Jim Young
Director of Technology
ITW Insulation Systems

In addition to reducing heat gain from the ambient environment, one of the main purposes of insulation on pipe and mechanical equipment operating at below ambient temperatures is to prevent condensation on the outer surface of the insulation system. Preventing this surface condensation is simple in concept. Merely design the system to keep the surface temperature of the insulation system above the dewpoint temperature of the surrounding air. This simple relationship is made complicated because each of these two temperatures is dependent on the interrelationship of a myriad of factors. All of these factors must be fully and properly considered or selected to assure optimum control of insulation system surface condensation – commonly called condensation control.

The influence of each of the design and climatic factors influencing condensation control is discussed and recommendations are made on how to select or identify the appropriate value for each factor. Lastly, some common mistakes, tips, and tricks related to achieving condensation control are described.

Pipe, tanks, ducts, vessels, and other mechanical equipment operating at below ambient temperatures are insulated for various reasons with a key one being to prevent condensation of water vapor from the ambient atmosphere on the exterior surface of the insulation system. Condensation can lead to numerous problems including:

• Safety hazards as the water drips onto the floor below
• Damage to inventory as the water drips onto the merchandise below • Poor aesthetics when dripping water stains ceiling tiles
• Damage to the insulation system materials
• Reduced insulating ability of the insulation (increased k-Factor) • Shortened insulation system lifespan
• Corrosion of jacketing or pipe
• Growth of mold on the insulation system surface or on other building materials where condensed water drips

Because of these potential problems, prevention of condensation on the surface of cold mechanical insulation systems is of critical importance. This document will discuss the causes of surface condensation, the factors influencing it, and how to best identify design conditions and select system components to prevent surface condensation on mechanical insulation systems.

Various tables or charts are presented below that show the insulation thickness necessary to prevent condensation under various conditions. These thicknesses were not generated by experimentation but, rather, are based on common modeling or thickness calculations using the ASTM C680 standard thickness calculation method. This is the normal method in which insulation thicknesses are designed in the mechanical insulation industry. All of the thickness charts or tables presented assume that the material properties and ambient conditions are correctly known. These thickness charts or tables presented also assume that the insulation system is working perfectly and is impervious to water and water vapor penetration. While the water resistance of various system components is important, especially in a cold pipe application, that is a subject for another time. There will be no discussion here related to which insulation or vapor retarder materials have better or worse resistance to water. These assumptions about material properties and system performance are often wrong but are useful and necessary for the purpose of this discussion.

This discussion will be limited solely to insulating to achieve condensation control. Other design criteria including meeting energy code requirements, achieving heat gain limits, maintaining temperature control, and freeze protection will not be addressed.

The theory of surface condensation will be presented first followed by the influence of climatic conditions and system components on surface condensation. Lastly, recommendations are made on how to best select climatic conditions and system components to prevent surface condensation.

THEORY OF SURFACE CONDENSATION

The cause of surface condensation is quite simple in concept. Water vapor in the air will condense on a surface that is below the dewpoint temperature of the surrounding air. This is a complicated topic when applied to mechanical insulation systems because there are so many factors which influence either the dewpoint or the surface temperature of the insulation system. Figure 1 illustrates this concept and lists the various factors influencing each component of the equation. The factors shown in red are discussed in detail in this document.

The system designer must understand this theory, select the appropriate climatic design conditions, the proper insulation system components, and then determine the required insulation thickness to achieve their desired performance.

INFLUENCE OF CLIMATIC CONDITIONS

Ambient Temperature

The first climatic condition to be examined for its influence on insulation surface condensation is ambient temperature. Table 1 shows how insulation thickness has to be adjusted to prevent surface condensation as the ambient temperature is changed. This is shown for a very cold pipe at -80°F as well as for a pipe at 20°F. The insulation material used for this table is polyisocyanurate (PIR) which is specified by ASTM C591, Grade 2, Type IV. The specific constant conditions used for this table were 90% r.h., 7 mph wind, aluminum jacket with an emittance (ε) of 0.1, and horizontal pipe. The pipe size, ambient temperature, and pipe temperature were varied as shown.

