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A

Report on

PRECISION AIR-CONDITIONING

Submitted

By

S.SABOOR

(Research scholar)

Roll no: 110669ME11F05

Under the guidance of

Prof.T.P.ASHOK BABU

MECHANICAL ENGINEERING DEPARTMENT

NATIONAL INSTITUTE OF TECHNOLOGY, KARNATAKA

SURATHKAL.

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NOMENCLATURE

CRAC - Computer Room Air-Conditioner

TR - Ton of refrigeration (Tons)

HACA - Hot Aisle/Cold Aisle

RH -Relative humidity (%)

Cfm -Cubic foot per minute (ft3/min)

ESD -Electrostatic discharge

T - Temperature (oC)

Q - Heat load from data center components (W/ft2 )

m - Air flow rate (cfm/ft2)

[ ] - indicates reference numbers in references

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CONTENTS PAGE

1. Abstract 04

2. Introduction 04

3. Study of NITK server room 06

Necessity of humidity in server rooms 08

Electrostatic discharge method 08

Humidity control methods 09

Computer server room acoustics 09

4. Conclusions 10

5. References 10

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ABSTRACT:

The computer equipment installed in a data center or office space must be maintained

within acceptable temperature and humidity specifications for reliable operation. A typical

cooling arrangement consists of installing the equipment on a raised floor and using several air-

conditioning units to force air into the space under the raised floor. While most the raised floor

is impermeable, perforated tiles are installed at desired locations to provide cool air at the inlets

of the data processing equipment. Temperature and humidity requirements for computer

equipment are as broad as possible to permit customers the flexibility of the type of equipment

they install as well as the degree of control. This paper mainly focuses on the concept of

understanding precision air-conditioning with a case study of NITK server room air-

conditioning arrangement.

1. INTRODUCTION:

A data center or server room is a facility housing for high-performance computers,

storage servers, computer servers, networking or other IT equipment. It provides various services

such as storage, management, processing and exchange of digital data and information for

Information and Communication Technology (ICT). Data centers consume huge amount of

energy. Temperature and humidity requirements for computer equipment are as broad as possible

to permit customers the flexibility of the type of equipment they install as well as the degree of

control. The most common temperature and humidity specifications for components that are

installed in today's computer rooms are summarized here:

Temperature Range = 16 to 32°C [1]

Relative Humidity Range = 20 to 80% [1]

Recommended Temperature = 22 ±10C [1]

Recommended Relative Humidity =50 ± 5% [1]

With the increased computer equipment heat loads the air flow rate through the system is

increased. Generally, the flow rate through the equipment is approximately 1 cubic foot per

minute (cfm) for every 5 to 7 watts of heat load. For example, for an equipment rack/frame with

20 kW of heat load, approximately 2500 cfm of air flow through the rack will be required. In

addition, the inlet air conditions for this system must meet the temperature and humidity

requirements specified by the manufacturer. The challenge for the customer with the increased

rack heat loads is in providing stable environmental conditions within the manufacturers

specifications.

In general, cooling systems can be classified into two categories: air-forced cooling and

liquid cooling. Air cooling is still predominant. In this technique, cold air is pushed through

racks, containing IT equipment, for heat removal. The aim is to keep the rack (IT equipment)

inlet temperature within an acceptable range for reliable operation of equipment in data centers.

The complexity of the rather high internal heat dissipation and cold and warm outdoor

environments requires a smart solution, capable of providing acceptable indoor climate

management. Many efforts have been put to advance air cooling systems. Introduced in 1992 by

IBM, Hot Aisle/Cold Aisle (HACA) protocol for air cooling is probably the most popular

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technique to date. The majority of modern data centers are still using HACA. Some other

alternatives include In Row Cooling with Hot Aisle Containment, Cold Aisle Containment and

Overhead Cooling. However, with the growth of heat dissipation in data centers, air cooling and

HACA are stumbling and many problems have occurred, such as hot spots and oversized cooling

equipment. Consequently, liquid cooling is making its way back to data centers. It is considered

to be more efficient and the future cooling technique since liquid can carry much more heat than

air. In fact, liquid cooling is not new in data centers, but its acceptance has been and continues to

be difficult.

In IT server rooms the air distribution system plays a very vital role in cooling the server

racks. Reducing the air-conditioning temperature is not good practice to reduce the hot spots in

the room due to the congested spaces in the racks (due to bulk wires). Mixing of cold and hot air

steams adversely affect the cooling performance.

Prevention of the air from the cold aisle and the hot aisle intermixing; air recirculation and by-

pass must be considered in order to prevent a drop in the rack cooling efficiency. However, there

is a limitation in effectively preventing such phenomenon in an open space – IT server room with

only the location of supply and return air infrastructures. The reality is that the problem of rising

temperature (hot spot) continues to occur in currently operating data centers (see Fig.1 (a)).

