bs 2 hvac air conditioning 1

BUILDING SERVICES TWO (BLD60503)

HVAC Systems & Building

Prepared by Tan Hee Chai

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Air Conditioning System

BUILDING | SERVICES | TWO (BLD60503)

BUILDING | SERVICES | TWO (BLD60503)

Internal Climate: Human comfort

To maintain thermal equilibrium and internal body temperature of aprox 36.8 deg Celsius, body lose

internal heat.

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Heat is lost through:-

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Internal Climate -Human comfort

What is your ideal comfort zone (room)?Temperature?Speed of Fan?

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Internal Climate – Human comfort

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Internal Climate - Human comfortInternal environment may be classified as follows:

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Internal ClimateHuman comfortInternal climate can be defined as ‘a band of internal conditions’ that satisfy most of the people, most of the time.’ for the given function of the room/workshop.

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Internal Climate - Human comfortFactors influencing heat loss thus feeling of comfort are as follows:

1.Dry bulb temperature of the room air – narrow band 19 to 23 Celsius acceptable

2.Humidity of the room air (%RH) – wide band of 40% to 60% acceptable

3.Mean temperature of the surrounding surfaces (Celsius) – narrow band 9 to 26 Celsius

4.Velocity of the air across the body (m/s) – narrow band 0.1 to 0.25 m/s

5.Extent of clothing worn by occupants – personal choice

6.Degree of physical activity of occupant (ie metabolic rate) mets or W/m2

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Internal Climate - Human comfortOperative temperature (previously known as resultant temperature or dry resultant temperature, but renamed to align with ASHRAE and ISO standards) is ‘a simplified measure of human thermal comfort derived from air temperature, mean radiant temperature and air speed.

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Internal ClimateResultant Temperature (Operative Temperature) Comfort Zone for Sedentary Occupation

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Internal ClimateDiagram for continuous occupation

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Internal Climate

Mean radiant temperature (MRT) is a measure of the average temperature of the surfaces that surround a particular point, with which it will exchange thermal radiation

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Internal ClimateThe predicted mean vote (PMV) was developed by Povl Ole Fanger at Kansas State University and the Technical University of Denmark as an empirical fit to the human sensation of thermal comfort. It was later adopted as an ISO standard.

PMV = method to measure human sensation of thermal comfort

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Internal ClimateIt predicts the average vote of a large group of people on the a seven-point thermal sensation scale where:

+3 = hot+2 = warm+1 = slightly warm0 = neutral-1 = slightly cool-2 = cool-3 = cold

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Internal Climate

The predicted percentage dissatisfied (PPD) index provides information on thermal discomfort or thermal dissatisfaction by predicting the percentage of people likely to feel too warm or too cool in a given environment.

The PPD can be obtained from the PMV.

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Internal Climate

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Internal Climate - Air qualityComfort building’s occupants affected by Contaminants and Lack of oxygen

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Internal Climate - Air qualityAtmospheric contaminants include•Coarse dirt •Dust – soot, mineral, organic particles•Fumes – form by condensation vapours•Smoke – from incomplete combustion•Gases – carbon monoxide, sulphur dioxide etc•Bacteria – small organisms

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Internal Climate - Air quality•Air quality is measured using CO2 sensors.•Solids removed by filtration•Vapours – by cooling & washing•Gases remain in solution in the air•Dilution of air inside and with air outside increases oxygen content and reduces concentrations of pollutants.•Bacteria – by sterilisation facility, ultra-violet light sources

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Internal Climate - Ventilation air

Dilution air requirements may be found in the Chartered Institute of Building Services Engineers (CIBSE) guide,Typical ventilation rate : 8 to 20 litres per second per person

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Humidity of air:Properties of humid airHumidity of air is defined by1.Absolute humidity or moisture content.

is the measure of water vapour(moisture) in the air, regardless of temperature.

Measured in kg/kg or g/kg Max AH of warm air at 30º Celsius is approx. 30gm of water

vapour (30g/m3) Max AH of cold air at 0º Celsius is approx. 5gm of water

vapour (5g/m3)

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Humidity of air is defined by (cont’d)2. Relative humidity

also measures water vapour but RELATIVE to the temperature of the air.

It is expressed as the amount of water vapour in the air as a percentage of the total amount that could be held at its current temperature.

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Humidity of air is defined by (cont’d)

BUILDING | SERVICES | TWO (BLD60503)Properties of humid air

Container one has a maximum volume of 30g of water and is half full- it contains 50% of its capacity.

Container two has a maximum volume of 5g of water and is three quarters full- it contains 75% of its capacity

Container one contains six times as much water as container two, yet actually contains a lower percentage.

Warm air can hold far more moisture than cold air meaning that the relative humidity of cold air would be far higher than warm air if their absolute humidity levels were equal.

BUILDING | SERVICES | TWO (BLD60503)Properties of humid air

Human body temperature is dependent on the air as it absorbs and removes moisture from our skin to cool us down.  If the relative humidity is high, the amount of water evaporating from our skin is limited so we feel warm and stifled.

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Humidity of air is defined by (cont’d)Dew-point temperature – air holding

max water (saturated)

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BUILDING | SERVICES | TWO (BLD60503)

Properties of humid airDry bulb temperature – is registered by an ordinary thermometer often referred as air temperature.Wet bulb temperature – is temperature registered by a bulb covered by a watered- moistened wick exposed to rapidly moving current of airWhen bulb reading is the same as wet bulb reading this indicates 100 percent relative humidity.Dew point temperature is the temperature at which air can no longer ‘hold’ all the water vapour which is mixed with it and some of water vapour must condense into liquid water

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Adverse effects of high and low levels of relative humidity for the occupants as well as the dwelling itself.Hygiene, health, retention of value, aesthetics and comfort which can be affected by a failure to maintain an optimal level of relative humidity.

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A relative humidity (RH) of 40-60% is generally considered to be optimal for a comfortable and healthy home.

Too much moisture can lead to mold and overheating.

Too little causes dry eyes, chapped lips and an environment in which bacteria and viruses can thrive.  

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Psychrometric data

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Psychrometric Data The relationship between dry bulb and wet bulb temperature, relative humidity and moisture content can be seen by reference to the psychrometric chart.

It provides a measure of air state at a variety of temperature and humidity levels.

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Psychrometric data

Included in the chart also Specific volume (inverse of density – m3/kg Specific enthalpy (heat energy) – kJ/kg

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Psychrometric Chart

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Psychrometric Chart

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AC provide desirable climate for the purpose of:Comfort to occupantsOperation of sensitive equipment (eg computers)The storage of commodities in good condition.Carrying out of industrial processes without harmful effectsOccasionally the well-being of exotic plants and animals.

BUILDING | SERVICES | TWO (BLD60503)

BUILDING | SERVICES | TWO (BLD60503)

What have we learnt today?

Internal room climate – human comfortAir qualityTemperatureHumidity

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The End