Overall Heat Transfer Coefficient

The overall heat transfer coefficient (U) is a measure of the rate at which heat is transferred through a material or system. It is a combination of the individual heat transfer coefficients of the different components involved in the heat transfer process.

Explanation

The overall heat transfer coefficient depends on various factors such as the nature of the fluids involved, their flow rates, the type of heat exchanger used, and the temperature difference between the hot and cold fluids.

In the given options, the lowest value of the overall heat transfer coefficient is associated with option 'C' - Air to heavy tars. This is because of the following reasons:

1. Nature of Fluid

Air to heavy tars involves the transfer of heat between air and heavy tars. Heavy tars are high-viscosity materials that have poor heat transfer properties. The low thermal conductivity and high viscosity of heavy tars result in a low overall heat transfer coefficient.

2. Flow Rate

The flow rate of the fluids also affects the overall heat transfer coefficient. In the case of air to heavy tars, the flow rate of heavy tars is typically low due to its high viscosity. This results in a lower overall heat transfer coefficient compared to other options where the flow rates may be higher.

3. Temperature Difference

The temperature difference between the hot and cold fluids plays a significant role in determining the overall heat transfer coefficient. In the case of air to heavy tars, the temperature difference may not be as large as in other options such as steam or ammonia condensers. A smaller temperature difference leads to a lower overall heat transfer coefficient.

Conclusion

In conclusion, the lowest value of the overall heat transfer coefficient is associated with option 'C' - Air to heavy tars. This is due to the poor heat transfer properties of heavy tars, the low flow rate, and the smaller temperature difference compared to other options.