Original subtitles

To provide cooling, an air handler uses chilled water and a control valve

whereas an air conditioner uses refrigerant and a compressor

whereas an air conditioner uses refrigrant and a compressor

whereas an air conditioner uses refrigerant and a compressor

this slide demonstrates the type of hybrid air conditioning system,

with a mixture of a chilled water circuit and a direct expansion system

The air conditioner unit is equipped with two evaporators in a single unit

Under the normal conditions, the chiller system is operating,

supplying cold water to the chilled water circuit

When the chiller system undergoes maintenance or setters a failure,

this cold water is no longer available

At this moment, the direct expansion system takes over,

to provide the required cooling to the data center

Therefore, the hybrid solution

can minimize the risk of cooling interruption to the computer room

There is a large variety of chiller system types in the market

of which the basic split is between air cooled and water cooled systems

The principle is to have one central chiller plant,

which then fowls the entire building

Redundancy needs to be well planned, with multiple chillers,

including redundancy on the electrical system and piping,

to ensure that the facility is either concurrently maintainable,

as per the Rated 3 requirements,

or fault tolerant in case of Rated 4

Direct and/or indirect air handlers can be used it outdoor temperature

conditions in the location of the data center allow you to do so

Direct air handlers will use the outdoor air,

and channel this directly into the computer room

Appropriate filtering of the outdoor air needs to take place,

to avoid potential dust and gas contamination

You also need to consider the humidity levels of the outdoor air,

compared to the desired computer room humility levels

Indirect handlers will use the outdoor air

and channel this to an air to air heat exchanger

the advantage is that the outdoor air will not touch the indoor air,

and hence no additional filtering and/or humidity control

is required in the computer room

In areas where the outdoor temperature conditions are against you

it is possible to use adiabatic cooling of the air

before it goes to the air handlers

There are a number of benefits for direct/indirect air handlers

I he main advantage is the potential tor savings on the cooling of the data center

I ess space is required indexes,

and lower cost of the electrical installation and power consumption

The drawback is that in the case of adiabatic cooling being used,

water usage can be high, which in some countries

may be restricted by local regulations

As with all systems, you also need to consider redundancy for the water supply,

to avoid this trom becoming your single point of failure

Direct air handlers required to be equipped with filters,

which need to he maintained on a regular basis

If outdoor humidity needs to be corrected by indoor units,

this could add additional cost to build, and of course, cost to operate

Another drawback is a possible legionella contamination of the water

therefore, regular testing of the water needs to take place

nP$

ACADEMY

Now that you know all about cooling systems

Now that you know all about cooling systems,

let's have a closer look at cooling principles

looking at air flow guidance,

air conditioners are based on up flow which sometimes is referred to as up throw,

or down flow, sometimes referred to as "down throw

When the principal is based on up flow,

the air conditioner in the data center is pushing

the cold air out from the top of the unit

Cold air will drop, and therefore falls into various rack rows,

and gets sucked into the intake of the equipment

The difficulty with this principle is

that it is very hard to regulate the amount of cold air tor each area required

therefore, when this principle is applied,

air flow uidance is needed by means of ducting

Another concern with this type of air conditioner unit is,

that the hot air return is commonly located

at the bottom front or bottom back of the air conditioner

Since hot air will rise,

it will be very hard to get this hot air routed back to the air conditioner

When the principal is based on down flow

the air conditioner in the data center is pushing

the cold air out from the bottom of the unit

Doing so, the air flow gets pushed into the raised floor area

I his will result in the pressure being built up under the raised floor

after which the air is pushed into the computer room

via perforated tiles in the floor

I he cold air gets sucked into the intake of the equipment,

and hot air will be expelled from the back of the rack

Hot air will rise, and through the ceiling return,

it is routed back to the top of the air conditioner, the hot air return path

lookmg at the image on the slide

the principle of up flow can be applied with or without a raised floor present,

but when the principle of down flow is applied,

you must have a raised floor installed, otherwise it will not work

There are debates going on whether or not to use a raised floor

There is no perfect answer to that as what is the best

since it is very much depending on many factors,

such as the type of equipment being deployed,

the height that you have available in the room,

cooling capacity and flexibility required, and so on and so on

Since most of the equipment is now working on a front to rear principle,

there is a trend that more and more data centers

are moving towards a set up without a raised floor

In the following units, we will have a closer look at the properties

for botti raised floor and non raised floor cooling

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Raised floors are still used for many computer rooms,

