Cooling tower loss calculation

Working principle of cooling tower:

 

Cooling tower is a kind of equipment which USES the contact of water and air to disperse waste heat generated in industry or refrigeration and air conditioning through evaporation. The basic principle is: dry (low enthalpy) air after the fan's pumping, from the air network into the cooling tower; High temperature water molecules with high saturated steam pressure flow into the air with low pressure, and the water-seeding system with moist heat (high enthalpy) sprinkles into the tower. When the water droplets come into contact with air, on the one hand, the direct heat transfer between air and water; on the other hand, there is pressure difference between water surface and air. Under the action of pressure, evaporation occurs, which is carried to the present latent heat of evaporation. The heat of water is taken away by evaporation, so as to achieve the purpose of cooling.

 

Working process of cooling tower:

 

The working process of the circular counter-current cooling tower is taken as an example: the water circulating water is pressed into the water seeding system of the cooling tower by the water pump at certain pressure through pipes, horizontal pipes, curved pipes and central pipes, and the water is evenly spread on the filler through the holes in the water seeding pipe. When hot water flows through the packing surface, water film and air are formed for heat exchange. Hot air with high humidity and high han value is drawn from the top, and cooling water drops are put into the bottom basin, and then flow into the main engine through the outlet pipe. In general, into the tower air is dry, low wet bulb temperature of air, water and air obviously exists between the concentration of water molecules and the kinetic pressure difference, when the fan operation, under the action of static pressure inside the tower, the water molecules to evaporate in the air, to become water vapor molecules, the average kinetic energy of the rest of the water molecules will be reduced, thus make the temperature of the circulating water drop. From the above analysis, it can be seen that the evaporation cooling has nothing to do with the temperature of the air (usually dry bulb temperature) lower or higher than the water temperature. As long as the water molecules can continuously evaporate into the air, the water temperature will decrease. But the evaporation of water into the air will not go on forever. When the air in contact with water is unsaturated, water molecules continue to evaporate into the air, but when the air on the contact surface is saturated, the water molecules do not evaporate, but are in a state of dynamic equilibrium. The number of water molecules that evaporate is equal to the number of water molecules that return from the air to the water, and the water temperature remains the same. It can be seen that the drier the air in contact with water, the easier it is to evaporate, and the lower the water temperature.

 

Classification of cooling towers:

 

There are natural ventilation cooling tower, mechanical ventilation cooling tower and mixed ventilation cooling tower.

 

According to the contact way of hot water and air, there are wet cooling tower, dry cooling tower and dry and wet cooling tower.

 

Flow direction of hot water and air can be divided into reverse flow cooling tower, cross flow (ac) cooling tower and mixed flow cooling tower.

 

According to the shape, there are round cooling towers, square cooling towers and rectangular cooling towers.

 

According to the cooling temperature, there are standard cooling tower, medium temperature cooling tower and high temperature cooling tower.

 

6. Classified into common type cooling tower, low noise type cooling tower, ultra-low noise type cooling tower and ultra-quiet type cooling tower according to noise level.

 

There are special cooling towers for plastic machines, special cooling towers for generators, special cooling towers for medium frequency furnaces, central air-conditioning cooling towers and cooling towers for power plants.

 

Other cooling tower with jet flow, fan free cooling tower, hyperbolic cooling tower, etc.

 

Cooling water refill problem

 

Cooling tower water loss includes three parts: evaporation loss, wind blow loss and discharge loss, namely:

 

Qm=Qe+ Qw+Qb :Qm is water loss of cooling tower; Qe is water loss of swallow-hair; Qw refers to wind volume loss; Qb refers to the loss of sewage water.

 

(1) evaporation loss

 

 

Qe = (0.001 + 0.00002 theta) Δ t Q (1)

 

 

Where, Qe is the loss of evaporation; Δ t for cooling tower in and out of the water temperature difference; Q is the circulating water quantity; Is the air's dry bulb temperature.

 

 

 

(2) the amount of water lost by wind

 

 

For the mechanical ventilation cooling tower with water remover, the air loss is

 

've = (0.2% ~ 0.3%), Q (2)

 

 

(3) loss of sewage discharge and leakage

 

The loss is a relatively mechanical one, which is related to the water quality requirements of cyclic cooling, treatment methods, water quality of supplementary water and concentration ratio of circulating water. Calculation formula of concentration ratio:

 

N = Cr/Cm

 

 

Where :N is the concentration multiple; Cr is the salt content of the recirculated cooling water. Cm is the salt content of supplementary water.

 

According to the salinity balance of the circulating cooling water system, the salinity added by the water into the system should be equal to the salinity taken away by the sewage discharge, wind blowing and osmotic water.

 

 

According to the salinity balance of the circulating cooling water system, the salinity added by the water into the system should be equal to the salinity taken away by the sewage discharge, wind blowing and osmotic water.

 

 

QmCm = ('ve + Qb) Cr

 

 

N =Cr/Cm=Qm/(Qw+Qb)=(Qe+ Qw+Qb)/(Qw+Qb) (3)

 

 

 

Qm = QeN/(N 1)

 

The concentration multiple is the ratio of the salt content of the supplementary water to that of the cooled water after concentration. The recommended N value in the building water supply and drainage design manual is generally no more than 5 ~ 6. If N value is too large, sewage discharge and leakage loss is large, water waste will be inevitable. If N value is too small, water replenishment is small, and the concentration of cooling water is large, system dirt and corrosion will be caused.

 

The amount of evaporation can be calculated from equation (1), and then the amount of lost water from wind blowing can be calculated from equation (2). Finally, the amount of lost water from sewage discharge and leakage can be calculated from equation (3).

 

Example: the cooling water circulation of the cold engine is 570M3/h, and the cooling system of the circular counter-current cooling tower is adopted.

 

Solution: loss of cooling tower:

 

1. Evaporation loss water amount: the supplementary water amount of the cooling tower is preliminarily determined, which can be calculated according to the following formula

 

Qe = (0.001+0.00002X) tQ =KtQ

 

Qe-- -- water lost by evaporation; T -- water temperature difference in and out of the cooling tower; Q--- circulating water quantity; X--- wet-bulb temperature (28.6 degrees)

 

2. Blown water loss: 0.2-0.3%

 

3. Discharge loss water quantity: Qb

 

Total water supply: Qm=Qe+Qw+Qb Qm=Qe x N/(n-1)

 

Concentration multiple: N=3

 

Qm = (0.001 + 0.00002 x 28.6) x 3 x 5 Q/(3, 1) = 0.012 Q = 1.2% Q = 6.8 m3 / h.

 

How to determine the concentration ratio? (usually 3-6)