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Asked on 23.01.2020.

Fouling on direct oven flue heat exchanger


I was wondering if anyone can me some insight into fouling issues on a potential combustion air preheater, for use on a direct oven. I am aware that combustion air preheaters have been installed successfully on indirect oven lines, but I am unsure if it is feasible on a direct oven. The flue stream will contain small solid particles and possibly fat alongside the obvious combustion products and water vapour.

Has anyone here successfully installed a heat exchanger on a direct oven flue line? Or heard of such a project?

I have created a model with expected savings which make the idea tempting, as long as the heat exchanger doesn’t cake up with biscuits residue!

Any and all responses appreciated.

Answered on 24.01.2020.

Fouling of the heat exchanger with oils and particulates  will reduce the heat transfer efficiency of the heat exchanger. One option to consider is to filter the oven exhaust before  it enters the heat exhanger. Otherwise, be prepared to clean the exchanger regularly. You might want to discuss this with a local manufacturer of heat exchangers

Answered on 24.01.2020.

Thanks Joe, 

I will have a look into filtration prior to the proposed HEX. 

Answered on 11.02.2020.

Dear Mr Duncan,

In direct oven there is no convection heat.  So your idea is to increase the convection heat?

if circulation of preheated done along with direct heat the total heat efficiency will get affected.

more over because of external force of the combustion air there will be increase in oxygen supply more than what is required for burning the direct fire and efficiency will decrease

and need to study on total fuel cost as in baking industry  fuel cost is always higher than man and electricity required for production.

please check these aspects

Answered on 11.02.2020.

Hello Duncan, 

not sure to understand your question but

 if you preheat fresh air with air pipe in contact of the chimney below insulation or inside the chimney (serpentine around), it could be work and below 100°C no specific issue. You save energy from delta of air temperature but no specific risk to have small particle in air. chimney have to be clean in consequence of air temperature inside and risk of condensation 

If you want to install heat exchanger in upstream of chimney is not easy for direct oven because it could be very difficut to maintain constant extraction with the fouling of heat exchanger (with or without filter) and some perturbation of baking can appears.  Nevertheless, if you want to try :

    1-avoid to maximize the heat exchange and take a margin to avoid dew point in heat exchanger and in chimney 

     2-with or without filter, it is really difficult to manage constant pressure drop and important to add control of delta P with variable fan speed to compensate the fouling impact 

     3- Design Heat exchanger & cleaning procedure to maintain the system easier

hope could help



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