but data centers without a raised floor are more and more deployed

Data centers with a raised floor do provide a high level of flexibility,

to provide cooling as and where it is needed,

as it is a matter of placing perforated floorboards where they are required

By changing the type of perforated floorboard,

it is possible to change the cooling capacity where you have such need present

Cooling through a raised floor has limitations,

in terms of how much cooling capacity it can provide,

and it is relatively costly to install and maintain

In addition, it also otten providos a limitation

loading capabilities

T he raised floor is making use of down flow air conditioners,

pushing cold air under the floor

this will create pressure, and having perforated floor tiles

allows the air pressure to escape into the computer room void itself

Since cold air needs to be available at the intake of the equipment,

perforated floor tiles can be found in the front of the rack

the internal fans suck the cold air into the intake of the ICT systems,

releasing hot air at the back

The hot air will rise automatically,

and will flow back to the air conditioner either via the suspended ceiling,

or via the room void itself

This image demonstrates the traditional raised tloor in a classroom setup

This setup leads to various issues, since the hot air of one rack

is thrown into the intake of the equipment of the next rack

This results in the hot air being mixed with the cold air

that comes from underneath the raised floor,

and will load to higher intake temperatures

this set up is outdated, and it is not recommended tor current data center

A more appropriate set up is what is often

referred to as the hot and the cold aisle set up

The racks placed facing front to front and back to back

It moans that there is a better separation between the hot and the cold air areas

loading to better efficiency

There is, however, still some issue of hot and cold air mixing throughout the room

In order to create a better separation between the hot and cold aisles,

is to guide the hot air to the intake of the air conditioner

this can be done using a hot air return duct,

or a suspended ceiling, such as can be seen in this image

Both solutions will assist in routing back the hot air to the air conditioner

such that mixing of hot and cold air is minimised,

and therefore, optimising both cooling effectiveness and efficiency

nR

Let's have a look at how to make the most of cooling

based on the down flow principle

In order to make the hot and cold aisle principle work,

make sure that the air conditioners are placed at the right locations

Air conditioners should be placed perpendicular to the hot aisles

as can be seen in this image

This set up will ensure that the hot air from the back of the racks

is returnin to the air conditioner in the shortest path possible

The other advantage is that the cold air leaving the air conditioner

is flowing underneath the hot aisle here no erforated tiles exist

This results in a more equalised air ressure within the cold aisle area,

and therefore a more equalised cooling capacity within each rack

Having a raised floor will result in having more

air supply volume at he bottom of the racks

For this reason ou wish to place the highest heat loads

in the lower part of the rack

1 oss volume of air is present in the top of the rack,

and therefore less cooling capacity,

so this is the part of the rack where, ideally, you place the lower heal loads

When using a raised floor,

you need to make sure that cold air is only going where it is supposed to go,

which is the area where cold air is required in the front of the rack

All other openings in the raised floor are considered to be leakage

and therefore needs to be closed

The most common leakage rea is the cable ent coming

from under the raised floor into the back of the rack

You should close these cable entries in the back of the rack,

and there are multiple options available in doing so

You also need to avoid air leakage within the racks

Hot air is always attracted to cold air areas,

hence the hot air at the back of the rack

is travelling back to the front of the rack

This will lead to inefficiencies,

and hence, you need to block the hot air from the back travelling to the front

which can bo done by installin blankin panels,

which arc also referred to as blind panels

Cooling of equipment requires the temperature and humidity

to be within the values recommended by ASHRAE

In addition to this, you need to make sure that enough air volume is available

to flow through the ICT equipmerit within the rack

Air volume displacement is often expressed in multiple units,

such as CFM, which is short for "Cubic Feet per Minute",

or CMH, which is short for "Cubic Meter per hour"

