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Closed cooling tower basics-you are half an expert!


Release time:

2023-09-28

A closed cooling tower is a device that uses the contact between air and water (directly or indirectly) to cool water. Water is used as a circulating coolant to absorb heat from a system and discharge it to the atmosphere, thereby reducing the temperature in the tower and making cooling water recyclable equipment.

A closed cooling tower is a device that uses the contact between air and water (directly or indirectly) to cool water. Water is used as a circulating coolant to absorb heat from a system and discharge it to the atmosphere, thereby reducing the temperature in the tower and making cooling water recyclable equipment.

 

cooling tower in the heat dissipation relationship:

In a wet cooling tower, the temperature of hot water is high, and the temperature of the air flowing through the surface of the water is low. The water transfers heat to the air, is taken away by the air, and is scattered into the atmosphere. There are three forms of heat dissipation from water to the air:

① Contact heat dissipation;

② Evaporative heat dissipation;

③ Radiation heat dissipation.

Cooling tower mainly depends on the first two kinds of heat dissipation, radiation heat dissipation is very small, not slightly.

evaporation heat dissipation principle:

Evaporative heat dissipation is accomplished by material exchange, I .e., by the constant diffusion of water molecules into the air. Water molecules have different energies, and the average energy is determined by the water temperature. Some water molecules with large kinetic energy near the water surface overcome the attraction of adjacent water molecules and escape from the water surface to become water vapor. As the water molecules with large energy escape, the energy of the water body near the water surface becomes smaller.

Therefore, the water temperature of the closed cooling tower is reduced, which is evaporative heat dissipation. It is generally believed that the evaporated water molecules first form a thin saturated air layer on the water surface, and its temperature is the same as the water surface temperature, and then the water vapor from the saturated layer to the atmosphere The diffusion speed depends on the water vapor pressure of the saturated layer and the water vapor pressure difference of the atmosphere, that is, Dalton (Dolton) law, which can be represented by the following figure.

Basic structure of cooling tower

Bracket and tower: external support

Packing: Provide the largest possible heat exchange area for water and air

Cooling water tank: located at the bottom of the cooling tower, receiving cooling water

Water collector: Recycling water droplets taken away by air flow

Air inlet: cooling tower air inlet

Water spraying device: spray cooling water

Fan: air supply to cooling tower

Axial fans are used in induced draft cooling towers.

Axial flow/centrifugal fans are used in forced draft cooling towers.

Cooling tower louvers: average intake air flow; retain moisture in the tower.

Types of cooling towers and their advantages and disadvantages
Natural draft cooling tower

The less dense hot air flows from the top of the cooling tower;

The cold air with high density enters the cooling tower from the bottom of the tower to fill;

No fan is required;

Concrete tower <200 m;

For cooling of large heat.

mechanical ventilation cooling tower

 

High-power fan forced air and circulating water heat exchange;

The water film on the surface of the filler can maximize heat exchange with the air;

There are many determinants of cooling efficiency;

Multiple cooling capacity options;

Multiple cooling towers can work at the same time, such as 8-tower joint control.

Forced ventilation:

Air is blown into the vent by the centrifugal fan; advantages: suitable for towers with large airflow resistance; centrifugal fan noise is relatively small.

countercurrent cooling tower:

The cooling water is sprayed on the packing and flows down into the cooling water tank.

Air is forced to blow in from the bottom, and part of the cooling water is evaporated in contact with the water in the packing, thereby reducing the water temperature.

induced draft cooling tower

Cross-flow induced draft cooling tower

counterflow induced draft cooling tower

Advantages: the degree of reflux is lower than that of the forced draft cooling tower, and the operating cost of the fan is less than that of the forced draft cooling tower.

Disadvantages: The mechanical transmission of the fan and the motor needs a waterproof design.

1) Hot water enters the cooling tower from the top

2) The air is forcibly induced by the fan and enters the cooling tower from the bottom; a forced induction fan is used.

The cooling water enters from the top and flows through the packing layer; the air enters from one or both sides, and the induced fan causes the air to flow laterally through the packing layer.

Due to the natural flow of hot water distribution system of such cooling towers:

Advantages:

Low pump head

Lower initial investment in pumps

Lower annual operating energy consumption and costs

Large changes in flow will not adversely affect the water distribution system.

Disadvantages:

The low pressure head will cause the nozzle to be easily blocked and the cooling water cannot be well dispersed into a fine mist when it is sprayed.

Direct exposure of hot water tanks to air can lead to algae growth

It covers a large area.

Because the pressurized water distribution spray device in this type of cooling tower:

Advantages:

By increasing the height of the tower to obtain a longer heat exchange process and a smaller cold amplitude.

Since the pressurized spray device can spray smaller water droplets, the heat exchange efficiency is higher.

