Design and Fabrication of a Compact Evaporator–Absorber Unit with Mechanical Enhancement for LiBr–H₂O Vertical Falling-Film Absorption, Part I: Experimental Validation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear authors,

The commissioning of an absorption chiller, even partially, is a hard-working task. I recognize the originality of the design (demonstrated with the utility model) and the originality of using an integrated fan for the forced convection between evaporator and absorber. The article is quite well structured. However, there are some significant points that should be better explained:

• Please clearly explain the advantages of the present design in the introduction. Apart from the possibility of forced convection of the vapour, are there any additional ones? Is the number of weldings/joints minimized, maybe?
• Figure 2. I understand how the fluid is distributed between the different tubes. However, it is not clear how the falling film is formed. Is any kind of flow distributor used? Could you provide details of this?
• How is the LiBr concentration measured? Is it measured outside the test rig? How is it assured that if the measurement is done in atmospheric conditions, there is no absorbed mass from the air in the process? Is the initial concentration the same for all the experiments?
• What is the pressure provided by the solution pump to assure a fair distribution between tubes? What type of pump is used? Is its price limiting the development of a small-sized machine? For the complete assembly, could a pump in the return line of the generator-absober be avoided?
• Figure 2. It is unclear to me whether the coolant flow circulates inside or outside the falling film absorber tubes.
• Figure 2. Where the 20W power is allocated. What are their characteristics, i. e. flow vs. pressure curve? What is their position with respect to the pressure probes? We have to keep in mind that the experiment should be reproducible or simulated.
• Figure 2. The water vapour circulates inside or outside the inner shell of the middle part?
• Point 2.2, about the experimental procedure: once it is checked that the internal pressure is about 1 kPa, how is it assured that the amount of air is minimized?  I suppose that the whole unit has been tested under vacuum conditions.  What is its leakage rate in Pa⋅m3/s/?
• Figure 6. What is the difference between green and red points? Does it mean fan on/off? Maybe a legend is needed.
• Table 5. I do not understand how the 65% absorption improvement is calculated from this data. Please clarify because it is the main result of the research.
• Table 6. I suggest revising this reference [1], which reports the impact of different kinds of absorption improvements.

[1] J Zheng, J Castro, G Papakokkinos, O Assensi.  Sensitivity study to an absorption system performance considering heat and mass transfer enhancements. International Refrigeration and Air Conditioning Conference at Purdue, 2022 

Author Response

We sincerely thank you for your careful reading and constructive comments, which helped us significantly clarify and strengthen the manuscript. In response:

• We have clarified the distinctive advantages of the proposed compact evaporator–absorber design in the Introduction, including its integrated cylindrical architecture, reduced welds, modular flanged construction, improved sealing under vacuum, ease of maintenance, and scalability for different capacities.
• The internal configuration and operation of the unit are now described in greater detail:
  • The role and configuration of the drip-pan distributor and formation of the falling film in the absorber tubes.
  • The recirculation loop with a magnetic-drive pump, its specifications, cost implications, and its function strictly within the evaporator–absorber subsystem.
  • The coolant-water path (shell side, outside the tube bank, guided by deflector plates).
  • The location, nominal power, and function of the 20 W axial fan, its integration in the middle compartment, its relation to the pressure taps, and how it can be represented for modeling and reproducibility purposes.
  • The intended vapor path through the inner shell in the middle section has been clarified to remove any ambiguity.
• We have added a clear description of LiBr–H₂O solution preparation, initial concentration, handling under minimized air exposure, and its use in a single continuous test, along with an explicit statement on vacuum integrity and leakage assessment.
• Figure 2 has been redrawn and its caption rewritten to explicitly distinguish: refrigerant pool and heaters in the lower compartment, fan-assisted vapor conveyance in the middle compartment, internal falling-film absorption in the vertical tubes, and external coolant-water flow on the shell side.
• Figure 6 has been updated with an explicit legend (fan-OFF vs fan-ON), and the text explains how the ≈65% relative improvement in the index n is computed, emphasizing that this metric is a local indicator of thermal separation under matched conditions.
• Table 6 has been revised to correct and update the comparative overview of enhancement strategies, including the sensitivity analysis by Zheng et al., and to avoid non-traceable or misleading entries.
• A dedicated uncertainty analysis (Section 2.5) and complementary phase-resolved statistics have been incorporated to reinforce transparency, data reliability, and reproducibility.

