How does the fouling coefficient affect the selection of closed cooling towers (Part 2)

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. When many design institutes and users choose heat exchangers, they are entangled in open cooling towers or closed cooling towers due to various reasons such as budget. Today, Huatal Cooling analyzes the impact of the fouling coefficient on heat exchange efficiency to facilitate everyone to choose a suitable cooling tower.

Condition 4, air side fouling coefficient rk=0, water side fouling coefficient rl=0.000344m2.℃/W, other conditions are the same as condition 1, substitute into calculation: compared with condition 1, the heat transfer coefficient of condition 4 is 0.73 times the set heat transfer coefficient;

Condition 5, air side fouling coefficient rk=0, water side fouling coefficient rl=0.0004m2.℃/W, other conditions are the same as condition 1, substitute into calculation: compared with condition 1, the heat transfer coefficient of condition 5 is 0.70 times the set heat transfer coefficient;

Condition 6, air side fouling coefficient rk=0, water side fouling coefficient rl=0.0005m2.℃/W, other conditions are the same as condition 1, substitute into calculation: Compared with working condition 1, the heat transfer coefficient of working condition 6 is 0.65 times that of working condition 1;

Condition 7, the fouling coefficient of the gas side rk=0.000344m2.℃/W, the fouling coefficient of the water side rl=0, the other conditions are the same as the working condition, substitute into the calculation: Compared with working condition 1, the heat transfer coefficient of working condition 7 is 0.98 times that of working condition 1;

Condition 8, the fouling coefficient of the gas side rk=0.000344m2.℃/W, the fouling coefficient of the water side rl=0.000344, the other conditions are the same as the working condition, substitute into the calculation: Compared with working condition 1, the heat transfer coefficient of working condition 8 is 0.72 times that of working condition 1; Compared with working condition 4, the heat transfer coefficient of working condition 8 is 0.99 times that of working condition 4.

Condition 9, the air side fouling coefficient rk=0.000344m2.℃/W, the water side fouling coefficient rl=0.0004, the other conditions are the same as the working condition, substitute into the calculation: Compared with working condition one, the heat transfer coefficient of working condition nine is 0.69 times that of working condition one; compared with working condition five, the heat transfer coefficient of working condition nine is 0.99 times that of working condition five. From the above comparison of the calculation of several combinations of fouling thermal resistance, it can be seen that when different fouling thermal resistances are taken on the water side, the influence on the heat transfer coefficient is different, and all exceed the original coefficient of 0.85. When the water side fouling coefficient rl=0.000344m2.℃/W, the coefficient becomes 0.73; when the water side fouling coefficient rl=0.0004 m2.℃/W, the coefficient becomes 0.70; when the water side fouling coefficient rl=0.0005m2.℃/W, the coefficient becomes 0.65; in short, the fouling thermal resistance on the water side greatly weakens the heat transfer performance. When the fouling coefficient on the gas side changes, the heat transfer coefficient does not change much, that is to say, the influence of the fouling thermal resistance on the gas side on the heat transfer coefficient can still be ignored.

Obviously, it is not applicable to the current design calculation method of taking the original 0.85 coefficient, but should be calculated according to the specific fouling coefficients in the design of the closed cooling tower heat exchanger. Therefore, in the design of heat exchangers, compensation measures must be considered to reduce heat transfer due to dirt thermal resistance, such as increasing flow rate, total average temperature difference or heat transfer area. In this way, how to consider the influence of dirt in the design of heat exchangers often becomes a key factor in the success or failure of closed cooling tower manufacturers.

We understand the influence of dirt on heat transfer. In the design of closed cooling tower heat exchangers, how large a dirt thermal resistance is reasonable? Some people think that it is more reliable to select a larger dirt thermal resistance. In fact, this often brings more serious consequences, because under the condition of a certain amount of heat transfer, it is necessary to increase the heat transfer area or the total average temperature difference, thereby increasing the cost of the cooling tower heat exchanger. However, if the heat transfer surface is too large, the outlet temperature of the hot fluid will be too low and the outlet temperature of the cold fluid will be too high, which not only affects the process requirements, but also sometimes in order to avoid this result, the medium flow rate is often reduced during operation, causing the wall temperature to rise, which in turn promotes the growth of scale more rapidly. Similarly, if the average temperature difference is too large, it is required to increase the temperature of the hot medium or reduce the temperature of the cooling medium, resulting in a larger temperature difference between the medium and the wall, which also results in the growth of scale, especially for the boiling and evaporating heating surface. If the temperature difference is too large, film boiling may occur, which not only greatly weakens the heat transfer and rapidly develops scale, but may even cause the heat transfer surface to overheat and be destroyed.