Heating , ventilating and air

conditioning

Objectives :

1. Recognize the significance of HVAC in the total

function of a building

2. Explain how HVAC principles influence an

architect’s overall design.

Introduction

Le Corbusier, one of the great masters of the 20th century

architecture, turned to more integral architectural ways to

control the environment than employing elaborate air conditioning

system which could not always be relied upon.

His contemporary, Meis van de Rohe, never felt so compelled,

and retained pure glazed forms and their dependence on

extensive mechanical systems.

In 1948, when Meis designed the Lake Shore Apartments in

Chicago, he was able to have the all-glass wall he had dreamt of

since 1990, but the air-conditioning equipment originally

specified was deleted to reduce building costs.

In 1954, with the help of an elaborate cooling apparatus atop his

Seagram Building, Meis was able to achieve the sealed box that Le

Corbusier had attempted in the Salvation Army Building.

By the time the Seagram Building was designed, mechanical

systems for vertical transportation, lighting, heating, and cooling

were consuming more than half of the budgets of new buildings. It

was as if the building now was the mechanical system, wrapped in a

membrane

Description

HVAC is an acronym that stands for "heating, ventilating, and air

conditioning". This is sometimes referred to as climate control. In

certain regions the term "Building Services" is also used, but may

also include plumbing and electrical systems. Refrigeration is

sometimes added to the field's abbreviation as HVAC&R or HVACR.

The HVAC seek s to provide:

thermal comfort, acceptable indoor air quality, reasonable

installation, operation, and maintenance costs, provide ventilation ,

reduce infiltration, maintain pressure relationships between spaces

Terms

Air handler, or air handling unit (AHU)

can mean a whole unit including the blower, heating and cooling

elements, filter racks or chamber, dampers, humidifier, and other central

equipment in direct contact with the airflow. This does not include the

ductwork through the building.

Makeup Air Unit (MAU)

is an air handler that conditions 100% outside air. MAUs are typically

used in industrial settings, or in once-through, low-flow, or primary-

secondary commercial HVAC systems.

Rooftop Unit (RTU)

is an air handling unit, of recirculating or once-through design, that is

for outdoor installation. They most often include, internally, their own

heating and cooling

devices. RTUs are very popular -- particularly for single-story commercial

buildings.

Fan Coil Unit (FCU)

is a small terminal unit that is often composed of only a blower and

either a heating or a cooling coil. Often used in hotels, condos, or

apartments.

Constant Air Volume (CAV)

is an all-air or air-water HVAC system that has variable supply air

temperature, but a constant flow rate of air. Most residential forced-air

systems are small CAV systems with on/off control.

Variable Air Volume (VAV)

is an all-air or air-water HVAC system that has a relatively fixed supply air

temperature, but the volumetric flow rate of air varies to meet the thermal

load. Most new commercial buildings have VAV systems due to their

reduced fan energy consumption, as compared to CAV. contractor

expects will have similar thermal loads. Zones are defined to reduce

the number of HVAC subsystems, and thus initial cost. Small

residences typically have only one conditioned thermal zone, plus

unconditioned spaces such as attached garages, attics, and

crawlspaces. Basements may be either conditioned or

unconditioned.

Heating Systems

Heating systems may be classified as central or local.

Central heating is often used in cold climates to heat private houses and

public buildings. Such a system contains a boiler, furnace, or heat pump to

heat water, steam, or air, all in a central location such as a furnace room in a

home or a mechanical room in a large building.

The system also contains piping or ductwork to distribute the heated

fluid, and radiators to transfer this heat to the air. The term radiator in

this context is misleading since most heat transfer from the heat exchanger is

by convection, not radiation. The radiators may be mounted on walls or

buried in the floor to give under-floor heating.

1. Fireplace

The fireplace was developed as a method of heating rooms by means of an

open fire. The first fireplaces were hearths, recessed into the walls of

buildings, with short flues that communicated with the open air. The useful

heat given off by a fireplace consists of both direct radiation from the

burning fuel and indirect radiation from the hot sidewalls and back wall.

2. Stoves

The stove, an enclosure of metal or ceramic materials in which fuel is

burned, is an improvement over the fireplace because its surfaces are in

contact with the air of the room and by convection deliver heat to the air

passing over them.

The heat can be provided from electric coils or strips used in varying

patterns—for example, convectors in or on the walls, under windows, or

as baseboard radiation in part or all of a room.

4. Heat Pump

A heat pump is a system designed to provide useful heating and

cooling, and its actions are essentially the same for either process.

Instead of creating heat, as does a furnace, the heat pump

transfers heat from one place to another. In heating season, a

liquid refrigerant, such as Freon, is pumped through a coil that is

outside the area to be heated.

5. Solar Heating

During each sunlight hour of the day approximately 0.9 kw per sq m

(280 Btu per hour per sq ft) of solar energy reaches the surface of

the earth. The actual energy received varies with time of day, time

of year, latitude, clarity of the atmosphere, and the direction

relative to the sun that an absorbing surface faces at any given

time. This energy can often be more than enough to heat a well-

designed building, provided enough solar absorbing surface can

be installed and enough heat storage is made available to carry the

building during periods of darkness and inclement weather. A

common method employed uses roof panels with

built-in water circuits.

