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What are the ideal stack temperatures I should try to achieve with a direct fire burner? I'm currently in the process of commissioning a 5BBL brewhouse with a 400K Midco modulating burner. The HVAC professional has dialed in the burner to run at 23% power in order to achieve a burner efficiency of 65% with stack temperatures exiting the kettle exhaust at around 850F.

I have no idea what temperatures I should be targeting in the stack. Is 850F too high? We were able to dial the burner back down to 10% power with stack temperatures in the 450F range, but the temperature rise in the kettle was extremely slow.

Any feedback would be greatly appreciated.

The flue gas temperatures at the boiler outlet are roughly 60 degrees above the temperature of the product inside the steam boiler. 

At an operating pressure of 1 bar (assuming you are running an atmospheric low-pressure 15 psi boiler), which corresponds to a saturated steam temperature of 249 degrees F, the flue gas temperature is roughly 309 degrees F. Combustion efficiency affects Flue Gas temperatures, but Themo efficiently decreases or increases it also. You have a heat transfer issue. Open the boiler a confirm the water side isn't scaled up. If you want to know more about your Boiler Operation, I will send you a site to download a free book, 10 Huge Mistakes Facilities Make with Boiler Operation.

Cheers

What are the ideal stack temperatures I should try to achieve with a direct fire burner? I'm currently in the process of commissioning a 5BBL brewhouse with a 400K Midco modulating burner. The HVAC professional has dialed in the burner to run at 23% power in order to achieve a burner efficiency of 65% with stack temperatures exiting the kettle exhaust at around 850F.

I have no idea what temperatures I should be targeting in the stack. Is 850F too high? We were able to dial the burner back down to 10% power with stack temperatures in the 450F range, but the temperature rise in the kettle was extremely slow.

Any feedback would be greatly appreciated.

What are the ideal stack temperatures I should try to achieve with a direct fire burner? I'm currently in the process of commissioning a 5BBL brewhouse with a 400K Midco modulating burner. The HVAC professional has dialed in the burner to run at 23% power in order to achieve a burner efficiency of 65% with stack temperatures exiting the kettle exhaust at around 850F.

I have no idea what temperatures I should be targeting in the stack. Is 850F too high? We were able to dial the burner back down to 10% power with stack temperatures in the 450F range, but the temperature rise in the kettle was extremely slow.

Any feedback would be greatly appreciated.

What are the ideal stack temperatures I should try to achieve with a direct fire burner? I'm currently in the process of commissioning a 5BBL brewhouse with a 400K Midco modulating burner. The HVAC professional has dialed in the burner to run at 23% power in order to achieve a burner efficiency of 65% with stack temperatures exiting the kettle exhaust at around 850F.

I have no idea what temperatures I should be targeting in the stack. Is 850F too high? We were able to dial the burner back down to 10% power with stack temperatures in the 450F range, but the temperature rise in the kettle was extremely slow.

Any feedback would be greatly appreciated.

Hi Kyle

You're going to hate this answer, but "it depends".

I assume you have a stainless kettle. 

Brew Kettles are not built for max efficiency like a steam boiler. Stainless steel has awful heat transfer plus has a tendency to foam and burn on as it doesn't "wet" like copper. Direct fired brew kettles historically were made of copper (or normal steel) as heat transfer thru those metals is not too bad. Sometimes, the exhaust gas traveled thru a Hot Water Tank before exiting as it was still very hot. 

Efficiency depends on many factors - most beyond your control. For example: 

• Heating surface area. The more surface the better. Some kettles have heating zones that extends up the side wall to allow more contact time between hot gas and wort. Some even include multiple passes like a Scandinavian tile stove. This is not a new problem! Also, you may have seen old drawings of direct fired kettles where the bottom was inverted to increase surface area, dwell time, as well as force the boiling wort column up the center of the kettle. 
• The more turbulence the better as the combustion gas travels thru the chamber. Having a mixer on the wort side helps but it doesn't solve mixing on the combustion side. That's a question of kettle design. 
• The thickness of the stainless bottom. Thinner stainless transfers heat better but might warp under the stress of high heat. 304 stainless steel doesn't like high temps.
• You don't want to push too much heat thru the burner chamber as stainless burns on so easily plus the gas needs time to give up its heat before exiting. That takes time.

The good news is this is a solvable problem, but you need to rethink your question. 

You're going to have to accept a high exhaust temp. You have to balance opposing needs - apply enough BTUs to boil as you wish but no more! I cannot give you an exhaust temp value as it depends on your kettle design. My gut feeling is 450 - 850 deg F is "in the ballpark" but it's been 35 years since I ran a direct fired stainless kettle.

