By Grant Laidlaw
Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavour to enlighten. I am going back to basics as I have questions coming in which indicate that the basic understanding necessary to work in industry is not in place.
Jannie asks: “I have often wondered why do we fit insulation to pipes on split units, does it really make such a difference when looking at costs, trunking looks better anyway? On larger air conditioning systems what other types of insulation can we use? I would like a better understanding around this subject.”
Hi Jannie, let us start by asking: ‘Why insulate?’
Insulation is used to prevent heat loss or heat gain, to control noise (acoustic insulation) and to prevent condensation on cold surfaces (the vapour barrier). Although its primary purpose is economic, insulation also provides more accurate control of process temperature and protection (health, safety and comfort) of people. We need our air-conditioning and refrigeration systems to operate at maximum efficiency levels especially as our electricity supply is so limited. Remember, some buildings run hundreds of units, and the energy savings can therefore be significant.
Insulation is the energy stabiliser – it keeps the wanted energy in and the unwanted energy out. It protects, controls and saves.
Building and industrial safety standards must be adhered to when choosing the type of insulation products, vapour barriers, coverings, coatings and/or claddings. Knowledge of building and industrial fire standards is extremely important. One must know where and for which application combustible materials may be used. The rules for that building/room/industry apply to all materials used therein.
The suppliers of insulation materials supply data sheets containing the thermal, acoustic (where applicable), fire and safety properties of their material, as well as any handling, storing and installation instructions.
Thermal and acoustic insulation is used externally and internally on ducts. Pipe-work is insulated to save energy, prevent condensation and in some applications to reduce noise. Understanding the basics of heat transfer is vital in understanding the subject of insulation.
In order to understand thermal insulation, it is necessary that we first understand the basic principles of heat transmission and the important influence of surfaces and shape on thermal calculations. Heat is transferred via three mechanisms:
- Conduction to other bodies that are in contact
- Convection to the surrounding fluid or gasses
- Radiation (infra-red)
Heat transfer by conduction
Heat passes through solid materials that are in contact by means of conduction. The rate at which this happens depends on the thermal conductivity of the material in question. Metals generally have high thermal conductivities, while those of non-metals are considerably lower. The lowest thermal conductivity is shown by materials where the amount of solid matter is quite small, but where there is a considerable proportion of voids that are not big enough to carry heat by means of convection.
The Coefficient of Thermal Conductivity (k-value): The thermal conductivity (k-value) is defined as the heat flow in watts (joules/second), which takes place across a cube 1m2 in cross section and 1 metre thick, if the difference between the two faces is 1 K (1 Celsius degree) – Units W/mK.
The Thermal Resistance (R- value): The R-value is calculated by dividing the thickness of insulation materials (m) by the coefficient of thermal conductivity – Units m2K/W. The thermal resistance is defined as the opposition to the passage of heat through the material.
R-value = thickness (m)
Thermal resistances of components can be added together and thus the total thermal resistance of combined structures can be obtained.
Heat transfer by convection
Convection is the bodily transfer of heat by fluid molecules. Liquid or gas is warmed up by contact with a surface and moves away carrying the heat with it. It is called forced convection if the movement is caused by an external agent, for example stirrer, pump, fans or wind. Alternately, if the movement is simply due to a change in density of the fluid-due to the temperature, we talk of free convection.
Convection is important in considering the transfer of heat to surrounding air, and the influence of wind.
Heat transfer by radiation
Whenever there are two surfaces at different temperatures facing each other, or there is a single surface surrounding or facing an open space, there is a heat exchange by radiation. It occurs at the speed of light by emitting or absorbing infrared radiation.
Emissivity and reflectivity
Emissivity, (symbol ‘e’), is a dimensionless number that states which fraction of heat supplied to a surface is emitted in the form of radiant heat, at a specified temperature. For example, 18% of radiation from the sun onto a surface of polished aluminium will be absorbed, while 82% will be reflected. Only 3% of the low-grade heat that passes to the sheet will be further transmitted.
Types of insulation
When looking at other insulation materials, there is a tendency to look upon thermal conductivity as the sole factor, but this is not always the most important factor. Other considerations are: mechanical strength, physical and chemical resistance, temperature resistance, fire resistance, chemical resistance, hygroscopy, protection against animal and fungal attack. When looking at different thermal insulation materials, one must consider thermal performance in the main, but all systems and applications must be seen holistically.
The best thermal insulation materials have a very high void content, such as small bubbles of air/gas, which are stopped from moving around by solid materials, which surround them. In practice a k-value of about 0.030W/mK is as low as one can achieve with insulation based on air encapsulation such as high void open cell foams and mineral wools. In reality the k-value is rarely below 0.035W/mK.
Looking at the types and forms of insulation, we find that there are four main types of insulation:
- Fibrous insulation is composed of small diameter fibres that divide the air space finely. The fibres may be parallel or perpendicular to the surface being insulated and they may be separated or bonded together. Mineral wools are typical examples of this type of insulation. The occupational health and safety aspects of these products have been extensively studied and they are considered worldwide to be safe.
- Cellular insulation is composed of small individual cells of gas separated from each other by a polymer/plastic or rubber. The cellular material may be glass or foamed plastic such as polystyrene, polyurethane or rubber.
