basics of hvac & r

8/12/2019 Basics of HVAC & R

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WELCOME TO

HVAC(VRF) & R TRAINING


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BASICS OF AIR CONDITIONING

PART1


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Part 1

Introduction Definition Expectations Categories of Air Conditioning Methods of Heat Transfer Respiration Losses Factors Affecting Human Comfort Outdoor Air


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Introduction:

Air Conditioning is a form of refrigeration.When we use air as a medium for cooling, wecall it Air Conditioning.Refrigeration is a process of producing cold or a

process of taking away heat.As per laws of nature, The heat is always flow fromhigh temperature region to low temperature region.Based on this phenomenon only we can done the AirConditioning.Whenever the room temperature below thesurrounding temperature then cooling process takes

place.


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Definition of Air Conditioning :

Air Conditioning is the process of treating airso as to control simultaneously its temperature,humidity, cleanliness and distribution to meetthe requirements of the conditioned space.

The primary objective of Air Conditioning iscooling.


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Expectations:

An Air conditioning system must be able to treat air

so as to: Cool or Heat to regulate temperature. Humidify/Dehumidify to regulate moisture content. Filter to reduce contaminants. Ventilate to regulate fresh air requirements air quality

and odor. Circulate to regulate air motion no drafts/no

suffocation.

The system which takes care of all the these aspectscan truly called a Complete Air Conditioning Process.But in real life it cant possible.


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Categories of Air Conditioning:

Air Conditioning process can be divided in twocategories. Such as

Comfort Air Conditioning. Process Air Conditioning.

Comfort Air Conditioning:The comfort air conditioning is related to human comfort.The primary objective of this process is human comfort for the peopleworking in the premises.

Process Air Conditioning:The objective of process air conditioning is to maintain the conditionswhich will help in carrying out the process effectively and satisfactorily.The conditions to be maintained in process or material being handled andthe controlled parameters therefore vary greatly from process to process.


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Some of the major process air conditioning applications and their primaryobjectives are mentioned below.

Textile air conditioning the primary concern is humidity to be maintained properly.Printing and paper manufacturing Close temperature andhumidity control.Electronics/computer rooms air cleanliness, lowertemperatures and close humidity.Pharmaceuticals IAQ, positive static pressure in the space.Hospitals/operation theaters quality of air/contaminationcontrol.Music and sound recording rooms acoustics.CNC machine control panels/air conditioners for controlrooms and switch rooms unit design with very high sensibleheat load capacity and controlled humidity to ensure nocondensation of moisture.


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Methods of Heat transfer:

There are four ways by which heat transfer takes place:

Conduction

Convection

Radiation

Evaporation


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Conduction whenever our body parts come in contact withany object or air which is at a lower temperature than the bodytemperature, then the heat transfer takes place.

Convection when the body is rejecting heat, the air closerto the body becomes warmer than the air which is away fromthe body. As warm air is lighter than the cold air. This is done

by convection method.

Radiation The heat transfer takes place due to temperaturedifference between body and the objects in the premises whichare at a lower than the body. The radiant heat is transferredfrom a warmer body to colder objects or surfaces withoutheating the medium between the two.

Evaporation the rate at which the body is rejecting heat isfaster than the rate at which the surrounding air can absorb it.


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Respiration Losses:During respiration, the body looses both sensible and latentheat by convection and evaporation.

The velocity over the skin is also important influencing thefeel of comfort.Temperature levels considered as comfortable vary dependingon many factors. Such as,

1. Level of Activity.2. Nature of Activity.3. Type of clothing.4. Personal preferences.5. Age.6. Sex.7. Geographical conditions.8. Non uniform temperatures within the space

etc.,


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Factors Affecting Human Comfort:TemperatureRelative HumidityAir motion

Temperature Greater is the temperaturedifference, faster is the heat flow.

Relative Humidity whenever temperature israised or lowered the relative

humidity changes.Air motion Air motion affects the heat rejection

from the body. Forced air convectionaccelerates the rate of evaporation.


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PART2


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Part 2

Properties of Air Daltons law of partial pressures Saturated Air

Dew point, dry bulb & Wet bulb temperature Standard Air Atmospheric Pressure Specific Density

Specific Volume Relative, specific Humidity Types of heat


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Properties of Air:

76% N2 23% O2 1% Other

Dry Air

1 Kg

Fig1: Composition of Dry Air


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Daltons law of Partial Pressures: If dry air and water vapour is mixed in a container, the

pressure exerted by this mixture is the addition of the pressureexerted by each of two gases.The volume occupied however, is the same as if the entirevolume is occupied by each gas at its partial pressure.

Dry Air Water Vapour Total

Partial Pressure Partial Pressure Pressure

The total enthalpy of the mixture is the sum of the enthalpiesof each constituent at this partial pressure.


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Saturated Air:When the air and saturated water vapour occupied same volume, we say air issaturated.In reality only water vapour is saturated.

Dew Point Temperature:The temperature at which the moisture starts condensing is called dew pointtemperature.

Dry Bulb Temperature:This is the temperature of air as indicated by an ordinary thermometer.

Wet Bulb Temperature:This is the temperature registered by the thermometer whose bulb is covered

by a wetted wick over which air is moving at a velocity of 2.5 10.0 m/s.

The difference between the readings of wet and dry bulb thermometer iscalled wet bulb depression.


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Standard AirASHRAE defines standard air as dry air at sea level having anatmosphere pressure of 101.325 kPa, at 20

0C (or) 70

0F,14.696

Psia.Atmospheric PressureThe weight of the air pushing down the earth is referred to asatmospheric pressure. A Barometer is used to measure the atmospheric pressure.The atmospheric pressure at sea level is 101.325 kPa or 760 mmof mercury column.

Specific Density:The air is heated at constant pressure, the air would expand andtherefore weigh less per unit volume. this property is defined asspecific density. Specific density means weight of air per unit volume. The unit ofmeasure is kg / m 3.The specific density of air at standard condition is 1.204 kg /m3.(0.075 lb/ft 3)


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Specific Volume:Specific volume is reciprocal of specific density.

Specific Volume indicates how much volume orspace one kilogram of air will occupy. The unit ofmeasure is m3/kg.

The air is heated then its specific volume wouldIncrease, which means the specific density willdecrease.

This property of air helps us in determining fan performance and selecting fan motor sizes.The specific volume of air at standard condition is1/1.204 or 0.830 m 3/kg.(13.33 ft 3/lb).


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Relative Humidity:It is the ratio of the actual partial pressure of water vapour to itssaturation pressure corresponding to the same dry bulb temperature.

It indicates the ability of air to absorb additional moisture and isexpressed as a percentage.

Fig: Dry air is heavier than moist air Fig: Cold air is heavier and warm air is lighter.


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Specific Humidity:Specific humidity indicates the amount of

moisture present in the air and is expressedin terms of grams of moisture per kg of air.Specific humidity is a more dependable

indication of actual moisture present in theair.

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