Chapter 70- Cooling Towers Flashcards
The average amount of water volume which evaporates
5%
The purpose of a cooling tower
To reduce the temperature of the cooling tower
The four ways of heat transfer
- velocity of air and water
- area of water surface in contact with air
- length of contact time
- difference between the inlet water temp and the inlet air wet bulb temp
Transfer heating air passing through the tower
Sensible heat
Removed of ________ by evaporation of the water
Latent heat
Circulation of atmosphere air is accomplished by
Fans, convection currents, natural wind currents, induction effects
When is cooling tower the most common type of heat exchanger
When water is scarce expensive and temp within 4-6 degrees
Chimney is also referred to as
Hyperbolic tower
How do you Ensure effective operation
Regular inspection and maintenance
Advantages of Venturi towers (4)
- quiet
- no moving parts
- no electrical connections
- reduced maintenance
Inhibit growth leads to
Legionnaires disease…. Very deadly
Give cause and solution on airborne particles
Cause: dust and debris
Solution: strainers and filters are clean and maintain
Cause and solution if scaling
Cause: hardness forming salts in the cooling water consisting of ca and mg
Solution: add sulphuric acid…increase ph and temp
Ph in a cooling tower should be
6-7.5
Cause and solution of corrosion
Cause: ph levels below 8.5 and dissolved oxygen Solution: * corrosion resistant metals * protective coatings *sacrificial metals *protective films
Cause and solutions of biological fouling
Cause: algae fungi and bacterial growth
Solution: mechanical and chemical
Cause and solution of wood deterioration
Cause: due to attack be choline and alkaline solution and biological attack of wood cellulose Solution: *choosing natural wood *treatment by wood preservatives *limited nails screws or iron hard wear *water treatment
The rate of heat transfer in a cooler tower system depends on how many factors?
4
Which of the following are factors that determine the heat transfer rate of any cooling tower system
The difference between wet bulb air
The surface area of the water in contact with the air
What are two classes of cooling towers?
Natural air circulation
Mechanical air circulation
What determines how much makeup water is required to maintain the level in a cooling tower water basin
The blowdown rate
Natural draft cooling towers operate most efficiently in large open spaces and
Where wind is relatively constant
Excessive wind velocity in a natural draft cooling tower may cause
High water drift or losses
Atmospheric towers and chimney towers are the two subclasses of what type of cooling tower
Natural draft cooling towers
What are atmospheric cooling towers dependant on?
Atmospheric conditions
What causes airflow in a hyperbolic cooling tower?
Convection currents
Compared to a cross flow tower, a counter flow tower
Requires less floor space
Water contained in the airstream at the discharge of a cooling tower is removed
With drift eliminators
How many subclasses of mechanical draft are there?
2
What are the two classifications of mechanical draft cooling towers?
Forced draft
Induced draft
A cross flow mechanical draft cooling tower
Has a low pressure drop
What is used to remove water from the air which exits a cooling tower
Drift eliminators
In a Venturi type cooling tower, how is airflow created
By spray nozzles
A car radiator works on the principle of
A dry cooling tower
The by-pass valve, which prevents water flow over the fill of a tower during cold weather should not be closed until the temperature in the water basin reaches
+ 27 Celsius
To prevent cooling tower basin water from freezing during cold weather many units are equipped with
Electric basin heaters
In cold climates, ice formation in a cooling tower may be dealt with by
Slowing the fans down
Reversing the fans
Scaling is not as severe in towers compared to boilers because
Temperatures are not as high
What is the purpose of a bleed off line in a cooling tower?
To control the amount of scale forming salts
Water treatment is necessary in cooling towers because of
Scaling
Biological fouling
Corrosion
Wood deterioration
To control scaling in a cooling tower, which should be done first?
Use the bleed off line
What purpose do both water softener and the addition of sulphuric acid to cooling water serve
They treat scaling
What is steel corrosion in cooling towers mainly due to
Dissolved oxygen
Corrosion rates in a cooling tower increase as water temperature increases up to this point
70 degrees celcius
For water temperatures up to 70 degrees Celsius corrosion rates in cooling towers
Double
In cooling towers, protective films and sacrificial metals are two methods of protecting against what?
Corrosion
Which chemicals are environmentally friendly alternatives for providing corrosion prevention in cooling towers?
Zinc
Polysilicates
Molybdates
What could happen if the initial treatment of film forming chemicals in a cooling tower is to weak?
Severe corrosion in unprotected areas is possible
What makes cooling towers such a good environment for biological growth?
An oxygen rich atmosphere
Warm water
What are trash bars and chemical methods used to protect a cooling tower against
Biological fouling
Chemical methods used to prevent biological fouling in cooling towers include
Oxidizing chemicals
Non- oxidizing chemicals
Heavy metals
The use of oxidizing chemicals is one of the methods used to protect against biological fouling in cooling towers. An example of an oxidizing chemical is
Chlorine
How are microbiological organisms most efficiently killed in a cooling tower water system.
By temporarily raising the concentration of the chemicals to a high level
Delignification in a cooling towers wood is a result of
Chemicals such as chlorine
What gives wood it’s strength
Cellulose
Microorganisms require carbon to develop. Where would they get this carbon from a cooling tower?
The wood
Excessive water drift in a cooling tower could be the result of drift eliminator a being out of place or.
The water flow rate is to high
Reasons for a noisy motor in a cooling tower include
Bad bearings
An unbalanced rotor
The motor is running single phase
Electrical unbalance
More water droplets =
Greater surface area
Atmosphere spray towers
Have the cheapest and simplest design
Packed atmospheric towers have
Baffles to create turbulence
Maintenance required in cooling towers may include
Cleaning louvers, piping and nozzles or debris, scale or algae
Cleaning water basin to check for leaks
Cleaning suction screen
Checking level control operates correctly
Advantages of a forced draft fan include
Minimal vibration
Fans handle dry air reducing erosion and condensation issues
Advantages of an induced draft fan include
Increased heat transfer
No recirculating warm air
- note: these may develop vibration issues and require more maintenance
Dry cooling towers
Have fluid move through finned tubes
Ex: the radiator in a car works on the same idea
A cooling tower that needs no fan or fill is
Venturi
You are replacing the cooling tower fan motor bearing because
The rotor was rubbing the stator
