Heat Exchanger Design Fundamentals

Study Notes

Heat Exchanger Design

Heat Duty

Heat exchangers are essential components in many industrial processes, responsible for efficiently transferring heat from one medium to another without mixing them. One crucial aspect of designing a heat exchanger is determining the heat duty, which refers to the quantity of energy transferred per unit time from one medium to another. Heat duty is calculated using the formula:

q = m * c \* (t\_hot - t\_cold)

where q stands for heat duty, m denotes mass flow, c signifies specific heat capacity, t\_hot represents hot fluid's temperature, and t\_cold indicates cold fluid's temperature.

Temperature Difference

Another vital aspect of heat exchanger design is managing the temperature difference between the hot and cold fluids. This difference is known as the logmean temperature difference (ΔT_lmtd) and is typically measured between the hot fluid's inlet and the cold fluid's outlet. The hot fluid enters the heat exchanger at a higher temperature than the cold fluid, allowing for effective heat transfer to occur. Efficient heat exchanger designs aim to minimize ΔT_lmtd while maintaining acceptable performance levels.

Overall Heat Transfer Area

The overall heat transfer area of a heat exchanger determines the effectiveness of the device in transferring heat. It is calculated by multiplying the individual heat transfer coefficients of both sides of the heat exchanger by the corresponding temperature differences and then summing these products. The equation for overall heat transfer area is given by:

U \* A = q / (T\_hot - T\_cold)

where U represents the overall heat transfer coefficient and A stands for the total heat transfer area.

Heat Transfer Area

Heat exchangers are designed with specific heat transfer areas to maximize heat transfer efficiency while minimizing energy consumption. The heat transfer area refers to the surface area through which heat exchange occurs between two fluids or solid bodies. The value of the heat transfer area depends on factors such as the type of heat exchanger used, its design, and the flow rates of the involved fluids.

Nusselt Number

The Nusselt number (Nu) is an important dimensionless parameter that describes the effectiveness of convective heat transfer from a fluid flow over a heated surface. It relates the heat transfer coefficient (h), characteristic length scale (L), and kinematic viscosity (ν). The Nusselt number can be calculated using the formula:

Nu = h \* L / ν

High values of Nu indicate efficient heat transfer, while low values suggest poor performance.

Reynolds Number

The Reynolds number (Re) is another dimensionless quantity that characterizes the nature of fluid flow around a body. It measures the ratio of inertial forces to viscous forces within a fluid. Re is defined as:

Re = \rho \* v \* L / \mu

where ρ indicates the fluid density, v represents fluid velocity, L denotes the length scale, and μ specifies the fluid's dynamic viscosity.

Prandtl Number

The Prandtl number (Pr) compares the rate of momentum transfer to the rate of heat transfer in a fluid. It is given by:

Pr = c\_p \* μ / k

The parameters include the specific heat capacity at constant pressure (cp), the dynamic viscosity (μ), and thermal conductivity (k) of a fluid. High values of Pr suggest larger differences in temperature compared to other factors affecting fluid behavior, while low values imply similarities between these factors.

In conclusion, the design of heat exchangers requires careful consideration of several key factors, including heat duty, temperature difference, overall heat transfer area, heat transfer area, Nusselt number, Reynolds number, and Prandtl number. Proper understanding of these concepts enables engineers to develop efficient heat exchangers tailored to various industrial applications.

Description

Explore the essential principles of designing heat exchangers, such as heat duty calculations, temperature difference management, and overall heat transfer area computations. Learn about key parameters like Nusselt number, Reynolds number, and Prandtl number that impact heat exchanger efficiency.