airconditioningforlargemultistorybuildings-110522204801-phpapp02

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Most large multistory buildings use highlycentralized air conditioning equipment. Theroof and basement are the usual choice forthese central station systems.

The basement has the advantage of easyutility connections, noise isolation, not beingvaluable rental area and the fact that

structural loads are not a problem.


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The roof is the ideallocation for fresh air

intakes and heat rejectionto the atmosphere.

Cooling tower are noisy,produce very hot andhumid exhaust air, so thebest location for coolingrower is placed on the roof.

SECTION OF A TYPICALMULTISTORY BUILDING WITH

A ROOFTOP CENTRALSTATION MECHANICALSYSTEM


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In most buildings, themechanical equipment isshown to be on the roof. Thissection shows an all-airsystem served by a singlecentral air handling unit onthe roof.

To avoid the large verticalducts, separate air handlingunits can be placed on eachfloor and only water circulatesvertically. This saves much

energy because moving airgreat distances requires muchpower.

MECHANICAL EQUIPMENT ISSTILL ON THE ROOF, EACHFLOOR HAS A SEPARATE AHU


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The advantage of all-air system is completecontrol over air quality is possible. Thedisadvantage is that all-air systems are verybulky and a significant part of the buildingvolume must be devoted to them.

There are 5 system in all-air system categories.i. Central Plants

ii. Variable Air Volume (V.A.V)

iii. Terminal Reheat System

iv. Multizone Systemv. Double Duct System


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The central air conditioning system has thelimitation of the same air quality being deliveredthroughout the building.

If the structure is zoned or divided and

compartmented into several different functionssuch as offices, workshops, canteen.

The temperature of each element can becontrolled with zoned thermostats.

The fan precedes the chiller and reheater coils

and blow air through the air-handling unit. Several ducts radiate from the unit to designated

zones within the building.


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This is a single duct system that can easilyhave many zones. A variable air volumecontrol box is located wherever a duct entersa separate zone.

A thermostat in each zone controls the airflow by operating a damper in the V.A.Vcontrol box. More cooling is required, morecool air is allowed to enter the zone.


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Floor plan of a VAV system.

VAV system.

VAV control box.


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VARIABLE AIR VOLUME SYSTEM


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In this system, every zone receives air at therequired temperature through a separateduct.

These ducts are supplied by a special

multizone air handling unit that custommixes hot and cold air for each zone.

This is accomplished by motorized damperslocated in the air handling unit but controlled

by thermostats in each zone. Depending onthe temperature, the ratio of hot and cold airvaries but the total amount of air is constant.


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The multizone unit is supplied with hot water,chilled water and a small amount of fresh air.

Each multizone unit can handle about 8zones.

Because moderate air temperatures arecreated by mixing hot and cold air, this

system also wasteful of energy. Disadvantage this system are costs relatively

high while the thermal control is relativelypoor.


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Like the multizone system, the double duct system mixeshot and cold air to achieve the required air temperature.

Instead of mixing the air at a central air handling unit,mixing boxes are dispersed throughout the building.

The double duct system creates a high level of thermalcomfort and allows for great zoning flexibility, it is veryexpensive, requires much building space and its wastefulof energy.

Double duct system.


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To reduce the size of the ducts, a high-velocity version of this system

High-velocity air systems (6000 feet/minute)consume more fan power than normalvelocity systems(2000 feet/minute).

Because of these problem and because V.A.Vsystem are a good alternative, dual ductsystems are not used much anymore.


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These system supply both airand water to eachzone of a building.

It greatly decreases the size of the equipmentbecause of the immense heart-carrying capacity

of water as opposed to air. Air is supplied mainly because of the need for

ventilation.

There are 2 system in air-water systemcategories.

i. Induction Systemii. Fan- coil with supplementary air

But in this topic is cover induction system only


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A small quantity of high-velocity air issupplied to each zone to supply the requiredfresh air and to induce room air to circulate.

Most induction terminal units are found

under windows where they can effectivelyneutralize the heat gain.

As the high-velocity air shoots into the roomit induces a large amount of room air to

circulates. This combination of room air and fresh air,

then passes over heating or cooling coils.


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Local thermostats regulate the temperature bycontrolling the flow of either hot or cold waterthrough the coils.

It takes a lot of fan power to circulate air at highvelocity throughout a building.

Because of the high cost of energy and theexpense of high-velocity ductwork, the use ofinduction systems is limited.

Induction system.


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Induction system.

Induction unit with coolingor heating coils


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These systems supply no air. There are 2 system in all-water system

categories.

i. Fan-coil System

ii. Water Loop Heat Pump System

But in this topic is cover Fan-coil system only.


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The fan-coil unit basically consists of a fanand a coil within water circulates.

The units are in the form of cabinets forplacement under windows.

The fan blows room air across coilscontaining either hot or cold water.

Thermostatically controlled valves regulatethe flow of water through the coils.

A 4 pipe system, which is has 2 pipes for hotwater supply and return and another 2 pipesfor cold water supply and return.


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Condensation on the cooling coils must becollected in a pan and drained away.

When the fan-coil unit is on an outside

wall, it is possible to have an outdoor airintake connected to the unit.

A three-speed fan switch allows occupantsof the zone to have some control over the

temperature. Fan-coil units are most appropriate for air

conditioning buildings with small zones(eg. Apartments, condominiums, motels,hotels, hospitals and schools).

Fan coil units also located above windows,in small closets or in the dropped ceilingabove a bathroom.


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Fan Coil System.

Schematic diagram of an under window FCU ( 4

pipe system).


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FCU placed in a small closet.

FCU placed above a dropped ceiling.

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