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Optimising chimney flue designs

Steen Hagensen, President of Enervex, spoke on optimising the chimney flue design for boilers and water heaters at an ASHRAE technical seminar on October 25 at Arjaan Rotana, Dubai Media City. The event was organised and conducted by ASHRAE Falcon Chapter. As part of our coverage of the seminar, we bring you an overview of the key issues covered.

| | Nov 30, 2011 | 4:43 pm
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Steen Hagensen, President of Enervex, spoke on optimising the chimney flue design for boilers and water heaters at an ASHRAE technical seminar on October 25 at Arjaan Rotana, Dubai Media City. The event was organised and conducted by ASHRAE Falcon Chapter. As part of our coverage of the seminar, we bring you an overview of the key issues covered.

Steen Hagensen arrived at the presentation wearing two hats – as President of Enervex and as Owner of Exhausto, a company that specialises in the design of demand-controlled mechanical draught systems for heating applications and demand-controlled ventilation systems.

Hagensen began his presentation by saying that optimising chimney flue design led to space saving, which, in turn, led to reduction of footprint. It also saved energy and reduced emissions. Further, he said, it improved boiler room safety through optimal chimney flue design and installation.

According to the International Fuel Gas Code (venting of equipment), the requirements include:

  1. Minimum Safe Performance
    A venting system shall be designed and constructed so as to develop positive flow adequate to remove flue or vent gases to the outside atmosphere
  2. Equipment Draught Requirement
    A venting system shall satisfy the draught requirement of the equipment in accordance with the manufacturer’s instructions

Hagensen went on to say that it was possible to make adjustments to boilers. “Chimney design is based on theoretical models where you can put information and get simulated results,” he said. “You have no guarantee that the results will work in actual situations, unless you have adequate experience.”

In the past, most boiler systems were designed in a way that each boiler had a chimney. “Today, it is a case of multiple boilers with one chimney,” Hagensen said, adding that it was an advancement of technology. But, he said, engineers had to be careful while designing, because they needed the right type of boilers. Engineers also had to pay attention to what the draught was going to be, because there were draught variations during operation, he added.

Hagensen said there were four equipment categories to consider while deciding on the chimney flue:

Appliance categories:

  1. Atmospheric
  2. Fan-assisted
  3. Forced draught – non condensing
  4. Forced draught – condensing

Hagensen made the point that understanding the categories was important, because if one were to use a condensing boiler, one had to use a SS316 inner and SS304 outer chimney flue. He then said modern system design includes the following elements:

  1. Design software
  2. Vent type application
  3. Operating temperature
  4. Resistance factors
  5. Ambient temperature variations

The above five would lead to demand-controlled chimney flue systems, he said.

The President of Enervex went on to add that there were a lot of tall buildings in the region. “When you get into chimneys, 30 metres tall, you will have an overdraught situation,” he said, “and the boilers could explode. So you will need to tweak your system.

“If you have too much draught, the boiler will start to close a bit and vice versa. So you have to modulate.

Speaking, then, on the benefits of demand-controlled chimney flue systems, he added: “They are ideal for hotels, resorts, apartment buildings and hospitals. They come into play in the case of steam boilers and calorifiers/water heaters.”

Having made his point, Hagensen then examined packaged chimney flue systems. This included chimney flue with safety controls, chimney flue with over-draught controls and lastly, chimney flue with over-draught controls as well as modulating draught control.

An inherent benefit of packaged chimney flue systems is space saving – one needs just one flue for multiple boilers, he said. And there are real examples of such installations in Abu Dhabi and Dubai, he added.

Hagensen said these scored high on aesthetics: “Nobody wants to see a chimney on the roof,” he said. “It needs to be at a height above the roof parapet. If not, gas gets blown back into the building.

“But then, why on the roof, why not on the side? This is possible. So you do have design freedom. If you take a trip to Burj Khalifa, the chimney flue terminates underground, which is unusual, because normally, a chimney flue goes up.

“In Burj Khalifa, you have six demand-controlled chimney flue systems, with horizontal venting feature for sidewall termination, which has provided the engineers full flexibility in design.”

Why do we need them?

  • Well, correct vent design is your insurance
  • Plus, they will save space – they will reduce the number of chimneys or vents or they will reduce the diameter.
  • They will also avoid tight boiler-room space
  • Further, they will save material
  • They also score high, aesthetics-wise
  • They also score high on safety and redundancy

 

 


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