Closed Systems - Passive Systems and Active Systems
Cooling
Most industrial processes that generate heat use water to maintain
the temperature at a suitable level. As long as the temperature is kept
controlled throughout the year, the company's production will not stop. This
heat, we can keep it within an ideal range with different systems, each
one with operating characteristics that sometimes force us to select one or
two systems that work in combination.
In water cooling systems we have two types, open and closed.
Open Systems
In the past, open circuits were common, water at ambient temperature was passed through the process and discarded. This practice was not very environmentally friendly since the water used was discarded. Today, this practice is no longer used in most industrialized countries.
Closed Systems - Passive Systems and Active Systems
Currently, closed circuit systems allow maintaining the temperature
both in winter and summer, these systems can be classified as passive or active
.
Passive transfer systems provide minimum temperatures of 10ºC
in winter and 35ºC in summer. Examples are cooling towers and air coolers
.
Cooling tower (35ºC in summer)
The cooling towers operate by evaporation of water releasing the heat
into the environment.
Disadvantages
- Increasing encrustation of minerals in pipes, panels, louvers, etc. Already
that these minerals are present in the water.
- Contamination by algae and other pathogens by the action of the sun and the air.
- Water cost by evaporation and by replenishment.
E.g. A 50 ton tower can transfer 150,000 kcal/h, in summer, the
ambient temperature in some regions can reach 30ºC
The specific heat of evaporation is 540 kcal per liter of water
150,000 kcal/h divided by 540 kcal = 277.8 Liters per hour of evaporated water
If the mass to evaporate is 277.8 liters per hour + the mass of water replacement is
277.8 liters per hour = We have an expense of 555.6 liters of water per hour.
In a working day of 9 hours we will have an approximate expense of 5,000 liters of
water per day.
It is worth mentioning the water losses due to entrainment in an open or closed
cooling tower. When sending the water vapor resulting from
heat transfer to the atmosphere, there is a percentage of water loss. The percentage
depends on the design and condition of the Dew Eliminators. Normally this
loss is very small, practically negligible 0.005% but if the eliminators
are in bad condition, it can reach up to 1% of the recirculated water flow.
In these water droplets that are sent to the atmosphere, can be present a
bacteria that can be very harmful to health, called legionella.
Legionella (Legionnaires' disease and Pontiac fever)
Common causes and sources of infection
Legionella bacteria are naturally found in freshwater environments,
such as lakes and streams. These bacteria can become a health concern
when they multiply and spread in man-made water systems in buildings,
such as:
- Showerheads and sink faucets
- Cooling towers (structures containing water and a fan as
part of the centralized air cooling system in buildings or industrial
processes)
- Whirlpool bathtubs
- Decorative fountains and water features
- Hot water tanks and water heaters
- Complex and large piping systems
Air conditioning systems in automobiles and homes do not use water
to cool the air; therefore, they do not present a risk of multiplication of
Legionella bacteria.
https://www.cdc.gov/legionella/about/causes-transmission-sp.html
Air cooler (45ºC in summer)
It allows heat to be transferred to the environment as long as the outlet temperature of the wateris higher than the ambient temperature.
It is a totally closed system with no water consumption, it works by convection between the hot water circulating inside the tubes and an air current generated by the equipment's fans. The water temperature is higher than in the towers , but below 45ºC in summer. The water is not consumed, it is not contaminated and there isno accumulation of minerals that can become embedded in the system.
Chiller
By successively evaporating and condensing a fluid in a closed circuit it cools down,
removing the heat in the evaporation stage, sending it to the environment in the condensation stage
. Currently, the water chiller is the way to obtain
constant temperatures below the ambient temperature. With the possibility
of reaching temperatures below 0ºC, using antifreeze in the water.
Considerations when selecting one system or another.
If the summer process water temperature can be 45ºC in the process, the tower or the
Air Cooler is an option.
If the summer process water temperature is above 35ºC, the tower is the
alternative. If you need temperatures below 30ºC or below ambient temperature, the chiller is the solution.
References
https://www.youtube.com/watch?v=UVNmaw2XMiU
https://www.cdc.gov/legionella/about/causes-transmission-sp.html
https://www.cdc.gov/legionella/downloads/fs-legionnaires-sp.pdf
