a data center by compressing and expanding a refrigerant to keep chilled water at a set supply temperature, typically 45°F/7°C. When the outdoor temperature is about 19°F/11°C colder than the chilled water temperature, the chiller can be turned off. The cooling tower now bypasses the chiller and removes the heat directly from the data center.

By increasing the chilled water supply temperature, the number of hours that the chiller can be turned off (economizer hours) increases. For example, there may be 1000 hours per year when the outdoor temperature is at least 19°F/11°C below the 45°F/7°C chilled water temperature. But if the chilled water is increased to 55°F/13°C, the economizer hours increase to 3,700.

By this we have covered all critical mechanical equipment’s that are involved in a data center cooling infrastructure.

Knowledge Credits : www.energystar.gov and www.42u.com

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