Each ICT system will suck a certain amount of air volume through,

in order to achieve the cooling capacity required

the air volume corning out of he raised floor should at least match

the total required by all equipment within the rack

The shortage of air volume will result in cooling issues,

which is one of the most common issues for cooling in computer rooms

Therefore, be aware that when you have cooling issues,

it is most likely not because of the temperature

and humidity settings on the air conditioner,

but a shortage of air volume instead so this is what ou need to verify first

Air flow can be measured with devices like a flow hood,

can be seen on this slide

When performing measurements using a flow hood,

make sure that there are no tiles taken out ot the floor,

since that will influence the measured values

Also, if you make use of an air conditioner rotation system,

you may wish to measure over multiple days,

to record all the values tor each rotation schedule that you are using

Do not place too many perforated tiles,

since it will limit the air volume needed to cool the IT equipment

Too many openings in the raised floor will ultimately lead

to a too low static pressure of air

Understand that cold air needs to be delivered

only at the intake of the equipment

Heat load coming out of the equipment is at the back of the rack

and this heat load needs to be extracted from the computer room

this is the exact reason why computer room set-ups

are based on the cold/hot aisle principle

So, place perforated tiles only in the front of the rack,

and where equipment is installed

If possible use return air ducts,

for the hot air to be moved out of the room as fast as possible,

because this will avoid heat load problems,

and will at the same time improve he efficiency of our cooling cycle

High powered equipment requires more cold air than low powered equipment,

and because of this it is preferred to install the high powered equipment

in the lower part of the rack

finally, looking at the physical position of the air conditioners,

which can only throw air at a certain distance

The distance capability normally depends

on the capacity of the air conditioner itself

For the capacity ratings commonly found in a data center,

you can expect approximately 12-18 meters

which is equivalent to 40-60 feet

Where long rack rows are deployed,

it may be required to have air conditioners at both sides of the aisle

n^

For a non-raised floor set up to work very well,

all equipment should be based on the front to rear type air flow principle

In a non raised floor set up,

racks are placed directly on the concrete floor or slab,

which has been treated with a proper paint coating,

to avoid contamination from the building slab

Placing racks directly onto the building floor

will remove investment costs for the raised floor structure,

and it commonly creates an easier way of handling the fairly heavy equipment loads

Furthermore, since there is no raised floor,

you no longer need to worry about cleaning underneath

Also, it will force you to run all cabling overhead,

although some data centres till refer a ve low raised floor,

for the purpose of cabling only

In that case, the raised floor is no longer used for the purpose of cooling

Without the raised floor, you need to provide cold air

directly to the racks from the side or the top

In-row cooling is based on the principle

of having the air conditioner in between the racks

Having air conditioner equipment close to the ICT equipment

will assist in cooling efficiency improvements

Placing cooling equipment inside computer rack rows

will however, result in less equipment racks to be installed

Also, not every data center is keen on installing

cooling equipment close to the ICT systems,

knowing that the maintenance for the cooling systems

is commonly performed by third party vendors

Some in row units are based on water and although water is a very good coolant,

any water leakage may cause damage to the data center,

and if water gets in contact with the electrical infrastructure,

there is a higher risk of equipment unavailability

Obviously, water is not the only option

In row units are also based on refrigerant

Non raised floor environments can also be cooled

using cold and hot air ducts placed in the computer room

I ho cold air ducts are positioned in the front of the racks,

and cold air will fall down and will gel sucked into the intake of the equipment

At the back of the rack the hot air will exhaust,

through the hot air ducts

It is important that the power and network cabling are over the rack rows,

so that the do not hinder the air flow

It is common for the ducts to have vents installed,

so to be able to regulate the air volume,

based on the Cubic Feet per Minute requirements of the rack

The ducts need to be properly designed,

so that it can carry the volume capacity requirement

In addition, you need to review the redundancy requirements,

so that it an air conditioner tails or requires maintenance,

the ICT equipment will not be impacted

Do not paint the ducts, as overtime the paint will dry out and splinter oil

leading to particles floating around ending up inside the ICT equipment

Proper maintenance of the ducts must be performed,

this includes regular inspections and cleaning

Doing so, reaise that involves overhead works,

and therefore, safety practices must apply at all times

A relative new concept is the wall flow, also referred to as a fan wall system

I his concept is more or less similar to the set up

having CRAH systems in the service corridor,

with the difference that this concept does not require a raised floor,

since the air is now coming from a large

service area in the wall of the computer room

To avoid mixing of cold and hot air using the duct based system,

you need to make sure that the hot air

is properly channelled back to the service corridor

This can be a hot aisle containment system,

with a direct air duct back to the service corridor,

or by means of the suspended ceiling

 a suspended ceiling is available

nR

ACADEMY

In some cases, air cooling need to be forced because the normal

In some cases air cooling need to be forced because the normal

cooling system is not able to deliver enough volume ot air

One of the systems that can be used is the cool air ducting system

There are some limitations to this system

it is a single point of failure, the device creates noise,

and it emits a low level of EMF,

so some distance between the unit and IC I equipment

needs to be taken into consideration

Cool air ducting systems tit under a perforated tile or a high flow rate tile

and can be placed in front of a mesh door rack

The air flow should not be too fast,

as then it would potentially limit the amount ot air the server is able to draw in