As Table 1 shows, a higher ambient temperature can lead to a slightly increased insulation thickness being needed to prevent surface condensation but this influence is small and only typically seen at higher pipe temperatures. For the system designer, this means that it is acceptable to determine the ambient design temperature only roughly. There is no need to expend significant effort pinning down this design variable. While the ambient temperature plays only a small role in condensation control, it is a key factor in energy conservation and other design criteria which are not being addressed in this document.

Ambient Relative Humidity

The influence of ambient relative humidity on surface condensation is shown in Figure 2 which graphically displays the insulation thickness necessary to prevent surface condensation as the ambient relative humidity is changed. This is shown for a very cold pipe at -80°F as well as for a pipe at 20°F. The insulation material used for these charts is again polyisocyanurate (PIR). The specific constant conditions used for these charts were 90°F ambient temperature, 7 mph wind, aluminum jacket with an emittance (ε) of 0.1, and horizontal pipe. The pipe size, ambient relative humidity, and pipe temperature were varied as shown.

As Figure 2 shows, the influence of relative humidity on surface condensation is very large especially as the r.h. gets above around 70-80%. As the relative humidity increases, the insulation thickness necessary to prevent surface condensation increases. This effect is present regardless of pipe size and pipe temperature and is particularly pronounced at r.h. above about 80%. It is important to note that the insulation thickness required to prevent surface condensation asymptotically approaches infinity as the r.h. approaches 100%. In other words, designing a system to prevent condensation at 100% relative humidity would require the use of an infinite thickness of insulation which is obviously impossible. As a result of this asymptotic behavior, above a relative humidity of around 90-95% it takes an unrealistic and impractical insulation thickness to prevent condensation. This leads to a practical design limit for relative humidity of around 90-95%.

In Figure 2 and many later graphs and tables showing insulation thickness, sections are highlighted in yellow to indicate “unrealistic thicknesses”. These are insulation thicknesses which the owner/engineer/specifier would likely consider too large to be considered practical in the specified application. There would certainly be debate as to what thickness is considered “unrealistic” and these yellow highlights are not meant to indicate some specific point at which insulation thickness becomes unrealistic. Rather, these are shown to help illustrate the point that there are practical limits that play a role in the system design as well as the theoretical factors that are being discussed.

Since most cold pipe systems are designed using a fairly high relative humidity, the influence of this factor is of paramount importance. Consider the five insulation system scenarios shown in Table 2 for the usually important 80-95% r.h. range. For each scenario, the insulation thickness required to prevent surface condensation is shown as a function of high % relative humidity. As this table shows, the insulation thickness required increases very rapidly above about 80-85% r.h., especially at colder pipe temperatures. Impractical insulation thicknesses are reached at 85-95% r.h. depending on the pipe temperature.

As the pipe temperature in an application gets colder, the specifier of an insulation system will typically reduce the design relative humidity or introduce other design features such as higher jacket emittance in order to avoid the need for unrealistic insulation thicknesses.

Ambient Wind Speed

In a cold pipe situation, the surface temperature of the insulation system will be below that of the surrounding atmosphere. Wind will increase the rate of heat transfer and warm the insulation surface thus leading to a reduced likelihood of surface condensation. The influence of wind speed on surface condensation is fairly large but reaches diminishing returns above the 5-7 mph range. Table 3 shows the influence of wind speed on the insulation thickness required to prevent condensation for several scenarios. As this table shows, the required insulation thickness increases at lower wind speed and is especially high at zero wind speed.

When considering the influence of wind speed, remember that 0 mph is also a speed and in most indoor applications, the wind speed will indeed be zero. In outdoor applications, it is typical to assume the presence of some wind in the design of the insulation system. A commonly assumed wind speed in the industry when there is not a specific reason to use a higher or lower value is 7 mph.

With the discussion of climatic design conditions complete, the next category of factors to examine is the system components and their influence on surface condensation.

INFLUENCE OF SYSTEM COMPONENTS

Jacket Type – Emittance (ε)

Emittance is an important factor in the radiative component of heat transfer and is defined in ASTM C168 as:

“The ratio of the radiant flux emitted by a specimen to that emitted by a blackbody at the same temperature and under the same conditions.”

This is certainly not as simple to understand as wind speed or ambient temperature which are straightforward even to a layperson. Emittance ranges from 0 to 1 with lower values being representative of materials that have comparatively low ability to transfer heat through radiation such as aluminum or stainless steel metal jacketing and higher values being representative of the performance of plastic, paper, or other non-metallic surfaces which typically have a greater ability to transfer heat through radiation.

