PLUME FORMATION MECHANISM
Figure 1 describes the condition of the exhaust stream during mixing with ambient air. Point 1 represents the condition of the moist stream at the discharge location. Point 2 represents the condition of ambient air at a typical winter condition. During mixing with the cold air the stream condition moves from point 1 to point 2.

When line 1-2 crosses the H2O saturation line condensation does occur and plume is formed. Such conditions prevail mainly during winter when plume formation is most severe.

During summer months when the air temperature is much higher (point 3) the mixing line 1-3 does not cross usually the saturation line and plume formation is minimal or even does not occur at all.

PLUME ABATEMENT SYSTEMS
The most effective method of minimizing plume formation consists in controlling the condition of the exhaust stream at the location of discharge into atmosphere.
When this point is situated on the saturation line (point 1 in Figure 1) plume formation in the winter is unavoidable. An effective plume abatement system must displace point 1 away from the saturation line inside the superheated area to such an extent that line 1-2 does not cross the saturation line. This displacement moves point 1 to a lower relative humidity and therefore has a drying effect on the exhaust stream before discharging it into atmosphere. This drying process can be achieved through a number of technical solutions.

Figure 1

One solution consists in re-heating the exhaust stream and displacing the steam condition from point 1 to point 1’. This process is shown in Figure 2. A relative high degree of superheating is required as made evident by the geometrical shape of the curves. Re-heating can be achieved by fuel combustion in the exhaust stream, by steam coils or by a recuperative exchanger recovering heat from a hotter stream. For many applications it is attractive to depress the gas dew point by condensing part of the moisture using atmospheric air as a cooling medium. This step is shown as line 1-1’ in Figure 3. The condensate is removed and the exhaust gas is further mixed with the air preheated in the previous step according to the line 1’-1”. This mixing process reduces the temperature of the gas but also reduces the humidity content of the mixture resulting into an effective plume abatement solution. Cooling and condensation process, line 1-1’ can be achieved using such exchangers as condensation inside finned tubes, stainless steel and plastic exchangers of tubular or plate type, glass coated plate exchangers, etc. The equipment selection is dictated mainly by the corrosion and plugging tendency of the gas. Adequate washing systems must be used. When a waste stream of hot air is available as in case of some gas cooling processes, mixing of this hot air with the humid exhaust can be very effective as shown in Figure 4. Special precautions must be taken to ensure a uniform mixing of the two streams. The above typical examples illustrate only the main aspects of plume abatement problems. In practice the designer is faced with complex environmental and technological problems.

Figure 2 Figure 3 Figure 4

PLUME & ODOR ABATEMENT SYSTEM

Figure 5

Fig 5 shows a practical implementation of a plume abatement system making use of two plate-type heat exchangers. HE-1 is condensing part of the moisture in the humid gas using ambient air as cooling medium. HE-2 is reheating the exhaust stream recovering heat from the incoming humid stream.

FUNCTIONS 1. GAS SCRUBBING 2. DEW POINT CORRECTION 3. GAS REHEATING TYPICAL DATA INLET DEW POINT: 50°C

OUTLET CONDITION AMB. AIR °C DEW POINT °C SUPERHEAT °C 15 35 20 0 30 25 -15 25 30