- Granular insulation is composed of small nodules that contain voids or hollow spaces. It is not considered to be true cellular material since gas can be transferred between the individual spaces.
- Flexible/reflective foils are also known as radiant barriers. These are laminated foils or ‘bubble’ foils.
Insulation is produced in a variety of forms suitable for specific functions and applications. The combinations and type of insulation determines the proper method of insulation. The forms most widely used are:
- Rigid board comes in blocks, sheets or pre-formed shapes. Fibrous, cellular and granular insulations are produced in these forms.
- Blankets: fibrous insulations are produced in this form.
- Cement (insulating and finishing) can be produced from fibrous and granular insulation. They may be of the hydraulic air-setting or air-drying types.
- Foam materials, preformed pipe insulation, sheets.
How do you select the correct product for your application? As outlined before, one must look at this as holistically as possible.
When comparing the thermal performance of insulation materials, a few factors must be considered:
- Thermal conductivity changes as temperature changes. Thus, when comparing thermal results, one must take note of the temperature at which the product was tested.
- Thermal conductivity is different for different densities of product. Thus, the thermal test results must be for that particular product and specific product density.
Thermal Resistance (R-value) is the thickness of the product (m) divided by the thermal conductivity (k-value). The higher the R-value, the better the thermal performance of the product. Once you have decided on the thermal performance required, you can calculate the needed R-value. The R-value of insulation products can be compared directly (at the same temperature). Economics and other factors can then be used in the decision-making process.
One can achieve the correct R-value by thickness or density of product. A 10% increase in thickness gives a 10% increase in thermal resistance. Sometimes there are limitations on thickness due to space considerations.
Is the material going to be exposed to possible mechanical damage in application or in use? Must the product be rigid or flexible? Is a high compressive strength needed?
If the material is to be used in – or potentially will be in contact with – water, one must consider the porosity of the materials. Closed pore materials, such as polystyrene will be virtually impervious to water, while open pore materials will absorb water.
What are the actual service temperatures? There are a number of dimensions to temperature considerations:
- At very low temperatures insulation material becomes brittle and may crumble.
- At high temperatures the material may char, will soften, sinter, and finally melt.
- In some cases chemical reactions may occur, creating a fire hazard if exothermic.
- If the material is to be used under intermittent conditions, it must be able to withstand the thermal cycling – in terms of thermal expansion and elasticity.
Insulation materials must have the fire properties that comply with the fire regulations applicable to the area where they are being installed. The following properties must be considered: fire resistance (where applicable); flame spread; smoke generated; generation of toxic fumes; and the potential dripping hazard.
In general, materials that are acidic (pH less than 7) should not be used in alkaline environments, and materials that are basic (alkaline, pH above 7) should not come into contact with acidic materials or metals such as aluminium. Solvent resistance must also be considered.
If water vapour pressure inside the insulation is lower than that of the surrounding air, the material will attract water vapour, which condense on the surface or may react chemically with the insulation material. If hygroscopic materials are used, they must be fully protected by an impervious layer/vapour barrier.
Protective coverings and finishes
The efficiency and service of insulation is dependant on its protection from moisture entry and mechanical or chemical damage. Choices of jacketing and finishing materials are based on the mechanical, chemical, thermal and moisture conditions of the insulation, as well as cost and appearance. Particular attention should be paid to ultraviolet from sunlight degrading the insulation.
Protective coatings can be divided into functional types
The basic function of weather barriers is to prevent ingress of water. Applications are usually metal jackets, plastic or coatings of weather barrier mastics.
These are designed to retard the passage of moisture vapour from the atmosphere to the surface of the insulation. Joints and overlaps must be sealed with vapour tight adhesive or sealer. There are three types of vapour barriers:
- Rigid jacketing; reinforced plastics; aluminium or stainless steel fabricated to exact dimensions and sealed vapour tight.
- Membrane jacketing metal foils, laminated foils or treated paper, which are generally factory applied to the insulation.
- Mastic applications, either emulsion or solvent type, which provide a seamless coating, but require time to dry.
Mechanical abuse covering
Metal jacketing/cladding provides the strongest protection against mechanical damage from personnel, equipment
and machinery. The compressive strength of the insulation should also be taken into consideration when assessing mechanical protection.
Low flame spread and corrosion resistant coverings
When selecting material for potential fire hazard areas, the insulation and the covering must be considered as a composite unit.
Appearance coverings and finishes
Coatings, finishing cements, covers and jackets can be chosen with regards to their appearance in exposed areas.
Coatings and jackets must present a smooth surface, which resists fungal and bacterial growth, especially in food processing areas and hospitals.
Thank you for your question, Jannie. I hope that this gives a better understanding of what insulation is about and its application. One special note, when applying insulation (armorflex) to small air conditioning system pipework do you use cable ties? If so, how tight do you pull up the cable ties? Remember you must not compress the armorflex to more than a maximum of 10% or the insulation will be compromised.
Thanks to everybody for the overwhelming response. I receive on average, over sixty questions per month and cannot publish all of them. But keep them coming, as I may answer you directly.
Looking forward to hearing from you.
About Grant Laidlaw
Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale. He holds a Bachelor of Business Administration and an associate degree in educational administration. He has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical. He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018. Currently he is the SAIRAC national treasurer.