Copyright 2010 stefaniarocca adv
WINTER PLUME IN COOLING TOWERS
WHAT IS IT?
The theory of operation behind equipment for evaporative cooling of water is well known: the fluid to be cooled (water) is forced into close contact with a substantial stream of air. Part of the water evaporates, absorbed and taken away by the air. On leaving the tower, the air is notably warmer, with a greater water content and a higher dew point. During winter months, in particular, when the outside temperature falls, as soon as the stream of air - which is warm and saturated with moisture - comes into contact with the atmosphere outside, it is suddenly cooled. When the temperature of the airflow falls below the dew point, part of the water contained in it condenses giving rise to that characteristic visual phenomenon: the vapour plume. Heavier droplets fall, causing a slight rain effect.
WHAT CAN WE DO TO CONTROL IT?
This brief introduction to how the phenomenon is formed (however modest and incomplete) was required to help us illustrate the principles behind the control system we are proposing.
First of all, we took into consideration a post-heating system for the escaping vapour. This was to be achieved with a water/air heat exchanger located at the vapour outlet. The heating fluid was to be the actual water to be cooled; for use before cooling starts, if course.
The exchanger located at the outlet creates constant flow resistance on the air side, and therefore requires greater energy consumption for the fans (even in summer, when it should not be needed), and hence a rather small total surface area would be necessary (2 to 4 rows).
There are various problems involved, which we shall attempt to outline below.
a) Even in winter with air at 0°C, the temperature of the vapour is in a range between the inlet water temperature and outlet temperature.
(See the evolution of the saturation curves on the psychrometric chart).
Hence, the heat contribution transferred to the vapour is slight, with the consequent rise in temperature and deviation from the saturation line being equally slight.
The only real effect is the physical opposition as the vapour leaves, thus reducing the vapour's compactness and encouraging its absorption into the atmosphere.
b) How do we tackle the exchanger issue?
If an arrangement is adopted whereby water remains inside even with the pumps stopped, we run the risk of bursting owing to the cold (the freezing time of the water in the exchanger's pipes is considerably lower than that of the water in the rest of the plant, including the main pipes).
If, instead, the exchanger is to be drained every time it is stopped, oxygen will inevitably enter the unit meaning we must allow for the risk of corrosion; a stainless steel exchanger would be perfect, but somewhat expensive.
Our research therefore took a different direction. Based on the assumption that less air is required to remove the heat during the colder months, we have come up with the following system:
- penalising the fans, which would therefore have to work at full speed the whole time, we lower the vapour's dewpoint temperature with the controlled introduction of a certain amount of air from outside;
- by lowering the dewpoint temperature, the difference in temperature between the external air and the vapour is reduced by approx. 50 increasing the vapour's actual cooling times.
Much of the condensation owing to the sudden cooling of the vapour occurs inside the plenum chamber, which is part of the tower.
The lower the external temperature, the greater the external air demand, i.e. in those conditions where the air required by the tower is at its lowest. The introduction of the external air is controlled by servo-controlled gates.
In practice, the system can be applied to evaporative cooling equipment to great advantage, whether a forced draught system is in operation or suction fans used.
Where possible, the plenum chamber should be installed between the water distribution system and drift eliminators, i.e. it features servo-controlled dampers between the eliminators and fan.
During summer months, the dampers remain closed and are disabled by the system. During winter months, a proportional thermostat checking water temperature will control the opening of the dampers. When fully open, it will also be responsible for the proportional stopping of the fans, where necessary. Of course, the system may be complemented with water/air heat exchangers on the dampers. In this case, the difference in temperature between the water to be cooled and the external air is higher, with the resulting advantage that:
a) the temperature of the vapour is raised, whilst the dewpoint temperature remains the same (deviation from the saturation curve);
b) heat is removed with a dry system, hence meaning no evaporation is required for removal.
Unfortunately, however, the above-mentioned problems of frozen pipes and corrosion remain.
WINTER PLUME PHENOMENON IN COOLING TOWERS
As all operators in the sector well know, equipment designed for the thermal recovery of service cooling water - known as cooling towers or evaporative towers - enables even substantial volumes of water to be cooled at relatively little cost by evaporating a small percentage (usually around 2-3 of the actual water.
Evaporation is encouraged by a considerable volume of air, circulated by fans usually located at the top of the towers. The air comes into close contact with the water to be cooled.
The air flow delivered by the fans is usually warmer and much more humid than the surrounding atmosphere.
Harmless plumes of saturated air escaping from cooling towers installed to serve industrial plants have become, in some cases, a traditional characteristic of the landscape.
As mentioned, the phenomenon is entirely harmless. Nonetheless, whilst in summer effects are decidedly marginal, during the colder season they may be grounds for contention with neighbours who find their premises assailed by damp, and their forecourts made slippery and icy. Similarly, contention may arise with authorities in charge of traffic should the phenomenon affect roadways. More recently, authorities concerned with environmental protection have also raised objection as increasing attention is turned to the impact the spectacle has on the territory, which is not always seen in a good light.
The question Boldrocchi T.E.'s technical department asked itself is whether this annoying phenomenon of winter plume in cooling towers can be eliminated or at least reduced without excessive cost.
Today's answer is mostly positive.
The phenomenon can be reduced drastically and proves relatively inexpensive in terms of both initial investment and running costs.
The original device that enables partial control of winter plume is based on recognised principles. It does not involve obstructing the fan outlet (which would penalise the tower during summer months) and does not call for any energy other than that required for regular operation.
Unfortunately, this system is not usually applicable to existing towers, whilst it can already be requested on all towers produced by Boldrocchi T.E..