It is important to note that emittance is not the same as solar reflectance. In solar reflectance the color of the jacketing is important. A black colored plastic jacket would have a much lower solar reflectance than a white jacket and would therefore absorb more heat from the incident sunlight. In emittance, the color of the jacket has a minimal influence. A black plastic jacket might have an emittance of 0.92 while a white plastic jacket might have an emittance of 0.9 which is an insignificant difference.

Table 4 shows the influence of jacket emittance on the likelihood of surface condensation. Jacket materials with lower emittance like most metals yield a colder outer surface which makes surface condensation more likely and increases the required insulation thickness. Materials with higher emittance like paper, plastic, or mastic yield a warmer outer surface which makes surface condensation less likely. This has an obvious and significant effect on the insulation thickness needed to prevent the condensation from occurring.

The use of PVC jacketing is not typically recommended for outdoor use due to sensitivity to ultra violet light and is included in Table 4 just to illustrate the impact on insulation thickness from using jacketing with high emittance. The use of painted metal jacketing in outdoor applications is an often overlooked but excellent way to reduce the required insulation thickness by raising the jacket emittance.

Insulation Type – Thermal Conductivity

The insulating ability of the insulation material used has an obvious and significant impact on the likelihood of surface condensation and the insulation thickness necessary to prevent this condensation. There are many ways to characterize insulating ability with the most common in the N. American mechanical insulation industry being the thermal conductivity (k-Factor) at 75°F mean temperature. This simple characterization is useful when discussing insulation materials but should not be used in actual thickness or other heat transfer calculations. The often complicated relationship of thermal conductivity to mean temperature requires that any heat transfer calculations be done using this actual and full curve not a single point representation of this curve such as the 75°F value. When comparing k-Factor, note that a lower value is better. Table 5 shows the k-Factor of several mechanical insulation types at 75°F mean temperature taken from the respective ASTM material standards.

Of course, there are many properties besides k-Factor that should be considered when selecting an insulation material including cost, water resistance, flammability, availability, and more. Nonetheless, the insulation material used and its thermal conductivity have a direct and strong impact on the thickness required to prevent surface condensation and must be considered when selecting an insulation material and certainly when designing the thickness of the insulation.

Table 6 shows the strong influence of the insulation thermal conductivity on the insulation thickness needed to prevent surface condensation. As the thermal conductivity goes down (gets better), the required insulation thickness also decreases.

System Geometry – Pipe Size and Flat Surface Orientation

The last factor to be discussed is the geometry of the system. What this means is the NPS for pipe scenarios and the orientation of the surface for flat tank or duct scenarios. The cold flat surface can be vertically oriented, horizontal facing downward, or horizontal facing upward. The influence of the flat surface orientation is a phenomenon that is often overlooked leading to the mistake wherein all surfaces of a tank or duct are insulated with the same thickness. In reality, the convective component of heat transfer is different in each of these orientations and this leads to a need for different insulation thicknesses for each orientation. The difference in convective heat transfer is caused by the cold air sinking and hot air rising phenomena coupled with the possible interference of this movement by the tank or duct. As an example, on the top of a tank or duct, the cold air next to the surface of the insulation system should naturally sink but it is “trapped” by the presence of the duct and insulation system below it. This causes the cold air to stay longer at the insulation system surface which leads to a colder insulation system surface, and a greater tendency to get condensation on this surface. To account for this, the insulation thickness on that top surface must be increased.

Table 7 shows the influence of pipe size and flat surface orientation on the insulation thickness required to prevent condensation. As can be seen, as the pipe size (NPS) increases, the insulation thickness required also increases and this is a moderate effect growing in significance at colder pipe temperatures. As can also be seen, the insulation thickness required on cold flat surfaces is greatest on the top of a tank/duct, lowest on the bottom, and intermediate on the sides of the tank/duct.

SUMMARY INFLUENCE OF THE VARIOUS FACTORS

The influence of all the factors is summarized in Table 8 which shows both the effect of each factor on insulation thickness and also a qualitative assessment of the magnitude or size of the effect of each factor.

Another way of summarizing the influences is to categorize the factors as either helpful – that is they reduce the likelihood of condensation or reduce the insulation thickness required to prevent condensation or harmful – that is they increase the likelihood of condensation or increase the insulation thickness required to prevent condensation. This is shown in Figure 3.

