Combustion air heat recovery

Consider this as a guess, or if you may an educated guess: the hotter inlet temperature caused unexpected vaporization of the oil from the supply line causing a drop of the oil temperature due to the heat of vaporization transferred into the vapor, and at the same time caused the lower oil-content but higher temperature combustion mixture to ignite. The result is the ignition of combustion mixture with lower heat content because of the lower oil content, and of course the temperature reduction of the oil supply line

Again this is merely a matter of taking a stab at rational explanation of the observation.

Good luck

what is the rate of flow of the oil? Was combustion complete (smoke = free carbon)?

Did the bypass impede the draft of air, thus reducing the amount burned?

You may have to adjust the air/fuel ratio.

This is a common search subject.

see if you can find any more hints here,

http://www.google.ca/search?hl=en&q=boiler+waste+heat+recovery&meta=&aq=0&oq=boiler+%2B%22wast

Gentlemen, thank you for your inputs.

What I am doing now is to check the excess air again. It was at around 30% with CO2 at about 12%.

What you also mentioned regarding the vaporization of the oil after coming in contact with the hot combustion air is plausible.

I may have to replace some components inside the burner to accomodate the new increased combustion air temperature. The boiler manufacturer is now looking into this matter.

Presently, fuel oil feed rate is 600 liters/hr . Fuel has GCV of 39MJ/kg.

Any other comments and guidance is appreciated.

A couple of thoughts:

1. If you are preheating the combustion air before the combustion air fan it will still move the same volume but because of the reduced density the mass will be lower. If you have trimmed the oil flow rate to suit you will have down rated the burner and hence the reduced output temperature.
2. The extra resistance of the heat exchange will tend to reduce the potential air supply to the burner producing a similar effect to 1 above.
   
3. If you have not reset the combustion during commissioning then the combination of 1 & 2 could be running rich and producing soot which wastes energy and reduces heat transfer rates. This would reduce output but would probably raise the exhaust temperature.

By the way where are you measuring the exhaust temperatures referred to. Before or after you new heat exchanger?

Hi Gasman

Responses to your queries;

1) We have increased air to the burner during commissioning of the unit. Yes, hotter air contains less kg mass.

2)The flue gas resistance of the air preheater is not significant.

Upon installing it, ( and before commisioning the hot air side) we did not observe any drop in boiler duty. We only noted drop in duty when we started to feed hotter combustion air to the burner.

3) I believe that the air/fuel ratio is adequate. In fact the exhaust gas temperature reduced dramatically by 25 deg C.

Gas exhaust temperature is measured after new heat exchanger.

Any comments or guidance much appreciated.

Hi Gasman

Responses to your queries;

1) We have increased air to the burner during commissioning of the unit. Yes, hotter air contains less kg mass.

2)The flue gas resistance of the air preheater is not significant.

Upon installing it, ( and before commisioning the hot air side) we did not observe any drop in boiler duty. We only noted drop in duty when we started to feed hotter combustion air to the burner.

3) I believe that the air/fuel ratio is adequate. In fact the exhaust gas temperature reduced dramatically by 25 deg C.

Gas exhaust temperature is measured after new heat exchanger.

Any comments or guidance much appreciated.