Heat Removal
Ventilation on the basis of heat removal from rooms where some considerable temperature drop is a main factor, needs special consideration. The amount of heat produced in the room must be estimated from:
Body heat of occupants
Electrical apparatus
Other heat-producing processes
Solar heat gain through structure
The volume of air required for ventilation can then be assessed by using the following formula:
| m³/h or air required = | Heat gain in kW x 3000 |
| Temp. diff. °C |
The ‘Temp. diff’ °C is the difference between outside shade temperature and the slightly higher required inside temperature.
e.g., If the total heat gain is say 11.75 kW per hour, the outside shade temperature. 21°C and the maximum inside temperature required 27°C then the volume of air required is:
| 11.75 x 300 | = 5875 m³/h |
| 6 |
Note that a small reduction in the temperature difference makes a considerable increase in the volume of air required. For instance, if in the example above the temperature difference is reduced by say 2°C, i.e., required inside temperature 25°C, then the volume of air required would be increased to:
| 11.75 x 3000 | = 8812m³/h, a 50% increase in air volume |
| 4 |
Always extract from as high as possible to prevent the collection of heated air under the roof or ceiling from extending downwards to near breathing level. Fans at or near the ridge in such cases are ideal, with air replacement from side windows above the head level. This method assists the natural tendency for warmer air to rise, and the quicker this ‘fug’ is removed from high level the better will be the conditions at breathing level.
Note that with ventilation alone, the temperature inside a room cannot be reduced to the outside shade temperature - it will always be a few degrees above. However, it is worth considering pulling the replacement air from the “cold” side of the building - the north facing (north of the equator).
It should be borne in mind particularly when dealing with general purpose single-storey factories that there are two separate ventilation problems, one for summer and one for winter. A high rate of ventilation is required in summer to deal with the build-up of heat from solar radiation, production processes and the workers themselves, whereas in winter a very low ACH is required, simply to prevent vitiation of the air and to remove odours and water vapour.
This variation in requirements may range from 6-15 ACH in summer and 2 ACH in winter, and is a good case for installing a larger number of well-distributed smaller (300mm) fans with speed control, rather than one or two large fans (600 to 900mm).
Radiant Heat
Note that radiant heat from hot surfaces, such as cooking vessels, steam presses, pipes etc., does not warm the surrounding air, but only solid bodies in the path of the radiation. The extraction of air therefore will not reduce or remove the radiant heat, but it will create air movement which would give relief by removing convected heat. To reduce radiated heat the hot surfaces should be insulated.
Source:The Vent Axia-Ventilation Hand Book