When examining the impact of a design condition, the SIZE of this effect must also be considered. For example, more care should be taken in considering relative humidity than in ambient temperature due to the former having a much larger effect.

SELECTING DESIGN CONDITIONS & SYSTEM COMPONENTS

Design Ambient Temperature

Recall from earlier in this document that the impact of the ambient temperature on the likelihood of condensation control or insulation thickness needed to prevent condensation is small. As a result, accurate selection of an ambient temperature is simply not very important. The most common and a perfectly acceptable approach is to select a reasonably harsh temperature for the situation.

Table 9 shows some examples of reasonable ambient temperatures for use in the design of insulation systems for several different applications.

Of course, if energy efficiency is a separate design criteria, selection of ambient temperature is of critical importance.

Design Wind Speed

In indoor applications, it is usually best to select 0 mph (no wind) unless it is certain that a forced ventilation will always be present and providing a wind speed above zero.

In outdoor applications there are two approaches that can be used. A reference source for climatic information can be used such as the ASHRAE Handbook of Fundamentals, an online database such as weatherbase. com, or a computer program with weather data such as WYEC2 or TMY2. The problem with this approach is that it is not clear which type of wind speed value should be used. Should it be a yearly average, the highest speed recorded, some high percentile value like 99th, or something else? As an alternative the industry standard 7 mph value can be used unless the system is known to be in a high or low wind location. Examples of locations where a higher wind speed may be appropriate include on or near the ocean shore, under bridges, and in mountain passes. An example of a location where a lower wind speed may be appropriate is on a rooftop location where the pipe is blocked from the prevailing winds by some solid structure. While it is certainly less accurate to use a single 7 mph value as a rule of thumb for most locations, this approach does have the advantage of simplicity and is very widely practiced.

Insulation Type and Thermal Conductivity

There are many factors that should influence the selection of the insulation material type and one of the key factors is the insulating ability (thermal conductivity) of the material. Where it is reasonable and appropriate, select an insulation material type that has the better (lower) thermal conductivity. Once an insulation material type has been selected or where it has already been specified, obtain thermal conductivity data for that insulation material from the most recent version of the appropriate ASTM material standard. Be cautious of using thermal conductivity claims by individual manufacturers. There are many ways to obtain thermal conductivity test values that are better than those published in the ASTM standards and these “better” values may not be truly indicative of long-term material performance.

System Geometry – Pipe Size and Flat Surface Orientation

This is not really a factor that can be controlled during insulation system design. The pipe size and orientation of any flat surfaces have already been set by the needs of the facility. The response by the insulation system designer on this factor is merely to understand that the pipe size and flat surface orientation can have an influence on the likelihood of surface condensation and on the insulation thickness needed to prevent this condensation. The required insulation thickness for each pipe size and surface orientation must be determined independently.

Jacket Type – Emittance (ε)

In most applications in N. America, the jacket choices are straightforward. In outdoor locations aluminum jacketing (ε = 0.1) is used for UV resistance and strength. In indoor locations PVC jacketing (ε = 0.9) is used. However, there are exceptions to these general practices when a different jacket type is dictated by conditions specific to the application. In an indoor environment where a great deal of physical abuse is likely such as in a loading dock, aluminum jacketing (ε = 0.1) should be considered. It might even be prudent to use a greater thickness of aluminum jacketing to provide even more physical abuse resistance. In an indoor or outdoor environment where there will be either excessive exposure to corrosive chemicals or a need for an especially high resistance to fire, stainless steel jacketing (ε = 0.3) should be considered.

As mentioned briefly earlier in this document in the section on the influence of emittance on surface condensation, the use of painted metal and especially painted aluminum (ε = 0.8) can be quite helpful in reducing the likelihood of surface condensation or reducing the thickness of insulation necessary to prevent surface condensation. This benefit of painted metal jacketing arises solely as a result of its higher emittance than bare metal. Consider, for example, Table 10 which shows the insulation thickness required to prevent surface condensation on an ammonia refrigeration line with standard aluminum and painted aluminum jacketing. The use of painted aluminum jacketing in this scenario yields an almost 50% reduction in insulation thickness. The use of painted aluminum jacketing also provides an increased resistance to exterior jacket corrosion and is particularly useful on rooftop refrigeration lines operating at pipe temperatures in the -60 to +20°F range.

