Choice of Design Choice of Design ConditionsConditions

April 18, 2023 choise of design condition 1

At the end of the lecture the student should be able to:

Select suitable outdoor design conditions based on weather data and applications

Find the supply conditions based on the psychrometry of air conditioning systems, and

Analyze various summer air conditioning systems

Choice of Inside Design Choice of Inside Design ConditionsConditions

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Comfort ZoneComfort ZoneAccording to ASHRAE (approximately)Dry bulb temperature from 20oC to 26oC Dew point temperature from 1.7 to 16.7 oC Relative Humidity between 30% and 80% at 20 oC and

between 26 and 72 at 22 oCAir movement: is related to air temperature

for lightly clothed people

Air velocity(m/s)

0.1 0.2 025 0.3 0.35

Dry Bulb T 25 26.8 26.9 27.1 27.2

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Thermal comfort depends on creating an environment of dry-bulb temperature, humidity, and air motion, that is appropriate for the activity level of the people in the space.

This environment allows the body’s rate of heat generation to balance with the body’s rate of heat loss.

April 18, 2023 choise of design condition 5

Using Psychometric ChartUsing Psychometric ChartASHRAE comfort zone can be shown

using the s.chart

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Choice of Inside Design Choice of Inside Design ConditionsConditions For a person to feel comfortable the following conditions

are desirable1. The air temp should be higher than the mean radiant

temperature in summer, but lower in winter2. Average air velocity should not exceed 0.15m/s3. Relative humidity should lie between 20% and 60% and

should not exceed 70%( >60 % > smell)4. The temperature difference between the feet and the

head should be as small as possible (<1.5oC)5. Thermal radiation is best not directed on the tops of the

heads

6. CO2 concentration should not exceed about 0.1%

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Selection of Outdoor Design Selection of Outdoor Design ConditionsConditions

• The ambient temperature & moisture content vary from hour to hour and day to day & from place to place

• For most of the major locations of the world, meteorological data are available in the form of mean daily or monthly maximum and minimum temperatures and corresponding relative humidity or wet bulb temperature

• Selection of worst possible condition leads to excessively large cooling/heating equipment as worst conditions prevail for a few hours in an a year

Choice of Out side Design Choice of Out side Design ConditionsConditions

There are three ways to decide outside summer design states

1.Follow the technique described belowi. choose the highest mean monthly maximum

temperature as the design dry bulb temperature

ii. For the same month used in (i) couple the mean daily maximum temperature and the afternoon record of relative humidity

iii. With the temperature and humidity coupled in (ii) determine the corresponding moisture content from psychometric chart

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Cont…Cont…

iv. Use the moisture content in (iii) with the mean monthly maximum temperature to establish the corresponding wet bulb temperature

v. Select the mean monthly maximum temperature (i) and wet bulb obtained in (iv) as the outside design state

2. Choose the outside dry and wet bulb temperatures in terms of their frequency of occurrence in the summer months

3. Adopt what is chosen by common custom, locally

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