LECTURE 3 HVAC Intro.ppt

SERVICES VI ( AR-310B)
LECTURE 3
INTRODUCTION TO HVAC

Introduction to HVAC
HVAC is Heating, Ventilationand Air-conditioning.
We sometime refer to it only as Air-conditioning.
Air Conditioning means:
 the total control of temperature, moisture in the air (humidity), supply of
outside air for ventilation, filtration of airborne particles, and air
movement in the occupiedspace.
 There are seven main processes involved in air conditioning :

1. HEATING—the process of adding thermal energy (heat) to the conditioned space for the
purposesof raisingor maintainingthe temperature of the space.
2. COOLING—the process of removing thermal energy (heat) from the conditioned space for
the purposes of lowering or maintaining the temperatureof the space.
3. HUMIDIFYING—the process of adding water vapour (moisture) to the air in the conditioned
spacefor the purposesof raising or maintainingthe moisturecontent of the air.
4. DEHUMIDIFYING—the process of removing water vapour (moisture) from the air in the
conditioned space for the purposes of lowering or maintaining the moisture
content of the air.
Introduction to HVAC

HUMAN COMFORT
5. CLEANING—the process of removing particulates, (dust etc.,) and biological contaminants,
(insects, pollen etc.,) from the air delivered to the conditioned space for the purposes
of improving or maintainingthe air quality.
6. VENTILATING—the process of exchanging air between the outdoors and the conditioned
spacefor the purposes of diluting the gaseous contaminants in the air and improving or
maintaining air quality, compositionand freshness.
7. AIR MOVEMENT—the process of circulating and mixing air through conditioned spaces in
the building for the purposes of achieving the proper ventilation and facilitating the thermal
energytransfer.

Purpose of the building, area classification, occupancy and usage patterns.
Type of building structure, orientation, geographical location, altitude, shape,
modules- size and height.
 Materials and thickness of walls, roof, ceilings, floors and partitions and their
relative positions in the structure, thermal and vapour transmittance coefficients,
areas and types of glazing, external building finishes and color as they affect solar
radiation, shading devices at windows, overhangs, etc.
 Ratio of internal to external zones, glazing, plant room siting, space for service
distribution;
HVAC & ARCHITECTURAL FACTORS

The HVAC designer must thoroughly evaluate the ceiling space for air distribution
ducts.
Low ceiling height clearance between suspended ceilings and roof (also called
plenum space) require close co-ordination with structural group for location and size
of structural beams.
Inadequate spaces to run ducts, probably force the system designer to use
decentralizedor unitary air conditioningunits.
HVAC & ARCHITECTURE

No equipmentshouldbe visibleor should suitablyblendwithenvironment
Size and appearance of terminal devices in ceiling shall harmonize with
lighting layout, fire sprinklers, detectors, communication systems and ceiling
design;
Acceptability of componentsobtruding into the conditionedspace.
Accessibility for installation of equipment, spacefor maintenance.
HVAC & ARCHITECTURE

The HVAC designer needto consider the likelihood of space changes.
It is likely that from time to time the users may need or wish to change the
layout of rooms or the intended use of the internal environment.
They may prefer systems that facilitate this to minimize consequent
disruption and cost. Some systems lend themselves to adaptation of cellular
layouts or fromopenplanto cellular.
HVAC & ARCHITECTURE

HUMAN COMFORT – A REVISIT
 A Comfortable air temperature is between 18-22°C and the relative humidity is
between 40-65%.
This zone can be shown on the psychometric chart. Andis known as the comfort zone.
Outside air is at a different conditionfrom the requiredcomfortzone condition.
To bring the condition of the air to within the comfort zone it is required to do one or
more of the following:-
Heat it;
Cool it;
Dehumidifyit;
Humidifyit;
or mix it.

HUMAN COMFORT
LATENTHEATANDSENSIBLEHEAT: Important Aspects Of Comfort Conditioning-
Sensible heat is the heat a thermostat senses. When the temperature goes up, there has
been an increase in the sensible heat.
Latent heat is hidden. It involves adding or removing heat without changing the
temperature.
Air contains latent heat in the water vapour that is in the air. Removing the vapour
removes heat but doesn’t lowerthe temperature.

HUMAN COMFORT
LATENTHEATANDSENSIBLEHEAT: Important Aspects Of Comfort Conditioning-
These heats create different types of loads on air- conditioningsystem.
The design cooling/heating load (or heat gain) is the amount of heat energy to be
removed or added from the space by the HVAC equipment to maintain the conditions at
indoor design temperature when worst case outdoor design temperature is being
experienced.

HUMAN COMFORT
SENSIBLEHEAT LOADS
Heat which a substance absorbs, as its temperature goes up, it does not change state.
Sensible heat gain is directly added to the conditioned space by conduction, convection,
and/or radiation.
But sensible heat gain entering the conditioned space does not equal the sensible cooling
load duringthe same time intervalbecause of the stored heat in the building envelope.
SENSIBLE HEATING is addition of heat to moist air without the addition of moisture. It
follows a constant humidity ratio (W) line on the psychrometric chart. It is the removal of
heat frommoist air
SENSIBLE COOLING is without the removal of moisture. It also follows a constant W on
the psychrometric chart.

