Comparative Experimental Analysis of Wet-State Thermal Performance in Pipe Mineral Wool Insulation with Different Hydrophobic Treatments

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

thank you very much for your paper.

I congratulate to it.

The paper is fine and looks nice.

There is only couple of things to improve.

The format af the figures are a littbe misuderstandable.

I recommend to replot the without green background. I recommend white background.

The conclusions can be improved.

Scanning elcetron microscopy measurements would be better to see.

Or with in higher magnifications.

Some calculataions would be also good. to find the heat loss with varying mositure content.

Author Response

Dear Reviewer,

Thank you for taking the time to review our manuscript. We appreciate your valuable feedback and constructive comments on our paper. We have addressed all your concerns and suggestions, and below you will find a point-by-point response.

 Reviewer Comment 1:

The format af the figures are a littbe misuderstandable.

I recommend to replot the without green background. I recommend white background.

Our Response:

We would like to clarify that the appearance of the figures in the previous version was slightly affected by the journal's formatting requirements. This process caused certain annotations to become misaligned and the graph backgrounds to change. We have since made the necessary corrections in this revised submission and ensured all graphics are accurate.

 Reviewer Comment 2:

The conclusions can be improved.

Our Response:

The Conclusion section has been revised for clarity and impact.

 Reviewer Comment 3:

Scanning electron microscopy measurements would be better to see.

Or with in higher magnifications.

Our Response:

Thank you for suggesting the use of an electron microscope for measuring the average fiber diameter. While standards do allow for this technique, electron microscopy is costly, and we lack access to the necessary equipment. We therefore utilized the alternative method detailed in the same standards, which employs an optical microscope. We consider this optical approach more reliable for our specific task because positioning the fibers in a thin layer provides a near two-dimensional image. In contrast, the 3D perspective of an electron microscope can introduce measurement ambiguities. We believe the SEM is excessive for measuring average fiber diameters and better suited for analyzing finer structural elements.

Reviewer Comment 4:

Some calculataions would be also good. to find the heat loss with varying mositure content.

Our Response:

Detailed heat loss calculations for varying moisture contents were not included in the manuscript as they depend heavily on specific boundary conditions (e.g., temperature differences, surface area).

However, these calculations can be easily performed by a user as needed, using Fourier's Law (Equation 5) in combination with the provided linear correlations for thermal conductivity (Table 3). This approach allows for flexible application of our results to a wide range of specific scenarios.

Reviewer 2 Report

Comments and Suggestions for Authors

This study evaluates the effects of mineral wool on the thermal performance of pipe insulations. This paper is well-organized. Here are some comments.

1. The manuscript needs to be carefully rechecked. There are some minor errors. For example, Line 169, “in the kg.m-3 sample”; Figs. 5-7, “thermmal”, “resudal”; Line 292, an experimental installation are shown in Fig. 1; etc.
2. The charts should follow the introduction text rather than be presented together at the end.
3. Please provide a more detailed explanation regarding the comparability between the direct water injection method and the actual situation.
4. Line 437, the authors mentioned that based on the comprehensive characterization results, two samples are selected. Please justify further these reasons.
5. Please pay attention to the layout of the manuscript. Many symbols and characters are not aligned with the text.

Author Response

Dear Reviewer,

Thank you for taking the time to review our manuscript. We appreciate your valuable feedback and constructive comments on our paper. We have addressed all your concerns and suggestions, and below you will find a point-by-point response.

 

Reviewer Comment 1:

The manuscript needs to be carefully rechecked. There are some minor errors. For example, Line 169, “in the kg.m-3 sample”; Figs. 5-7, “thermmal”, “resudal”; Line 292, an experimental installation are shown in Fig. 1; etc.

Our Response:

The paper was rechecked and corrected.

 

Reviewer Comment 2:

The charts should follow the introduction text rather than be presented together at the end.

Our Response:

The paper was corrected.

 

Reviewer Comment 3:

Please provide a more detailed explanation regarding the comparability between the direct water injection method and the actual situation.

Our Response:

Thank you for asking for a more thorough explanation. We maintain that the direct water injection method is particularly suitable for simulating real-world moisture ingress scenarios resulting from damaged pipelines. The key justification lies in the nature of the moisture source: A damaged pipeline results in a localized, often pressurized, influx of heat carrier (water or steam). This scenario differs significantly from gradual moisture absorption from ambient air or capillary rise from the ground (Cai et al., 2014). Our direct injection method mimics this specific localized ingress mechanism. Water is introduced directly into the insulation layer, simulating the internal origin of the leak source (the pipe surface). The injection process forces water into the porous structure of the mineral wool, filling the voids in a manner consistent with a leak under pressure. Therefore, this method provides a realistic representation of how insulation performance degrades rapidly following a pipeline failure event, which was the focus of this study.

The following was added in Section 3.3 of the article:

Specifically, the direct injection mimics the localized, internal influx of water or steam, which represents a forced penetration of fluid into the porous structure under pressure, rather than a gradual absorption process. This approach allows for a realistic assessment of the rapid degradation of insulation performance following a pipeline failure.

