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Air Conditioning Systems for Tropical Humid
Countries
Eng. Chandana DalugodaChandana Dalugoda Consultants
Tel: +94112700276Email: [email protected]
Contents
» Tropical & humid climates & effects on Air conditioning
» Thermal comfort & ASHRAE Standard 55
» Areas that affect the cooling load
» Reducing Solar gain
» Fresh air for ventilation & ASHRAE Standard 62
» Prevent condensation
» Reducing energy consumption
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Tropical climates & effects on Air conditioning
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POSITION OF EARTH IN ITS ORBIT AROUND THE SUN
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TRAVEL OF THE SUN REALTIVE TO EARTH
Sun will be directly overhead Tropic of Cancer on June 21st and Tropic of Capricorn on December 21st. So countries situated between Tropic of Cancer & Capricorn are known as Tropical climates or Tropics.
During December South Pole has 24 hr daylight and during June North Pole has 24 hr daylight.
Seasonal Changes
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Tropical countries on world map
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Tropical Climates03/03/2012
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What is tropics?‘Tropics’ are the countries situated between Tropic of Cancer & Tropic of Capricorn latitudes of the earth.
This has created as a result of earths tilt of 23.5°
Figure below shows the tropical countries;
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Temperature & RH
» Comparison between tropics and temperate climate
˃ Figure below shows London outdoor conditions are closer to standard indoor conditions in summer cooling.
Unit Colombo Chennai Delhi London
dry bulb °C 33 38.5 43.8 28.3
wet bulb °C 28 28.3 29.6 19.8
RH % 68 46 36 52
India & Sri Lanka
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Tropical weather
Tropic of cancer
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ACTUAL SUN PATH MODEL
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SUN PATH DIAGRAM- LONDON, UK
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SUN PATH DIAGRAM- NEW DELHI, INDIA
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SUN PATH DIAGRAM- CHENNAI, INDIA
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SUN PATH DIAGRAM- COLOMBO, SRI LANKA
Climates
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Climates
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Climates
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Outdoor conditions
» Comparison outdoor RH% in summer months between tropics and temperate climate
Unit Colombo Chennai Delhi London
dry bulb °C 33 38.5 43.8 28.3
wet bulb °C 28 28.3 29.6 19.8
RH % 68 46 36 52
Humidity, comfort & health
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Humidity
» What is humidity?
˃ Humidity is a term for the amount of water in the air
˃ Relative humidity: the ratio of actual water vapour pressure to the saturated water vapour pressure at the same dry bulb temperature
RH % = Pwv / Pwvs X 100
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Effect of Humidity
» perspire?
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Humid Climates
» Is it raining outside?
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Effects of Moulds
» Effects of moulds is shown here.
» After organic materials absorb enough moisture, some form of mould will be able to digest them and the result is visible clearly.
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Moulds insidevinyl wall coverings
Too often, vinyl wall coverings trap moisture behind walls, leading to mould growth in the face of the wall board and in the adhesive as shown here
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Optimum Humidity Range
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Case study‐Mould growth due to faulty equipment operation‐SL
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Treated Fresh Air unit (TFU) operates with defective 2‐way control valve & TFU dumps FA continuously at lower leaving conditions throughout the day.As a result , guest bed room s which not occupied, temperature has dropped to 19C and developed mould growth
Thermal comfort &ASHRAE Standard 55
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ASHRAE Standard 55Thermal Environmental Conditions for Human Occupancy
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The purpose of this standard is to specify the combinationsof indoor thermal environmental factors and personalfactors that will produce thermal environmental conditionsacceptable to a majority of the occupants within the space.
Comfort
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As we live in variable external environment, heating & cooling systems area required in buildings (in conjunction with adequate ventilation) to provide satisfactory thermal conditions.
1. for the occupants of the buildings
2. for the contents, e.g. Stored equipment, machinery,
furnishings etc.
3. For the structure and fabric of the building itself.
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Thermal comfort
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Thermal comfort can be defined as the condition of mind which satisfaction is expressed with environment.
