December 1, 2005
By Marie-Judith Jean-Louis, ing. & Gilles Desmarais, ing., Dessau-Soprin
Like many historic buildings in Montreal, Collge Jean-de-Brbeuf was operating with an inefficient central heating plant. Much of the equipment could no longer meet the heating demands of the school,...
Like many historic buildings in Montreal, Collge Jean-de-Brbeuf was operating with an inefficient central heating plant. Much of the equipment could no longer meet the heating demands of the school, and the ever-increasing cost of fuel was becoming a concern. In order to find a remedy, the building’s administrator called Dessau-Soprin engineers to retrofit the heating plant.
The college is located minutes from downtown Montreal. It was founded by Jesuit priests in 1928 and the oldest parts of the building date from that time. It is now one of Quebec’s prestigious private schools, giving secondary and collegiate degrees.
A team of experts, led by Gilles Desmarais, ing., co-author of this article, targeted the main source of the problem: the heating plant. After we reconfigured the plant, the energy consumption was reduced to give an annual saving of over $100,000. As well, the innovative design won a 2005 Technology Award from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).
The college used to consume over 32,000,000 cu.ft. of natural gas annually. The main energy consumer was the thermal plant. It consisted of three 45-year-old hot water boilers, a 10-year-old steam boiler and a 65-year-old steam boiler. The hot water boilers were unsuitable for the building and had a two-stage burner causing constant cycling during mild seasons. To solve this problem, two main tactics were used.
First tactic: out with the old, in with the new
The outdated boilers were replaced and new ones were selected keeping in mind their life expectancy and energy efficiency. Fire-tube boilers are recognized as being able to last 40 to 50 years and can be very efficient according to the number of passes inside the boilers. As such, four-pass fire-tube boilers were used for the project. Their efficiency varies between 82% and 84%. This efficiency is obtained by the number of passes and by preheating combustion air. Their high performance is also characterized by modulating burners that have a turndown ratio of 8 to 1, thus minimizing the on/off cycles. The boilers are designed to reduce the emission of nitrogen oxide (NOx) gases to a maximum of 40 parts per million.
Second tactic: Recover, Recover, Recover
One of the relatively simplest ways to increase the energy performance of the building was to recuperate any energy that would otherwise be thrown away. Therefore, a direct-contact heat recovery unit and a flooded steam-to-water heat exchanger were installed.
* Direct-Contact Heat Recovery Unit. The direct contact heat recovery unit considerably increases the plant’s performance by recovering heat from combustion gases coming from the boilers. Instead of exiting the building, the flue gases flow through the lower portion of a large cylindrical reservoir and rise to the top. A direct contact heat recovery unit is an open heat exchanger in the form of a large vertical cylinder with a reservoir. The top portion contains a heat transfer media essentially composed of stainless steel modules. As cold water is sprayed to the top of the equipment onto the heat transfer modules, the hot combustion gases are injected at the bottom of the cylinder. When the flue gases come into contact with the cold water mist, a small amount of energy is transferred through the process of evaporation, but the major part of the energy is transferred directly from the flue gases to the water. Through this clever process, 3,500 gallons of water are heated at no extra energy cost. The process is used to preheat the low temperature hot water heating circuit, the sanitary water, and the coil heating the fresh air for the boiler room and the locker room. The complete installation — boilers and heat recovery unit — operates with an average energy efficiency of 92%.
The efficiency increase expected by this process is mainly related to the return hot water process. If the temperature of the return hot water is around that of the condensing temperature of the flue gases (140F for the natural gas), the heat recovery on the flue gases will be limited to the sensible portion of energy contained in combustion gases (about 50% of energy losses). But if the energy transferred to the building’s heating load allows the decrease of the temperature of the return hot water process to approximately 70 to 80F, the major part of energy losses (sensible and latent heat) in flue gases will be recovered (90% of energy losses). Usually, the flue gases can be cooled to a temperature that is only 10F higher than the temperature of the return hot water process. By knowing the temperature of the outlet flue gases at the top of the cylinder, it is possible to obtain a good indication of the efficiency of this process. The goal is to transfer most of the energy losses from flue gases to the heating load of the building. In this case, the most interesting load transfers in the heat recovery process involve the sanitary water, laundry water, low temperature glycol used to preheat ventilation systems and low temperature hot water used for heating.
* Flooded Steam-to-Water Vertical Heat Exchanger. The flooded steam-to-water vertical heat exchanger was installed as a low capacity boiler to operate efficiently during off-peak hours. It uses a new efficient technology that is different to a conventional shell-and-tube heat exchanger. By modulating condensate, the new heat exchanger can modify its exchange surface, therefore optimizing the net energy output produced from the steam. This optimization is due to the decrease in the temperature of the condensate and of the production of flash steam. The exchanger uses only 85% of the energy input required by a conventional heat exchanger for the same net energy output.
Reducing the peak energy input can be a major concern in existing buildings where the heating needs have significantly increased but the system still relies on the original main steam and condensate pipes from the heating plant.
The renovated plant saves 7,000,000 cu.ft. of natural gas every year, representing over $85,000 of annual savings. It is now one of the most economical composite plants in Canada.
The existing controls were mainly pneumatic and electric in the boiler rooms. The modifications enabled the experts to integrate new DDC controls to increase the level of precision and monitor the efficiency of the energy recovered by the new system.
The renovations also helped reduce the maintenance costs for the heating plant by $15,000. Only routine inspections are required now. Since the combustion of natural gas is cleaner, transfer-surface fouling is no longer a problem. And the use of natural gas and condensing systems simplifies the maintenance requirements.
The reduction in energy use also saves 735,000 pounds of greenhouse gas emissions every year, helping Canada meet its Kyoto Protocol commitments.
Owner/client: Collge Jean-de-Brbeuf
Mechanical and electrical engineers: Dessau-Soprin (Gilles Desmarais, ing., David Damboise, ing, Claude Malette, techn.)
Other key players: Sofame Technology (direct contact heat recovery unit), Cleaver-Brooks (boilers), Maxi-Therm (flooded steam-to-water vertical heat exchanger), Regulvar (controls contractor)
Chapel/library restoration: Beaupre et Michaud, Dupuis Letourneux (architect); Saia Deslaurier Kadanoff Leconte Brisebois Blais (structural engineer); Cloutier, Powney (structural glass wall engineer)