Seven Ways Precision Air Conditioning Outperforms Comfort Systems In Controlled Environments

8/7/2019 Seven Ways Precision Air Conditioning Outperforms Comfort Systems In Controlled Environments

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Engineering White Paper

Seven Ways Precision Air Conditioning OutperformsComfort Systems In Controlled Environments


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SUMMARYEfficient, reliable operation of business critical computer systems is essential to the successof companies in virtually every industry. The cost of downtime is high, with even a shortinterruption causing the potential loss of productivity, profits and customer goodwill. And,business critical computers are no longer clustered only in one centralized location. Many

organizations today have multiple data centers, including smaller centers at remote loca-tions.

Often small or remote data centers dont benefit from the same attention to supportsystem design as large data centers, despite the fact that the costs associated with down-time from the systems they house can be remarkably high. One area, in particular, that maybe receiving inadequate attention is environmental control: Whether through rapid growth-- results in the data center outgrowing its support systems or lack of understanding of the cooling requirements of sensitive electronics, some users end up trying to use comfortcooling for precision electronics.

The result is increased risk for premature failure of protected equipment and higher thannecessary operating costs.

Precision cooling systems are designed specifically to meet the needs of dense electronicloads, which generate a dryer heat than typical comfort-cooling environments and requireyear-round, 24-hour cooling. Electronics are also more sensitive to variations in humidityand air quality than typical comfort-cooling environments.

When an decision is made to accept the increased risk of using comfort cooling systemsfor sensitive electronics, it is usually based on higher initial costs of the precision coolingsystem. But, because precision cooling systems are designed and sized for the high sensible

cooling requirements and year-round operation of the data center, they are less expensiveto operate over their life than comfort systems. Consequently, when initial costs and operat-ing costs are both taken into account, precision cooling systems actually represent the mostcost-effective solution to cooling sensitive electronics.


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Precision vs. Comfort Cooling

Consideration given to the planning anddesign of equipment essential to reliabledata center operation is often less thangiven to other aspects of the facility plan-ning process, particularly in smaller orremote facilities. Areas such as environ-mental control, power reliability and effec-tive monitoring of critical computer sup-port systems receive far less attention thandecisions about servers, operating systemsand network configurations. Yet, the perfor-mance of these systems is as dependent onpower and environmental support systemsas on the network connection.

The combination of heat generation andsensitivity of electronic components indata center computer and network systemsrequires that temperature, humidity, airmovement and air cleanliness be main-tained within stringent limits.

Precision cooling systems have beendesigned specifically for this purpose. Avail-able comfort cooling systems are occasion-ally used in these applications; however,that is usually because the differencesbetween comfort and precision cooling sys-tems are not well understood.

Comfort air systems include room air con-ditioners, residential central air condition-ers and air conditioning systems for officeand commercial buildings. They are engi-neered primarily for the intermittent use

required to maintain a comfortable envi-ronment for people in facilities with amoderate amount of in-and-out traffic.

Precision air systems are engineered pri-marily for facilities that require year-round constant cooling, precise humiditycontrol and a higher cooling capacity persquare foot than comfort systems.

Specifically, seven major differences existbetween precision air conditioning andcomfort systems.

1. Cooling Optimized to ElectronicSystem Requirements

Dense loads of electronics generate a dryerheat than typical comfort-cooling environ-ments and this significantly changes thedemands on the cooling system.

There are two types of cooling: latent andsensible.

Latent cooling is the ability of the airconditioning system to remove moisture.This is important in typical comfort-coolingapplications, such as office buildings, retailstores and other facilities with high humanoccupancy and use. The focus of latent cool-ing is to maintain a comfortable balance of temperature and humidity for people work-ing in and visiting such a facility. These facil-ities often have doors leading directly tothe outside and a considerable amount of entrance and egress by occupants.

Sensible cooling is the ability of the air con-ditioning system to remove heat that can

be measured by a thermometer. Data cen-ters generate much higher heat per squarefoot than typical comfort-cooling buildingenvironments, and are typically not occu-


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pied by large numbers of people. In mostcases, they have limited access and nodirect means of egress to the outside of thebuilding except for seldom-used emergencyexits.