While the use of jacketing with a higher emittance (e.g. painted metal) has a strong influence on insulation thickness related to control of surface condensation, it should be noted that a higher emittance jacketing has a very minor influence on overall heat transfer. Therefore, the use of higher emittance jacketing will have minimal impact on energy efficiency.

Regardless of the type of metal jacketing used (aluminum, painted aluminum, stainless steel, or some other metal) it is critical that the metal jacketing have a 3 mil thick polysurlyn moisture barrier factory heat laminated to the interior surface to help prevent galvanic and pitting/crevice corrosion on the interior surface of the jacketing.1

Relative Humidity

Selection of the proper relative humidity for control of surface condensation in mechanical insulation system design is definitely more complicated than selection of the other factors and also has the largest influence on the necessary insulation thickness. The first step is to distinguish between indoor and outdoor locations.

Relative Humidity Indoors

In indoor locations surface condensation on mechanical insulation systems is usually a disaster leading at best to demands that the contractor fix the system and at worst to lawsuits. Surface condensation can drip on floors causing slip hazards. It can drip onto manufactured goods below damaging them. It can drip onto food either during processing or storage contaminating it. It can drip onto ceiling tiles causing unsightly water stains. It can lead to mold growth on the surface of the vapor retarder particularly when the outer surface of the vapor retarder is made of paper. It can lead to mold growth inside the insulation system especially when the insulation material offers little or no resistance to water absorption and water vapor permeability. It can lead to reduced insulating ability of the system which exacerbates the surface condensation problem even further.

Indoors, surface condensation must be avoided 100% of the time. This is possible with proper insulation system design provided the indoor air is dehumidified and controlled at a relative humidity well below 100%.

Relative humidity in indoor locations can vary widely. Consider these examples:

• Food/beverage processing areas can have high r.h. and be subject to washdowns
• Machine rooms of commercial buildings can have high r.h. and can even be ventilated with outside air
• Concealed areas of commercial buildings can have higher r.h. • Plenum areas of commercial buildings can be higher in r.h.
• Office areas and other occupied spaces of commercial buildings are usually low in r.h.

Insulated pipe in indoor locations should be designed to prevent surface condensation at a very high r.h. compared to what is likely in that area. Designing for an ambient relative humidity of 85% or higher is completely reasonable since it is necessary to prevent surface condensation 100% of the time indoors. Designing for such a high ambient relative humidity usually has less impact on required insulation thickness indoors because of the high emittance of the jacketing (usually 0.9), the pipe size usually being smaller, and the pipe temperature usually not being that cold – especially on chilled water lines in commercial buildings.

Relative Humidity Outdoors

The key fact to remember in order to understand the philosophy of insulating to prevent surface condensation in outdoor locations is that it is impossible to prevent surface condensation 100% of the time. Sooner or later, the relative humidity outdoors will reach 100% which would require an obviously impossible infinite thickness of insulation to prevent surface condensation. Even if you design to a high relative humidity like 90-95% r.h. it will eventually reach a relative humidity above this limit. This high humidity might be reached during or immediately after a rainstorm. It might be reached on a cool morning when there is a heavy dew. It might be reached when there is fog on the ground. Any concern with the inevitability of surface condensation on outdoor cold pipe is tempered once it is realized that periodic surface condensation on outdoor pipe is perfectly acceptable. After all, the pipe surface gets wet from rain, dew, fog, and snow. It is not a significant problem if the frequency of surface wetness is increased slightly due to actual surface condensation. The key for insulated pipe in outdoor locations is to design the system to prevent surface condensation a reasonably high percentage of the time but how should this be done? There are several approaches that can be considered.

1. 2009 ASHRAE Handbook of Fundamentals, Chapter 14

This is the trickiest approach. The American Society of Heating, Refrigeration, and Air conditioning Engineers Handbook of Fundamentals, Chapter 14 contains climatic information on extreme conditions for numerous global locations. The tables in this chapter include the 0.4th percentile dewpoint and mean coincident dry bulb (MCDB) temperature. From this information and a psychrometric chart or program, the percent relative humidity can be determined. However, field experience has suggested to this author that the conditions arising from this approach are not harsh enough. As an example, consider the information in this chapter for Charlotte, NC. The 0.4th percentile dewpoint is 74°F with a MCDB = 80.8°F. This yields a relative humidity of 80%. However, consider Figure 4 which shows the frequency distribution of outdoor relative humidity based on the typical meteorological year data for Charlotte.2 As this figure shows, there is a very significant portion of the year (32.6%) when the relative humidity is above 80%. Too often, this approach seems to yield design conditions which are not harsh enough and which would allow surface condensation too frequently.