Additionally:

• We provide a point-by-point response to each of your comments in the attached document, structured as:
  • Answer – explanation, justification, or clarification.
  • Action – exact modification implemented in the manuscript (with section/figure/table reference).
• The revised manuscript includes updated text, clarified methodology, refined figures (notably Fig. 2 and Fig. 6), the new uncertainty analysis, and supplementary/appendix material supporting reproducibility.
• For ease of verification, we are submitting:
  • A clean revised manuscript (PDF).
  • A marked version (Word, Track Changes) showing all modifications.
  • The detailed “Response to Reviewer 1” document with the Answer/Action structure.

We hope these revisions fully address your concerns and reflect our commitment to a rigorous, transparent, and reproducible presentation of the proposed compact evaporator–absorber.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The paper presents the design and validation of a compact LiBr–H₂O evaporator–absorber unit enhanced by a low-power fan. The topic is relevant for sustainable cooling technologies, and the integration of evaporator and absorber in one vessel is an interesting direction. However, the study’s scientific rigor, methodological clarity, and modeling approach could be improved to strengthen its contribution.

1. The experimental protocol lacks control: thermal loads were manually adjusted, dwell times were unequal, and steady-state conditions were not defined. This introduces uncertainty and makes quantitative comparison questionable.

2. There is no uncertainty analysis for the derived quantities (ΔT, ΔP, n). This omission weakens the credibility of conclusions.

3. The choice of using median/IQR is justified by non-stationarity, but the authors should still provide confidence intervals or repeatability tests to validate trends.

4. The test duration (≈2 h 46 min) seems too short to assess system stability and repeatability under multiple load transitions.

5. The fan effect is not isolated from natural variations in the process (e.g., manual switching, transient effects). Controlled automation would give cleaner data.

6. The index n lacks physical interpretation or validation against real heat transfer performance (e.g., mass flux, absorption rate, or COP). It serves more as an empirical indicator than a verified performance metric.

7. The pressure in the evaporator is not measured, but inferred via correlation. This can produce large errors, especially under transient conditions. Validate the calculated evaporator pressure by direct measurement or comparison with independent data.

8. There is no heat or mass balance, meaning the paper does not evaluate actual absorption capacity or efficiency. Include at least an estimated heat transfer rate (Qabs) to connect ΔT and ΔP changes to actual performance.

9. Experimental part: Grouping thermal loads (Qin,1–Qin,12) is somewhat arbitrary and hides the dynamic variability between tests. The influence of environmental temperature (ambient ~17 °C) is ignored in the analysis, even though it could affect ΔT. The sample sizes for each phase are inconsistent and not justified. 

Normalize results to ambient conditions. Report total test uncertainty (combined error of sensors, timing, and manual adjustments). Include a table summarizing test duration, steady-state verification, and main outcomes for each phase.

10. Include a simplified mass and heat transfer model to correlate observed ΔT, ΔP, and n with theoretical expectations. Discuss possible errors due to pressure inference. Use dimensionless analysis (Re, Pr, Sc, Gr) to better explain how fan-induced flow changes performance.

 

Author Response

We sincerely thank Reviewer 2 for the careful reading of our manuscript and for the detailed, technically rigorous, and constructive comments. Your observations have been extremely valuable to improve the clarity, transparency, and robustness of this work.

The revised manuscript emphasizes more clearly that its primary scope is the experimental validation of a novel, patented evaporator–absorber unit, tested as a standalone subsystem. The prototype represents an original configuration, and given the high cost and limited availability of LiBr–H₂O and the complexity of the setup, our main objective in this Part I was to (i) demonstrate functional feasibility, (ii) characterize the thermal and pressure behavior under controlled but realistic operating conditions, and (iii) quantify the effect of a low-power fan using directly measured local indicators.

