Cooling Tower

Cooling Tower (CT) Principle ?

• To decrease the temperature of hot water entering the CT.
• Cooling by evaporation is the principle used.
• Heat transfer from Water to Air.

• High difference between Wet Bulb Temperature ( WBT ) and Dry Bulb Temperature ( DBT ) encourages more Heat Transfer ( HT ) between air and water inside the cooling tower. This is due to the fact that when WBT = DBT , air is fully saturated i.e 100% relative humidity (R.H) and no longer accepts water, thus no more HT.

• Thus more difference between WBT and DBT => low R.H. => greater capacity to hold water => effective lowering of water temperature.

• Final outlet water temperature will always be < WBT of entering air. ( suppose DBT = 30°C, WBT = 25°C find R.H from humidity chart, this implies the capacity of air to hold water i.e. hot water can maximum transfer heat till its drops 

A cooling tower is a heat rejection device which rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or, in the case of closed circuit dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature.

Common applications include cooling the circulating water used in oil refineries, petrochemical and other chemical plants, thermal power stations and HVAC systems for cooling buildings. The classification is based on the type of air induction into the tower: the main types of cooling towers are natural draft and induced draft cooling towers.

Cooling towers vary in size from small roof-top units to very large hyperboloid structures (as in the adjacent image) that can be up to 200 metres (660 ft) tall and 100 metres (330 ft) in diameter, or rectangular structures that can be over 40 metres (130 ft) tall and 80 metres (260 ft) long. The hyperboloid cooling towers are often associated with nuclear power plants, although they are also used to some extent in some large chemical and other industrial plants. Although these large towers are very prominent, the vast majority of cooling towers are much smaller, including many units installed on or near buildings to discharge heat from air conditioning.

With respect to drawing air through the tower, there are three types of cooling towers:

·         Natural draft  — Utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential compared to the dry, cooler outside air. Warm moist air is less dense than drier air at the same pressure. This moist air buoyancy produces an upwards current of air through the tower.

·         Mechanical draft — Uses power-driven fan motors to force or draw air through the tower.

·         Induced draft — A mechanical draft tower with a fan at the discharge (at the top) which pulls air up through the tower. The faninduces hot moist air out the discharge. This produces low entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intake. This fan/fin arrangement is also known as draw-through.

·         Forced draft — A mechanical draft tower with a blower type fan at the intake. The fan forces air into the tower, creating high entering and low exiting air velocities. The low exiting velocity is much more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. Another disadvantage is that a forced draft design typically requires more motor horsepower than an equivalent induced draft design. The benefit of the forced draft design is its ability to work with high static pressure. Such setups can be installed in more-confined spaces and even in some indoor situations. This fan/fill geometry is also known as blow-through.

·         Fan assisted natural draft — A hybrid type that appears like a natural draft setup, though airflow is assisted by a fan.

Working of CT ?

• Hot water from HE enters cooling tower
• It is sprinkled from top with the help of nozzles
• fills are used for better Heat Transfer between air and water.
• Normally water is brought down to room temperature and is collected in the concrete basin at the bottom.
• This water is re-used again and the same process follows.
• small amount of water gets evaporated during the entire process hence make up water is used .

Types of CT

1. Natural Draft ( Hyperbolic CT ) :

• ADVANTAGES : No fans, motors or gearboxes required, usually used for large quantity of water flow.
• DISADVANTAGE : Large space required
• uses the difference between ambient air temperature and the air inside tower.
• Hot air rises upwards and cooler air is drawn inside through bottom.
• Two types :

A) Cross flow Natural Draft CT: water and air flow are perpendicular. Fill is located outside CT.

NPSH (Net Positive Suction Head)

NPSH:

Net positive suction head measures the difference in head (differential head) & not the difference in pressure.

NPSHA	NPSHR
Absolute Pressure required at the pump suction above the vapor pressure of the liquid at that temperature. (Psuction > Pvap)	It is the minimum absolute pressure required at the pump suction to avoid vaporization. (Psuction = Pvap)
It is the function / requirement of your actual process / system	It is pump specific.
Hence it is calculated with the help of process parameters & conditions.	It is provided by pump manufacturer. It is calculated using water at room temperature by the manufacturer.

 Vapor Pressure (VP):

In order to understand cavitation, it is must to first understand the concept of vapor pressure. 

It is the pressure required (or pressure exerted by the vapor on the liquid surface) to boil the liquid at a given temperature.

When VP ≥ Patm, the liquid boils (at equality condition) and vaporizes (at greater than condition).

 It is a unique characteristic of every liquid.

The following Antoine equation shows the relation between Vapor

pressure (VP) & Temperature (T)

where A, B, C are constants.

Thus we see that as T ↓ VP ↓ i.e. T α VP. So when the surrounding pressure decreases (usually as you go to higher altitudes) the temperature at which the vaporization occurs also decreases. Thus we see that water boils at lower temperature at higher altitudes compared to that at sea level.  

We see that as temperature increases, vapor pressure increases. 

