Question

A process stream of dilute aqueous solution flowing at the rate of 10 kg/s is to be heated. Steam condensate at 95°C is available for heating purpose, also at a rate of 10 kg/s. A 1-1 shell and tube heat exchanger is available. The best arrangement is

• A. counter flow with process stream on shell side
• B. counter flow with process stream on tube side
• C. parallel flow with process stream on shell side
• D. parallel flow with process stream on tube side

Hint:

Counterflow heat exchangers are more efficient than parallel flow, as they maintain a greater temperature difference throughout the exchanger.

Answer 1

The Correct Option is A

Step 1: Understanding heat exchangers.
In a heat exchanger, the effectiveness is maximized when the temperature difference between the two streams is maximized throughout the exchanger. This is achieved in countercurrent flow configurations, where the two fluids flow in opposite directions. In this case, placing the process stream on the shell side and the steam condensate on the tube side enhances heat transfer efficiency.
Step 2: Conclusion.
The best arrangement for this heat exchanger is a counterflow configuration with the process stream on the shell side. Therefore, the correct answer is (1).

Answer 2

This question is really testing two separate ideas: (1) whether flow direction matters when one side is a condensing, isothermal stream, and (2) which side of the exchanger the process stream should go on. Let's assess each option.

1. Counter flow with process stream on shell side: Since the steam condensate stays at a constant 95°C throughout (an isothermal phase change), the temperature profile of the process stream approaching it is the same whether the two streams move in the same or opposite directions, so flow direction alone doesn't change the driving force here. What does matter is placing the dilute aqueous process stream on the shell side, where the larger, more open flow passages reduce the risk of fouling deposits building up and are easier to inspect and clean if scaling does occur, while the clean condensing steam occupies the tube side.
2. Counter flow with process stream on tube side: This places the process stream in the smaller tube passages, more prone to plugging or fouling buildup and comparatively harder to inspect than a shell-side arrangement for a stream of this nature.
3. Parallel flow with process stream on shell side: Since the temperature-difference profile with an isothermal condensing stream doesn't depend on flow direction, this option isn't worse than the counter-flow case on driving-force grounds, but conventionally counter-current arrangements remain the default choice in exchanger design when there is no reason to choose otherwise.
4. Parallel flow with process stream on tube side: This combines the same drawback of tube-side process placement discussed above with no advantage over the counter-flow alternative.

With the steam condensing isothermally, the meaningful choice comes down to stream placement, and putting the process stream on the shell side with a counter-current arrangement is the best practice here.

Therefore, the correct answer is counter flow with process stream on shell side.