Comfort air conditioning systems have asensible heat ratio of 0.60 to 0.70. Thismeans that they are 60 to 70 percent dedi-cated to lowering temperature, and 30 to40 percent dedicated to lowering humidity.Data center environments require a 0.80 to0.90 sensible heat ratio for effective andefficient data center cooling. Precision airconditioning systems have been designedwith a sensible heat ratio of 0.85 to 1.0.This means 80 to 100 percent of their effortis devoted to cooling and only 0 to 20 per-cent to removing humidity. So more nomi-nal 20-ton comfort units will be required tohandle the same sensible load as nominal20-ton precision units

As the need for latent heat removal lessens,so too does the need for dehumidification.Data centers have a minimal need forlatent cooling and require minimal moistureremoval. Because precision cooling systems

are engineered with a focus on heatremoval rather than moisture removal andhave a higher sensible heat ratio, they arethe most useful and appropriate choice forthe data center.

2. Systems Sized to the Higher Densitiesof Data Center Environments

Heat densities in electronics environmentsare three to five times higher than in a typi-cal office setting, and increasing at a fasterrate than ever before.

To illustrate, one ton of comfort air condi-tioning capacity (12,000 BTU/hour or 3413watts) is required per 250-300 square feetof office space. This translates into 15 wattsper square foot. In contrast, one ton of pre-cision air conditioning capacity is required

per 50-100 square feet of data center space.That translates into a much larger 75 wattsper square foot. And this is an averagenumber that is increasing yearly. Some sitescan have load densities as high as 200 300watts per square foot.

From an airflow standpoint, precision airsystems are designed differently than com-fort air systems to manage the larger loaddensities in data centers. A precision airsystem achieves a higher sensible heat ratio,helps maintain target temperature and

Sensible Heat Ratio

General Office Computer Room

.65

1.0

SENSIBLE SENSIBLE

LATENTLATENT .90

1.0

Figure 1. General office environments haveloads that are 60-70 percent sensible (.6-.7SHR), while computer rooms have 90-100percent sensible loads (.85-1.0 SHR)


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humidity levels and contributes to better airfiltration via the movement of significantlylarger volumes of air. Precision air equip-ment typically supply 500 to 900 cubic feetper minute (cfm) per cooling ton. This con-trasts with the much smaller range of 350 to400 cfm typically delivered by comfort airequipment.

In addition, the use of blade servers and

other newer rack-mounted equipment hascreated unprecedented new data centerheat rejection requirements. New serversand communications switches generate asmuch as ten times heat per square foot assystems manufactured just ten years ago.Data center designers and engineers nowneed to consider the need for specializedheat rejection units to work in conjunctionwith raised floor precision cooling systems.Comfort air systems are simply ill-equippedto deal effectively with this new challengeand the associated heat rejection required.

3. Precise Humidity Control

Ignoring the impact of humidity can resultin serious long-term problems, includingdamage to equipment and other resources,and to the facilitys infrastructure. The opti-mal relative humidity range for a datacenter environment is 45-50 percent. Anabove-normal level of moisture can corrodeswitching circuitry, which can cause mal-functions and equipment failures. At theother end of the spectrum, low humiditycan cause static discharges that interferewith normal equipment operation. This is amore likely scenario in a data center sinceit is typically cooled 24x7, creating lowerlevels of relative humidity.

Comfort air systems typically have nohumidity control, which makes it difficultto maintain stable relative humidity levels.If the necessary controls and componentsare added, they have to be set-up to oper-ate as a complete system. Precision air sys-tems have multi-mode operation to providea proper ratio of cooling, humidification and

dehumidification. This makes them muchmore suitable for the low tolerance range of humidity levels in a data center.

Figure 2. Heat densities are typically 3-5times higher in computer room environ-ments than in office environments.

General Office Computer Room

75 Watts/ sq.ft.

15 Watts/ sq.ft.

HeatDensity

0

10

20

30

40

50

60

70

80

Figure 3. Infrared humidifiers enableprecision air conditioning systems toprovide fast, precise humidity control.


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4. Protection Against AirborneContaminants

Even small amounts of dust or otherparticles can damage storage media andcharged electronic components. Most com-fort air systems use residential-type air fil-ters that are 10 percent efficient, makingthem inadequate for a data center envi-ronment. Precision air system filters havehigher quality internal filter chambers thatare 20-30 percent efficient and ASHRAEcompliant.

