By Jacques Lagac
Montreal-Trudeau Airport Thermal PlantEngineering
In 2000, Aroports de Montral had to expand their existing terminal building at the Montreal-Trudeau airport to cope with increasing passenger traffic. They had to double the overall surface area of ...
In 2000, Aroports de Montral had to expand their existing terminal building at the Montreal-Trudeau airport to cope with increasing passenger traffic. They had to double the overall surface area of the existing terminal building.
The existing remote heating plant built in 1960 had been located away from the runways and control tower in order to prevent the vapour plume given off by the plant’s exhaust combustion gases from hindering the activities of the air traffic controllers. The plume, usually very dense and opaque, is caused by the condensation of vapour found in exhaust gases when mixed with cold and/or humid outside air.
The capacity of the existing plant was insufficient to meet the needs of the expansion so the airport decided to build a new thermal plant. The new plant was to be integrated into the new building construction so long as the plant had a dependable system to eliminate the plume.
This new thermal plant was completed in 2003. It houses chilled water (CW), low temperature water (LTW) and medium temperature water (MTW) equipment. Four new chillers providing a total cooling capacity of 7,946 kW are connected in series by pairs. This approach was used in order to increase the temperature differential to 10C and reduce the chilled water flow rate and the size of distribution piping.
The heat generated by the first chillers is released to the atmosphere using dedicated cooling towers. The heat from the second chillers is reclaimed (5,004 kW) to generate part of the low temperature water. When heating loads are low or non-existent, the heat is rejected into the fresh air systems, allowing those systems to bring in more fresh air into the building, resulting in a better indoor air quality, better energy efficiency and reduction of the possibility of creating a vapour plume associated with using cooling towers.
Medium temperature water is produced by four 4,414 kW boilers, water tube type, heating the water from 74C to 115.6C.
The plume reduction process is quite complex and required several analyses. The final process is summarized in figure 1. The first step is to run the boiler flue gases (171C) through a direct contact economizer. “Grey water,” is sprinkled in direct contact to the flue gas. The grey water is cooled from 60 to 10C by passing it through a series of heat plate exchangers.
The major part of the heat recovered is supplied to the low temperature water loop through a dedicated heat plate exchanger Ex-1. Heat plate exchanger Ex-2, is supplied with chilled water; the temperature of saturated gas leaving this exchanger will thus be 15.6C.
Technical data for the exchanger requires that the effluent gases temperature averages 5.6C higher than the grey water temperature. Heat given off to the chilled water loop is recovered by the chillers capable of heat recovery. The overall efficiency of the combustion process is around 99% instead of 82% (without exhaust treatment). The heat reclaimed represents 1,251 kW per boiler.
The primary purpose of the grey water circulated in the direct contact economizer is to absorb the latent and sensible heat contained in the hot exhaust gas. In doing so, it also absorbs a large part of pollutants which otherwise would be released in the atmosphere. The direct contact economizers act as air scrubbers and help reduce the amount of greenhouse gas emissions.
The saturated flue gases leaving the direct contact economizer have to be mixed with hot dry air so that the mixture can be exhausted to the atmosphere with minimal chance of forming a visible plume.
The psychrometric chart in figure 2 represents the process lines in the exhaust flue gases. Line A-E is tangential to the saturation line at point A corresponding to the expected minimum outside temperature (-29C), which is considered the theoretical no plume process line. Any moist air process taking place along this line will not be subject to vapour condensation formation at point A.
Point C represents the gas leaving the direct contact economizer after being cooled by heat plate exchangers. Point B represents the temperature of the dilution air system (100% transferred air, dry and heated at 37.8C or 100F).
The mixture of gases that will be released to the atmosphere (humid exhaust gases with dry air) corresponds to point E. The proportion of dilution air (line C-E) to total evacuated gases (line B-E) is very high. The dilution air has to be heated, mixed and then exhausted, resulting in an important reduction of the overall efficiency. After discussions, it was agreed that a certain visible plume could be tolerated, as long as it was not opaque. Line A-D shows an area where condensation can take place only if the outside air is still or may be dispersed if any wind is present.
The primary philosophy of the heating plant concept is to use coherent and simple systems along with standard components to lower the cost of operation and maintenance.
The intent to eliminate the plume from the boilers is achieved, at the same time as obtaining a high energy efficiency for all the cold and hot water production processes, and better indoor air quality.
Client: Aroports de Montral
Thermal plant design: Consortium Bouthillette Parizeau/Pageau Morel/Groupe HBA, Montreal (Jacques Lagac, ing., Pierre Roussel, ing.)1
Contractor: Plomberie Richard Jubinville