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Consider a heat exchanger in which both fluids have the same specific heats but different mass flow rates. Which fluid will experience a larger temperature change: the one with the lower or higher mass flow rate?

Short Answer

Expert verified
Answer: The fluid with a lower mass flow rate will experience a larger temperature change.

Step by step solution

01

Review heat transfer relations for a heat exchanger

The heat transfer between two fluids in a heat exchanger can be described using the following equation: Q = m1 * c1 * (T1_out - T1_in) = m2 * c2 * (T2_out - T2_in), where Q is the heat transfer, m1 and m2 are the mass flow rates of each fluid, c1 and c2 are the specific heats of each fluid, and T1_out, T1_in, T2_out, T2_in are the inlet and outlet temperatures of each fluid. Since both fluids have the same specific heats, we can simplify the equation as: Q = m1 * c * (T1_out - T1_in) = m2 * c * (T2_out - T2_in).
02

Determine the temperature change for each fluid

Now we will determine which fluid will experience a larger temperature change depending on their mass flow rates. To do so, we'll examine how the temperature difference in each fluid relates to their mass flow rates. Divide both sides of the equation by their respective mass flow rates and specific heats: (T1_out - T1_in) / m1 = (T2_out - T2_in) / m2. Let ΔT1 = T1_out - T1_in and ΔT2 = T2_out - T2_in be the temperature changes of each fluid. Then we have: ΔT1 / m1 = ΔT2 / m2.
03

Analyze the relationship between mass flow rates and temperature changes

Since we are given that the fluids have different mass flow rates (m1 ≠ m2), we now study the relationship between the mass flow rates and the corresponding temperature changes. From the equation we derived in Step 2, suppose m1 > m2, then from ΔT1 / m1 = ΔT2 / m2, it follows that ΔT1 < ΔT2. This means that if Fluid 1 has a higher mass flow rate, it will experience a smaller temperature change compared to Fluid 2. On the other hand, if m1 < m2, then from the same equation, we would have ΔT1 > ΔT2. This means that if Fluid 1 has a lower mass flow rate, it will experience a larger temperature change compared to Fluid 2.
04

Conclusion

The fluid with a lower mass flow rate will experience a larger temperature change in the heat exchanger, while the fluid with a higher mass flow rate will experience a smaller temperature change.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Specific Heat
In the world of heat exchangers, understanding specific heat is essential. Specific heat (\( c \)) is a property of a material that measures the amount of heat needed to change its temperature. Think of it as the thermal capacity of a fluid. In a heat exchanger scenario, if two fluids have the same specific heat, they can absorb and release heat at the same rate per unit mass. This property helps balance out heat dynamics between the fluids. Specific heat is typically measured in joules per gram per degree Celsius (J/g°C).
A higher specific heat means more energy is needed to change the temperature, while a lower specific heat requires less energy.
When both fluids have the same specific heat, like in our exercise, it simplifies the problem as this factor cancels out when analyzing the heat exchange process.
Mass Flow Rate
Mass flow rate (\( m \)) is all about how much fluid moves through the heat exchanger in a given period of time. It's like the 'speed' of the fluid in the heat exchange process and is generally expressed in kilograms per second (kg/s).
The mass flow rate is crucial in determining how effectively a heat exchanger can transfer heat between two fluids. In our exercise, two fluids with differing mass flow rates can experience different temperature changes even when they have the same specific heat.
  • A higher mass flow rate means more fluid is moving through, keeping the temperature change smaller.
  • A lower mass flow rate results in a larger temperature change, as less fluid can absorb or release the heat energy.
This property of mass flow rate is central to solving which fluid experiences a greater temperature change.
Temperature Change
The temperature change (\( \Delta T \)) is the difference in temperature before and after the fluid passes through the heat exchanger. Defined mathematically as \( \Delta T = T_{out} - T_{in} \), it tells us how much heat is absorbed or released by the fluid.
In the exercise scenario, temperature change is directly influenced by the mass flow rate of the fluid. When two fluids have the same specific heat but different mass flow rates, the one with the lower mass flow rate will have a larger temperature change. This happens because less mass means less heat capacity, leading to a more significant change in temperature as it absorbs or loses heat.
Understanding the temperature change is vital as it highlights the effectiveness of heat transfer and the operational efficiency of the heat exchanger.
Heat Transfer Equation
The heat transfer equation is fundamental for calculating heat exchange between two fluids. It's a tool that ties together specific heat, mass flow rate, and temperature change. The equation is expressed as:
\[ Q = m \cdot c \cdot \Delta T \]Where \( Q \) is the heat transfer, \( m \) is the mass flow rate, \( c \) is the specific heat, and \( \Delta T \) is the temperature change.
In our specific heat exchanger scenario, since both fluids have the same specific heat, the equation simplifies and highlights the relationship between mass flow rate and temperature change without the influence of specific heat differences.
This equation shows us that by manipulating factors like mass flow rate, we can predict and control temperature changes, effectively managing the heat transfer process within a heat exchanger. Understanding this balance is crucial for designing efficient systems to maintain desired thermal conditions.