6. Portable Heating Units

Houses lacking central-heating systems are equipped with various types of

portable and semi portable heating devices, many of which can be moved

from room to room as needed. The most common types are kerosene

stoves and electric heaters.

Ventilation

Buildings in which people live and work must be ventilated to replenish

oxygen, dilute the concentration of carbon dioxide and water vapor, and

minimize unpleasant odors.

A certain amount of air movement or ventilation ordinarily is provided by

air leakage through small crevices in the building's walls, especially around

windows and doors. Such haphazard ventilation may suffice for homes, but

not for public buildings such as offices and theaters, or for factories.

Engineers estimate that for adequate ventilation the air in a room should be

changed completely from one and a half to three times each hour, or

that about 280 to 850 liters (about 10 to 30 cu ft) of outside air per

minute should be supplied for each occupant. Providing this amount of

ventilation usually requires mechanical devices to augment the natural

flow of air.

Simple ventilation devices include fans or blowers that are

arranged either to exhaust the stale air from the

building or to force fresh air into the building, or

both.

Ventilating systems may be combined with heaters, filters,

humidity controls, or cooling devices. Many systems include

heat exchangers. These use outgoing air to heat or cool

incoming air, thereby increasing the efficiency of the system

by reducing the amount of energy needed to operate it.

Ventilation is the changing of air in any space to remove:

moisture, odors, smoke, heat, airborne bacteria. Ventilation

includes both the exchange of air to the outside as well

as circulation of air within the building. It is one of the most

important factors for maintaining acceptable indoor air quality

in buildings

Methods for ventilating a building

A. Mechanical or Forced ventilation

used to control indoor air quality. Excess humidity, odors, and

contaminants can often be controlled via dilution or replacement

with outside air. But in humid climates, much energy is required to

remove excess moisture from ventilation air.

Kitchens and bathrooms typically have mechanical exhaust to

control odors and sometimes humidity. Factors in the design of

such systems include the flow rate (which is a function of the fan

speed and exhaust vent size) and noise level. If the ducting for the

fans traverse unheated space (e.g., an attic), the ducting should

be insulated as well to prevent condensation on the ducting.

Direct drive fans are available for many applications, and can

reduce maintenance needs.

B. Natural ventilation

HVAC Energy Efficiency

Heating Energy

Water heating is more efficient for heating buildings and was the

standard many years ago. Today forced air systems can double for air

conditioning and are more popular. The most efficient central heating

method is geothermal heating.

Air Conditioning Energy

The performance of vapor compression refrigeration cycles is limited by

thermodynamics. These AC and heat pump devices move heat rather

than convert it from one form to another, so thermal efficiencies do not

appropriately describe their performance.

Theoretically, an air-conditioning system consists of centralized

equipment that provides an atmosphere with controlled temperature,

humidity, and purity at all times, regardless of weather conditions.

Centralized air-conditioning systems, providing fully controlled

heating, cooling, and ventilation, as required, are employed

widely in theaters, stores, restaurants, and other public

buildings. Such systems, being complex, generally must be

installed when the building is constructed; in recent years,

these systems have increasingly been automated by computer

technology for purposes of energy conservation.

The design of an air-conditioning system depends

on:

the type of structure in which the system is to be placed,

the amount of space to be cooled, the number of

occupants, and the nature of their activity.

Air-conditioning

An air conditioning system, or a stand-alone air conditioner, provides

heating, cooling, ventilation, and humidity control for all or part of a

building.

'Central', 'all-air' air

conditioning systems are often installed in modern residences,

offices, and public buildings, but are difficult to retrofit (install in a

building that was not designed to receive it) because of the bulky air

ducts required. A duct system must be carefully maintained to

prevent the growth of pathogenic bacteria in the ducts.

An alternative to large ducts to carry the needed air to heat or cool

an area is the use of remote fan coils or split systems. These

systems, although most often seen in residential applications, are

gaining popularity in small commercial buildings. The remote coil is

connected to a remote condenser unit using piping instead of ducts.

A dehumidifier is an air-conditioner-like device that

controls the humidity of a room or building. They are often

employed in basements which have a higher relative humidity

because of their lower temperature (and propensity for damp

floors and walls). In food retailing establishments, large open

chiller cabinets are highly effective at dehumidifying the

internal air. Conversely, a humidifier increases the humidity of

a building.

Air-conditioned buildings often have sealed windows,

because open windows would disrupt the attempts of the

HVAC system to maintain constant indoor air conditions.

Types

1. Central air conditioning units live entirely outside the building,

separate from the area to be cooled. Hot air is pumped out of the

house while cool air is distributed throughout the building by way of

ducts.

2. Split-systems have 2 separate components; the motor of the air

conditioning unit lives outside the building whilst the air outlet of the

system remains inside.

3. A Stand Alone/Portable type of air conditioner is a single movable unit

can be easily moved to whatever room needs to be cooled.

