Our biscuit baking industry is a major consumer of gas throughout all international markets. Ovens have a large carbon footprint and emit greenhouse gases. Now in many countries the baking industry faces high cost increases for gas. Ultimately electricity generated from renewables will become more widely used for new ovens. However for existing lines the maximum efficiency and reduction of gas consumption is a priority. Typical profiles for energy usage for gas fired ovens are shown from two major oven suppliers.
The baking of the products accounts for 38.2 – 40% of the heat input. Heat loss through the extraction flues accounts for an average of 38%. Heat loss through the insulation and the oven conveyor accounts for an average of 22%.
For Direct Gas Fired ovens and Direct Convection ovens, all the combustion air and products of combustion enter the baking chamber and are extracted with the moisture from the products. This accounts for the considerable heat loss though the flues, 35% - 41.9% in the examples above.
Image 1: Direct Convection baking system
For Indirect Radiant (or cyclotherm) ovens, the combustion air and products of combustion are continuously re-cycled in the radiant heating system. The extraction flues only extract sufficient air from the baking chamber to reduce the moisture content of the products. The Indirect Fired ovens have less heat loss from the baking chamber through flues. This heat loss is reduced to a minimum.
Image 2: Indirect Radiant oven
The oven burners draw in air for combustion. In the indirect ovens, a burner flue with natural convection allows the ingress of combustion air to escape and maintain a constant pressure in the system. This hot air is dry and can be used in a heat recovery system.
Burner flues 5%
Oven band and fans 9%
Heat transfer for baking the product represents 52% of total heat input representing an important energy saving compared to direct fired ovens.
The Heat Recovery System (HRS) uses the heat from the burner flues which exhaust the air drawn into the burners for combustion. The hot air and burnt gas in the burner flues of an Indirect Radiant oven are at a high temperature, typically over 200oC and this hot air can be recovered and used for baking in a Heat Recovery System. This may be used to heat one or two final zones of the oven. These zones would not require burners, giving a saving in capital and running costs.
A proportion of the hot gases in the burner flues are diverted to an HRS collection pipe which runs along the top of the oven. The hot flue gases are drawn along the collection pipe by a fan and blown into radiant or convection ducts in the Heat Recovery Zone.
Image 4: Heat Recovery Zone. Top right of the image shows the collection pipe, fan casing and delivery of hot gases to the zone
Mineral wool slabs are used, 50, 100 and 150mm thick and with 70kg/m3 density up to over 140kg/m3 density for particularly hot areas. The insulation should be 200mm thick at the sides of the baking chamber, 250mm thick at the top and 150mm on the bottom. Thermal conductivity of mineral wool at baking temperatures is typically 0.03 – 0.05 W/mK.
High temperature areas around the burner tube may be insulated by high temperature thermal ceramics, for example Morgan Superwool Plus, density 128kg/m3 which has excellent performance up to 1000oC.
Image 6: Thermal conductivity for mineral wool from https://www.engineeringtoolbox.com
Heat is lost by the return of the oven band and this can be covered by insulated panels. These may be easily lifted off for access to band support rollers.
Image 7: Oven return band covers
The length of the oven ends should be as short as possible within the requirements of the products being baked. The movement of the oven band and products through the oven draws a large amount of heated air along the oven. This is exhausted at the delivery end of the oven. A hood over the oven delivery end with a fan and flue will avoid this air escaping into the bakery.
A large energy loss is in the products of combustion and air extracted from the oven in direct fired ovens. The combustion efficiency is dependent on using the optimum amount of excess air for combustion to consume all the gas and minimize the excess air lost to the flues.
To ensure complete combustion of the fuel, excess air is drawn in by the burners. For most applications, every 1 m3 of oxygen, approximately 15 m3 of air is required to provide complete combustion of natural gas. The combustion efficiency will increase with increased excess air, until the heat loss in the excess air is larger than the heat provided by more efficient combustion.
When fuel and oxygen in the air are in perfectly balance - the combustion is said to be stoichiometric.
Heat recovery systems are available to recover heat from the oven flues and partly transfer the heat to air or to water. This reduces the heat exhausted to the atmosphere from the flues and provides heat available for heating systems within the bakery requiring heated air or water. There are heat recovery units for installation in the flues or large units installed outside the bakery.
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Leading image by Yevhen Prozhyrko/shutterstock.com
Baker Pacific. www.bakerpacific.net
Baker Perkins. www.bakerperkins.com
GEA Imaforni. www.gea.com
Engineering ToolBox. www.engineeringtoolbox.com
Turnbull Scott. www.turnbull-scott.co.uk/heat-exchangers
Mineral wool insluation
Morgan Thermal Ceramics. www.morganthermalceramics.com