The Basic Principle And Structure Of Cooling Tower

Principle and Basic Structure

1. The basic principle of cooling tower

A cooling tower is a device that uses the contact (direct or indirect) of air and water to cool water. It uses water as a circulating coolant, absorbs heat from a system and discharges it into the atmosphere, thereby reducing the temperature in the tower and making equipment that can be recycled for cooling water.

The heat dissipation relationship in the cooling tower:

In a wet cooling tower, the temperature of the hot water is high, and the temperature of the air flowing over the water surface is low. The water transfers heat to the air, which is carried away by the air and dissipated into the atmosphere. There are three forms of water dissipating heat to the air:

① Touch to dissipate heat;

② Evaporative heat dissipation;

③ Radiation heat dissipation.

The cooling tower mainly relies on the first two kinds of heat dissipation, and the radiation heat dissipation is very small, so it should not be ignored.

Evaporative heat dissipation principle:

Evaporative heat dissipation is accomplished through material exchange, that is, through the continuous 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 and become water vapor. As the water molecules with large energy escape, the water body near the water surface energy is reduced.

Therefore, the water temperature decreases, which is evaporation and heat dissipation. It is generally believed that the evaporated water molecules first form a thin layer of saturated air on the surface of the water, whose temperature is the same as the temperature of the water surface, and then the speed of the diffusion of water vapor from the saturated layer to the atmosphere depends on The difference between the water vapor pressure of the saturated layer and the water vapor pressure of the atmosphere, that is, Dolton's law, can be represented by the following figure.

2. The basic structure of cooling tower

✦ Brackets and towers: 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: recovers the water droplets carried away by the air flow

✦ Air Inlet: Cooling tower air inlet

✦ Water spray device: spray cooling water out

✦ Fan: supply air to the cooling tower

✦ Axial fans are used in induced draft cooling towers.

✦ Axial/Centrifugal fans are used in forced draft cooling towers.

✦ Cooling tower shutters: Average intake airflow; retains moisture in the tower.

Types and their pros and cons

1. Natural ventilation cooling tower

Hot air with less density flows out from the top of the cooling tower;

The denser cold air enters the cooling tower from the bottom of the tower to fill;

No fan required;

Concrete tower < 200 m;

For cooling of large heat.

3. Mechanical ventilation cooling tower

High-power fans force the heat exchange between air and circulating water;

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

There are many factors that determine cooling efficiency;

A variety of cooling capacity options;

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

Forced ventilation:

The air is blown into the vent by the centrifugal fan; Advantages: It is suitable for towers with large airflow resistance; the centrifugal fan has relatively low noise.

Counterflow Cooling Tower:

Cooling water is sprayed over the packing and flows down into the cooling water tank.

Air is forced in from the bottom, and in the packing, it contacts with water to evaporate part of the cooling water, thereby reducing the water temperature.

3. Induced draft cooling tower

Advantage

The degree of backflow is lower than that of forced draft cooling towers; the operating cost of fans is less than that of forced draft cooling towers.

disadvantage

The mechanical transmission of the fan and motor requires a waterproof design.

Hot water enters the cooling tower from the top

Air is forced induction by a fan and enters the cooling tower from the bottom; use a forced induction fan.

Cross Flow Induced Draft Cooling Tower

Counterflow Induced Draft Cooling Tower

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

Due to the hot water natural flow distribution system of this type of cooling tower:

Advantage:

Low water pump head;

Lower initial pump investment;

Lower annual operating energy consumption and costs;

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

Disadvantage:

The low 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 out;

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

Covers a large area.

Because of the pressurized water distribution sprinklers in such cooling towers:

Advantage:

By increasing the height of the tower to obtain a longer heat exchange process and a smaller cooling width;

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

Disadvantage:

The system water pump head increases;

Increased energy demand and increased operating costs;

The cooling water nozzle is not easy to maintain and clean;

A water distribution system and related piping are required, so the initial investment increases.

Operating parameters and selection design

1. Cooling water temperature difference

inlet temperature - outlet temperature

Large temperature difference = high performance

2. Cold width

The difference between the cooling tower outlet water temperature and the inlet air wet bulb temperature:

Small cooling range = high performance

4. Efficiency:

4. Cooling tower capacity

The unit of cooling tower capacity is "kcal per hour" or "cooling ton";

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

Large capacity = high performance

5. Calculation of make-up water

Evaporative loss of water (E)

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

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

Q stands for heat load (Kcal/h);

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

T1 represents the water temperature (°C);

T2 represents the water temperature (°C);

L represents the circulating water volume (kg/h).

Calculation of make-up water:

Splash loss (C)

The splash loss of the cooling tower is determined by the cooling tower design type, wind speed and other factors. Under normal circumstances, its value is about 0.1~0.2% of the circulating water volume.