Disadvantages:

System pump head increase

Increased energy demand and increased operating costs

Cooling water nozzles are not easy to maintain and clean

The need for water distribution systems and associated piping increases initial investment.

Operation Parameters and Type Selection Design of Cooling Tower
1. Cooling water temperature difference: inlet temperature-outlet temperature

Large temperature difference = high performance

2. Cold range: the difference between the outlet temperature of the cooling tower and the wet bulb temperature of the inlet air:

Small cold amplitude = high performance

 

3. Efficiency: closed cooling tower

 

4. Cooling tower capacity

The unit of cooling tower capacity is "kilocalories per hour" or "cold tons";

Cooling tower capacity = mass flow of cooling water × specific heat capacity of water × temperature difference;

Large Capacity = High Performance

5. Calculation of recharge water

Amount of water lost by evaporation (E)

E = Q/600 = (T1-T2)* L /600

E represents the amount of water evaporated (kg/h);

Q represents heat load (Kcal/h);

600 represents the latent heat of evaporation of water (Kcal/h);

T1 represents the inlet water temperature (℃);

T2 represents the outlet water temperature (℃);

L represents the amount of circulating water (kg/h).

Calculation of recharge water quantity

Water loss by splashing (C)

The amount of splash loss of the cooling tower depends on the design type of the cooling tower, wind speed and other factors. Under normal circumstances, its value is approximately equal to about 0.1~0.2% of the circulating water volume.

Periodic discharge loss (D)

The loss of periodic discharge water is determined by factors such as water quality or solid concentration in the water. Generally about 0.3 per cent of the amount of circulating water.

M=E C D

Water loss by evaporation (E); Water loss by splashing (C); Water loss by periodic discharge (D).

 

When the cooling tower is used for air conditioning, the temperature difference is designed at 5 ℃, and the supply water required by the cooling tower is about 2% of the circulating water.

6. Cooling water flow

K · Q = C · M · ΔT

K: Estimation coefficient

Q: Maximum refrigerating capacity of the unit

C: specific heat capacity of water

ΔT: temperature difference between supply and return water

M: Cooling water mass flow

 

1.3 times the maximum refrigerating capacity of the compression refrigeration unit;

Absorption refrigeration unit (lithium bromide) cooling capacity of 2.5 times.

Selection example

Example: A project with a 640RT unit cooling tower water flow and water supply.

Q=640RT=2251KW

K=1.3

C = 4. 2kj/(kg· ℃)

ΔT=5℃

Water replenishment m = M · 2% = 140 kg/s · 2% = 2.8 kg/s

Cooling Tower Selection Often Design Problems

1. Determinants of cooling tower energy consumption?

Answer: fan power, cooling water flow, cooling water replenishment?

2, the cooling tower temperature conditions, what temperature efficiency economy is good?

Answer: The inlet water temperature of the cooling tower varies according to the use situation. For example, the outlet water temperature of the central air-conditioning condenser is generally 30-40°C, and the outlet water temperature of the cooling tower is generally 30°C. The ideal cooling temperature (return water temperature) of the cooling tower is 2-3 ℃ higher than the wet bulb temperature. This value is called "approximation degree" (public number: pump housekeeper). The smaller the approximation degree, the better the cooling effect and the more economical the cooling tower.

3, open and closed comparison

Open type: the first phase of the investment is relatively small, but the operating cost is higher (water consumption, electricity consumption).

Closed: This equipment is suitable for use in harsh environments such as drought, water shortage, and frequent sandstorms. It can cool a variety of media such as water, oil, alcohol, quenching fluid, brine and chemical fluid, with no loss and stable composition. Low energy consumption.

Disadvantages: Closed cooling towers cost three times as much as open towers.

 

Installation, piping, operation and common faults of cooling tower
Cooling tower noise source

The cooling towers used above are mechanical ventilation cooling towers. During their operation, the noise sources of the water tower are mainly as follows:

1. Fan noise:

The noise is mainly composed of mechanical noise and fluid noise;

2. Motor noise:

Electromagnetic sound when the main motor is running;

3. Ventilation noise:

It mainly has the air fluid noise inside and outside the tower body and the tower body resonance noise.

For the solutions, please refer to the relevant courseware of Nanshe Encyclopedia, "A Comprehensive Understanding of" Noise "and Noise and Vibration Reduction Methods of Equipment in HVAC Systems".

Precautions for installation and piping

 

Closed cooling tower operation matters needing attention
Preparation before operation:

1) Foreign matters on the air inlet side or around the wind trunk shall be removed;

2) Make sure that there is enough clearance between the tail of the windmill and the trunk of the windmill to avoid damage during operation;

3) Check whether the V-belt of the reducer is properly adjusted;

4) The position of the V-shaped pulley must be kept at the same level;

5) After the above inspection is completed, start the switch intermittently to check whether the operation mode of the windmill is correct and whether there is abnormal noise and vibration?

