Oven Heat Recovery System (HRS)

Heat recovery: two zones without burners

The Heat Recovery System (HRS) uses the waste heat from the burner flues of an indirect fired oven. 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 and achieving high efficiency.

All gas burners draw in a large amount of air for combustion. 1.0m3 of gas requires 3.0m3 of oxygen, (approximately 15m3 of air) for complete combustion. Excess combustion air improves efficiency. This air is exhausted through the extraction system of a direct gas fired oven and through the natural draught burner flue of an indirect-fired oven.

>Oven Efficiency - Comparison of Direct and Indirect Fired Ovens

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.

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. Hot flue gases are collected from each zone with a burner. 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.

Baker Pacific Indirect Radiant Oven with Heat Recovery System

Burner flue connected to the collection pipe

The burner flue (on the right) is connected to the HRS collection pipe and the flow of hot gases is controlled by dampers. One damper controls the quantity of flue gas to the exhaust flue and one damper controls the quantity of flue gas to the connecting pipe for the Heat Recovery System. These dampers are set by the commissioning engineer to allow sufficient quantity of heat for the Heat Recovery Zone.

The HRS zone is constructed with ducts above and below the oven band. The hot gases recovered from the burner flues are fed by the fan to the ducts. The fan is located on top of the oven at the end of the collection pipe.

Baker Pacific Heat Recovery Zone

The final zone of the oven has radiant or convection ducts above and below the oven band. These ducts are divided along their length into 3 sections (control side, centre and non-control side). The flow of hot flue gas into each section is controlled by a damper. These dampers may be adjusted to ensure the optimum heat balance top to bottom and across the width of the oven.

>Calculation of oven zone lengths

Heat Recovery Zone with radiant ducts

The Heat Recovery Zone may also be designed as a convection zone. This is particularly beneficial for products requiring an even bland colour and even low moisture content.

Heat recovery zone with convection system

Calculations Of Hot Air Flow To The Hrs Zone

Calculations are based on the following oven specification: 

• 1.2m x 100m Indirect Radiant oven 
• 8 zones / 7 burners x 350 kW / 1 HRS zone 
• Product: Rotary moulded 
• Output: 3200kg/hour 
• Energy used: 0.4043 kWh/kg of biscuits 

Total energy used per hour: 0.4043 kWh x 3200kg = 1294 kWh 
Natural gas energy (average): 13.5 kWh/m3
Gas consumption per hour = 1294 / 13.5 m3  = 95.8 

Air volume required per hour for complete combustion (approx.) 1438 m3  / hour. 
Total volume of products of combustion exhausted = 1533 m3 /hour 
This is the total volume of hot flue gas/air available for HRS.

If we assume that the temperature of the hot air delivered to the final zone is 180°C (150°C above ambient), the energy available in the HRS zone can be calculated as follows:

Energy (kWh) = air volume (m3 /h) x density of air at 180oC (0.75kg/m3) x specific heat (1.02 kJ/kgoC = 0.00028 kWh/kg°C) x temperature above ambient (°C) 
Energy = 1533 x 0.75 x 0.00028 x 150 = 48.28 kWh 

>Energy for Biscuit Baking

REFERENCES 
Baker Pacific Ltd. www.bakerpacific.net 2021 
The Engineering ToolBox. Air Heating Systems. 
www.engineeringtoolbox.com 2021 
Esspee Engineers, Kolkata, India. www.espenger.com 2021 
S. Eldridge Design Ltd. www.seldridgedesign.co.uk 2021