The fouling coefficient tells you why you should choose a closed cooling tower

In the increasingly fierce competition in the current heat exchanger and evaporative cooling market, a problem has become increasingly prominent and should be given enough attention, that is, the fouling coefficient problem.

The fouling coefficient refers to the degree of influence of fouling on heat exchange during the use of the heat exchanger. Since the heat transfer surface of the heat exchanger itself has a large thermal conductivity, its thermal resistance can usually be ignored. However, if there is fouling on the wall, it will have a great impact on the heat transfer performance and pressure drop, and its thermal resistance can sometimes reach the order of magnitude of control. The effective heat transfer area of ​​a severely fouled cooler is only 1/2 of that in the clean state. Therefore, the influence of fouling thermal resistance on heat transfer performance must be considered in the design of the heat exchanger.

Calculation of heat transfer coefficient: In actual work, there are three sets of standards for the selection of fouling coefficient: one standard is the standard value clearly proposed by the user when designing the heat exchanger. Referring to the national standard, the fouling coefficient requirements are proposed for industrial water, circulating cooling water and clean tap water respectively; the second standard is proposed by the project technicians. Because they are worried that the heat transfer performance of the heat exchanger will not meet the requirements during operation, the fouling thermal resistance is greatly raised; the third is the reference value proposed by the closed cooling tower manufacturer. In the past closed cooling tower design, users generally do not mention the requirements of fouling thermal resistance. The influence of fouling thermal resistance is not considered in the heat exchanger design calculation process. Only when the heat transfer coefficient is taken at the end, a coefficient of 0.85 (i.e. 85% of the calculated value) is taken as the final heat transfer coefficient after considering the fouling thermal resistance.

To determine whether the actual selected fouling coefficient standard is reasonable, we make a calculation comparison based on commonly used cooling media and working conditions:

Working condition 1: air side heat transfer coefficient hk=65.5W/(m2.℃), water side heat transfer coefficient hl=7353W/(m2.℃), rib coefficient ψ=19.7, heat exchange tube wall thickness δ=0.001m, heat exchange tube thermal conductivity λ=39　W/(m2　.℃), air side fouling coefficient rk=0, water side fouling coefficient rl=0, calculate the heat transfer coefficient K of the heat exchanger, and substitute the values ​​to calculate.

Working condition 2: air side fouling coefficient rk=0, water side fouling coefficient rl=0.000172m2.℃/W (fouling coefficient taken when clean tap water), the rest of the conditions are the same as the settings, substitute them to calculate.

The three working conditions are as follows: the air side dirt coefficient rk=0.000172 m2.℃/W (normal pressure air), the water side dirt coefficient rl=0.000172 m2.℃/W (the dirt coefficient taken when using clean tap water), and the other conditions are the same as the settings, which can be substituted into the calculation.

Comparing working condition 1 and working condition 2, it is not difficult to find that the heat transfer coefficient after taking the dirt thermal resistance on the water side is 0.85 times the heat transfer coefficient when the dirt thermal resistance is not considered. Comparing working condition 1 and working condition 3, it can be seen that the heat transfer coefficient when both the gas side and the water side take the conventional dirt thermal resistance is 0.84 times the heat transfer coefficient when the dirt thermal resistance is not considered. This shows that the coefficient of 0.85 used in the past is appropriate. At the same time, it also shows that the dirt thermal resistance on the gas side has less influence on the heat transfer coefficient than the dirt thermal resistance on the water side in the area calculation of our closed cooling tower heat exchanger. The influence of the dirt thermal resistance on the gas side on the overall heat transfer coefficient can be ignored, that is, the dirt thermal resistance outside the tube has less influence than the dirt thermal resistance inside the tube. This shows that in our conventional design in the past, taking 0.85 times the heat transfer coefficient is appropriate and feasible, and it is also the simplest empirical method when considering the dirt thermal resistance.

The influence of dirt on the heat transfer of closed cooling towers: In recent years, with the rapid development of my country's closed cooling tower industry, the use conditions and customers of closed cooling towers have undergone fundamental changes. Users have put forward higher, stricter and more specific requirements for the design of closed cooling tower products, such as product pressure, area, volume and process medium. The most obvious point is that users have put forward clearer requirements for the dirt thermal resistance of water, and clearly stated that the dirt thermal resistance of water is 0.000344m2.℃/W (twice the original clean tap water, which is generally the minimum requirement of users), 0.0004m2.℃/W, and some even mentioned 0.0005m2.℃/W. The gas side is generally compressed air, and users generally do not clearly put forward requirements, but according to the book "Principles and Calculations of Heat Exchangers", it is clearly stipulated that its dirt thermal resistance is 0.000344m2.℃/W. From these data, we can see that the fouling coefficient is 2 or even 3 times that of conventional products. This will make people question whether the previous method of considering the fouling coefficient is applicable and appropriate.

Through more than ten years of accumulation, Jiangsu Huatal has obtained the following sets of data on the fouling coefficient, fouling thickness and energy consumption increase. The unit of fouling coefficient is m2.℃/kW; the unit of fouling thickness is mm; the unit of energy consumption increase is %, and the relevant data correspond to the following:

Data 1: 0.044m2.℃/kW 0.1mm 3.1%;

Data 2: 0.088m2.℃/kW 0.2mm 6.2%;

Data 3: 0.176m2.℃/kW 0.4mm 12.4%;

Data 4: 0.352m2.℃/kW 0.8mm 24.8%.

The above Huatal Cooling summarizes a few points for your reference. Jiangsu Huatal Cooling Technology Co., Ltd. focuses on the research and development, manufacturing and service of industrial cooling equipment. Its products include medium cooling, air cooling and hybrid combined cooling systems. With high-quality products and professional services, Huatal makes cooling systems more energy-saving, environmentally friendly, efficient, reliable, durable and convenient, and continuously creates the greatest value for customers.