Stephen Smith offers tips on what types of mobile heaters to use for your site in cold snaps.
05 March 2018 | Stephen Smith
Heating air increases its capacity to hold water and therefore reduces its relative humidity.
Air can be heated in two ways:
1. Directly (the air comes into contact with the heating element) or
2. Indirectly (the air bypasses the heating element through a heat-exchanger).
Direct heating gives a heating efficiency of almost 100 per cent as most of the energy from the heating element is absorbed into the air stream (only a small amount of heat is absorbed within the body of the heater).
The most clean and dry form of direct heating is electric, as nothing is absorbed by the air passed over the heating element apart from thermal energy.
The use of heating fuels is also an effective form of direct heating. The most economical fuel is gas oil (diesel) or paraffin. Burning these fuels produces moisture, combustion gases and smell, along with other hazards such as soot and combustion particles. This can cause health complications if used in an enclosed area without adequate ventilation. In addition, soot can settle on surfaces. Operatives can also suffer breathing difficulties unless the correct precautions are taken and, ultimately, carcinogenic emissions can have a serious affect on health.
An alternative heating fuel is LPG, which can be cleaner and easier to handle but its combustion produces moisture and gases that carry a smell, causing similar problems as burning gas oil or paraffin if the correct safety procedures are not followed.
Indirect heating has traditionally produced efficiencies as low as 70 per cent but developments in heater design mean that we are now achieving results in excess of 90 per cent. This represents an enormous cost saving, considering the reduction in heating energy exhausted through the flue along with the residues of combustion, moisture, gases, and soot.
Indirect heaters are usually powered by gas oil, paraffin, or LPG and circulate clean, dry air through the heat exchanger.
They can be used safely in enclosed areas as long as the flue is correctly ducted away from any enclosed areas. In many installations of this nature the flue itself will radiate an amount of thermal energy into the enclosed areas, thus increasing the overall efficiency of the system. Alternatively, indirect heaters can be placed outside the enclosed area and the warm air ducted in. This is especially useful where a supply of fresh air is required or where an area (such as a marquee) is to be kept under slight positive internal pressure.
Drying or draughty areas
Mobile heaters are usually fan-assisted for effective air movement. Where it is important to minimise airflow or noise, radiant heaters can be used which rely on natural convection currents to circulate the warmed air.
They should not be used where flammable fumes or dust can be drawn into the air stream. In contaminated and flammable areas, indirect-fired heaters sited externally may be suitable if used correctly.
Electric quartz radiant heaters conduct thermal energy by light, heating whatever material the light falls on. As these materials warm up, they will themselves radiate some thermal energy, which is absorbed by the air, although the air is not heated directly by the heater itself. This form of heating is especially effective in draughty or open areas.
Where heat is being applied for purposes of drying - building materials, crops, water-damaged buildings - it would be most effective to use electric heat, although this is not often practical owing to the amount of power required to achieve the heat output. Taking into consideration that a 3KW electric heater on an 110V supply draws the best part of 32A, it's not surprising that alternative forms of heating are looked for on site.
In most drying situations the optimal solution is the use of indirect-fired heaters but if the fumes and soot aren't going to be an issue, direct-fired heaters can still be effective because the water-holding capacity of warming air exceeds the volume of water produced by the combustion process.
Stephen Smith is managing director of Airflo Envirorental, rental solutions for site ventilation and air quality management