Also, make sure that the tans are not sucking air volume away from other racks

Scavenging the hot xhaust air is an approach

to handle increased ower density

I he principle is simple,

if you are able to collect all the hot exhaust air and make it go away,

it will not create any hotspots,

and the equipment is always getting cool air horn the air conditioner

I his system goes up to Z kilowatt per rack,

and is available for snap on retrofit

Depending on the system, variable speed may be available

Do realise the system is focusing on

extracting heat load from the back ot the rack

In other words, it will not create more cold air volume at the front of the rack

You still need to supply a sufficient volume of air

for the equipment to be properly cooled

Another option to consider is the use of in row cooling systems,

which can be placed in between the equipment racks

The benefit of in row units is the relative short delivery path

for cold air to be delivered and the hot air to be returned,

and this will increase the efficiency on the air flow management

In row units can be combined with under the tloor cooling,

for areas where high or higher cooling capacity requirement exists

for which the under the floor cooling

can no longer deliver sufficient cooling capacity

Depending on the in row unit,

the flow can be thrown forward or sideways

I ho units which throw sideways can also be deployed in non contained areas

whore the ones that throw the air forward

are more suitable to be deployed in contained aisle concepts

I he downside is that these units can be quite noisy,

and some of the models are based on chilled water supply,

which means that water will be in the computer room,

and it poses a certain risk factor

Rear door heat exchangers are cooling modules

which are placed at the hack of the rack

work in a simiar way as radiators in cars

I he hot air at the back ot the rack will go through the door,

which therefore pre cools the hot air before it enters the computer room area

I his is a solution which allows high density racks

to bo placed into a low density computer room

Very often this solution is based on chilled water supply

With this, again, there is a certain risk involved,

and therefore, it is recommended to install a water leak detection system

In some cases, heat loads are so high

that the racks demands a built in cooling system

A self contained air conditioner system is a full enclosure,

and this may result in potential problems

since there is not a lot ot redundancy present

Vendors claim redundancy,

but this is sometimes at the expense? of reduced cooling capacity

at the time of a failure occurring

If the cooling system in the rack fails, it will heat up very fast

and equipment failure will appear within a few minutes only

these units can be placed in any location, as it is a self cooling rack

and it is therefore not reliant on other cooling systems in the computer room

these racks are able to cool high heat loads

up to 35 kilowatt in the rack

Self contained racks are available in a number of options,

looking at the coolant being used

Each option has benefits and drawbacks

Focusing only on the safety for humans and the equipment involved

this slide provides the information for each option

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ACADEMY

Liquid immersion cooling is a cooling practice

by which IT component and other electronics,

including complete service and storage devices,

are submerged in a thermally conductive dielectric liquid or coolant

In doing so, it cools as effectively as 1200 cubic feet of air,

and requires significantly less energy input

Immersion cooling can handle compute densities exceeding 10kW per bath or unit,

which is 10 to 20 times higher than the per rack footprint

By eliminating the need for cooling fans within each server,

and air conditioning equipment for the computer room,

savings up to 40% can be achieved,

significantly reduce

Now that we have the technical complex details out ot the way,

let's look at the practical side of this solution

As with every solution,

some drawbacks can be identified using liquid immersion cooling

It requires modified IT equipment that can operate in a submerged environment,

which means no fans or hard disk drives

Also, when servicing or maintenance is needed,

this system requires work spaces that are capable ot containing drips and spills

In terms of investment, liquid immersion cooling has a higher initial cost,

compared to traditional cooling

How doos it work?