Several of your key concerns have been addressed as follows:

1. Unequal dwell times, manual operation, and data consistency.
   We have clarified that the unequal sample sizes and manually adjusted thermal loads arise from the intentional choice to operate the bench in a realistic, exploratory mode during a single continuous test. To ensure that this does not compromise the quantitative comparison:
   • Signals are tagged by phase and fan state, and analyzed separately for each (Qin,k, OFF/ON) interval.
   • Median and IQR are used as robust statistics for non-stationary data with unequal dwell times.
   • A detailed uncertainty and error-propagation analysis (Section 2.5, Appendix B) confirms that the observed OFF→ON differences in ΔT, ΔP, and n are significantly larger than the combined measurement uncertainties.
     Together, these revisions directly address the concern that methodological variability could invalidate the comparisons.
2. Manual thermal loads and fan operation.
   The revised text now explains more explicitly that:
   • The manual implementation of Qin,k and the OFF→ON switching sequence was chosen to emulate realistic dynamic conditions and to probe the response of the prototype under varying loads.
   • The phase definition, tagging strategy, and removal of short transients are described in Section 2.3.
   • Future work is explicitly oriented toward automated control (heater power and fan/pump speeds) to perform more systematic mapping once this baseline has been established.
     This clarifies that the present data treatment is consistent with the exploratory, subsystem-validation character of the study.
3. Thermodynamic balances, efficiency metrics, and modeling scope.
   In line with your suggestions, we now state more clearly that:
   • Part I intentionally focuses on local, directly measured indicators (ΔT, ΔP, and the dimensionless index n) within the integrated evaporator–absorber.
   • Full heat and mass balances, absorber-side capacity, COP-related metrics, and detailed dimensionless analysis (Re, Pr, Sc, Gr) require additional modeling assumptions and are therefore reserved for a dedicated follow-up study, to avoid mixing experimental validation with partial or truncated models.
     We have reinforced this scope in Sections 2.3, 4, and 5. If the Editor considers it appropriate, we would warmly welcome the continuity of your expert assessment in the review of that balance- and modeling-focused work, so that you can directly verify the consistency between the subsystem data presented here and the subsequent thermodynamic analysis.

Additionally:

• We provide a point-by-point response to each of your comments in the attached document, structured as:
  • Answer – our explanation, justification, or clarification.
  • Action – the exact modification implemented in the manuscript (with section/figure/table reference).
• The revised manuscript includes updated text, clarified methodology, new uncertainty analysis (Section 2.5), updated figures (e.g., Fig. 2 and Fig. 6), and complementary appendices/supplementary material (including phase-resolved statistics and uncertainty tables) to improve transparency and reproducibility.
• For ease of verification, we are submitting:
  • A clean version (PDF) of the revised manuscript.
  • A marked version (Word, with Track Changes) showing all modifications relative to the originally submitted version.
  • The detailed Response to Reviewers document with the Answer/Action structure.

We hope that these revisions adequately address your concerns and demonstrate our commitment to rigorous experimental practice and clear definition of scope. We sincerely trust that the improved manuscript will meet your expectations and be suitable for acceptance in its revised form.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Questions and suggestions are as follows：

1）The singularity of the operating conditions. Table 1 only provides one operating condition, and the basis for selecting the data in the table is lacking. Secondly, the 12 different heating powers of the heater correspond to what application scenarios in actual industry, and the basis for selecting these values also needs to be supplemented.

2）The validity and reproducibility of the experimental data are questionable. Because the heater power and fan start-stop time are manually adjusted in the experiment, the duration of each stage is inconsistent, which may affect the reproducibility and validity of the results. Secondly, the uniqueness of the number of times may also bring measurement deviations to the results. How did the authors evaluate these situations.

3）What is the speed of the fan? The phenomenon that the fan significantly increases the n value under low thermal load has not been fully explained from the physical mechanism. Is it due to enhanced convective heat transfer, improved distribution of the absorbent, or the combined effect of other factors. In addition, compared with other strategies, is the innovation only the introduction of the fan. The operation of the fan, its electrical consumption, and the reliability of long-term operation may bring challenges to the cost and reliable operation of the miniaturized device.

4）The limitation of a single index. The dimensionless temperature separation index (n) is introduced in the paper to evaluate the thermal separation effect of the system. However, relying only on this index may not be able to comprehensively reflect the performance of the system. Other important performance parameters such as the system's heat transfer rate and absorption efficiency lack direct representation. In order to make the comparison convincing, a comprehensive evaluation of the multi-performance performance under different operating conditions should be made in comparison with other enhancement strategies.

