Bernard Doyle looks at how direct gas heating of drying air has become economical and a means of reducing greenhouse gasses.
Except for certain specialised processes, the drying of foodstuffs involves either blowing hot air through a wet material, as in the case of drum, fluid bed or belt dryers, or alternatively passing the wet material in droplet form through hot air as in spray drying.
The energy cost in terms of heat represents a significant percentage of the total drying costs and the majority of manufacturers in the food industry are spending money unnecessarily on their heating costs by using inefficient methods such as secondary or indirect methods of heating.
There are two main categories of heater used in the food industry, indirect and direct.
Indirect heating involves the use of a secondary system to transfer heat from the fuel into the drying air, this secondary system being either a heat exchanger in the drying air stream or a specially designed heater similar in construction to a steam boiler but heating air rather than water.
Although all forms of indirect heater can claim up to 70percent utilisation of the heat content of the fuel, additional heat losses in the distribution systems reduce the overall efficiencies closer to 50percent or even less.
The type of heater based on boiler design gives the highest thermal efficiency, about 60percent, but maintenance costs tend to be high.
In general, indirect heaters discharge a minimum of 40percent of the available heat to atmosphere together with large amounts of carbon dioxide.
For example, an average size of spray dryer evaporating 1000kgs of water per hour would exhaust about 250kgs per hour or 40tons per week of CO2 into the atmosphere.
With increasing costs of oil and the levelling out of gas prices, direct gas heating of drying air has become, not only an economical advantage but a means of reducing greenhouse gasses.
Direct heating of drying air is, as the name implies, a system whereby the fuel is combusted directly into the air to raise the temperature.
Whilst burning oil to dry foods is obviously unacceptable, natural gas is highly efficient, clean, has no detrimental affect on food materials, and the heat wastage is insignificant it is surprising that this form of heating has not been accepted as standard considering that most people use direct gas heating in their homes for cooking.
Apart from the fact that almost 100percent of the heat content of the fuel is transferred into the drying air and discharge of CO2 is halved, there is an additional advantage arising from the speed at which drying temperatures can be reduced in emergencies thus limiting any thermal damage or charring of delicate powders.
In the USA, direct natural gas has been the most common method used for heating air for dairy and food dryers for several decades and heater designs have been developed which are safe, reliable and relatively maintenance free.
In the UK and Europe there is no national or EEC legislation restricting the use of gas burners for drying air, it has been an acceptable form of heating for over a century, but there is still a reluctance to use direct gas.
Concern over the its use arose in the 1970s when hops from a newly installed dryer using direct gas heating were found to contain unacceptably high levels of oxides of nitrogen (Nox) but after replacing the gas heater with an indirect steam heater and measuring exactly the same Nox level it became apparent that the nitrogen oxides arose from nitrate based fertilisers used by the farmer, not from the gas.
A logical consideration of the drying process would confirm that it is not possible for ny impurities in the air to penetrate into the powder because, when removing moisture, vapour migrates outwards from the powder core and prevents any flow of impurities back into the powder.
In 1998, samples of powdered egg white dried by using both direct and indirect heating were analysed by the University of Manchester Science and Technology (UMIST) and the Nox levels in all samples were found to be between 1.5 and 3 parts per million, which the Food Standards Agency advised could be considered as being zero.
Bernard Doyle is Technical Director, Spray Processes Ltd, Bedford, UK. " target="_blank">www.spraypro.com"