With single phase immersion cooling,

the coolant remains in a liquid form and is pumped to a heat exchanger,

where the heat is transferred to a cool water circuit

I he cooling baths have an open top

as there is little risk of the coolant evaporating

the good news is,

a single phase coolant does not boil or undergo a phase change

at anytime during the cooling process

This eliminates pressure, fumes, vapours and corrosion due to microcavitation.

created by a coolant’s state transition from liquid to gas

Apart from reducing the risk of IT and cooling systems breakdown

as a result of micro cavitation,

it is also important from a health and safety perspective,

since it means it is not possible to inhale coolants,

they cannot re condense and remain in the lungs,

or form a "toxic dew" throughout the workplace

two phase immersion cooling

This system takes advantage of a concept known as latent heat,

which is the heat or thermal energy that required to change the phase ot a fluid

this occurs whon the two phase coolant comes

in contact with the floated electronics in the bath

that are above the coolant’s boiling point

Two phase immersion cooling has a greater heat rejection capacity

due to the energy transferred from the IT systems into the two phase coolant

that will cause a portion of it to boil off into a gas

Purpose designed cooling liquid is required with a low boiling point

At 56 degrees Celsius or 133 degrees Fahrenheit,

versus 100 degrees Celsius or 212 Fahrenheit in water,

the fluid boils on the surface of heat generating components,

and rising vapour passively takes care of the heat transter

The not so good news is, boiling action of the coolant creates micro cavitation,

which actually erodes the heated metals in the IT systems,

destroying it and contaminating the coolant with metallic particles,

making the coolant conductive

Cavitation consists of the formation,

growth and rapid collapse of cavities in a liquid.

These vapour cavities, or in simple term bubbles,

are formed whenever fluid pressure falls below the vapour pressure of the liquid

These bubbles collapse if the pressure again rises above the vapour pressure

wR

A C A 0 E H V

Another option is aisle containment

Containment solutions are getting more and more popular

since they could lead to efficiency improvements

the principle of air containment is to have total

separation between the hot and the cold air airflows

This can be done by containing the hot air,

the cold air, or in some cases both areas are contained

Containment can be implemented with or without the raised floor,

let's have a look at some details

On the previous slides, a raised floor could be identified,

and on this slide an example of hot aisle and cold aisle containment

without a raised floor

Again, the principle is to have a total separation

between the hot and the cold air airflows

In this example, in row cooling units are being deployed,

to provide the cooling tor the ICT equipment

Apart from exploring which solution is the best fit for your data center

there is another important onsideration to look into,

which is the potential impact on fire detection and suppression systems

A potential downside of containment is that you

are effectively building a room within a room,

which forces you to rethink your existing fire detection and suppression systems

Also, before you make any decisions approving investment plans,

you often need to verify with the local authorities

on what is allowed and/or required

There are various scenarios to work around the fire suppression challenges,

let's have a look on the details n the following slides

Who says you must do a full enclosure?

partial containment,

which means that you create an overhang on the top of the rack row,

as can be seen on this particular slide

This will allow for most hot and cold air to be separated,

and having an opening at the top of the rack,

there are no changes required for the existing tire suppression system

Another option is to use a retractable roof system

Under normal operating conditions this system will provide a full enclosure

The roof is put over the rack row and fixed with a magnetic lock

that is connected to the tire panel

Once the suppression system is activated and the gas will discharge soon,

the fire panel will release the lock and the roof automatically retracts

This is done mechanically, so no power is required

Another option is to bring the fire su ression nozzles

and sprinklers within the contained area

This is technically possible,

but it would require a substantial amount of modification to an existing data center,

which could be intrusive and very expensive

I ven for new computer rooms,

it would be quite an expensive option and hence this is hardly used

* hi t*t r 11 d All P qI

So which typo of containment option is the best?

I here are various views as to what is potentially the best solution

As usual, there is no best for every situation.

and you need to look at the requirements and set up of the computer room

before you make a decision

Ultimately, both solutions have the same goal and principle,

separate the hot and the cold air

Cold aisle containment has the advantage that

the cold air only goes where it needs to be,

which is in front of the equipment, where the intake is present

You can calculate the volume required for the cold aisle

by reviewing the Cubic Feet per Minute requirements

for all the equipment inside the contained area

In addition, you can also calculate the number ot perforated tiles required

to bring that volume of air into the contained area

Cold aisle containment is commonly a good idea

if you want to contain every aisle in the computer room

Since the volume of cold air is matching the requirement of the equipment in the aisle,