Major indicators of cavitation:

1. Pitting
2. Loss of capacity
3. Noise & vibrations.
4. Varied power consumption
5. Unstable head

Prevention: 

Install a pressure gauge at the suction line. This will help you indicate the actual pressure value (corresponding to Ha + Hs - Hf).

Now, you just have to subtract the Hvp from the above pressure value indicated by the pressure gauge, to get the actual NPSH available at the pump suction any point during the plant operation. Compare this value with the NPSHR of that pump to know if there is any cavitation or the possibility of cavitation in the near future.

Hvp value is only dependent on the pumping temperature, so also note the pumping temperature when you are subtracting the Hvp value. If the temperature is not as you expected/desired, the Hvp value (new value) will change (from the old/desired value) and hence when you subtract this new value, u'll end up with a wrong/undesired NPSHA for your system, which when compared with NPSHR will give you wrong indications about the cavitation problem. Basically, putting a pressure gauge at the pump suction serves no purpose then.

Calculating NPSHA:

NPSHA = Hp + Hst - Hf - Hvp

(NOTE: All units are in 'm of liquid')

    1. Hst =

       (+)ve when the liquid level is above the pump suction line. (Static head)
       (-)ve when the liquid level is below the pump suction line (Suction lift)

   2. Hp = Pressure applied on the liquid surface in the tank.
                For open tank, P = Atmospheric pressure,
                For closed tank, P = pressure above the liquid surface

   3. Hvp = Vapor pressure head at the pumping temperature.

   4. Hf = Frictional head.
     

                                                                                      

Example:

First we will see the steps to calculate the differential head (total head) for the pump system, post that we will calculate the NPSH available.

Suction Head:

1. Hp = 101.3 / (ρsg)

2. Hst = H1 (height of the tank above the pump center line. In this case, the pump is designed for LLL)

3. Hf = ?

• Calculate velocity (using area [A] & flow rate [Q])
• Calculate Reynold's number [Re]

• Calculate Relative roughness [e/D]

• Use Moody's chart to find out the moody's (darcy's) friction factor [f] using Re & e/D. 

• Determine the 'K' (Frictional resistance coefficient) value (standard) for the total number of bends, turns, fittings, valves in the suction side of piping.
• Add all the 'K' values & multiply it by the pipe diameter to get the equivalent length [Le].
• Calculate Line Pressure Drop by using Darcy equation:

• Calculate the head [Hf] now using the same equation as in 1. 

Total suction head (Hs) =  Hp + Hst + Hf

Discharge Head:

1. Hp = 101.3 / (ρsg)

2. Hst = H2 (height of the liquid level in the tank above the pump centerline.)

3. Hf = ?

• Calculate velocity (using area [A] & flow rate [Q])
• Calculate Reynold's number [Re]

• Calculate Relative roughness [e/D]

• Use Moody's chart to find out the friction factor [f] using Re & e/D. 

• Determine the 'K' value (standard) for the total number of bends, turns, fittings, valves in the suction side of piping.
• Add all the 'K' values & multiply it by the pipe diameter to get the equivalent length [Le].
• Calculate Line Pressure Drop by using Darcy equation:

• Calculate the head [Hf] now using the above pressure drop and the equation used in 1.

Total Discharge head (Hd) =  Hp + Hst + Hf

Total Differential head (H) = Hd - Hs

Now to calculate NPSHA:

Hp, Hst, Hf all are calculated in the calculation of total suction head. Only now left to calculate is Hvp.

 Hvp = VP (Pa) / (ρsg)

NPSHA = Hp + Hst - Hf - Hvp

Compare this calculated value of NPSHA with the value of NPSHR provided by the manufacturer.

Generally, to be on safer side, 

NPSHA = NPSHR + 2m

2m (case specific) is added to compensate for the various losses in the line  due to following reasons:

1. Mechanical losses (flow restriction losses): Valves, orifice, turns, bends, pipe friction, bearings etc.
2. Volumetric losses: Leakage
3. Hydraulic losses: Friction at entry or exit, vortices separation etc. 

In case if we fail to keep the NPSHA above NPSHR, the liquid will then start to boil and vaporize and hence the phenomenon of cavitation will take place.

NOTE: The margin of 2m is just to keep enough safety margin over NPSHA. It is not mandatory or thumb rule. In fact, any value of NPSHA greater than 1m will be helpful in most cases, depending on the case. The more the margin the better it is to operate safely without any problem for a long period.

Critical Suction Variables:(guide during trouble-shooting)

• Tank pressure / source pressure check.

It should not decrease, since in NPSHA formula, this term is added. So if this term decreases NPSHA decreases and there might be problems of cavitation. So always ensure that the tank pressure is maintained. Increase in tank pressure will cause no harm, but decrease in pressure might do.

• Tank level.

A good design practice is to design the pump for lowest liquid level [LLL] in the tank. The reason is explained as follows in the next section.

• Pressure & temperature of the pumping fluid:

Pressure at the suction gauge and temperature of the pumping fluid must be checked frequently to ensure that the fluid is pumped and operated well within the expected pressure, temperature limits.  