6) Remove the sundries inside the hot water tray and tower body;

7) Remove the dirt and foreign matter in the hot water tray, and then fill the water to the overflow position;

8) Start the circulating water pump intermittently to remove the air in the pipe until the pipe and the cold water tray are filled with circulating water;

9) When the circulating water pump operates normally, the water level in the cold water tray will drop slightly, and the float valve must be adjusted to a certain water level;

10) Circuit system, reconfirm whether the circuit switch, fuse and wiring specifications match the motor load.

Precautions for water tower starting:

a. Start the windmill intermittently and check whether it is running in reverse or abnormal noise and vibration occur? Then start the water pump again;

B. Check whether the running current of the windmill motor is overloaded. Avoid the phenomenon of motor burning or voltage drop;

C, use the control valve to adjust the water volume, so that the water level of the hot water tray is maintained between 30~50mm;

D, check whether the running water level in the cold water tray is normal.

Matters needing attention during operation of water tower:

A, after 5 to 6 days of operation, re-check the windmill reducer V belt is normal, if loose, you can use the adjustment bolt to re-lock properly;

B, cooling tower after a week of operation, must replace the circulating water, in order to remove the debris in the pipeline dust;

C, the cooling efficiency of the cooling tower will be affected by the circulating water level, for this reason, it is necessary to ensure a certain water level of the hot water tray;

D, if the water level in the cold water tray drops, the performance of the circulating water pump and air conditioner will be affected, so the water level must also be maintained;

Water tower routine maintenance matters needing attention:

Circulating water is generally replaced once a month, or if there is a dirty phenomenon, it must be replaced. The replacement of circulating water is determined according to the solid concentration in the water. At the same time, the hot water pan and the cold water pan are cleaned. If there is dirt in the hot water pan, it will affect the cooling efficiency.

Water tower seasonal downtime maintenance matters needing attention:

a. Loosen the V-belt in the reducer, and fill the bearing with lubricating oil;

B The circulating water in the pipeline must be completely removed to avoid cracks caused by freezing in winter. The drain pipe of the cold water tray should be opened at any time so that rainwater and dissolved snow can flow out;

c. when the cooling tower is shut down for a period of time, it is necessary to check whether the motor insulation is normal or not. then refer to the instructions of the preparation items before operation for operation.

Maintenance Precautions

Fault

Reason

countermeasures

Temperature increase of cooling water

1 Excessive circulating water;

2 Uneven air volume;

3 hot air recirculation phenomenon

4 Insufficient air volume;

5 The heat sink is blocked;

6. Blocking of the water distribution pipe;

7 The air inlet network is blocked;

1 Adjust the water volume to the design standard;

2 improve the ventilation environment;

3 improve the ventilation environment;

4 Adjust the wind blade angle (within the rated current)

5 Remove the blockage of the heat sink;

6 remove dust and algae;

7 Remove the blockage of the air inlet network.

Too little cooling water

1. Blocking of the water distribution hole;

2 Filter screen is blocked;

3 The water level is too low;

4 cycle pump selection error;

1 Remove dust and algae;

2 Take out the filter screen and clean it;

3 Adjust the floating ball valve to the operating water level;

4 Replace the pump that is consistent with the design water volume;

Abnormal noise and vibration

1 The wind blade touches the inner wall of the wind trunk;

2. Improper installation of fan blades;

3 Unbalance of windmill;

4 reducer lubricating oil is too little;

5 Bearing failure;

1 Adjust the blade length;

2 Tighten the nut again;

3 Correct the blade angle;

4 Replenish the oil to the specified oil level;

5 Replace the bearing or shaft seal;

Motor overload

1 Pressure drop is too low;

2. The fan blade angle is inappropriate;

3 The air volume is too large;

4 Motor failure;

1 Check the power supply;

2 Adjust the angle of the fan blade;

3 Adjust the angle of the fan blade;

4 Replacement or repair;

Excessive splashing of water droplets

1 The loose water pipe rotates too fast;

2 The water level of the scattered water tank overflows;

3 The heat sink is blocked;

4 Failure of water deflector;

5 Excessive circulating water;

1 Adjust the angle of the water distribution pipe;

2. Revised the number of holes for scattering water;

3 Remove the blockage of the heat sink;

4 Replace the water baffle again;

5 reduce the amount of circulating water;

Requirements for circulating water quality (with water quality limit value)

Project

make-up water

Circulating water

PH(25℃)

6~8

6~8

Conductivity (uv/CM)

Below 200

Below 500

Full hardness (CaCO3) p p m

Below 50

Below 200

M Alkalinity (CaCO3) p p m

Below 50

Below 100

Chloride ion (CL) p p m

Below 50

Below 200

Sulfate ion (SO4) p p m

Below 50

Below 200

铁(Fe) p p m

Below 0.3

Below 1.0