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Chapter 11: Problem 128

There are two heat exchangers that can meet the heat transfer requirements of a facility. One is smaller and cheaper but requires a larger pump, while the other is larger and more expensive but has a smaller pressure drop and thus requires a smaller pump. Both heat exchangers have the same life expectancy and meet all other requirements. Explain which heat exchanger you would choose and under what conditions.

Short Answer

Expert verified
Answer: When comparing heat exchanger options, the factors to consider include initial costs, energy consumption over time, specific facility conditions, and requirements, such as noise level and space limitations.

Step by step solution

01

Evaluate the cost of each heat exchanger option

Determine the total initial cost of both heat exchanger options, including the cost of each heat exchanger and their respective required pumps. Let's denote the total cost of smaller and cheaper heat exchanger (Option A) as C_A and the total cost of the larger and more expensive heat exchanger (Option B) as C_B.
02

Compare Initial Costs

Analyze whether C_A < C_B or C_A > C_B. If the initial cost of Option A is cheaper than Option B, then you might choose Option A. Conversely, if Option B is cheaper upfront, you might choose Option B.
03

Consider Energy Costs

Calculate the energy consumption for each heat exchanger option over their life expectancy, factoring in their respective pump sizes and pressure drops. Denote these values as E_A and E_B for Options A and B, respectively.
04

Compare Energy Costs

Analyze whether E_A < E_B or E_A > E_B. If energy consumption for Option A is lower than Option B, then it might be more beneficial to choose Option A in the long run, regardless of their initial costs. Conversely, if Option B consumes less energy over time, then it might be the better choice despite being more expensive upfront.
05

Assess Facility Conditions and Requirements

It is essential to consider any specific facility conditions or requirements that may impact the overall efficiency of the chosen heat exchanger. For example, if the facility requires a low operating noise level, you may prioritize the heat exchanger with the smaller pump, which may produce less noise. Similarly, if space is limited within the facility, the smaller heat exchanger may be the better choice to conserve space.
06

Make the Final Decision

Based on the comparisons and analyses made in the previous steps, choose the heat exchanger that best suits the facility's requirements, economic preferences, and operating conditions.

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Most popular questions from this chapter

Consider a closed loop heat exchanger that carries exit water \(\left(c_{p}=1 \mathrm{Btu} / \mathrm{lbm} \cdot{ }^{\circ} \mathrm{F}\right.\) and \(\left.\rho=62.4 \mathrm{lbm} / \mathrm{ft}^{3}\right)\) of a condenser side initially at \(100^{\circ} \mathrm{F}\). The water flows through a \(500 \mathrm{ft}\) long stainless steel pipe of 1 in inner diameter immersed in a large lake. The temperature of lake water surrounding the heat exchanger is \(45^{\circ} \mathrm{F}\). The overall heat transfer coefficient of the heat exchanger is estimated to be \(250 \mathrm{Btu} / \mathrm{h} \cdot \mathrm{ft}^{2}{ }^{\circ} \mathrm{F}\). What is the exit temperature of the water from the immersed heat exchanger if it flows through the pipe at an average velocity of \(9 \mathrm{ft} / \mathrm{s}\) ? Use \(\varepsilon-\mathrm{NTU}\) method for analysis.

Saturated water vapor at \(100^{\circ} \mathrm{C}\) condenses in a 1 -shell and 2-tube heat exchanger with a surface area of \(0.5 \mathrm{~m}^{2}\) and an overall heat transfer coefficient of \(2000 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). Cold water \(\left(c_{p c}=4179 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) flowing at \(0.5 \mathrm{~kg} / \mathrm{s}\) enters the tube side at \(15^{\circ} \mathrm{C}\), determine \((a)\) the heat transfer effectiveness, \((b)\) the outlet temperature of the cold water, and \((c)\) the heat transfer rate for the heat exchanger.

Oil in an engine is being cooled by air in a cross-flow heat exchanger, where both fluids are unmixed. Oil \(\left(c_{p h}=\right.\) \(2047 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\) ) flowing with a flow rate of \(0.026 \mathrm{~kg} / \mathrm{s}\) enters the heat exchanger at \(75^{\circ} \mathrm{C}\), while air \(\left(c_{p c}=1007 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) enters at \(30^{\circ} \mathrm{C}\) with a flow rate of \(0.21 \mathrm{~kg} / \mathrm{s}\). The overall heat transfer coefficient of the heat exchanger is \(53 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) and the total surface area is \(1 \mathrm{~m}^{2}\). Determine \((a)\) the heat transfer effectiveness and \((b)\) the outlet temperature of the oil.

Cold water \(\left(c_{p}=4.18 \mathrm{~kJ} / \mathrm{kg} \cdot \mathrm{K}\right)\) enters a cross-flow heat exchanger at \(14^{\circ} \mathrm{C}\) at a rate of \(0.35 \mathrm{~kg} / \mathrm{s}\) where it is heated by hot air \(\left(c_{p}=1.0 \mathrm{~kJ} / \mathrm{kg} \cdot \mathrm{K}\right)\) that enters the heat exchanger at \(65^{\circ} \mathrm{C}\) at a rate of \(0.8 \mathrm{~kg} / \mathrm{s}\) and leaves at \(25^{\circ} \mathrm{C}\). Determine the maximum outlet temperature of the cold water and the effectiveness of this heat exchanger.

A single-pass cross-flow heat exchanger uses hot air (mixed) to heat water (unmixed), flowing with a mass flow rate of \(3 \mathrm{~kg} / \mathrm{s}\), from \(30^{\circ} \mathrm{C}\) to \(80^{\circ} \mathrm{C}\). The hot air enters and exits the heat exchanger at \(220^{\circ} \mathrm{C}\) and \(100^{\circ} \mathrm{C}\), respectively. If the overall heat transfer coefficient is \(200 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), determine the required surface area.
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