Periodic discharge water loss (D)

The loss of regular discharge water is determined by factors such as water quality or solid concentration in the water. Generally, it is about 0.3% of the circulating water volume.

M=E+C+D

Evaporative water loss (E); splash water loss (C); periodic discharge water loss (D).

When the cooling tower is used for air conditioning, the temperature difference is designed to be 5°C. At this time, the water supply 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: The maximum cooling 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 cooling capacity of the compression refrigeration unit;

2.5 times the cooling capacity of absorption refrigeration units (lithium bromide).

1. Selection example

Example: A project with a 640RT unit cooling tower water flow and make-up.

Q=640RT=2251KW

K=1.3

C=4.2KJ/(kg·℃)

ΔT=5℃

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

2. Common design problems in cooling tower selection

(1) What are the determinants of cooling tower energy consumption?

A: Fan power, cooling water flow, cooling water replenishment amount?

(2) The temperature conditions of the cooling tower, at what temperature is the efficiency and economical good?

Answer: The inlet water temperature of the cooling tower varies according to the usage. For example, the outlet water temperature of the central air-conditioning condenser is generally 30-40°C, and the outlet water temperature of Guo Pengxue HVAC and cooling tower is generally 30°C. The ideal cooling temperature (return water temperature) of the cooling tower is 2-3°C higher than the wet bulb temperature. This value is called "approximation degree" (public account: pump housekeeper). The smaller the approximation degree, the better the cooling effect. Thai-Vietnamese economy.

(3) Comparison of open and closed

Open type: The investment in the first phase is relatively small, but the operating cost is relatively high (water consumption, power consumption).

Closed: This equipment is suitable for use in harsh environments such as drought, water shortage, and frequent sandstorms. The cooling medium can be multi-media such as water, oil, alcohol, quenching liquid, salt water and chemical liquid. The medium has no loss and stable composition. Low energy consumption.

Disadvantages: The cost of a closed cooling tower is three times that of an open tower.

Installation, piping, operation and common failures

1. The source of cooling tower noise

The cooling towers used above are all mechanical ventilation cooling towers. When they are running, the main sources of water tower noise are as follows:

(1) Fan noise:

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

(2) Motor noise:

Electromagnetic sound when its main motor is running;

(3) Ventilation noise:

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

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

2. Precautions for installation and piping

The ground bearing should refer to the operating weight of the cooling tower and the design installation factor to check the bearing capacity of the installation foundation.

Environmental conditions

1. The shortest distance between the air inlet end of the cooling tower and the adjacent buildings should not be less than 1.5 times the tower height.

2. It should not be installed in places with heat sources such as substations and boilers. Keep the top of the tower away from open flames.

3. It should not be installed in places where corrosive gases exist, such as beside chimneys and hot springs.

Installation Instructions

1. The foundation of the cooling tower should be pre-buried with horizontal steel plates according to the specified size. The height of each foundation surface should be on the same horizontal plane, the elevation error should be within 1mm, and the deviation center error should be within 2mm.

2. The tower body should be placed horizontally, and it should be based on the overall condition.

3. When installing the water tower, the installer should step on the reinforcing ribs of the chassis to avoid crushing the chassis. In addition, when installing the card shell, chassis and other fiber parts, the screws should be worn first, and then gradually tightened in order to avoid deformation of the shell and chassis. After confirming that the chassis is not deformed, and that the contact area and its vicinity are clean. When it is dry, the fiber blanket and rubbing resin can be supplemented at the joints to avoid water leakage during use.

Preparation before start

1. Open the drain valve of the water basin to clean the mud dust and dirt in the water basin. Flush the tower body parts.

2. Adjust the fan so that the angle of the fan blades is the same, and the clearance between the fan and the tower shell is uniform.

3. Check whether the running parts are flexible.

4. Adjust the float valve so that the water level of the basin is guaranteed to be 20cm below the overflow. 

Start up

Start the water pump intermittently to completely discharge the air in the circulating water pipe, and then start the fan.

1. When opening, check whether the air inlet and outlet environment are normal. Check whether the wind direction is upward when the fan is running.

2. Adjust the water flow to the normal water flow of the water tower.

3. Check that the operating voltage and current of each phase of the motor cannot exceed that shown on the motor nameplate.

4. The user's power supply circuit should have phase loss protection and overload protection measures.

Run check

The inside of the tower should be kept clean to prevent fouling and its algae formation. Maintain the circulating water volume to ensure the cooling load of the cooling tower. Regularly check the operating water level, cooling water temperature, motor voltage, motor current, vibration and noise value of the cooling tower in the water basin.

Something else

1. When the installation is completed, check whether there are tools and other items placed in the tower or the exhaust fan port in time.

