Many people request assistance in the practical aspects of the industry. In response, I investigate the problem and endeavour to present the best possible explanation.
Thank you for all your questions sent in. Send your problems (and sometimes your creative solutions) to firstname.lastname@example.org with problem page in the subject line. You may include pictures.
In the last issue, we answered Chris from Johannesburg’s question regarding insulation. While a large portion of his query was explained, insulation will be scrutinised closer in this issue for further clarification.
Having looked at why insulate, we move on to the selection of insulation which best suits the application.
Chris from Johannesburg asked:
“Hi Grant, I have often wondered why we fit insulation to pipes on split units. Does it really make such a difference when looking at costs, as trunking looks better anyway? How thick should insulation be, as everybody uses it in their own way? I would like to know more please.”
When comparing thermal performance of insulation materials, a few factors must be considered:
Thermal conductivity differences such as temperature changes. As a result, when comparing thermal results, it is important to take note of the temperature at which the product was tested.
Thermal conductivity varies for different densities of product, so thermal test results must be for the respective products and their 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. Do you require insulation to minimise heat gain/loss to a certain value, or give you a certain cold face temperature to reduce risk of condensation?
Once the thermal performance required has been identified, calculate the R-value. The R-value of insulation products can be compared directly, and at the same temperature. Economics and other factors can then be used to help make a decision.
The correct R-value can be achieved by thickness or density of product. A 10 percent increase in thickness gives a 10 percent increase in thermal resistance. Sometimes there are limitations on thickness due to space considerations. If the product is clad with a metal sheet, an increase in thickness means an increase in metal cladding.
Considerations to be taken into mind include: Is the material going to be exposed to possible mechanical damage in application or in use? Must the product be rigid or flexible? And is a high compressive strength needed?
If the material is to be used in, or potentially have contact with water, porosity of the materials must be considered. Closed pore materials, such as polystyrene are virtually impervious to water, while open pore materials will absorb water.
What is the service temperature? There are a number of dimensions to temperature considerations:
At very low temperatures insulation material becomes brittle and crumbles, and
At high temperatures the material may char, soften, sinter and finally melt.
If exothermic, a chemical reaction may create a fire hazard. 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 fire properties that comply with the fire regulations applicable to the region they are being installed. The following properties must be considered:
Fire resistance (where applicable)
Generation of toxic fumes
Potential dripping hazard
In general, acidic materials with a pH less than seven should not be used in alkaline environments, and basic materials with alkaline -- pH above seven -- 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 condenses 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 dependent 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. Protective coatings are divided into six functional types (vapour barriers will be covered in more detail).
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, and
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. Most of the available jacketing/cladding and mastics have low flame spread rating. The manufacturers must supply this information.
Appearance coverings and finishes
Coatings, finishing cements, covers and jackets are chosen primarily for their appearance value in exposed areas. Typically for piping, jacketed insulation is covered with a reinforcing canvas and coated with mastic to give a smooth, even finish.
Coatings and jackets must present a smooth surface, which resists fungal and bacterial growth, especially in food processing areas and hospitals. High temperature steam or high-pressure water wash down conditions require coverings with high mechanical strength and temperature rating.
The effect of relative humidity on insulation effectiveness:
Condensation control. Exposure to moisture and lack of condensation control are factors often missed in the selection and maintenance of an insulating system. To understand the importance of moisture in the insulation, it is useful to keep in mind:
Insulation saturated with water transfers heat approximately 15 to 20 times faster than dry insulation, and
Insulation saturated with ice transfers heat approximately 50 times faster than dry insulation.
Condensation will occur if the surface temperature falls below the dew point temperature, the temperature at which the ambient air of a certain relative humidity, will become saturated if cooled. The insulation thickness must be sufficient enough to ensure the surface temperature of the vapour barrier is above the dew point temperature for the worst anticipated conditions of temperature and humidity.
The following table gives the dew point temperature for any given set of conditions:
Dew point temperature at various temperatures and relative humidities.
Economic thickness of insulation
In the vast majority of industries, energy is purchased at high cost and in many cases, it is the greatest single operational cost factor. As commented in the introduction, world shortages of energy, greenhouse gas considerations, and the threat of global warming are all forcing governments to take action against wasteful use of energy.
Insulation is an integral part of any industrial cost control programme and this must be recognised by engineers in the design of insulation systems for both new projects and for re-insulation of existing facilities. The decisions are economic. The aim is to specify insulation thicknesses that will achieve the minimum over-all cost over the planned lifetime of a project. In other words, it is to determine the economic thickness of the insulation.
Economic thickness is defined as the thickness of insulation that produces the lowest total cost for a given period. This total cost being the installed cost of the insulation plus the cost of energy lost or gained (where refrigeration is concerned) through this thickness. In most cases, the insulation material to be specified for a particular unit or pipeline can be decided by factors which will be covered later. In these instances, the installed cost of different thicknesses of the selected material will be the basis for determining optimum thickness.
Sometimes however, it may be necessary to evaluate the installed cost at different thicknesses of two or more grades or types of insulation.
In either case, the objective will be to design a system to show the greatest Rand value over a specified period in both capital and running costs.
The planned lifetime of the project should be the period over which the costs are evaluated. The long-term stability of modern insulating materials is such that large-scale replacement, apart from damage caused during plant maintenance, will not be necessary within the lifetime of the plant. Thus, the evaluation period will usually be 10, 15 or 20 years.
Accounting practices of some companies require the installed cost of an insulation system to be ‘written off’ over a shorter period of two to three years. If the cost appraisal to determine economic thickness is made on such a basis, however, it must be realised that any extension of operational life beyond this period will be costly from the point of view of lost energy, as insulation thickness will be less than optimum.
To establish for an economic design study, the installed insulation cost must first be expressed as an annual cost. This can be added to the average annual cost of lost energy over the expected life of the insulation system.
Cost of insulation
The cost of insulation is expressed as an annual cost factor by adding:
The annual insulation investment cost, and
The annual cost of maintenance of the insulation system.
The annual insulation investment cost is the original installed cost of the insulation system proportioned over the number of years of the planned project life making allowance for the appropriate cost of money or the required rate of return on an increment of applied insulation.
Some allowance should be made for the annual maintenance cost of the insulation system. Left undisturbed, most insulation systems are virtually maintenance free, but there is always the prospect of mechanical damage and also the chance that some replacement of material may be necessary where it may have to be removed to allow access for plant maintenance and modification. Therefore it is wise to allow for this by a factor of five to 10 percent of the annual insulation investment cost. The installed cost of insulation is by far the most critical variable in the consideration of economic thickness. Wherever possible, prices for each item should be obtained, allowing for a range of thicknesses of selected insulation material. The unit-installed prices must allow for the cost of insulating valves, flanges and fittings, as all of these must be insulated.
Unfortunately Chris, you will have to read the next issue where I shall continue.
Just as a matter of interest, here is the state of the insulation on a plant I came across in Johannesburg. The plant is operational and used to cool a fairly important building. So much for maintenance.
Thanks to everybody for the overwhelming response. I receive on average, over 60 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.