Science - Thermal Transmission - Page 1 2 3 4 5 6 7
Natural Ventilation Heat Loss
This is the heat loss
associated with air flow through a building by natural
means, that is, through small openings and cracks in the structure.
The rate of natural
ventilation (infiltration and exfiltration) depends on several factors such as;
wind strength and direction.
Other factors are air tightness and stack effect
especially in high buildings as warm air rises.
The formula for
ventilation heat loss is:
Q = N . V .
Sp.ht . dt
Where;
Q = heat loss (
N = Number of air changes per hour. An air
change is one room volume.
V = Room volume (m3)
Sp.ht. = Specific
heat factor for air. This is found from the following formula.
Sp. Ht. Factor = ( Specific heat capacity of air x 1000 to convert from kJ to Joules x density of air ) / 3600 to convert from hr
to secs.
Sp. Ht. Factor = (1.01 x 1000 x 1.2 ) / ( 3600 )
Sp. Ht. Factor = 0.34
dt = temperature difference between inside
and outside (oC)
Natural Ventilation Rates
CIBSE guide A (2006)
section 4.6 gives details of natural ventilation and infiltration in buildings.
CIBSE guide A (2006) -
Tables 4.13 to 4.21 give air infiltration rates for various buildings.
The maximum average air change rates are given in the
Table below based on data from the above tables.
|
Maximum Average Air Infiltration
rates in Air Changes per hour (AC/h) |
|||
|
CIBSE
Guide A –Table ref. |
Building |
‘Leaky’
building (does not comply with
current regulations) |
Moderately ‘tight’ building (complies with 2005
regulations) |
|
Table
4.13 |
Office Type 1: naturally
ventilated, 100 – 3000 m2 |
0.90 |
0.30 |
|
Table
4.14 |
Office Type 2: naturally
ventilated, 500–4000 m2 |
0.70 |
0.25 |
|
Table
4.15 |
Office Type 3: air conditioned,
2000–8000m2 |
0.60 |
0.20 |
|
Table
4.16 |
Office Type 4: air
conditioned HQ-type building, 4000–20000
m2 |
0.65 |
0.25 |
|
Table
4.17 |
Factories, warehouses, halls |
0.65 |
0.25 |
|
Table
4.18 |
Schools |
0.70 |
0.25 |
|
Table
4.19 |
Hospitals and Health Care buildings |
0.60 |
0.25 |
|
Table
4.20 |
Hotels |
0.85 |
0.30 |
|
Table
4.21 |
Dwellings – 1 storey |
1.15 |
0.40 |
|
Dwellings – 2 storeys |
1.00 |
0.35 |
|
|
Apartments – 1 to 5 storeys |
1.00 |
0.50 |
|
|
Apartments
– 6 to 10 storeys |
1.60 |
0.55 |
|
Notes:
On severely exposed
sites, a 50%
increase
above the tabulated values should be allowed.
On sheltered sites,
the infiltration rate may be reduced by 33%.
Mechanical
Ventilation
When mechanical ventilation is installed in a building
then the amount of heat loss is increased.
In large Mechanical
Ventilation systems the air change rates are not
included in heat loss calculations.
Heat loss calculations primarily include infiltration only and allowances for extract
ventilation in small rooms such as; Bathroom, Toilet, Kitchen or Utility room.
This is because heat
emitters such as radiators can be sized to overcome this additional extract
ventilation heat loss.
In large ventilation systems the heat loss incurred by
bringing in large amounts of cold air in winter
is allowed for in psychrometric chart sizing of plant such as the air heater
battery.
We would not expect radiators
to be sized to cope with a mechanical ventilation rate of 12 air changes per
hour of outside air, a heater battery in an
air system is the way to heat this air.
Mechanical Ventilation rates are given in the Ventilation
section of these notes.
If a heating system is to be sized to allow for mechanical
extract ventilation such as in a Bathroom, Toilet, Kitchen or Utility room the running time for the fan(s) should be considered.
In a Bathroom in a dwelling the extract fan may be on for
1 or 2 hours per day for example.
The mechanical ventilation rate may be 3 AC/h but this may
only occur for short periods.
If
3 AC/h of air is removed and the fan is on for 20 minutes (1/3 hour) in that hour then the overall ventilation
rate is; 3 AC/h x 1/3
= 1
AC/h.
Example 7
Calculate the ventilation
heat loss from the building shown below.
The air change rate due
to natural ventilation is 2 air changes per hour.
Q = N
. V . Sp.ht. . dt
Q = 2.0 x 6.0 x
3.0 x 2.5 x 0.34 ( 20 - - 2)
Q = 2.0 x 45 x
0.34 x 22
Q = 673.2 Watts