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Most popular questions from this chapter

Write an essay on the static and dynamic types of regenerative heat exchangers and compile information about the manufacturers of such heat exchangers. Choose a few models by different manufacturers and compare their costs and performance.

Cold water \(\left(c_{p}=4180 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) leading to a shower enters a thin-walled double-pipe counter-flow heat exchanger at \(15^{\circ} \mathrm{C}\) at a rate of \(1.25 \mathrm{~kg} / \mathrm{s}\) and is heated to \(45^{\circ} \mathrm{C}\) by hot water \(\left(c_{p}=4190 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) that enters at \(100^{\circ} \mathrm{C}\) at a rate of \(3 \mathrm{~kg} / \mathrm{s}\). If the overall heat transfer coefficient is \(880 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), determine the rate of heat transfer and the heat transfer surface area of the heat exchanger.

A shell-and-tube heat exchanger is used for heating \(10 \mathrm{~kg} / \mathrm{s}\) of oil \(\left(c_{p}=2.0 \mathrm{~kJ} / \mathrm{kg} \cdot \mathrm{K}\right)\) from \(25^{\circ} \mathrm{C}\) to \(46^{\circ} \mathrm{C}\). The heat exchanger has 1 -shell pass and 6-tube passes. Water enters the shell side at \(80^{\circ} \mathrm{C}\) and leaves at \(60^{\circ} \mathrm{C}\). The overall heat transfer coefficient is estimated to be \(1000 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). Calculate the rate of heat transfer and the heat transfer area.

Consider a shell and tube heat exchanger in a milk be heated from \(20^{\circ} \mathrm{C}\) by hot water initially at \(140^{\circ} \mathrm{C}\) and flowing at a rate of \(5 \mathrm{~kg} / \mathrm{s}\). The milk flows through 30 thin-walled tubes with an inside diameter of \(20 \mathrm{~mm}\) with each tube making 10 passes through the shell. The average convective heat transfer coefficients on the milk and water side are \(450 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) and \(1100 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), respectively. In order to complete the pasteurizing process and hence restrict the microbial growth in the milk, it is required to have the exit temperature of milk attain at least \(70^{\circ} \mathrm{C}\). As a design engineer, your job is to decide upon the shell width (tube length in each pass) so that the milk exit temperature of \(70^{\circ} \mathrm{C}\) can be achieved. One of the design requirements is that the exit temperature of hot water should be at least \(10^{\circ} \mathrm{C}\) higher than the exit temperature of milk.

Steam is to be condensed on the shell side of a 1 -shellpass and 8-tube-passes condenser, with 50 tubes in each pass, at \(30^{\circ} \mathrm{C}\left(h_{f g}=2431 \mathrm{~kJ} / \mathrm{kg}\right)\). Cooling water \(\left(c_{p}=4180 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) enters the tubes at \(15^{\circ} \mathrm{C}\) at a rate of \(1800 \mathrm{~kg} / \mathrm{h}\). The tubes are thin-walled, and have a diameter of \(1.5 \mathrm{~cm}\) and length of \(2 \mathrm{~m}\) per pass. If the overall heat transfer coefficient is \(3000 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), determine \((a)\) the rate of heat transfer and \((b)\) the rate of condensation of steam.

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