• NPSHA v/s NPSHR.

NPSHA has to be greater than a minimum of 0.5m to ensure no cavitation. Even 0.5m margin is a boundary case, for better and safe system take >1m or 2m margin over and above the manufacturers NPSHR.

• Suction reducer positioning 

Reducer is installed to ensure that no air is trapped and the pump is always flooded with liquid. Pay attention during the installation of suction piping and the position of the eccentric reducer. 

• Straight run

A general practice of minimum of 5D to 10D of straight run is given without any valve, bend or elbow at the pump suction before the reducer, to make the flow laminar and stable. The above picture is right only in the positioning of the reducer and not with the concept of straight run. 

This picture gives a clear view of how the suction piping should be. The suction nozzle of the pump should immediate follow the reducer. 

• Suction Length:

Suction length must be as small as possible with minimum bends, for the very simple reason that the pressure loss increases with the length and thus NPSHA value decreases. 

• Fluid properties

Density, viscosity, liquid feed condition i.e. saturated or not. 

Important parameters to keep in mind:

Keep the following parameters in mind whenever there is an issue in your pump.

1. Pressure (tank + losses + vapor head)
2. Temperature
3. Reducer 
4. Suction length
5. NPSHA

Fixing Errors:

1. Hs:

Hs is governed/controlled by the amount of liquid in the tank and the height of the tank above the pump center line. 

• Raise the liquid level in the tank. More the liquid, more will be the pressure at the pump suction line and hence more will the NPSHA, so higher the chances of avoiding cavitation and keeping the system running safely.

• Raise the tank to a higher level. Again the      same, higher the tank is from the center line of the     pump, more will be the static head developed at the suction side & hence more will be the NPSHA.

NOTE: Raising the tank is not always a feasible option in many running plants, so raising / maintaining the liquid level in the tank can help you gain the necessary NPSHA (this is only when the tank is above the pump center line) 

 Consider & design the pump for the Lowest Liquid Level (LLL) in the tank for safety reasons. This will help your system to run satisfactorily and avoid cavitation, irrespective of the amount of liquid in the tank.

Say, 
Ha = 1 atm,
H = 5m (in left tank) + 10m (length) 
   = 0m (in right tank) + 10m (length)
H(losses) = 0.5m
Hvp = 0.5m

NPSHA (Left tank) = 1 + (10+5) - 0.5 - 0.5 = 15m
NPSHA (right tank) = 1 + (10+0) -0.5 - 0.5 = 10m

Now, here we see that for the right tank, with LLL condition, has NPSHA of 10m and now when u add the liquid in the tank you see that H value increases (editable in above figure and calculation of left tank). This clearly ensures that the margin is very well over and above NPSHR, which ensures smooth running of the pump over a long period of time.

If the pump is selected on the basis of the liquid level in the left tank, then the NPSHA would vary as the liquid level in the tank changes, thus we have to maintain the liquid level or raise the liquid level to ensure NPSHA that was initially calculated doesn't decrease. 

Whereas, in the right side figure, NPSHA is designed for LLL condition, wherein NPSHA will only increase with the rise in the liquid level and will never decrease or pose a problem of cavitation.

2. Hf:

Friction head is the easiest parameter to change among all other variables. Remember larger the frictional losses in the line, lesser will be the NPSHA (It is subtracted in the NPSHA equation above), so we have to reduce the losses in the line.

Frictional losses in the line can be reduced by:

1. Increasing pipe DIAMETER of the suction line. This will reduce the velocity in the suction side (by equation of continuity, AV=constant) thus allowing the flow to be laminar, where the line problems like corrosion & erosion due to high velocities (turbulent nature of flow) are significantly reduced and hence the line pressure drop is reduced.
2. Reducing the LENGTH of the suction piping. This will directly reduce the frictional losses in the suction line.
3. Reducing OBSTRUCTIONS like valves, fittings, turns ,bends, etc. in the suction piping. This is because across each of this component there will be a pressure drop & will thus increase the overall pressure drop in the suction pipeline, and since pressure drop in the pipeline is in direct proportion to the friction factor, the frictional losses will increase with the increase in overall pressure drop. (del P equation with f & proportionality explain)
4. Putting ELECTRIC HEAT TRACING lines. This are used especially to avoid freezing of solid components present in the liquid or the liquid as a whole (in cold countries). It is also used to reduce the viscous losses while pumping highly viscous liquids.

3. Hvp:

We know that T α VP, so control the temperature to make sure that the vapor pressure is always less than the suction pressure by a significant margin to ensure that the possibility of vaporization of the liquid is reduced, this will in turn reduce the cavitation inside your pump. 

Often Tanks & pipes are painted with light colors to avoid natural (sun) heating of high vapor pressure liquids contained in them.

If the above options doesn't work, the only & the final option left is to ask the pump manufacturer to reduce the NPSHR of the system.

So in a nut-shell,

Ways to Increase NPSHA.

Increase	Decrease
Pipe diameter	Obstructions
Tank height	Suction lift
Liquid level in tank	Operating temperature
	Run length.