2. 2009 ASHRAE HANDBOOK OF FUNDAMENTALS, CHAPTER 23

This chapter recommends the use of 90% r.h. for all outdoor applications and indoor applications vented to outdoor conditions. This is coupled with the 0.4th percentile dew-point and with the use of a psychrometric chart, the dry-bulb temperature can be determined. Using the Charlotte, NC example again, the 0.4th percentile dewpoint is 74°F. Coupling this with 90% relative humidity yields a dry-bulb temperature of 77°F so the design conditions recommended according this this approach would be 77°F and 90% relative humidity. This approach ignores the fact that the 0.4th percentile dewpoint has a mean coincident dry-bulb temperature already associated with it as was discussed in the first approach. The approach of always using 90% r.h. would probably yield acceptable results in commercial building chilled water applications where the pipe temperature is seldom lower than 40°F. For colder temperature applications and especially those at cryogenic temperatures like liquid oxygen (-297°F), liquid nitrogen (-320°F), liquid natural gas (LNG) (-265°F), and even very cold ammonia refrigeration lines at -40 to -60°F, the use of 90% r.h. yields a requirement for an insulation thickness to prevent surface condensation that is often considered impractical. As an example, consider the 15.5 inches of PIR insulation thickness required on a large diameter LNG pipe to prevent surface condensation at 90% r.h. from Table 2. This thickness is impractical and would never be used. Instead, the system designer would design to a lower relative humidity, accept the consequence of more frequent surface condensation, and design other aspects of the insulation system to prevent damage from the more frequent surface condensation.

3. RECOMMENDED APPROACH TO SELECTING OUTDOOR DESIGN RELATIVE HUMIDITY

Unless there are detailed specific reasons to use a higher or lower relative humidity, use a value in the range of 80-90%. Within this recommended range, base the specific value selected on knowledge of the climate at the job location and the pipe temperature. At warmer pipe temperature including chilled water in the 35-45°F range, it is reasonable to use 90% r.h. At colder pipe temperature, use the lower design relative humidity values in this range (80-85%) to determine the required insulation thickness and examine this thickness to see if it is impractical. If it is too thick, then take some step to reduce the required insulation thickness. This could be the use of a higher emittance jacket such as painted metal instead of bare metal or it could be to design to a lower relative humidity with commensurate changes to other aspects of the system design such as vapor retarder permeance and quality to prevent system damage from the more frequent surface condensation.

COMMON MISTAKES, TRICKS, AND TIPS

Common Mistakes in Mechanical System Design Related to Surface Condensation

• Owners, system designers, or others involved want to design the insulation system to prevent condensation at 100% relative humidity. This issue most commonly manifests itself as a request for the insulation thickness that will prevent condensation at 100% r.h. This cannot be done since it would require an infinite thickness of insulation to accomplish. The proper approach is to design to between 80-90% r.h. outdoors depending on various factors described above and to about 85% r.h. indoors.
• Owners, system designers, or others involved want to design the insulation system to prevent condensation 100% of the time in an outdoor or non-climate controlled indoor location. This cannot be done. Sooner or later the relative humidity will rise to above any design value and will occasionally reach 100% r.h. To accomplish this impossible design goal would require an infinite insulation thickness. The proper approach is to design the insulation system to allow surface condensation a small but non-zero fraction of the time and to design other aspects of the system so that this infrequent but inevitable surface condensation does not damage the insulation system.
• Designing an insulation system for climate controlled indoor conditions and then starting up the system before the building has been enclosed and completed. In effect, this is simply operating the system in an environment with a higher r.h. than the system was designed to handle. Surface condensation in this situation is common and has been the cause of some high profile system failures.
• Unrepaired damage to the insulation system during construction – often by other trades. This type of damage almost always causes breaches in what is supposed to be a continuous vapor retarder. If the system is started up without repair to the damaged insulation system, water vapor quickly enters the insulation system and condenses. This is the beginning of a classic vicious circle. The condensed water leads to poorer insulating ability of the insulation (higher/worse k-Factor) which leads to more condensation and ever worsening k-Factor. The water intrusion into the insulation system can also cause pipe and jacket corrosion, mold growth, ice formation, and loss of process control.
• Designing for 50% relative humidity in indoor locations. This level of r.h. may make sense in the occupied portion of an office building if the air conditioning/dehumidification system can guarantee this value is never exceeded. However, there are other parts of a commercial building which may have higher r.h. including machine rooms, kitchens, locker/shower rooms, and even concealed spaces like pipe chases. It is critical that the humidity is not assumed to be always less than 50% in these other portions of the building. In light industrial facilities such as food and beverage manufacturing, high humidity processing areas can readily exist despite the air conditioning of the building in general or portions of the building. The same pipe may need different thicknesses of insulation or other changes to the insulation system depending on which portion of a building it is in.
• Using an emittance of 0.4 for aluminum jacketing. This is a common error based on some older specifications and handbooks that list this value as the emittance of aluminum jacketing. Using this incorrectly high value will result in inadequate insulation thickness to prevent surface condensation at the specified conditions. An accurate value to use for standard oxidized in service aluminum jacketing of all finishes (plain, stucco, and 3/16” corrugated) is 0.1. This emittance is also contained in the new ASTM standard on aluminum jacketing, C1729.