 

Reviewer Comment 4:

Line 437, the authors mentioned that based on the comprehensive characterization results, two samples are selected. Please justify further these reasons.

Our Response:

We appreciate the request for further clarification. The two samples were specifically selected to represent the extremes of the quality range observed during the initial comprehensive characterization phase. Sample A was chosen as representative of low-quality materials due to its significantly higher water absorption capacity (above four times higher than Sample B) and elevated shot content. Sample B was selected as the superior-quality counterpart, characterized by the lowest water absorption capacity and minimal non-fibrous inclusions observed in our study. This selection strategy allows for a clear, comparative demonstration of how these divergent quality parameters affect thermal performance across different moisture conditions. The measured properties substantiating this choice are provided in detail in Table 1.

Section 4.1. was revised to better clarify the criteria for choosing samples A and B.

 

Reviewer Comment 5:

Please pay attention to the layout of the manuscript. Many symbols and characters are not aligned with the text.

 Our Response:

The paper was corrected.

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript presents an experimental investigation into the thermal conductivity of two types of mineral wool pipe insulation materials. The authors use a guarded hot pipe method to measure thermal conductivity (λ) at temperatures from 20°C to 85°C and moisture contents up to 12% by weight. The authors report a paradoxical key finding that the superior quality material (sample B) exhibits a higher λ when wet compared to the inferior sample A. This effect is especially pronounced at 85°C, where sample B's λ increases by 276% at 12% moisture, while sample A's increases by only 83%. The authors attribute this to sample B's uniform structure creating more continuous liquid water pathways and its superior hydrophobicity actively pushing moisture and vapor out, creating convective heat transfer. The experimental dataset is potentially valuable. However, there are several major methodological and interpretation issues that must be addressed before this manuscript can be considered for publication.

1. The guarded hot pipe method is designed to measure thermal conductivity under purely conductive heat transfer conditions. However, the authors attribute the observed behavior of sample B to internal convective or fluid transport processes. If convection significantly contributes to heat transfer, the fundamental assumption of purely conductive heat flow in Equation 5 is violated. In that case, the authors are not measuring a true thermal conductivity.

2. The guarded hot pipe method relies on quasi-steady conditions. Figures 7 and 10 indicate that sample A achieved a steady plateau, whereas sample B began drying immediately, never reaching steady-state. The authors relaxed their residual criterion (from 1 % to 2–3 %) to obtain values. Consequently, the reported λ for Sample B likely represents a transient, drying-affected effective value, not a steady-state conductivity. The comparison between samples A and B therefore mixes steady-state and transient-state data. The authors should clarify how it affects both the reported λ values and the linear regression in Table 3.

3. The counterintuitive finding that Sample B with better hydrophobic treatment shows larger increases in λ at high T is interesting but requires stronger support. For example, more quantitative microstructural evidence such as pore size distribution, porosity, and connectivity.

4. The proposed mechanism that hydrophobic treatment can concentrate moisture and create high-conductivity pathways needs more proof such as direct liquid distribution imaging.

5. The "the first and second samples" in the abstract is confusing. Need to be consistent using sample A and B to avoid ambiguity.

6. There is a complete mismatch between the figure callouts in the text and the actual figures. For example, the manuscript writes "Technical details of an experimental installation are shown on Fig. 3. As shown in the section view in Fig. 3a, the central pipe is divided into three parts ..." but it should be figure 1. Figure 3 is the fragments of the injection procedure. The authors say "The heating/cooling coil was fitted (Fig. 4c) and connected to the chiller." which is actually Figure 2c. This confusion of figure numbers persists throughout the entire Methods and Results sections.

Author Response

Dear Reviewer,

Thank you for taking the time to review our manuscript. We appreciate your valuable feedback and constructive comments on our paper. We have addressed all your concerns and suggestions, and below you will find a point-by-point response.

 

Reviewer Comment 1:

The guarded hot pipe method is designed to measure thermal conductivity under purely conductive heat transfer conditions. However, the authors attribute the observed behavior of sample B to internal convective or fluid transport processes. If convection significantly contributes to heat transfer, the fundamental assumption of purely conductive heat flow in Equation 5 is violated. In that case, the authors are not measuring a true thermal conductivity.

Our Response:

When studying the thermal conductivity of complex systems such as fibrous porous media, the effective thermal conductivity is considered, which takes into account all heat and mass transfer effects, including the thermal conductivity of air and the matrix, air convection, water vapor diffusion, water moving, radiative heat exchange, etc. “Pure” thermal conductivity does not exist in such systems. In this area of research, 'thermal conductivity' commonly denotes effective thermal conductivity. For example:

544. Abdou, I. Budaiwi, The variation of thermal conductivity of fibrous insulation materials under different levels of moisture content, Construction and Building Materials 43 (2013) 533–544. https://doi.org/10.1016/j.conbuildmat.2013.02.058.
545. Jiřičková, Z. Pavlík, L. Fiala, R. Černý, Thermal Conductivity of Mineral Wool Materials Partially Saturated by Water, Int J Thermophys 27 (2006) 1214–1227. https://doi.org/10.1007/s10765-006-0076-8.
546. Ochs, H. Müller-Steinhagen, Temperature and Moisture Dependence of the Thermal Conductivity of Insulation Materials, Proceedings of the 9th International Conference on Thermal Engineering, 1–6. (2005).

