Question

Crude oil at \(22^{\circ} \mathrm{C}\) enters a 20 -cm-diameter pipe with an average velocity of \(20 \mathrm{~cm} / \mathrm{s}\). The average pipe wall temperature is \(2^{\circ} \mathrm{C}\). Crude oil properties are as given below. Calculate the rate of heat transfer and pipe length if the crude oil outlet temperature is \(20^{\circ} \mathrm{C}\). $$ \begin{array}{lcccc} \hline T & \rho & k & \mu & c_{p} \\ { }^{\circ} \mathrm{C} & \mathrm{kg} / \mathrm{m}^{3} & \mathrm{~W} / \mathrm{m} \cdot \mathrm{K} & \mathrm{mPa} \cdot \mathrm{s} & \mathrm{kJ} / \mathrm{kg} \cdot \mathrm{K} \\ \hline 2.0 & 900 & 0.145 & 60.0 & 1.80 \\ 22.0 & 890 & 0.145 & 20.0 & 1.90 \\ \hline \end{array} $$

Short answer

The rate of heat transfer for the crude oil is -21.28 kJ/s, which means the heat flows out of the crude oil. The required pipe length to achieve the desired temperature change is approximately 53.58 meters.

Step-by-step solution

Calculate mass flow rate

First, we need to find the mass flow rate of the crude oil. We can use the equation: \(\dot{m} = \rho \cdot A \cdot v\) where \(\dot{m}\) is the mass flow rate, \(\rho\) is the density of crude oil, \(A\) is the cross-sectional area of the pipe, and \(v\) is the average velocity of crude oil. Using the given average velocity \(v = 20\) cm/s (converting to m/s: \(v = 0.2\) m/s), diameter of the pipe \(D = 20\) cm (converting to meters: \(D = 0.2\) m), and density of crude oil at \(22^{\circ} \mathrm{C}\), which is \(\rho = 890 \,\mathrm{kg} / \mathrm{m}^{3}\), we can calculate the mass flow rate. Cross-sectional area of the pipe is: \(A = \frac{\pi D^2}{4} = \frac{\pi (0.2)^2}{4} = 0.0314 \,\mathrm{m^2}\) Mass flow rate: \(\dot{m} = 890 \,\mathrm{kg/m^3} \cdot 0.0314 \,\mathrm{m^2} \cdot 0.2 \,\mathrm{m/s} = 5.6 \,\mathrm{kg/s}\)

Calculate the enthalpy change

Next, we'll calculate the enthalpy change for the crude oil, which can be found using the equation: \(\Delta H = \dot{m} \cdot c_p \cdot \Delta T\) Where \(\Delta H\) is the enthalpy change, \(c_p\) is the specific heat capacity of crude oil, and \(\Delta T\) is the temperature change. Given the specific heat capacity at \(22^{\circ} \mathrm{C}\) is \(c_p = 1.9 \,\mathrm{kJ/kg \cdot K}\), and the temperature change is \(\Delta T = 20 - 22 = -2\,^{\circ} \mathrm{C}\). The enthalpy change is: \(\Delta H = 5.6 \,\mathrm{kg/s} \cdot 1.9 \,\mathrm{kJ/kg \cdot K} \cdot (-2\,^{\circ} \mathrm{C}) = -21.28 \,\mathrm{kJ/s}\)

Calculate heat transfer rate

Now we will find the rate of heat transfer, which is the same as the enthalpy change for this problem. The heat transfer rate (\(\dot{Q}\)) is thus: \(\dot{Q} = -21.28 \,\mathrm{kJ/s}\) The negative sign indicates that the heat flows out of the crude oil.

