1. The Internal Environment & Human Comfort
5508BESG Services and Utilities Lecture 3
The Internal Environment & Human Comfort

2. Are you sitting comfortably?
Heating, Ventilation, Air Conditioning, Lighting Installations are intended to manage the internal environment, largely aimed at providing conditions necessary to maintain human comfort, but…..
what is comfort,
what contributes to a comfortable environment, and
how do we measure it?

3. The Internal Environment
Thermal Environment
Visual Environment
Acoustic Environment

4. The Thermal Environment
To provide human comfort
To protect building fabric
The maintain conditions for industrial / commercial processes.
To maintain conditions for storage of goods and artefacts.

5. What is thermal comfort?
“The state of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation”.
But … the sensation of thermal comfort is fundamentally a defence mechanism.

6. What determines thermal comfort?37
The human body is a heat generator but it can only function if its core temperature is maintained at a constant 37oC. A variation of more than ± 2oC can be fatal. The thermoregulatory system has a range of mechanisms to maintain the core temperature.

7. What determines thermal comfort?
Metabolic Rate
130w
Heat Rejection Rate
150w 37
Result = Discomfort
For Comfort: Rate of heat Generation = Rate of Heat Rejection

9. Methods Of Heat Rejection
Method of Heat Transfer Room Environmental Factors that effect heat rejection rate
Conduction
Convection
Radiation
Evaporation
Area of body in direct contact with cold surfaces.
Room air temperature
Air velocity
Average temperature of room surfaces
Humidity
Air Velocity

11. Factors that effect thermal comfort.
Environmental Factors
Room surface temperature (radiant temperature).
Air Temperature.
Air Velocity.
Humidity
Personal Factors
Activity Level (Metabolic Rate) (Watts)
Clothing Level (Clo)
Acclimatisation
Health
Gender
Age
Psychological factors

13. Indoor air temperature (ai) or (tai ) oC
The dry bulb temperature of the air in the space
Mean radiant temperature (r) or ( tr ) oC
For practical purposes, the mean radiant temperature is considered to be the weighted average surface temperature in a space.
Relative humidity (or Percentage Saturation) %
The ratio of moisture vapour content in the air expressed as percentage of the maximum moisture the air could hold at same dry bulb temperature. (The difference between humidity and the percentage saturation is the quantities used to measure the moisture vapour, the difference is very slight). Humidity or % Saturation is an indication of the ease with which the air will absorb water vapour. Water will evaporate easily with low humidity and visa-versa with high humidity.
Air Velocity m/s
The air speed within a room , speeds below 0.1m/s can feel stuffy, 0.25m/s and above can be felt on the skin.

15. = 0.5 x air temp + 0.5 x mean radiant temp
Question: If we are sensitive to both air temperature and mean radiant temperature (surface temperature) which do we use to specify what temperature we want a space to be?
Answer: we use the average of the two i.e.
= 0.5 x air temp x mean radiant temp
θc= 1/2 θai+ 1/2 θr
This temperature has been given several names: Dry resultant temperature, comfort temperature or Operative Temperature.
CIBSE recommend the use of this as the temperature to specify the design temperature for spaces.

17. Environmental criteria for design
Design Conditions
Environmental design
CIBSE Guide A
Environmental criteria for design

18. Comfort Conditions
When we talk about comfort conditions we are referring to a set of conditions within a room; whereby any person in that room should feel ‘comfortable’
Typically:
Room temperature 16°C - 23°C
Air changes 1½ - 2 per hour
Humidity 65% - 75%
These are taken as approximations but give a good indicator if temperature and ventilation air change are correct then the humidity will usually be correct also.

19. Extract from CIBSE Guide A Table 1.5.
Based on an assumed air velocity of 0.15m/s and a RH of 50%.

20. Air Velocity: Is it a draught by any other name?

21. Our perception of Room Air Velocity.
Our perception of air velocity varies with the temperature of the air .
Published summer and winter design temperatures assume moderate air velocities (circa 0.1 to 0.15m/s) These are typically found in naturally ventilated rooms and those rooms with moderate requirements for mechanical ventilation.
Rooms with high mechanical air supply rates can experience air velocities above 0.15m/s but should not exceed 0.25m/s in the occupied zone. (exceptionally 0.5m/s)
Air velocities above 0.15m/s can be felt as a draught unless air temperatures are elevated.
Air velocities much above 0.5m/s tend to be annoying irrespective of temperature.

23. Humidity
Humidity is the term used for the amount of moisture in the air i.e. the concentration of water vapour in the atmosphere.

24. Relative Humidity
Is a percentage ratio of the amount of water vapour in the air at a particular temperature and pressure compared with the maximum amount of water vapour the air could hold at the same temperature.

25. Humidity & Comfort
Air with low humidity can easily and quickly absorb vapour from any source. Air with a high humidity does not absorb vapour easily.
Therefore relative humidity provides an indication of how readily air will absorb moisture vapour.
if there is too much moisture in the air it can feel humid and uncomfortable, whereas if there is too little the eyes, throat and skin can all feel uncomfortably dry and static electricity can build up.

26. Humidity
Design conditions are generally only specified for specialist rooms or locations where air conditioning is expected.
Humidity control only required for:
specialist controlled environments for specific tasks,
rooms with high density occupancy rooms (theatres etc) or
where high levels of water vapour are produced e.g. swimming pools, restaurants, wet processes.
For most “dry” activities, humans are comfortable within a humidity range of 40 to 70%, with 50% being a common design value for general use rooms.

27. Air Quality
Air quality: a feeling of freshness, rather than stuffiness and a build up of odours, depends on how much fresh air is supplied and what contaminants are present or are produced in the space.

28. Ventilation and air quality
Fresh air for ventilation is required to both provide air for respiration and to achieve acceptable air quality.
People tend to assess air quality in two ways,
firstly by smell and secondly by sensitivity to irritants, such as pollen, tobacco smoke or other pollution, by the eyes, nose and throat.

29. How much fresh air? To provide oxygen 0.2 L/s
To dilute carbon dioxide 1 L/s
To dilute occupation contaminants 5 L/s
To give a feeling of freshness 10 L/s

30. Managing Thermal Comfort
Heating systems modify and control in spaces during cold weather.
Air conditioning systems modify and control the temperature in spaces during both warm and cold weather and can additionally modify and control the humidity in a space if required.
Ventilation systems modify and can control the air quality in a space and can effect the air velocity

31. Recommended Reading CIBSE Knowledge Series KS6 “Comfort” 2006
CIBSE Guide A “Environmental Design