5）Although the paper mentions the future research directions, they are not specific and clear enough. For example, no detailed plans and expected goals have been given for the improvement of the experimental device, the optimization of the experimental method, and the further improvement of the system performance.

Author Response

We sincerely thank Reviewer 3 for the constructive and detailed feedback, which has helped us improve the clarity, rigor, and defined scope of the manuscript. The main changes are summarized as follows:

1. Operating conditions and heat-load rationale clarified
   We clarified that Table 1 defines the unique initial charge/operating condition for a single continuous experiment, and that the twelve heater powers Qin,1–Qin,12Q_{in,1}–Q_{in,12}Qin,1​–Qin,12​ (Table 2) were selected to span low, intermediate, high, and zero input levels representative of small-scale LiBr–H₂O applications, including deliberate zero-load phases to probe the behavior of the compact module under weak or absent heat supply.
2. Uncertainty analysis for derived metrics incorporated
   A formal uncertainty and error-propagation analysis for ΔT\Delta TΔT, ΔP\Delta PΔP, and nnn has been added (Section 2.5 and Appendix B). We show that the expanded uncertainties are significantly smaller than the observed OFF→ON differences, thereby reinforcing the robustness of the conclusions despite the single continuous run and unequal dwell times.
3. Fan characteristics, mechanism, and innovation explicitly defined
   We now report the nominal characteristics and placement of the integrated 4-inch, 20 W axial fan (Section 2.1), and provide a clearer physical explanation in the Discussion of how the fan assists vapor conveyance and stabilizes hot–cold separation, especially under low/zero thermal load. We emphasize that the core innovation is not the fan alone, but the compact vertical integration of evaporator and absorber within a single shared-pressure vessel, with localized vapor guidance and minimal mechanical complexity.
4. Scope and role of the index nnn clarified
   We explicitly state that the dimensionless temperature separation index nnn, together with ΔT\Delta TΔT and ΔP\Delta PΔP, is used strictly as a local indicator for comparing fan-OFF/fan-ON operation under identical conditions in this prototype. We clarify that system-level performance metrics (e.g., QabsQ_{abs}Qabs​, mass flux, COP) are intentionally not derived here and are instead addressed in a separate modeling and balance-oriented companion work.
5. Future work made specific and structured
   The Conclusions have been revised to present a concrete roadmap along three axes: (i) device improvements (added sensors, refined distribution, direct evaporator pressure, long-term sealing and durability tests), (ii) methodological refinement (automated control, fixed dwell times, repeated sequences), and (iii) performance-oriented studies (variable-speed fan/pump, alternative geometries and working pairs, and full heat/mass-balance evaluation using the same platform).

Additionally:

• We provide a point-by-point response to each of your comments in the attached document, using the Answer / Action structure to clearly link our explanations with the exact changes made in the manuscript.
• The revised manuscript includes clarified descriptions of the operating strategy, added uncertainty analysis, strengthened discussion of the fan-assisted mechanism and innovation, and refined conclusions to better delimit scope and future work.
• For ease of verification, we are submitting:
  • A clean version of the revised manuscript.
  • A marked version (Track Changes) highlighting all modifications.
  • The detailed Response to Reviewers document.

We hope these revisions satisfactorily address your concerns and demonstrate our commitment to transparency, reproducibility, and a well-defined experimental scope for this first-part study on the compact mechanically assisted evaporator–absorber.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear authors,

The article is acceptable, but Table 5 requires revision. I think the titles of the columns are not placed in the right order.

Regards

Author Response

Answer. We appreciate the careful catch and apologize for the confusion caused by the header placement. You are right—the column titles were not aligned with the left-to-right data sequence.

Action. We have corrected Table 5 as follows: the headers have been reordered to match the displayed data and units throughout the table; header alignment and spacing were standardized; footnotes and abbreviations were verified; and all in-text references to Table 5 were rechecked.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Accept in present form

Author Response

Answer. We sincerely thank you for the careful reading in the first round and for your present recommendation to accept. Your earlier suggestions helped us clarify the scope, strengthen the methods description, and improve figure/table readability.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have addressed all my concerns in a thorough manner. I recommend the acceptance of this manuscript.

Author Response

We are grateful for your thorough and constructive review and for recommending acceptance. Your comments on fan operation, physical mechanism at low load, and practicality guided clarifications that substantially improved the manuscript.