Common Useful Tricks in Mechanical System Design Related to Surface Condensation

•  Use painted metal jacketing to significantly increase the emittance. Bare aluminum has an emittance of 0.1 while painted aluminum is 0.8. This is a very large increase since the scale of emittance only goes from 0 to 1. Making this change will raise the surface temperature significantly which will either reduce the insulation thickness required to prevent surface condensation or prevent surface condensation to a higher percent relative humidity. This trick is most helpful in outdoor applications where metal is the preferred type of jacketing. While not widely practiced in cryogenic applications, this is a trick that engineers and other system designers should consider when designing insulation systems to prevent surface condensation on pipes and other mechanical equipment operating at cryogenic temperatures. Painted aluminum jacketing has the added advantage of being more corrosion resistant than standard (bare) aluminum jacketing.

Tips Related to Preventing Surface Condensation on Mechanical Insulation Systems

• Contractors, facility owners, and insulation system designers should work with manufacturers who understand the complex design issues described in this paper
• Some insulation manufactures know as much or more about some aspects of insulation system design than do the persons charged with designing these systems. Contractors, facility owners, and insulation system designers should seek input from these knowledgeable manufacturers on selecting appropriate design conditions and on insulation system design but should also educate themselves on these issues so manufacturer’s recommendations can be properly assessed.
• When comparing insulation materials, comparing insulation thickness tables, or preparing new insulation thickness tables, it is very important that the same conditions be used for all materials. Even a seemingly small change like one manufacturer using an aluminum emittance of 0.4 while another uses the correct value of 0.1 can have a significant impact on the recommended/calculated insulation thickness.
• While this paper has focused only on condensation control, remember that this is only one design criterion. There are many other possible design criteria including code compliance, process control, energy efficiency, personal protection, and fire protection.
• This document is merely an overview of the impact of various factors on the likelihood of surface condensation and is not intended to replace proper system design by an engineer experienced with mechanical insulation on cold surface. Insulation system design has many subtleties that are not addressed by the more simplistic review presented here.

CONCLUSIONS 

Surface condensation on insulation systems for cold mechanical equipment (pipe, tanks, vessels, etc.) is a simple concept. Surface condensation will occur if the surface temperature of the insulation system is less than the dewpoint temperature of the surrounding air. This simple relationship is made complicated because each of these two temperatures is dependent on the interrelationship of a myriad of factors as shown in Figure 5.

All of these factors must be fully and properly considered or selected to assure optimum control of insulation system surface condensation – commonly called condensation control.

Of these factors, selection of the proper relative humidity to use for system design is the most important and also the most complicated to handle.

One last but very important point to emphasize is that in outdoor applications, surface condensation cannot be prevented 100% of the time.

REFERENCES:

1. J. Young, “Preventing Corrosion on the Interior Surface of Metal Jacketing”, Insulation Outlook, November, 2011.
2. ASHRAE 2009 Handbook of Fundamentals, Chapter 23, p. 3