Therefore, there is a need to improve air distribution efficiency through additional physical

barrier installation, and cooling efficiency can be improved by installing a simple partition wall

on the rack server. Two types of formation are possible here[2]: the aisle partition system

vertically dividing the cold aisle and the hot aisle, as shown in Fig.1(b): and the aisle enclosure

system that blocks off the upper part of the cold aisle, as shown in Fig.1(c). In terms of cooling

efficiency, the aisle enclosure system that completely surrounds the cold aisle can more

effectively prevent air re-circulation in comparison with the aisle partition system that simply

forms a vertical wall. However, because the aisle enclosure system can act as an obstructing

factor when relocating the IT server, setting up fire suppression systems as clean agent systems

and water based systems (gas and sprinkler heads installation location), and responding quickly

to a fire, the installation of the aisle partition system is more reasonable. In addition, since a large

data center has a large amount of IT equipment, the matter of initial cost cannot be overlooked.

In the enclosed space of a server room, the heat that all those boxes generate can quickly increase

the ambient temperature beyond equipment specifications. The results can be ugly: hardware

failure, loss of data, and an uncomfortable working environment are all distinct possibilities.

It’s critical to keep server room’s temperature within the listed tolerances of hardware. Tallying

up the heat dissipation from servers and other hardware can help to ensure server room is

designed and built with adequate ventilation and cooling.

In case of raised floors, the air intake vents also are in the ceiling in this type of setup, which

would lead to cool air being sucked back into the system before it has a chance to reach the

server racks. When using this type of forced air system, you need to place air return vents at

locations throughout the room to provide for the proper flow of air. The best place for the vents

when using a raised floor is in the ceiling of the room, since hot air rises and will more likely be

drawn out of the room by the elevated vents.

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Fig.1 Air distribution system applications in high compute density data centers. (a) Typical under-floor air cooling

system configuration, (b) typical configuration with aisle partition system, and (c) typical configuration with aisle

enclosure system.

The main difference between the split air conditioner and the precision air conditioner can be

explained as below:

In split air conditioner, outside unit has compressor and condenser with insulated suction and

discharge lines. Where as in precision air conditioner outside unit has only condenser and

compressor is located in the inside unit.

3. STUDY OF NITK SERVER ROOM:

NITK server room is designed to accommodate various servers of the institution like servers of

Board room main building, CCB main building, Silver jubilee auditorium, ladies hostel, Director

residency, Resident engineer office, Humanities, Megatower1, Megatower2, Megatower3,

Hostels-1,2,3,4,5,6,7,8, SACA, Chemical engineering department, Industrial bio technology

department, IT ,EE,CSE,ECE Departments, Guest house and Industrial structures lab as shown

in Fig.2.

These servers release large amount of heat which is removed by two precision raised floors Air-

conditioner. Two CRAC units are located on the raised floor. Set point control data is fed to the

CRAC units in the form of a chip. These units can also be operated via internet. Scroll

compressors of 10TR capacity and using moisture free R-22 refrigerant are used in the CRAC

units. Air-conditioning unit is charged with 7kgs of refrigerant.

Set point control for the server room is as follows:

Return air temperature: 210C

Return air humidity: 50%

Supply air temperature: 180C

Allowable Temperature: ±10C

Allowable Relative Humidity: ± 5%

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Fig.2.Precision air-conditioning of NITK server room.

Fig.3.Raised floor section of the NITK server room.

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Raised floors are provided with the perforate tiles which can be opened at any desired location

for air supply to the server racks. This precision air conditioning involves no duct work as the

floor is raised to accommodate conditioned air between the ground and the raised floor as shown

in Fig.3. Without adequate chilled air flow supplied from the perforated tiles, recirculation cells

above the racks are generated where the hot air exhausting from the rear of a rack passes over the

top of the rack and enters back into the front of the racks. In addition those racks at the ends

of rows experience hot air entering into the rack from exhaust of nearby racks.

A common problem occurs in computer rooms that have multiple, floor-mounted air

handling units with their own reheat and moisture addition system for humidity control. The

Units begin to “fight” each other. One unit is in humidification mode while one nearby may be

in dehumidification mode The solution is to allow the floor mounted units to provide only

sensible cooling while humidification/dehumidification is performed from a central location. The

raised floor technique has been found to provide better humidity control as shown in Fig.3.

Necessity of humidity in server rooms:

Humidity relates to the moisture content in a specific unit of air and the term relative humidity is

how we often define humidity levels. Relative humidity is the ratio of the amount of the water

vapor in the air compared to the maximum it could hold at the same temperature. In most

installations cold air is drawn in to mix with the internal circulation of air both to provide fresh

air intake to reduce any contamination problems within the room. This cold air has low moisture

content depending on how cold it is. Therefore, moisture must be added to maintain the

RH level of the room. In addition there is some loss of moisture in room from convection to

outside areas. Computer data centers should have vapor barriers to limit the loss of moisture, but

even with a vapor barrier some loss in moisture is inevitable.