 

For clarification, the following was added to the article:

In the Nomenclature, thermal conductivity was replaced by effective thermal conductivity

In Section 1.1 Footnote 1 was added: ^[1] Hereinafter, thermal conductivity refers to the effective thermal conductivity of a fibrous porous medium which accounts all heat and mass transfer effects.

 

Reviewer Comment 2:

The guarded hot pipe method relies on quasi-steady conditions. Figures 7 and 10 indicate that sample A achieved a steady plateau, whereas sample B began drying immediately, never reaching steady-state. The authors relaxed their residual criterion (from 1 % to 2–3 %) to obtain values. Consequently, the reported λ for Sample B likely represents a transient, drying-affected effective value, not a steady-state conductivity. The comparison between samples A and B therefore mixes steady-state and transient-state data. The authors should clarify how it affects both the reported λ values and the linear regression in Table 3.

Our Response:

With short-term moisture introduction, processes in thermal insulation are non-stationary. Therefore, to measure thermal conductivity using the hot pipe method, it was necessary to identify at least a short period of quasi-steady-state conditions. This is a condition in which the heat flux and wall temperatures remain constant. Indeed, for sample A, this period was longer and resulted in a lower residual (Figure 7c). For sample B, the processes occurred more rapidly, but if we look at Figure 10c, we also see a section (80–160 min) in which the residual thermal conductivity changes insignificantly, albeit with a large value of up to 2%. This can also be considered a quasi-steady-state condition. Thus, in this paper, we compare not steady-state and non-steady-state thermal conductivity, but two "non-steady-state" thermal conductivities with different time constants. Nevertheless, we have specifically described the thermal conductivity calculation method in detail and provided time dependences so that readers can correctly account for these time effects in their research.

 

Reviewer Comment 3:

The counterintuitive finding that Sample B with better hydrophobic treatment shows larger increases in λ at high T is interesting but requires stronger support. For example, more quantitative microstructural evidence such as pore size distribution, porosity, and connectivity.

Our Response:

Yes, indeed, to gain a deeper understanding of the observed effects, it would be useful to conduct more complex studies of the thermal insulation morphology. However, this is a separate, complex technical task that is beyond the scope of our study. In our study, we used standard quality indicators for mineral wool (GOST 17177-94, GOST 32025-2012) used in the manufacture and practical use of pipe insulation. At the same time, we presented our interpretation of the relationship between morphology and the observed effects in the Discussion section (4.7).

 

For clarity, the following was added to the text:

Perhaps this creates continuous liquid water pathways that significantly enhance heat transfer through conduction mechanisms.

 

Reviewer Comment 4:

The proposed mechanism that hydrophobic treatment can concentrate moisture and create high-conductivity pathways needs more proof such as direct liquid distribution imaging.

Our Response:

The technique of "direct liquid distribution imaging" seems fantastic. We haven't encountered its application to mineral wool insulation studies in our review. We believe this is technically impracticable when testing pipe insulation in a climatic chamber in real conditions. A more detailed study of the physical relationships occurring in insulation can currently only be accomplished using mathematical modeling (Lu et al. 2024; Liu and Wang 2024), and the data obtained in our study will be useful for validating such models.

 

Reviewer Comment 5:

The "the first and second samples" in the abstract is confusing. Need to be consistent using sample A and B to avoid ambiguity.

Our Response:

The abstract was corrected as follows:

“In a low-temperature regime, the inferior quality sample (Sample A) at a maximum moisture content of 12% exhibited thermal conductivity of 0.042 W∙m⁻¹·K⁻¹, and the sample with the best hydrophobic treatment (Sample B) of 0.05 W∙m⁻¹·K⁻¹. At an elevated temperature at a moisture content of 12%, Sample A and Sample B had thermal conductivity of 0.077 W∙m⁻¹·K⁻¹, and 0.109 W∙m⁻¹·K⁻¹, respectively.”

 

Reviewer Comment 6:

There is a complete mismatch between the figure callouts in the text and the actual figures. For example, the manuscript writes "Technical details of an experimental installation are shown on Fig. 3. As shown in the section view in Fig. 3a, the central pipe is divided into three parts ..." but it should be figure 1. Figure 3 is the fragments of the injection procedure. The authors say "The heating/cooling coil was fitted (Fig. 4c) and connected to the chiller." which is actually Figure 2c. This confusion of figure numbers persists throughout the entire Methods and Results sections.

Our Response:

In Section 3, Figure 3 was replaced by Figure 1 and Figure 4 by Figure 2.

 

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have addressed my comments.