4. A Thru-Wall/Window air conditioner is a single unit mounted through a

wall or window. Part of it is inside the building and part sticks through to

the outside. with several speeds would have a broader range of options

like low, medium and high.

Max Cooling Capacity

Capacity is the principle gauge of how much cooling power an

air conditioner has, and is measured in British Thermal Units per

hour (BTU/hr). Room air conditioners range in

capacity from about 5000 BTU/hr to about 30,000 BTU/hr.

Air conditioner

An air conditioner (AC or A/C) is an appliance, system, or

mechanism designed to extract heat from an area using a

refrigeration cycle. The most common uses of modern

Air conditioners are for comfort cooling in buildings and

transportation vehicles.

A combined system that also provides heating and ventilation is

often called an HVAC system.

1. Window and through-the-wall air conditioners

2. Portable air conditioners

A portable air conditioner or portable A/C is an air

conditioner on wheels that can be easily transported inside

a home or office. They are currently available with capacities

of about 6,000 to 14,000 BTU/h (1800 to 4100 watts

output) and with and without electric resistance heaters.

Portable air conditioners come in two forms, split and mono

block.

3. Mono block systems

These are vented to the outside via air ducts. A single duct

mono block unit draws air out of the room to cool its

condenser. This air is then replaced by hot air from outside

or other rooms, thus reducing efficiency.

4. Central air conditioners

Central air conditioning, commonly referred to as central air

(US) or air-con (UK), is an air conditioning system which uses

ducts to distribute cooled and/or dehumidified air to typically

more than one room, or uses pipes to distribute chilled water

to heat exchangers.

•With a typical split system, the condenser and compressor

are located in an outdoor unit; the evaporator is mounted in the

air handling unit (which is often a forced air furnace).

•With a packaged system, all components are located in a

single outdoor unit that may be located on the ground or roof.

Central air conditioning has several benefits as compared to

having many smaller distributed units:

•When the air handling unit turns on, room air is drawn in from

various parts of the house through return-air ducts. This air is

pulled through a filter where airborne particles such as

dust and lint are removed.

Sophisticated filters may remove microscopic pollutants as

well. The filtered air is routed to air supply ductwork that

carries it back to rooms. Whenever the air conditioner is

running, this process repeats continually.

•Because the central air conditioning's condenser unit is

located outside the home, it typically offers a lower level of

noise indoors than window or through-the-wall air conditioning

units, for example. However, the air ducts do become dirty

over time, and pose a risk of growth and spread of harmful

microorganisms.

5. "Ductless", "duct-free", or "mini-split" air

conditioners

Ductless mini-split air conditioners combine some traits of

central air conditioning systems with some traits of window or

through-the-wall units. They were invented as an alternative to

window air conditioners for buildings where the cool-air

distribution ducts of a central air conditioning system could not

be installed or would be prohibitively

expensive to install.

Evaporative coolers

In very dry climates, evaporative coolers are popular for

improving comfort during hot weather. The evaporative cooler is

a machine that draws or forces outside air through a wet pad.

The sensible heat of the incoming air is measured by a dry bulb

thermometer.

The total heat (sensible heat plus latent heat) of the entering air

is unchanged. Some of the

sensible heat of the entering air is converted to latent heat by the

evaporation of water in the wet cooler pads. If the entering air is dry

enough, the resulting supply air can be quite comfortable.

Absorption air coolers and washers

There is a process called absorptive refrigeration which uses heat to

produce cooling. In one instance, a three-stage absorptive cooler first

dehumidifies the air with a spray of saltwater or brine. The brine

somatically absorbs water vapor from the air. The second stage

sprays water in the air, cooling the air by evaporation.

Finally, to control the humidity, the air passes through another brine

spray.

Absorptive chillers

Some buildings use gas turbines to generate electricity. The exhausts of

these are hot enough to drive an absorptive chiller that produces cold

water. The cold water is then run through heat exchangers in air handlers

to provide cooling and dehumidification.

The dual use of the energy, both to generate electricity and cooling,

makes this cogeneration technology attractive when regional utility and

fuel prices are more expensive than average

Thermostats

Thermostats control the operation of HVAC systems, turning on

and off, or modulating, the heating or cooling systems to bring the

building to the set temperature. Thermostats may also be

incorporated into facility energy management systems in which the

electrical power utility customer may control the overall energy

expenditure. In addition, a growing number of electric utilities have

made available a device which, when professionally installed, will

control or limit the power to an HVAC system during peak use

times in order to avoid necessitating the use of rolling blackouts.

There are many widely used thermostat technologies such as

mechanical, electromechanical (EM), pneumatic, digital and hybrid.

Equipment capacity

Air conditioner equipment cooling capacity is described in

terms of "tons of refrigeration".

A "ton of refrigeration" is defined as the cooling power of

one short ton (2000 pounds or 907 kilograms) of ice

melting in a 24-hour period. This is equal to 12,000 BTU

per hour, or

3517 watts. Single-family residential "central air" systems

are usually from 2 to 5 tons (3 to 20 kW) in capacity