WHAT IS IT?
The theory of operation behind equipment for evaporative cooling of water is well known: the fluid to be cooled (water) is forced into close contact with a substantial stream of air. Part of the water evaporates, absorbed and taken away by the air. On leaving the tower, the air is notably warmer, with a greater water content and a higher dew point. During winter months, in particular, when the outside temperature falls, as soon as the stream of air - which is warm and saturated with moisture - comes into contact with the atmosphere outside, it is suddenly cooled. When the temperature of the airflow falls below the dew point, part of the water contained in it condenses giving rise to that characteristic visual phenomenon: the vapour plume. Heavier droplets fall, causing a slight rain effect.
WHAT CAN WE DO TO CONTROL IT?
This brief introduction to how the phenomenon is formed (however modest and incomplete) was required to help us illustrate the principles behind the control system we are proposing.
First of all, we took into consideration a post-heating system for the escaping vapour. This was to be achieved with a water/air heat exchanger located at the vapour outlet. The heating fluid was to be the actual water to be cooled; for use before cooling starts, if course.
The exchanger located at the outlet creates constant flow resistance on the air side, and therefore requires greater energy consumption for the fans (even in summer, when it should not be needed), and hence a rather small total surface area would be necessary (2 to 4 rows).
There are various problems involved, which we shall attempt to outline below.
a) Even in winter with air at 0°C, the temperature of the vapour is in a range between the inlet water temperature and outlet temperature.
(See the evolution of the saturation curves on the psychrometric chart).
Hence, the heat contribution transferred to the vapour is slight, with the consequent rise in temperature and deviation from the saturation line being equally slight.
The only real effect is the physical opposition as the vapour leaves, thus reducing the vapour's compactness and encouraging its absorption into the atmosphere.
b) How do we tackle the exchanger issue?
If an arrangement is adopted whereby water remains inside even with the pumps stopped, we run the risk of bursting owing to the cold (the freezing time of the water in the exchanger's pipes is considerably lower than that of the water in the rest of the plant, including the main pipes).
If, instead, the exchanger is to be drained every time it is stopped, oxygen will inevitably enter the unit meaning we must allow for the risk of corrosion; a stainless steel exchanger would be perfect, but somewhat expensive.
Our research therefore took a different direction. Based on the assumption that less air is required to remove the heat during the colder months, we have come up with the following system:
- penalising the fans, which would therefore have to work at full speed the whole time, we lower the vapour's dewpoint temperature with the controlled introduction of a certain amount of air from outside;
- by lowering the dewpoint temperature, the difference in temperature between the external air and the vapour is reduced by approx. 50 increasing the vapour's actual cooling times.
Much of the condensation owing to the sudden cooling of the vapour occurs inside the plenum chamber, which is part of the tower.
The lower the external temperature, the greater the external air demand, i.e. in those conditions where the air required by the tower is at its lowest. The introduction of the external air is controlled by servo-controlled gates.
In practice, the system can be applied to evaporative cooling equipment to great advantage, whether a forced draught system is in operation or suction fans used.
Where possible, the plenum chamber should be installed between the water distribution system and drift eliminators, i.e. it features servo-controlled dampers between the eliminators and fan.
During summer months, the dampers remain closed and are disabled by the system. During winter months, a proportional thermostat checking water temperature will control the opening of the dampers. When fully open, it will also be responsible for the proportional stopping of the fans, where necessary. Of course, the system may be complemented with water/air heat exchangers on the dampers. In this case, the difference in temperature between the water to be cooled and the external air is higher, with the resulting advantage that:
a) the temperature of the vapour is raised, whilst the dewpoint temperature remains the same (deviation from the saturation curve);
b) heat is removed with a dry system, hence meaning no evaporation is required for removal.
Unfortunately, however, the above-mentioned problems of frozen pipes and corrosion remain.
WINTER PLUME PHENOMENON IN COOLING TOWERS
As all operators in the sector well know, equipment designed for the thermal recovery of service cooling water - known as cooling towers or evaporative towers - enables even substantial volumes of water to be cooled at relatively little cost by evaporating a small percentage (usually around 2-3 of the actual water.
Evaporation is encouraged by a considerable volume of air, circulated by fans usually located at the top of the towers. The air comes into close contact with the water to be cooled.
The air flow delivered by the fans is usually warmer and much more humid than the surrounding atmosphere.
Harmless plumes of saturated air escaping from cooling towers installed to serve industrial plants have become, in some cases, a traditional characteristic of the landscape.
As mentioned, the phenomenon is entirely harmless. Nonetheless, whilst in summer effects are decidedly marginal, during the colder season they may be grounds for contention with neighbours who find their premises assailed by damp, and their forecourts made slippery and icy. Similarly, contention may arise with authorities in charge of traffic should the phenomenon affect roadways. More recently, authorities concerned with environmental protection have also raised objection as increasing attention is turned to the impact the spectacle has on the territory, which is not always seen in a good light.
The question Boldrocchi T.E.'s technical department asked itself is whether this annoying phenomenon of winter plume in cooling towers can be eliminated or at least reduced without excessive cost.
Today's answer is mostly positive.
The phenomenon can be reduced drastically and proves relatively inexpensive in terms of both initial investment and running costs.
The original device that enables partial control of winter plume is based on recognised principles. It does not involve obstructing the fan outlet (which would penalise the tower during summer months) and does not call for any energy other than that required for regular operation.
Unfortunately, this system is not usually applicable to existing towers, whilst it can already be requested on all towers produced by Boldrocchi T.E..
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