A person’s sensation of warmth is influenced by the following main physical parameters, which constitute the thermal environment.
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1. Air temperature
2. Mean radiant temperature
3. Relative air speed
4. Humidity
Physical Parameters
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Beside these environmental factors there are personal factors that affect thermal comfort;
1. Metabolic heat production
2. Clothing
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Transformation of chemical energy into heat and mechanical work by metabolic activities within an organism
met: a unit used to describe the energy generated inside the body due to metabolic activity, defined as 58.2 W/m2
(18.4 Btu/h∙ft2), which is equal to the energy produced per unit surface area of an average person, seated at rest. The surface area of an average person is 1.8 m2 (19 ft2)
Metabolic Heat production
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Clothing
The resistance to sensible heat transfer provided by a clothing
clo: a unit used to express the thermal insulation provided bygarments and clothing ensembles, where 1 clo = 0.155 m2 °C/ W (0.88 ft2∙h∙°F/Btu)
Personal Factors
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Operative Temperature
“The uniform temperature of an imaginary black enclosure in which an occupant would exchange the same amount of heat by radiation plus convection as in the actual nonuniform environment”
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Operative Temperature
Predicted Mean Vote (Pmv)
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PMV represent the ‘predicted mean vote’ of a large population of people exposed to a certain environment.The PMV equation only strictly applies to human exposed to constant conditions at a constant metabolic rate.
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Predicted Mean Vote
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PMV is an index that predicts the mean value of the votes of a large group of people on a 7 point scale
Acceptable Thermal Environment for General Comfort
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Table below defines the recommended PPD & PMV range for typical applications.
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Predicted Mean Vote
PMV
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Acceptable Range of Operative Temperature & Humidity for Spaces
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Adaptive Comfort
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The Adaptive Principle
The fundamental assumption of the adaptive approach is expressed by the adaptive principle
“If a change occurs such as to produce discomfort, people react in ways which tend to restore their comfort”
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Adaptive Comfort
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Adaptive ComfortThe adaptive relationship between comfort temperature and the outdoor temperature can be used to help design comfortable buildings.
Here the indoor comfort temperature is calculated from the mean outdoor temperature and plotted on a monthly basis together with the monthly mean of the daily outdoor 55 maximum, minimum and mean air temperatures.
Such a diagram helps the designer to judge whether passive heating and/or cooling are a possibility in the climate under consideration.
The relationship between the desired indoor temperature and the range of outdoor temperatures shows whether, for instance, night cooling is likely to be a viable way to keep the building comfortable in summer, or to calculate whether passive solar heating will be enough in winter.
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Adaptive Comfort
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Finally….. Do we really need to specify indoor climates?
Something like 24°C DB/50%RH ?
Indoor temperature, 24°C can be changed as per the outdoor temperature to satisfy the
occupants!
Adaptive Comfort
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Reducing Energy Consumption
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Effects on air conditioning˃ Tropical countries get sunshine throughout the year, except for monsoon season. Very high solar loads, needs special attention to fenestration, shading, walls & roofs
˃ Some places specially between 10° latitudes from equator, high humidity prevails. 70% ~ 80% RH. Needs special attention specially costal areas to prevent condensation. High RH% leads to Bugs, Mold & Rot. ASHRAE Standard 62; Occupied space relative humidity shall be designed to be limited to 65% or less
˃ Dust and particulate mater is found increased quantities. Filtration is essential
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Effects on air conditioning
It takes lot of energy to cool a building in a hot humid country. So the key is to using less energy to cool a building. Smaller loads allow less energy use.
Suggestions by ASHRAE guide is given below
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Exterior glazing
Minimize exterior glazing, install insulating low –e glass and shad it.
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Low‐e glass with exterior solar shading
Avoid glazing which faces west.
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Avoid glazing which faces west.
