Building Services Engineering 5th Edition Handbook

Built environment 11
Atmospheric vapour pressure p is that part of the barometric pressure produced by the water
vapour in humid air. Standard atmospheric pressure at sea level is 1013.25 mb (millibars) and this
comprises about 993.0 mb from the weight of dry gases and 20.25 mb from the water vapour,
depending on the values of barometric pressure, air temperature and humidity.
The comfort equation balances to zero when heat transfers between the body and surroundings
are stable. This is the thermally neutral condition when there are no feelings of discomfort.
It is unlikely to be satisfied for a group of people, and a comfort zone is used to specify the range
of acceptable levels for the majority.
Comfort measurement
Figure 1.1 shows the percentage of people dissatisfied (PPD) and predicted mean vote (PMV)
that are used for the assessment of indoor thermal environments, from Fanger. The chart
demonstrates that dissatisfaction among less than one-third of the occupants is achieved
in indoor environments within the slightly cool to slightly warm categories. Personal variation
in clothing, sedentary position and control over the microclimate being the final control
mechanism.
A thermal comfort analogue computer may be used to make this assessment with an
ellipsoidal-shaped sensor that is around the size of a straight banana. This shape has the same
mathematical relationship between its heat loss by radiation and convection as the human body.
To obtain values of temperature, velocity, carbon dioxide and humidity in the atmosphere,
each is measured separately as 0–10 V or 0–20 mA analogue signals, converted into digital bytes
with an analogue-to-digital converter and then stored in a data logger in random access memory.
Numerous readings are easily retained in memory for later access through a desktop or portable
computer. Lists of data numbers and their time of occurrence can be stored on hard and floppy
disks for processing with dedicated software or any spreadsheet program. Graphical output can
show the history of the recorded temperature, or other variable, during a period of time. Such
graphs are termed trends. The software can be programmed to take readings at, say, intervals of
2 s or longer, or only whenever a significant change takes place. Figure 1.2 shows the schematic
arrangement of a data-logging system with a trend display of room air temperature. Figure 1.3
is a photograph of a portable data-logging cart created to assess existing building conditions.
100
Predicted percentage of
dissatisfied people
80
60
40
20
Comfort
zone
–3
Cold
–2
Cool
1.1 PPD versus PMV.
–1
Slightly
cool
0
Neutral
1
Slightly
warm
Predicted mean vote
2
Warm
3
Hot