Question

How is the thermal resistance due to fouling in a heat exchanger accounted for? How do the fluid velocity and temperature affect fouling?

Short answer

Answer: Thermal resistance due to fouling refers to the additional resistance to heat transfer that occurs due to the accumulation of undesirable materials on heat exchanger surfaces. Fluid velocity and temperature play crucial roles in fouling formation. Higher fluid velocities can help reduce fouling by increasing turbulence and scouring the surfaces, while fluid temperature can have varying effects on fouling rates depending on fluid properties and type of fouling mechanisms involved. Engineers use fouling factors to account for this resistance, allowing for better prediction and control of heat transfer performance.

Step-by-step solution

Understand thermal resistance due to fouling

Thermal resistance due to fouling is the additional resistance to heat transfer, introduced by the accumulation of undesirable materials on heat exchanger surfaces. Fouling can be caused by scaling, deposition of particles, or the growth of biological organisms in the heat exchanger. The presence of fouling increases the overall thermal resistance, which in turn reduces the heat exchanger's efficiency.

Accounting for thermal resistance due to fouling

To account for thermal resistance due to fouling, engineers introduce a fouling factor, also known as the fouling resistance, into the calculation of the overall heat transfer coefficient (U) in a heat exchanger. The modified formula for U becomes: U = 1 / [(1/ h_1) + (1/ h_2) + R_f1 + R_f2] where h_1 and h_2 are the heat transfer coefficients for the two fluids, and R_f1 and R_f2 are the fouling resistances for the two fluids, respectively. The fouling resistance, expressed in square meter kelvin per Watt (m²K/W), is determined based on experimental data and depends on the type of fluids, operating conditions, and the design of the heat exchanger.

Fluid velocity and its effect on fouling

Fluid velocity plays an important role in the fouling process. Higher fluid velocities usually result in lower fouling rates in the heat exchanger. At high velocities, the fluid turbulence can help to prevent particle deposition and scour the surfaces, reducing the formation of fouling layers. However, if the fluid velocity is too high, it may cause erosion of the heat exchanger surfaces or excessive vibrations, which can damage the equipment.

Fluid temperature and its effect on fouling

The temperature of the fluid can also influence fouling rates. In general, an increase in temperature can accelerate fouling processes, such as chemical reaction or precipitation. However, in some cases, higher temperatures can reduce fouling by increasing the solubility of certain substances or by promoting an autocatalytic cleaning process. The effect of temperature on fouling depends on the specific properties of the fluid and the type of fouling mechanisms involved. In conclusion, thermal resistance due to fouling is an important consideration when designing and operating heat exchangers. It can significantly impact the efficiency of the heat exchanger, and can be affected by several factors, including the fluid velocity and temperature. Engineers use fouling factors to account for this resistance, allowing for better prediction and control of heat transfer performance.