Choosing the right heater

The right choice of heater to be used in conjunction with drying equipment is important. Clearly any direct fired type of heater will contribute moisture to the atmosphere locally and this will be counter productive to the drying process.

Gas or liquid fuel fired heaters which exhaust their products of combustion via a flue to the outside will provide dry heat to the area as will any form of electric heating.

A calculation of the amount of heat needed to maintain the temperature to promote drying of the fabric of the building will need to be made. If negative pressure is used to contain any spores or other contamination then this will need to be taken into account.

Calculation of heating load

For the correct temperature to be maintained within the area to be dried an assessment of the heating load needs to be made. This will be made up with 3 components:-

• Sensible heat to fresh air introduced to the area.
• Latent heat of evaporation to dry out the fabric.
• Fabric loss.

It may not be necessary to go into too much detail on the calculation of fabric loss but a basic understanding of the impact on heating load, particularly where very high air change rates exist, is relevant to ensuring effective drying in the shortest time.

Sensible heat

An estimate of the volume of air being introduced into the area each hour, along with the required temperature rise will allow a sensible heating load to be assessed. For these purposes we can simplify the calculation as follows:-

KW of heating = 1.005KJ/KG.deg K x 1.2KG/cubic mt for air x volume in cubic mt of air x change in temp all divided by 3600.

Or

Sensible heating KW = 0.000335 x air volume (m³) x temp rise (°K)

So, for example if a room of 40 cubic mt has an air change rate of 120 cu mt per hour (3 air changes per hour) and the outside temperature is 5 deg C and we wish to maintain 20 deg C within the room then the sensible heat needed will be :-

0.000335 x 120m³ x 15 °C = 0.6KW (this is the Power needed to heat the incoming air)

Latent heat

If we are expecting to use the dehumidifier to collect 30 litres/day of water then this requires 1.25 litres of water to be evaporated from the internal surfaces of the room and latent heat is required for this.

Each KG of water evaporated requires 2465KJ of heat so the formula is :-

KW of heating = 2465KJ/KG x 1.25KG divided by 3600 = 0.85KW (this is the heat needed to evaporate the moisture from the wet surfaces)

Conclusion

It is clear that a around twice the expected heating needed to heat a dry room is needed to heat a room which needs to be dried out. The additional heat goes into the drying process and the drying time will be reduced significantly by heating.