you need to make sure that a failure on an air conditioner

is not creating a shortage of air flow

It would be best, therefore, to create a slight overpressure,

so to cater for air flow variances

This since in toda s ICT equipment

the fan speed is relative to the heat inside the equipment,

and hence the CFM requirement is not always constant

Ideally, you measure the pressure in the aisle,

and when the overpressure drops too low

a sensor connected to the air conditioners

will result in an increase of the fan its speed,

providing more cold air

When hot aisle containment is applied, the hot air separates from the cold air,

this allows for the hot air to be guided back directly to the air conditioners

or is exhausted out of the building,

depending on the cooling principles that you have

In this set up, cold air will be present throughout the entire computer room

Hot aisle containment is commonly deployed when in a large computer room

you have a few racks with a high heat load

By containing the hot air for this section,

the hi h heat load will not add to the overall computer room

Now cold air is flowing through the entire room,

which at first goes against the principles of cooling,

since you don't wish to cool the room, you want to cool the equipment

The good part, however, is that the entire room acts as a cold air butter /one

So if your air conditioner equipment fails and you do not have proper redundancy,

you would still be able to potentially handle a temporary failure

In-row cooling units is often a very good

option to be used in hot aisle contained areas

Something that you need to consider is that

hot aisles can potentially be very noisy and hot,

and this may force you to review local regulations,

to ensure that you are not violating any rules,

for staff to be allowed working in this type of environment

wR

ACADEMY

The concept of thermal storage is the one whereby

energy is supplied to a storage system for removal,

• :• • : *. : • : i a later point in time

Thermal energy in the form of heat or cold can be effectively stored

and used to offset the required cooling and/or heating demand in buildings,

and of course data centers, using seasonal thermal energy storage,

or Sell S as it is shortly known

As the name suggests, seasonal storage technologies are primarily intended

for storing thermal energy during one seasonal condition

Which, for example, could be the summer or the winter,

and discharging the stored energy in the other seasonal condition,

depending on the load demand

SeH S makes use of large basins or earth subsurface as the key source

for enabling the energy storage,

to serve multiple buildings through a district cooling and/or heating network

So, how does it work?



produces chilled water within a centralised energy plant,

and distributes it through underground pipes to buildings connected to the system

and provides them with air conditioning

Therefore individual buildings do not need split systems,

chillers and/or cooling towers

The chillers of the district cooling system can be coupled to cold storage

a large tank that holds chilled water and functions as a thermal battery

I his technology allows the district cooling system to chill water at night,

using off peak electr icily,

and then store the water for distribution to customers during the day

Obviously, the initial expense of a district cooling plant will bo high,

but there is the economies of scale due to the set up of centralised plants

instead of individual cooling plants in each building

An example of self S is aquifer thermal energy storage, shortly known as AI I S

ATES systems work by using two separate wells of cold well and a warm well

In the summer, cold groundwater From the cold well is pumped out,

and used to cool the building using a heat exchanger

The heat extracted from the building is injected into the warm well

and stored until winter

And in the winter the process is reversed

Water from the warm well is used to heat the building,

the cool air extracted from the system is then stored in the cold well

In this module, you have learned that it ASI IRAt who sets the recommended

and allowable temperature and humidity levels tor computer room equipment

In cooling, various units of measure are being used,

but it is recommended to stick to one unit, preferably being watt or kilowatts

Cooling systems are classified as comfort cooling units and precision cooling units

Within the precision cooling units, various cooling systems are available

Selecting a cooling system for your data center depends on various criteria

Cooling can be based on the concept of a raised floor and a non raised floor,

each has its own benefits and drawbacks

There are two important factors in cooling temperature and air flow volume,

which is expressed in Cubic Feet Minute (CFM) or Cubic Metre per Hour (CMH)

When lacing air conditioner units on the floor,

these units must be facing the hot aisle

or as it is commonly called "positioned perpendicular to the hot aisle"

Sometimes additional cooling is required, and when it applies,

various supplemental cooling options are available

Hot and cold aisle containment,

if done correctly, will improve the cooling efficiency and effectiveness

but take into consideration the possible impact on tire suppression systems

Seasonal thermal energy storage is a concept

whereby energy is supplied to a storage system for removal,

to be used at a later point in time

R

e dir conditioners perpendicular to t le disks W id

e air-conditioners perpendicular to the aisle. Wha

ike use of a coolai s< fe for both ec lipmet t and I

e air-conditioners perpendicular to the aisle. Wha