HUMAN COMFORT
SENSIBLEHEAT LOADS
Onlythe convective heat becomes coolingload instantaneously.
Sensible heat loadis total of:
Heat transmitted thrufloors, ceilings, walls.
Occupant’sbody heat.
Appliance & Light heat
Solar Heat gainthru glass
Infiltration of outside air
Air introduced by Ventilation

HUMAN COMFORT
LATENTHEATLOADS
Latent heat gain occurs when moisture is added to the space either from internal
sources (e.g. vapour emitted by occupants and equipment) or from outdoor air as a result
of infiltration or ventilation to maintain proper indoor air quality.
Latent heat loadis total of:
Moisture-ladenoutside air fromInfiltration& Ventilation
Occupant Respiration & Activities
Moisture fromEquipment & Appliances
Sensible load affects the dry bulb temperature, while the Latent load affects the wet bulb
temperature.

HUMAN COMFORT
The DRY-BULB TEMPERATURE (DB) of air is the temperature taken with an unwetted
bulbthat is shielded from radiant exchange.
Dry-bulbtemperature affects the rateof convective and evaporativebodyheat loss.
The capability of air to hold moisture depends on the dry-bulb temperature. The higher
the temperature, the more moisture the air can hold.

HUMAN COMFORT
Dry BulbTemperature……
It is the most important determinant of comfort, since a narrow range of comfortable
temperatures can be established almost independently of the other variables.
A wide range of temperatures can provide comfort when combined with the proper
combination of relative humidity, MRT, and air flow.
But as any one of these conditions varies, the dry-bulb temperature must be adjusted in
order to maintain comfort conditions.

HUMAN COMFORT
Wet Bulb Temperature (WB)……
The wet-bulb temperature is the temperature a parcel of air would have if it were cooled
to saturation (100% relative humidity) by the evaporation of water into it, with the latent
heat beingsupplied by the parcel.
 The wet-bulb temperature is the lowest temperature that can be reached under
current ambient conditions by the evaporationof water only.
Wet-bulb temperature is largely determined by both actual air temperature (dry-bulb
temperature) and the amount of moisture in the air (humidity). At 100% relative humidity,
the wet-bulbtemperature equalsthe dry-bulbtemperature.

HUMAN COMFORT
Wet Bulb Temperature (WB)……
Lower wet-bulb temperatures that correspond with drier air in summer can translate to
energysavings in air-conditionedbuildings due to:
Reduceddehumidification load for ventilationair
Increasedefficiency of coolingtowers
 A psychrometer, or wet-and-dry-bulb thermometer, consists of two thermometers,
one that is dry and one that is kept moist with distilled water on a sock or wick. At
temperatures above the freezing point of water, evaporation of water from the wick lowers
the temperature, so that the wet-bulb thermometer usually shows a lower temperature than
that of the dry-bulbthermometer.

HUMAN COMFORT
A sling psychrometer for outdoor use
Wet and Dry Bulb thermometer
 A psychrometer is a device that includes both a dry-bulb
and a wet-bulbthermometer.

FEW MORE DEFINITIONS
 Dew Point Temperature (DP) – Temperature at which the water
vapour in the air would begin to condense if the air were cooled at
constant pressure.
 Humidity Ratio (W) – This is known as Moisture Content also.
The weight of water vapour per pound of dry air – lb/lbof d.a.
 Specific Volume (v) – Volume of air per unit weight of dry air –
cubic ft/lb

Dew Point
• Condensation
occursat the Dew
Point Temperature

Humidity Ratio
• Actual weight of water in an
air – water vapour mixture
• Amount of moisture per unit
of dry air
• Canbe definedas,
W=m/G

HUMAN COMFORT & HUMIDITY
Humidity is the amount of water vapor in a given space. The density of water vapor per
unit volume of air is called absolutehumidity.
Relative humidity (RH) is the ratio of the actual vapor pressure of the air-vapor mixture
to the pressure of saturatedwater vapor at the same dry-bulb temperature times 100.
RH describes how far the air is from saturation. It is a useful term for expressing the
amount of water vapor when discussing the amount and rate of evaporation. It is therefore
useful to knowhow much the air needs to be cooled to reach saturation.

Relative Humidity
Relative
Humidity
Amount of moisture that a given amount
of air is holding
=
Amount of moisture that a given amount
of air can hold
50% RH 100% RH -
Saturated
(percentage)
HUMAN COMFORT & HUMIDITY

 Specific Enthalpy (h) – The heat content (enthalpy) of air per
unit weight in BTU/lbor Calorie/gmof dryair.
WHAT IS A PSYCHROMETRIC CHART?
Graphical form of representation of properties of atmospheric air. It
is universally used since, it contains a great deal of information very
simply and because it is helpful in understanding Air-conditioning
processes.

Dry
Blub
Relative
Humidity
Dew
Point
100%
Saturation
Enthalpy
Wet
Blub
Grains of
Moisture
Specific
Volume
Breakdown of the Lines

LECTURE 3 HVAC Intro.ppt

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LECTURE 3 HVAC Intro.ppt