Electrostatic discharge effects:

Relative humidity levels are important not only for ESD reasons but also for thermal comfort

health, condensation control, control of growth of pathogenic and allergenic organism, and

corrosion. ESD damage is through electrostatic induction. This occurs when an electrically

charged object is placed near a conductive object isolated from ground. The presence of the

charged object creates an electrostatic field that causes electrical charges on the surface of the

other object to redistribute. Even though the net electrostatic charge of the object has not

changed, it now has regions of excess positive and negative charges. An ESD event may occur

when the object comes into contact with a conductive path.

Humidity helps to reduce ESD in two ways:

1. As a conductor: when humidity levels are such that an invisible film of moisture can

form which can allow the electrons that accumulate to conduct and flow to the ground.

This is due to the grounding process, which allows the electrons to flow away rather than

accumulate.

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2. As a lubricant: the moisture film reduces friction between moving bodies thereby

voltages can’t accumulate as fast on the surface.

Below a relative humidity of 50%, the failures increase exponentially with decreasing relative

humidity [1]. For example, at 5% RH, the number of failures is approximately 2.5 times that at

50% relative humidity.

Humidity control methods:

Several commercially available methods are available to maintain RH levels. They are, 1. Steam

2. Water Spray

3. Evaporative pan

To use steam to increase relative humidity can either be direct steam provided by a boiler or

steam to steam where the steam h m a boiler heats fresh water. This later method would

eliminate any concern with chemicals that may be used to treat boiler feed water. Water spray

can take various forms to inject water into the environment - compressed air or ultrasonic

transducers to atomize the water. Finally evaporative pan uses steam, hot water or electricity to

provide energy for heating the coils which in turn heats the water. Steam is recommended for

most applications.

Computer server room acoustics:

With the increased heat loads of computer equipment the air flow rates required to cool

this equipment has also increased. Of course, this increase in system air flow rates has had a

negative impact on the acoustical levels generated in the data center. Some measurements

comparing room air flow rates, room level heat fluxes and sound power levels are shown in

Fig.4.

Fig.4. Data center acoustical power

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The German Workplace Noise Laws have a limit of 70 dbA sound power level for rooms.

This would indicate that room air flow rates exceeding approximately 18 cfm/ft2 or room heat

fluxes exceeding 110 watts/ft2 would exceed the room level sound power levels allowed by the

German Workplace Noise Laws. These measurements were made in data centers that had racks

with powers less than 5 kW. With power levels exceeding 20 kW for some racks being shipped

today, the acoustical noise levels would appear to greatly exceed allowable levels in room.

Conclusions:

NITK server data centre was studied with the aim of addressing some of the thermal challenges.

CAD drawings are drawn to understand the clear concept of raised floor precision air-

conditioning. The effect of humidity and sound on the performance of the CRAC unit is studied.

To improve the data centre performance, some design solutions can be performed using

computational fluid dynamics.

References:

[1] Chang-Yu Wu, Roger RSchmidt, Brian P. Rawson, Interaction of EMC, acoustics and

thermal management in the reliable operations of data center, Proc.of IEEE conference (2002)0-

7803-7277.

[2] Jinkyun Cho, Byungseon Sean Kim, Evaluation of air management system’s thermal

performance for superior cooling efficiency in high-density data centers, Energy and Buildings

43 (2011) 2145–2155.

[3] Tao Lu, Xiaoshu Lu, Matias Remes, Martti Viljanen, Investigation of air management and

energy performance in a data center in Finland: Case study, Energy and Buildings 43 (2011)

3360–3372.

[4] B. Fakhim, M. Behnia, S.W. Armfield, N. Srinarayana, Cooling solutions in an operational

data centre: A case study, Applied Thermal Engineering 31 (2011) 2279-2291.

[5] Chandrakant D. Patel, Cullen E. Bash, Ratnesh Sharma, Abdlmonem Beitelmal, Christopher

G. Malone, Smart Chip, System and Data Center Enabled by Advanced Flexible Cooling

Resources, 21st IEEE SEMI-THERM Symposium (2005) 7803-8985.

[6] Madhusudan Iyengar, Roger Schmidt, and Joe Caricari, Reducing energy in data centers

through control of room air conditioning units, Proc.of IEEE conference (2010) 978-1-4244-

5343.

[7] Paolo Cremonesi, Andrea Sansottera, Stefano Gualandi, Optimizing Cooling and Server

Power Consumption, Proc.of IEEE conference (2011) 978-1-4577-1481.

[8] Michael K Patterson, The Effect of Data Center Temperature on Energy Efficiency, Proc.of

IEEE conference (2008) 978-1-4244-1701.