2. Pay attention to check the piping and water pan for water leakage when starting up.

3. When the water supply source is lower than that of the cooling tower or the water pressure is not enough for water supply, an additional water pump or a higher water supply tank should be installed to supply water for filling.

4. When adjusting and installing, it is not allowed to step on the filler directly. If you need to step on it, you should temporarily pad the filler with a wooden board.

3. Operation precautions

Preparation before operation:

(1) Foreign objects on the side of the air inlet or around the wind carcass must be removed;

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

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

(4) The position of the V-belt pulley must be kept at the same level with each other;

(5) After the above inspection is completed, start the switch intermittently to check whether the windmill is running correctly? And whether there is abnormal noise and vibration?

(6) Clean the hot water pan and the sundries inside the tower body;

(7) Remove the dirt and foreign matter in the hot water pan, 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 pan are filled with circulating water;

(9) When the circulating water pump operates normally, the water level in the cold water pan will drop slightly, at this time, the float valve must be adjusted to a certain water level;

(10) Circuit system, re-confirm whether the circuit switch, fuse and wiring specifications match the motor load.

Precautions for starting the water tower:

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

(2) Check whether the running current of the windmill motor is overloaded? Avoid motor burnout or voltage drop;

(3) Use the control valve to adjust the water volume to keep the water level of the hot water pan between 30 and 50 mm;

(4) Check whether the running water level in the cold water pan remains normal.

Precautions during the operation of the water tower:

(1) After 5~6 days of operation, re-check whether the V-belt of the windmill reducer is normal? If it is loose, it can be properly locked again with the adjusting bolt;

(2) After a week of operation of the cooling tower, the circulating water must be replaced to remove the debris and dirt in the pipeline;

(3) 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 in the hot water pan;

(4) If the water level in the cold water pan drops, the performance of the circulating water pump and the air conditioner will be affected, so the water level must also be kept constant;

Water tower routine maintenance precautions:

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

Precautions for water tower seasonal shutdown and maintenance:

(1) Loosen the V-belt in the reducer and fill the bearing with lubricating oil;

(2) All the circulating water in the pipeline must be removed to avoid cracks caused by freezing in winter;

(3) The drain pipe of the cold water pan should be opened at any time so that rainwater and melted snow can flow out;

(4) The cooling tower restarts after a period of shutdown. At this time, it is necessary to check whether the motor insulation is normal? Then refer to the instructions for preparations before operation to operate.

3. Maintenance precautions

Fault

Reason

Countermeasures

Cooling water temperature rises

1 too much circulating water;

2 The air volume is uneven;

3 The phenomenon of hot air recirculation occurs

4 Insufficient air volume;

5 The heat sink is blocked;

6. The diffuser pipe is blocked;

7 The air inlet net is blocked;

1. Adjust the water volume to the design standard;

2 to improve the ventilation environment;

3 Improve the ventilation environment;

4 Adjust the wind blade angle (within rated current)

5 Clear the blockage of the heat sink;

6 Remove dirt and algae;

7 Clear the blockage of the air inlet net.

Too little cooling water

1 The diffuser hole is blocked;

2 The filter is blocked;

3 The water level is too low;

4 Circulation pump selection error;

1 Remove dirt and algae;

2 Take out the filter and clean it;

3 Adjust the float valve to the operating water level;

4. Replace the pump with the designed water volume;

Abnormal noise and vibration

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

2. Improper installation of fan blades;

3 The windmill is unbalanced;

4 Too little lubricating oil in the reducer;

5 Bearing failure;

1 Adjust the length of the fan blade;

2 Re-tighten the nut;

3 Correct the blade angle;

4 Add oil to the specified oil level;

5 Replace the bearing or shaft seal;

Motor overload

1 The pressure drop is too low;

2 The angle of the fan blade is not appropriate;

3 The air volume is too large;

4 motor failure;

1 Check the power supply;

2 Adjust the blade angle;

3 Adjust the angle of the fan blade;

4 Replace or send for repair;

Excessive splash of water droplets

1. The water distribution pipe rotates too fast;

2 The water level of the bulk water tank is too high and overflows;

3 The heat sink is blocked;

4 The water flap fails;

5 too much circulating water;

1 Adjust the angle of the diffuser pipe;

2 Change the number of apertures of the diffuser holes;

3 Clear the blockage of the heat sink;

4 Re-replace the water baffle;

5 Reduce the amount of circulating water;

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

Project

Make-up water

Recycled water

pH (25℃)

6~8

6~8

Conductivity (uv/CM)

Below 200

Below 500

Total hardness (CaCO3) ppm

Below 50

Below 200

MAlkalinity (CaCO3) ppm

Below 50

Below 100

Chloride (CL) ppm

Below 50

Below 200

Sulfate ion (SO42-) ppm

Below 50

Below 200

Iron (Fe) ppm

Below 0.3

1.0 or less