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Suppose there is hot water supply at fixed flow rate and inlet temperature around 40~60C, also there is cold air to be heated by the water at fixed flow rate and inlet temperature around -10~20C.

Heat exchange system 1: use water coils to exchange heat directly.

Heat exchange system 2: first use plate heat exchanger to transfer from the incomming hot water to secondary water, then use water coils to transfer secondary water to air.

Both system have the same water coil. The only thing varies is the design of the plate heat exchanger subsystem.

System 2 has the advantage of isolation and disadvantage of extra pipes/pump/tank.

What about the heat exchange capability comparison between them?

Suppose the heat exchange capability of system 1 is $P_1$ (i.e. the power at which heat is taken away from the incomming hot water), can there be an estimation about the maximum heat exchange capability that system 2 can reach? e.g. $P_2 \le 0.5P_1$, $P_2 \le 1.5P_1$, etc?

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  • $\begingroup$ Evaluate the losses at each stage. $\endgroup$
    – Solar Mike
    Nov 14, 2023 at 6:40
  • $\begingroup$ "System 2 has the advantage of isolation" what does this mean? Is your first set of hot water corrosive or toxic? All you're doing is making it more complicated. $\endgroup$
    – Tiger Guy
    Nov 14, 2023 at 13:30
  • $\begingroup$ @TigerGuy it is muddy. particles in the water will bounce and wear the copper tube at the each turning point. Isolation can extends the lifetime of the water coil. $\endgroup$
    – jw_
    Nov 15, 2023 at 0:42
  • $\begingroup$ @SolarMike Suppose there is no loss. Only want to know will cascaded one always have much lower transfer rate then non cascaded ones. $\endgroup$
    – jw_
    Nov 15, 2023 at 0:43
  • $\begingroup$ You have no specifics regarding the sizes of each system. If you measure them in thermal resistance, the system with the lower one will allow greater flow of energy per time per temperature difference. If the isolation objective is to reduce flow rate of water by providing increased flow rate of intermediary fluid, it will come at a cost of energy to flow that intermediary. $\endgroup$
    – Abel
    Nov 15, 2023 at 3:16

2 Answers 2

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Question: In a residential setting, how does the heat exchange capability of a cascaded heat exchange system (System 2) compare to a direct heat exchange system (System 1), given the provided parameters?

Answer:

Comparing System 1 and System 2:

System 1: Direct heat exchange using water coils. System 2: Cascaded heat exchange involving a plate heat exchanger and water coils. Key Considerations:

Efficiency of Direct vs. Cascaded Heat Transfer: System 1's direct approach typically offers higher efficiency due to fewer stages of heat transfer, thus less thermal resistance. System 2, with its additional stage (plate heat exchanger), introduces more potential points for heat loss.

Impact at Residential Scale: The efficiency difference between these systems is less pronounced in residential applications compared to larger, industrial ones.

Complexity of System 2: The added components in System 2 (extra pipes, pump, tank) mean more opportunities for heat loss, but also offer the benefit of isolation.

Rough Estimation of Heat Exchange Capability:

If System 1's heat exchange capability is denoted as P1, then System 2’s capability (P2) is likely to be somewhat lower, considering the additional components and stages involved. Estimated Range: A reasonable estimation might place P2 in the range of 0.7P1 to 0.9P1. This suggests a decrease in heat exchange capability for System 2 compared to System 1, but not a significantly drastic one, especially in a residential context. Conclusion:

While System 2’s cascaded approach does introduce complexity and potential points for efficiency loss, in a residential setting, this decrease in efficiency might not be as significant as it would be in industrial applications. The estimated range reflects a balance between the typical efficiencies of residential-grade heat exchangers and the inherent losses due to the additional components in System 2. It's important to note that this is a general estimation and actual performance can vary based on specific system design and operational conditions.

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  • $\begingroup$ Why not just modify your previous answer? Beside why does this answer feel like ChatGPT? $\endgroup$
    – jw_
    Nov 25, 2023 at 1:51
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The heat exchange capability of a cascaded system like the one you described in System 2, compared to a direct heat exchange system (System 1), can vary depending on several factors. Here's a breakdown of the considerations:

  1. Efficiency of Each Heat Exchanger: The efficiency of each heat exchanger in the system plays a crucial role. In a cascaded system, each stage of heat transfer introduces potential losses. The plate heat exchanger's efficiency in transferring heat from the incoming hot water to the secondary water will impact the overall heat transfer efficiency. The water coil's efficiency in transferring heat from the secondary water to the air is also a factor.

  2. Temperature Gradient: The effectiveness of heat transfer depends largely on the temperature difference between the hot and cold mediums. In System 1, where the water coils exchange heat directly with the air, the temperature gradient might be larger compared to System 2, where the heat is first transferred to a secondary water system. This could potentially make System 1 more efficient in terms of heat transfer.

  3. Heat Losses in Additional Components: System 2 involves more components (extra pipes, pump, tank), which could lead to additional heat losses. This might reduce the overall heat transfer efficiency compared to System 1.

  4. Control and Stability: System 2 offers better isolation and control, which could lead to more stable operation. This can be an advantage in certain applications where maintaining specific temperatures or flow rates is crucial.

Regarding the estimation of the maximum heat exchange capability of System 2 (P2) compared to System 1 (P1), it's challenging to give a precise figure like P2 ≤ 0.5P1 or P2 ≤ 1.5P1 without detailed information about the efficiency of each component and the specific operating conditions. Typically, a cascaded system might have lower heat transfer efficiency due to the additional stages of heat transfer and potential losses in each stage, but this is not a universal rule and depends heavily on the specific design and operating conditions of the systems in question.

To get a more accurate comparison, you would need to conduct detailed thermal analyses or simulations of both systems, taking into account the specific properties of the heat exchangers, the flow rates, the temperature ranges, and the materials involved.

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  • $\begingroup$ The purpose of the question is to skip the analyses or simulations process:), instead ask some folks with experiences in such situations for a rough estimation: will 2 be much worse than 1 - for a residential configuation? $\endgroup$
    – jw_
    Nov 17, 2023 at 1:13

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