Windows for day lightingDesign the exterior enclosure and its glazing so that the sun provides day lighting at the perimeter of the building
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Fresh air for ventilation &ASHRAE Standard 62
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Ventilation for Acceptable Indoor Air Quality
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ASHRAE Standard 62.1-2004
The purpose of this standard is to specify minimum ventilation rates and indoor air quality that will be acceptable to human occupants and are intended to minimize the potential for adverse health effects
Ventilation Rate Procedure. This is a prescriptive procedure in which outdoor air intake rates are determined based on space type/application, occupancy level, and floor area.
IAQ Procedure. This is a design procedure in which outdoor air intake rates and other system design parameters are based on an analysis of contaminant sources, contaminant concentration targets, and perceived acceptability targets.
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Calculation for fresh air ventilation
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Example
Determine the amount of fresh air required for ventilation of a lecture room with an occupancy of 75 people.
Usual practise is to use ventilation rate of 8 L/s. person to determine the ventilation air quantity. This was based on (Room CO2 ppm- Outdoor CO2 ppm) ≤700 ppm
Required air volume then be 8 L/s.p x No. people, say 600 L/s
However ASHRAE standard 62.1-2004 provides the ventilation rate procedure to determine the air volume, which is more energy efficient.
ASHRAE Standard 62.1
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ASHRAE Standard 62.1-2004Breathing Zone Outdoor Airflow
The design outdoor airflow required in the breathing zone of the occupiable space or spaces in a zone, i.e., the breathing zone outdoor airflow (Vbz), shall be determined in accordance with Equation
Vbz = Rp.Pz + Ra.Az
Where:Az = zone floor area:Pz = zone population:Rp = outdoor airflow rate required per person as determinedfrom Table 6-1.Ra = outdoor airflow rate required per unit area asdetermined from Table 6-1
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ASHRAE Standard 62
Calculation for fresh air ventilation
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ASHRAE Standard 62.1-2004Example: 200 m2 lecture room of with 75 people, determine the fresh air ventilation volume?
Vbz = RpPz + RaAz
Rp= 3.8 l/s.p - from Table 6-1Pz= 75 peopleRa= 0.3 l/s - from Table 6-1Az= 200 m2
Vbz = (3.8 x 75) + (0.3 x 200) = 345 l/s
If we use rate of 8 l/s.p then air flow would be 600 l/s, which is waste of energy.
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Energy required to cool fresh air for ventilation
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Psychrometrics conditions for a comparison of fresh air ventilation energy required
Unit Colombo Chennai Delhi London
dry bulb °C 33 38.5 43.8 28.3
wet bulb °C 28 28.3 29.6 19.8
RH % 68 46 36 52
h (OA) kJ/Kg 90 92 97 60
h (RA) kJ/Kg 48 48 48 48
Δh kJ/Kg 42 44 49 12
Energy required to cool fresh air for ventilation
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Fresh air for a room requires 400 L/s
Total heat absorbed Q = V x 1.20 x Δh
Q (kW) Q (TR) Q (kW/p)
Colombo 20.2 5.7 0.4032
Chennai 21.1 6.0 0.4224
Delhi 23.5 6.7 0.4704
London 5.8 1.6 0.1152
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Rotary enthalpy heat exchanger
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Hot & humid climate, extract air is valuable asset
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Cooling incoming air by leaving extract air
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Enthalpy wheel
Conclusions
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Air conditioning systems for Tropical Humid climates shall address the following in order meet with energy goals;
1. Avoid east west glasses as much as possible
2. Use low-e glasses for windows sufficient shading
3. Never install larger cooling equipment than necessary which causes high
humidity during off periods
4. For indoor comfort use adaptive method
5. Use ASHRAE Standard 62.1 to calculate the ventilation air requirement.
6. Use demand control ventilation method to reduce the load during non
occupying periods.
7. Use rotary enthalpy heat exchanger to reduce ventilation loads.
8. In high humidity areas, specially hotel rooms, use dual set point thermostat
to prevent condensation during un-occupied periods.
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Thank You
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