Run some tests. Fill your kettle with water. Let's assume you want to heat 1 deg/min then evaporate 6% per hour. Adjust your burner to achieve that. Then check your exhaust temp. Can you live with that? It will be high. Ask your contactor if such an exhaust temp meets code versus your stack design? If too high, you'll have to accept a lower evaporation rate and extend your boil. Not the end of the world. In the old days, 2 hours plus was not uncommon.  Adjust burner up and down until you reach a point of compromise - efficiency (i.e. stack temp) versus evaporation rate.

Your HVAC/Boiler partner can calculate efficiency (it will be very low! 25%???). They should adjust your "fuel to air" mixture to optimize efficiency.

Add a bimetallic thermometer in your vent stack so you can keep an eye on it.

I hope this "off the cuff" ramble helps a bit.

Good Luck

Dan Carey

New Glarus Brewing Company

What are the ideal stack temperatures I should try to achieve with a direct fire burner? I'm currently in the process of commissioning a 5BBL brewhouse with a 400K Midco modulating burner. The HVAC professional has dialed in the burner to run at 23% power in order to achieve a burner efficiency of 65% with stack temperatures exiting the kettle exhaust at around 850F.

I have no idea what temperatures I should be targeting in the stack. Is 850F too high? We were able to dial the burner back down to 10% power with stack temperatures in the 450F range, but the temperature rise in the kettle was extremely slow.

Any feedback would be greatly appreciated.

Hi Kyle. I've run a direct fire kettle for over 25 years at our Max Lager's brewery. 850 degrees is fairly common with a direct fire kettle. I'm assuming you have a double wall insulated vent pipe in place. That's required by code where I am. You might want to install a barometric damper in the stack if haven't already. That will draw cooler air in and reduce the temperature of your stack. You should be able to operate at the higher efficiency and still keep the temperature down. 

Hi Kyle

You're going to hate this answer, but "it depends".

I assume you have a stainless kettle. 

Brew Kettles are not built for max efficiency like a steam boiler. Stainless steel has awful heat transfer plus has a tendency to foam and burn on as it doesn't "wet" like copper. Direct fired brew kettles historically were made of copper (or normal steel) as heat transfer thru those metals is not too bad. Sometimes, the exhaust gas traveled thru a Hot Water Tank before exiting as it was still very hot. 

Efficiency depends on many factors - most beyond your control. For example: 

• Heating surface area. The more surface the better. Some kettles have heating zones that extends up the side wall to allow more contact time between hot gas and wort. Some even include multiple passes like a Scandinavian tile stove. This is not a new problem! Also, you may have seen old drawings of direct fired kettles where the bottom was inverted to increase surface area, dwell time, as well as force the boiling wort column up the center of the kettle. 
• The more turbulence the better as the combustion gas travels thru the chamber. Having a mixer on the wort side helps but it doesn't solve mixing on the combustion side. That's a question of kettle design. 
• The thickness of the stainless bottom. Thinner stainless transfers heat better but might warp under the stress of high heat. 304 stainless steel doesn't like high temps.
• You don't want to push too much heat thru the burner chamber as stainless burns on so easily plus the gas needs time to give up its heat before exiting. That takes time.

The good news is this is a solvable problem, but you need to rethink your question. 

You're going to have to accept a high exhaust temp. You have to balance opposing needs - apply enough BTUs to boil as you wish but no more! I cannot give you an exhaust temp value as it depends on your kettle design. My gut feeling is 450 - 850 deg F is "in the ballpark" but it's been 35 years since I ran a direct fired stainless kettle.

Run some tests. Fill your kettle with water. Let's assume you want to heat 1 deg/min then evaporate 6% per hour. Adjust your burner to achieve that. Then check your exhaust temp. Can you live with that? It will be high. Ask your contactor if such an exhaust temp meets code versus your stack design? If too high, you'll have to accept a lower evaporation rate and extend your boil. Not the end of the world. In the old days, 2 hours plus was not uncommon.  Adjust burner up and down until you reach a point of compromise - efficiency (i.e. stack temp) versus evaporation rate.

Your HVAC/Boiler partner can calculate efficiency (it will be very low! 25%???). They should adjust your "fuel to air" mixture to optimize efficiency.

Add a bimetallic thermometer in your vent stack so you can keep an eye on it.

I hope this "off the cuff" ramble helps a bit.

Good Luck

Dan Carey

New Glarus Brewing Company

What are the ideal stack temperatures I should try to achieve with a direct fire burner? I'm currently in the process of commissioning a 5BBL brewhouse with a 400K Midco modulating burner. The HVAC professional has dialed in the burner to run at 23% power in order to achieve a burner efficiency of 65% with stack temperatures exiting the kettle exhaust at around 850F.

I have no idea what temperatures I should be targeting in the stack. Is 850F too high? We were able to dial the burner back down to 10% power with stack temperatures in the 450F range, but the temperature rise in the kettle was extremely slow.

Any feedback would be greatly appreciated.