Calculate the required pipe length

To calculate the pipe length, we will use the equation for heat transfer due to convection: \(\dot{Q} = h A \Delta T\) where \(h\) is the heat transfer coefficient, \(A\) is the surface area, and \(\Delta T\) is the temperature difference between the crude oil and the wall of the pipe. We can further simplify this equation as follows: \(L = \frac{\dot{Q}}{h \cdot \pi \cdot D \cdot \Delta T}\) where \(L\) is the pipe length, and \(D\) is the diameter of the pipe. As we are not given the heat transfer coefficient, we will assume a value for it. Let's assume \(h = 100 \,\mathrm{W/(m^2 \cdot K)}\). The temperature difference between the crude oil and the wall is \(\Delta T = 22 - 2 = 20\,^{\circ} \mathrm{C}\). Now, we can calculate the pipe length: \(L = \frac{-21.28\, \mathrm{kJ/s}}{100 \,\mathrm{W/(m^2 \cdot K)} \cdot \pi \cdot 0.2\, \mathrm{m} \cdot 20\,^{\circ} \mathrm{C}}\) Converting \(\dot{Q}\) from \(\mathrm{kJ/s}\) to \(\mathrm{W}\), we have \(\dot{Q} = -21280\, \mathrm{W}\). \(L = \frac{-21280 \,\mathrm{W}}{100 \,\mathrm{W/(m^2 \cdot K)} \cdot \pi \cdot 0.2\, \mathrm{m} \cdot 20\,^{\circ} \mathrm{C}} = 53.58\,\mathrm{m}\) The required pipe length is approximately 53.58 meters.

Key concepts

Mass Flow Rate Calculation

The mass flow rate, often denoted as \(\dot{m}\), is a fundamental concept in thermodynamics and fluid dynamics, representing the amount of mass passing through a given surface per unit time. It's critical to calculating various processes in mechanical and chemical engineering, such as the design of heat exchangers or determining the performance of a heating system.

In the context of heat transfer in pipes, the mass flow rate can be understood through the equation \(\dot{m} = \rho \cdot A \cdot v\), where \(\rho\) is the fluid density, \(A\) is the cross-sectional area of the pipe, and \(v\) is the fluid velocity. Calculating the mass flow rate accurately is crucial for determining the rate of heat transfer in the system.

For example, given a fluid velocity (\(v\)), pipe diameter (\(D\)), and density of fluid (\(\rho\)) at a specific temperature, we can calculate the cross-sectional area (\(A\)) of the pipe as \(A = \frac{\pi D^2}{4}\). Once these parameters are known, the mass flow rate can be easily calculated, providing us with key information needed to evaluate how effectively a fluid can carry or remove heat from a system.

Enthalpy Change

Enthalpy change (\(\Delta H\)) is another fundamental concept in thermodynamics that describes the total heat content change within a system during a process. It's very important in understanding how heat is transferred within fluids as they undergo temperature changes. The enthalpy change can be determined by the equation \(\Delta H = \dot{m} \cdot c_p \cdot \Delta T\), where \(\dot{m}\) is the mass flow rate, \(c_p\) is the specific heat capacity of the fluid, and \(\Delta T\) is the temperature difference between the inlet and outlet of the pipe.

For convective heat transfer in pipes, the enthalpy change will inform us how much heat is lost or gained by the fluid between the entry and exit points. If the fluid cools down, the enthalpy change will be negative, indicating that heat is removed from the fluid as it travels through the pipe. Conversely, a positive enthalpy change signifies heat gain by the fluid. Understanding enthalpy change is essential in designing systems where temperature regulation of the fluid is key to the process efficiency.

Convection Heat Transfer

Convection heat transfer is a mechanism that involves the movement of molecules within fluids (liquids and gases) to transfer heat. It is one of the primary modes of heat transfer in systems where fluids are in motion, such as in the scenario of heat transfer in pipes. The rate at which heat is transferred by convection can be expressed by the equation \(\dot{Q} = h A \Delta T\), where \(\dot{Q}\) is the heat transfer rate, \(h\) is the convective heat transfer coefficient, \(A\) is the surface area through which heat is transferred, and \(\Delta T\) is the temperature difference driving the heat transfer.

In our example with the crude oil in a pipe, the temperature difference between the pipe's wall and the oil initiates heat transfer through the process of convection. The convective heat transfer coefficient (\(h\)), which can vary based on fluid properties and flow conditions, plays a critical role in determining how efficiently heat can be transferred. Accurately estimating \(h\) is often a complex task and may require empirical correlations or experimentation. Ultimately, the pipe length required to achieve a desired temperature change can be derived by manipulating the convection heat transfer equation, highlighting the interdependent relationship between the flow properties, heat transfer rate, and physical dimensions of the system.