5. Efficient Continuous Year-RoundCooling

Comfort air conditioning systems for mostbuildings are designed to operate an aver-age of 8 hours per day, 5 days per week. Thistranslates into about 1,200 hours per year,assuming cooling is required only duringthe summer months.

Most data centers require heat rejection 24

hours per day, 365 days per year regardless

of outside weather conditions. Precision airconditioning systems and their componentsare engineered to meet this high coolingdemand. A precision air units circulating fanoperates continually, 8,760 hours per year,

with other components turning on and off as needed.

Moreover, comfort air systems with out-door heat exchangers are typically inoper-able due to lack of head pressure controlwhen outside ambient temperatures dropbelow 32 F. A precision system can operateeffectively to minus 30 F., and glycol-cooled models cool effectively down tominus 60 degrees F. outside ambient tem-peratures.

6. Lower Operating Costs

Because of basic engineering, design andequipment differences, a purchase pricecomparison of comfort versus precision airconditioning systems does not tell the com-plete story. A more accurate comparison

will consider the difference in operatingcosts between the two systems.

To following is a basic example of an oper-ating cost comparison between the twoapproaches, using the assumptions outlinedbelow. A more detailed analysis can be doneon specific equipment.

Each ton of cooling requires 1.0horsepower (or .747 kw)

The compressor motors and fans are90 percent efficient

Electricity costs $.06 per kilowatt-hour.

Figure 4. Precision air conditioningsystems are available in a range of configurations to provide room-level andrack-level protection.


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Humidification is required Novemberthrough March (3,650 hours)

The precision air system has a SHR of 0.90; the comfort system has a SHR of 0.60.

First, calculate the cost per ton of coolingfor a year:

0.746 kw/ton x 8760 hrs/yr x $0.06/kwh

0.90 efficiency

This results in a cost of $436 ton/yr.

Then determine the cost per sensible ton of cooling by dividing the total cost by the SHRfor each system. For the precision air systemthe cost per sensible ton is

$4360.90 SHR

For the comfort cooling system, the costper sensible ton is :

$4360.60 SHR

In this example, the operating cost to run a

comfort air system for one year exceeds thecost to run a precision air system by $243 per ton of sensible load. This is consistentwith the generally accepted principle that ittakes three tons of comfort cooling capacityto equal two tons of precision capacity.

A second point of comparison is the costof rehumidification, which is determined bycalculating the latent cooling that occursper ton of sensible cooling.

For a precision system:

12000 Btu/ton0.90 SHR

For a comfort system:

12000 Btu/ton0.60 SHR

The comfort system expends 6,667 Btu

of energy per ton of sensible cooling toremove humidity that must be replacedto maintain required data center moisturecontent of 45-50 percent.

The added cost is:

6667Btu/ton x 3650hrs/yr x $0.06/kwh3413 Btu/hr/kw

In this scenario, when all cooling andrehumidification costs are considered, the

operating cost of a comfort-based systemexceeds the operating cost of a precision airsystem by $670 per ton of sensible coolingannually.

7. Enhanced Service and Support

Critical environments require high availabil-ity of critical support systems. Therefore,it is important that these systems operatereliably, and that their performance is tunedspecifically to control the environment forthe computer systems with which they aredeployed.

Precision air systems often feature greaterinternal redundancy of components thancomfort cooling systems, allowing them tocontinue operating in the event of somefailures. In addition, they are supported

by factory-trained, locally-based installa-tion, service and support partners that areaccustomed to the needs and sensitivitiesof working in the data center environment.Depending upon the manufacturer, 24-hour

= $484 ton/yr.

12,000 Btu/ton = 1,333 latentBtu/ton

12,000 Btu/ton = 8,000 latentBtu/ton

= $427 ton/yr.

= $727 ton/yr.


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emergency service and preventive mainte-nance service may be available on precisioncooling systems.

Because even a short amount of downtimecan impact the bottom line, leadingprecision air systems are designed forserviceability.

Conclusion

Computer systems have unique environ-mental requirements and necessitate cool-ing systems that match those require-ments. Comfort cooling systems are appro-priate for comfort environments facil-ities that are occupied by people or thathouse routine equipment and supplies.

Precision cooling systems provide the effi-ciency, reliability and flexibility to meetthe increasing demands for heat rejection,humidity control, filtration, and otherrequirements in data centers and other highavailability computer facilities.


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Seven Ways Precision Air Conditioning Outperforms Comfort Systems In Controlled Environments

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